System and method for radio frequency calibration utilizing channel reciprocity in distributed input distributed output wireless communications - Patents.com

Abstract

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

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application may be related to the following co-pending U.S. patent applications:

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

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

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

[0005] U.S. patent application Ser. No. 13 / 633,702, entitled "Systems and Methods for wireless backhaul in distributed-input distributed-output wireless systems."

[0006] U.S. patent application Ser. No. 13 / 475,598, entitled "Systems and Methods to enhance spatial diversity in distributed-input distributed-output wireless systems."

[0007] U.S. Patent Application No. 13 / 233,006, entitled "System and Methods for Planned Evolution and Obsolescence of Multiuser Spectrum."

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

[0009] U.S. patent application Ser. No. 13 / 464,648, entitled "System and Methods to Compensate for Doppler Effects in Distributed-Input Distributed Output Systems."

[0010] U.S. patent application Ser. No. 12 / 917,257, entitled "Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering."

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

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

[0013] 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."

[0014] 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."

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

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

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

[0018] 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."

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

[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."

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

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

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

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

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

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

[0027] Long Term Evolution (LTE) is a standard developed by the Third Generation Partnership Project (3GPP) for fourth-generation (4G) wireless cellular systems. LTE can theoretically achieve up to a four-fold improvement in downlink spectral efficiency over the previous 3G and HSPA+ standards by exploiting the spatial components of the wireless channel with multiple-input multiple-output (MIMO) technology. LTE-Advanced, an evolution of LTE that is currently being standardized, will theoretically enable up to an eight-fold increase in spectral efficiency over 3G standard systems.

[0028] Despite this technological evolution, there is a strong possibility that in the next three years wireless carriers will not be able to meet the growing demand for data rates due to the increasing market penetration of smartphones and tablets offering more data-hungry applications such as real-time HD video streaming, videoconferencing, and gaming. Wireless network capacity is estimated to expand five-fold in Europe from 2011 to 2015, due to improved technologies such as LTE, as well as additional spectrum made available by governments.

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

[24] . Unfortunately, the forecast for capacity usage by 2015 is 23 times higher than in 2011 in Europe

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

[28]

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

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

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

[0030] One promising technology that offers a large increase in spectral efficiency over wireless links without the constraints of traditional cellular systems is the Distributed Input Distributed Output (DIDO) technology (see related patents and applications cited in [0002-0020] above). To provide significant performance benefits over traditional wireless systems, the present invention describes the DIDO technology used in conjunction with cellular systems (such as LTE or LTE-Advanced), both within and outside the constraints of cellular standards. Starting with an overview of MIMO, we review the various spatial processing techniques used by LTE and LTE-Advanced. We then show how the present invention offers significant capacity gains for next generation wireless communication systems compared to traditional approaches.

[0031] MIMO uses multiple antennas at the transmitter and receiver of a wireless link to improve link reliability through diversity techniques (i.e., diversity gain) or to use spatial processing to provide higher data rates through multiplexing (i.e., multiplexing gain) [1-2]. Diversity gain is a measure of enhanced robustness against signal fading, resulting in a higher signal-to-noise ratio (SNR) for a fixed data rate. Multiplexing gain is obtained by exploiting the additional spatial degrees of freedom of the wireless channel to increase the data rate at a fixed error probability. The basic tradeoff between diversity and multiplexing in MIMO systems is described in [3-4].

[0032] In practical MIMO systems, link adaptation techniques can be used to dynamically switch between diversity and multiplexing schemes based on the propagation conditions [20-23]. For example, link adaptation methods described in [22-23] have shown that in low SNR regimes or channels characterized by low spatial selectivity, beamforming or orthogonal space-time block coding (OSTBC) are the preferred schemes. On the other hand, spatial multiplexing can provide significant gains in data rate for channels with high SNR and high spatial selectivity. For example, Figure 1 shows that a cell can be divided into two regions: i) the multiplexing region 101, characterized by high SNR (due to the proximity to the cell tower or base station), where spatial multiplexing can exploit the spatial freedom of the channel to increase the data rate, and ii) the diversity region 102 or cell edge, where spatial multiplexing techniques are not as effective and diversity methods can be used to improve the SNR and coverage (yielding 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" and the unshaded outer region as the "diversity region." This same region designation, where the shaded region is the "multiplexing region" and the unshaded region is the "diversity region," is used throughout Figures 1 and 3-5, even though not labeled. For example, the same designation is used for small cell 104 in Figure 1.

[0033] The LTE (Release 8) and LTE-Advanced (Release 10) standards define a set of 10 transmission modes (TM) 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) Summary of the Invention [Means for solving the problem]

[0034] In the following, diversity and multiplexing schemes commonly used in cellular systems and 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 (exploiting micro-diversity in cellular systems), which use multiple antennas to improve link reliability or data rate within a cell, and ii) inter-cell methods (exploiting macro-diversity), which allow cooperation between cells to provide additional diversity or multiplexing gain. Next, we describe how the present invention provides significant advantages over the prior art, including spectrum capacity gains.

[0035] 1. Intra-cell diversity method Intra-cell diversity methods operate within one cell and are intended to enhance the SNR in scenarios with poor link quality (e.g., cell-edge users experiencing high path loss from the central tower or base station). Typical diversity schemes used in MIMO communications are beamforming [5-11] and orthogonal space-time block coding (OSTBC) [12-15].

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

[31] . Rank-1 precoding generates a beam dedicated to one user based on quantized weights selected from a codebook (pre-designed using limited feedback techniques [36-42]) to reduce the feedback overhead from the user equipment (UE) to the base transceiver station (BTS 105 in Fig. 1, i.e., eNodeB in LTE terminology). Alternatively, the dedicated beamforming weights can be calculated based on a UE-specific reference signal.

[0037] 2. Intra-cell multiplexing method MIMO multiplexing schemes [1, 19] provide data rate gains in the high SNR regime and in scenarios with sufficient spatial freedom in the channel (e.g., rich multipath environments with high spatial selectivity [16–18]) to support multiple parallel data streams over a wireless link.

[0038] The LTE standard supports various multiplexing techniques for Single User MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO)

[31] . The SU-MIMO scheme has two modes of operation: i) closed loop, which utilizes feedback information from the UE to select DL precoding weights; and ii) open loop, which is used when feedback from the UE is not available or the UE is moving too fast to support the closed loop scheme. The closed loop scheme uses a set of precomputed weights selected from a codebook. These weights can support two or four transmit antennas and one to four parallel data streams (specified by the number of layers of the precoding matrix), depending on the UE requirements and the BTS scheduler's decision. LTE-Advanced can include new transmission modes up to MIMO 8x8 to provide an increase in spectral efficiency of up to eight times through spatial processing

[62] .

[0039] MU-MIMO schemes have been specified for both UL and DL channels [31, 50]. In the UL, every UE transmits a reference signal (consisting of a circularly shifted version of the Zadoff-Chu sequence

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

[0040] Intra-cell multiplexing techniques using spatial processing provide good performance only in propagation scenarios characterized by high SNR (or SINR) and high spatial selectivity (multipath-rich environments). For conventional macrocells, these conditions may be more difficult to realize since the BTS is usually far from the UE and the distribution of SINR is usually concentrated at low values ​​

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

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

[85] , thereby giving higher data rates. Intra-band CA combines different bands within the same part of the spectrum. It can therefore use the same RF chain for multiple channels, and multiple data streams are recombined in software. Inter-band CA requires different RF chains to operate in different parts of the spectrum, as well as signal processing to recombine multiple data streams from different bands.

[0042] The main idea of ​​small cells [30, 47] is to reduce the size of conventional macro cells, thereby enabling higher cell density and greater throughput per coverage area. Small cells are usually deployed through inexpensive access points 106 with low power transmission (as depicted in Fig. 1) in contrast to the tall and expensive cell towers used for macro cells. Two types of small cells are specified in LTE-Advanced: i) metro cells, for outdoor installations in urban areas, supporting 32-64 simultaneous users, and ii) femto cells, for indoor use, which can accommodate up to 4 active users. One advantage of small cells is that the density of UEs near the BTS is statistically higher, thereby giving a better SNR that can be exploited through spatial multiplexing to enhance data rates. However, there are still many concerns regarding the practical deployment of small cells, especially those related to the backhaul. Indeed, reaching every small cell BTS through a high-speed wired connection may be challenging, especially considering the high density of metro and femto cells in a given coverage area. Compared to wired backhaul, using line-of-sight (LOS) backhaul for small cells is often inexpensive to implement, but there are often no practical LOS backhaul paths available for the preferred small cell BTS placement, and no general solution for non-line-of-sight (NLO) wireless backhaul to the small cell BTS. Furthermore, 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 (more complex than traditional cellular systems due to the densification of small cells) to plan their optimal locations [48, 49]. Finally, handoff is a limiting factor for small cell deployments, especially in scenarios where groups of subscribers simultaneously switch cells, causing a large amount of handoff overhead over the backhaul, resulting in long waiting times and unavoidable missed calls.

[0043] It can be shown trivially that there is no practical general solution that allows small cells to coexist with macro cells and achieve optimal or necessarily even improved throughput. Among countless such unsolvable situations, there is a situation where the small cell is located such that its UE inevitably overlaps with the macro cell transmission, and the small cell and the macro cell use the same frequency to reach each UE. In this situation, obviously, the macro cell transmission will interfere with the small cell transmission. There may be some approaches to mitigate such interference for the specific situation of a specific macro cell, a specific small cell, the specific macro cell and small cell UEs involved, the throughput requirements of those UEs, and the environmental situation. However, any such approach is very specific not only for the static planning of the macro cell and the small cell, but also for the dynamic situation of a specific time interval. Usually, the full throughput of the channel to each UE cannot be achieved.

[0044] 3. Inter-cell diversity method In heterogeneous networks (HetNets)

[90] , where macrocells coexist with small cells (e.g., metro, pico, and femtocells), various techniques must be used to eliminate inter-cell interference. While HetNets provide better coverage through small cells, the gain in data rate is small because it requires sharing spectrum through different forms of frequency reuse patterns or using spatial processing to eliminate interference rather than obtaining multiplexing gains. The LTE standard uses Inter-Cell Interference Control (ICIC) methods to eliminate interference, especially at the cell edge. There are two types of ICIC methods: cell-autonomous and coordinated between BTSs.

[0045] The cell autonomous ICIC scheme avoids inter-cell interference through different frequency repetition patterns as shown in Figure 2. In Figure 2, the hexagons represent the cells and the colors represent the different carrier frequencies. Three schemes are considered for LTE: i) Full Frequency Repetition (or Repetition 1), where the cell uses the entire available bandwidth as shown in Figure 2a, generating high interference at the cell edges; ii) Hard Frequency Repetition (HFR), where every cell is assigned a different frequency band (typically with a repetition factor of 3) as shown in Figure 2b; and iii) Fractional Frequency Repetition (FFR), where the cell center is assigned the entire available bandwidth as frequency repetition 1, while the cell edges operate in HFR mode to mitigate inter-cell interference as shown in Figure 2c.

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

[53] . The system in

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

[0047] More recently, this class of cooperative wireless cellular networks has been defined in the MIMO literature as "Network MIMO" or "Cooperative Multipoint" (CoMP) systems. Theoretical analysis and simulated results on the benefits of Network MIMO by eliminating inter-cell interference are presented in [54-61]. The advantage of Network MIMO and CoMP is that it eliminates inter-cell interference in the cell overlap region, which for a macrocell 302 is labeled as the "interference region" 301 in Figure 3.

[0048] 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]. Three CoMP solutions have been proposed so far in the standard to eliminate inter-cell interference: i) Cooperative Scheduling / Beamforming (CS / CB), where a UE receives its data stream from only one BTS via beamforming and coordination across BTSs allows interference to be removed via beamforming or scheduling techniques; ii) Dynamic Cell Selection (DCS), which dynamically selects a cell for every UE on a subframe basis, transparent to the UE; and iii) Joint Transmission (JT), where data for a given UE is jointly transmitted from multiple BTSs to improve the received signal quality and eliminate inter-cell interference. CoMP-JT brings larger gains than CoMP-CS / CB at the expense of higher overhead in the backhaul to enable coordination among BTSs.

[0049] 4. Inter-cell multiplexing method Prior art multi-user wireless systems add complexity and introduce constraints to wireless networks, resulting in a situation where a given user's experience (e.g., available throughput, delay, predictability, reliability) is affected by the utilization of spectrum by other users in the area. Given the increasing demand for aggregate throughput in a wireless spectrum shared with multiple users, and the increasing growth of applications that can depend on the reliability, predictability, and low latency of a multi-user wireless network for a given user, it is clear that prior art multi-user wireless technologies suffer from many constraints. Indeed, with limited availability of spectrum suitable for certain types of wireless communication (e.g., at wavelengths effective for penetrating building walls), prior art wireless technologies will be inadequate to meet the increasing demand for reliable, predictable, and low latency bandwidth.

[0050] Prior art intra-cell diversity and multiplexing methods can only provide a theoretical maximum of four times increase in throughput over current cellular networks for LTE (with MIMO 4x4) and at most eight times for LTE-Advanced (with MIMO 8x8). It should be noted that for higher order MIMO, the improvement in throughput increase decreases in a given multipath environment, especially as UEs (such as smartphones) become smaller and more constrained in terms of antenna placement. Other slight throughput gains in next generation cellular systems may come from further spectrum allocations (e.g., FCC National Broadband Plan) exploited by carrier aggregation techniques, and from denser distribution of BTSs by small cell networks and SONs ​​[30, 46]. However, all the above mentioned techniques are still highly dependent on spectrum or time sharing techniques that enable multi-user transmissions, since the spectral efficiency gains obtained by spatial processing are limited.

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

[0052] 4 shows that the power transmitted simultaneously on the same frequency from all three BTSs 401 is increased, thereby allowing a high level of interference throughout the cell 402. In prior art systems, such interference would result in incoherent interference (impairing UE signal reception) throughout the interference region of the BTSs, but this interference is actually exploited in the present invention by a new inter-cell multiplexing method. This method uses spatial processing to create a region of coherent interference (enhancing UE signal reception) around every UE, thereby providing a simultaneous non-interfering data stream to every UE and increasing their SINR throughout the cell.

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

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

[89] consists of several BTSs that identify different cells grouped into clusters. Cooperation is only allowed between BTSs from adjacent cells in the same cluster. In this case, it was shown that there is a limit to the capacity (or spectral efficiency) that can be obtained by the inter-cell multiplexing method as the power transmitted from the BTS increases. In fact, as the transmit power increases, the out-of-cluster interference increases proportionally, generating a saturation regime for the SINR and, consequently, for the capacity. As a result of this effect, the system in

[89] can theoretically obtain up to a three-fold increase in capacity (i.e., up to three cells in a cluster), while any additional cells included in the cluster will reduce the capacity due to the increased out-of-cluster interference (e.g., 21 cells per cluster obtains a lower capacity than 3 cells per cluster). It was observed that the basic capacity limit in

[89] holds, since the BTSs are restricted to predefined locations as in the cellular system, and the multiplexing gain is obtained by increasing the transmit power from the BTSs. To obtain theoretically infinite capacity through the inter-cell multiplexing method, the constraints on BTS placement must be eliminated, which allows conveniently for the BTS to be placed anywhere.

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

[0055] It can be seen that the prior art inter-cell methods [53-64] avoid incoherent interference by purposely limiting the transmission power from every BTS as in Fig. 3 and eliminate the remaining inter-cell interference (for the overlapping regions between cells) by spatial processing, thereby providing improved SINR and inter-cell diversity gain. It is further observed that

[89] limits the obtained capacity due to out-cluster interference by restricting the BTS placement to a cell design while increasing the transmission power, thus still being interference limited. On the other hand, the present invention exploits the incoherent interference by transmitting higher power from every BTS that happens to be placed to generate coherent interference around the UE. This improves the signal quality at the UE, which is a prerequisite for obtaining inter-cell multiplexing gain over the whole cell by spatial processing. Therefore, the systems described in the prior art cannot be used to realize unlimited inter-cell multiplexing gain by spatial processing, since there is not enough SINR throughout the cell to enable an inter-cell multiplexing method like the present invention (due to the limited transmission power from the BTS or the out-cluster interference when the transmission power is increased). Furthermore, the systems described in the prior art cannot be implemented to achieve the multiplexing gains achieved in the present invention shown in Figures 4-5, taking into account that the prior art systems are designed to avoid inter-cell interference in the diversity regions shown in the shaded regions of Figures 1 and 3-5, rather than exploiting inter-cell interference in the multiplexing region to obtain the inter-cell multiplexing gains achieved in the present invention.

[0056] The present invention can be better understood from the following detailed description taken in conjunction with the drawings. [Brief description of the drawings]

[0057] [Figure 1] 1 illustrates multiplexing and diversity areas for a macro cell and a small cell. [Figure 2a] FIG. 1 illustrates a complete frequency reuse pattern in a conventional cellular system. [Figure 2b]FIG. 1 illustrates a hard frequency repetition (HFR) pattern in a conventional cellular system. [Figure 2c] FIG. 1 illustrates a fractional frequency reuse (FFR) pattern in a conventional cellular system. [Diagram 3] FIG. 2 is a diagram showing an interference region between adjacent macrocells. [Figure 4] FIG. 2 shows multiple BTSs transmitting at high power to increase the degree of interference between cells. [Diagram 5] FIG. 2 shows an example in which many access points are added to intentionally increase incoherent interference levels throughout a cell. [Figure 6] A diagram showing network elements in an LTE network. [Figure 7a] FIG. 1 illustrates the LTE frame structure for FDD operation. [Figure 7b] FIG. 1 illustrates the LTE frame structure for TDD operation. [Figure 8a] FIG. 2 illustrates LTE “resource elements” and “resource blocks” in an OFDM DL channel. [Figure 8b] FIG. 2 illustrates LTE "resource elements" and "resource blocks" in an SC-FDMA UL channel. [Figure 9] FIG. 1 illustrates an embodiment of a multi-user (MU) multiple antenna system (MAS), or MU-MAS, consisting of antenna clusters and user clusters. [Figure 10] FIG. 1 illustrates an embodiment of MU-MAS in which a different cell ID is associated with every antenna sub-cluster. [Figure 11] FIG. 1 illustrates an embodiment of MU-MAS in which the same set of cell IDs are assigned to antenna sub-clusters with a predetermined repeating pattern. [Figure 12] FIG. 1 shows SNR distributions for a practical deployment of a MU-MAS system in metropolitan San Francisco, California, which has sparsely populated and densely populated areas. [Figure 13] A diagram showing one embodiment of a MU-MAS consisting of a CP, a distributed BTS, and multiple UEs. [Figure 14] 1 illustrates an embodiment of a MU-MAS consisting of a CP, distributed BTSs, multiple devices, and one UE connected to the multiple devices and BTSs via a network interface. [Figure 15] 1 illustrates an embodiment of the MU-MAS in which the UE is in a case that is physically attached to the user equipment. [Figure 16] A diagram showing an embodiment of a MU-MAS in which distributed antennas communicate to a UE over an UL channel and a DL channel. [Figure 17] A diagram showing an embodiment of a MU-MAS in which distributed antennas communicate to a beacon over an UL channel and a DL channel. [Figure 18] FIG. 13 shows the bit error rate (SER) performance of MU-MAS with linear pre-coding with and without RF mismatch and with and without RF calibration. [Figure 19] FIG. 13 illustrates the bit error rate (SER) performance of MU-MAS with linear and nonlinear pre-coding with and without RF mismatch and with and without RF calibration. [Figure 20a] FIG. 1 illustrates a 4QAM constellation at a UE (before modulo operation) when THP nonlinear precoding is applied. [Figure 20b] FIG. 1 illustrates a 4QAM constellation at a UE (before modulo operation) when THP nonlinear precoding is applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] One solution that overcomes many of the limitations of the prior art discussed above is an embodiment of Distributed Input Distributed Output (DIDO) technology. DIDO technology is described in the following patents and patent applications, all of which are assigned to the assignee of this patent and are incorporated herein by reference. These patents and applications are referred to at various times herein in their entirety as "Related Patents and Applications."

[0059] U.S. patent application Ser. No. 13 / 633,702, entitled "Systems and Methods for wireless backhaul in distributed-input distributed-output wireless systems."

[0060] U.S. patent application Ser. No. 13 / 475,598, entitled "Systems and Methods to enhance spatial diversity in distributed-input distributed-output wireless systems."

[0061] U.S. Patent Application No. 13 / 233,006, entitled "System and Methods for Planned Evolution and Obsolescence of Multiuser Spectrum."

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

[0063] U.S. patent application Ser. No. 13 / 464,648, entitled "System and Methods to Compensate for Doppler Effects in Distributed-Input Distributed Output Systems."

[0064] U.S. patent application Ser. No. 12 / 917,257, entitled "Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering."

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

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

[0067] 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."

[0068] 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."

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

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

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

[0072] 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."

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

[0074] 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."

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

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

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

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

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

[0080] In order to reduce the volume and complexity of this patent application, the disclosures of some of the related patents and applications are not explicitly described below, and the reader is referred to the related patents and applications for a complete description of their disclosures.

[0081] The present invention describes a system and method for exploiting inter-cell multiplexing gains through spatial processing in wireless communication networks, using a multiple antenna system (MAS) with multiple-user (MU) transmissions where multiple antennas are coincidentally located (multi-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 (like in traditional 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 are within regulatory, safety or practical (e.g., available power, transmitter and / or antenna specifications) limits). Thereby, high order inter-cell interference is intentionally generated throughout the cells utilized to realize the inter-cell multiplexing gains, increasing the capacity of the wireless communication network.

[0082] In one embodiment, the wireless communication network is a cellular network as in Fig. 1 and Fig. 3, such as a cellular network based on the LTE standard, and the randomly deployed antennas are macrocell or small cell transceivers. In another embodiment of the present invention, the wireless communication network is not bound to any particular cell layout, and the cell boundaries can be spread over a larger area as in Fig. 4-5. For example, the wireless communication network can be a wireless local area network (WLAN) where the multiple antennas are Wi-Fi access points, or a mesh, ad-hoc or sensor network, or a distributed antenna system, or a DIDO system with randomly deployed access points without any transmit power restrictions. However, such network structure examples should not be considered as limiting the general application of the present invention to wireless communication networks. The present invention applies to any wireless network where multiplexing gain is realized by transmitting signals from multiple antennas. The signals interfere where they are received by multiple UEs, generating simultaneous non-interfering data streams to the multiple UEs.

[0083] The MU-MAS consists of a centralized processor, a network, and M transceiver stations (or distributed antennas) that communicate wirelessly to N client devices or UEs. The centralized processor unit receives N streams of information (e.g., videos, web pages, video games, text, audio, etc., streams from web servers or other network sources) for various client devices regarding various network content. In the following, we use the term "stream of information" to refer to any stream of data transmitted over the network. It includes information that can be demodulated or decoded as an independent stream according to a particular modulation / coding scheme or protocol to generate any data, including but not limited to 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.

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

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

[0085] Precoding is computed at a centralized processor using channel state information (CSI) and applied on the DL or UL channel to multiplex data streams to or from multiple users. In one embodiment of the present invention, the centralized processor knows the CSI between the distributed antennas and the client device and uses the CSI to precode data transmitted on the DL or UL channel. In the same embodiment, the CSI is estimated at the client device and returned to the distributed antennas. In another embodiment, the DL-CSI is derived at the distributed antennas from the UL-CSI using radio frequency (RF) calibration and by using UL / DL channel reciprocity.

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

[0087] In the following, we describe how the above-mentioned MU-MAS framework is incorporated into the LTE standard for cellular systems (as well as non-cellular systems utilizing the LTE protocol) to realize additional gains in spectral efficiency. We start with a general overview of the LTE framework and the modulation techniques used in DL and UL channels. Then, we provide a brief description of the physical layer frame structure and resource allocation in the LTE standard. Finally, we clearly state the MU-MAS precoding method for the downlink (DL) and uplink (UL) channels in a multi-user scenario using the LTE framework. For the DL scheme, we propose two solutions: an open-loop and a closed-loop DIDO scheme.

[0088] LTE is designed with a flat network architecture (as opposed to the hierarchical architecture of previous cellular standards) and offers: reduced delay, reduced packet loss through ARQ, reduced call setup time, and improved coverage and throughput through macro diversity. The network elements in the LTE network, depicted in Figure 6, are as in

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

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

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

[0091] LTE uses Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) modulation for DL ​​and Single Carrier Frequency Division Multiple Access (SC-FDMA) for UL. A "resource element" (RE) is the smallest modulation structure in LTE, consisting of one OFDM subcarrier in frequency and one OFDM symbol period in time, as shown in FIG. 8a for DL ​​channel and FIG. 8b for UL channel. A "resource block" (RB) consists of 12 subcarriers in frequency and one 0.5 ms slot in time (consisting of 3-7 OFDM symbol periods, depending on DL vs. UL channel and cyclic prefix type). Resource blocks for every UE are assigned on a subframe basis. Since the MU-MAS in the present invention uses spatial processing to transmit multiple data streams to different UEs, all resource blocks in every subframe can be assigned to the same UE. In one embodiment, all or a subset of resource blocks are assigned to every UE, and simultaneous non-interfering data streams are transmitted to the UE via precoding.

[0092] To set up a link between the BTS and the UE, the LTE standard specifies a synchronization procedure. The BTS transmits two consecutive signals to the UE: the Primary Synchronization Signal (P-SS) transmitted on the Primary Synchronization Channel (PSCH) and the Secondary Synchronization Signal (S-SS) transmitted on the Secondary Synchronization Channel (SSCH). Both signals are used by the UE for time / frequency synchronization as well as for obtaining the cell identity. The P-SS consists of a Zadoff-Chu sequence of length 63 from which the UE derives the physical layer identity (0-2). The S-SS is an interleaved concatenation of two length 31 binary sequences and is used to derive the cell identity group number (0-167). From these two identification numbers, the UE derives the physical cell identity (PCI, specified from 0-503).

[0093] In the MU-MAS system described in this invention, there are no cell boundaries because the power transmitted from the BTS is intentionally increased to generate interference that is used to generate an interference region around the UE. In this invention, the various BTSs are grouped into "antenna clusters" or "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 on May 1, 2012. For example, FIG. 9 shows a main antenna cluster 901 and one adjacent antenna cluster 902. Every antenna cluster consists of multiple BTSs 903.

[0094] Cell IDs can be used in MU-MAS and DIDO systems to distinguish antenna clusters. In one embodiment of the present invention, the same Cell ID is transmitted over P-SS and S-SS from all BTSs of the same antenna cluster. In the same embodiment, different antenna clusters use different Cell IDs. In another embodiment of the present invention, all BTSs in the same antenna cluster 1001 are grouped into "antenna sub-clusters" 1003 with different shaded colors as illustrated in FIG. 10, and a different Cell ID 1004 is associated with every antenna sub-cluster. In one embodiment, the antenna sub-clusters are defined statistically according to a predefined cluster design or GPS location information. In another embodiment, the antenna sub-clusters are defined dynamically based on relative signal strength measurements between BTSs or based on GPS location information. In a different embodiment of the present invention, a different Cell ID is assigned to every coherence area (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") associated with the UE.

[0095] When all BTSs in the same antenna cluster or antenna sub-cluster transmit LTE broadcast channels (e.g., P-SS and S-SS) to the UE, destructive interference may degrade the performance of the time or frequency synchronization enabled by the broadcast channel. Destructive interference may be caused by multipath generated from BTSs distributed with intervals and recombining non-coherently at some UE locations. To avoid or mitigate this effect, in one embodiment of the present invention, only one BTS out of all BTSs in the same antenna cluster or antenna sub-cluster transmits the LTE broadcast channel (e.g., P-SS and S-SS) to all UEs. In the same embodiment, the BTS transmitting the LTE broadcast channel is selected to maximize the power received at the UE on the broadcast channel. In another embodiment, only a limited set of BTSs is selected to simultaneously transmit the LTE broadcast channel to all UEs so that destructive interference is avoided at the UE. In a different embodiment of the present invention, the LTE broadcast channel is transmitted with a power higher than the payload to reach all UEs in the same antenna cluster or antenna sub-cluster.

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

[0097] 1. Downlink closed-loop MU-MAS precoding method in LTE The MU-MAS closed loop scheme can be used in either Time Division Duplex (TDD) or Frequency Division Duplex (FDD) systems. In FDD systems, the DL and UL channels operate on different frequencies. Therefore, DL channel state information (CSI) must be estimated at the UE side and reported back to the CP via the BTS or CTR by the UL channel. In TDD systems, the DL and UL channels are configured on the same frequency and the system may use either closed loop techniques or open loop schemes, taking advantage of channel reciprocity (as described in the following sections). The main disadvantage of closed loop schemes is that they require feedback, which results in a larger overhead for control information on the UL.

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

[0099] As described in paragraph

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

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

[0100] 1.1 Downlink MU-MAS signaling method within the LTE standard The LTE standard defines two types of reference signals (RS) that can be used for closed-loop DL signaling [33, 50, 82-83]: i) cell-specific reference signals (CRS); ii) UE-specific RS, e.g., Channel State Information (CSI) Reference Signals (CSI-RS) and Demodulation RS (DM-RS). Cell-specific RS is not precoded, whereas UE-specific RS is precoded

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

[0101] 1.1.1 MU-MAS signalling using CRS CRS is used in LTE (Release 8) systems to estimate the CSI from all transmit antennas at the UE to the BTS [80, 84]. The CRS is obtained as the product of a two-dimensional orthogonal sequence and a two-dimensional pseudorandom number (PRN) sequence. There are three orthogonal sequences (i.e., placed on an orthogonal set of OFDM subcarriers) and 168 possible PRN sequences, for a total of 504 different CRS sequences. Every sequence uniquely identifies one cell. Each of the three orthogonal CRSs is associated with one of three physical layer IDs (0 to 2) that generate different cell IDs, as described in the previous subsection. The CRS is transmitted in the first and third-to-last OFDM symbol of every slot, as well as in every sixth subcarrier. Orthogonal patterns in time and frequency are designed for every transmit antenna of the BTS in order for the UE to uniquely estimate the CSI from each of the transmit antennas. Release 8 specifies up to four orthogonal patterns per CRS, one for each of the four transmit antennas used for MIMO 4×4. This dense CRS in time and frequency (i.e., transmitted every 0.5 ms slot and on every 6th subcarrier) generates a 5% overhead and was intentionally designed to support scenarios with fast channel variations in time and frequency

[83] .

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

[0103] In a scenario where more than 12 BTSs are deployed in the same cluster, it is desirable to increase the number of available orthogonal CRSs (i.e. BTSs that transmit precoded signals simultaneously to the UE) that support a larger number of active BTSs. One way to achieve this is to define more than three antenna subclusters 1003 per antenna cluster 1101 and assign the same three physical layer IDs (or cell IDs 1104 from 0 to 2) to the antenna subclusters 1103 with a repeating pattern as shown in Figure 11. It is observed that the antenna subclusters can be obtained in various ways and are defined in such a way that any user cluster 1102 cannot reach two antenna subclusters with the same physical layer ID, thereby avoiding interference to the CRS. For example, one way to achieve this is to define an area of ​​antenna subclusters 1103 that is larger than the user cluster 1102, avoiding that its neighboring antenna subclusters use the same physical layer ID. In one embodiment of the present invention, multiple antenna subclusters are deployed in the same antenna cluster with a repeating pattern such that the corresponding CRSs do not interfere, thereby allowing non-interfering simultaneous transmissions from more than 12 BTSs.

[0104] In a real MU-MAS system, every UE may see more than only four BTSs in its user cluster. For example, Fig. 12 shows the SNR distribution for a real deployment of a DIDO or MU-MAS system in the San Francisco, California metropolitan area. The propagation model is based on the 3GPP path loss / shadowing model

[81] and assumes a carrier frequency of 900 MHz. The dots on the map indicate the locations of the DIDO-BTSs, while the black circles indicate the user clusters (UEs are located at the centers of the circles). In a sparsely populated area 1201, the UE sees only a few BTSs in its user cluster (e.g., only three BTSs for the example of Fig. 12), while in a densely populated area 1202, each user cluster may contain as many as 26 BTSs as in Fig. 12.

[0105] The high redundancy of CRS can be exploited in MU-MAS to enable CSI estimation from any number of transmit antennas, 5 or more. For example, if the channel is fixed radio or characterized by low Doppler effect, it is not necessary to calculate CSI from all four transmit antennas every 0.5 ms (slot duration). Similarly, if the channel is frequency flat, it is redundant to estimate CSI every sixth subcarrier. In that case, the resource elements (RE) occupied by the redundant CRS can be reallocated to another transmit antenna or BTS in the MU-MAS. In one embodiment of the present invention, the system allocates the resource elements of the redundant CRS to the extra antennas or BTS in the MU-MAS system. In another embodiment, the system estimates the time and frequency selectivity of the channel and dynamically allocates CRS for different BTSs or only BTSs in a user cluster to different resource elements.

[0106] The number of BTSs included in every user cluster depends on the signal power level from all BTSs in the user cluster measured at the UE relative to the noise power level or signal-to-noise ratio (SNR). In one embodiment, the UE estimates the SNR from all BTSs in its neighborhood and selects the BTSs that belong to its user cluster based on the SNR information. In another embodiment, the CP knows the SNR from the BTSs to every UE (based on feedback information from the UE or information obtained from the UL channel by inferring UL / DL channel reciprocity) and selects the set of BTSs to be included in every user cluster.

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

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

[0109] 1.1.2 MU-MAS signalling using CSI-RS and DM-RS In the LTE-Advanced (Release 10) standard, CSI-RS is used by every UE to estimate CSI from the BTS [33, 83]. The standard defines orthogonal CSI-RS for different transmitters in the BTS, so that the UE can distinguish CSI from different BTSs. Up to eight transmit antennas in the BTS are supported by CSI-RS, as per Table 6.10.5.2-1, 2 in

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

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

[0110] When there are more than eight antennas (the maximum number of transmitters supported by the LTE-Advanced standard) in a user cluster, an alternative technique must be used to enable DIDO precoding while maintaining system compatibility with the LTE-Advanced standard. In one embodiment of the invention, every UE uses the CSI-RS or DM-RS or a combination of both to estimate the CSI from all active BTSs in its own user cluster. In the same embodiment, the DIDO system detects the number of BTSs in the user cluster and detects whether the user cluster is compliant with the LTE-Advanced standard (which supports up to eight antennas). If not, the DIDO system uses an alternative technique to enable DL signaling from the BTS to the current UE. In one embodiment, the transmission power from the BTS is reduced until up to eight BTSs can be reached by the UE in that user cluster. However, this solution may result in a reduction in data rate since the coverage is reduced.

[0111] Another solution is to split the BTSs in a user cluster into subsets and transmit one set of CSI-RS for all subsets at the same time. For example, if the CSI-RS periodicity is 5 subframes (i.e., 5 ms) as per Table 6.10.5.3-1 of

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

[0112] The solutions proposed so far for CSI-RS are all LTE standard compliant and can be deployed within the framework of a conventional LTE system. For example, the proposed method to allow more than eight antennas per user cluster does not require any modification of the UE LTE hardware and software implementation, but only slight modifications of the protocols used by the BTS and CP to allow the selection of a BTS subset at any time. These modifications can be easily implemented in a cloud-based software-defined radio (SDR) platform, which is one promising deployment paradigm for DIDO and MU-MAS 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 or MU-MAS operation modes similar to LTE but not compliant with LTE, it allows the UE to operate in a full LTE compliant mode or a modified mode supporting non-LTE compliant DIDO or MU-MAS operation. For example, it allows another solution to increase the amount of CSI-RS to allow a larger number of BTSs in the system. In another embodiment of the invention, different CSI-RS patterns and periodicities are possible as a means to increase the number of supported BTSs 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, even if hardware modifications are required to the chipset, the changes will be small.

[0113] 1.2 Uplink MU-MAS 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 Forward Error Correction (FEC) coding scheme and modulation to be used on the DL to maintain a defined error rate performance for given channel conditions.

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

[0115] In one embodiment of the present invention, the DIDO or MU-MAS system uses RI, PMI and CQI to report the current channel condition and precoding information to the BTS and CP. In one embodiment, the UE uses the PUCCH channel to report these indicators to the CP. In another embodiment, if more indicators are needed for DIDO precoding, the UE uses the PUSCH to report additional indicators to the CP. If the channel is frequency flat, the UE can utilize the extra UL resources to report PMI for more antennas in the DIDO system. In one embodiment of the invention, the UE or the BTS or the CP estimates the channel frequency selectivity, and if the channel is frequency flat, the UE utilizes the extra UL resources to report PMI for more BTSs. 2. Downlink open-loop MU-MAS precoding method in LTE

[0116] The open-loop MU-MAS precoding scheme can only be used in time division duplex (TDD) systems that employ RF calibration and exploit channel reciprocity. The general mechanism of the open-loop scheme in MU-MAS consists of: i) UEs transmit signaling information to the BTS or CTR on the UL; ii) the BTS or CTR utilizes the signaling information to estimate UL CSI from all UEs; iii) the BTS or CTR converts the UL CSI to DL CSI using RF calibration; iv) the BTS or CTR transmits DL CSI or codebook index to the CP via the BSN; v) based on the DL CSI, the CP calculates precoding weights for data transmission on the DL. Similar to the closed-loop MU-MAS precoding scheme, user clusters can be used to reduce the amount of CSI from UEs 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, the open-loop precoding technique is used to transmit simultaneous non-interfering data streams from the BTS to the UEs on the DL channel.

[0117] In LTE, there are two types of reference signals for the uplink channel [31,33,87]: i) Sounding Reference Signal (SRS), used for scheduling and link adaptation, and ii) Demodulation Reference Signal (DMRS), used for data reception. In one embodiment of the present invention, DMRS is used in an open-loop precoding system to estimate the UL channel from every UE to every BTS. In the time domain, DMRS is transmitted in the fourth OFDM symbol (when using normal cyclic prefix) of every LTE slot (of duration 0.5 ms). In the frequency domain, DMRS transmitted on PUSCH is mapped for every UE to the same resource block (RB) used by that UE for UL data transmission. 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, up to 12 DMRSs are generated from one basic Zadoff-Chu

[88] or computer-generated constant amplitude zero autocorrelation (CG-CAZAC) sequence by 12 possible cyclic shifts of the 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. It is observed that the 30 base sequences are not guaranteed to be orthogonal and are designed to reduce, without completely eliminating, interference across cells. In contrast, 12 cyclic shifts of the same base sequence are orthogonal, thereby allowing up to 12 UEs to transmit in the UL on the same RB without interference. The value of the cyclic shift used by any UE is provided by the BTS through a Downlink Control Information (DCI) message transmitted over the PDCCH. The DCI in Release 8 consists of 3 bits, which allows the UE to use only up to 8 cyclic shifts in a pool of 12 possible choices.

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

[0119] A method for increasing the number of UEs simultaneously served through MU-MIMO in UL or MU-MAS precoding in DL is to multiplex the DMRSs of the UEs in the frequency domain. For example, when a 10 MHz bandwidth is used in TDD mode, there are 50 RBs that can be assigned to the UEs. In this case, 25 interleaved RBs can be assigned to one set of 8 UEs, and the remaining 25 interleaved RBs can be assigned to another set of UEs, resulting in a total of 16 simultaneously served UEs. The CSI is then calculated by interpolating the estimates from the DMRSs transmitted on the interleaved RBs. A larger number of simultaneously served UEs can be supported by increasing the number of interleaving patterns of the UL RBs. These patterns can be statically or dynamically assigned to different UEs by specific frequency hopping sequences. In one embodiment of the present invention, to increase the number of UEs supported through MU-MIMO or MU-MAS precoding, the DMRSs are assigned to the UEs on orthogonal interleaved RBs. In the same embodiment, the interleaved RBs are statically assigned. In another embodiment, the interleaved RBs are dynamically allocated according to a specific frequency hopping pattern.

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

[0121] Another solution is to spatially separate different groups of UEs using the same set of DMRSs. For example, the same set of orthogonal DMRSs can be used for all UEs from different antenna sub-clusters in FIG. 11 that are identified by the same cell ID. In one embodiment of the present invention, groups of UEs using the same set of orthogonal DMRSs are spatially separated to avoid interference between the groups. In the same embodiment, the same set of orthogonal DMRSs is used by different antenna sub-clusters that are identified by the same cell ID. The MU-MAS may assign UEs to "virtual cells" to maximize the number of DMRSs that can be used in the UL. In one exemplary embodiment, the virtual cell is a coherence area (described in related co-pending U.S. patent application Ser. No. 13 / 232,996, entitled "Systems and Methods to Exploit Areas of Coherence in Wireless Systems") around the UE, and the DIDO system creates up to 3300 coherence areas for different UEs. In another embodiment of the present invention, each of the 30 base sequences is assigned to a different antenna cluster in order to reduce inter-cluster interference between adjacent antenna clusters (clusters are defined in related 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").

[0122] 3. Uplink MU-MAS method in LTE 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. The UL open-loop MU-MIMO scheme consists of the following steps: i) UEs transmit signaling information and data payloads to all BTSs, ii) BTSs use the signaling information to calculate channel estimates from all UEs, iii) BTS transmits channel estimates and data payloads to the CP, iv) CP uses the channel estimates to remove co-channel interference from the data payloads of all UEs via 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 UEs to the BTS and multiplexes them in the frequency domain.

[0123] In one embodiment, synchronization between UEs is achieved by signaling from the DL, and assumes that all BTSs 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 by GPSDO in one embodiment. Variations in channel delay across different UEs can generate jitter between the time references of different UEs, which can affect the performance of the MU-MIMO method on the UL. In one embodiment, to reduce the relative propagation delay across different UEs, only UEs in the same antenna 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 at the BTS.

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

[0125] 4. Integration with existing LTE networks In the United States and other regions of the world, LTE networks are already in operation, are being deployed, and / or are to be deployed. It would be of significant benefit to LTE operators if they could gradually deploy DIDO or MU-MAS capabilities into existing or already committed deployments. In this way, they could deploy DIDO or MU-MAS in areas where it provides the most immediate benefit, gradually expanding DIDO or MU-MAS capabilities across more and more of the network. Eventually, once they have sufficient DIDO or MU-MAS coverage in an area, they could choose to decommission the cells entirely and instead switch completely to DIDO or MU-MAS, achieving much higher spectrum density at a much lower cost. Through this complete transition from cellular to DIDO or MU-MAS, the LTE operator's wireless customers would never suffer a loss in service. Rather, they would simply see improvements in their data throughput and reliability, while the operator would see a decrease in its costs.

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

[0127] The LTE standard supports a variety of channel bandwidths (e.g., 1.4, 3, 5, 10, 15, and 20 MHz). In one embodiment, an operator of an existing LTE network would support a conventional LTE BTS in a cellular configuration with one block of spectrum and a DIDO LTE BTS with another block of spectrum, either by allocating new bandwidth for the LTE-DIDO BTS or by subdividing the existing LTE spectrum (e.g., 20 MHz could be subdivided into two 10 MHz blocks). In effect, this would establish two separate LTE networks, and UE devices would be configured to use one or the other network, or to choose between both. In the case of a subdivided spectrum, the spectrum could be divided evenly or unevenly between the conventional LTE and DIDO-LTE BTS networks, and more spectrum could be allocated to the network that can best utilize it, taking into account the degree of deployment of cellular LTE BTS and DIDO-LTE BTS, and / or UE usage patterns. This division can change as needed over time, and at some point, when there are enough DIDO-LTE BTSs deployed to provide the same or better coverage as the cellular BTSs, all of the spectrum can be allocated to the DIDO-LTE BTSs and the cellular BTSs can be retired.

[0128] In another embodiment, conventional cellular LTE BTSs can be configured to cooperate with DIDO-LTE BTSs and share the same spectrum, but use the spectrum in a rotating manner. For example, if they share the spectrum equally, each BTS network will take turns using one 10 ms frame time, e.g., one 10 ms frame for the cellular LTE BTS followed by one 10 ms frame for the DIDO-LTE BTS. The frame time can also be divided into non-uniform intervals. This division of intervals 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 BTSs, all of the time can be assigned to the DIDO-LTE BTSs and the cellular BTSs can be retired.

[0129] In another embodiment of the present invention, DIDO or MU-MAS is used as a LOS or NLOS wireless backhaul for small cells in LTE and LTE-Advanced networks. As small cells are deployed in LTE networks, DIDO or MU-MAS 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 more small cells to a given area without causing inter-cell interference. In the same embodiment of the present invention, DIDO-LTE BTS is used to gradually replace the small cells, thereby taking advantage of the inter-cell interference to provide increased network capacity.

[0130] 5.MU-MAS LTE Scheduler In MU-MAS, a distributed antenna or BTS transmits simultaneous precoded data streams to multiple UEs. As described in the related patents and applications, the number of BTSs must be equal to or greater than the number of UEs to allow simultaneous data transmission. In practical deployments, the number of UEs may exceed the number of BTSs. In this case, the excess UEs may be selected to transmit in different time slots or frequency bands according to a specific scheduling policy. The scheduler utilizes the channel quality information of the UEs to determine the best set of UEs to be served at the serving time and frequency. Different scheduling methods are used in the present invention, including a proportional fair scheduler, a round robin or a greedy method.

[0131] As described in the previous section, the LTE standard defines two parameters that inform the scheduler about the link quality of every UE: CQI and SRS. CQI measures the quality of the DL channel and is provided back from the UE to the BTS. SRS is the signaling information sent from the UE to the BTS to measure the UL channel quality. Both indicators provide information of the UL / DL channel quality over time and in the frequency domain. Since the DL and UL channel quality may vary with different carrier frequencies, in FDD systems the DL scheduler must use CQI as a performance measure. In TDD mode, the DL scheduler makes scheduling decisions using either CSI or SRS or a combination of both. Similar performance metrics can be used for UL scheduling. In one embodiment of the present invention, the MU-MAS scheduler uses CQU and SRS as the performance metrics used by the scheduling algorithm.

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

[0133] 9, 10 and 11, in certain scenarios, orthogonal signaling sequences may not be sufficient to allow a large number of BTSs in the same antenna cluster or antenna sub-cluster. In this case, some degree of interference may occur when additional BTSs are activated to cover an area with a larger number of active UEs. In one embodiment of the present invention, the scheduler measures the degree of interference between antenna clusters or antenna sub-clusters and schedules UEs to minimize the effect of the interference on the radio link.

[0134] The antenna selection algorithm described in related U.S. Patent Application No. 13 / 475,598, entitled "Systems and Methods to enhance spatial diversity in distributed-input distributed-output wireless systems," is used in the present invention to select the optimal set of active BTSs based on SSI. However, this antenna selection algorithm may require high computational complexity since the MU-MAS precoding process must be applied to all possible antenna subset permutations before making a decision about the best subset based on the SSI performance metric. In MU-MAS with a large number of cooperating BTSs, this computational burden may be expensive or infeasible in practical deployments. It is therefore desirable to develop an alternative method to reduce the number of antenna subsets while maintaining good performance of the antenna selection method. In one embodiment of the present invention, the MU-MAS uses a method based on a queue of antenna subset ID numbers, hereinafter referred to as the "antenna shuffling method." In one embodiment of the present invention, the antenna shuffling method subdivides the queue containing all possible antenna subset IDs (i.e. all possible permutations of active BTSs for a given set of available BTSs) into various groups and assigns different priorities to these groups. These groups are defined to assign a fair chance to all subset IDs to be selected, while the SSI metric is calculated only for a limited number of subsets (e.g. the ones with the highest priority), thus reducing computational complexity. In one exemplary embodiment, the queue of subset IDs is divided into three groups, each group being assigned a different rule: i) group 1 contains the IDs with the highest priority, which are only pulled from the group when a new subset with a higher priority is identified; ii) group 2, in which a new antenna subset (selected from group 3) is included at every iteration of the method; iii) group 3, in which the antenna subset IDs are shuffled according to a round robin rule.The subset IDs in groups 1 and 2 are sorted based on their priority at each iteration of the method, and subset IDs from group 2 are given the opportunity to be upgraded to group 1. The SSI is calculated only for subsets in groups 1 and 2, and the antenna selection algorithm is applied only to these subsets.

[0135] 6. MU-MAS LTE user equipment The present invention encompasses various designs for an LTE UE. In one embodiment, the UE is an LTE UE compatible with MU-MAS using precoding, as described above and illustrated in FIG.

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

[0137] In one embodiment of the invention, the UE 1501 is housed in a case that is physically attached to the user equipment 1502 as shown in Figure 15. In some embodiments, the case serves as a cosmetic addition to the user equipment. In other embodiments, the case serves to protect the user equipment from physical damage. The UE consists of a battery 1503, and one or more network interfaces 1504.

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

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

[0140] In another embodiment, the same RF chains are used for MU-MAS communications as well as the alternative network, hi another embodiment, different RF chains are used for MU-MAS communications and the alternative network.

[0141] 7. Radio Frequency (RF) Calibration Using Channel Reciprocity Conventional closed-loop MU-MAS methods employ the UL channel to feed back quantized CS index or codebook index from the UE to the BTS or CP (like the codebook-based limited feedback method). However, this method results in large feedback overhead and high protocol complexity for enabling the CSI feedback channel. Therefore, in TDD systems where the UL and DL are set to the same frequency, it is desirable to avoid CSI feedback by utilizing UL / DL channel reciprocity. In practical systems, the transmit RF chain and receive RF chain in the BTS or UE typically have different characteristics due to different RF components and circuit layouts. Therefore, in order to maintain UL / DL interaction, it is necessary to employ an RF calibration method to compensate for the RF mismatch between the transmit chain and the receive chain.

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

[91] , and a hardware solution to mitigate the impact of RF mismatch on the performance of adaptive digital beamforming systems is discussed in

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

[96] , and for a system employing antenna selection in

[96] .

[0143] However, the prior art assumes that all RF chains are arranged on the same circuit board as in a MIMO system, which simplifies the problem of RF calibration since information about RF mismatch between all RF chains is available locally. In contrast, the present invention consists of distributed antennas located geographically remotely such that communication between the distributed antennas occurs only through the network. Therefore, the applicant defines a novel system unit called a "beacon station" specifically designed to enable RF calibration in MU-MAS with distributed antennas. Furthermore, in the prior art MIMO system, significant RF coupling occurs between transmit / receive chains due to the close proximity of the RF chains on the same board. In contrast, in the present invention, RF coupling occurs only between one transmit chain and one receive chain of the same distributed antenna. Therefore, the technique employed for RF calibration is significantly different from that described in the prior art described below. Finally, the RF calibration methods disclosed in the prior art are limited to single user systems (e.g., single user equipment devices). As will be shown in the derivations in the following paragraphs, multiple user systems (e.g., MU-MAS) are particularly sensitive to RF mismatch because it causes inter-user interference. Therefore, special techniques must be employed to enable RF calibration while exploiting channel reciprocity, as described below.

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

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

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

number

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

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

number

number

number

number

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

number

number

number

number

[0150] When linear pre-coding (eg, zero forcing

[65] , block diagonalization or BD [66-67], matrix inversion, etc.) is employed, the received symbol at the mth UE is given by:

number

number

number

number

number

number

number

number

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

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

[0153] In a practical MU-MAS, the beacon station is a radio transceiver dedicated to RF calibration. Since the beacon needs a feedback channel to communicate the estimated effective DL channel from all distributed antennas for calibration purposes, the beacon communicates to the CP via a wireless or wired link. In another embodiment, the beacon station is any of the distributed antennas for which the calibration parameters are calculated. In the same embodiment, the distributed antennas are organized like a mesh network, and pair-wise RF calibration between adjacent distributed antennas is calculated to ensure good link quality. RF calibration is performed across all antennas, and the calibration information is fed back to the CP so that all distributed antennas are calibrated to each other. In another embodiment, the beacon is any of the UEs that uses any wireless or wired link to feed back calibration information to the CP.

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

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

[0156] In one embodiment of the present invention, the beacon stations are not only used for RF calibration, but also to transmit signal information to the distributed antennas and / or UEs, including a time and frequency synchronization reference, which the distributed antennas and / or UEs employ to maintain time and frequency synchronization with the MU-MAS master reference clock. In one embodiment, this reference clock distribution from the beacons to the distributed antennas and UEs is enabled via an LTE Multimedia Broadcast Single Frequency Network (MBSFN) communication channel.

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Claims

1. 1. A system comprising: a plurality of more than two radio transceiver units (BTSs) of a radio access network (RAN) distributed throughout a coverage area, at least one antenna of each BTS configured to operate as a beacon station; three or more of the plurality of BTSs are configured to transmit a plurality of training signals therebetween; The training signal is processed to estimate a plurality of radio frequency (RF) calibration coefficients; the RF calibration coefficients are used to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; the UL CSI is determined from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of a plurality of antennas of one or more of a plurality of UEs; The DL CSI matrix is ​​used to precode multiple wireless signals simultaneously transmitted by the multiple BTSs; the precoding purposefully causes the wireless signals to interfere to generate simultaneous non-interfering channels at one or more locations of one or more of the antennas of one or more of the UEs; system.

2. 2. The system of claim 1, wherein downlink (DL) channel state information (CSI) is derived from uplink (UL) CSI using the RF calibration coefficients and exploiting UL and DL channel reciprocity.

3. the DL CSI is derived from the UL CSI using the RF calibration coefficients and exploiting UL and DL channel reciprocity; the UL CSI is determined from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of user equipment units (UEs); The system of claim 1 .

4. The system of claim 1 , wherein the RF calibration coefficients are used to pre-adjust a UL CSI matrix to obtain a DL CSI matrix.

5. the RF calibration coefficients are used to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; the UL CSI is determined from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; The system of claim 1 .

6. the RF calibration coefficients are used to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; the UL CSI is determined from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; The DL CSI matrix is ​​used to precode multiple wireless signals simultaneously transmitted by the multiple BTSs. The system of claim 1 .

7. The system of claim 1 , wherein the RAN does not have cells.

8. The system of claim 1 , wherein the multiple BTSs are interconnected to a centralized processor (CP).

9. 1. A method implemented in a RAN, comprising: transmitting a plurality of training signals among a plurality of BTSs distributed throughout a coverage area, at least one antenna of each BTS being configured to operate as a beacon station; processing the training signal to estimate a plurality of RF calibration coefficients; using the RF calibration coefficients to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; determining the UL CSI from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of a plurality of antennas of one or more of a plurality of UEs; using the matrix of DL CSI to precode multiple wireless signals simultaneously transmitted by the multiple BTSs; Including, the precoding purposefully causes the wireless signals to interfere to generate simultaneous non-interfering channels at one or more locations of one or more of the antennas of one or more of the UEs; method.

10. 10. The method of claim 9, further comprising deriving DL CSI from UL CSI using the RF calibration coefficients and exploiting UL and DL channel reciprocity.

11. deriving DL CSI from UL CSI using the RF calibration coefficients and exploiting UL and DL channel reciprocity; determining the UL CSI from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; The method of claim 9 further comprising:

12. 10. The method of claim 9, further comprising using the RF calibration coefficients to precondition a matrix of UL CSI to obtain a matrix of DL CSI.

13. using the RF calibration coefficients to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; determining the UL CSI from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; The method of claim 9 further comprising:

14. using the RF calibration coefficients to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; determining the UL CSI from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; using the matrix of DL CSI to precode multiple wireless signals simultaneously transmitted by the multiple BTSs; The method of claim 9 further comprising:

15. The method of claim 9 , wherein the RAN does not have any cells.

16. The method of claim 9 , wherein the plurality of BTSs are interconnected to a CP.

17. 1. A system comprising: a plurality of more than two radio transceiver stations (BTSs) of the RAN distributed throughout a coverage area, at least one antenna of each BTS configured to operate as a beacon station; three or more of the plurality of BTSs are configured to transmit a plurality of training signals therebetween, the training signals being processed to estimate a plurality of radio frequency (RF) calibration coefficients; the DL CSI is derived from the UL CSI using the RF calibration coefficients and exploiting UL and DL channel reciprocity; the RF calibration coefficients are used to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; the UL CSI is determined from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of a plurality of antennas of one or more of a plurality of UEs; The DL CSI matrix is ​​used to precode multiple wireless signals simultaneously transmitted by the multiple BTSs; the precoding purposefully causes the wireless signals to interfere to generate simultaneous non-interfering channels at one or more locations of one or more of the antennas of one or more of the UEs; system.

18. the UL CSI is determined from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; 20. The system of claim 17.

19. 20. The system of claim 17, wherein the RF calibration coefficients are used to pre-adjust a matrix of UL CSI to obtain a matrix of DL CSI.

20. the RF calibration coefficients are used to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; the UL CSI is determined from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; 20. The system of claim 17.

21. the RF calibration coefficients are used to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; the UL CSI is determined from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; The DL CSI matrix is ​​used to precode multiple wireless signals simultaneously transmitted by the multiple BTSs.

20. The system of claim 17.

22. The system of claim 17 , wherein the RAN does not have cells.

23. The system of claim 17 , wherein the plurality of BTSs are interconnected to a CP.

24. 1. A method implemented in a RAN, comprising: transmitting a plurality of training signals between a plurality of more than two BTSs distributed throughout a coverage area, where at least one antenna of each BTS is configured to operate as a beacon station; processing the training signal to estimate a plurality of RF calibration coefficients; deriving DL CSI from UL CSI using the RF calibration coefficients and exploiting UL and DL channel reciprocity; using the RF calibration coefficients to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; determining the UL CSI from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of a plurality of antennas of one or more of a plurality of UEs; using the matrix of DL CSI to precode multiple wireless signals simultaneously transmitted by the multiple BTSs; Including, the precoding purposefully causes the wireless signals to interfere to generate simultaneous non-interfering channels at one or more locations of one or more of the antennas of one or more of the UEs; method.

25. 25. The method of claim 24, further comprising: determining the UL CSI from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs.

26. 25. The method of claim 24, further comprising using the RF calibration coefficients to precondition a matrix of UL CSI to obtain a matrix of DL CSI.

27. using the RF calibration coefficients to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; determining the UL CSI from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; 25. The method of claim 24, further comprising:

28. using the RF calibration coefficients to pre-adjust a UL CSI matrix to obtain a DL CSI matrix; determining the UL CSI from a plurality of UL transmissions received by a subset or all of the plurality of BTSs from one or more of the plurality of antennas of one or more of the plurality of UEs; using the matrix of DL CSI to precode multiple wireless signals simultaneously transmitted by the multiple BTSs; 25. The method of claim 24, further comprising:

29. The method of claim 24 , wherein the RAN has no cells.

30. The method of claim 24 , wherein the plurality of BTSs are interconnected to a CP.

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