Utilizing inter-cell multiplexing gains 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, filed November 26, 2012, entitled "Systems And Methods For Exploiting Inter-Cell Multiplexing Gain In Wireless Cellular Systems Via Distributed Input Distributed Output Technology," which is assigned to the assignee of the present application and is incorporated herein 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 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 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] Over the past 30 years, 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 a rapid evolution of wireless technology through different standards, starting with first-generation analog AMP and TACS (for voice services) in the early 1980s, moving to 2G and 2.5G digital GSM, IS-95, and GPRS (for voice and data services) in the 1990s, 3G with UMTS and CDMA2000 (for web browsing) in the early 2000s, and finally LTE (for high-speed Internet access), 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 fourfold 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 currently under standardization, will enable up to an eightfold increase in spectral efficiency over 3G standard systems.

[0023] Despite this technological advancement, there is a strong possibility that wireless carriers will be unable to meet the growing demand for data rates over the next three years due to the increasing market penetration of smartphones and tablets offering more data-intensive applications such as real-time HD video streaming, videoconferencing, and gaming. Wireless network capacity is estimated to expand fivefold in Europe from 2011 to 2015, driven by improved technologies such as LTE and additional spectrum made available by governments.

[25] For example, as part of the National Broadband Initiative

[24] , the FCC plans 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. Unfortunately, projected capacity utilization by 2015 is 23 times higher in Europe than in 2011

[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 below their capital expenditures (CAPEX) and operating expenses (OPEX), with potentially devastating effects on the wireless market.

[28]

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

[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, to enable cooperation between cells, there is a limit to the number of cells that can fit in a given area without introducing interference problems or increasing backhaul complexity.

[0025] This patent or application document 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 explanation of the drawings]

[0027] [Figure 1] FIG. 1 illustrates a cell divided into multiplexing and diversity regions. [Figure 2] FIG. 1 illustrates inter-cell interference in different regions. [Figure 3] FIG. 1 illustrates an embodiment in which the power transmitted simultaneously on the same frequency from all three base transceiver stations (BTSs) is increased, thereby allowing for 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 the cell. [Figure 5] FIG. 1 illustrates several LTE network elements used in one embodiment of the present invention. [Figure 6A] FIG. 2 illustrates an example of an LTE frame and associated details. [Figure 6B] FIG. 2 illustrates an example of an LTE frame and associated details. [Figure 6C] FIG. 2 illustrates an example of an 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. 1 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 INVENTION

[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 the present patent and are incorporated herein by reference. These patents and applications are referred to at various times herein collectively 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 No. 13 / 475,598, entitled "Systems and Methods to Enhance Spatial Diversity in Distributed Input Distributed Output Wireless Systems."

[0032] U.S. Patent Application 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 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 will not be explicitly described below, and reference should be made 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 conventional cellular systems is the Distributed Input Distributed Output (DIDO) technology (see related patents and applications cited above in [0002-0020]). To provide significant performance benefits over conventional wireless systems, this invention describes DIDO technology used in conjunction with cellular systems (such as LTE or LTE-Advanced), both within and outside the constraints of cellular standards. Beginning with an overview of MIMO, we consider various spatial processing techniques used by LTE MIMO and LTE-Advanced. We then show how this invention offers significant capacity gains for next-generation wireless communication systems compared to conventional 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 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 propagation conditions [20-23]. For example, link adaptation methods described in [22-23] have shown that beamforming or orthogonal space-time block coding (OSTBC) is the preferred scheme in low SNR regimes or channels characterized by low spatial selectivity. 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 proximity to the cell tower or base station), where spatial multiplexing can exploit the spatial freedom of the channel to increase data rates; 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 not labeled.

[0053] The LTE (Release 8) and LTE-Advanced (Release 10) standards define a set of 10 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 (which exploit microdiversity in cellular systems), which use multiple antennas to improve link reliability or data rates within a cell; and ii) inter-cell methods (which exploit macrodiversity), which allow cooperation between cells to provide additional diversity or multiplexing gains. Next, we describe how the present invention offers significant advantages over the prior art, including spectral capacity gains.

[0055] 1. Intra-cell diversity method Intra-cell diversity methods operate within a 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 on the uplink (UL). It is achieved by space-frequency block coding (SFBC) combined with frequency-switched transmit diversity (FSTD) to exploit spatial and frequency selectivity in the DL channel

[31] . Rank-1 precoding generates a dedicated beam for a single 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, or 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 high SNR regimes and in scenarios with sufficient spatial degrees of 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 multi-user MIMO (MU-MIMO)

[31] . SU-MIMO has two modes of operation: i) closed-loop, which uses feedback information from the UE to select DL precoding weights; and ii) open-loop, which is used when feedback from the UE is unavailable or the UE is moving too fast to support the closed-loop approach. The closed-loop approach 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 (determined by the number of layers in the precoding matrix), depending on the UE's requirements and the BTS scheduler's discretion. 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] .

[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. These 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 (e.g., each UE receives only one data stream).

[0060] Intra-cell multiplexing techniques using spatial processing only provide good performance in propagation scenarios characterized by high SNR (or SINR) and high spatial selectivity (multipath-rich environments). For conventional macrocells, these conditions can be more difficult to achieve because the BTS is usually far from the UE and the SINR distribution 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 achieve further multiplexing gains (without requiring MIMO spatial processing) is carrier aggregation (CA) and small cells. CA [30, 44-47] combines different parts of the RF spectrum to increase the signal bandwidth by up to 100 MHz

[85] , thereby providing higher data rates. Intra-band CA combines different bands within the same portion 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 and 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 macrocells, thereby enabling higher cell density and greater throughput per coverage area. Small cells are typically deployed through inexpensive access points with low-power transmissions (as depicted in Figure 1), in contrast to the tall, expensive cell towers used for macrocells. Two types of small cells are defined in LTE-Advanced: i) metrocells, which are for outdoor installations in urban areas and support 32–64 simultaneous users; and ii) femtocells, which are for indoor use and can serve up to four active users. 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 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 backhaul. In fact, reaching every small cell BTS through a high-speed wired connection may be challenging, especially considering the high density of metrocells and femtocells in a given coverage area. Although using line-of-sight (LOS) backhaul for small cells is often cheaper than wired backhaul, there are often no practical LOS backhaul paths available for preferred small cell BTS placements, and no general solution exists for non-line-of-sight (NLO) wireless backhaul to small cell BTSs. Finally, small cells require complex real-time coordination between BTSs to avoid interference, as in self-organizing networks (SONs) [30, 51-52], and require sophisticated 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 trivially shown that there is no practical general solution that allows small cells to coexist with macro cells and achieve optimal or necessarily improved throughput. Among countless such unsolvable situations, one is when a small cell is located such that its UE inevitably overlaps with macro cell transmissions, and the small cell and 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 mitigating 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. However, any such approach is highly specific not only to the static planning of macro cells and small cells, but also to the dynamic conditions of a particular time interval. Normally, the full throughput of the channel to each UE cannot be achieved.

[0064] 3. Intercell diversity method Inter-cell transmission techniques enable cooperation between BTSs to improve wireless network performance. 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 simultaneously using the same frequency. For the specific case of a cellular system with a single UE at a given frequency and 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 subcells by using dedicated beamforming from the coordinated BTSs. The use of dedicated beamforming from the coordinated BTSs improves link robustness for a single frequency and a single UE as the single UE moves along the subcell boundaries.

[0065] 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 from 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]. To eliminate inter-cell interference, two CoMP solutions have been proposed in the standard: i) Coordinated Scheduling / Beamforming (CS / CB). A UE receives data streams from only one BTS through beamforming, and cooperation between BTSs is enabled to eliminate interference through beamforming or scheduling techniques. ii) Joint Processing (JP). Data for a given UE (UE) is jointly transmitted from multiple BTSs to improve received signal quality and eliminate inter-cell interference. CoMP-JP offers greater 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 use of spectrum by other users in that area. Given the increasing demand for aggregate throughput in wireless spectrum shared with multiple users, and the increasing growth of applications that can depend 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 from many constraints. Indeed, with limited availability of spectrum suitable for certain types of wireless communications (e.g., at wavelengths effective for penetrating building walls), prior art wireless technologies will be inadequate to meet the increasing demand for reliable, predictable, and low-latency bandwidth.

[0068] Prior art intra-cell diversity and multiplexing methods can only theoretically provide up to a fourfold increase in throughput over current cellular networks for LTE (with MIMO 4x4) and at most an eightfold increase for LTE-Advanced (with MIMO 8x8). It should be noted that for higher-order MIMO, the improvement in throughput increases diminishes in a given multipath environment, especially as UEs (e.g., smartphones) become smaller and more constrained in terms of antenna placement. Other small throughput gains in next-generation cellular systems may come from additional spectrum allocations (e.g., the FCC National Broadband Plan) utilized by carrier aggregation techniques and denser distribution of BTSs through small cell networks and SONs ​​[30, 46]. However, due to the limited spectral efficiency gains achieved through spatial processing, all of the above techniques still rely heavily on spectrum- or time-sharing techniques to enable multi-user transmission.

[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 modest. In fact, these systems are only effective at constraining the power transmitted from any BTS contained within a 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. Because these systems operate in the low SINR regime in the interference region, their spectral efficiency gains, similar to intra-cell methods for SU-MIMO, are only modest. To achieve truly significant capacity gains in inter-cell cooperative networks, the power restrictions limited to 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 would therefore be 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 exploiting inter-cell multiplexing gains through spatial processing. Figure 3 shows that the power transmitted simultaneously on the same frequency from all three BTSs 301-303 is increased, thereby allowing for high levels of interference throughout the cell. In prior art systems, such interference would result in incoherent interference (impairing UE signal reception) throughout the BTS's interference region, but this interference is actually exploited in embodiments of the present invention through a novel inter-cell multiplexing method. This method uses spatial processing to create a region of coherent interference (enhancing UE signal reception) around every UE, thereby simultaneously providing non-interfering data streams to every UE and increasing their SINR throughout the cell.

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

[0072] It can be seen that the prior art inter-cell method avoids incoherent interference by intentionally limiting the transmit power from every BTS 210-212 as shown in FIG. 2 and eliminates the remaining inter-cell interference (related to the overlapping regions between cells) through spatial processing, thereby providing improved SINR and inter-cell diversity gain. In contrast, the present invention exploits incoherent interference by transmitting higher power from every BTS to generate coherent interference around the UE. This improves signal quality at the UE, which is a prerequisite for achieving inter-cell multiplexing gain across cells through spatial processing. Therefore, the systems described in the prior art cannot be used to achieve inter-cell multiplexing gain through spatial processing, since there is insufficient signal quality throughout the cells (due to the limited transmit 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 by the present invention as depicted in Figures 3-4, given 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 achieve the inter-cell multiplexing gains achieved by the present invention.

[0073] Embodiments of the present invention include systems and methods for utilizing inter-cell multiplexing gains through spatial processing in wireless communication networks, 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). This intentionally generates high-order inter-cell interference throughout the cells and utilizes it to realize inter-cell multiplexing gains, increasing the capacity of the wireless communication network.

[0074] In one embodiment, the wireless communication network is a cellular network such as that shown 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 cell boundaries can span a larger area such as that shown in FIGS. 3-4. For example, the wireless communication network could 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 located access points without any transmit power limitations. However, such example 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 received by multiple UEs, generating simultaneous non-interfering data streams to the multiple UEs.

[0075] As illustrated in FIG. 9 , one 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 equipment (UE) (illustrated as UE 1-4). The centralized processor unit 901 receives N streams of information (e.g., video, web pages, video games, text, audio, 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 in terms of various network content C1-5. Hereinafter, we use the term “stream of information” to refer to any stream of data transmitted over the network 900, which 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, audio, 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 transform that combines N streams of information from the network content and converts them into M streams of bits (using algorithms such as those described in related patents and applications). By way of example and not limitation, the precoding transform can be linear (e.g., zero-forcing

[65] , block diagonalization [66-67], matrix inversion, etc.) or nonlinear (e.g., dirty-paper coding [68-70] or Tomlinson-Harashima precoding [71-72], lattice or trellis precoding [73-74], vector perturbation techniques [75-76]). Hereinafter, 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 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 consists of: 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 channel. In one embodiment, the BTS is an access point consisting of a DAC / 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 RF-over-fiber technology as described in related patents and applications. Controller (CTR) 905: The CTR 905 is a specific type of BTS designed for the following specific functions: transmitting training signals for time / frequency synchronization of the BTS and / or UEs, receiving and transmitting control information from and to UEs, and receiving channel state information (CSI) or channel quality information from UEs. One or more CTR stations can be included in any DIDO system. When multiple CTRs are available, information to and from these stations can be combined to increase diversity and improve link quality. In one embodiment, CSI is received from multiple CTRs using maximal ratio combining (MRC) techniques to improve CSI demodulation. In another embodiment, control information is transmitted from multiple CTRs using maximal ratio transmission (MRT) to improve the SNR at the receiver. The scope of the invention is not limited to MRC or MRT; any other diversity technique (e.g., antenna selection) 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 to 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 the network connecting the CP to the distributed BTSs that carry information on either DL or UL channels. The BSN may be a wired or wireless network, or a combination of both. For example, the BSN may be a DSL, cable, fiber optic network, or a line-of-sight (LOS) or non-line-of-sight (NLO) wireless link. Furthermore, the BSN may be a proprietary network, or a local area network, or the Internet.

[0078] In the following, we describe how the above-described 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 begin with a general overview of the LTE framework and the modulation schemes used for DL ​​and UL channels. This is followed by a brief description of the physical layer frame structure and resource allocation in the LTE standard. Finally, we define DIDO precoding methods for downlink (DL) and uplink (UL) channels in a multi-user scenario using the LTE framework. For DL ​​systems, we propose two solutions: open-loop and closed-loop DIDO.

[0079] LTE is designed with a flat network architecture (as opposed to the hierarchical architecture of previous cellular standards) and offers: reduced latency, reduced packet loss through ARQ, reduced call setup time, 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 external networks (i.e., the Internet). The GWs are divided into a Serving Gateway (S-GW) 502, which forms the boundary of the E-UTRAN interface, and a PDN Gateway (P-GW) 501, which interfaces with external networks. The S-GW 502 and 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-6C, an LTE frame has a duration of 10 ms and consists of 10 subframes [33, 80]. Each subframe is divided into two slots, each with a duration of 0.5 ms. 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 the downlink (DL) or uplink (UL) channel. ii) Type 2 for TDD operation, as shown in Figure 6B. Some subframes are assigned to the DL and some to the UL (depending on the selected configuration), but a few subframes are reserved for "special use." Each frame has at least one special subframe, which consists 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.

[0082] LTE uses Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFMDA) modulation for the DL and Single Carrier FDMA (SC-FDMA) for the 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 (consisting of 3 to 7 OFDM symbol periods, depending on the DL vs. UL channel and cyclic prefix type).

[0083] 1. Downlink Closed-Loop DIDO in LTE DIDO closed-loop schemes 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 and reported back to the CP via the BTS or CTR over 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 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 BTS 903 or CTR 905; v) BTS 903 or CTR 905 reports the CSI information or codebook index to CP 901, which calculates precoding weights for data transmission on the DL. "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 to reduce the amount of CSI estimated from the BTSs for a given UE, thereby reducing feedback overhead on the UL and the complexity of the DIDO precoding calculations in CP 901.

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

[50] . CRS is used in LTE Release 8, which uses SU / MU-MIMO codebook-based technology, where each cell uses up to four antennas. LTE-Advanced Release 10 supports non-codebook-based SU / MU-MIMO, uses up to eight transmit antennas, and uses CoMP, where antennas are distributed across different cells. Therefore, Release 10 enables flexible signaling schemes using CSI-RS. In this invention, we describe how either 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 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 third-to-last OFDM symbols of every slot and in every sixth subcarrier. Orthogonal time and frequency patterns are designed for every transmit antenna at the BTS so that the UE can uniquely estimate 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) generates 5% overhead and is intentionally designed to support scenarios with fast channel variations in time and frequency

[83] .

[0087] In a practical DIDO system, every UE may see more than just four BTSs in its user cluster. For example, Figure 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 DIDO-BTSs, while the black circles indicate user clusters (UEs are located at the centers of the circles). In sparsely populated areas, UEs see only a few BTSs within their user cluster (e.g., only three BTSs for the example in Figure 8), while in densely populated areas, each user cluster may contain as many as 26 BTSs as in Figure 8.

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

[0089] 1.1.2 DIDO 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 a BTS [33, 83]. The standard defines orthogonal CSI-RS for different transmitters at the BTS, allowing the UE to distinguish CSI from different BTSs. Up to eight transmit antennas at the BTS are supported by CSI-RS, as shown in Tables 6.10.5.2-1 and 6.10.5.2-2 of

[33] . CSI-RS is transmitted with a periodicity spanning 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 CSI-RS in LTE-Advanced is intentionally larger than that of CRS in conventional LTE to avoid excessive control information overhead, especially for legacy LTE terminals that cannot use these extra resources. Another reference signal used for CSI estimation is the demodulation-modulation-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 in a user cluster (the maximum number of transmitters supported by the LTE-Advanced standard), 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, each UE uses CSI-RS or DM-RS, or a combination thereof, to estimate 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 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 transmit power from the BTS is reduced until a maximum of 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 divide the BTSs in a user cluster into subsets and transmit one set of CSI-RS for all subsets simultaneously. 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 cover 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, then we can define up to 20 subsets of 8 BTSs each, for a total of 160 BTSs in the user cluster. SIn 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] All solutions for CSI-RS proposed to date are compliant with the LTE standard and can be deployed within the framework of a conventional LTE system. For example, proposed methods that allow more than eight antennas per user cluster do not require modifications to the UE LTE hardware and software implementation, but only slight modifications to the protocols used by the BTSs and CPs to enable BTS subset selection at any time. These modifications can be easily implemented on 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 that are similar to LTE but not compliant with LTE, it would allow the UE to operate in either a fully LTE-compliant mode or a modified mode that supports non-LTE-compliant DIDO operation. For example, another solution is to increase the amount of CSI-RS to enable a larger number of BTSs in the system. In other embodiments 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 slight modifications to the LTE standard may be small enough that existing LTE UE chipsets can be used simply with software modifications, or 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): Determines 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] While 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 RI, PMI, and CQI reporting procedure 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 LTE-Advance transmission mode 9 with eight transmitters).

[0095] In one embodiment of the present invention, the DIDO system uses RI, PMI, and CQI to report current channel conditions 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 channel 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, BTS, or 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 BTS 903 or CTR 905; ii) BTS 903 or CTR 905 uses the signaling information to estimate UL CSI from all UEs 1-4; iii) BTS 903 or CTR 905 uses RF CSI to convert UL CSI to DL CSI. calibration) 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 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 a normal cyclic prefix) of every LTE slot (of 0.5 ms duration). In the frequency domain, DMRS transmitted on the PUSCH is mapped, for every UE, to the same resource blocks (RBs) used by that UE for UL data transmission.

[0098] 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 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 around the UE (as 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"), and the DIDO system generates up to 3,300 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 (as 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 take advantage of the frequency diversity of the channel.

[0100] If there are not enough orthogonal SRS or DMRS for all UEs to be simultaneously provided in the 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 these 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 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 a DL data stream to the UE by DIDO precoding. In one embodiment of the present invention, the system divides 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 payloads 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 payloads to CP 901; iv) CP 901 uses the channel estimates to remove co-channel interference from all UEs' data payloads 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 the DL, assuming all BTSs 903 are 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 GPSDO. Variations in channel delay across different UEs can create jitter between the time references of different UEs, which can affect the performance of MU-MIMO methods 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 BTS 903.

[0103] The techniques for enabling signaling information for data demodulation on the UL can be the same methods used for signaling in the downlink open-loop DIDO scheme 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 nonlinear 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 demodulate 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 operational, in the process of being deployed, and / or to be deployed. It would be a significant benefit for 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 would provide the most immediate benefit, gradually expanding DIDO capabilities across more and more of their networks. Eventually, once they have sufficient DIDO coverage in their area, they could choose to discontinue using cellular 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, LTE operators' wireless customers would not suffer any loss in service. Rather, they would simply experience improvements in their data efficiency and reliability, while the operator would experience lower 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 DIDO compatible), 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 hereafter be referred to as a DIDO-LTE BTS.

[0106] The LTE standard supports various bandwidths (e.g., 1.4, 3, 5, 10, 15, and 20 MHz). In one embodiment, an operator of an existing LTE network can support cellular-configured conventional LTE BTSs in one block of spectrum and DIDO LTE BTSs in another block of spectrum, either by allocating new bandwidth for LTE-DIDO BTSs 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 network or the other, or to choose between both. In the case of subdivided spectrum, the spectrum can be divided evenly or unevenly between the conventional LTE and DIDO-LTE networks, with more spectrum allocated to the network that can best utilize it, taking into account the degree of deployment of cellular LTE BTSs and DIDO-LTE BTSs 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, conventional cellular LTE BTSs can be configured to cooperate with DIDO-LTE BTSs, sharing the same spectrum but using it in alternating fashion. For example, if they share spectrum equally, each BTS network would alternately utilize a 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 allocated to the DIDO-LTE BTSs and the cellular BTSs can be decommissioned.

[0108] In another embodiment of the present invention, DIDO is used as a line-of-sight or non-line-of-sight (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 this same embodiment of the present invention, DIDO BTSs are used to gradually replace the small cells, thereby taking advantage of the inter-cell interference to provide increased network capacity.

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Claims

1. 1. A system including a multiple antenna system (MAS) with multiple user (MU) transmissions (“MU-MAS”), The multiple antenna system comprises: a plurality of more than one wireless user equipments (UEs); a plurality of more than eight base transceiver stations (BTSs) or cooperative distributed antennas communicatively coupled to the one or more wireless user equipments (UEs) via a wireless communication network, the base transceiver stations or cooperative distributed antennas having overlapping coverage at locations of the one or more wireless user equipments (UEs); spatial processing means for employing spatial processing to perform inter-cell interference to form multiple simultaneous non-interfering downlink or uplink data links with the more than one wireless user equipment (UE) in the same frequency band; the wireless communication network is a cellular network such as a Third Generation Partnership Project (3GPP) standard network; RF calibration is used to convert UL CSI between the one or more wireless user equipments (UEs) and the eight or more wireless transmit / receive units (BTSs) or cooperative distributed antennas into DL CSI between the one or more wireless user equipments (UEs) and the eight or more wireless transmit / receive units (BTSs) or cooperative distributed antennas, thereby utilizing 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 the SRS or DMRS are allocated based on a frequency hopping pattern to exploit frequency diversity of the channel.

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

5. 5. The system of claim 4, 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.

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

7. 7. The system of claim 6, 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 via a Global Positioning System Disciplined Oscillator (GPSDO).

8. The system of claim 6 , wherein relative propagation delays between UEs are pre-compensated at the UE side before UL transmission to ensure time synchronization of the UEs with the BTS.

9. 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.

10. 10. The system of claim 9, 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.

11. 10. The system of claim 9, wherein the open-loop precoding method includes a linear spatial filter including a zero-forcing (ZF) or minimum mean square error (MMSE) receiver.

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

13. 1. A method comprising: a multiple antenna system (MAS) with multiple user (MU) transmissions ("MU-MAS"); a plurality of more than one wireless user equipments (UEs); a plurality of more than eight base transceiver stations (BTSs) or cooperating distributed antennas communicatively coupled to the one or more wireless user equipments (UEs) over a wireless communication network with overlapping coverage at locations of the one or more wireless user equipments (UEs); using spatial processing by a spatial processing means to perform inter-cell interference to form multiple simultaneous non-interfering downlink or uplink data links with the more than one wireless user equipment (UE) within the same frequency band; Including, the wireless communication network is a cellular network such as a Third Generation Partnership Project (3GPP) standard network; The method comprises: using RF calibration to convert UL CSI between the one or more wireless user equipments (UEs) and the eight or more wireless transmit / receive units (BTSs) or cooperative distributed antennas into DL CSI between the one or more wireless user equipments (UEs) and the eight or more wireless transmit / receive units (BTSs) or cooperative distributed antennas, thereby utilizing UL / DL channel reciprocity; method.

14. The method of claim 13 , wherein SRS or DMRS are used to estimate channel impulse responses from all UEs to the BTS.

15. 15. The method of claim 14, wherein the SRS or DMRS are assigned based on a frequency hopping pattern to exploit frequency diversity of the channel.

16. 15. The method of claim 14, wherein active UEs are divided into groups and the same set of SRSs or DMRSs is assigned to each group on consecutive time slots.

17. 15. The method of claim 14, 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.

18. The method of claim 13, wherein time and frequency synchronization between UEs is achieved by utilizing DL signaling information.

19. 20. The method of claim 18, 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).

20. 20. The method of claim 18, wherein relative propagation delays between UEs are pre-compensated at the UE side before UL transmission to ensure time synchronization of the UEs with the BTS.

21. The method of claim 13 , 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.

22. 22. The method of claim 21, 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.

23. 22. The method of claim 21, wherein the open-loop precoding method includes a linear spatial filter including a zero-forcing (ZF) or minimum mean square error (MMSE) receiver.

24. The method of claim 13, wherein SC-FDMA is used to multiplex the UEs in the frequency domain.

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