Utilization of inter-cell multiplexing gain in wireless cellular systems

Abstract

Achieves multiplexing gain in a multiple antenna system (MAS) ("MU-MAS") with multiple user (MU) transmissions. [Solution] The power transmitted from multiple antennas is not constrained by any particular power level (as long as their power emission levels fall within regulatory or safety limits). This allows for the deliberate generation of higher-order inter-cell interference through the cells, which is then used to achieve inter-cell multiplexing gain and increase the capacity of the wireless communication network. One embodiment of MU-MAS includes a wireless cellular network having multiple distributed antennas that work together to eliminate inter-cell interference.

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This application claims the benefit of U.S. Provisional Application No. 61 / 729,990, filed on November 26, 2012, entitled "Systems And Methods For Exploiting Inter - Cell Multiplexing Gain In Wireless Cellular Systems Via Distributed Input Distributed Output Technology". This provisional application has been assigned to the assignee of this application. This application 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] <0000​​​​​​​​U.S. Patent No. 8,542,763, issued September 24, 2013, titled "Systems and Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering".

[0006] U.S. Patent Application No. 12 / 802,988, titled "Interference Management, Handoff, Power Control and Link Adaptation in Distributed-Input Distributed-Output (DIDO) Communication Systems".

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0021] Over the past 30 years, the wireless cellular market has experienced a global increase in subscriber numbers, as well as a demand for better services shifting from voice to web browsing and real-time HD video streaming. This growing demand for services requiring higher data rates, lower latency, and improved reliability has driven the rapid evolution of wireless technology through various standards. Beginning with the first generation of analog AMP and TACS (for voice services) in the early 1980s, followed by 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 connectivity), which is now deployed in various countries around the world.

[0022] Long-Term Evolution (LTE) is a standard developed by the 3rd Generation Partnership Project for Fourth Generation (4G) Wireless Cellular Systems (3GPP). LTE utilizes the spatial components of the radio channel through Multiple Input Multiple Output (MIMO) technology, enabling up to four times the downlink spectral efficiency compared to previous 3G and HSPA+ standards. LTE-Advanced is an advanced version of LTE, currently under standardization, and is expected to enable up to eight times the spectral efficiency compared to 3G standard systems.

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

[25] For example, the FCC plans to release 500 MHz of spectrum by 2020 (of which 300 MHz is expected to be available by 2015) to advance wireless internet connectivity across the United States as part of the National Broadband Plan

[24] . Unfortunately, projections for capacity use by 2015 are 23 times that in Europe compared to 2011

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

[28]

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

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

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

[0025] This patent or application document includes at least one drawing finished in color. Copies of this patent or patent publication having the color drawing(s) will be provided by the United States Patent and Trademark Office upon payment of the claims and required fees.

[0026] The present invention can be better understood from the following detailed description together with the drawings.

Brief Description of the Drawings

[0027] [Figure 1] It is a diagram illustrating cells divided into a multiplexing region and a diversity region. [Figure 2] It is a diagram showing inter-cell interference in a plurality of different regions. [Figure 3] It is a diagram illustrating an embodiment in which the power transmitted simultaneously at the same frequency from all three transceiver base stations (BTSs) is increased, thereby enabling a high level of interference throughout the cell. [Figure 4] It is a diagram illustrating an embodiment in which many additional access points are added to deliberately increase the incoherent interference level throughout the cell. [Figure 5] It is a diagram illustrating a plurality of LTE network elements used in an embodiment of the present invention. [Figure 6A] It is a diagram illustrating details related to the LTE frame. [Figure 6B] It is a diagram illustrating details related to the LTE frame. [Figure 6C] It is a diagram illustrating details related to the LTE frame. [Figure 7A] It is a diagram illustrating a "resource element", which is the smallest modulation structure in LTE and consists of one OFDM sub-carrier in frequency and one OFDM symbol duration in time. [Figure 7B]This diagram illustrates the smallest modulation structure in LTE, which is a "resource element" consisting of one OFDM subcarrier in frequency and one OFDM symbol duration in time. [Figure 8] This figure illustrates the SNR distribution for a practical deployment of one embodiment of the present invention in downtown San Francisco, California. [Figure 9] This diagram illustrates a system architecture used in one embodiment of the present invention. [Modes for carrying out the invention]

[0028] One solution that overcomes many of the limitations of the prior art described 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 have been assigned to the assignee of this patent and are incorporated herein by reference. These patents and applications are referred to several times herein collectively as “Related Patents and Applications.”

[0029] U.S. Patent Application No. 13 / 233,006, title: "System and Methods for planned evolution and obsolescence of multiuser spectrum".

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

[0031] U.S. Patent Application No. 13 / 475,598, titled "Systems and Methods to Enhance Spatial Diversity in Distributed Input / Distributed Output Wireless Systems".

[0032] U.S. Patent Application No. 13 / 464,648, titled "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, titled "Systems and Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering".

[0034] U.S. Patent Application No. 12 / 802,988, titled "Interference Management, Handoff, Power Control and Link Adaptation in Distributed-Input Distributed-Output (DIDO) Communication Systems".

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0049] To reduce the volume and complexity of this patent application, some disclosures in related patents and applications are not explicitly described below. For a complete description of those disclosures, please refer to the related patents and applications.

[0050] One promising technique that offers a significant increase in spectral efficiency over wireless links without the constraints of conventional cellular systems is Distributed Input Distributed Output (DIDO) technology (see relevant patents and applications cited above in [0002-0020]). To provide a significant performance benefit over conventional wireless systems, the present invention describes DIDO techniques used in connection with cellular systems (such as LTE or LTE-Advanced), both within and outside the constraints of cellular standards. Beginning with an overview of MIMO, the invention examines the various spatial processing techniques used by LTE MIMO and LTE-Advanced. Next, it shows how the present invention provides a significant capacity gain for next-generation wireless communication systems compared to conventional approaches.

[0051] MIMO uses multiple antennas at the transmitting and receiving ends of a radio link to improve link reliability through diversity techniques (i.e., diversity gain) or to provide higher data rates through multiplexing techniques (i.e., multiplexing gain) [1-2]. Diversity gain is a measure of enhanced robustness against signal fading and results in a higher signal-to-noise ratio (SNR) for a fixed data rate. Multiplexing gain is achieved by taking advantage of additional spatial degrees of freedom in the radio channel to increase the data rate with a fixed error probability. The fundamental trade-offs between diversity and multiplexing in MIMO systems are described in [3-4].

[0052] In practical MIMO systems, link adaptive techniques can be used to dynamically switch between diversity and multiplexing schemes based on propagation conditions [20-23]. For example, the link adaptive schemes described in [22-23] showed that beamforming or orthogonal space-time block coding (OSTBC) is a 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 rates for channels with high SNR and high spatial selectivity. For example, Figure 1 shows that a cell can be divided into two regions: i) Multiplexing region 101, characterized by high SNR (due to proximity to the cell tower or base station), where spatial multiplexing can be used to utilize the spatial degrees of freedom of the channel to increase the data rate; ii) Diversity region or cell edge 102, where spatial multiplexing techniques are not equally effective, and diversity methods can be used to improve SNR and coverage (providing only a slight increase in data rate). Note that in Figure 1, the circles of the macrocells label the shaded central area of ​​the circle as the "multiplexing region" 101, and the unshaded outer area as the "diversity region" 102. This same regional designation, where the shaded area is the "multiplexing region" and the unshaded area is the "diversity region," is used throughout Figures 1 to 4, even if the areas are not labeled.

[0053] The LTE (Release 8) and LTE-Advanced (Release 10) standards define a set of 10 transmission modes™, which include either diversity or multiplexing schemes [35, 85-86]. • Mode 1: Single antenna port, port 0 Mode 2: Transmit diversity • Mode 3: Extensions to open-loop space multiplexing for large-delay cyclic-delay diversity (CDD) and single-user MIMO (SU-MIMO) Mode 4: Closed-loop spatial multiplexing related to SU-MIMO • Mode 5: Multi-user MIMO (MU-MIMO) • Mode 6: Closed-loop spatial multiplexing, using a single transmit 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 layers of closed-loop SU-MIMO (added in Release 10) • Mode 10: Up to 8 layers of multi-layer closed-loop SU-MIMO (added in Release 10)

[0054] Below, we describe the diversity and multiplexing schemes commonly used in cellular systems, as well as the specific methods used in LTE outlined above, and compare them with technologies specific to DIDO communication. First, we identify two types of transmission methods: i) Intracell method (utilizing microdiversity in cellular systems): Using multiple antennas within a single cell to improve link reliability or data rate. ii) Intercell method (utilizing macrodiversity): Enabling inter-cell coordination to provide further diversity or multiplexing gain. Next, we describe a method in which the present invention offers significant advantages (including spectral capacity gain) over the prior art.

[0055] 1. Intracell Diversity Methods Intracell diversity methods operate within a single cell and are designed to enhance the signal-to-noise ratio (SNR) in scenarios with poor link quality (e.g., users at the cell edge experiencing high path loss from the central tower or base station). Typical diversity methods used in MIMO communications include beamforming [5-11] and orthogonal space-time block coding (OSTBC) [12-15].

[0056] The diversity technologies supported by the LTE standard are transmit diversity, closed-loop rank-1 precoding, and dedicated beamforming [31-35]. Transmit diversity supports two or four transmit antennas on the downlink (DL) and two antennas for the uplink (UL). It is implemented in the DL channel by spatial frequency block coding (SFBC) combined with frequency switching transmit diversity (FSTD) to take advantage of spatial and frequency selectivity

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

[0057] 2. Intracell Multiplexing Method MIMO multiplexing schemes [1, 19] provide data rate gains in high SNR regimes and in scenarios with sufficient spatial freedom in the channel (e.g., rich multipath environments with high spatial selectivity [16-18]) in order to support multiple parallel data streams on a wireless link.

[0058] The LTE standard supports various multiplexing techniques for single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO)

[31] . The SU-MIMO scheme has two operating modes: i) Closed-loop, which utilizes feedback information from the UE to select DL precoding weights; ii) Open-loop, which is used when feedback from the UE is unavailable or when the UE is moving too fast to support the closed-loop scheme. The closed-loop scheme uses a set of pre-calculated weights selected from a codebook. These weights can support two or four transmitting antennas, as well as one to four parallel data streams (specified by the number of layers in the precoding matrix), depending on the UE's request and the BTS scheduler's judgment. LTE-Advanced can include new transmission modes up to MIMO 8x8 to provide up to an 8x increase in spectral efficiency through spatial processing

[62] .

[0059] The MU-MIMO scheme is defined for both UL and DL channels [31, 50]. In UL, every UE sends a reference signal (consisting of a cyclically shifted version of the Zadoff-Chu sequence

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

[0060] Intracell multiplexing techniques using spatial processing offer good performance only 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 typically far from the UE and the SINR distribution is typically concentrated at low values ​​

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

[0061] Another technology and network solution conceivable by LTE-Advanced to achieve further multiplexing gain (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 signal bandwidth up to 100 MHz

[85] , thereby giving higher data rates. Intraband CA combines different bands within the same portion of the spectrum. Thus, it can use the same RF chain for multiple channels, and the multiplexed data streams are recombined in software. Interband CA requires different RF chains to operate in different portions of the spectrum and also requires signal processing to recombine multiple data streams from different bands.

[0062] The main idea behind small cells [30, 47] is to reduce the size of conventional macrocells, thereby enabling higher cell density and greater processing power per coverage area. Small cells are typically deployed through inexpensive access points with low-power transmission (as shown 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, for outdoor installations in urban areas, supporting 32 to 64 concurrent users; and ii) Femtocells, for indoor use, capable of serving up to 4 active users. One advantage of small cells is the statistically higher density of UEs near the BTS, thereby giving better SNR which can be utilized through spatial multiplexing to augment data rates. However, there are still many concerns regarding the practical deployment of small cells, particularly those related to backhaul. In fact, reaching any small cell's BTS through high-speed wired connections may be challenging, especially considering the high densities of metrocells and femtocells in a given coverage area. Compared to wired backhaul, using line-of-sight (LOS) backhaul for small cells is often less expensive, but there are often no practical LOS backhaul paths available for preferred small cell BTS placements, and there is no general solution 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 advanced cell planning tools (which are even more complex than in conventional cellular systems due to the high density of small cells) to plan their optimal locations [48, 49].

[0063] It can be shown self-evidently that there is no practical general solution that enables small cells to coexist with macrocells and achieve optimal or necessarily improved throughput. One of countless such intractable situations is when a small cell is positioned such that its UE inevitably overlaps with macrocell transmission, and both the small cell and macrocell use the same frequency to reach their respective UEs. In this situation, macrocell transmission will obviously interfere with small cell transmission. There may be some approaches to mitigate such interference in specific situations, such as specific macrocells, specific small cells, specific macrocell and small cell UEs involved, the throughput conditions of those UEs, and environmental conditions. However, any such approach is highly specific, not only for static planning of macrocells and small cells, but also for dynamic situations over specific time intervals. Typically, full channel throughput for each UE is not achievable.

[0064] 3. Intercellular diversity methods Intercell transmission technologies enable coordination between BTSs to improve the performance of wireless networks. These technologies are special cases of the methods taught in related patents and applications [0002-0020] to enable coordination between radio transceivers in the common case of a distributed antenna network for multiplexed UEs where all are using the same frequency simultaneously. Coordination between BTSs to eliminate intercell interference for a specific case of a cellular system relating to a single UE at a given frequency and time is described in

[53] . The system in

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

[0065] Recently, this class of cooperative wireless cellular networks has been clearly defined in the MIMO literature as "Network MIMO (MIMO)" or "Cooperative Multipoint" (CoMP) systems. Theoretical analyses and simulated results regarding the benefits of Network MIMO by eliminating inter-cell interference are shown in [54-61]. The main advantage of Network MIMO and CoMP is the elimination of inter-cell interference in the cell overlap region 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 in the standard to eliminate inter-cell interference: i) Coordinated Scheduling / Beamforming (CS / CB). A UE receives a data stream from only one BTS by beamforming, and coordination between BTSs is enabled by beamforming or scheduling techniques to eliminate interference. ii) Combined Processing (JP). Data for a given UE is transmitted jointly from multiple BTSs to improve the received signal quality and eliminate inter-cell interference. CoMP-JP yields greater gain than CoMP-CS / CB at the expense of higher overhead in backhaul to enable coordination between BTSs.

[0067] 4. Intercell Multiplexing Method Prior art multi-user wireless systems add complexity and constraints to wireless networks, resulting in a situation where a given user's experience (e.g., available throughput, latency, predictability, reliability) is affected by the use of the spectrum by other users in that domain. Given the increasing demand for total throughput within the wireless spectrum shared by multiple users, and the growing number of applications that can rely 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 are subject to many constraints. Indeed, with respect to the limited availability of spectrum suitable for certain types of wireless communication (e.g., at wavelengths effective for penetrating building walls), prior art wireless technologies would be insufficient to meet the increasing demand for bandwidth that is reliable, predictable, and low latency.

[0068] Prior art intra-cell diversity and multiplexing methods can theoretically provide up to a fourfold increase in throughput on current cellular networks (with MIMO 4x4) for LTE (LTE), and at most an eightfold increase for LTE-Advanced (with MIMO 8x8). Furthermore, for higher-order MIMO, the improvement in throughput decreases in given multipath environments, 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 spectral allocation utilized by carrier aggregation techniques (e.g., the FCC National Broadband Programme), and from higher-density distributions of BTSs by small cell networks and SONs ​​[30, 46]. However, all of the above techniques still heavily rely on spectral or time-sharing techniques that enable multi-user transmission, as the spectral efficiency gains obtained by spatial processing are limited.

[0069] Prior art intercell methods (e.g., network MIMO and CoMP systems [53-64]) can improve the reliability of cellular networks by eliminating intercell interference, but their capacity gain is minimal. In fact, these systems are only effective in eliminating intercell interference due to power leakage between cells by restricting the power transmitted from any BTS that falls within the range of a cell boundary. Figure 2 shows an example of a cellular network with three BTSs 210-212, each characterized by its own coverage region or cell. The power transmitted from each BTS 210-212 is restricted to limit the amount of intercell interference represented in Figure 2 by the region where the cells overlap. Since these systems operate in a low SINR regime in the interference region, their spectral efficiency gain is minimal, similar to intracell schemes for SU-MIMO. To obtain truly significant capacity gain in intercell cooperative networks, the power limitations that are restricted to cell boundaries must be relaxed. Furthermore, spatial multiplexing technology must be enabled throughout the entire cell, not just at the cell edges where SINR performance is poor, as in conventional approaches.

[0070] Therefore, it is desirable to provide a system that eliminates all power constraints transmitted from distributed BTSs and achieves a large-scale increase in spectral efficiency by utilizing inter-cell multiplexing gain through spatial processing. Figure 3 shows that the power transmitted simultaneously at the same frequency from all three BTSs 301-303 is amplified, thereby enabling 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 BTS, but this interference is actually utilized in embodiments of the present invention by a novel inter-cell multiplexing method. This method uses spatial processing to generate a region of coherent interference (enhancing UE signal reception) around every UE, thereby simultaneously providing every UE with a non-interfering data stream and increasing their SINR through the cell.

[0071] In exemplary embodiments of the present invention, this inter-cell multiplexing gain is realized by a distributed input distributed output (DIDO) system [0014-0020] and [77-78]. Figure 4 shows an example in which one many additional access points 401 are added to deliberately increase the level of incoherent interference throughout the cell. It is utilized in the present invention to generate a region of coherent interference around the UE to give inter-cell multiplexing gain. Their added BTS can be low-power transceivers similar to inexpensive Wi-Fi access points, thereby providing a smaller region 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 deliberately limiting the transmit power from all BTS 210-212 as shown in Figure 2, and removes the remaining inter-cell interference (relating to the overlap region between cells) through spatial processing, thereby providing improved SINR and inter-cell diversity gain. On the other hand, the present invention utilizes incoherent interference by transmitting higher power from all BTS to generate coherent interference around the UE. This improves the signal quality at the UE, which is a necessary condition for obtaining inter-cell multiplexing gain across cells through spatial processing. Therefore, since there is insufficient signal quality through the cells (due to the limited transmit power from the BTS) to enable an inter-cell multiplexing method such as the present invention, the systems described in the prior art cannot be used to achieve inter-cell multiplexing gain through spatial processing. Furthermore, considering that the systems described in the prior art avoid inter-cell interference in the diversity region shown in the shaded areas of Figures 1-4, rather than utilizing inter-cell interference in the multiplexing region to obtain the inter-cell multiplexing gain realized in the present invention, it is impossible to implement the prior art systems in a way that achieves the multiplexing gain realized in the present invention as shown in Figures 3-4.

[0073] Embodiments of the present invention include systems and methods for utilizing inter-cell multiplexing gain through spatial processing of a wireless communication network, using a multiple-antenna system (MAS) with multiple-user (MU) transmission (a multiple-user multiple-antenna system, i.e., "MU-MAS"). In one embodiment of the present invention, the power transmitted from multiple antennas is constrained to minimize interference at cell boundaries (as in a conventional cellular system), and the spatial processing method is used solely to eliminate inter-cell interference. In another embodiment of the present invention, the power transmitted from multiple antennas is not constrained to any particular power level (as long as their power emission levels fall within regulatory or safety limits). Thereafter, higher-order inter-cell interference is deliberately generated through the cells and used to achieve inter-cell multiplexing gain and increase the capacity of the wireless communication network.

[0074] In one embodiment, the wireless communication network is a cellular network, such as a cellular network based on the LTE standard, as shown in Figures 1-2. In another embodiment of the present invention, the wireless communication network is not constrained to any particular cell layout, and cell boundaries can extend across a larger area, as shown in Figures 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 scheme, or a DIDO system with randomly placed access points without any transmit power limitations. However, such examples of network structures 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 achieved by transmitting signals from multiple antennas. These signals interfere where they are received by multiple UEs, generating simultaneous non-interfering data streams to the multiple UEs.

[0075] As illustrated in Figure 9, one embodiment of MU-MAS comprises a centralized processor 901, a base station network (BSN) 902, and M transmitting / receiving base stations (BTS) 903 that communicate wirelessly with N client devices. The client devices are also called user equipment UEs (described as UEs 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) on a network 900 (e.g., the Internet) intended for various client devices UEs 1-4 with respect to various network content C1-5. Hereafter, we use the term “stream of information” to refer to any stream of data transmitted on the network 900. It contains information that can be demodulated or decoded like a standalone stream according to a specific 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 set of bits carrying network content that can be demodulated or decoded like a standalone stream.

[0076] The centralized processor 901 utilizes a precoding transform that combines N streams of information from network content and converts them into a stream of M bits (by algorithms such as those described in the relevant patents and applications). 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 techniques or trellis precoding [73-74], vector perturbation techniques [75-76]). Hereafter, the term “stream of bits” is used to refer to any sequence of bits that does not necessarily contain any useful bits of information, and therefore cannot be decoded or demodulated as an independent stream for reading 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 transmitting and receiving base stations.

[0077] In one embodiment, the MAS is a distributed input distributed output (DIDO) system as described in the relevant patents and applications. In this embodiment, the DIDO system consists of the following: User equipment (UE) 1-4: RF transceivers for fixed or mobile clients that receive data streams on the downlink (DL) channel from the DIDO backhaul and transmit data to the DIDO backhaul via the uplink (UL) channel. • Transceiver / Receiver Base Station (BTS) 903: The BTS connects the DIDO backhaul to the radio 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 equipped with a power amplifier / antenna, and the RF signal is carried to the BTS by RF overfiber technology as described in the relevant patents and applications. • Controller (CTR) 905: The CTR 905 is a specific type of BTS designed for the following specific special functions: transmitting training signals for time / frequency synchronization of the BTS and / or UE, receiving and transmitting control information from the UE, and receiving channel status 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 those stations can be combined to increase diversity and improve link quality. In one embodiment, CSI is received from multiple CTRs by Maximum Ratio Combination (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 techniques (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 DIDO baseband processing and transmits waveforms to distributed BTSs via DL transmission. • Base Station Network (BSN) 902: A BSN is a network connecting a CP to a distributed BTS that carries information on either DL or UL channels. A BSN can be a wired or wireless network, or a combination of both. For example, a BSN can be a DSL, cable, fiber optic network, or a line-of-sight (LOS) or non-line-of-sight (NLO) wireless link. Furthermore, a BSN can be a unique network, a local area network, or the internet.

[0078] Below, we describe how the aforementioned DIDO system framework can be incorporated into LTE standards for cellular systems (and furthermore, non-cellular systems utilizing the LTE protocol) to achieve additional gains in spectral efficiency. We begin with a general overview of the LTE framework and the modulation schemes used in DL and UL channels. Next, we provide a brief explanation of the physical layer frame structure and resource allocation in the LTE standard. Finally, we clearly describe the DIDO precoding methods for downlink (DL) and uplink (UL) channels in multi-user scenarios using the LTE framework. For DL ​​schemes, we propose two solutions: open-loop and closed-loop DIDO schemes.

[0079] LTE is designed with a flat network architecture (in contrast to the layered 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 the LTE network, as shown in Figure 5, are as follows

[79] . • Gateways 501-502 (GWs): These are routers that connect the LTE network to the external network (i.e., the Internet). The GWs are divided into Serving Gateways (S-GWs) 502, which form the boundary of the E-UTRAN interface, and PDN Gateways (P-GWs) 501, which are interfaces with the external network. S-GWs 502 and P-GWs 501 are part of the so-called Evolutionary Packet Core (EPC). • MME (Mobility Management Entity) 503: Manages mobility, protection parameters, and UE identity. MME 503 is also part of LTE EPC. • eNodeB (Enhanced Node-B) 504: A base station that handles wireless resource management, user mobility, and scheduling. UE (User Equipment) 505: This is a mobile station.

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

[0081] As shown in Figures 6A to 6C, an LTE frame has a duration of 10 milliseconds and consists of 10 subframes [33, 80]. Each subframe is divided into two slots, each with a duration of 0.5 milliseconds. The LTE standard defines two types of frames: i) Type 1 for FDD operation, 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 DL and some to 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) Downlink Pilot Time Slot (DwPTS) reserved for DL ​​transmission; ii) Guard Period (GP); iii) Uplink Pilot Time Slot (UpPTS) for UL transmission.

[0082] LTE uses orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiplexing 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-millisecond 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 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 at different frequencies. Therefore, DL channel status information (CSI) must be estimated on the UE side and reported back to the CP via the UL channel through the BTS or CTR. In TDD systems, the DL and UL channels are set to the same frequency, and the system may use either closed-loop or open-loop techniques, taking advantage of channel reciprocity (as described in the following sections). The main disadvantage of closed-loop techniques is that they require feedback, which introduces greater overhead for control information on the UL.

[0084] One embodiment of the closed-loop mechanism in the DIDO system is as follows: i) BTS 903 transmits signaling information to UE via DL. ii) UE uses this signaling information to estimate DL channel state information (CSI) from all "active BTS". iii) UE quantizes the DL CSI or uses a codebook to select precoding weights to use for the next transmission. iv) UE transmits the quantized CSI or codebook index to BTS 903 or CTR 905 via the UL channel. v) BTS 903 or CTR 905 reports the CSI information or codebook index to CP 901, and CP 901 calculates precoding weights for data transmission on DL. "Active BTS" is defined as a set of BTS reachable by a given UE. For example, in related concurrently pending U.S. Patent Application No. 12 / 802,974, entitled "System And Method For Managing Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDO Clusters," and related concurrently pending U.S. Patent Application No. 12 / 917,257, entitled "Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering," a "user cluster" is defined as a set of BTSs reachable by a given UE. The number of active BTSs is limited to user clusters to reduce the amount of CSI estimated from the BTSs to a given UE, thereby reducing feedback overhead on the UL and the complexity of 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 status information (CSI-RS) and demodulated RS (DM-RS). Cell-specific RS are not precoded, but UE-specific RS are

[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 schemes, using up to eight transmit antennas and CoMP schemes, where antennas are distributed across different cells. Thus, Release 10 enables flexible signaling schemes with CSI-RS. In this invention, we describe how each type of signaling scheme can be used in the DIDO system to enable precoding.

[0086] 1.1.1 DIDO signaling using CRS CRS is used in LTE (Release 8) systems to estimate the CSI from all transmitting antennas in the UE to the BTS [80, 84]. CRS is obtained as the product of a two-dimensional orthogonal sequence and a two-dimensional pseudorandom (PRN) sequence. There are three orthogonal and 170 possible PRN sequences for a total of 510 different CRS sequences. Each sequence uniquely identifies one cell. CRS is transmitted in the first and third to last OFDM symbols of every slot, as well as in every sixth subcarrier. The orthogonal patterns of time and frequency are designed for every transmitting antenna in the BTS so that the UE can uniquely estimate the CSI from each of the four antennas. This high-density CRS in time and frequency (i.e., transmitted in every 0.5 millisecond slot and every sixth subcarrier) generates a 5% overhead and is intentionally designed to support scenarios with fast channel variations in time and frequency

[83] .

[0087] In a real-world DIDO system, every UE may allow more than four BTSs within a cluster for that user. For example, Figure 8 shows the SNR distribution for a real-world 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 center of the circles). In sparsely populated areas, an UE might allow only two or three BTSs within its user cluster (for example, only three BTS in the example in Figure 8), while in densely populated areas, each user cluster could contain as many as 26 BTS, as shown in Figure 8.

[0088] The high redundancy of CRS can be utilized in a DIDO system to enable CSI estimation from any number of transmitting antennas greater than four. For example, if a channel is fixed radio or characterized by a low Doppler effect, it is not necessary to calculate the CSI from all four transmitting antennas every 0.5 milliseconds (slot duration). Similarly, if a channel is frequency flat, estimating the CSI for each sixth subcarrier is redundant. In this case, the resource elements (REs) occupied by redundant CRSs can be reallocated to another transmitting antenna or BTS in the DIDO system. In one embodiment of the present invention, the system allocates the resource elements of redundant CRSs to an extra antenna or BTS 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 BTS within 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 any UE to estimate the CSI from a BTS [33, 83]. The standard defines orthogonal CSI-RS for different transmitters in a BTS, allowing UEs to distinguish CSIs from different BTSs. Up to eight transmitting antennas in a BTS are supported by CSI-RS as shown in Table 6.10.5.2-1, 2 of

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

[33] . The periodicity of the LTE-Advanced CSI-RS is deliberately designed to be larger than that of the LTE CRS, especially for older LTE terminals that cannot use these extra resources, in order to avoid excessive overhead of control information. Another reference signal used for CSI estimation is demodulated RS (DM-RS). DM-RS is a demodulation reference signal intended for a specific UE, and is simply transmitted within a resource block 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, alternative techniques must be used to enable DIDO precoding while maintaining system compliance with the LTE-Advanced standard. In one embodiment of the invention, any UE uses CSI-RS or DM-RS or a combination thereof to estimate the CSI from all active BTS in its own user cluster. In the same embodiment, the DIDO system detects the number of BTS in the user cluster and whether the user cluster complies with the LTE-Advanced standard (supporting up to eight antennas). If it does not comply, the DIDO system uses alternative techniques to enable DL signaling from the BTS to the current UE. In one embodiment, the transmit power from the BTS is reduced until up to eight BTS are reachable by the UEs in that user cluster. However, this solution may result in a reduction in data rate because coverage is reduced.

[0091] Another solution is to divide the BTS within the user cluster into subsets and send a set of CSI-RS for all subsets simultaneously. For example, if the periodicity of the CSI-RS is five subframes (i.e., 5 milliseconds) as shown in Table 6.10.5.3-1 of

[33] , then every 5 milliseconds, the CSI-RS will be sent from a new subset of the BTS. 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 milliseconds and the channel coherence time is 100 milliseconds, it is possible to define up to 20 subsets of 8 BTS each, resulting in 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 how many BTS can be supported within the user cluster to avoid degradation due to channel fluctuations and Doppler effects with respect to a given CSI-RS periodicity.

[0092] All proposed solutions for CSI-RS so far comply with the LTE standard and can be deployed within the framework of conventional LTE systems. For example, a proposed method enabling more than eight antennas per user cluster requires no modification to the UE LTE hardware and software implementation, and only minor modifications to the protocols used in the BTS and CP to allow selection of BTS subsets 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 the LTE UE to support LTE-like but non-LTE-compliant DIDO operating modes, the UE can 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 BTS in the system. In another embodiment of the invention, different CSI-RS patterns and periodicities are possible as means of increasing the number of supported BTS per user cluster. Such minor modifications to the LTE standard may be small enough that existing LTE UE chipsets could be used simply with software updates. Or, even if hardware modifications are required for the chipset, those changes would likely be small.

[0093] 1.2 Uplink DIDO CSI Feedback Method within the LTE Standard In the LTE and LTE-Advanced standards, the UE feeds information back to the BTS to communicate its current channel status and precoding weights for closed-loop transmissions on DL channels. Three different channel indices are included in those 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 transmitting antennas. • Precoding Matrix Index (PMI): An index of the codebooks used for precoding on DL channels. • Channel Quality Index (CQI): Defines a forward error correction (FEC) coding scheme to maintain the predefined error rate performance for the modulation used on the DL and for given channel conditions.

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

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

[0096] 2. Downlink Open Loop DIDO in LTE The DIDO open-loop scheme can only be used in time-division duplex (TDD) systems that utilize channel interoperability. One embodiment of the mechanism for the open-loop scheme in a DIDO system is as follows: i) UEs 1-4 transmit signaling information to BTS 903 or CTR 905 on the UL. ii) BTS 903 or CTR 905 uses this signaling information to estimate the UL CSI from all UEs 1-4. iii) BTS 903 or CTR 905 uses RF correction to convert the UL CSI to DL CSI. iv) BTS 903 or CTR 905 transmits the DL CSI or codebook index to CP via BSN 902. v) Based on the DL CSI, 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 estimated from the UE in the BTS, thereby reducing the computational burden in the BTS and the amount of signaling required on the UL. In one embodiment of the present invention, the open-loop precoding technique is used to transmit a simultaneous non-interfering data stream from the BTS to the UE over the DL channel.

[0097] LTE has two types of reference signals with respect to uplink channels [31, 33, 87]: i) Voice Reference Symbol (SRS), used for scheduling and link fitting; ii) Demodulation Reference Signal (DMRS), used for data reception. In one embodiment of the present invention, the SRS or DMRS is used in an open-loop DIDO system to estimate the UL channel from all UEs to all BTSs. In the time domain, the DMRS is transmitted in the fourth OFDM symbol (usually using a cyclic prefix) of all LTE slots (with a duration of 0.5 milliseconds). In the frequency domain, the DMRS transmitted over the PUSCH is mapped for all UEs to the same resource block (RB) used by that UE for UL data transmission.

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

[88] or computer-generated constant amplitude zero autocorrelation (CG-CAZAC) sequences by cyclic shifting the base sequence. The base sequence is divided into 30 groups, and neighboring LTE cells select DMRS from different groups to reduce intercell 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 SRS or DMRS that can be used in UL. In one exemplary embodiment, the virtual cell is a coherence area around a UE (as described in the related concurrently pending U.S. Patent Application No. 13 / 232,996 entitled “Systems and Methods to Exploit Areas of Coherence in Wireless Systems”), and the DIDO system generates up to 3300 coherence areas with respect to different UEs. In another embodiment of the present invention, each of 30 basic sequences is assigned to a different DIDO cluster to reduce inter-cluster interference between adjacent DIDO clusters (clusters are defined in the 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 SRS or DMRS are assigned according to a specific frequency hopping pattern to take advantage of the frequency diversity of the channel.

[0100] If, for all UEs, there are not enough orthogonal SRS or DMRS to be provided simultaneously in DL by DIDO precoding, one alternative is to multiplex the SRS or DMRS of different UEs in the time domain. For example, UEs are divided into different groups, and the SRS or DMRS for those groups are transmitted on consecutive time slots (each with a duration of 0.5 milliseconds). However, in this case, it is necessary to ensure that the periodicity of the SRS or DMRS allocation for different groups (multiplexed communications) is lower than the channel coherence time of the fastest moving UE. In fact, this is a requirement to ensure that the channel does not change for all UEs from the time the CSI estimates by the SRS or DMRS until the time 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 UE groups and the periodicity of the SRS or DMRS time-division multiplexing based on that information.

[0101] 3. Uplink DIDO technology in LTE Embodiments of the present invention use an open-loop MU-MIMO scheme on a 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 transmit signaling information and data payloads to all BTS 903. ii) BTS 903 uses the signaling information to calculate channel estimations from all UEs. iii) BTS 903 transmits the channel estimations and data payloads to CP 901. iv) CP 901 demodulates the data streams from all UEs, using the channel estimations to remove inter-channel interference from the data payloads of all UEs by spatial filtering. In one embodiment, the open-loop MU-MIMO system uses single-carrier frequency-division multiplexing (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, either by direct wiring to the same clock or by sharing a common time / frequency reference via GPSDO, with all BTS 903s considered locked to the same time / frequency reference clock. Variations in channel delay across different UEs can generate jitter between the time references of different UEs, potentially affecting the performance of the MU-MIMO method on the UL. In one embodiment, to reduce relative propagation delays across different UEs, only UEs in the same DIDO cluster (e.g., adjacent UEs) are processed by the MU-MIMO method. In another embodiment, relative propagation delays between UEs are compensated in the UEs or in the BTS, ensuring simultaneous reception of data payloads from different UEs 1-4 in the BTS 903.

[0103] The techniques for enabling signaling information regarding data demodulation on the UL may be the same methods used for signaling in the downlink open-loop DIDO scheme described in a previous section. CP 901 may use different spatial processing techniques to remove inter-channel interference from the UE data payload. In one embodiment, CP 901 uses a nonlinear spatial processing method such as a maximum likelihood (ML), decision feedback equalization (DFE), or successive interference rejection (SIC) receiver. In another embodiment, CP 901 demodulates the uplink data streams individually using a linear filter such as a zero-forcing (ZF) or least mean squares error (MMSE) receiver to cancel out same-channel interference.

[0104] 4. Integration with existing LTE networks In the United States and other parts of the world, LTE networks are already operational, in the process of being deployed, and / or are scheduled to be deployed. It would be of significant benefit to LTE operators if they could gradually deploy DIDO capabilities into their existing or already-promised deployments. In this way, they could deploy DIDO in the areas where it would provide the most immediate benefits, and gradually expand DIDO capabilities to span more networks. Eventually, once they have sufficient DIDO coverage in their areas, they could choose to completely phase out the use of cells and switch entirely to DIDO instead, achieving much higher spectrum density at a very low cost. Through this complete transition from cellular to DIDO, LTE operators' wireless customers would never suffer any loss of service. Rather, they would simply experience improvements in their data efficiency and reliability, while operators would experience cost reductions.

[0105] There are several embodiments that would enable the gradual integration of DIDO into existing LTE networks. In all cases, the DIDO BTS will be called a DIDO-LTE BTS, and it 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 minor modification of the LTE standard, as described above, and the UE will either be updated (for example, if a software update is sufficient to modify the UE to be compatible with DIDO), or a new generation of DIDO-compatible UE will be deployed. In either case, the new BTS that supports DIDO, whether within the constraints of the LTE standard or as a modification 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 conventional LTE BTS in a cellular configuration on one block of spectrum and DIDO LTE BTS on another block of spectrum, either by allocating new bandwidth for LTE-DIDO BTS or by subdividing the existing LTE spectrum (e.g., 20 MHz can be subdivided into two 10 MHz blocks). In effect, this would establish two separate LTE networks, and UE devices would be configured to use one or the other network, or to choose between the two. In the case of a subdivided spectrum, the spectrum can be divided uniformly or non-uniformly between the conventional LTE network and the DIDO-LTE network, and more spectrum can be allocated to the network that is most readily available, taking into account the degree of deployment of cellular LTE BTS and DIDO-LTE BTS, and / or patterns of UE usage. This subdivision can change as needed over time, and at some point, when there are enough DIDO-LTE BTS deployed to provide the same or better coverage as cellular BTS, the entire spectrum can be allocated to DIDO-LTE BTS and the cellular BTS can be decommissioned.

[0107] In another embodiment, conventional cellular LTE BTS can be configured to cooperate with DIDO-LTE BTS, sharing the same spectrum but alternating its use. For example, if they share spectrum usage equally, each BTS network would alternately utilize one 10ms frame time, for example, one 10ms frame for a cellular LTE BTS followed by one 10ms frame for a DIDO-LTE BTS. The frame time can also be subdivided into non-uniform intervals. This segmentation can change as needed over time, and when there are enough DIDO-LTE BTS deployed to provide the same or better coverage as the cellular BTS, all the time can be allocated to the DIDO-LTE BTS and the cellular BTS can be decommissioned.

[0108] In another embodiment of the present invention, DIDO is used as a radio backhaul for LOS or NLO to small cells in LTE and LTE-Advanced networks. As small cells are deployed in the LTE network, DIDO provides high-speed radio 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 radio network cannot add any more small cells to a given area without causing inter-cell interference. In the same embodiment of the present invention, DIDO BTS is used to gradually replace small cells, thereby leveraging inter-cell interference to provide increased network capacity.

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Claims

1. A multiple antenna system (MAS) with multiple user (MU) transmission ("MU-MAS") utilizes inter-cell interference to achieve multiplexing gain through spatial processing, thereby increasing the capacity of the wireless communication network.

2. The system according to claim 1, wherein the power transmitted from the plurality of antennas is constrained to minimize interference at the cell boundaries, and a spatial processing method is used to eliminate inter-cell interference.

3. The system according to claim 1, wherein the power emitted from the plurality of antennas is not constrained to any particular power level so that inter-cell interference is intentionally generated throughout the cells and used to increase the capacity of the wireless communication network.

4. The system according to claim 1, wherein the wireless communication network is a cellular network such as an LTE network.

5. The system according to claim 1, wherein the wireless communication network is a distributed antenna system having randomly placed access points without any restrictions on transmitted power (as long as the transmitted power emission satisfies FCC regulations).

6. The system according to claim 4, wherein Dido-UE is LTE UE, Dido-BTS is LTE eNodeB, Dido-CTR is LTE eNodeB or MME, and Dido-CP is LTE GW.

7. The system according to claim 4, wherein a closed-loop precoding method is used to transmit a simultaneous non-interfering data stream from the BTS to the UE over a downlink (DL) channel.

8. The system according to claim 7, wherein all UEs use cell-specific reference signals (CRS) to estimate channel state information (CSI) from all BTSs or only from BTSs in its own user cluster.

9. The system according to claim 8, wherein the system estimates the time and frequency selectivity of the channel and dynamically reallocates CRS for different BTSs to different resource elements.

10. The system according to claim 7, wherein every UE uses a CSI reference signal (CSI-RS) or a demodulated reference signal (DM-RS) or a combination of both to estimate the CSI from all BTS or from only the BTS in its own user cluster.

11. The system according to claim 10, wherein the transmission power from the BTS is reduced so that the number of BTS in the user cluster is less than the maximum number of antennas supported by the LTE standard CSI-RS scheme (i.e., 8).

12. The system according to claim 10, wherein the BTS in the user cluster is divided into subsets of eight antennas, and the CSI-RS is transmitted simultaneously from one subset according to a predetermined periodicity.

13. The system according to claim 12, wherein the periodicity of the CSI-RS with respect to different subsets is determined based on the channel coherence time of the UE and periodicity values ​​supported by the LTE standard.

14. The system according to claim 10, wherein different patterns and periodicities from the LTE standard are possible for the CSI-RS in order to enable a larger number of BTS in the system.

15. The system according to claim 7, wherein the UE reports RI, PMI and CQI to the CP via PUCCH.

16. The system according to claim 7, wherein the UE reports RI, PMI and CQI to the CP via PUSCH.

17. The system according to claim 16, wherein the system estimates the frequency selectivity of a channel and dynamically adjusts the PMI to support a further number of BTSs with respect to the same available uplink (UL) resources.

18. The system according to claim 4, wherein an open-loop precoding method is used to transmit a simultaneous non-interfering data stream from the BTS to the UE over the DL channel.

19. The system according to claim 4, wherein the open-loop MU-MIMO method is used to receive a simultaneous non-interfering data stream from the UE to the BTS on the UL channel.

20. The system according to claim 18, wherein SRS or DMRS is used to estimate the channel impulse response from all UEs to the BTS.

21. The system according to claim 20, wherein different SRS or DMRS are assigned to different coherence areas around the UE.

22. The system according to claim 20, wherein different SRS or DMRS are assigned to different DIDO clusters in order to reduce inter-cluster interference.

23. The system according to claim 20, wherein the SRS or DMRS is assigned based on a frequency hopping pattern in order to take advantage of the frequency diversity of the channel.

24. The system according to claim 20, wherein the active UEs are divided into groups such that the same set of SRS or DMRS is assigned to each group on a continuous time slot.

25. The system according to claim 24, wherein the shortest channel coherence time for all active UEs is estimated, and based on that information, the maximum number of UE groups and the periodicity of the time-division multiplexing scheme of the SRS or DMRS are calculated.

26. The system according to claim 19, wherein the synchronization of time and frequency between UEs is achieved by utilizing DL signaling information.

27. The system according to claim 26, wherein the BTS is synchronized to the same reference clock by direct wiring to the same physical clock or by sharing a common time and frequency reference via GPSDO.

28. The system according to claim 26, wherein relative propagation delays between UEs are avoided by processing only UEs within the same DIDO cluster using UL MU-MIMO, thereby ensuring time synchronization of UEs.

29. The system according to claim 26, wherein relative propagation delays between UEs are compensated on the UE side before UL transmission to ensure time synchronization of the UEs at the UL MU-MIMO receiver.

30. The system according to claim 19, wherein a nonlinear spatial filter, such as a maximum likelihood (ML), decision feedback equalization (DFE), or successive interference rejection (SIC) receiver, is used to remove interference between data streams of the UE.

31. The system according to claim 19, wherein a linear spatial filter, such as a zero-forcing (ZF) or least mean squares error (MMSE) receiver, is used to remove interference between data streams of the UE.

32. The system according to claim 19, wherein SC-FMDA is used to multiplex the UE in the frequency domain.

33. The system according to claim 4, wherein the DIDO technology is gradually integrated into an existing LTE network.

34. The system according to claim 33, wherein the DIDO BTS and UE are compatible with LTE.

35. The system according to claim 33, wherein the aforementioned DIDO BTS and UE utilize a modification of the LTE standard.

36. The system according to claim 35, wherein the LTE UE is updated to be compatible with DIDO.

37. The system according to claim 35, wherein a new generation of UE compatible with Dido is deployed.

38. The system according to claim 33, wherein the LTE spectrum is subdivided so as to support a conventional LTE BTS in a cellular configuration with one spectral block and a DIDO LTE BTS with another spectral block.

39. The system according to claim 33, wherein a conventional cellular LTE BTS is configured to cooperate with a DIDO-LTE BTS to operate according to a time-division multiple access (TDMA) scheme while sharing the same spectrum.

40. The system according to claim 33, wherein the DIDO is used as a wireless backhaul for LOS or NLOS to an LTE small cell.

41. The system according to claim 33, wherein LTE small cells are gradually replaced by DIDO BTS.

42. The system according to claim 18, wherein RF correction is used to convert UL CSI to DL CSI, thereby taking advantage of the interoperability of UL / DL channels.

43. The system according to claim 19, wherein SRS or DMRS is used to estimate the channel impulse response from all UEs to the BTS.

44. A method implemented within a Multiple Antenna System (MAS) having multiple user (MU) transmissions ("MU-MAS"), A method comprising the step of increasing the capacity of a wireless communication network by utilizing inter-cell interference to achieve multiplexing gain through spatial processing.

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