Exploiting inter-cell multiplexing gain in wireless cellular systems

Abstract

To achieve multiplexing gain in a multiple antenna system (MAS) with multi-user (MU) transmissions ("MU-MAS").SOLUTION: Power transmitted from multiple antennas is not constrained to any particular power level (as long as their power emission level falls within regulatory or safety limits), thereby creating intentionally higher levels of inter-cell interference throughout the cell to be exploited to achieve inter-cell multiplexing gain and increase the capacity of a wireless communications network. A MU-MAS of one embodiment comprises a wireless cellular network with multiple distributed antennas operating cooperatively to eliminate inter-cell interference.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of co-pending U.S. Provisional Application No. 61 / 729,990, entitled "Systems And Methods For Exploiting Inter-Cell Multiplexing Gain In Wireless Cellular Systems Via Distributed Input Distributed Output Technology," filed Nov. 26, 2012, which is assigned to the assignee of the present application and which is hereby incorporated by reference in its entirety.

[0002] (Related Applications) This application may be related to the following co-pending U.S. patent applications: U.S. Patent Application No. 13 / 233,006, entitled "System and Methods for Planned Evolution and Obsolescence of Multiuser Spectrum."

[0003] U.S. Patent Application No. 13 / 232,996, entitled "Systems and Methods to Exploit Areas of Coherence in Wireless Systems."

[0004] U.S. patent application Ser. No. 13 / 464,648, entitled "System and Methods to Compensate for Doppler Effects in Distributed-Input Distributed Output Systems."

[0005] U.S. Patent No. 8,542,763, issued September 24, 2013, entitled "Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering."

[0006] U.S. patent application Ser. No. 12 / 802,988, entitled "Interference Management, Handoff, Power Control And Link Adaptation In Distributed-Input Distributed-Output (DIDO) Communication Systems."

[0007] U.S. Patent No. 8,170,081, issued May 1, 2012, entitled "System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements."

[0008] U.S. Patent Application No. 12 / 802,974, entitled "System And Method For Managing Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDO Clusters."

[0009] U.S. Patent Application No. 12 / 802,989, entitled "System And Method For Managing Handoff Of A Client Between Different Distributed-Input-Distributed-Output (DIDO) Networks Based On Detected Velocity Of The Client."

[0010] U.S. Patent Application No. 12 / 802,958, entitled "System And Method For Power Control And Antenna Grouping In A Distributed-Input-Distributed-Output (DIDO) Network."

[0011] U.S. Patent Application No. 12 / 802,975, entitled "System And Method For Link Adaptation In DIDO Multicarrier Systems."

[0012] U.S. Patent No. 8,571,086, issued October 29, 2013, entitled "System And Method For DIDO Precoding Interpolation In Multicarrier Systems."

[0013] U.S. Patent Application No. 12 / 630,627, entitled "System and Method For Distributed Antenna Wireless Communications."

[0014] U.S. Patent No. 7,599,420, issued October 6, 2009, entitled "System and Method for Distributed Input Distributed Output Wireless Communication."

[0015] U.S. Patent No. 7,633,994, issued December 15, 2009, entitled "System and Method for Distributed Input Distributed Output Wireless Communication."

[0016] U.S. Patent No. 7,636,381, issued December 22, 2009, entitled "System and Method for Distributed Input Distributed Output Wireless Communication."

[0017] U.S. Patent No. 8,160,121, issued April 17, 2012, entitled "System and Method For Distributed Input-Distributed Output Wireless Communications."

[0018] U.S. Patent No. 7,711,030, issued May 4, 2010, entitled "System and Method For Spatial-Multiplexed Tropospheric Scatter Communications."

[0019] U.S. Patent No. 7,418,053, issued August 26, 2008, entitled "System and Method for Distributed Input Distributed Output Wireless Communication."

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

[0021] In the past three decades, the wireless cellular market has experienced an increase in the number of subscribers worldwide and a demand for better services moving from voice to web browsing and real-time HD video streaming. This increasing demand for services requiring higher data rates, lower latency, and improved reliability has driven the rapid evolution of wireless technology with different standards, starting with the first generation analog AMP and TACS (for voice services) in the early 1980s, moving to 2G and 2.5G digital GSM (for voice and data services) in the 1990s, IS-95 and GPRS (for voice and data services), 3G with UMTS and CDMA2000 (for web browsing) in the early 2000s, and finally LTE (for high-speed Internet connection) which is currently being deployed in various countries around the world.

[0022] Long Term Evolution (LTE) is a standard developed by the Third Generation Partnership Project (3GPP) for fourth-generation (4G) wireless cellular systems. LTE can achieve up to a four-fold improvement in downlink spectral efficiency over the previous 3G and HSPA+ standards by exploiting the spatial components of the wireless channel with multiple-input multiple-output (MIMO) technology. LTE-Advanced, an evolution of LTE that is currently being standardized, will enable up to an eight-fold increase in spectral efficiency over 3G standard systems.

[0023] Despite this technological advancement, there is a strong possibility that in the next three years wireless carriers will not be able to meet the growing demand for data rates due to the increasing market penetration of smartphones and tablets offering more data-hungry applications such as real-time HD video streaming, videoconferencing, and gaming. Wireless network capacity is estimated to expand five-fold in Europe from 2011 to 2015, due to improved technologies such as LTE, as well as additional spectrum made available by governments.

[25] For example, the FCC is scheduled to release 500 MHz of spectrum by 2020 (of which 300 MHz will be available by 2015) to advance wireless Internet connectivity throughout the United States as part of the National Broadband Initiative

[24] . Unfortunately, the forecast for capacity usage by 2015 is 23 times higher than 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, revenues for wireless carriers may fall short of their capital expenditures (CAPEX) and operating expenses (OPEX), with a potentially devastating impact on the wireless market.

[28]

[0024] The only foreseeable solution to prevent this approaching spectrum crisis is to promote new radio technologies, since the capacity gains provided by the deployment of LTE and increased spectrum availability are insufficient.

[29] LTE-Advanced (an evolution of the LTE standard) promises further gains over LTE through more advanced MIMO techniques and by increasing the density of "small cells".

[30] However, there is a limit to the number of cells that can fit in a given area without incurring interference problems or increasing the backhaul complexity to allow for cooperation between cells.

[0025] This patent or application contains at least one drawing executed in color. Copies of this patent or patent publication with color drawing(s) will be provided by the U.S. Patent and Trademark Office upon request and payment of the necessary fee.

[0026] The present invention can be better understood from the following detailed description taken in conjunction with the drawings. [Brief description of the drawings]

[0027] [Figure 1] FIG. 2 illustrates a cell divided into multiplexing and diversity regions. [Diagram 2] FIG. 2 illustrates inter-cell interference in different regions. [Diagram 3] FIG. 1 illustrates an embodiment in which the power transmitted simultaneously from all three base transceiver stations (BTSs) on the same frequency is increased, thereby allowing high levels of interference throughout the cell. [Figure 4] FIG. 1 illustrates an embodiment in which many additional access points are added to intentionally increase incoherent interference levels throughout a cell. [Diagram 5] FIG. 2 illustrates a number of LTE network elements used in one embodiment of the present invention. [Figure 6A] FIG. 2 illustrates an example LTE frame and associated details. [Figure 6B] FIG. 2 illustrates an example LTE frame and associated details. [Figure 6C] FIG. 2 illustrates an example LTE frame and associated details. [Figure 7A] FIG. 1 illustrates the smallest modulation structure in LTE, a "resource element", consisting of one OFDM subcarrier in frequency and one OFDM symbol duration in time. [Figure 7B]FIG. 1 illustrates the smallest modulation structure in LTE, a "resource element", consisting of one OFDM subcarrier in frequency and one OFDM symbol duration in time. [Figure 8] FIG. 2 illustrates an example SNR distribution for a practical deployment of an embodiment of the present invention in downtown San Francisco, California. [Figure 9] FIG. 1 illustrates an exemplary system architecture used in one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] One solution that overcomes many of the limitations of the prior art discussed above is an embodiment of Distributed Input Distributed Output (DIDO) technology. DIDO technology is described in the following patents and patent applications, all of which are assigned to the assignee of this patent and are incorporated herein by reference. These patents and applications are referred to at various times herein in their entirety as "Related Patents and Applications."

[0029] U.S. Patent Application No. 13 / 233,006, entitled "System and Methods for Planned Evolution and Obsolescence of Multiuser Spectrum."

[0030] U.S. Patent Application No. 13 / 232,996, entitled "Systems and Methods to Exploit Areas of Coherence in Wireless Systems."

[0031] U.S. patent application Ser. No. 13 / 475,598, entitled "Systems and Methods to Enhance Spatial Diversity in Distributed Input Distributed Output Wireless Systems."

[0032] U.S. patent application Ser. No. 13 / 464,648, entitled "System and Methods to Compensate for Doppler Effects in Distributed-Input Distributed Output Systems."

[0033] U.S. Patent No. 8,542,763, issued September 24, 2013, entitled "Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering."

[0034] U.S. patent application Ser. No. 12 / 802,988, entitled "Interference Management, Handoff, Power Control And Link Adaptation In Distributed-Input Distributed-Output (DIDO) Communication Systems."

[0035] U.S. Patent No. 8,170,081, issued May 1, 2012, entitled "System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements."

[0036] U.S. Patent Application No. 12 / 802,974, entitled "System And Method For Managing Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDO Clusters."

[0037] U.S. Patent Application No. 12 / 802,989, entitled "System And Method For Managing Handoff Of A Client Between Different Distributed-Input-Distributed-Output (DIDO) Networks Based On Detected Velocity Of The Client."

[0038] U.S. Patent Application No. 12 / 802,958, entitled "System And Method For Power Control And Antenna Grouping In A Distributed-Input-Distributed-Output (DIDO) Network."

[0039] U.S. Patent Application No. 12 / 802,975, entitled "System And Method For Link Adaptation In DIDO Multicarrier Systems."

[0040] U.S. Patent No. 8,571,086, issued October 29, 2013, entitled "System And Method For DIDO Precoding Interpolation In Multicarrier Systems."

[0041] U.S. Patent Application No. 12 / 630,627, entitled "System and Method For Distributed Antenna Wireless Communications."

[0042] U.S. Patent No. 7,599,420, issued October 6, 2009, entitled "System and Method for Distributed Input Distributed Output Wireless Communication."

[0043] U.S. Patent No. 7,633,994, issued December 15, 2009, entitled "System and Method for Distributed Input Distributed Output Wireless Communication."

[0044] U.S. Patent No. 7,636,381, issued December 22, 2009, entitled "System and Method for Distributed Input Distributed Output Wireless Communication."

[0045] U.S. Patent No. 8,160,121, issued April 17, 2012, entitled "System and Method For Distributed Input-Distributed Output Wireless Communications."

[0046] U.S. Patent No. 7,711,030, issued May 4, 2010, entitled "System and Method For Spatial-Multiplexed Tropospheric Scatter Communications."

[0047] U.S. Patent No. 7,418,053, issued August 26, 2008, entitled "System and Method for Distributed Input Distributed Output Wireless Communication."

[0048] U.S. Patent No. 7,885,354, issued February 8, 2011, entitled "System and Method For Enhancing Near Vertical Incidence Skywave ("NVIS") Communication Using Space-Time Coding."

[0049] In order to reduce the volume and complexity of this patent application, the disclosures of some of the related patents and applications are not explicitly described below, and the reader is referred to the related patents and applications for a complete description of their disclosures.

[0050] One promising technology that offers a large increase in spectral efficiency over wireless links without the constraints of traditional 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 traditional wireless systems, the present invention describes the DIDO technology used in conjunction with cellular systems (such as LTE or LTE-Advanced), both within and outside the constraints of cellular standards. Starting with an overview of MIMO, we review the various spatial processing techniques used by LTE MIMO and LTE-Advanced. We then show how the present invention offers significant capacity gains for next generation wireless communication systems compared to traditional approaches.

[0051] MIMO uses multiple antennas at the transmitter and receiver of a wireless link to improve link reliability through diversity techniques (i.e., diversity gain) or to use spatial processing to provide higher data rates through multiplexing (i.e., multiplexing gain) [1-2]. Diversity gain is a measure of enhanced robustness against signal fading, resulting in a higher signal-to-noise ratio (SNR) for a fixed data rate. Multiplexing gain is obtained by exploiting the additional spatial degrees of freedom of the wireless channel to increase the data rate at a fixed error probability. The basic tradeoff between diversity and multiplexing in MIMO systems is described in [3-4].

[0052] In practical MIMO systems, link adaptation techniques can be used to dynamically switch between diversity and multiplexing schemes based on the propagation conditions [20-23]. For example, link adaptation schemes described in [22-23] have shown that in low SNR regimes or channels characterized by low spatial selectivity, beamforming or orthogonal space-time block coding (OSTBC) are the preferred schemes. On the other hand, spatial multiplexing can provide significant gains in data rate for channels with high SNR and high spatial selectivity. For example, Figure 1 shows that a cell can be divided into two regions: i) the multiplexing region 101, characterized by high SNR (due to closeness to the cell tower or base station), where spatial degrees of freedom of the channel can be exploited by spatial multiplexing to increase the data rate; and ii) the diversity region or cell edge 102, where spatial multiplexing techniques are not as effective and where diversity methods can be used to improve SNR and coverage (providing only a small increase in data rate). Note that the macrocell circle in Figure 1 labels the shaded center of the circle as the "multiplexing region" 101 and the unshaded outer region as the "diversity region" 102. This same region designation, where the shaded region is the "multiplexing region" and the unshaded region is the "diversity region," is used throughout Figures 1-4 even though it is not labeled.

[0053] The LTE (Release 8) and LTE-Advanced (Release 10) standards define a set of ten transmission modes (TMs) that include either diversity or multiplexing schemes [35, 85–86]. Mode 1: Single antenna port, port 0 Mode 2: Transmit diversity Mode 3: Large-delay Cyclic Delay Diversity (CDD), an extension of open-loop spatial multiplexing for 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: Multi-layer closed-loop SU-MIMO up to 8 layers (added in Release 10)

[0054] In the following, diversity and multiplexing schemes commonly used in cellular systems, as well as the specific methods used in LTE outlined above, are described and compared with techniques specific to DIDO communications. First, two types of transmission methods are identified: i) intra-cell methods (utilizing micro-diversity in cellular systems), which use multiple antennas to improve link reliability or data rates within a cell; and ii) inter-cell methods (utilizing macro-diversity), which allow cooperation between cells to provide additional diversity or multiplexing gains. Next, we describe how the present invention provides significant advantages over the prior art, including spectrum capacity gains.

[0055] 1. Intra-cell diversity method Intra-cell diversity methods operate within one cell and are intended to enhance the SNR in scenarios with poor link quality (e.g., cell-edge users experiencing high path loss from the central tower or base station). Typical diversity schemes used in MIMO communications are beamforming [5-11] and orthogonal space-time block coding (OSTBC) [12-15].

[0056] 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 two antennas for the uplink (UL). It is achieved by space-frequency block coding (SFBC) combined with frequency-switched transmit diversity (FSTD) to exploit the spatial as well as frequency selectivity in the DL channel

[31] . Rank-1 precoding generates a beam dedicated to one user based on quantized weights selected from a codebook (pre-designed using limited feedback techniques [36-42]) to reduce the feedback overhead from the user equipment (UE) to the base transceiver station (BTS, i.e., eNodeB in LTE terminology). Alternatively, the dedicated beamforming weights can be calculated based on a UE-specific reference signal.

[0057] 2. Intra-cell multiplexing method MIMO multiplexing schemes [1, 19] provide data rate gains in the high SNR regime and in scenarios with sufficient spatial freedom in the channel (e.g., rich multipath environments with high spatial selectivity [16–18]) to support multiple parallel data streams over a wireless link.

[0058] The LTE standard supports various multiplexing techniques for Single User MIMO (SU-MIMO) and Multiple User MIMO (MU-MIMO)

[31] . The SU-MIMO scheme has two modes of operation: i) closed loop, which utilizes feedback information from the UE to select DL precoding weights; and ii) open loop, which is used when feedback from the UE is not available or the UE is moving too fast to support the closed loop scheme. The closed loop scheme uses a set of precomputed weights selected from a codebook. These weights can support two or four transmit antennas and one to four parallel data streams (specified by the number of layers of the precoding matrix), depending on the UE requirements and the BTS scheduler's decision. LTE-Advanced can include new transmission modes up to MIMO 8 × 8 to provide an increase in spectral efficiency of up to eight times through spatial processing

[62] .

[0059] MU-MIMO schemes have been specified for both UL and DL channels [31, 50]. In the UL, every UE transmits a reference signal (consisting of a circularly shifted version of the Zadoff-Chu sequence

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

[0060] Intra-cell multiplexing techniques using spatial processing provide good performance only in propagation scenarios characterized by high SNR (or SINR) and high spatial selectivity (multipath-rich environments). For conventional macrocells, these conditions may be more difficult to realize since the BTS is usually far from the UE and the distribution of SINR is usually concentrated at low values ​​

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

[0061] Another technology and network solution considered by LTE-Advanced to realize further multiplexing gains (without requiring spatial processing by MIMO) is carrier aggregation (CA) and small cells. CA [30, 44-47] combines different parts of the RF spectrum to increase the signal bandwidth up to 100 MHz

[85] , thereby giving higher data rates. Intra-band CA combines different bands within the same part of the spectrum. It can therefore 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, as well as signal processing to recombine multiple data streams from different bands.

[0062] The main idea of ​​small cells [30, 47] is to reduce the size of conventional macro cells, thereby allowing higher cell density and more throughput per coverage area. Small cells are usually deployed through inexpensive access points with low power transmission (as depicted in Figure 1) in contrast to the tall and expensive cell towers used for macro cells. Two types of small cells are defined in LTE-Advanced: i) metro cells, for outdoor installations in urban areas, supporting 32-64 simultaneous users; and ii) femto cells, for indoor use, serving up to four active users. One advantage of small cells is that the density of UEs near the BTS is statistically higher, thereby giving a better SNR that can be exploited through spatial multiplexing to enhance data rates. However, there are still many concerns regarding the practical deployment of small cells, especially those related to the backhaul. Indeed, reaching every small cell BTS through a high-speed wired connection may be challenging, especially considering the high density of metro and femto cells in a given coverage area. Compared to wired backhaul, using line-of-sight (LOS) backhaul for small cells is often cheaper to implement, but there are often no practical LOS backhaul paths available for the preferred small cell BTS placement, and there is no general solution for non-line-of-sight (NLO) wireless backhaul to the small cell BTS. Finally, small cells require complex real-time coordination between BTSs to avoid interference, as in self-organizing networks (SONs) [30, 51-52], and require advanced cell planning tools (even more complex than traditional cellular systems due to the densification of small cells) to plan their optimal locations [48, 49].

[0063] It can be shown trivially that there is no practical general solution that allows a small cell to coexist with a macro cell and achieve optimal or necessarily even improved throughput. Among countless such unsolvable situations, there is a situation where the small cell is located such that its UE inevitably overlaps with the macro cell transmission, and the small cell and the macro cell use the same frequency to reach each UE. In this situation, obviously, the macro cell transmission will interfere with the small cell transmission. There may be some approaches to mitigate such interference for the specific situation of a specific macro cell, a specific small cell, the specific macro cell and small cell UEs involved, the throughput requirements of those UEs, and the environmental situation. However, any such approach is very specific not only for the static planning of the macro cell and the small cell, but also for the dynamic situation of a specific time interval. Usually, the full throughput of the channel to each UE cannot be achieved.

[0064] 3. Intercell diversity method Inter-cell transmission techniques allow cooperation between BTSs to improve the performance of wireless networks. These techniques are special cases of methods taught in related patents and applications [0002-0020] to enable cooperation between radio transceivers in the general case of a distributed antenna network for multiple UEs all using the same frequency at the same time. For the specific case of a cellular system with a single UE at a given frequency and at a given time, cooperation between BTSs to eliminate inter-cell interference was described in

[53] . The system in

[53] divides any macrocell into multiple subcells and enables flexible handoff between the subcells by using dedicated beamforming from the coordinated BTSs. And, by using dedicated beamforming from the coordinated BTSs, it improves the robustness of the link at a single frequency and a single UE as the single UE moves along the subcell boundaries.

[0065] More recently, this class of cooperative wireless cellular networks has been defined in the MIMO literature as "Network MIMO (MIMO)" or "Cooperative Multipoint" (CoMP) systems. Theoretical analyses and simulated results on the benefits of Network MIMO by eliminating inter-cell interference are presented in [54-61]. The main advantage of Network MIMO and CoMP is the elimination of inter-cell interference in the cell overlap regions 201-203 shown in Figure 2.

[0066] CoMP networks are actively becoming part of the LTE-Advanced standard as a solution to mitigate inter-cell interference in next generation cellular networks [62-64]. Two CoMP solutions have been proposed so far in the standard to eliminate inter-cell interference: i) Coordinated Scheduling / Beamforming (CS / CB), where a UE receives data streams from only one BTS by beamforming, and cooperation between BTSs is enabled to eliminate interference by beamforming or scheduling techniques; and ii) Joint Processing (JP), where data for a given UE (UE) is jointly transmitted from multiple BTSs to improve the received signal quality and eliminate inter-cell interference. CoMP-JP brings larger gains than CoMP-CS / CB at the expense of higher overhead in the backhaul to enable cooperation between BTSs.

[0067] 4. Inter-cell multiplexing method Prior art multi-user wireless systems add complexity and introduce constraints to wireless networks, resulting in a situation where a given user's experience (e.g., available throughput, delay, predictability, reliability) is affected by the utilization of spectrum by other users in the area. Given the increasing demand for aggregate throughput in a wireless spectrum shared with multiple users, and the increasing growth of applications that can depend on the reliability, predictability, and low latency of a multi-user wireless network for a given user, it is clear that prior art multi-user wireless technologies suffer from many constraints. Indeed, with limited availability of spectrum suitable for certain types of wireless communication (e.g., at wavelengths effective for penetrating building walls), prior art wireless technologies will be inadequate to meet the increasing demand for reliable, predictable, low latency bandwidth.

[0068] Prior art intra-cell diversity and multiplexing methods can only provide a theoretical maximum of four times increase in throughput over current cellular networks for LTE (with MIMO 4×4) and at most eight times for LTE-Advanced (with MIMO 8×8). It should be noted that for higher order MIMO, the improvement in throughput increase decreases in a given multipath environment, especially as UEs (such as smartphones) become smaller and more constrained in terms of antenna placement. Other slight throughput gains in next generation cellular systems may come from further spectrum allocations (e.g., FCC National Broadband Plan) exploited by carrier aggregation techniques, and from denser distribution of BTSs by small cell networks and SONs ​​[30, 46]. However, all the above mentioned techniques are still highly dependent on spectrum or time sharing techniques that enable multi-user transmissions, since the spectral efficiency gains obtained by spatial processing are limited.

[0069] Prior art inter-cell methods (e.g., network MIMO and CoMP systems [53-64]) can improve the reliability of cellular networks by eliminating inter-cell interference, but their capacity gains are only marginal. In fact, those systems are only effective to constrain the power transmitted from any BTS contained within the cell boundary to eliminate inter-cell interference due to power leakage between cells. Figure 2 shows an example of a cellular network with three BTSs 210-212, each characterized by its own coverage area or cell. The power transmitted from each BTS 210-212 is constrained to limit the amount of inter-cell interference, represented in Figure 2 by the cell overlap area. Since these systems operate in the low SINR regime in the interference area, their spectral efficiency gains are only marginal, as are the intra-cell approaches for SU-MIMO. To obtain truly significant capacity gains in inter-cell cooperative networks, the power restrictions limited to the cell-boundaries must be relaxed. And spatial multiplexing techniques must be enabled throughout the cell where SINR is high (not just at the cell edge, which has poor SINR performance, as in conventional approaches).

[0070] It is therefore desirable to provide a system that achieves a large increase in spectral efficiency by removing any constraints on the power transmitted from distributed BTSs and by exploiting the inter-cell multiplexing gains through spatial processing. FIG. 3 shows that the power transmitted simultaneously on the same frequency from all three BTSs 301-303 is increased, thereby allowing a high level of interference throughout the cell. In prior art systems, such interference results in incoherent interference (hindering UE signal reception) throughout the interference region of the BTSs, but this interference is actually exploited in embodiments of the present invention by a new inter-cell multiplexing method. This method uses spatial processing to create regions of coherent interference (enhancing UE signal reception) around every UE, thereby providing a simultaneous non-interfering data stream to every UE and increasing their SINR throughout the cell.

[0071] In an exemplary embodiment of the invention, this inter-cell multiplexing gain is achieved by a distributed input distributed output (DIDO) system [0014-0020] and [77-78]. Figure 4 shows an example where one many additional access points 401 are added to intentionally increase the level of incoherent interference throughout the cell, which is exploited in the present invention to create a region of coherent interference around the UE and provide inter-cell multiplexing gain. These added BTSs can be low power transceivers similar to inexpensive Wi-Fi access points, thereby providing smaller areas of overlapping coverage within the macro cell as shown in Figure 4.

[0072] It can be seen that the prior art inter-cell method avoids incoherent interference by intentionally limiting the transmission power from every BTS 210-212 as in Fig. 2 and removes the remaining inter-cell interference (for overlapping regions between cells) by spatial processing, thereby providing improved SINR and inter-cell diversity gain. On the other hand, the present invention exploits the incoherent interference by transmitting higher power from every BTS to generate coherent interference around the UE, thereby improving the signal quality at the UE, which is a prerequisite for obtaining inter-cell multiplexing gain across cells by spatial processing. Therefore, the system described in the prior art cannot be used to realize inter-cell multiplexing gain by spatial processing, since there is not enough signal quality throughout the cells (due to the limited transmission power from the BTSs) to enable an inter-cell multiplexing method such as the present invention. Furthermore, the systems described in the prior art cannot be implemented to achieve the multiplexing gains achieved in the present invention as depicted in Figures 3-4, taking into account that the prior art systems avoid inter-cell interference in the diversity regions shown in the shaded regions of Figures 1-4, rather than exploiting inter-cell interference in the multiplexing region to obtain the inter-cell multiplexing gains achieved in the present invention.

[0073] An embodiment of the present invention includes a system and method for exploiting inter-cell multiplexing gains from spatial processing in a wireless communication network, using a multiple antenna system (MAS) with multiple user (MU) transmissions (multiple user multiple antenna system or "MU-MAS"). In one embodiment of the present invention, the power transmitted from the multiple antennas is constrained to minimize interference at cell boundaries (as in conventional cellular systems), and spatial processing methods are used to only eliminate inter-cell interference. In another embodiment of the present invention, the power transmitted from the multiple antennas is not constrained to any particular power level (as long as their power emission levels fall within regulatory or safety limits), thereby intentionally generating high order inter-cell interference throughout the cells and utilizing it to realize inter-cell multiplexing gains and increase the capacity of the wireless communication network.

[0074] In one embodiment, the wireless communication network is a cellular network as in Figs. 1-2, such as a cellular network based on the LTE standard. In another embodiment of the present invention, the wireless communication network is not bound to any particular cell layout, and the cell boundaries can be spread over a larger area as in Figs. 3-4. For example, the wireless communication network can be a wireless local area network (WLAN), or a mesh, ad-hoc or sensor network, or a distributed antenna system, or a DIDO system with haphazardly placed access points without any transmit power restrictions. However, such network structure examples should not be considered as limiting the general application 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.

[0075] As illustrated in FIG. 9, an embodiment of the MU-MAS comprises a centralized processor 901, a base station network (BSN) 902, and M base transceiver stations (BTSs) 903 that communicate wirelessly to N client devices. The client devices are also referred to as user equipments UE (illustrated as UE 1-4). The centralized processor unit 901 receives N streams of information (e.g., videos, web pages, video games, text, voice, etc., streams from web servers or other network sources) over a network 900 (e.g., the Internet) intended for various client devices UE 1-4 for various network contents C1-5. In the following, we use the term "stream of information" to refer to any stream of data transmitted over the network 900. It includes information that can be demodulated or decoded as an independent stream according to a particular modulation / coding scheme or protocol to generate any data, including but not limited to voice, web, and video content. In one embodiment, the stream of information is a series of bits carrying network content that can be demodulated or decoded as an independent stream.

[0076] The centralized processor 901 utilizes a precoding transformation that combines the N streams of information from the network content and transforms them into M streams of bits (by algorithms such as those described in the 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 non-linear (e.g., dirty paper coding [68-70] or Tomlinson-Harashima precoding [71-72], lattice or trellis precoding [73-74], vector perturbation techniques [75-76]). In the following, we use the term "stream of bits" to refer to any sequence of bits that does not necessarily contain any useful bits of information, and therefore cannot be decoded or demodulated as a standalone stream to retrieve the network content. 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 the M base transceiver stations.

[0077] In one embodiment, the MAS is a Distributed Input Distributed Output (DIDO) system as described in the related patents and applications. In this embodiment, the DIDO system comprises: User Equipment (UE) 1-4: RF transceiver for fixed or mobile clients that receives data streams on a downlink (DL) channel from the DIDO backhaul and transmits data to the DIDO backhaul via an uplink (UL) channel. Base Transceiver Station (BTS) 903: The BTS connects the DIDO backhaul to the wireless channels. In one embodiment, the BTS is an access point consisting of a DAC / ADC and a radio frequency (RF) chain that converts the baseband signal 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 RF-over-fiber technology as described in the related patents and applications. Controller (CTR) 905: The CTR 905 is one particular type of BTS designed for certain special functions: transmitting training signals for time / frequency synchronization of the BTS and / or UE, receiving and transmitting control information from and to the UE, and receiving channel state information (CSI) or channel quality information from the UE. One or more CTR stations can be included in any DIDO system. When multiple CTRs are available, information to and from them can be combined to increase diversity and improve link quality. In one embodiment, CSI is received from multiple CTRs by Maximum Ratio Combining (MRC) technique to improve CSI demodulation. In another embodiment, control information is transmitted from multiple CTRs by Maximum Ratio Transmission (MRT) to improve SNR at the receiver side. The scope of the invention is not limited to MRC or MRT, and any other diversity technique (e.g. antenna selection, etc.) can be used to improve the radio link between the CTR and the UE. Centralized Processor (CP) 901: The CP is a DIDO server that connects the DIDO backhaul with the Internet or another type of external network. In one embodiment, the CP computes the DIDO baseband processing and transmits waveforms to distributed BTSs via DL transmission. Base Station Network (BSN) 902: The BSN is a network that connects the CP to distributed BTSs that carry information on either DL or UL channels. The BSN can be a wired or wireless network, or a combination of both. For example, the BSN can be a DSL, cable, fiber optic network, or a line-of-sight (LOS) or non-line-of-sight (NLO) wireless link. Furthermore, the BSN can be a proprietary network, or a local area network, or the Internet.

[0078] In the following, we describe how the above-mentioned DIDO system framework can be incorporated into the LTE standard for cellular systems (as well as non-cellular systems utilizing the LTE protocol) to realize additional gains in spectral efficiency. We start with a general overview of the LTE framework and the modulation schemes used in the DL and UL channels. Then, we provide a brief description of the physical layer frame structure and resource allocation in the LTE standard. Finally, we define the DIDO precoding method for the downlink (DL) and uplink (UL) channels in a multi-user scenario using the LTE framework. For the DL scheme, we propose two solutions: an open-loop and a closed-loop DIDO scheme.

[0079] LTE is designed with a flat network architecture (as opposed to the hierarchical architecture of previous cellular standards) to provide: reduced latency, reduced packet loss through ARQ, reduced call setup times, and improved coverage and throughput through macro diversity. The network elements in an LTE network, depicted in Figure 5, are as follows

[79] : GW (Gateway) 501-502: Routers that connect the LTE network to an external network (i.e., the Internet). The GW is divided into a Serving Gateway (S-GW) 502 that forms the boundary of the E-UTRAN interface and a PDN Gateway (P-GW) 501 that is an interface with an external network. The S-GW 502 and the P-GW 501 are part of the so-called Evolved Packet Core (EPC). · MME (Mobility Management Entity) 503: Manages mobility, protection parameters and UE identity. The MME 503 is also part of the LTE EPC. · eNodeB (enhanced Node-B) 504: A base station that handles radio resource management, user mobility and scheduling. · UE (User Equipment) 505: A mobile station.

[0080] In one embodiment of the present invention, when the DIDO-UE is a UE of an LTE network, the LTE network is a DIDO network, the DIDO-BTS is an LTE eNodeB, the DIDO-CTR is an LTE eNodeB or MME, and the DIDO-CP is an LTE GW.

[0081] As shown in Figures 6A-C, an LTE frame has a duration of 10 ms and consists of 10 subframes [33, 80]. Every subframe is divided into two slots of 0.5 ms duration each. The LTE standard defines two types of frames: i) Type 1 for FDD operation as shown in Figure 6A. All subframes are assigned to either downlink (DL) or uplink (UL) channels. ii) Type 2 for TDD operation as shown in Figure 6B. Some of the subframes are assigned to DL and some to UL (depending on the selected configuration), while a few subframes are reserved for "special use". For each frame, there is at least one special subframe, which consists of three fields: i) Downlink Pilot Time Slot (DwPTS) reserved for DL ​​transmissions; ii) Guard Period (GP); and iii) Uplink Pilot Time Slot (UpPTS) for UL transmissions.

[0082] LTE uses Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFMDA) modulation for DL ​​and Single Carrier FDMA (SC-FDMA) for UL. 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 Figure 7. A "Resource Block" (RB) consists of 12 subcarriers in frequency and one 0.5 ms slot in time (3 to 7 OFDM symbol periods, depending on DL vs. UL channel and cyclic prefix type).

[0083] 1. Downlink Closed-Loop DIDO in LTE The DIDO closed loop scheme can be used in either Time Division Duplex (TDD) or Frequency Division Duplex (FDD) systems. In FDD systems, the DL and UL channels operate on different frequencies. Therefore, DL channel state information (CSI) must be estimated at the UE side and reported back to the CP via the BTS or CTR by the UL channel. In TDD systems, the DL and UL channels are configured on the same frequency and the system may use either closed loop techniques or open loop schemes (as described in the following sections) to exploit the channel reciprocity. The main disadvantage of closed loop schemes is that they require feedback, which results in a larger overhead for control information on the UL.

[0084] One embodiment of the mechanism for the closed loop approach in a DIDO system is as follows: i) BTS 903 sends signaling information to the UE on the DL; ii) The UE uses the signaling information to estimate DL channel state information (CSI) from all "active BTSs"; iii) The UE quantizes the DL CSI or uses a codebook to select precoding weights to use for the next transmission; iv) The UE sends the quantized CSI or codebook index over the UL channel to the BTS 903 or CTR 905; v) The BTS 903 or CTR 905 reports the CSI information or codebook index to the CP 901, which calculates the precoding weights for the data transmission on the DL. The "active BTSs" are defined as the set of BTSs that can be reached by a given UE. For example, in 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 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" is defined as the set of BTSs that can be reached by a given UE. The number of active BTSs is limited to a user cluster in order to reduce the amount of CSI estimated for a given UE from the BTSs, thereby reducing the feedback overhead on the UL and the complexity of the DIDO precoding calculations in CP 901.

[0085] 1.1 Downlink DIDO signalling within the LTE standard The LTE standard defines two types of reference signals (RS) that can be used for DL ​​signaling in a closed-loop manner [33, 50, 82-83]: i) cell-specific reference signals (CRS); ii) UE-specific RS, such as Channel State Information (CSI) reference signals (CSI-RS) and Demodulation RS (DM-RS). The cell-specific RS is not precoded, whereas the UE-specific RS is precoded

[50] . CRS is used in LTE Release 8, which uses SU / MU-MIMO codebook-based techniques, where every cell uses up to four antennas. LTE-Advanced Release 10 supports non-codebook-based SU / MU-MIMO schemes, with up to eight transmit antennas as well as CoMP schemes, where the antennas are distributed on different cells. Thus, Release 10 allows flexible signaling schemes with CSI-RS. In this invention, we describe how any type of signaling scheme can be used in a DIDO system to enable precoding.

[0086] 1.1.1 DIDO signaling using CRS The CRS is used in LTE (Release 8) systems to estimate the CSI from all transmit antennas at the UE to the BTS [80, 84]. The CRS is obtained as the product of a two-dimensional orthogonal sequence and a two-dimensional pseudorandom number (PRN) sequence. There are three orthogonal and 170 possible PRN sequences for a total of 510 different CRS sequences. Every sequence uniquely identifies one cell. The CRS is transmitted in the first and penultimate OFDM symbols of every slot and in every sixth subcarrier. Orthogonal patterns in time and frequency are designed for every transmit antenna of the BTS for the UE to uniquely estimate the CSI from each of the four antennas. This dense CRS in time and frequency (i.e., transmitted every 0.5 ms slot and every sixth subcarrier) creates a 5% overhead and was intentionally designed to support scenarios with fast channel variations in time and frequency

[83] .

[0087] In a practical DIDO system, there may be cases where every UE sees more than just four BTSs in its user cluster. For example, Fig. 8 shows the SNR distribution for a practical deployment of a DIDO system in downtown San Francisco, California. The propagation model is based on the 3GPP path loss / shadowing model

[81] and assumes a carrier frequency of 900 MHz. The dots on the map indicate the locations of the DIDO-BTSs, while the black circles indicate user clusters (UEs are located at the centers of the circles). In sparsely populated areas, the UE sees only a few BTSs in its user cluster (e.g., only three BTSs for the example in Fig. 8), while in densely populated areas, each user cluster can contain as many as 26 BTSs as in Fig. 8.

[0088] The high redundancy of the CRS can be exploited in the DIDO system to enable CSI estimation from any number of transmit antennas greater than four. For example, if the channel is fixed radio or characterized by low Doppler effect, it is not necessary to calculate the CSI from all four transmit antennas every 0.5 ms (slot duration). Similarly, if the channel is frequency flat, estimating the CSI every sixth subcarrier is redundant. In that case, the resource elements (RE) occupied by the redundant CRS can be reallocated to another transmit antenna or BTS in the DIDO system. In one embodiment of the present invention, the system allocates the resource elements of the redundant CRS to the extra antennas or BTS in the DIDO system. In another embodiment, the system estimates the time and frequency selectivity of the channel and dynamically allocates CRS for different BTSs or only BTSs in a user cluster to different resource elements.

[0089] 1.1.2 DIDO signalling 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]. The standard defines orthogonal CSI-RS for different transmitters in the BTS, so that the UE can distinguish CSI from different BTSs. Up to eight transmit antennas in the BTS are supported by CSI-RS, as per Table 6.10.5.2-1, 2 in

[33] . CSI-RS is transmitted with a periodicity spanning 5 to 80 subframes (i.e., CSI-RS is transmitted every 5 to 80 ms), as per Table 6.10.5.3-1 in

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

[0090] When there are more than eight antennas (the maximum number of transmitters supported by the LTE-Advanced standard) in a user cluster, an alternative technique must be used to enable DIDO precoding while maintaining system compatibility with the LTE-Advanced standard. In one embodiment of the invention, every UE uses the CSI-RS or DM-RS or a combination thereof to estimate the CSI from all active BTSs in its own user cluster. In the same embodiment, the DIDO system detects the number of BTSs in the user cluster and detects whether the user cluster is compliant with the LTE-Advanced standard (which supports up to eight antennas). If not, the DIDO system uses an alternative technique to enable DL signaling from the BTS to the current UE. In one embodiment, the transmission power from the BTS is reduced until up to eight BTSs can be reached by the UE in that user cluster. However, this solution may result in a reduction in data rate due to reduced coverage.

[0091] Another solution is to split the BTSs in a user cluster into subsets and transmit one set of CSI-RS for all subsets at the same time. For example, if the CSI-RS periodicity is 5 subframes (i.e., 5 ms) as per Table 6.10.5.3-1 of

[33] , then every 5 ms, 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 UE's channel coherence time (which is a function of the UE's Doppler velocity). For example, if the selected CSI-RS periodicity is 5 ms and the channel coherence time is 100 ms, it is possible to define up to 20 subsets of 8 BTSs each, for a total of 160 BTs in the user cluster. In another embodiment of the invention, the DIDO system estimates the channel coherence time of the UE and determines, for a given CSI-RS periodicity, how many BTSs can be supported in a user cluster to avoid degradation due to channel variations and Doppler effects.

[0092] The solutions proposed so far for CSI-RS are all LTE standard compliant and can be deployed within the framework of a conventional LTE system. For example, the proposed method to allow more than eight antennas per user cluster does not require any modification of the UE LTE hardware and software implementation, but only slight modifications of the protocols used by the BTS and CP to allow the selection of a BTS subset at any time. These modifications can be easily implemented in a cloud-based software-defined radio (SDR) platform, which is one promising deployment paradigm for DIDO systems. Alternatively, if it is possible to relax the constraints of the LTE standard and develop slightly modified hardware and software for LTE UEs to support DIDO operation modes similar to LTE but not compliant with LTE, it would allow the UE to operate in a full LTE compliant mode or in a modified mode supporting non-LTE compliant DIDO operation. For example, another solution is to increase the amount of CSI-RS to allow 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 to increase the number of BTSs supported per user cluster. Such slight modifications to the LTE standard may be small enough that existing LTE UE chipsets can be used with merely software modifications, or even if hardware modifications are required to the chipset, the changes will be small.

[0093] 1.2 Uplink DIDO CSI feedback method in LTE standard In the LTE and LTE-Advanced standards, the UE feeds back information to the BTS to communicate its current channel conditions and precoding weights for closed-loop transmission on the DL channel. Three different channel metrics are included in the standards

[35] . Rank Index (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 Index (PMI): The index of the codebook used for precoding on the DL channel. · Channel Quality Indicator (CQI): defines the modulation used on the DL and the Forward Error Correction (FEC) coding scheme to maintain a defined error rate performance for given channel conditions.

[0094] Whereas only one RI is reported for the entire bandwidth, PMI and CQI reporting can be wideband or per subband, depending on the frequency selectivity of the channel. These indicators are transmitted in the UL on two different physical channels: i) the Physical Uplink Control Channel (PUCCH), used only for control information; and ii) the Physical Uplink Shared Channel (PUSCH), used for data and control information and allocated per resource block (RB) and on a subframe basis. For the PUCCH, the procedure of reporting RI, PMI and CQI is periodic and the indicators can be either wideband (for frequency-flat channels) or selected per UE on a subband basis (for frequency-selective channels). For the PUSCH, the feedback procedure is aperiodic and can be selected per UE on a subband basis (for frequency-selective channels) or on a higher layer configured subband (e.g. for transmission mode 9 of LTE-Advance with 8 transmitters).

[0095] In one embodiment of the present invention, the DIDO system uses RI, PMI and CQI to report the current channel condition and precoding information to the BTS and CP. In one embodiment, the UE uses the PUCCH channel to report these indicators to the CP. In another embodiment, if more indicators are needed for DIDO precoding, the UE uses the PUSCH to report additional indicators to the CP. If the channel is frequency flat, the UE can utilize the extra UL resources to report PMI for more antennas in the DIDO system. In one embodiment of the invention, the UE or the BTS or the CP estimates the channel frequency selectivity, and if the channel is frequency flat, the UE utilizes the extra UL resources to report PMI for more BTSs.

[0096] 2. Downlink open-loop DIDO in LTE The DIDO open loop scheme can only be used in a time division duplex (TDD) system that exploits channel reciprocity. One embodiment of the mechanism for the open loop scheme in a DIDO system is as follows: i) UEs 1-4 send signaling information on the UL to the BTS 903 or CTR 905. ii) The BTS 903 or CTR 905 uses the signaling information to estimate the UL CSI from all UEs 1-4. iii) The BTS 903 or CTR 905 uses RF correction to convert the UL CSI to DL CSI. iv) The BTS 903 or CTR 905 sends the DL CSI or codebook index to the CP through the BSN 902. v) Based on the DL CSI, the CP 901 calculates the precoding weights for data transmission on the DL. Similar to the closed-loop DIDO scheme, user clusters can be used to reduce the amount of CSI from the UE that is estimated at the BTS, thereby reducing the computational burden at the BTS as well as the amount of signaling required on the UL. In one embodiment of the present invention, open-loop precoding techniques are used to transmit simultaneous non-interfering data streams from the BTS to the UE on the DL channel.

[0097] In LTE, there are two types of reference signals for the uplink channel [31, 33, 87]: i) Speech Reference Symbols (SRS), used for scheduling and link adaptation; and ii) Demodulation Reference Signals (DMRS), used for data reception. In one embodiment of the present invention, SRS or DMRS are used in an open-loop DIDO system to estimate the UL channel from every UE to every BTS. In the time domain, DMRS is transmitted in the fourth OFDM symbol (when using normal cyclic prefix) of every LTE slot (of duration 0.5 ms). In the frequency domain, DMRS transmitted on PUSCH is mapped for every UE to the same resource block (RB) used by that UE for UL data transmission.

[0098] The length of the DMRS is MRS = mN RB where m is the number of RBs and N RB = 12 is the number of subcarriers per RB. To support multiple UEs, several DMRSs are generated from 1-based Zadoff-Chu

[88] or computer-generated constant amplitude zero autocorrelation (CG-CAZAC) sequences by circular shifting the base sequence. The base sequence is divided into 30 groups, and neighboring 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.

[0099] In one embodiment of the present invention, the DIDO system assigns UEs to "virtual cells" to maximize the number of SRSs or DMRSs that can be used in the UL. In one exemplary embodiment, the virtual cells are coherence areas (described in related co-pending U.S. patent application Ser. No. 13 / 232,996, entitled "Systems and Methods to Exploit Areas of Coherence in Wireless Systems") around the UE, and the DIDO system creates up to 3300 coherence areas for different UEs. In another embodiment of the present invention, each of the 30 base sequences is assigned to a different DIDO cluster to reduce inter-cluster interference between adjacent DIDO clusters (clusters are defined in related U.S. Patent No. 8,170,081, entitled "System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements," issued May 1, 2012). In another embodiment, the SRSs or DMRSs are assigned according to a specific frequency hopping pattern to exploit the frequency diversity of the channel.

[0100] If there are not enough orthogonal SRS or DMRS for all UEs to be simultaneously provided in DL by DIDO precoding, one alternative is to multiplex the SRS or DMRS of different UEs in the time domain. For example, the UEs are divided into different groups, and the SRS or DMRS for those groups are transmitted on consecutive time slots (0.5 ms duration each). However, in this case, it is necessary to ensure that the periodicity of the SRS or DMRS assignment for the different groups (multiplexes) 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 the CSI is estimated by the SRS or 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 SRS or DMRS to each group on consecutive time slots. In the same embodiment, the system estimates the shortest channel coherence time for all active UEs, as well as the maximum number of UEs and the periodicity of time division multiplexing of SRS or DMRS based on that information.

[0101] 3. LTE uplink DIDO technology An embodiment of the present invention uses an open-loop MU-MIMO scheme on the UL channel to receive simultaneous UL data streams from all UEs to the BTS. One embodiment of the UL open-loop MU-MIMO scheme includes the following steps: i) UEs 1-4 send signaling information and data payload to all BTSs 903; ii) BTS 903 uses the signaling information to calculate channel estimates from all UEs; iii) BTS 903 sends the channel estimates and data payload to CP 901; iv) CP 901 uses the channel estimates to remove co-channel interference from the data payloads of all UEs by spatial filtering 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 UEs to the BTS and multiplexes them in the frequency domain.

[0102] In one embodiment, synchronization between UEs is achieved by signaling from DL, and assumes that all BTSs 903 are locked to the same time / frequency reference clock, whether by direct wiring to the same clock or by sharing a common time / frequency reference by GPSDO. Variations in channel delay across different UEs can generate jitter between the time references of different UEs, which can affect the performance of the MU-MIMO method on the UL. In one embodiment, to reduce the relative propagation delay across different UEs, only UEs in the same DIDO cluster (e.g., UEs adjacent to each other) are processed by the MU-MIMO method. In another embodiment, the relative propagation delay between UEs is compensated at the UE or at the BTS, ensuring simultaneous reception of data payloads from different UEs 1-4 at the BTS 903.

[0103] The technique for enabling signaling information for data demodulation on the UL can be the same method used for the downlink open loop DIDO scheme signaling described in the previous section. The CP 901 may use different spatial processing techniques to remove co-channel interference from the UE data payload. In one embodiment of the present invention, the CP 901 uses a non-linear spatial processing method such as a maximum likelihood (ML), decision feedback equalization (DFE), or successive interference cancellation (SIC) receiver. In another embodiment, the CP 901 uses a linear filter such as a zero forcing (ZF) or minimum mean square error (MMSE) receiver to cancel co-channel interference and demodulates the uplink data streams separately.

[0104] 4. Integration with existing LTE networks In the United States and other regions of the world, LTE networks are already in operation, are being deployed, and / or are to be deployed. It would be of significant benefit to LTE operators if they could gradually deploy DIDO capabilities into existing or already committed deployments. In this way, they could deploy DIDO in areas where it provides the most immediate benefit, gradually expanding DIDO capabilities across more and more of the network. Eventually, once they have sufficient DIDO coverage in an area, they could choose to phase out the use of cells entirely and instead switch completely to DIDO, achieving much higher spectrum density at a much lower cost. Through this complete transition from cellular to DIDO, the wireless customers of the LTE operators would never suffer a loss in service. Rather, they would simply see improvements in their data efficiency and reliability, while the operator would see a drop in costs.

[0105] There are several embodiments that allow for the gradual integration of DIDO into existing LTE networks. In all cases, the DIDO BTS will be referred to as a DIDO-LTE BTS, which will utilize one of the LTE-compatible DIDO embodiments described above, or other LTE-compatible embodiments that may be developed in the future. Alternatively, the DIDO-LTE BTS will utilize a slight variant of the LTE standard, as described above, and either the UEs will be updated (e.g., if a software update is sufficient to modify the UE to be compatible with DIDO), or a new generation of UEs will be deployed that is DIDO compatible. In either case, the new BTS that supports DIDO, whether within the constraints of the LTE standard or as a variant of the LTE standard, will be referred to hereafter as a DIDO-LTE BTS.

[0106] The LTE standard supports a variety of bandwidths (e.g., 1.4, 3, 5, 10, 15, and 20 MHz). In one embodiment, an operator of an existing LTE network can support a conventional LTE BTS in a cellular configuration in one block of spectrum and a DIDO LTE BTS in another block of spectrum, either by allocating new bandwidth for the LTE-DIDO BTS or by subdividing the existing LTE spectrum (e.g., 20 MHz could be subdivided into two 10 MHz blocks). In effect, this establishes two separate LTE networks, and UE devices would be configured to use one or the other network, or to choose between both. In the case of a subdivided spectrum, the spectrum can be divided evenly or unevenly between the conventional LTE network and the DIDO-LTE network, and more spectrum can be allocated to the network that can be best utilized, taking into account the degree of deployment of cellular LTE BTS and DIDO-LTE BTS, and / or UE usage patterns. This division can change as needed over time, and at some point, when there are enough DIDO-LTE BTSs deployed to provide the same or better coverage as the cellular BTSs, all of the spectrum can be allocated to the DIDO-LTE BTSs and the cellular BTSs can be retired.

[0107] In another embodiment, a conventional cellular LTE BTS can be configured to cooperate with a DIDO-LTE BTS and share the same spectrum, but to use the spectrum in a rotating manner. For example, if they share the spectrum equally, each BTS network will take turns using one 10 ms frame time, e.g., one 10 ms frame for the cellular LTE BTS followed by one 10 ms frame for the DIDO-LTE BTS. The frame time can also be subdivided into non-uniform intervals. This interval division can change as needed over time, and when there are enough DIDO-LTE BTSs deployed to provide the same or better coverage as the cellular BTS, all of the time can be assigned to the DIDO-LTE BTS and the cellular BTS can be retired.

[0108] In another embodiment of the invention, DIDO is used as a LOS or NLO wireless backhaul for small cells in LTE and LTE-Advanced networks. As small cells are deployed in LTE networks, DIDO provides 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 a 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 invention, DIDO BTSs are used to gradually replace the small cells, thereby taking advantage of the inter-cell interference to provide increased network capacity.

[0109] 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 [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 [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 [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 [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。 [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。 [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。 [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。 [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。

[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。

[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.

[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。

[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。

[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。

[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。

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

[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

[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.

[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

[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。

[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。

[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

[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

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

[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

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

[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

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

[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

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

[31] J.Lee,J.-K.Han,J.Zhang,「MIMO technologies in 3GPP LTE and LTE-advanced」,EURASIP Journal on Wireless Communications and Networking,Hindawi,May 2009

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

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

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

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

[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。

[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。

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

[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。 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。

[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

[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 Communication Networks,vol.26,no.8,pp.1505~1515,Oct.2008

[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

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

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

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

[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

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

[49] NICTA,「InterfereX」, http: / / www.nicta.com.au / research / archive / research_themes / networked_systems / interferex

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

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

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

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

[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

[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。

[0110]

[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。

[0111]

[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。

[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

[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

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

[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

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

[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

[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

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

[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;

[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。

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

[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。

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

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

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

[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。

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

[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

[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

[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

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

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

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

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

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

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

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

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

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

[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

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

Claims

1. 1. A system comprising: a multiple antenna system (MAS) with multiple user (MU) transmissions ("MU-MAS"); a plurality of wireless UEs; a plurality of cooperative distributed antennas or wireless transceiver devices communicatively coupled to the wireless UE via a wireless communication network and having overlapping coverage at a location of the wireless UE; spatial processing logic that employs spatial processing to exploit inter-cell interference to generate multiple simultaneous non-interfering downlink or uplink data links with the UE within the same frequency band; the wireless communication network is a cellular network such as a 3GPP standard network; RF correction is used to convert the ULCSI to DLCSI, thereby taking advantage of UL / DL channel reciprocity; system.

2. The system of claim 1 , wherein SRS or DMRS is used to estimate the channel impulse response from all UEs to the BTS.

3. The system of claim 2 , wherein different SRS or DMRS are assigned to different antennas of every UE.

4. The system of claim 2 , wherein different SRS or DMRS are assigned to different subsets of BTSs to reduce interference between non-cooperative BTSs.

5. The system of claim 2 , wherein the SRS or DMRS are allocated based on a frequency hopping pattern to exploit frequency diversity of the channel.

6. The system of claim 2 , wherein the active UEs are divided into groups and the same set of SRSs or DMRSs is assigned to each group on consecutive time slots.

7. 7. The system of claim 6, wherein the shortest channel coherence time is estimated for all active UEs, and based on that information, the maximum number of UEs and the periodicity of the time division multiplexing scheme of the SRS or DMRS are calculated.

8. The system of claim 1 , wherein time and frequency synchronization between UEs is achieved by utilizing DL signaling information.

9. 9. The system of claim 8, wherein the BTSs are synchronized to the same reference clock by direct wiring to the same physical clock or by sharing a common time and frequency reference by a Global Positioning System Disciplined Oscillator (GPSDO).

10. 9. The system of claim 8, wherein relative propagation delays between UEs are avoided by processing the UL frames only for those UEs linked to the same set of BTSs, thereby ensuring time synchronization between UEs.

11. 9. The system of claim 8, wherein relative propagation delays between UEs are pre-compensated at the UE side before UL transmission to ensure time synchronization of the UEs at the BTS receiver.

12. The system of claim 1 , wherein an open-loop precoding method is used to transmit simultaneous non-interfering data streams from the BTS to the UE over the DL channel.

13. 13. The system of claim 12, wherein the open-loop precoding method includes a nonlinear spatial filter including a maximum likelihood (ML), decision feedback equalization (DFE), or successive interference cancellation (SIC) receiver.

14. The system of claim 12 , wherein the open-loop method includes a linear spatial filter including a zero-forcing (ZF) or minimum mean square error (MMSE) receiver.

15. The system of claim 1 , wherein SC-FDMA is used to multiplex the UEs in the frequency domain.

16. 1. A method comprising: a multiple antenna system (MAS) with multiple user (MU) transmissions ("MU-MAS"); a plurality of wireless UEs; communicatively connecting a plurality of cooperative distributed antennas or wireless transceiver devices to the wireless UE via a wireless communication network, with overlapping coverage at a location of the wireless UE; using spatial processing with spatial processing logic to exploit inter-cell interference to generate multiple simultaneous non-interfering downlink or uplink data links with the UE within the same frequency band; Including, the wireless communication network is a cellular network such as a 3GPP standard network; The method comprises: using RF compensation to convert ULCSI to DLLCSI, thereby taking advantage of UL / DL channel reciprocity; method.

17. The method of claim 16, wherein SRS or DMRS are used to estimate channel impulse responses from all UEs to the BTS.

18. The method of claim 17, wherein different SRS or DMRS are assigned to different antennas of every UE.

19. The method of claim 17, wherein different SRS or DMRS are assigned to different subsets of BTSs to reduce interference between non-cooperative BTSs.

20. 18. The method of claim 17, wherein the SRS or DMRS are assigned based on a frequency hopping pattern to exploit frequency diversity of the channel.

21. 18. The method of claim 17, wherein the active UEs are divided into groups and the same set of SRSs or DMRSs is assigned to each group on consecutive time slots.

22. 18. The method of claim 17, wherein the shortest channel coherence time is estimated for all active UEs, and based on this information, the maximum number of UEs and the periodicity of the time division multiplexing scheme of the SRS or DMRS are calculated.

23. 17. The method of claim 16, wherein time and frequency synchronization between UEs is achieved by utilizing DL signaling information.

24. 24. The method of claim 23, wherein the BTSs are synchronized to the same reference clock by direct wiring to the same physical clock or by sharing a common time and frequency reference by a Global Positioning System Disciplined Oscillator (GPSDO).

25. 24. The method of claim 23, wherein relative propagation delays between UEs are avoided by processing the UL frames only for those UEs linked to the same set of BTSs, thereby ensuring time synchronization between UEs.

26. 24. The method of claim 23, wherein relative propagation delays between UEs are pre-compensated at the UE side before UL transmission to ensure time synchronization of the UEs at the BTS receiver.

27. 17. The method of claim 16, wherein an open-loop precoding method is used to transmit simultaneous non-interfering data streams from the BTS to the UE over the DL channel.

28. 28. The method of claim 27, wherein the open-loop precoding method includes a nonlinear spatial filter including a maximum likelihood (ML), decision feedback equalization (DFE), or successive interference cancellation (SIC) receiver.

29. 28. The method of claim 27, wherein the open loop method includes a linear spatial filter including a zero-forcing (ZF) or minimum mean square error (MMSE) receiver.

30. The method of claim 16, wherein SC-FDMA is used to multiplex the UEs in the frequency domain.

Images