System and method for radio frequency calibration exploiting channel reciprocity in distributed input distributed output wireless communication

Abstract

To provide a system and method for radio frequency (RF) calibration in a multiple antenna system (MAS) with multi-user (MU) transmissions ("MU-MAS") exploiting uplink / downlink channel reciprocity.SOLUTION: The RF calibration is used to compute open-loop downlink precoder on the basis of uplink channel estimates, thereby avoiding feedback overhead for channel state information as in closed-loop schemes. For example, an MU-MAS of one embodiment comprises a wireless cellular network with one or multiple beacon stations, multiple client devices and multiple distributed antennas operating cooperatively via precoding methods to eliminate inter-client interference and increase network capacity.SELECTED DRAWING: Figure 16

Description

Technical Field

[0001] (Cross - reference to Related Applications) This application may be related to the following co - pending U.S. patent applications.

[0002] U.S. Patent Application No. 13 / 797,984, titled "Systems and Methods for exploiting inter - cell multiplexing gain in wireless systems via distributed input distributed output technology".

[0003] U.S. Patent Application No. 13 / 797,971, titled "Systems and Methods for exploiting inter - cell multiplexing gain in wireless systems via distributed input distributed output technology".

[0004] U.S. Patent Application No. 13 / 797,950, titled "Systems and Methods for exploiting inter - cell multiplexing gain in wireless systems via distributed input distributed output technology".

[0005] U.S. Patent Application No. 13 / 633,702, titled "Systems and Methods for wireless backhaul in distributed - input distributed - output wireless systems".

[0006] U.S. Patent Application No. 13 / 475,598, titled "Systems and Methods to enhance spatial diversity in distributed-input distributed-output wireless systems".

[0007] U.S. Patent Application No. 13 / 233,006, titled "System and Methods for planned evolution and obsolescence of multiuser spectrum".

[0008] U.S. Patent Application No. 13 / 232,996, titled "Systems and Methods to Exploit Areas of Coherence in Wireless Systems".

[0009] U.S. Patent Application No. 13 / 464,648, titled "System and Methods to Compensate for Doppler Effects in Distributed-Input Distributed Output Systems".

[0010] U.S. Patent Application No. 12 / 917,257, titled "Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering".

[0011] U.S. Patent Application No. 12 / 802,988, titled "Interference Management,Handoff,Power Control And Link Adaptation In Distributed-Input Distributed-Output(DIDO)Communication Systems".

[0012] U.S. Patent Application No. 12 / 802,974, titled "System And Method For Managing Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDO Clusters".

[0013] U.S. Patent Application No. 12 / 802,989, titled "System And Method For Managing Handoff Of A Client Between Different Distributed-Input-Distributed-Output(DIDO)Networks Based On Detected Velocity Of The Client".

[0014] U.S. Patent Application No. 12 / 802,958, titled "System And Method For Power Control And Antenna Grouping In A Distributed-Input-Distributed-Output(DIDO)Network".

[0015] U.S. Patent Application No. 12 / 802,975, titled "System And Method For Link adaptation In DIDO Multicarrier Systems".

[0016] U.S. Patent Application No. 12 / 802,938, titled "System And Method For DIDO Precoding Interpolation In Multicarrier Systems".

[0017] U.S. Patent Application No. 12 / 630,627, titled "System and Method For Distributed Antenna Wireless Communications".

[0018] U.S. Patent No. 8,170,081, issued on May 1, 2012, titled "System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements".

[0019] U.S. Patent No. 8,160,121, issued on April 17, 2012, titled "System and Method For Distributed Input-Distributed Output Wireless Communications".

[0020] U.S. Patent No. 7,885,354, issued on February 8, 2011, titled "System and Method For Enhancing Near Vertical Incidence Skywave(「NVIS」)Communication Using Space-Time Coding".

[0021] U.S. Patent No. 7,711,030, issued on May 4, 2010, titled "System and Method For Spatial-Multiplexed Tropospheric Scatter Communications".

[0022] U.S. Patent No. 7,636,381, issued on December 22, 2009, titled "System and Method for Distributed Input Distributed Output Wireless Communication".

[0023] U.S. Patent No. 7,633,994, issued on December 15, 2009, titled "System and Method for Distributed Input Distributed Output Wireless Communication".

[0024] U.S. Patent No. 7,599,420, issued on October 6, 2009, titled "System and Method for Distributed Input Distributed Output Wireless Communication".

[0025] U.S. Patent No. 7,418,053, issued on August 26, 2008, titled "System and Method for Distributed Input Distributed Output Wireless Communication".

Background Art

[0026] In the past 30 years, the wireless cellular market has experienced an increase in the number of subscribers worldwide and a demand for better services that shift from voice to web browsing and real-time HD video streaming. This increasing demand for services that require higher data rates, lower latency, and improved reliability has driven the rapid evolution of wireless technologies by different standards. Starting from the first-generation (for voice services) analog AMPS and TACS in the early 1980s, to 2G and 2.5G digital GSM (registered trademark), IS-95 and GPRS (for voice and data services) in the 1990s, 3G related to UMTS and CDMA2000 (for web browsing) in the first half of the 2000s, and finally, to LTE (for high-speed Internet connection) that is currently deployed in various countries around the world.

[0027] Long Term Evolution (LTE) is a standard developed by the 3rd Generation Partnership Project (3GPP) for 4th generation (4G) wireless cellular systems. In theory, LTE can achieve up to a 4-fold improvement in downlink spectral efficiency over previous 3G and HSPA+ standards by utilizing the spatial component of the wireless channel through Multiple-Input Multiple-Output (MIMO) technology. LTE-Advanced is an evolved version of LTE and is currently under standardization. In theory, it will enable up to an 8-fold increase in spectral efficiency over 3G standard systems.

[0028] Despite this technological evolution, in the next three years, wireless carriers are likely to struggle to meet the increasing demand for data rates due to the growing market penetration of smartphones and tablets that provide more data-intensive applications such as real-time HD video streaming, video conferencing, and gaming. The capacity of wireless networks is estimated to expand five-fold in Europe from 2011 to 2015 through improved technologies such as LTE and additional spectrum made available by governments

[25] . For example, the FCC plans to free up 500 MHz of spectrum (300 MHz of which will be available by 2015) by 2020 to promote wireless Internet connectivity across the United States as part of the National Broadband Plan

[24] . Unfortunately, the projected capacity usage by 2015 is 23 times that in 2011 in Europe

[25] , and a similar spectrum shortage is expected to occur in the United States by 2014 [26 - 27]. As a result of this data crisis, the revenues of wireless carriers may fall below their capital expenditure (CAPEX) and operating expenses (OPEX), potentially having a disruptive impact on the wireless market

[28] .

[0029] Due to the insufficient capacity gains provided by the deployment of LTE and the increased spectrum availability, the only foreseeable solution to prevent this impending spectrum crisis is to promote new radio technologies

[29] . LTE-Advanced (an evolved version of the LTE standard) promises further gains over LTE by means of more advanced MIMO technologies and by increasing the density of "small cells"

[30] . However, in order to enable inter-cell cooperation, there is a limit to the number of cells that can be adapted to a specific area without causing interference problems or increasing the complexity of the backhaul.

[0030] One promising technology that provides a large increase in spectral efficiency over the radio link without the constraints of conventional cellular systems is the distributed input distributed output (DIDO) technology (see related patents and applications cited in [0002 - 0020] above). To provide significant performance benefits over conventional wireless systems, the present invention describes the DIDO technology used in relation to cellular systems (such as LTE or LTE-Advanced), both within and outside the constraints of cellular standards. Starting from an overview of MIMO, various space processing technologies used by LTE and LTE-Advanced are examined. Next, the present invention shows how to provide significant capacity gains for next-generation wireless communication systems compared to conventional approaches.

[0031] MIMO uses multiple antennas at the transmitting and receiving sides of a radio link to improve link reliability by means of diversity techniques (i.e., diversity gain) or to provide a higher data rate by means of multiplexing schemes (i.e., multiplexing gain), using space processing [1 - 2]. Diversity gain is a measure of enhanced robustness against signal fading and results in a higher signal-to-noise ratio (SNR) for a fixed data rate. Multiplexing gain is obtained by utilizing additional spatial degrees of freedom of the wireless channel to increase the data rate at a fixed error probability. The basic trade-off between diversity and multiplexing in a MIMO system is described in [3 - 4].

[0032] In a practical MIMO system, link adaptation technology can be used to dynamically switch between diversity and multiplexing modes based on propagation conditions [20 - 23]. For example, the link adaptation methods described in [22 - 23] showed that beamforming or orthogonal space-time block code (OSTBC) is a preferred method in a low SNR regime or channel characterized by low spatial selectivity. On the other hand, spatial multiplexing can provide significant gains in data rates for channels with high SNR and high spatial selectivity. For example, Figure 1 shows that a cell can be divided into two regions. i) A multiplexing region 101 characterized by high SNR (due to proximity to the cell tower or base station) where the spatial degrees of freedom of the channel can be utilized by spatial multiplexing to increase the data rate, ii) A diversity region 102 or cell edge where spatial multiplexing technology is not as effective and diversity methods can be used to improve SNR and coverage (producing only a slight increase in data rate). Note that it should be noted that the circle of the macro cell in Figure 1 labels the shaded central part as the "multiplexing region" and the outer unshaded region as the "diversity region". This same region designation, where the shaded region is the "multiplexing region" and the unshaded region is the "diversity region", is used throughout Figures 1, 3 - 5 even if not labeled. For example, the same designation is also used for the small cell 104 in Figure 1.

[0033] The LTE (Release 8) standard and the LTE-Advanced (Release 10) standard define a set of 10 transmission modes (TMs) that include either diversity or multiplexing modes [35, 85 - 86]. · Mode 1: Single antenna port, port 0 · Mode 2: Transmit diversity · Mode 3: Extended open-loop spatial multiplexing for large delay cyclic delay diversity (CDD), single-user MIMO (SU-MIMO) · Mode 4: Closed-loop spatial multiplexing for SU-MIMO · Mode 5: Multi-User MIMO (MU-MIMO) · Mode 6: Closed-loop spatial multiplexing, using a single transmission layer · Mode 7: Single antenna port, UE-specific RS (port 5) · Mode 8: Single or dual layer transmission, UE-specific RS (ports 7 and / or 8) · Mode 9: Single or up to 8-layer closed-loop SU-MIMO (added in Release 10) · Mode 10: Up to 8-layer multi-layer closed-loop SU-MIMO (added in Release 10)

Summary of the Invention

Means for Solving the Problems

[0034] Hereinafter, the diversity and multiplexing methods commonly used in cellular systems, as well as the specific methods used in LTE outlined above, are described and compared with the technologies specific to Dido communication. First, two types of transmission methods are identified. i) An intracell method that uses multiple antennas to improve the reliability or data rate of links within one cell (utilizing microdiversity in a cellular system), ii) An intercell method that enables cooperation between cells to provide additional diversity or multiplexing gain (utilizing macro-diversity). Next, a method by which the present invention provides significant advantages (including spectral capacity gain) over the prior art is described.

[0035] 1. Intracell Diversity Method The intracell diversity method operates within one cell and is designed to enhance the SNR in scenarios with poor link quality (e.g., users at the cell edge experiencing high path loss from the central tower or base station). Representative diversity methods used in MIMO communication are beamforming [5-11] and orthogonal space-time block code (OSTBC) [12-15].[[]]

[0036] The diversity techniques supported by the LTE standard are transmit diversity, closed-loop rank-1 precoding, and dedicated beamforming [31-35]. The transmit diversity scheme supports two or four transmit antennas on the downlink (DL), and only two antennas for the uplink (UL). It is realized by the space-frequency block code (SFBC) combined with frequency-switching transmit diversity (FSTD)

[31] to utilize spatial as well as frequency selectivity in the DL channel. Rank-1 precoding generates a beam dedicated to a single user based on quantized weights selected from a codebook (predesigned using limited feedback techniques [36-42]) to reduce the feedback overhead from the user equipment (UE) to the transceiver base station (BTS 105 in FIG. 1, i.e., eNodeB in LTE terminology). Alternatively, the weights for dedicated beamforming can be calculated based on UE-specific reference signals.

[0037] 2. Intra-cell multiplexing method The MIMO multiplexing scheme [1, 19] provides a data rate gain in a high SNR regime and in scenarios with sufficient spatial degrees of freedom in the channel (e.g., a rich multipath environment with high spatial selectivity [16-18]) to support multiple parallel data streams on the wireless link.

[0038] The LTE standard supports various multiplexing techniques for single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO)

[31] . The SU-MIMO scheme has two operating modes. i) Closed-loop. It uses feedback information from the UE to select DL precoding weights. ii) Open-loop. It is used when feedback from the UE is not available or when the UE is moving too fast to support the closed-loop scheme. The closed-loop scheme uses a set of pre-computed weights selected from a codebook. These weights can support two or four transmit antennas and 1 to 4 parallel data streams (specified by the number of layers of the precoding matrix) according to the UE's requirements and the BTS scheduler's judgment. LTE-Advanced can include new transmission modes up to MIMO 8x8 to provide up to an eight-fold increase in spectral efficiency through spatial processing

[62] .

[0039] The MU-MIMO scheme is defined for both the UL channel and the DL channel [31, 50]. In the UL, every UE transmits a reference signal (consisting of a version of the Zadoff-Chu sequence

[33] circularly shifted) to the BTS. These reference signals are orthogonal so that the BTS can estimate the channels from all UEs and demodulate data streams from multiple UEs simultaneously through spatial processing. In the DL, the precoding weights for various UEs are selected from a codebook based on feedback from the UE and the scheduler (similar to the closed-loop SU-MIMO scheme), and only rank-1 precoding is possible for every UE (e.g., each UE receives only one data stream).

[0040] The intracell multiplexing technology using spatial processing provides good performance only in propagation scenarios characterized by high SNR (or SINR) and high spatial selectivity (rich multipath environment). For conventional macrocells, these situations may be more difficult to achieve because the BTS is usually far from the UE and the SINR distribution usually concentrates on low values

[43] . In these scenarios, the MU-MIMO scheme or diversity technology may be a better choice than SU-MIMO with spatial multiplexing.

[0041] Another technology and network solution considered by LTE-Advanced to achieve further multiplexing gain (without requiring spatial processing by MIMO) are carrier aggregation (CA) and small cells. CA [30, 44 - 47] combines different parts of the RF spectrum to enhance the signal bandwidth up to 100 MHz

[85] , thereby providing a higher data rate. In-band CA combines different bands within the same part of the spectrum. Therefore, it can use the same RF chain for multiple channels, and multiple data streams are recombined in software. Inter-band CA requires different RF chains to operate in different parts of the spectrum, and also requires signal processing to recombine multiple data streams from different bands.

[0042] The main idea of small cells [30, 47] is to reduce the size of conventional macro cells, thereby enabling higher cell density and greater throughput per coverage area. Small cells are typically deployed through low-power transmission via inexpensive access points 106 (as shown in Figure 1), as opposed to the tall and expensive cell towers used for macro cells. Two types of small cells are defined in LTE-Advanced. i) Metro cells that support 32 to 64 simultaneous users for outdoor installation in urban areas, and ii) Femto cells that can accommodate up to four active users for indoor use. One advantage of small cells is that the density of UEs near the BTS is statistically higher, thereby providing a better SNR that can be utilized through spatial multiplexing to enhance the data rate. However, there are still many concerns regarding the actual deployment of small cells, especially those related to the backhaul. In fact, reaching every small cell BTS through a high-speed wired connection may be challenging, especially considering the high density of metro cells and femto cells in a given coverage area. Compared to wired backhaul, using line-of-sight (LOS) backhaul for small cells can often be achieved at a lower cost, but there are often no practical LOS backhaul paths available for the preferred small cell BTS locations, and there is no general solution for non-line-of-sight (NLO) wireless backhaul to small cell BTSs. Furthermore, small cells require complex real-time coordination between BTSs to avoid interference, as in self-organizing networks (SON) [30, 51 - 52], and advanced cell planning tools (even more complex than conventional cellular systems due to the high density of small cells) are required to plan their optimal locations [48, 49]. Finally, handoff is a limiting factor in small cell deployment, especially in scenarios where a large number of subscribers in multiple groups simultaneously switch cells, causing a large amount of overhead handoff beyond the backhaul capacity, resulting in inevitable longer waiting times and inability to make calls.

[0043] It can be clearly shown that there is no practical general solution that enables small cells to coexist with macro cells and achieve optimal or necessarily further improved throughput. As one of countless such intractable situations, there is a situation where the small cell is located such that its UE inevitably overlaps with macro cell transmissions, and the small cell and the macro cell use the same frequency to reach their respective UEs. Clearly, in this situation, macro cell transmissions will interfere with small cell transmissions. There may be some approaches to reducing such interference for specific situations such as a specific macro cell, a specific small cell, the specific macro cell and small cell UEs involved, the throughput requirements of those UEs, and environmental conditions, etc. However, all such approaches are very specific, not only for the static planning of macro cells and small cells, but also for the dynamic situations at specific time intervals. Usually, the full throughput of the channel for each UE cannot be achieved.

[0044] 3. Inter-cell Diversity Method In a heterogeneous network (HetNet) where macro cells coexist with small cells (e.g., metro cells, pico cells, and femto cells)

[90] , various techniques need to be utilized to eliminate inter-cell interference. While HetNet provides better coverage through small cells, the gain in data rate is only marginal. This is because it is necessary to use spatial processing to eliminate interference rather than sharing the spectrum through different forms of frequency repetition patterns or obtaining multiplexing gains. The LTE standard utilizes an inter-cell interference control (ICIC) method, especially to eliminate interference at the cell edge. There are two types of ICIC methods: cell autonomous and coordinated between BTSs.

[0045] The cell autonomous ICIC method avoids inter-cell interference through different frequency repetition patterns shown in FIG. 2. In FIG. 2, the hexagons represent cells, and the colors represent various carrier frequencies. Three methods are being considered in LTE. i) Complete frequency repetition (or repetition 1) that generates high interference at the cell edge by using all the available bandwidth of the cell, as shown in FIG. 2a. ii) Hard frequency repetition (HFR) where different frequency bands are assigned to all cells (typically with a repetition factor of 3), as shown in FIG. 2b. iii) Fractional frequency repetition (FFR) where the center of the cell is assigned to the entire available bandwidth like frequency repetition 1, while the cell edge operates in HFR mode to reduce inter-cell interference, as shown in FIG. 2c.

[0046] Coordinated ICIC methods enable cooperation between BTSs to improve the performance of the wireless network. These techniques are special cases of the methods taught in related patents and applications [0002 - 0022] to enable cooperation between radio transceivers in the general case of a distributed antenna network for multiple UEs where all use the same frequency simultaneously. For the specific case of a cellular system for a single UE at a given frequency and given time, the cooperation between BTSs to remove inter-cell interference is described in

[53] . The system in

[53] divides every macro cell into a plurality of sub-cells and enables flexible handoff between sub-cells by using dedicated beamforming from the coordinated BTSs. And by using dedicated beamforming from the coordinated BTSs, the robustness of the link at a single frequency and for a single UE is improved when the single UE moves along the boundary of the sub-cells.

[0047] Recently, this class of coordinated wireless cellular networks has been clearly defined in the MIMO literature as "network MIMO" or "coordinated multipoint" (CoMP) systems. Theoretical analysis and simulated results regarding the benefits obtained with network MIMO by eliminating inter-cell interference are shown in [54 - 61]. The advantage of network MIMO and CoMP is to eliminate inter-cell interference in the overlapping area of cells described as the "interference area" 301 in FIG. 3 for macro cell 302.

[0048] CoMP networks are actively becoming part of the LTE-Advanced standard as a solution to reduce inter-cell interference in next-generation cellular networks [62 - 64]. To eliminate inter-cell interference, three CoMP solutions have been proposed in the standard so far. i) Coordinated scheduling / beamforming (CS / CB) where the UE receives its data stream from only one BTS via beamforming and cooperation across BTSs is made possible to remove interference via beamforming or scheduling techniques, ii) Dynamic cell selection (DCS) where for every UE, cells are dynamically selected on a subframe basis transparently to the UE, iii) Joint transmission (JT) where data for a given UE is jointly transmitted from multiple BTSs to improve the received signal quality and eliminate inter-cell interference. CoMP-JT provides a greater gain than CoMP-CS / CB at the expense of higher overhead in the backhaul to enable cooperation between BTSs.

[0049] 4. Inter-cell multiplexing method Prior art multi-user wireless systems add complexity and introduce constraints into the wireless network, such that the experience of a given user (e.g., available throughput, latency, predictability, reliability) is affected by the use of the spectrum by another user in that area. Considering the increasing demands on the total throughput within the wireless spectrum shared among multiple users, as well as the growth of applications that can rely on the reliability, predictability, and low latency of multi-user wireless networks for a given user, it is clear that prior art multi-user wireless technologies suffer many constraints. Indeed, with respect to the limited availability of suitable spectrum for certain types of wireless communications (e.g., at wavelengths effective for penetrating building walls), prior art wireless technologies will be insufficient to meet the increasing demands for high-reliability, predictable, and low-latency bandwidth.

[0050] Prior art intracell diversity and multiplexing methods can provide at most a theoretical four-fold increase in throughput on current cellular networks for LTE (by MIMO 4x4), and at most a theoretical eight-fold increase for LTE-Advanced (by MIMO 8x8). Note that for higher-order MIMO, in a given multipath environment, especially as the UE (such as a smartphone) becomes smaller and more constrained with respect to antenna placement, the improvement effect on throughput increase decreases. Some additional throughput gains in next-generation cellular systems may be obtained from additional spectrum allocations utilized by carrier aggregation technology (e.g., the FCC National Broadband Plan), and from the denser distribution of BTSs by small cell networks and SON [30, 46]. However, since the spectral efficiency gains obtained by spatial processing are limited, all of the above-mentioned technologies still rely very much on spectrum or time sharing technologies that enable multi-user transmission.

[0051] Prior art inter-cell methods (e.g., network MIMO and CoMP systems [53 - 64]) can improve the reliability of cellular networks by removing inter-cell interference, but their capacity gains are only marginal. In fact, these systems are only effective in removing inter-cell interference due to inter-cell power leakage by restricting the power transmitted from any BTS within the range of the cell boundary. FIG. 3 shows an example of a cellular network with three BTSs, each characterized by its own coverage area or cell. The power transmitted from each BTS is restricted to limit the amount of inter-cell interference represented in FIG. 3 by the overlapping areas of the cells. Since these systems operate in the low SINR regime in the interference region, similar to the intra-cell methods for SU-MIMO, their spectral efficiency gains are only marginal. To obtain truly significant capacity gains in an inter-cell cooperative network, the power limitations that are restricted to the cell boundary must be relaxed. And spatial multiplexing techniques must be enabled throughout the cells with high SINR (not just at the cell edge with poor SINR performance as in the conventional approach).

[0052] FIG. 4 shows that the power transmitted simultaneously at the same frequency from all three BTSs 401 is increased, thereby enabling a high level of interference throughout the cell 402. In prior art systems, such interference results in incoherent interference (which inhibits UE signal reception) throughout the interference region of the BTS, but this interference is actually utilized in the present invention by the new inter-cell multiplexing method. This method uses spatial processing to generate regions of coherent interference (which enhances UE signal reception) around any UE, thereby providing non-interfering data streams to any UE simultaneously and increasing their SINR throughout the cell.

[0053] The scenario illustrated in Figure 4 is described in

[89] for a specific case of a cellular system. The system in

[89] consists of several BTSs that identify various cells grouped into clusters. Cooperation is only permitted between BTSs from adjacent cells within the same cluster. In this case, it has been shown that as the power transmitted from the BTS increases, there are limitations on the capacity (or spectral efficiency) obtained by the inter-cell multiplexing method. In fact, as the transmitted power increases, the out-of-cluster interference increases proportionally, creating a saturation regime for the SINR and, consequently, for the capacity. As a result of this effect, the system in

[89] can theoretically obtain a maximum capacity increase of up to three times (i.e., a maximum of three cells within the cluster), and any additional cells included in the cluster reduce the capacity due to the increased out-of-cluster interference (e.g., for 21 cells per cluster, a lower capacity is obtained compared to 3 cells per cluster). It has been observed that the basic capacity limit in

[89] is maintained because the BTSs are restricted to predefined positions as in a cellular system and the multiplexing gain is obtained by increasing the transmitted power from the BTSs. To obtain theoretically infinite capacity via the inter-cell multiplexing method, the constraints in the BTS placement must be removed, which conveniently allows the BTSs to be placed anywhere.

[0054] Therefore, it is desirable to provide a system that realizes a large increase in spectral efficiency that utilizes inter-cell multiplexing gain by spatial processing by removing any power constraints transmitted from the distributed BTS 501 in addition to the constraints in their placement. FIG. 5 shows an example in which one of many additional access points 502 is added to deliberately increase the level of incoherent interference throughout the cell 503. It is utilized in the present invention to create a region of coherent interference around the UE, thereby providing a theoretically infinite inter-cell multiplexing gain. The additional access points are serendipitously placed in convenient locations and are not constrained by any particular cell design, such as the cellular systems described in the prior art. In an exemplary embodiment of the invention, the serendipitous access points are distributed input distributed output (DIDO) access points, and the inter-cell multiplexing gain is obtained by the DIDO method described in paragraphs [0014-0020] and [77-78]. In another embodiment, the serendipitous access points are low-power transceivers similar to inexpensive Wi-Fi access points or small cells [30,47], thereby providing smaller regions of overlapping coverage throughout the macro cell, as shown in FIG. 5.

[0055] Prior art inter-cell methods [53-64] avoid incoherent interference by deliberately limiting the transmission power from any BTS as shown in FIG. 3, and remove the remaining inter-cell interference (relating to the overlapping area between cells) by spatial processing, thereby providing improved SINR and inter-cell diversity gain. Further,

[89] limits the BTS placement in cell design while increasing the transmission power, thereby limiting the achievable capacity due to out-of-cluster interference, and it is thus observed that it is still limited by interference. On the other hand, the present invention utilizes incoherent interference by transmitting higher power from any randomly placed BTS in order to generate coherent interference around the UE. This improves the signal quality at the UE, which is a prerequisite for obtaining inter-cell multiplexing gain across the cell by spatial processing. Therefore, since there is not enough SINR to enable an inter-cell multiplexing method like the present invention throughout the cell (due to limited transmission power from the BTS or out-of-cluster interference when the transmission power increases), the systems described in the prior art cannot be used to achieve unlimited inter-cell multiplexing gain by spatial processing. Further, considering that the systems described in the prior art avoid inter-cell interference in the diversity regions shown in the shaded areas of FIGS. 1 and 3-5 rather than utilizing inter-cell interference in the multiplexing region to obtain the inter-cell multiplexing gain achieved in the present invention, it is impossible to implement to achieve the multiplexing gain achieved in the present invention shown in FIGS. 4-5.

[0056] The present invention can be better understood from the following detailed description together with the drawings.

Brief Description of the Drawings

[0057]

Figure 1

Figure 2a

Figure 2b

Figure 2c

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7a

Figure 7b

Figure 8a

Figure 8b

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20a

Figure 20b

DETAILED DESCRIPTION OF THE INVENTION

[0058] One solution that overcomes many of the limitations of the prior art described above is an embodiment of distributed input distributed output (DIDO) technology. The DIDO technology is described in the following patents and patent applications, all of which are assigned to the assignee of this patent and incorporated herein by reference. These patents and applications are referred to herein several times as "related patents and applications" in general.

[0059] U.S. Patent Application No. 13 / 633,702, titled "Systems and Methods for wireless backhaul in distributed-input distributed-output wireless systems".

[0060] U.S. Patent Application No. 13 / 475,598, titled "Systems and Methods to enhance spatial diversity in distributed-input distributed-output wireless systems".

[0061] U.S. Patent Application No. 13 / 233,006, titled "System and Methods for planned evolution and obsolescence of multiuser spectrum".

[0062] U.S. Patent Application No. 13 / 232,996, titled "Systems and Methods to Exploit Areas of Coherence in Wireless Systems".

[0063] U.S. Patent Application No. 13 / 464,648, titled "System and Methods to Compensate for Doppler Effects in Distributed-Input Distributed Output Systems".

[0064] U.S. Patent Application No. 12 / 917,257, titled "Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering".

[0065] U.S. Patent Application No. 12 / 802,988, titled "Interference Management, Handoff, Power Control And Link Adaptation In Distributed-Input Distributed-Output (DIDO) Communication Systems".

[0066] U.S. Patent Application No. 12 / 802,974, titled "System And Method For Managing Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDO Clusters".

[0067] U.S. Patent Application No. 12 / 802,989, titled "System And Method For Managing Handoff Of A Client Between Different Distributed-Input-Distributed-Output (DIDO) Networks Based On Detected Velocity Of The Client".

[0068] U.S. Patent Application No. 12 / 802,958, titled "System And Method For Power Control And Antenna Grouping In A Distributed-Input-Distributed-Output (DIDO) Network".

[0069] U.S. Patent Application No. 12 / 802,975, titled "System And Method For Link adaptation In DIDO Multicarrier Systems".

[0070] U.S. Patent Application No. 12 / 802,938, titled "System And Method For DIDO Precoding Interpolation In Multicarrier Systems".

[0071] U.S. Patent Application No. 12 / 630,627, titled "System and Method For Distributed Antenna Wireless Communications".

[0072] U.S. Patent No. 8,170,081, issued on May 1, 2012, titled "System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements".

[0073] U.S. Patent No. 8,160,121, issued on April 17, 2012, titled "System and Method For Distributed Input-Distributed Output Wireless Communications".

[0074] U.S. Patent No. 7,885,354, issued on February 8, 2011, titled "System and Method For Enhancing Near Vertical Incidence Skywave(「NVIS」)Communication Using Space-Time Coding".

[0075] U.S. Patent No. 7,711,030, issued on May 4, 2010, titled "System and Method For Spatial-Multiplexed Tropospheric Scatter Communications".

[0076] U.S. Patent No. 7,636,381, issued on December 22, 2009, titled "System and Method for Distributed Input Distributed Output Wireless Communication".

[0077] U.S. Patent No. 7,633,994, issued on December 15, 2009, titled "System and Method for Distributed Input Distributed Output Wireless Communication".

[0078] U.S. Patent No. 7,599,420, issued on October 6, 2009, titled "System and Method for Distributed Input Distributed Output Wireless Communication".

[0079] U.S. Patent No. 7,418,053, issued on August 26, 2008, titled "System and Method for Distributed Input Distributed Output Wireless Communication".

[0080] To reduce the volume and complexity of this patent application, some of the disclosures of related patents and applications are not explicitly described below. For a complete description of such disclosures, refer to the related patents and applications.

[0081] The present invention describes a system and method that utilize the inter-cell multiplexing gain by spatial processing in a wireless communication network, and uses a multiple antenna system (MAS) (multi-user multiple antenna system, i.e., "MU-MAS") with multiple randomly placed antennas having multi-user (MU) transmission. In one embodiment of the present invention, the power transmitted from multiple antennas is constrained to minimize interference at the cell boundary (such as in a conventional cellular system), and a spatial processing method is used only to cancel inter-cell interference. In another embodiment of the present invention, the power transmitted from multiple antennas is not constrained to any specific power level as long as their power emission levels are within the limits of regulation, safety, or practical (e.g., available power, transmitter, and / or antenna specifications). Thereby, higher-order inter-cell interference is deliberately generated across the entire cell utilized to realize the inter-cell multiplexing gain, increasing the capacity of the wireless communication network.

[0082] In one embodiment, the wireless communication network is a cellular network such as a cellular network as shown in FIGS. 1 and 3 based on the LTE standard, and the multiple randomly deployed antennas are macro cell or small cell transceivers. In another embodiment of the present invention, the wireless communication network is not constrained to any specific cell layout, and the cell boundary can extend over a larger area as shown in FIGS. 4-5. For example, the wireless communication network may be a wireless local area network (WLAN) where multiple antennas serve as Wi-Fi access points, or a mesh, ad hoc, or sensor network, or a distributed antenna system, or a DIDO system having access points randomly placed without any transmission power limitation. However, such examples of network structures should not be considered as limiting the general applicability of the present invention to wireless communication networks. The present invention applies to any wireless network in which multiplexing gain is realized by transmitting signals from multiple antennas. The signals interfere where they are received by multiple UEs, generating simultaneous non-interfering data streams to the multiple UEs.

[0083] MU-MIMO consists of a centralized processor, a network, and M transceiver stations (or distributed antennas) that communicate wirelessly with N client devices or UEs. The centralized processor unit receives N streams of information (e.g., streams from a web server or other network source such as video, web pages, video games, text, voice, etc.) for various client devices regarding various network contents. Hereinafter, we use the term "information stream" to refer to any stream of data transmitted over the network. It includes information that can be demodulated or decoded like an independent stream according to a specific modulation / coding scheme or protocol to generate any data including but not limited to voice, web, and video contents. In one embodiment, an information stream is a series of bits carrying network content that can be demodulated or decoded like an independent stream.

[0084] The centralized processor utilizes a precoding transformation that combines N streams of information from network content into M streams of bits (by algorithms such as those described in related patents and applications). By way of example and not limitation, the precoding transformation can be linear (e.g., zero-forcing

[65] , block diagonalization [66 - 67], matrix inversion, etc.). Or, it can be non-linear (e.g., dirty paper coding [68 - 70] or Tomlinson-Harashima precoding [71 - 72], lattice techniques or trellis precoding [73 - 74], vector perturbation techniques [75 - 76]). Hereinafter, the term "stream of bits" is used to refer to any sequence of bits that does not necessarily contain any useful bits of information, and thus, for reading network content, it cannot be decoded or demodulated as an independent stream. In one embodiment of the present invention, the stream of bits is a complex baseband signal generated by the centralized processor, quantized to a predetermined number of bits, and transmitted to one of M transceiver base stations.

[0085] Precoding is calculated at the centralized processor by using channel state information (CSI) and applying it on the DL or UL channel to multiplex data streams for or from multiple users. In one embodiment of the present invention, the centralized processor recognizes the CSI between the distributed antenna and the client device and utilizes the CSI to precode the data transmitted on the DL channel or UL channel. In the same embodiment, the CSI is estimated at the client device and returned to the distributed antenna. In another embodiment, the DL-CSI is obtained at the distributed antenna from the UL-CSI by using radio frequency (RF) calibration and by exploiting UL / DL channel reciprocity.

[0086] In one embodiment, the MAS is a Distributed Input Distributed Output (DIDO) system as described in related patents and applications. In another embodiment, the MU-MAS illustrated in FIG. 13 consists of the following. · User Equipment (UE) 1301: An RF transceiver for fixed and / or mobile clients, which receives a data stream on a downlink (DL) channel from the backhaul and transmits data to the backhaul via an uplink (UL) channel. · Base Transceiver Station (BTS) 1302: The BTS interfaces with the backhaul over a wireless channel. The BTS in one embodiment is an access point consisting of a Digital - Analog Converter (DAC) / Analog - Digital Converter (ADC) and a Radio Frequency (RF) chain that converts baseband signals to RF. In some cases, the BTS is a simple RF transceiver with a power amplifier / antenna, and the RF signal is carried to the BTS by the RF over fiber technology described in related patents and applications. · Controller (CTR) 1303: The CTR is a specific type of BTS designed for the following specific special functions. The functions are to transmit training signals for time / frequency synchronization of the BTS and / or UE, receive control information from the UE and transmit it to the UE, and receive channel state information (CSI) or channel quality information from the UE. One or more CTR stations can be included in any MU-MAS system. When multiple CTRs are available, information to or from those stations can be combined to increase diversity and improve link quality. In one embodiment, the CSI is received from multiple CTRs by the Maximum Ratio Combining (MRC) technique to improve CSI demodulation. In another embodiment, the control information is transmitted from multiple CTRs by the Maximum Ratio Transmission (MRT) to improve the SNR at the receiver side. The scope of the invention is not limited to MRC or MRT, and any other diversity technique (such as antenna selection, etc.) can be used to improve the wireless link between the CTR and the UE. · Centralized Processor (CP) 1304: The CP is a server that connects the backhaul to the Internet or another type of external network 1306. In one embodiment, the CP calculates MU-MAS baseband processing and transmits waveforms to the distributed BTS by DL transmission. · Base Station Network (BSN) 1305: The BSN is a network that connects the CP to the distributed BTS and carries information about either the DL channel or the UL channel. The BSN is a wired network, a wireless network, or a combination of both. For example, the BSN is DSL, cable, fiber optic network, or line-of-sight (LOS) or non-line-of-sight (NLO) wireless link. Further, the BSN is a proprietary network, or a local area network, or the Internet.

[0087] Below, it is described how the above MU-MAS framework is incorporated into the LTE standard for a cellular system (more specifically, a non-cellular system using the LTE protocol) to realize additional gains in spectral efficiency. Start with a general overview of the LTE framework and the modulation techniques used in the DL channel and UL channel. Next, a brief description of the physical layer frame structure and resource allocation in the LTE standard is given. Finally, we clearly describe the MU-MAS precoding methods for the downlink (DL) channel and uplink (UL) channel in a multi-user scenario using the LTE framework. For the DL scheme, two solutions, the open-loop and closed-loop DIDO schemes, are proposed.

[0088] LTE is designed by a flat network architecture (as opposed to the hierarchical architecture of previous cellular standards) and provides the following: namely, reduced latency, reduced packet loss by ARQ, reduced call setup time, improved coverage and throughput by macro diversity. The network elements in the LTE network shown in Figure 6 are as in

[79] . · GW (Gateway): A router that connects the LTE network to an external network (i.e., the Internet). The GW is divided into a Serving Gateway (S-GW) 601 that forms the boundary of the EUTRAN interface 608 and a PDN Gateway (P-GW) 602 that is the interface with the external network. The S-GW and P-GW are part of the so-called evolved packet core (EPC) 609. · MME (Mobility Management Entity) 603: Manages mobility, protection parameters, and UE identity. The MME is further part of the LTE EPC. · eNodeB (enhanced Node-B) 604: A base station that handles radio resource management, user mobility, and scheduling. · UE (User Equipment) 605: A mobile station. · S1 and X2 interfaces (606 and 607): Wired or wireless backhaul between the MME and the eNodeB (S1-MME), between the S-GW and the eNodeB (S1-U), and between multiple eNodeBs (X2).

[0089] In one embodiment of the present invention, when the UE is an LTE UE, the MU-MAS network is an LTE network, the BTS is an LTE eNodeB, the CTR is an LTE eNodeB or MME, the CP is an LTE GW, and the BSN is an S1 or X1 interface. Hereinafter, the distributed antenna, BTS, and eNodeB can be used interchangeably to refer to any base station in the MU-MAS, DIDO, or LTE system.

[0090] As shown in FIG. 7, an LTE frame has a duration of 10 milliseconds and consists of 10 subframes [33, 80]. Each subframe is divided into two slots, each with a duration of 0.5 milliseconds. The LTE standard defines two types of frames: i) Type 1 for FDD operation where all subframes are allocated to either the DL channel or the UL channel, as shown in FIG. 7a), and ii) Type 2 for TDD operation where (depending on the selected configuration) a part of the subframe is allocated to the DL and a part to the UL, but some subframes are set aside for "special use". These are at least one special subframe per frame and consist of three fields: i) a downlink pilot time slot (DwPTS) reserved for DL transmission, ii) a guard period (GP), and iii) an uplink pilot time slot (UpPTS) for UL transmission.

[0091] For the DL, LTE uses orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) modulation, and for the UL, it uses single carrier frequency division multiple access (SC-FDMA). A "resource element" (RE) is the smallest modulation structure in LTE and consists of one OFDM subcarrier in frequency and one OFDM symbol period in time, as shown in FIG. 8a for the DL channel and FIG. 8b for the UL channel. A "resource block" (RB) is composed of 12 subcarriers in frequency and one 0.5 - millisecond slot in time (consisting of 3 - 7 OFDM symbol periods, depending on the type of DL / UL channel and cyclic prefix). Resource blocks for all UEs are allocated on a subframe basis. Since MU-MAS in the present invention uses spatial processing to transmit multiple data streams to various UEs, all resource blocks in every subframe can be allocated to the same UE. In one embodiment, all or a subset of the resource blocks are allocated to all UEs, and simultaneous non - interfering data streams are transmitted to the UEs via precoding.

[0092] To set up a link between the BTS and the UE, the LTE standard defines a synchronization procedure. The BTS transmits to the UE two consecutive signals, namely, the Primary Synchronization Signal (P-SS) transmitted on the Primary Synchronization Channel (PSCH) and the Secondary Synchronization Signal (S-SS) transmitted on the Secondary Synchronization Channel (SSCH). Both signals are used by the UE for time / frequency synchronization in addition to obtaining the cell ID. The P-SS consists of a Zadoff-Chu sequence of length 63 from which the UE obtains the physical layer ID (0 - 2). The S-SS is an interleaved concatenation of two binary sequences of length 31 and is used to obtain the cell ID group number (0 - 167). From the above two identification numbers, the UE obtains the Physical Cell ID (PCI, defined from 0 - 503).

[0093] In the MU-MAS system described in the present invention, since the power transmitted from the BTS is intentionally increased to generate interference used to generate an interference region around the UE, there is no cell boundary. In the present invention, various BTSs are grouped into "antenna clusters" or "DIDO clusters" defined in the related U.S. Patent No. 8,170,081, issued on May 1, 2012, titled "System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements". For example, FIG. 9 shows one main antenna cluster 901 and an adjacent antenna cluster 902. Each antenna cluster consists of a plurality of BTSs 903.

[0094] The cell ID can be used in MU-MAS and DIDO systems to distinguish antenna clusters. In one embodiment of the present invention, the same cell ID is transmitted from all BTSs of the same antenna cluster via P-SS and S-SS. In the same embodiment, different antenna clusters use different cell IDs. In another embodiment of the present invention, all BTSs in the same antenna cluster 1001 are grouped into "antenna sub-clusters" 1003 having various shaded colors as illustrated in FIG. 10, and different cell IDs 1004 are associated with each antenna sub-cluster. In one embodiment, the antenna sub-clusters are statistically defined according to a predefined cluster design or GPS location information. In another embodiment, the antenna sub-clusters are dynamically defined based on measurements of relative signal strength between BTSs or based on GPS location information. In different embodiments of the present invention, different cell IDs are assigned to any coherence area associated with a UE (described in the related co-pending U.S. patent application Ser. No. 13 / 232,996, titled "Systems and Methods to Exploit Areas of Coherence in Wireless Systems").

[0095] When all BTSs within the same antenna cluster or antenna sub - cluster transmit LTE broadcast channels (e.g., P - SS and S - SS) to the UE, due to interfering with each other, the performance of time or frequency synchronization enabled by the broadcast channels may degrade. The interfering interference can be caused by multipaths generated from BTSs that are distributed with an interval and recombined non - interferingly at several UE positions. To avoid or mitigate this effect, in one embodiment of the present invention, only one BTS out of all BTSs within the same antenna cluster or antenna sub - cluster transmits LTE broadcast channels (e.g., P - SS and S - SS) to all UEs. In the same embodiment, the BTS that transmits the LTE broadcast channel is selected to maximize the power received by the UE on the broadcast channel. In another embodiment, only a limited set of BTSs is selected to simultaneously transmit LTE broadcast channels to all UEs so that interfering interference is avoided at the UE. In different embodiments of the present invention, the LTE broadcast channel is transmitted with higher power than the payload so as to reach all UEs within the same antenna cluster or antenna sub - cluster.

[0096] As described above, LTE-Advanced supports the carrier aggregation (CA) method to improve the data rate in the DL channel. In MU-MAS, CA can be used in combination with precoding to increase the data rate for each user. In one embodiment of the present invention, to increase the data rate for each user, transmission precoding is applied to different parts of the RS spectrum (inter-band CA) or different bands within the same part of the spectrum (intra-band CA). When using inter-band CA, the path loss in different bands may vary significantly because those bands are centered at different carrier frequencies. In a conventional LTE cellular system, the frequency band at a low carrier frequency may experience a lower path loss than that at a high carrier frequency. Therefore, applying inter-band CA to a cellular system may cause undesirable inter-cell interference at a low carrier frequency. In contrast, the MU-MAS of the present invention is not limited by interference at the cell boundary because the BTSs are dispersed and there is no concept of a cell. This more flexible system layout enables various methods of inter-band CA in MU-MAS. In one embodiment of the present invention, MU-MAS enables inter-band CA by operating one set of BTSs at a lower carrier frequency and another set of BTSs at a higher carrier frequency, and using them such that the two sets intersect or one set is a subset of the other set. In another embodiment, MU-MAS with precoding uses the CA method in combination with a frequency hopping pattern to improve the robustness against frequency selective fading or interference.

[0097] 1. Downlink Closed-Loop MU-MAS Precoding Method in LTE The MU-MAS closed-loop method can be used in either a time-division duplex (TDD) or a frequency-division duplex (FDD) system. In an FDD system, the DL and UL channels operate at different frequencies. Therefore, the DL channel state information (CSI) has to be estimated at the UE side and reported back to the CP via the UL channel through the BTS or CTR. In a TDD system, the DL and UL channels are set at the same frequency, and the system may use either a closed-loop technique or an open-loop method (as described in the following section) by taking advantage of channel reciprocity. The main drawback of closed-loop methods is that they require feedback, which results in a larger overhead for control information on the UL.

[0098] The general mechanism for the closed-loop scheme in MU-MAS is as follows. i) The BTS transmits signaling information to the UE on the DL. ii) The UE uses the signaling information to estimate the DL CSI from all "active BTSs". iii) The UE either quantizes the DL CSI or uses a codebook to select the precoding weights to be used in the next transmission. iv) The UE transmits the quantized CSI or the codebook index on the UL channel to the BTS or the CTR. v) The BTS or the CTR reports the CSI information or the codebook index to the CP, which calculates the precoding weights for data transmission on the DL. An "active BTS" is defined as a set of BTSs reachable by a given UE. For example, in the related co-pending U.S. patent application Ser. No. 12 / 802,974, entitled "System And Method For Managing Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDO Clusters", and the related co-pending U.S. patent application Ser. No. 12 / 917,257, entitled "Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering", a "user cluster" 905 is defined as a set of BTSs reachable by a given UE, as illustrated in FIG. 9. The number of active BTSs is limited to the user cluster in order to reduce the amount of CSI estimated by the BTSs for a given UE, thereby reducing the feedback overhead on the UL and the complexity of the MU-MAS precoding calculation at the CP.

[0099] As described in paragraph

[0060] , MU-MAS precoding uses either a linear or non-linear method. In the case of a non-linear method (e.g., dirty paper coding [68-70]) or Tomlinson-Harashima precoding [71-72], lattice techniques or trellis precoding [73-74], vector perturbation techniques [75-76]), successive interference cancellation is applied at the transmitter to avoid interference between users. In this case, the precoding matrix is calculated to account for the CSI to all UEs within the antenna cluster. Alternatively, a linear precoding method (e.g., zero forcing

[65] , block diagonalization [66-67], matrix inversion, etc.) can be used on a user cluster basis since the precoding weights for each UE are calculated independently of the other UEs. Depending on the number of UEs within the antenna cluster and user cluster and the number of eNodeBs, linear vs. non-linear precoding methods offer various computational performances. For example, if MU-MAS consists of K UEs per antenna cluster, M eNodeBs per antenna cluster, and C eNodeBs per user cluster, the complexity of linear precoding is O(K * C 3 ), while for non-linear precoding it is O(M * K 2) Therefore, it is desirable to develop a method for dynamically switching between two types of precoding techniques based on the number of UEs and eNodeBs in MU-MAS to reduce the computational complexity in CP. In one embodiment of the present invention, MU-MAS uses a linear precoding method. In another embodiment, MU-MAS uses a non-linear precoding method. In the same embodiment of the present invention, MU-MAS dynamically switches between linear and non-linear precoding methods based on the number of UEs and eNodeBs in the antenna cluster and user cluster to reduce the computational complexity in CP. In different embodiments, MU-MAS switches between a precoding multiplexing method (e.g., at the proximal of the eNodeB) for UEs experiencing good channel quality and a beamforming or diversity method for UEs with poor link quality (e.g., far from the eNodeB).

[0100] 1.1 Downlink MU-MAS Signaling Method within the LTE Standard The LTE standard defines two types of reference signals (RSs) that can be used for closed-loop DL signaling [33, 50, 82 - 83]. i) Cell-specific reference signal (CRS), ii) UE-specific RSs, such as channel state information (CSI) reference signal (CSI-RS) and demodulation RS (DM-RS). The cell-specific RS is not precoded, while the UE-specific RSs are precoded

[50] . The CRS is used in LTE Release 8, which uses a technique based on the SU / MU-MIMO codebook where each cell uses a maximum of four antennas. LTE-Advanced Release 10 supports a non-codebook-based SU / MU-MIMO scheme with a maximum of eight transmit antennas and a CoMP scheme where antennas are distributed on different cells. Therefore, Release 10 enables a more flexible signaling method using CSI-RS. In the present invention, how any type of signaling method can be used in the MU-MAS system to enable precoding is described.

[0101] 1.1.1 MU-MAS Signaling Using CRS CRS is used in the LTE (Release 8) system [80, 84] to estimate the CSI from all the transmitting antennas at the UE to the BTS. CRS is obtained as the product of a two-dimensional orthogonal sequence and a two-dimensional pseudo-random number (PRN) sequence. For a total of 504 different CRS sequences, there are three orthogonal sequences (i.e., placed on orthogonal pairs of OFDM subcarriers) and 168 possible PRN sequences. Each sequence uniquely identifies one cell. Each of the three orthogonal CRSs is associated with one of the three physical layer IDs (0 to 2) that generate different cell IDs as described in the previous subsection. CRS is transmitted in the first and last and third OFDM symbols of all slots, and in every sixth subcarrier. The orthogonal pattern in time and frequency is designed for every transmitting antenna at the BTS so that the UE can uniquely estimate the CSI from each of the transmitting antennas. Release 8 defines up to four orthogonal patterns for each CRS, one for each of the four transmitting antennas used in MIMO 4×4. This high-density CRS, transmitted in time and frequency (i.e., every 0.5 millisecond slot and every sixth subcarrier), generates a 5% overhead and is deliberately designed to support scenarios with high channel variations in time and frequency

[83] .

[0102] In Release 8, there are up to three orthogonal CRSs (or six orthogonal CRSs for single antenna mode) each having four orthogonal patterns for each multi-antenna mode. Therefore, within the same coverage area, up to 12 transmit antennas can be distinguished without causing interference to the CRS. In one embodiment of the present invention, the antenna cluster 1001 is divided into three antenna sub-clusters 1005 as shown in FIG. 10. Different physical layer IDs (or cell IDs) are associated with the respective antenna sub-clusters, and each antenna sub-cluster is assigned one of the three orthogonal CRSs having four orthogonal patterns (i.e., each antenna sub-cluster can support up to four BTSs without causing interference to the CRS from other BTSs). In this embodiment, any cluster can support up to 12 BTSs without causing interference to the CRS.

[0103] In a scenario where more than 12 BTSs are deployed within the same cluster, it is desirable to increase the number of available orthogonal CRSs (i.e., BTSs that simultaneously transmit pre-coded signals to the UE) that support a larger number of active BTSs. As one way to achieve this, more than three antenna sub-clusters 1003 are defined for each antenna cluster 1101, and the same three physical layer IDs (or cell IDs 1104 from 0 to 2) are assigned to the antenna sub-clusters 1103 having the repeating pattern shown in FIG. 11. It has been observed that antenna sub-clusters can be obtained in various forms, and each user cluster 1102 is defined in such a way that it cannot reach two antenna sub-clusters having the same physical layer ID, thereby avoiding interference with the CRS. For example, one way to achieve this is to define the area of the antenna sub-cluster 1103 that is larger than the user cluster 1102 and avoid having its adjacent antenna sub-cluster use the same physical layer ID. In one embodiment of the present invention, a plurality of antenna sub-clusters are arranged within the same antenna cluster with a repeating pattern so that the corresponding CRSs do not interfere with each other, thereby enabling non-interfering simultaneous transmission from more than 12 BTSs.

[0104] In an actual MU-MAS system, it is possible that all UEs may recognize only more than four BTSs within their user clusters. For example, FIG. 12 shows the SNR distribution for an actual deployment of a DIDO or MU-MAS system in the urban area of San Francisco, California. The propagation model is based on the 3GPP path loss / shadowing model

[81] , and a carrier frequency of 900 MHz is assumed. The dots on the map indicate the locations of the DIDO-BTSs, while the black circles indicate the user clusters (the UE is located at the center of the circle). In the sparse area 1201, the UE recognizes only two or three BTSs out of its user cluster (e.g., only three BTSs in the example of FIG. 12), while in the densely populated area 1202, each user cluster can include as many as 26 BTSs as shown in FIG. 12.

[0105] The high redundancy of CRS can be utilized in MU-MAS that enables CSI estimation from any number of five or more transmitting antennas. For example, if the channel is fixed wireless or characterized by a low Doppler effect, it is not necessary to calculate CSI from all four transmitting antennas every 0.5 milliseconds (slot duration). Similarly, if the channel is frequency flat, it is redundant to estimate CSI for every sixth subcarrier. In that case, the resource elements (REs) occupied by the redundant CRS can be reallocated to another transmitting antenna or BTS in MU-MAS. In one embodiment of the present invention, the system allocates the resource elements of the redundant CRS to the extra antennas or BTSs in the MU-MAS system. In another embodiment, the system estimates the time and frequency selectivity of the channel and dynamically allocates the CRS related to different BTSs or only the BTSs within a user cluster to different resource elements.

[0106] The number of BTSs included in any user cluster depends on the signal power levels from all BTSs in the user cluster, measured at the UE, with respect to the noise power level or signal-to-noise ratio (SNR). In one embodiment, the UE estimates the SNR from all BTSs in its vicinity and selects the BTSs belonging to its own user cluster based on the SNR information. In another embodiment, the CP recognizes the SNR from each BTS to every UE (based on the feedback information from the UE or the information obtained from the UL channel by inferring the UL / DL channel reciprocity), and selects the set of BTSs included in any user cluster.

[0107] The number of BTSs included in every user cluster determines the performance of the MU-MAS method described in the present invention. For example, when the number of BTSs per user cluster is low, the UE experiences a higher level of inter-cluster interference and, as a result, obtains a high signal-to-interference-plus-noise power ratio (SINR) and a low data rate. Similarly, when a large number of BTSs are selected for every user cluster, the SNR from the BTSs measured at the edge of the user cluster at the UE is low and can be affected by the inter-cluster interference from the adjacent BTSs outside the user cluster. There is an optimal number of BTSs per user cluster that yields the highest SINR and data rate. In one embodiment of the present invention, the CP selects the optimal number of BTSs per user cluster to maximize the SINR and data rate to the UE. In another embodiment of the present invention, the BTSs per user cluster are dynamically selected to adapt to changing conditions of the propagation environment or UE mobility.

[0108] Another drawback of using a large number of BTSs per user cluster is its high computational load. In fact, the more BTSs there are in a user cluster, the greater the computational complexity of the MU-MAS precoder. In one embodiment of the present invention, the BTSs per user cluster are selected to obtain an optimal trade-off between the SINR or data rate performance and the computational complexity of the MU-MAS precoder. In another embodiment, the BTSs per user cluster are dynamically selected based on a trade-off between the propagation conditions and the computational resources available in the MU-MAS.

[0109] 1.1.2 MU-MAS Signaling Using CSI-RS and DM-RS In the LTE-Advanced (Release 10) standard, CSI-RS is used by every UE to estimate CSI from the BTS [33, 83]. By defining orthogonal CSI-RS for different transmitters in the BTS, the UE can distinguish CSI from different BTSs. Up to 8 transmit antennas in the BTS are supported by CSI-RS as shown in Tables 6.10.5.2-1 and 2 of

[33] . CSI-RS is transmitted with a periodicity over 5 to 80 subframes (i.e., CSI-RS is transmitted every 5 to 80 milliseconds) as shown in Table 6.10.5.3-1 of

[33] . The periodicity of LTE-Advanced CSI-RS is deliberately designed to be larger than that of LTE CRS, especially with respect to legacy LTE terminals that cannot use these extra resources to avoid excessive overhead of control information. Another reference signal used for CSI estimation is the Demodulation RS (DM-RS). DM-RS is a demodulation reference signal intended for a specific UE and is only transmitted by being placed in the resource blocks allocated for transmission to that UE.

[0110] When there are more than eight (the maximum number of transmitters supported by the LTE-Advanced standard) antennas in the user cluster, alternative techniques must be used to enable DIDO precoding while maintaining system compliance with the LTE-Advanced standard. In one embodiment of the invention, each UE uses CSI-RS or DM-RS or a combination of both to estimate CSI from all active BTSs in its user cluster. In the same embodiment, the DIDO system detects the number of BTSs in the user cluster and detects whether the user cluster complies with the LTE-Advanced standard (supporting a maximum of eight antennas). If not compliant, the DIDO system uses alternative techniques to enable DL signaling from the BTS to the current UE. In one embodiment, the transmit power from the BTS is reduced until a maximum of eight BTSs are reached by the UE in its user cluster. However, this solution may result in a decrease in data rate as coverage is reduced.

[0111] Another solution is to divide the BTSs within a user cluster into subsets and transmit a set of CSI-RS for all subsets simultaneously. For example, if the CSI-RS periodicity is five subframes (i.e., 5 milliseconds) as in Table 6.10.5.3-1 of

[33] , then every 5 milliseconds, the CSI-RS is transmitted from a new subset of BTSs. Note that this solution works as long as the CSI-RS periodicity is short enough to span all BTS subsets within the channel coherence time of the UE (which is a function of the Doppler speed of the UE). For example, if the selected CSI-RS periodicity is 5 milliseconds and the channel coherence time is 100 milliseconds, it is possible to define up to 20 subsets of 8 BTSs each while adding a total of 160 BTSs in the user cluster. In another embodiment of the invention, the DIDO system estimates the channel coherence time of the UE and determines how many BTSs within the user cluster can be supported for a given CSI-RS periodicity in order to avoid degradation due to channel variations and Doppler effects.

[0112] The solutions proposed so far for CSI-RS are all compliant with the LTE standard and can be deployed within the framework of a conventional LTE system. For example, a proposed method that enables more than 8 antennas per user cluster requires no modification to the UE LTE hardware and software implementation, and only a minor modification to the protocol used in the BTS and CP to enable the selection of a BTS subset at any time. These modifications can be easily implemented on a cloud-based software radio (SDR) platform, which is a promising deployment paradigm for DIDO and MU-MAS systems. Alternatively, if it is possible to relax the LTE standard constraints and develop slightly modified hardware and software for the LTE UE to support a DIDO or MU-MAS operating mode that is similar to LTE but non-compliant with LTE, it is possible to enable the UE to operate in either the full LTE-compliant mode or a modified mode that supports non-LTE-compliant DIDO or MU-MAS operation. For example, another solution is possible to increase the amount of CSI-RS to enable a larger number of BTSs in the system. In another embodiment of the invention, different CSI-RS patterns and periodicities are possible as a means of increasing the number of BTSs supported per user cluster. Such minor modifications to the LTE standard may be small enough that existing LTE UE chipsets can be used with just a software modification. Or, even if a hardware modification is required for the chipset, the change will be small.

[0113] 1.2 Uplink MU-MAS CSI Feedback Method within the LTE Standard In the LTE and LTE-Advanced standards, the UE feeds back information to the BTS to communicate its current channel conditions and the precoding weights for closed-loop transmission on the DL channel. Three different channel metrics are included in those standards

[35] . · Rank Indicator (RI): Indicates how many spatial streams are transmitted to a given UE. This number is always equal to or less than the number of transmit antennas. · Precoding Matrix Indicator (PMI): Index of the codebook used for precoding on the DL channel. · Channel Quality Indicator (CQI): Determines the forward error correction (FEC) coding scheme and modulation used on the DL to maintain a defined error rate performance for a given channel condition.

[0114] While only one RI is reported for the entire bandwidth, the PMI and CQI reports can be for the wideband or for each sub-band, depending on the frequency selectivity of the channel. These indicators are transmitted on the UL on two different types of physical channels. i) Physical Uplink Control Channel (PUCCH) used only for control information, ii) Physical Uplink Shared Channel (PUSCH) used for both data and control information, allocated on a single resource block (RB) and on a sub-frame basis. For the PUCCH, the procedure for reporting the RI, PMI and CQI is periodic, and the indicators can be either wideband (for frequency-flat channels) or selected for each UE on a sub-band basis (for frequency-selective channels). For the PUSCH, the feedback procedure is aperiodic, and can be selected for each UE either on a sub-band basis (for frequency-selective channels) or on a higher layer configured sub-band (e.g., for transmission mode 9 of LTE-Advanced with 8 transmitters).

[0115] In one embodiment of the present invention, the DIDO or MU-MAS system uses RI, PMI, and CQI to report the current channel status and precoding information to the BTS and CP. In one embodiment, the UE uses the PUCCH channel to report those metrics to the CP. In another embodiment, if more metrics are required for DIDO precoding, the UE uses the PUSCH to report additional metrics to the CP. If the channel is frequency flat, the UE can utilize extra UL resources to report PMI for more antennas in the DIDO system. In one embodiment of the invention, the UE or BTS or CP estimates the channel frequency selectivity, and if the channel is frequency flat, the UE utilizes extra UL resources to report PMI for more BTSs. 2. Downlink Open-Loop MU-MAS Precoding Method in LTE

[0116] The open-loop MU-MAS precoding method can only be used in a time-division duplexing (TDD) system that employs RF calibration and utilizes channel reciprocity. The general mechanism of the open-loop method in MU-MAS consists of the following: i) The UE transmits signaling information on the UL to the BTS or CTR. ii) The BTS or CTR utilizes this signaling information to estimate UL CSI from all UEs. iii) The BTS or CTR uses RF calibration to convert UL CSI to DL CSI. iv) The BTS or CTR transmits the DL CSI or codebook index to the CP via the BSN. v) Based on the DL CSI, the CP calculates the precoding weights for data transmission on the DL. Similar to the closed-loop MU-MAS precoding method, user clustering can be used to reduce the amount of CSI from UEs estimated at the BTS, thereby reducing the computational burden at the BTS and the amount of signaling required on the UL. In one embodiment of the present invention, the open-loop precoding technique is used to transmit simultaneous non-interfering data streams from the BTS to the UEs on the DL channel.

[0117] In LTE, there are two types of reference signals for the uplink channel [31, 33, 87]. i) The sounding reference signal (SRS) used for scheduling and link adaptation. ii) The demodulation reference signal (DMRS) used for data reception. In one embodiment of the present invention, the DMRS is used in an open-loop precoding system to estimate the UL channel from all UEs to all BTSs. In the time domain, the DMRS is transmitted in the fourth OFDM symbol of all LTE slots (with a duration of 0.5 milliseconds) when using a normal cyclic prefix. In the frequency domain, the DMRS transmitted on the PUSCH is mapped to the same resource block (RB) used by the UE for UL data transmission for all UEs. The length of the DMRS is M RS =mN RB where m is the number of RBs and N RB= 12 is the number of sub - carriers per RB. To support multiple UEs, up to 12 DMRSs are generated from one basic Zadoff - Chu

[88] or computer - generated constant - amplitude zero - autocorrelation (CG - CAZAC) sequence by 12 possible cyclic shifts of the basic sequence. The basic sequence is divided into 30 groups, and adjacent LTE cells select DMRSs from different groups to reduce inter - cell interference. For example, if the maximum number of resource blocks in one OFDM symbol is 110 (i.e., assuming an overall signal bandwidth of 20 MHz), it is possible to generate up to 110×30 = 3300 different sequences. It is observed that the 30 basic sequences are not guaranteed to be orthogonal and are designed to reduce interference across cells without completely eliminating it. In contrast, the 12 cyclic shifts of the same basic sequence are orthogonal, thereby enabling up to 12 UEs to transmit on UL on the same RB without interference. The value of the cyclic shift used by each UE is provided by the BTS through the downlink control information (DCI) message transmitted via the PDCCH. The DCI in Release 8 consists of 3 bits, which allows the UE to use only up to 8 cyclic shifts out of the pool of 12 possible options.

[0118] The cyclic shift of the basic DMRS sequence is utilized in the present invention to enable the use of the MU-MIMO scheme on the UL channel and to enable the estimation of the CIS from multiple UEs for DL precoding when channel reciprocity is utilized in the TDD mode. In one embodiment of the present invention, an open-loop precoding method is used to transmit a simultaneous non-interfering data stream from a distributed BTS to a UE on the DL channel. In a different embodiment of the present invention, an open-loop MU-MIMO method is used to receive a simultaneous non-interfering data stream from a UE to a BTS on the UL channel. The same CSI estimated from all active UEs on the UL can be used to calculate a receive spatial filter for MU-MIMO operation in addition to the weights for DL precoding. Since Release 8 defines only up to eight orthogonal DMRSs (by the limited DCI bits as described above), the MU-MIMO scheme for the UL channel and the MU-MAS precoding scheme for the DL channel can support up to eight UEs assuming that all UEs utilize the entire UL bandwidth.

[0119] As a method of increasing the number of UEs simultaneously supported through MU-MIMO in UL or MU-MAS precoding in DL, multiplexing the DMRS of UEs in the frequency domain can be mentioned. For example, when a 10 MHz bandwidth is used in the TDD mode, there are 50 RBs that can be assigned to UEs. In this case, 25 interleaved RBs can be assigned to one group of 8 UEs, and the remaining 25 interleaved RBs can be assigned to another group of UEs, and the total number of UEs that can be supported simultaneously becomes 16. Next, the CSI is calculated by interpolating the estimation from the DMRS transmitted on the interleaved RBs. A larger number of simultaneously supported UEs can be supported by increasing the number of interleaving patterns of UL RBs. These patterns can be specified for different UEs statically or dynamically by a specific frequency hopping sequence. In one embodiment of the present invention, in order to increase the number of UEs supported through MU-MIMO or MU-MAS precoding, the DMRS is assigned to UEs on orthogonal interleaved RBs. In the same embodiment, the interleaved RBs are assigned statically. In another embodiment, the interleaved RBs are assigned dynamically by a specific frequency hopping pattern.

[0120] An alternative solution is to multiplex the DMRS of different UEs in the time domain. For example, UEs are divided into different groups, and the DMRS for those groups are transmitted on consecutive time slots (each with a duration of 0.5 milliseconds). However, in this case, it is necessary to ensure that the periodicity of the DMRS assignment for different groups is lower than the channel coherence time of the fastest moving UE. In fact, this is a necessary condition to ensure that the channel does not change for all UEs from the time when CSI is estimated by the DMRS to the time when the system transmits the DL data stream to the UE by DIDO precoding. In one embodiment of the present invention, the system divides the active UEs into groups and assigns the same set of DMRS to each group on consecutive time slots. In the same embodiment, the system estimates the shortest channel coherence time for all active UEs and calculates the periodicity of the time multiplexing of the DMRS based on the maximum number of UE groups and that information.

[0121] Another solution is to spatially separate different groups of UEs that use the same set of DMRSs. For example, the same set of orthogonal DMRSs can be used for all UEs from different antenna sub-clusters in FIG. 11 identified by the same cell ID. In one embodiment of the present invention, groups of UEs that use the same set of orthogonal DMRSs are spatially separated to avoid interference between the groups. In the same embodiment, the same set of orthogonal DMRSs is used by different antenna sub-clusters identified by the same cell ID. MU-MAS may assign UEs to "virtual cells" in order to maximize the number of DMRSs that can be used in the UL. In one exemplary embodiment, a virtual cell is the coherence area around a UE (described in the related co-pending U.S. patent application Ser. No. 13 / 232,996, titled "Systems and Methods to Exploit Areas of Coherence in Wireless Systems"), and the DIDO system generates up to 3300 coherence areas for different UEs. In another embodiment of the present invention, each of the 30 basic sequences is assigned to a different antenna cluster to reduce inter-cluster interference between adjacent antenna clusters (a cluster is defined in the related U.S. Patent No. 8,170,081, issued May 1, 2012, titled "System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements").

[0122] 3. Uplink MU-MAS Method in LTE Embodiments of the present invention use an open-loop MU-MIMO scheme on the UL channel to receive simultaneous UL data streams from all UEs to the BTS. The UL open-loop MU-MIMO scheme consists of the following processes. i) The UE transmits signaling information and data payloads to all BTSs. ii) The BTS calculates channel estimates from all UEs using the signaling information. iii) The BTS transmits the channel estimates and data payloads to the CP. iv) The CP removes inter-channel interference from the data payloads of all UEs via spatial filtering using the channel estimates and demodulates the data streams from all UEs. In one embodiment, the open-loop MU-MIMO system uses single-carrier frequency-division multiple access (SC-FDMA) to increase the number of UL channels from the UE to the BTS and multiplexes them in the frequency domain.

[0123] In one embodiment, synchronization between UEs is achieved by signaling from the DL, and all BTSs are considered to be locked to the same time / frequency reference clock, either by direct wiring to the same clock or by sharing a common time / frequency reference via a GPSDO of one embodiment. Variations in channel delays across different UEs can generate jitter between the time references of different UEs and may affect the performance of the MU-MIMO method on the UL. In one embodiment, only UEs in the same antenna cluster (e.g., UEs adjacent to each other) are processed by the MU-MIMO method to reduce the relative propagation delay between different UEs. In another embodiment, the relative propagation delay between UEs is compensated for at the UE or at the BTS to ensure simultaneous reception of data payloads from different UEs at the BTS.

[0124] The method for enabling signaling information regarding data demodulation on the UL is the same as that used for the signaling of the downlink open loop DIDO scheme described in the previous section. The CP uses different spatial processing techniques to remove the inter-channel interference from the UE data payload. In one embodiment of the present invention, the CP uses non-linear spatial processing methods such as maximum likelihood (ML), decision feedback equalization (DFE), or successive interference cancellation (SIC) receivers. In another embodiment, the CP uses linear filters such as zero forcing (ZF) or minimum mean square error (MMSE) receivers to cancel the co-channel interference and demodulate the uplink data stream individually.

[0125] 4. Integration with Existing LTE Networks In the United States of America and other regions of the world, LTE networks are already in operation or are being deployed and / or are scheduled to be deployed. If LTE operators can gradually deploy DIDO or MU-MAS capabilities into existing or already committed deployments, it will be of significant benefit to them. In this way, they can deploy DIDO or MU-MAS in the areas where DIDO or MU-MAS provides the most immediate benefits and gradually expand the DIDO or MU-MAS capabilities across more networks. Eventually, when they have sufficient DIDO or MU-MAS coverage in the area, they can choose to completely abolish the use of cells and instead switch completely to DIDO or MU-MAS to achieve a very high spectral density at a very low cost. Through this complete transition from cellular to DIDO or MU-MAS, the wireless customers of LTE operators will never suffer losses in service. Rather, they will simply experience an improvement in the throughput and reliability of their data, while the operator will experience a reduction in its costs.

[0126] There are several embodiments that enable a gradual integration of DIDO or MU-MAS into an existing LTE network. In all cases, the BTS for DIDO or MU-MAS is referred to as a DIDO-LTE BTS, which will utilize one of the LTE-compatible DIDO or MU-MAS embodiments described above, or some other LTE-compatible embodiment that may be developed in the future. Alternatively, the DIDO-LTE BTS will utilize a slight modification of the LTE standard, as described above, and the UE will either be updated (e.g., if a software update is sufficient to modify the UE to be compatible with DIDO or MU-MAS), or a new generation of UEs that are DIDO or MU-MAS compatible will be deployed. In either case, the new BTS that supports DIDO or MU-MAS, whether within the constraints of the LTE standard or as a modification of the LTE standard, will hereinafter be referred to as a DIDO-LTE BTS.

[0127] The LTE standard supports various channel bandwidths (e.g., 1.4, 3, 5, 10, 15, and 20 MHz). In one embodiment, an operator of an existing LTE network can either allocate a new bandwidth for the LTE-DIDO BTS or subdivide the existing LTE spectrum (e.g., 20 MHz can be subdivided into two 10 MHz blocks), and support a conventional LTE BTS of a cellular configuration with one block of spectrum and a DIDO LTE BTS with another block of spectrum. In effect, this establishes two separate LTE networks, and the UE device will be configured to use one or the other network, or to select between the two. In the case of subdivided spectrum, the spectrum can be divided evenly or unevenly between the conventional LTE network and the DIDO-LTE BTS network, and more spectrum can be allocated to the network that can be utilized best, taking into account the degree of deployment of the cellular LTE BTS and the DIDO-LTE BTS, and / or the UE usage pattern. This subdivision can change over time as needed, and at some point, when there are sufficient DIDO-LTE BTS deployed to provide the same or better coverage as the cellular BTS, all of the spectrum can be allocated to the DIDO-LTE BTS and the cellular BTS can be decommissioned.

[0128] In another embodiment, a conventional cellular LTE BTS can cooperate with a DIDO-LTE BTS to share the same spectrum, but be configured to use that spectrum alternately. For example, if they equally share the use of the spectrum, each BTS network will utilize one 10 ms frame time alternately, such that, for example, after one 10 ms frame for the cellular LTE BTS, one 10 ms frame for the DIDO-LTE BTS follows. The frame time can also be subdivided into non-uniform intervals. This interval division can change as needed over time and, when there are sufficient DIDO-LTE BTSs deployed to provide the same or better coverage as the cellular BTS, all of the time can be allocated to the DIDO-LTE BTS and the cellular BTS can be decommissioned.

[0129] In another embodiment of the present invention, DIDO or MU-MAS is used as a LOS or NLOS wireless backhaul for small cells in LTE and LTE-Advanced networks. As small cells are deployed in the LTE network, DIDO or MU-MAS provides a high-speed wireless backhaul to those small cells. As the demand for higher data rates increases, more small cells are added to the network until the limit is reached where the wireless network cannot add any more small cells to a given area without causing inter-cell interference. In the same embodiment of the present invention, the DIDO-LTE BTS is used to gradually replace the small cells, thereby leveraging inter-cell interference to provide increased network capacity.

[0130] 5. MU-MAS LTE Scheduler In MU-MAS, a distributed antenna or BTS transmits simultaneously pre-coded data streams to multiple UEs. As described in related patents and applications, the number of BTSs must be equal to or greater than the number of UEs to enable simultaneous data transmission. In an actual deployment, the number of UEs may exceed the number of BTSs. In this case, the excess UEs can be selected to be transmitted in different time slots or frequency bands according to a specific scheduling policy. The scheduler utilizes the channel quality information of the UEs to determine the best set of UEs to be paired at the provided time and frequency. In the present invention, different scheduling methods are used, including a proportional fair scheduler, round robin, or greedy algorithm.

[0131] As described in the previous section, the LTE standard defines two parameters that convey to the scheduler about the link quality of all UEs. CQI and SRS. CQI measures the quality of the DL channel and is supplied from the UE to the BTS and returned. SRS is signaling information transmitted from the UE to the BTS for measuring the UL channel quality. Both indicators provide information on the UL / DL channel quality over time and frequency domains. Since the DL and UL channel qualities can vary depending on different carrier frequencies, in an FDD system, the DL scheduler must use CQI as a performance metric. In the TDD mode, the DL schedule makes a scheduling decision using either CSI or SRS or a combination of both. Similar performance measurement criteria can also be used for UL scheduling. In one embodiment of the present invention, the MU-MAS scheduler uses CQU and SRS as performance metrics used by the scheduling algorithm.

[0132] The MU-MAS described in the present invention enables the use of one additional channel quality metric not disclosed in the prior art. The spatial selectivity index (SSI) described in related U.S. patent application Ser. No. 13 / 475,598, titled "Systems and Methods to enhance spatial diversity in distributed-input distributed-output wireless systems". The SSI can be calculated based on the CSI obtained from all UEs via a feedback mechanism or from the UL channel (applying UL / DL channel reciprocity). In one embodiment of the present invention, the scheduler utilizes the SSI as a performance metric. The SSI is a measure of the spatial diversity available in a wireless link. The SSI depends on the spatial characteristics of the BTS in addition to the UE. In an exemplary embodiment of the present invention, the scheduler obtains the SSI from all UEs and schedules the UEs with the "optimal" SSI according to certain scheduling criteria. When there are more available BTSs than active BTSs, the above user selection criteria are combined with the antenna selection method described in related U.S. patent application Ser. No. 13 / 475,598, titled "Systems and Methods to enhance spatial diversity in distributed-input distributed-output wireless systems". In one embodiment of the present invention, the scheduler selects an optimal subset of BTSs and UEs based on certain scheduling criteria.

[0133] Referring to FIGS. 9, 10, and 11, in certain scenarios, orthogonal signaling sequences that enable a large number of BTSs within the same antenna cluster or antenna subcluster may not be sufficient. In this case, some degree of interference may occur when additional BTSs are activated to cover areas with a larger number of active UEs. In one embodiment of the present invention, the scheduler measures the degree of interference between antenna clusters or antenna subclusters and schedules the UEs to minimize the effect of that interference on the wireless link.

[0134] The antenna selection algorithm described in related U.S. Patent Application No. 13 / 475,598, titled "Systems and Methods to enhance spatial diversity in distributed-input distributed-output wireless systems", is used in the present invention to select an optimal set of active BTSs based on SSI. However, this antenna selection algorithm may require high computational complexity because the MU-MAS precoding process must be applied to all possible permutations of all antenna subsets before making a determination about the best subset based on SSI performance measurement criteria. In MU-MAS with a large number of cooperating BTSs, this computational load may be prohibitively expensive or may not be achievable in actual deployment. Therefore, it is desirable to develop an alternative method that reduces the number of antenna subsets while maintaining good performance of the antenna selection method. In one embodiment of the present invention, MU-MAS uses a method, hereinafter referred to as the "antenna shuffling method", based on a queue of antenna subset ID numbers. In one embodiment of the present invention, the antenna shuffling method divides a queue containing all possible antenna subset IDs (i.e., all possible permutations of active BTSs for a given set of available BTSs) into different groups and assigns different priorities to these groups. These groups are defined to give all selected subset IDs a fair chance, but the SSI measurement criteria are calculated for only a limited number of subsets (e.g., the ones with the highest priority), thus reducing the computational complexity. In an exemplary embodiment, the queue of subset IDs is divided into three groups, and each group is assigned different rules. i) Group 1 contains the IDs with the highest priority, which are only pulled out of the group when a new subset with a higher priority is identified. ii) Group 2, which contains new antenna subsets (selected from Group 3) in every iteration of the method. iii) Group 3, in which the antenna subset IDs are shuffled according to a round-robin rule.The subset IDs within groups 1 and 2 are classified based on their priorities in each iteration of the method, and the subset IDs from group 2 are given the opportunity to be upgraded to group 1. The SSI is calculated only for the subsets within groups 1 and 2, and the antenna selection algorithm is applied only to these subsets.

[0135] 6.MU-MAS LTE User Equipment The present invention includes various designs of LTE UEs. In one embodiment, the UE is an LTE UE that is compatible with MU-MAS using precoding, as previously described and illustrated in FIG. 13.

[0136] In different embodiments, as shown in FIG. 14, the UE 1401 is connected to different devices 1402 and 1403 through a first network interface 1404 (e.g., Wi-Fi, USB, Ethernet, Bluetooth, optical fiber, etc.), and to the MU-MAS through a second network interface 1405. The UE in FIG. 14 has two different network interfaces implemented, and each network interface includes one or more antennas (however, in an alternative embodiment, the first network interface 1404 may be a wired interface without an antenna). The antennas of the first network interface are shown as circles, and the antennas of the second network interface are shown as triangles. In the same embodiment, the second network interface supports MU-MAS precoding, MU-MAS implementing an LTE-compliant protocol, or MU-MAS (with or without implementing an LTE-compliant protocol) and an alternative network. In the same embodiment, the alternative network is a cellular network, an LTE network, or a Wi-Fi network. In the same embodiment, the UE operates with one and / or both MU-MASs, and / or the UE and the alternative network select either one of the MU-MASs or the alternative network based on some criteria. In the same embodiment, the criteria are: i) whether only one network is available and selected, ii) whether one network has better performance, iii) whether one network is more economical, iv) whether one network is less congested, v) whether one network uses fewer UE resources.

[0137] In one embodiment of the present invention, as illustrated in FIG. 15, the UE 1501 is housed in a case that is physically attached to the user device 1502. In the same embodiment, the case serves as a decorative addition to the user device. In another embodiment, the case serves to protect the user device from physical damage. The UE consists of a battery 1503 and one or more network interfaces 1504.

[0138] In one embodiment, the UE electronic device is built into the case. In the same embodiment, the UE electronic device includes a battery 1503. The battery includes a power charger that couples through physical electrical contact or wireless contact. Exemplary power couplings are conductive, dielectric, RF, optical, or thermal, but the power coupling is not limited to these techniques. In the same embodiment, the UE electronic device is coupled to receive power from the user device. This power coupling is through physical contact, or through dielectric or wireless contact. In the same embodiment, the user device is coupled to receive power from the MU-MAS UE. This coupling is through physical contact, or through dielectric or wireless contact. In different embodiments, the same power charger powers both the user device and the MU_MAS UE.

[0139] In one embodiment of the present invention, the UE is configured to communicate with the user device. In the same embodiment, the UE is resettable so that the user device can initially connect to the UE (e.g., via a switch or by removing power), and once the connection is established, the UE is configurable by the user device. Such configuration includes configuring a private password and / or other security protocols. In different embodiments, the UE includes means for being configured to communicate with the user device. Such configuration is performed via a communication port that is a USB to another device, or via a control device and / or buttons on the UE, or via a display where buttons or touch inputs are used.

[0140] In another embodiment, the same RF chain is used for MU-MAS communication in addition to the alternative network. In another embodiment, different RF chains are used in MU-MAS communication and the alternative network.

[0141] 7. Radio Frequency (RF) Calibration Utilizing Channel Reversibility The conventional closed-loop MU-MAS method employs an UL channel to feedback the quantized CS index or codebook index from the UE to the BTS or CP (such as in the codebook-based limited feedback method). However, this method increases the feedback overhead for enabling the CSI feedback channel and results in higher protocol complexity. Therefore, in a TDD system where UL and DL are set to the same frequency, it is desirable to avoid CSI feedback by utilizing UL / DL channel reversibility. In an actual system, the transmitting and receiving RF chains in the BTS or UE typically have different characteristics due to different RF components and circuit layouts. Therefore, in order to maintain UL / DL interaction, it is necessary to adopt an RF calibration method to compensate for the RF mismatch between the transmitting chain and the receiving chain.

[0142] A model for RF mismatch in a typical wireless transceiver is described in

[91] , and a hardware solution for mitigating the impact of RF mismatch on the performance of an adaptive digital beamforming system is discussed in

[92] . Software techniques enabling RF calibration in multiple-input multiple-output (MIMO) systems have been proposed in [93, 94], and experimental results for a multiple-input single-output (MISO) system are shown in

[96] , and experimental results for a system adopting antenna selection are shown in

[96] .

[0143] However, the prior art assumes that since all RF chains are arranged on the same circuit board as in a MIMO system, information regarding RF mismatches between all RF chains is locally available, thus simplifying the problems related to RF calibration. In contrast, the present invention consists of distributed antennas that are geographically remote from each other such that communication between the distributed antennas occurs only through the network. Therefore, the applicant of the present application defines a novel system unit called a "beacon station" that is specifically designed to enable RF calibration in a MU-MAS equipped with distributed antennas. Further, in a MIMO system according to the prior art, significant RF coupling occurs between the transmit / receive chains due to the close proximity of the RF chains on the same substrate. In contrast, in the present invention, RF coupling occurs only between one transmit chain and one receive chain of the same distributed antenna. Therefore, the techniques employed for RF calibration are significantly different from those described in the prior art, which will be described later. Finally, the RF calibration methods disclosed in the prior art are limited to single-user systems (e.g., single-user equipment devices). As shown in the derivations in the following paragraphs, multi-user systems (e.g., MU-MAS) are particularly sensitive to RF mismatches because RF mismatches cause interference between users. Therefore, as described below, special techniques must be employed to enable RF calibration while utilizing channel reversibility.

[0144] The present invention consists of a MU-MAS comprising a plurality of distributed antennas, a plurality of user equipment devices (UEs), and one or more beacon stations, which employs radio frequency (RF) calibration and utilizes the interaction between the downlink (DL) channel and the uplink (UL) channel. In one embodiment, RF calibration may be employed to calculate the DL MU-MAS precoding weights from the UL channel estimates. FIG. 16 shows a block diagram of a system including a distributed antenna 1601, a plurality of UEs 1613, one beacon station 1619, one base station network (BSN) 1607 connecting the distributed antennas, one centralized processor (CP) 1621, and one feedback channel 1620 which is a calibration control channel from the beacon to the CP.

[0145] Each distributed antenna unit consists of a baseband unit 1602, a transmit RF chain 1603, a receive RF chain 1604, an RF switch unit 1605 that dynamically selects the transmit / receive RF chain for TDD operation, and an antenna 1606. In one embodiment, the baseband unit comprises baseband signal processing and a digital-to-analog converter (DAC). In another embodiment, all baseband processing is performed at the CP such that RF signals are sent to each distributed antenna (e.g., via RF over RF coaxial cables or fiber networks). Each UE consists of a baseband unit 1608, a transmit RF chain 1609 / receive RF chain 1610, an RF switch 1611, and an antenna 1612. The beacon station consists of a baseband unit 1614, a transmit RF chain 1615 / receive RF chain 1616, an RF switch 1617, and an antenna 1618.

[0146] The wireless link between the distributed antenna and the UE is modeled as a complex Gaussian channel matrix H of dimension M×N, where M is the number of UEs and N is the number of distributed antennas. The applicant of the present application, H DL , the DL channel matrix 1622, and H UL, UL channel matrix 1623. Channel reciprocity holds as long as DL and UL are set to the same carrier frequency. In this case, the following properties hold:

number

[0147] The above model holds for either single-carrier or multi-carrier systems. In a multi-carrier system (e.g., OFDM), the complex matrix H represents the channel of one subcarrier, and the same model extends to any subcarrier in the system. Figure 16 also shows the complex channel matrix A T and A R Similarly, the transmit and receive RF units in the UE are modeled using a matrix B T and B. R For MU-MAS with distributed antennas, A T , A R , B T and B. R Due to the relative separation of the antennas such that , is expressed as a diagonal matrix, RF coupling between distributed antennas and / or between UEs is negligible. Applicant has found that this is an inherent feature of MU-MAS with distributed antennas and distributed UEs. Thus, the present invention is novel over prior art for multiple-input multiple-output (MIMO) systems.

[0148] Based on the block diagram of FIG. 16, Applicant writes the effective DL channel matrix (which models the transmit / receive RF units and radio links) as follows:

number

number

Number

Number

[0149] As shown in FIG. 17, the matrix C is the effective DL between each distributed antenna 1701 and the beacon station 1719 defined as follows

Number

Number

Number

Number

[0150] When linear precoding (e.g., zero-forcing

[65] , block diagonalization or BD [66 - 67], matrix inversion, etc.) is employed, the symbol received at the m-th UE is obtained as follows.

Number

Number

Number

Number

Number

Number

Number

Number

[0151] In another embodiment of the present application, a non-linear precoding method (e.g., dirty paper coding [68-70]) or Tomlinson-Harashima precoding or THP [71-72], lattice techniques or trellis precoding [73-74], vector perturbation techniques [75-76]) is applied to the RF-calibrated channel matrix to cancel the inter-client interference in each UE. FIG. 19 shows that the SER obtained using non-linear precoding techniques using RF calibration and UL / DL reversibility matches the performance of linear precoding. FIG. 20a shows the constellation before the THP modulo operation for UE 1 in a MU-MAS with two distributed antennas and two UEs, and FIG. 20b shows the constellation before the THP modulo operation for UE 2 (THP beam structure) in a MU-MAS with two distributed antennas and two UEs. THP precoding is designed to completely cancel the interference to the "reference UE" and applies a successive interference cancellation scheme to other UEs. Therefore, it is expected that the SER performance for the reference UE can be better than that of other UEs. In one embodiment, a round-robin or proportional fairness scheduling technique, or another type of scheduling technique, is applied to the UEs to guarantee similar average SER performance for all UEs.

[0152] The computational performance of the BD method and the THP method can vary depending on the number of distributed antennas and / or UEs within each user cluster. In one embodiment of the present invention, the MU-MAS switches between linear precoding techniques and non-linear precoding techniques to minimize the computational complexity of the precoder calculation, depending on the number of distributed antennas and / or UEs in each user cluster.

[0153] In an actual MU-MAS, the beacon station is a radio transceiver dedicated to RF calibration. Since the beacon requires a feedback channel for communicating the effective DL channel estimated from all the distributed antennas for calibration purposes, the beacon communicates with the CP via a wireless link or a wired link. In another embodiment, the beacon station is any one of the distributed antennas, and calibration parameters are calculated for that antenna. In the same embodiment, the distributed antennas are organized like a mesh network, and pairwise RF calibration between adjacent distributed antennas is calculated to ensure good link quality. The RF calibration is performed across all the antennas, and calibration information is fed back to the CP so that all the distributed antennas are calibrated with each other. In another embodiment, the beacon is any one of the UEs that use an arbitrary wireless link or a preferred link to feed back calibration information to the CP.

[0154] The calibration information from the beacon to the CP is transmitted either by quantization over a limited number of bits or by a codebook-based limited feedback method to reduce overhead on the control channel. The applicant of the present application has found that RF calibration can operate at a slow rate (depending on the rate of variation of RF characteristics due to, for example, temperature changes). When the update rate of the calibration information is low, the wireless data channel can be used to transmit that information to the CP without causing any severe data loss rate. In an exemplary embodiment, in an LTE cellular network, the PUSCH is used to feed back calibration information from the UE to the CP.

[0155] One or more geographically dispersed beacons are employed per cluster according to the relative link quality between the beacons in a user cluster, or an antenna cluster or antenna sub-cluster, and the distributed antennas. In one embodiment, the beacon with the best signal quality for all distributed antennas in the cluster is used for RF calibration. In another embodiment, the beacon is dynamically selected at all times to adapt to the changing quality of the link to the distributed antennas due to variations in the propagation environment. In another embodiment, multiple beacons are cooperatively employed (e.g., by maximum ratio combining / transmission) to maximize the SNR or SINR via the link from / to the distributed antennas. In different embodiments, one or more RF calibrations are performed per cluster.

[0156] In one embodiment of the present invention, the beacon station is used not only for RF calibration but also for transmitting signal information to distributed antennas and / or UEs including time and frequency synchronization references. The distributed antennas and / or UEs employ a reference for maintaining time and frequency synchronization with the MU-MAS master reference clock. In one embodiment, this reference clock distribution from the beacon to the distributed antennas and UEs is made possible via the LTE multimedia broadcast single frequency network (MBSFN) communication channel.

[0157] "References" [1]A.Paulraj,R.Nabar,and D.Gore,Introduction to Space-Time Wireless Communications,Cambridge University Press,40 West 20th Street,New York,NY,USA,2003

[0158] [2] D.Gesbert, M.Shafi, D.Shiu, P.J.Smith and A.Naguib, 「From theory to practice: an overview of MIMO space-time coded wireless systems」, IEEE Journal on Selected Areas on Communications, vol.2, n.3, pp.281~302, Apr.2003

[0159] [3] L.Zheng and D.N.C.Tse, 「Diversity and multiplexing: a fundamental tradeoff in multiple-antenna channels,」 IEEE Trans.Info.Th., vol.49, no.5, pp.1073~1096, May 2003

[0160] [4] D.N.C.Tse, P.Viswanath, and L.Zheng, 「Diversity-multiplexing tradeoff in multiple-access channels」, IEEE Trans.Info.Th., vol.50, no.9, pp.1859~1874, Sept.2004

[0161] [5] E.Visotsky and U.Madhow, 「Space-time transmit precoding with im-perfect feedback」, IEEE Trans.Info.Th., vol.47, pp.2632~2639, Sep.2001。

[0162] [6] S.A. Jafar, S. Vishwanath, and A. Goldsmith, "Channel capacity and beamforming for multiple transmit and receive antennas with covariance feedback", Proc. IEEE Int. Conf. on Comm., vol. 7, pp. 2266~2270, Jun. 2001。

[0163] [7] S.A. Jafar and A. Goldsmith, "Transmitter optimization and optimality of beamforming for multiple antenna systems", IEEE Trans. Wireless Comm., vol. 3, pp. 1165~1175, July 2004。

[0164] [8] E.A. Jorswieck and H. Boche, "Channel capacity and capacity-range of beamforming in MIMO wireless systems under correlated fading with covariance feedback,", IEEE Trans. Wireless Comm., vol. 3, pp. 1543~1553, Sep. 2004

[0165] [9] A.L. Moustakas and S.H. Simon, "Optimizing multiple-input single-output (MISO) communication systems with general Gaussian channels: nontrivial covariance and nonzero mean", IEEE Trans. Info. Th., vol. 49, pp. 2770~2780, Oct. 2003。

[0166]

[10] M. Kang and M. S. Alouini, 「Water-filling capacity and beamforming performance of MIMO systems with covariance feedback,」 IEEE Work. on Sign. Proc. Adv. in Wire. Comm., pp. 556~560, June 2003。

[0167]

[11] S. H. Simon and A. L. Moustakas, 「Optimizing MIMO antenna systems with channel covariance feedback」, IEEE Jour. Select. Areas in Comm., vol. 21, pp. 406~417, Apr. 2003。

[0168]

[12] S. M. Alamouti, 「A simple transmit diversity technique for wireless communications」, IEEE Jour. Select. Areas in Comm., vol. 16, no. 8, pp. 1451~1458, Oct. 1998。

[0169]

[13] V. Tarokh, N. Seshadri, and A. R. Calderbank, 「Space-time codes for high data rate wireless communication: Performance criterion and code construction」, IEEE Trans. Info. Th., vol. 44, pp. 744~65, Mar. 1998

[0170]

[14] V. Tarokh, H. Jafarkhani, and A. R. Calderbank, 「Space-time block codes from orthogonal designs」, IEEE Trans. Info. Th., vol. 45, pp. 1456~467, July 1999。

[0171]

[15] E.N. Onggosanusi, A.G. Dabak, and T.A. Schmidl, 「High rate space-time block coded scheme: performance and improvement in correlated fading channels,」 Proc. IEEE Wireless Comm. and Net. Conf., vol.1, pp.194~199, Mar. 2002。

[0172]

[16] G.D. Durgin, Space-Time Wireless Channels, Prentice Hall, Upper Saddle River, NJ, USA, 2003

[0173]

[17] D.-S. Shiu, G.J. Foschini, M.J. Gans, and J.M. Kahn, 「Fading corre-lation and its effect on the capacity of multielement antenna systems,」 IEEE Trans. Comm., vol.48, no.3, pp.502~513, Mar. 2000

[0174]

[18] A. Forenza and R.W. Heath Jr., 「Impact of antenna geometry on MIMO communication in indoor clustered channels」, Proc. IEEE Antennas and Prop. Symp., vol.2, pp.1700~1703, June 2004

[0175]

[19] E.A. Jorswieck and H. Boche, 「Channel capacity and capacity-range of beamforming in MIMO wireless systems under correlated fading with covariance feedback,」 IEEE Trans. Wireless Comm., vol. 3, pp. 1543~1553, Sep. 2004

[0176]

[20] R.W. Heath Jr. and A. Paulraj, 「Switching between multiplexing and diversity based on constellation distance,」 Proc. of Allerton Conf. on 208, Comm. Control and Comp., Sep. 2000。

[0177]

[21] S. Catreux, V. Erceg, D. Gesbert, and R.W. Heath Jr., 「Adaptive modulation and MIMO coding for broadband wireless data networks,」 IEEE Comm. Mag., vol. 2, pp. 108~115, June 2002

[0178]

[22] A. Forenza, A. Pandharipande, H. Kim, and R.W. Heath Jr., 「Adaptive MIMO transmission scheme: Exploiting the spatial selectivity of wireless channels」, Proc. IEEE Veh. Technol. Conf., vol. 5, pp. 3188~3192, May 2005

[0179]

[23] C.B.Chae,A.Forenza,R.W.Heath,Jr.,M.R.McKay,and I.B.Collings,「Adaptive MIMO Transmission Techniques for Broadband Wireless Communication Systems,」IEEE Communications Magazine,vol.48,no.5,pp.112~118,May 2010

[0180]

[24] FCC,「Broadband action agenda」,National Broadband Plan,2010 http: / / www.broadband.gov / plan / national-broadband-plan-action-agenda.pdf

[0181]

[25] ,N.Delfas,F.Meunier,S.Flannery,T.Tsusaka,E.Gelblum and S.Kovler,「Mobile data wave:who dares to invest,wins」,Morgan Stanley Research Global,June 13,2012

[0182]

[26] D.Goldman,「Sorry,America:your wireless airwaves are full」,CNN Money http: / / money.cnn.com / 2012 / 02 / 21 / technology / spectrum_crunch / index.htm

[0183]

[27] P.Rysavy,「No silver bullets for FCC,NTIA spectrum challange」,Daily report for executives,Bloomberg BNA,Aug.2012 http: / / www.rysavy.com / Articles / 2012_09_No_Spectrum_Silver_Bullets.pdf

[0184]

[28] T.W. Hazlett, "Radio spectrum for a hungry wireless world", Sept. 22, 2011

[0185]

[29] B.J. Love, D.J. Love and J.V. Krogmeier, "Like deck chairs on the Titanic: why spectrum reallocation won't avert the coming data crunch but technology might keep the wireless industry afloat", Feb. 2012

[0186]

[30] Qualcomm, "The 1000x data challenge, the latest on wireless, voice, services and chipset evolution", 4G World, Oct. 31 st , 2012

[0187]

[31] J. Lee, J.-K. Han, J. Zhang, "MIMO technologies in 3GPP LTE and LTE-advanced", EURASIP Journal on Wireless Comm. and Net., Hindawi, May 2009

[0188]

[32] 3GPP, TS 36.201, "Evolved Universal Terrestrial Radio Access (E-UTRA); LTE Physical Layer-General Description (Release 8)"

[0189]

[33] 3GPP, TS 36.211, "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)"

[0190]

[34] 3GPP, TS 36.212, "Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 8)"

[0191]

[35] 3GPP, TS 36.213, "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)"

[0192]

[36] T. Yoo, N. Jindal, and A. Goldsmith, "Multi-antenna broadcast channels with limited feedback and user selection," IEEE Journal on Sel. Areas in Communications, vol. 25, pp. 1478~91, July 2007.

[0193]

[37] P. Ding, D. J. Love, and M. D. Zoltowski, "On the sum rate of channel subspace feedback for multi-antenna broadcast channels," in Proc., IEEE Globecom, vol. 5, pp. 2699~2703, November 2005.

[0194]

[38] N. Jindal, "MIMO broadcast channels with finite-rate feedback," IEEE Trans. on Info. Theory, vol. 52, pp. 5045 - 60, November 2006。

[0195]

[39] D. J. Love, R. W. Heath, Jr., V. K. N. Lau, D. Gesbert, B. D. Rao, and M. Andrews, "An Overview of Limited Feedback in Wireless Communication Systems", IEEE Journal on Sel. Areas in Comm., Special Issue on Exploiting Limited Feedback in Tomorrow’s Wireless Communication Networks, vol. 26, no. 8, pp. 1341 - 1365, Oct. 2008

[0196] R. W. Heath, Jr., D. J. Love, V. K. N. Lau, D. Gesbert, B. D. Rao, and M. Andrews, "Exploiting Limited Feedback in Tomorrow’s Wireless Communication Networks", IEEE Journal on Sel. Areas in Comm., Special Issue on Exploiting Limited Feedback in Tomorrow’s Wireless Communication Networks, vol. 26, no. 8, pp. 1337 - 1340, Oct. 2008

[0197]

[41] D.J. Love, R.W. Heath, Jr., and T. Strohmer, 「Grassmannian Beamforming for Multiple-Input Multiple-Output Wireless Systems,」 IEEE Trans. on Info. Theory special issue on MIMO Communication, vol. 49, pp. 2735~2747, Oct. 2003

[0198]

[42] C.B. Chae, D. Mazzarese, N. Jindal and R.W. Heath, Jr., 「Coordinated Beamforming with Limited Feedback in the MIMO Broadcast Channel」 IEEE Journal on Sel. Areas in Comm., Special Issue on Exploiting Limited Feedback in Tomorrow’s Wireless Comm. Networks, vol. 26, no. 8, pp. 1505~1515, Oct. 2008

[0199]

[43] A. Paulraj, 「Is OFDMA, MIMO and OS the right stuff for mobile broad-band?」 http: / / www.ieeevtc.org / vtc2005fall / presentations / paulraj.pdf, Sept. 2005

[0200]

[44] J. Wannstrom, 「Carrier aggregation explained」, 3GPP http: / / www.3gpp.org / Carrier-Aggregation-explained

[0201]

[45] 3GPP, TS 36.808, "Evolved Universal Terrestrial Radio Access (E-UTRA); Carrier Aggregation (Release 10)", v10.0.0, June 2012

[0202]

[46] Nokia Siemens Networks, "2020: beyond 4G, radio evolution for the gigabit experience", White Paper, 2011, www.nokiasiemensnetworks.com

[0203]

[47] S. Marek, "AT&T’s Rinne talks about carrier aggregation trials, small cells and more", http: / / www.fiercebroadbandwireless.com / story / atts-rinne-talks-about-carrier-aggregation-trials-small-cells-and-more / 2012-11-08

[0204]

[48] M. Reed, "InterfereX", Tech23, 2011 http: / / www.youtube.com / watch?v=YPpELm6iip8

[0205]

[49] NICTA, "InterfereX", http: / / www.nicta.com.au / research / archive / research_themes / networked_systems / intererex

[0206]

[50] J. Duplicity, et al., "MU-MIMO in LTE systems", EURASIP Journal on Wireless Communications and Netowrking, Mar. 2011

[0207]

[51] S. Feng and E. Seidel, "Self-organizing networks (SON) in 3GPP LTE", Nomor research, May 2008

[0208]

[52] NEC, "Self organizing networks", White paper, Feb. 2009

[0209]

[53] U.S. Patent No. 5,809,422, issued September 15, 1998, entitled "Distributed microcellular communications system", G.R. Raleigh, M.A. Pollack

[0210]

[54] G.J. Foschini, H.C. Huang, K. Karakayali, R.A. Valenzuela, and S. Venkatesan. The Value of Coherent Base Station Coordination. In Conference on In-formation Sciences and Systems (CISS 2005), Mar. 2005

[0211]

[55] M.K. Karakayali, G.J. Foschini, R.A. Valenzuela, and R.D. Yates, "On the maximum common rate achievable in a coordinated network," Proc. of the Int’l Conf. on Communications (ICC’06), vol. 9, pp. 4333~4338, June 2006

[0212]

[56] M.K. Karakayali, G.J. Foschini, and R.A. Valenzuela, "Network coor-dination for spectrally efficient communications in cellular systems," IEEE Wireless Communications Magazine, vol. 13, no. 4, pp. 56~61, Aug. 2006。

[0213]

[57] G.J. Foschini, M.K. Karakayali, and R.A. Valenzuela, "Coordinating multiple antenna cellular networks to achieve enormous spectral efficiency," Pro-ceedings of the IEEE, vol. 153, no. 4, pp. 548~555, Aug. 2006。

[0214]

[58] S. Venkatesan, A. Lozano, and R. Valenzuela, "Network MIMO: overcoming inter-cell interference in indoor wireless systems", Proc. of Asilomar conf., pp. 83~87, Nov. 2007

[0215]

[59] S. Venkatesan, H. Huang, A. Lozano, and R. Valenzuela, "A WiMAX-based implementation of network MIMO for indoor wireless systems", EURASIP Journal on Advances in Signal Processing, Sep. 2009

[0216]

[60] Y. Liang, R. Valenzuela, G. Foschini, D. Chizhik, and A. Goldsmith, "Interference suppression in wireless cellular networks through picocells", ACM SSC, pp. 1041~1045, Nov. 2007

[0217]

[61] A. Papadogiannis, H. J. Bang, D. Gesbert, and E. Hardouin, "Efficient selective feedback design for multicell cooperative networks", IEEE Trans. On Vehicular Techn., pp. 196~205, vol. 60, n. 1, Jan. 2011

[0218]

[62] I. F. Akyildiz, D. M. Guterrez-Estevez, E. C. Reyes, "The evolution to 4G cellular systems: LTE-Advanced", Physical comm, Elsevier, pp. 217~244, 2010

[0219]

[63] A. Barbieri, P. Gaal, S. Geirhofer, T. Ji, D. Malladi, Y. Wei, and F. Xue, "Coordinated downlink multi-point communications in heterogeneous cellular networks", (Qualcomm), Information Theory and App. Workshop, pp. 7~16, Feb. 2012

[0220]

[64] S.Parkvall,E.Dahlman,A.Furuskar,Y.Jading,M.Olsson,S.Wanstedt,and K.Zangi,「LTE-Advanced - evolving LTE towards IMT-Advanced」,(Ericsson)IEEE VTC,pp.1~5,Sep.2008

[0221]

[65] R.A.Monziano and T.W.Miller,Introduction to Adaptive Arrays,New York:Wiley,1980

[0222]

[66] K.K.Wong,R.D.Murch,and K.B.Letaief,「A joint channel diagonalization for multiuser MIMO antenna systems,」IEEE Trans.Wireless Comm.,vol.2,pp.773~786,Jul 2003

[0223]

[67] R.Chen,R.W.Heath,Jr.,and J.G.Andrews,「Transmit Selection Diversity for Unitary Precoded Multiuser Spatial Multiplexing Systems with Linear Receivers」,IEEE Trans.on Signal Proc.,vol.55,no.3,pp.1159~1171,Mar.2007

[0224]

[68] M.Costa,「Writing on dirty paper,」IEEE Transactions on Information Theory,Vol.29,No.3,Page(s):439~441,May 1983

[0225]

[69] G.Caire and S.Shamai, 「On the achievable throughput of a multiantenna Gaussian broadcast channel」, IEEE Trans.Info.Th., vol.49, pp.1691~1706, July 2003

[0226]

[70] N.Jindal&A.Goldsmith, 「Dirty Paper Coding vs.TDMA for MIMO Broadcast Channels」, IEEE Trans.on Info.Theory, vol.51, pp.1783~1794, May 2005

[0227]

[71] M.Tomlinson, 「New automatic equalizer employing modulo arithmetic,」 Electronics Letters, Page(s):138~139, March 1971

[0228]

[72] H.Miyakawa and H.Harashima, 「A method of code conversion for digital communication channels with intersymbol interference,」 Trans.of the Inst.of Electronic

[0229]

[73] U.Erez, S.Shamai(Shitz), and R.Zamir, 「Capacity and lattice-strategies for cancelling known interference,」 Proceedings of International Symposium on Information Theory, Honolulu, Hawaii, Nov.2000

[0230]

[74] W.Yu and J.M.Cioffi, 「Trellis Precoding for the Broadcast Channel」, IEEE Globecom, vol.2, pp.1344~1348, 2001

[0231]

[75] B.M.Hochwald, C.B.Peel, and A.L.Swindlehurst, 「A Vector-Perturbation Technique for Near-Capacity Multiantenna Multiuser Communication-Part I: Channel Inversion and Regularization」, IEEE Trans.On Communications, vol.53, n.1, pp.195~202, Jan.2005

[0232]

[76] B.M.Hochwald, C.B.Peel, and A.L.Swindlehurst, 「A Vector-Perturbation Technique for Near-Capacity Multiantenna Multiuser Communication-Part II: Perturbation」, IEEE Trans.On Communications, vol.53, n.3, pp.537~544, Mar.2005

[0233]

[77] S.Perlman and A.Forenza, 「Distributed-input distributed-output (DIDO) wireless technology: a new approach to multiuser wireless」, Rearden Labs White Paper, July 2011, http: / / www.reardenwireless.com / 110727-DIDO-A%20New%20Approach%20to%20Multiuser%20Wireless.pdf

[0234]

[78] A.Vance, "Steve Perlman’s wireless fix", Businessweek, July 2011 http: / / www.businessweek.com / magazine / the-edison-of-silicon-valley-07272011.html

[0235]

[79] M.Lindstrom(Ericsson), "LTE-Advanced Radio Layer 2 and RRC aspects", 3GPP TSG-RAN WG2

[0236]

[80] Anritsu, "LTE resource guide", www.us.anritsu.com

[0237]

[81] 3GPP, "Spatial Channel Model AHG(Combined ad-hoc from 3GPP & 3GPP2)", SCM Text V6.0, April 22, 2003

[0238]

[82] J.Lee, "Introduction of LTE-Advanced DL / UL MIMO", Samsung Electronics, Sep.2009

[0239]

[83] E.Dahlman, S.Parkvall and J.Skold, "4G:LTE / LTE-Advanced for mobile broadband", Elsevier, 2011

[0240]

[84] J.Syren, "Overview on the 3GPP long term evolution physical layer", Freescale White Paper, July 2007

[0241]

[85] M.Baker, "LTE-Advanced physical layer", Alcatel-Lucent, Dec. 2009

[0242]

[86] J.Xu, "LTE-Advanced signal generation and measurements using SystemVue", Agilent Technologies

[0243]

[87] X.Hou and H.Kayama, "Demodulation reference signal design and channel estimation for LTE-Advanced uplink", DOCOMO, Adv.in Vehic.Netw.Tech., Apr. 2011

[0244]

[88] D.C.Chu, "Polyphase codes with good periodic correlation properties", IEEE Trans.Info.Theory, vol.18, n.4, pp.531~532, July 1972

[0245]

[89] A.Lozano, R.W.Heath and J.Andrews, "Fundamental limits of cooperation", Mar. 2012. http: / / arxiv.org / pdf / 1204.0011.pdf

[0246]

[90] J.G.Andrews, "Seven ways that HetNet are a cellular paradigm shift" http: / / users.ece.utexas.edu / ~jandrews / pubs / And_HetNet_CommMag2012_v3.pdf

[0247]

[91] J-C. Guey, and L.D. Larsson, 「Modeling and evaluation of MIMO systems exploiting channel reciprocity in TDD mode」, 2004

[0248]

[92] N. Tyler, B. Allen, and H. Aghvami, 「Adaptive antennas: the calibration problem」, IEEE Comm. Mag., pp. 114~122, Dec. 2004

[0249]

[93] A. Bourdoux, B. Come, and N. Khaled, 「Non-reciprocal transceivers in OFDM / SDMA systems: impact and mitigation」, IEEE, pp. 183~186, 2003

[0250]

[94] M. Guillaud, D.T.M. Slock, and R. Knopp, 「A practical method for wireless channel reciprocity exploitation through relative calibration」, IEEE Proc. Of Sign Proc., pp. 403~406, vol. 1, Aug. 2005

[0251]

[95] P. Zetterberg, 「Experimental investigation of TDD reciprocity based zero-forcing transmit precoding」, EURASIP, June 2010

[0252]

[96] P. Uthansakul, K. Attakitmongkol, N. Promsuvana, and Uthansakul, "MIMO antenna selection using CSI from reciprocal channel", Int. Journ. Of Elect. And Info. Eng., 2010

Claims

1. A multi - antenna system (MAS) ( "MU - MAS") comprising a plurality of distributed antennas, a plurality of client devices, and one or more beacon stations, using multi - user (MU) transmission that employs radio frequency (RF) calibration and utilizes reciprocity between a downlink (DL) channel and an uplink (UL) channel.

2. The system according to claim 1, further comprising a plurality of distributed antennas interconnected to a centralized processor (CP) via the base station network (BSN) and using precoding to communicate with a plurality of client devices.

3. The system according to claim 2, wherein the CP recognizes channel state information (CSI) between the distributed antennas and the client devices and utilizes the CSI to precode data transmitted on the DL channel or the UL channel.

4. The system according to claim 3, wherein the CSI is estimated at the client device and fed back to the distributed antennas.

5. The system according to claim 4, wherein the DL - CSI is obtained from the UL - CSI at the distributed antennas by using radio frequency (RF) calibration and utilizing UL / DL channel reciprocity.

6. The system according to claim 1, wherein the RF calibration is employed to calculate the DL MU - MAS precoding weights from the UL channel.

7. The system according to claim 6, wherein precoding weights are calculated to cancel inter - client interference for each UE.

8. The system according to claim 6, wherein precoding weights are calculated from a linear precoding method (e.g., zero - forcing or block diagonalization, matrix inversion, etc.).

9. The system according to claim 6, wherein precoding weights are calculated from a non - linear precoding method (e.g., dirty paper coding, Tomlinson - Harashima, lattice trellis, vector perturbation, etc.).

10. The system according to claim 9, characterized in that round robin or proportional fair scheduling, or other types of scheduling methods are used to ensure the same symbol error rate (SER) performance for all UEs in the MU-MAS.

11. The system according to claim 6, characterized in that the MU-MAS dynamically switches between linear precoding technology and non-linear precoding technology in order to minimize the complexity of precoder calculation according to the number of distributed antennas and / or UEs in each user cluster.

12. The method according to claim 1, characterized in that RF calibration is obtained by pre-adjusting the matrix of UL channel estimation values using a complex RF calibration matrix.

13. The system according to claim 12, characterized in that the RF calibration matrix is obtained from the effective DL channel and UL channel between the distributed antenna and one or more beacon stations.

14. The system according to claim 13, characterized in that the effective DL channel and UL channel are estimated by training signals transmitted from the distributed antenna to the beacon station / from the beacon station to the distributed antenna.

15. The system according to claim 1, characterized in that the beacon station is any one of the distributed antennas.

16. The system according to claim 1, characterized in that the beacon station is any one of the client devices.

17. The system according to claim 2, characterized in that the RF calibration information is transmitted from the beacon to the CP via a wireless feedback channel or a wired feedback channel.

18. The system according to claim 17, characterized in that quantization or codebook-based limited feedback technology is adopted to feedback the RF calibration information from the beacon to the CP.

19. The system according to claim 1, wherein the MU-MAS is a cellular network such as the long term evolution (LTE) network, the client device is an LTE user equipment (UE), the distributed antenna is an LTE enhanced Node B (eNodeB) or a mobility management entity (MME), the CP is an LTE gateway (GW), and the BSN is an S1 or X1 interface.

20. The system according to claim 19, wherein the uplink demodulation reference signal (DMRS) is adopted in the Node B to estimate the UL channel from all UEs.

21. The system according to claim 19, wherein the downlink cell-specific reference signal (CRS), or CSI reference signal (CSI-RS), demodulation reference signal (DM-RS) is adopted to estimate the DL channel from the eNodeB to the beacon station, which is used for RF calibration.

22. The system according to claim 19, wherein the PUSCH physical channel is adopted to feedback the RF calibration information from the beacon to the CP.

Images