# Patent

## Title
Cooperative MIMO in multicell wireless networks

## Abstract

A method and system for cooperative multiple-input multiple output (MIMO) transmission operations in a multicell wireless network. Under the method, antenna elements from two or more base stations are used to from an augmented MIMO antenna array that is used to transmit and receive MIMO transmissions to and from one or more terminals. The cooperative MIMO transmission scheme supports higher dimension space-time-frequency processing for increased capacity and system performance.

## Background
FIELD OF THE INVENTION
The present invention relates to the field of communications systems; more particularly, the present invention relates to techniques for performing MIMO operations in a multicell wireless network.
BACKGROUND OF THE INVENTION
With high-speed wireless services increasingly in demand, there is a need for more throughput per bandwidth to accommodate more subscribers with higher data rates while retaining a guaranteed quality of service (QoS). In point-to-point communications, the achievable data rate between a transmitter and a receiver is constrained by the available bandwidth, propagation channel conditions, as well as the noise-plus-interference levels at the receiver. For wireless networks where a base-station communicates with multiple subscribers, the network capacity also depends on the way the spectral resource is partitioned and the channel conditions and noise-plus-interference levels of all subscribers. In current state-of-the-art, multiple-access protocols, e.g., time-division multiple access (TDMA), frequency-division multiple-access (FDMA), code-division multiple-access (CDMA), are used to distribute the available spectrum among subscribers according to subscribers' data rate requirements. Other critical limiting factors, such as the channel fading conditions, interference levels, and QoS requirements, are ignored in general.
The fundamental phenomenon that makes reliable wireless transmission difficult to achieve is time-varying multipath fading. Increasing the quality or reducing the effective error rate in a multipath fading channel may be extremely difficult. For instance, consider the following comparison between a typical noise source in a non-multipath environment and multipath fading. In environments having additive white Gaussian noise (AWGN), it may require only 1- or 2-db higher signal-to-noise ratio (SNR) using typical modulation and coding schemes to reduce the effective bit error rate (BER) from 10−2 to 10−3. Achieving the same reduction in a multipath fading environment, however, may require up to 10 db improvement in SNR. The necessary improvement in SRN may not be achieved by simply providing higher transmit power or additional bandwidth, as this is contrary to the requirements of next generation broadband wireless systems.
One set of techniques for reducing the effect of multipath fading is to employ a signal diversity scheme, wherein a combined signal is received via independently fading channels. Under a space diversity scheme, multiple antennas are used to receive and/or send the signal. The antenna spacing must be such that the fading at each antenna is independent (coherence distance). Under a frequency diversity scheme, the signal is transmitted in several frequency bands (coherence BW). Under a time diversity scheme, the signal is transmitted in different time slots (coherence time). Channel coding plus interleaving is used to provide time diversity. Under a polarization diversity scheme, two antennas with different polarization are employed for reception and/or division.
Spatial diversity is commonly employed in modern wireless communications systems. To achieve spatial diversity, spatial processing with antenna arrays at the receiver and/or transmitter is performed. Among many schemes developed to date, multiple-input multiple-output (MIMO) and beamforming are the two most studied and have been proved to be effective in increase the capacity and performance of a wireless network. (see, e.g., Ayman F. Naguib, Vahid Tarokh, Nambirajan Seshadri, A. Robert Calderbank, “A Space-Time Coding Modem for High-Data-Rate Wireless Communications”, IEEE Journal on Selected Areas in Communications, vol. 16, no. 8, October 1998 pp. 1459-1478). In a block time-invariant environment, it can be shown that in a system equipped with Nt transmit antennas and Nr receive antennas, a well designed space-time coded (STC) systems can achieve a maximum diversity of Nr*Nt. Typical examples of STC include space-time trellis codes (STTC) (see, e.g., V. Tarokh, N. Seshadri, and A. R. Calderbank, “Space-time codes for high data rate wireless communication: performance criterion and code construction”, IEEE Trans. Inform. Theory, 44:744-765, March 1998) and space-time block codes from orthogonal design (STBC-OD) (see, e.g., V. Tarokh, H. Jafarkhani, and A. R. Calderbank, “Space-time block codes from orthogonal designs”, IEEE Trans. Inform. Theory, 45:1456-1467, July 1999.)
Since the capacity and performance of an MIMO system depends critically on its dimension (i.e., Nt and Nr) and the correlation between antenna elements, larger size and more scattered antenna arrays are desirable. On the other hand, costs and physical constraints prohibit the use of excessive antenna arrays in practice.
SUMMARY OF THE INVENTION
A method and system is disclosed herein for cooperative multiple-input multiple output (MIMO) transmission operations in a multicell wireless network. Under one embodiment, antenna elements from two or more base stations are used to from an augmented MIMO antenna array that is used to transmit and receive MIMO transmissions to and from one or more terminals.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.
FIG. 1 depicts a multicell scenario where antenna elements from multiple base-stations are augmented to form a higher dimension MIMO transceiver array.
FIG. 2 shows a generic channel matrix H used for modeling the capacity of MIMO systems.
FIG. 3 shows the capacity increase of an MIMO system with respect to the number of transmitting antennas.
FIG. 4a shows a cooperative MIMO architecture under which antenna arrays from two base stations are employed in a cooperative MIMO transmission scheme to transmit downlink signals to one terminal;
FIG. 4b shows aspects of the cooperative MIMO architecture of FIG. 4a employed for transmitting and processing uplink signals received by the augmented antenna array.
FIG. 5 shows an extension to the cooperative MIMO architecture of FIG. 4a, wherein beamforming is used to direct a MIMO transmission toward one terminal while performing spatial nulling towards another terminal.
FIG. 6 shows a cooperative MIMO architecture under which two base-stations performing multiuser MIMO with two terminals simultaneously using joint encoding and decoding.
FIG. 7a shows a block diagram of an MIMO OFDM encoder/transmitter.
FIG. 7b shows the block diagram of an MIMO OFDM encoder/transmitter with beamforming.
FIG. 8 shows a block diagram of an MIMO OFDM receiver/decoder.
FIG. 9 shows a block diagram used to model a space-time coding transmission scheme.
FIG. 10 shows an exemplary PSK-based space-time trellis code (STTC) encoder.
FIG. 11 shows an exemplary QAM-based STTC encoder.
FIG. 12 shows a block diagram used to model a space-time block coding (STBC) transmission scheme.
FIG. 13a shows a block diagram modeling an STTC delay diversity scheme.
FIG. 13b shows a block diagram modeling an STBC delay diversity scheme.
FIG. 14 is a block diagram of an exemplary PSK-based STTC delay diversity encoder.
FIG. 15 is a schematic diagram illustrating a cooperative MIMO architecture under which STC encoding operations are performed at a master encoder;
FIG. 16 is a schematic diagram illustrating a cooperative MIMO architecture under which STC encoding operations are performed on respective instances of replicated data streams at multiple base stations.



DETAILED DESCRIPTION OF THE PRESENT INVENTION
In accordance with aspects of the present invention, a method and apparatus to augment antenna elements from two or more base-stations/terminals to perform higher dimensional MIMO operations is disclosed. In one implementation, MIMO/joint space-time coding is employed across multiple base stations in a cellular environment, wherein the cooperative transmission of signals is performed at the modulation and coding level. This novel approach introduces additional diversities and capacities to existing network components with minimal additional costs. Because of the increase in the number of transmit antennas, the number of simultaneous users increases, leading to better spectrum efficiency.
In the following description, numerous details are set forth to provide a more thorough explanation of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.
Some portions of the detailed descriptions which follow are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.
It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.
The present invention also relates to apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, and each coupled to a computer system bus.
The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear from the description below. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein.
A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium includes read only memory (“ROM”); random access memory (“RAM”); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc.
Overview
FIG. 1 depicts three cells 100, 102, and 104 for a typical wireless network with multiple base stations BS1, BS2 and BS3 and terminals A, B, and C. Each of base stations BS1 and BS2 includes a 4-element circular antenna array, while terminal A has two antennas 1 and 2.
From a theoretical viewpoint, the capacity between a transmitter and a receiver for a MIMO transmission scheme is determined by the vector channel H, which is also referred to as the channel matrix. As illustrated in FIG. 2, the channel matrix H includes M rows and N columns, wherein M is the number of receiver antennas (Rx) and N is the number of transmitter antennas (Tx). In the illustrated channel matrix H, each entry αij is the complex channel gain from the i-th transmit antenna to the j-th receive antenna.
The channel capacity for a Single-Input Single-Output (SISO) channel is,

C=log2(1+ρ)bits/sec/use  (1),

where ρ is the signal to noise ratio. The channel capacity for a MIMO channel is,





  
    
      
        C
        =
        
          
            log
            2
          
          ⁢
          
            
              det
              ⁡
              
                [
                
                  I
                  +
                  
                    
                      ρ
                      N
                    
                    ⁢
                    
                      HH
                      *
                    
                  
                
                ]
              
            
            .
          
        
      
    
    
      
        (
        2
        )
      
    
  




From above, the outage capacity can be shown to be,





  
    
      
        C
        =
        
          
            1
            2
          
          ⁢
          M
          ⁢
          
            
          
          ⁢
          
            
              
                log
                2
              
              ⁡
              
                (
                
                  1
                  +
                  
                    σ
                    ⁢
                    
                      
                        {
                        h
                        }
                      
                      2
                    
                    ⁢
                    ρ
                  
                
                )
              
            
            .
          
        
      
    
    
      
        (
        3
        )
      
    
  




It is observed that under equation 3, the capacity increases linearly with the number of receive antennas when M is large. The channel capacity limit grows logarithmically when adding an antenna array at the receiver side (Single-Input Multiple-Output—SIMO). Meanwhile, the channel capacity limit grows as much as linearly with min(M,N), which is the maximum number of spatial eigenmodes, in the case of a MIMO system. An illustration of a MIMO system capacity as a function of channel matrix dimension is shown in FIG. 3.
Since the system capacity is dictated by the dimension (number of antennas) and the condition (correlation between antenna elements) of the channel, it is desirable to have large size antenna array with more scattered elements. However, there is a point of diminishing return, wherein the costs of adding antenna elements and corresponding processing complexity for a given base station exceeds the benefit of the incremental increase in system capacity. Furthermore, to obtain the added benefit of extra capacity, it may be necessary to add additional antenna elements to many or all base stations within a given wireless network.
Embodiments of the present invention take advantage of the benefit of having large size antenna arrays with more scattered elements without requiring additional antenna elements to be added to base stations. This is accomplished by augmenting the operations of antenna elements from two or more base stations to form a larger size antenna array. The augmented array performs “cooperative MIMO” transmission operations for one or more terminals. For example, FIG. 1 shows an exemplary use of a cooperative MIMO transmission scheme, wherein the antenna elements for base stations BS1 and BS2 are augmented to cooperatively communicate via receive antennas 1 and 2 for terminal A.
FIG. 4a depicts a block diagram of one embodiment of a downlink (from base stations to terminals) cooperative MIMO architecture 400. For illustrative purposes, the architecture shown in FIG. 4a include two base stations 402 and 404 and a single terminal 406. It will be understood that an actual implementation of MIMO architecture 400 may include two or more base stations that transmit signals that are received by one or more terminals.
In the illustrated embodiment of FIG. 4a, base station 402 has an antenna array including Nt1 transmit antennas, while base station 404 has an antenna array including Nt2 antennas and terminal 406 includes Nr antennas. In view of the foregoing MIMO definitions, the cooperative use of the base station antennas increases the MIMO dimension to (Nt1+Nt2)*Nr. This increase in dimension is accomplished without requiring any additional antenna elements at the base stations, as well as the components use to drive the antennas.
According to aspects of various embodiments of the invention described herein, an information bit sequence corresponding to data to be transmitted to a subscriber (e.g., terminal 406) may be space-time, space-frequency, or space-time-frequency coded, as depicted by a block 408 in FIG. 4a. In some embodiments, space-time, space-frequency, or space-time-frequency codes may be augmented to support delay diversity, as described below. After appropriate encoding is performed in block 408, the coded data is then passed to the base stations, whereupon it is transmitted via applicable antenna elements at those base stations. The two or more base stations then perform joint MIMO transmissions (depicted as signals 410 and 412) towards the subscriber (e.g., a user operating terminal 406) in view of applicable MIMO channel configuration parameters. For example, signals 410 and 412 transmitted from base stations 402 and 404 may employ selected antenna elements for each of the base stations based on the coding scheme and/or MIMO scheme that is currently employed for a particular subscriber. In general, cooperative MIMO transmissions can be performed during regular communication, or during handoff, where a subscriber moves across the boundary between cells
In one embodiment, space-time coding is employed. For example, incoming information bits are space-time coded (using e.g., space-time block or trellis codes) at block 408, and the encoded data are forwarded to each of base stations 402 and 404. Further details of space-time block encoding and the use of space-time trellis codes are discussed below.
In one embodiment, the space-time (or space-frequency, or space-time-frequency) coding is performed at a master encoder. In another embodiment, the space-time (or space-frequency, or space-time-frequency) is performed at separate locations (e.g., within the base stations) based on a common (replicated) information bit sequence received at each of the separate locations.
FIG. 4b shows uplink signal processing aspects of cooperative MIMO architecture 400. In this instance, an uplink signal 414 is transmitted from terminal 406 via selected antennas from among transmit antennas 1-Nt. The uplink signal 414 is received by the respective receive antenna arrays (1-Nr1, 1-Nr2) for base stations 402 and 404. (It is noted that the same antennas may be used for both transmit and receive operations for some embodiments, while separate sets of transmit and receive antennas may be employed for other embodiments.) Upon being received at the base stations, initial signal processing is performed on the uplink signals, and the processed signals are forwarded to a block 416 to perform joint MIMO decoding and demodulation, thus extracting the information bits corresponding to the data transmitted by terminal 406. In general, the components for performing the operations of block 416 may be implemented in a master decoder that is centrally located with respect to multiple base stations (e.g., base stations 402 and 404), or may be located at one of the multiple base stations.
FIG. 5 depicts a multi-user cooperative MIMO architecture 500. Under this embodiment, the augmented antenna array (comprising selected transmit antenna elements for base stations 502 and 504) is used to perform MIMO operation towards one or more intended subscribers while limiting the radio signal at the location/direction of un-intended subscribers using a beamforming and nulling scheme. For example, techniques are known for steering transmitted signals toward selected locations, while transmitted signals sent toward other directions are nullified due to signal canceling effects and/or fading effects. Collectively, these selective transmission techniques are referred to as beamforming, and are accomplished by using appropriate antenna elements (an augmented array of antennas hosted by two or more base stations under the embodiments herein) and applicable control of the signals transmitted from those antenna elements (e.g., via weighted inputs derived from feedback returned from a targeted terminal). Under beamforming embodiments of the invention, current techniques employed for antenna arrays located at a single base stations (see, e.g., D. J. Love, R. W. Heath Jr., and T. Strohmer, “Grassmannian Beamforming for Multiple-Input Multiple-Output Wireless Systems,” IEEE Transactions on Information Theory, vol. 49, pp. 2735-2747, October 2003) are extended to support beamforming operations via selected antenna elements hosted by multiple base stations. As described below, it may be necessary to employ signal synchronization between multiple base stations to obtain the desired beamforming results.
In the embodiment of FIG. 5, information bits are encoded using one of space-time, space-frequency, or space-time-frequency coding schemes in a block 514. Block 514 is also employed to perform beamforming operations, as describe below in further detail with reference to FIG. 7b. The encoded output of block 514 is then provided to each of base stations 502 and 504, which in turn transmit respective signals 516 and 518. As depicted by lobes 520, 522, and 524, the channel characteristics of the combined signals 516 and 518 produce areas of higher gain in certain directions. At the same time, the gain of the combined signals 516 and 518 in other directions, such as depicted by a null direction 526, may be greatly reduced (e.g., to the point at which the signal cannot be decoded) due to spatial nulling. In one embodiment, spatial nulling is performed at the direction of un-intended subscribers.
For example, under the scenario illustrated in FIG. 5, the combined signals 516 and 518 are controlled so as to produce a high gain within lobe 522. As such, terminal 506 receives a good signal at its antenna array, and can decode the combined MIMO signal using appropriate MIMO decoding techniques that are well-known in the wireless communication system arts. Meanwhile, the strength of the combined signal received at a terminal 528 is nulled using spatial nulling. Accordingly, data corresponding to the information bits received at block 514 is transmitted to only terminal 506, and is not received by terminal 528.
FIG. 6 depicts another multi-user cooperative MIMO architecture 600. Instead of forming nulls to un-intended terminals, information from multiple users is jointed encoded, transmitted from multiple base stations via the augmented MIMO antenna array, and then decoded at the receiving terminals. In one embodiment of the invention, the information is decoded at the user ends independently. The signals intended for other users are treated as interference. In another embodiment, the information from all users are decoded jointly. In yet another embodiment, the information received at different user locations are consolidated for joint decoding.
The embodiment of FIG. 6 shows an example of joint decoding. In this instance, information to be sent to terminals 1 (606) and 2 (628) is jointly encoded using one of space-time, space-frequency, or space-time-frequency coding in a block 630. For clarity, the respective information to be sent to terminals 1 and 2 is depicted as data A and data B. The jointly encoded output of block 630 is provided as inputs to each of base stations 602 and 604. The base stations then transmit the jointly encoded data via selected antennas (corresponding to MIMO channels assigned to terminals 1 and 2) to terminals 606 and 628. Upon receipt of the jointly encoded data, it is decoded via operations performed in a block 632 for each of terminals 606 and 628. Upon being decoded, information intended for each recipient terminal is kept, while other information is discarded. Accordingly, terminal 606 keeps data A and discards data B, while terminal 628 keeps data B and discards data A. In one embodiment, information to keep and discard is identified by packet headers corresponding to packets that are extracted from the decoded data received at a given terminal.
A block diagram corresponding to one embodiment of an OFDMA (Orthogonal Frequency Division Multiple Access) encoding/transmitter module 700A for a base station having Nt transmit antennas is shown in FIG. 7a. Information bits for each of 1-N subcarriers are received at respective space-time coding (STC) blocks 7041-N. The size of the STCs is a function of the number of transmit antennas Nt. In general, the space-time codes may comprise space-time trellis codes (STTC), space-time block codes (STBC), as well as STTC or STBC with delay diversity, details of which are described below. Based on the applicable STC, each of blocks 7041-P outputs a set of code words c1[j,k] to cNt[j,k], wherein j represents the sub-channel index and k is the time index. Each of the code words is then forwarded to an appropriate Fast Fourier Transform (FFT) blocks 7061-Nt. The outputs of the FFT blocks 7061-Nt are then fed to parallel to serial (P/S) conversion blocks 7081-Nt, and cyclic prefixes are added via add cyclic prefix (CP) blocks 7101-Nt. The outputs of add CP blocks 7101-Nt are then provided to transmit antennas 1-Nt.to be transmitted as downlink signals to various terminals within the base station's coverage area.
A block diagram corresponding to one embodiment of an OFDMA receiver/decoder module 800 for a terminal having Nr receive antennas is shown in FIG. 8. The signal processing at the receive end of a downlink signal is substantially the inverse of the process used for encoding and preparing the signal for transmission. First, the cyclic prefix for each of the signals received at respective receive antennas 1-Nr is removed by a respective remove CP block blocks 8101-Nr. The respective signals are then fed into respective serial-to-parallel (S/P) conversion blocks 8081-Nt to produce parallel sets of data, which are then provided as inputs to FFT blocks 8061-Nr. The outputs of FFT blocks 8061-Nr are then forwarded to appropriate STC decoding blocks 8041-N for decoding. The decoded data is then output at the information bits for subcarriers 1-N.
A block diagram corresponding to one embodiment of an OFDMA encoding/beamforming/transmitter module 700B that performs beamforming is shown in FIG. 7b. As depicted by like-numbered blocks, much of the signal processing performed by the embodiments of FIGS. 7a and 7b is similar. In addition to these processing operations, OFDMA encoding/beamforming/transmitter module 700B further includes beamforming blocks 7051-N. Each of these beamforming blocks applies a weighted value W1-N to its respective inputs in view of control information provided by a beamforming control block 712, which is generated in response to beamforming feedback data 714. Further differences between the embodiments of FIGS. 7a and 7b include STC blocks 704A1-N, which now employ STCs having a size L, which represents the number of beamforming channels.
Space Time Encoding
Space-Time Codes (STC) were first introduced by Tarokh et al. from AT&T research labs (V. Tarokh, N. Seshadri, and A. R. Calderbank, Space-time codes for high data rates wireless communications: Performance criterion and code construction,” IEEE Trans. Inform. Theory, vol. 44, pp. 744-765, 1998) in 1998 as a novel means of providing transmit diversity for the multiple-antenna fading channel. There are two main types of STCs, namely space-time block codes (STBC) and space-time trellis codes (STTC). Space-time block codes operate on a block of input symbols, producing a matrix output whose columns represent time and rows represent antennas. Space-time block codes do not generally provide coding gain, unless concatenated with an outer code. Their main feature is the provision of full diversity with a very simple decoding scheme. On the other hand, space-time trellis codes operate on one input symbol at a time, producing a sequence of vector symbols whose length represents antennas. Like traditional TCM (trellis coded modulation) for a single-antenna channel, space-time trellis codes provide coding gain. Since they also provide full diversity gain, their key advantage over space-time block codes is the provision of coding gain. Their disadvantage is that they are difficult to design and generally require high complexity encoders and decoders.
FIG. 9 shows a block diagram of as STC MIMO transmission model. Under the model, data from an information source 900 is encoded using a STBC or STTC code by a space-time encoder 902. The encoded data is then transmitted over a MIMO link 904 to a receiver 906. The received signals are then decoded at the receiver to extract the original data.
An exemplary 8-PSK 8-state space-time trellis code for two antennas is shown in FIG. 10, while an exemplary 16-QAM 16-state STTC for two antennas is shown in FIG. 11. The encoding for STTCs are similar to TCM, except that at the beginning and the end of each frame, the encoder is required to be in the zero state. At each time t, depending on the state of the encoder and the input bits, a transition branch is selected. If the label of the transition branch is c1t; c2t: : : ; cnt, then transmit antenna i is used to send the constellation symbols cit, i=1; 2; : : : ; n and all these transmissions are in parallel. In general, an STTC encoder may be implemented via a state machine programmed with states corresponding to the trellis code that is to be implemented.
FIG. 12 shows a block diagram corresponding to an STBC model employing two antennas. As before, data is received from an information source 1200. Space time block encoding is then performed by the operations of space time block code 1202 and constellation maps 1204A and 1204B.
In further detail, an STBC is defined by a p×n transmission matrix G, whose entries are linear combinations of x1; : : : ; xk and their conjugates x1*; : : : ; xk* and whose columns are pairwise-orthogonal. In the case when p=n and {xi} are real, G is a linear processing orthogonal design which satisfies the condition that GTG=D, where D is a diagonal matrix with the (i; i)th diagonal element of the form (l1ix12+l2ix22+ . . . +lnixn2) with the coefficients l1i, l2i, . . . lni>0. An example of a 2×2 STBC code is shown in FIG. 12.
Another signal diversity scheme is to employ a combination of STC with delay. For example, FIGS. 13a and 13b respectively show models corresponding to an STTC with delay transmission scheme and an STBC with delay transmission scheme. In FIG. 13a, data from an information source 1300 is received by a code repetition block 1302, which produces a pair of replicated symbol sequences that are generated in view of the data. A first sequence of symbols is forwarded to an STTC encoder 1304A for encoding. Meanwhile, the replicated sequence of symbols is fed into a delay block 1306, which produces a one-symbol delay. The delayed symbol sequence output of delay block 1306 is then forwarded to STTC encoder 1304B for encoding. An exemplary 8-PSK 8-state delay diversity code for two antennas is shown in FIG. 14. As illustrated, the symbol sequence for transmission antenna Tx2 is synchronized with the input sequence, while the symbol sequence for transmission antenna Tx1 is delayed by one symbol.
Under the signal diversity scheme of FIG. 13b, data from information source 1300 is received at best block code selection logic 1308, which outputs replicated block codes to produce two block code sequences. The first block code sequence is forwarded to constellation mapper 1310A for encoding, while the second block code sequence is delayed by one symbol via a delay block 1312 and then forwarded to constellation mapper 1310B for encoding. The encoded signals are then transmitted via first and second transmit antennas.
The foregoing STTC and STBC schemes are depicted herein in accordance with conventional usage for clarity. Under such usage, the various encoded signals are transmitted using multiple antennas at the same base station. In contrast, embodiments of the invention employ selective antenna elements in antenna arrays from multiple base stations to form an augmented MIMO antenna array.
In order to implement an STC transmission scheme using multiple base stations, additional control elements may be needed. For example, if the base stations are located at different distances from a master encoder facility, there may need to be some measure to synchronize the antenna outputs in order to obtain appropriate MIMO transmission signals. Likewise, appropriate timing must be maintained when implementing a delay diversity scheme using antenna arrays at base stations at different locations.
FIG. 15 shows a cooperative MIMO architecture 1500 that employs a master encoder 1502. In general, the master encoder 1502 may be located at a separate facility from base stations 402 and 404, or may be co-located with one of the base stations. In respective embodiments, master encoder 1502 performs STC encoding and signal processing operations similar to the operations performed by the OFDMA encoding/transmitter module 700A of FIG. 7A (as depicted in FIG. 15) or OFDMA encoding/beamforming/transmitter module 700B of FIG. 7B. However, the transmission output are not fed directly to the transmission antennas, since the transmission antennas for at least one of the base stations will be located at a separate facility. Rather, master encoder 1502 produces respective sets of antenna drive signals 1504 and 1506 for base stations 402 and 404. Upon receipt of the antenna drive signals, corresponding downlink signals are transmitted by selected antennas hosted by base stations 402 and 404 based on the different MIMO channels supported by the system. Control inputs to master encoder 1502 corresponding to the MIMO channels are provided by a subscriber MIMO channel assignment register 1508.
If necessary, signal synchronization is performed by one or more sync/delay blocks 1510. For example, in the embodiment of FIG. 15, two sync/delay blocks 1510A and 1510B are shown, with each being employed at a respective base station. In other embodiments, some base stations may not require a delay block, particularly if a co-located master encoder is employed. In general, the sync/delay blocks for a system are employed to synchronize the antenna signals or synchronize the delay of antenna signals (when delay diversity is employed).
Signal synchronization may be performed in any number of ways using principles known in the communication arts. For example, in one embodiment separate timing signals or sequences are provided to each of the base stations in a cooperative MIMO system. The timing signals or sequences contain information from which corresponding antenna drive signals may be synchronized. To perform such synchronization, each sync/delay blocks add an appropriate delay to its antenna signals. Synchronization feedback information may also be employed using well-known techniques.
Under one embodiment of a variation of architecture 1500, antenna signal processing operations corresponding to the FFT, P/S, and add CP blocks are implemented at the respective base stations. In this instance, STC code sequences are provided to each of the base stations, with further antenna signal processing being performed at the base stations. Under this approach, timing signals or the like may be embedded in the data streams containing the code sequences.
Another approach for implementing an cooperative MIMO system is depicted by cooperative MIMO architecture 1600 in FIG. 16. Under this architecture, replicated instances of input information streams for multiple channel subscribers are generated by a block 1602 and provided to each of the base stations used to form the augmented MIMO antenna array. In this case, the STC encoding and signal processing operations are performed at each base station in a manner similar to that described with respect to the OFDMA encoding/transmitter module 700A of FIG. 7A (as depicted in FIG. 16) or OFDMA encoding/beamforming/transmitter module 700B of FIG. 7B.
In one embodiment, subscriber MIMO channel information is embedded in the input data streams received at each base station. Accordingly, there is a need to determine which antenna elements are used to support each MIMO channel. This information is stored in a subscriber MIMO channel register 1604, and is used to control signal processing in a collaborative manner at the base stations.
As before, there may be a need to synchronize the antenna signals. For example, if the components used to perform the operations of block 1602 are located at different distances from the base stations, the input streams will be received at different times. In response, the corresponding antenna signals will be generated at different times. To address this situation, one or more sync/delay blocks 1606 may be employed (e.g., as depicted by sync/delay blocks 1606A and 1606B in FIG. 16B. In one embodiment, timing signals are encoded in the input data streams using one of many well-known schemes. The timing signals, which may typically comprise timing frames, timing bits, and/or timing sequences, are extracted by 1606A and 1606B. In view of the timing information, a variable delay is applied by sync/delay block for the data streams that are received earlier, such that at the point the data streams are ready received at the STC blocks, they have been resynchronized.
In general, the processing operations performed by the process blocks depicted herein may be performed using known hardware and/or software techniques. For example, the processing for a given block may be performed by processing logic that may comprise hardware (circuitry, dedicated logic, etc.), software (such as is run on a general purpose computer system or a dedicated machine), or a combination of both.
Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as essential to the invention.

## Claims

1. A method comprising:
employing antenna elements from a plurality of base stations to support cooperative multiple-input multiple-output (MIMO) transmissions in a wireless network;
replicating a data stream received from an information source;
forwarding a replication of the data stream to each base station;
performing antenna signal processing operations at each base station on the data stream it receives to generate antenna signals; and
transmitting the antenna signals over selected antenna elements at each of the base station to form a cooperative MIMO transmission.

2. The method of claim 1, wherein the cooperative MIMO transmissions comprise orthogonal frequency division multiple access (OFDMA) MIMO transmissions.

3. The method of claim 1, further comprising encoding the cooperative MIMO transmissions using space-time coding.

4. The method of claim 3, wherein the space-time coding comprises space-time trellis coding.

5. The method of claim 3, wherein the space-time coding comprises space-time block coding.

6. The method of claim 3, further comprising encoding the cooperative MIMO transmissions using space-time coding with delay diversity.

7. The method of claim 1, further comprising encoding the cooperative MIMO transmissions using space-frequency coding.

8. The method of claim 3, further comprising encoding the cooperative MIMO transmissions using space-time-frequency coding.

9. The method of claim 1, further comprising performing spatial multiplexing on the cooperative MIMO transmissions.

10. The method of claim 1, wherein the cooperative MIMO transmissions include downlink transmissions from the plurality of base stations to terminals and uplink transmissions from the terminals to the plurality of base stations.

11. The method of claim 1, further comprising performing spatial beamforming for selected MIMO transmissions.

12. The method of claim 11, further comprising performing spatial beamforming in combination with one of space-time, space-frequency and space-time-frequency coding of the selected MIMO transmissions.

13. The method of claim 1, further comprising transmitting a cooperative MIMO transmission to at least two terminals simultaneously.

14. The method of claim 13, further comprising performing spatial beamforming on the cooperative MIMO transmissions such that MIMO transmissions are directed toward intended users while spatial nulling is effected toward unintended users

15. The method of claim 1, further comprising jointly decoding uplink MIMO transmissions received from multiple terminals.

16. The method of claim 1, further comprising jointly decoding downlink MIMO transmissions received at multiple terminals.

17. The method of claim 1, further comprising separately decoding uplink MIMO transmissions received from multiple terminals.

18. The method of claim 1, further comprising jointly encoding downlink MIMO transmissions sent to multiple terminals.

19. The method of claim 18, further comprising:
decoding a jointly encoded downlink MIMO transmission at a terminal; and
keeping portions of data sent via jointly encoded downlink MIMO transmission intended for the terminal, while discarding other portions of data that are not intended for the terminal.

20. The method of claim 18, further comprising separately decoding the jointly encoded downlink transmissions at each of the multiple terminals.

21. The method of claim 1, further comprising:
generating antenna drive signals via at a first facility;
sending respective portions of the antenna drive signals to each of the base stations; and
performing cooperative MIMO transmissions by driving selected antenna elements at the base stations with the respective portion of the antenna drive signals sent to those base stations.

22. The method of claim 21, wherein the first facility is co-located with one of the base stations.

23. The method of claim 21, further comprising synchronizing transmission of the respective portions of antenna drive signals sent to the base stations.

24. The method of claim 1, further comprising synchronizing performing the antenna signal processing operations at the base stations such that the generated antenna signals are transmitted over the selected antenna elements at different base stations in synchrony.

25. The method of claim 1, further comprising performing cooperative MIMO transmissions to facilitate terminal handoff between wireless network cells or sectors.

26. The method of claim 1, further comprising:
performing MIMO encoding on a data stream to be transmitted over a corresponding cooperative MIMO channel, the MIMO encoding producing respective sets of encoded data sequences on which antenna signal processing is to be performed at respective base stations;
sending each respective set of encoded data sequences to its corresponding base station; and
performing antenna signal processing operations to generate antenna signals at each of the base stations in view of the set of encoded data sequences received by that base station; and
transmitting the antenna signals over corresponding antenna elements at each of the base station to transmit the data stream over the cooperative MIMO channel.

27. The method of claim 26, further comprising synchronizing performing the antenna signal processing operations at the base stations such that the generated antenna signals are transmitted over the selected antenna elements at different base stations in synchrony.

28. A multicell wireless network, comprising:
a plurality of base stations, each associated with a respective cell and having a respective antenna array including at least one antenna element; and
a cooperative multiple-input multiple-output (MIMO) transmission mechanism that employs selected antenna elements from the plurality of base stations to form an augmented antenna array used to support cooperative MIMO transmissions over the wireless network;
a data replicator to:
replicate a data stream received from an information source; and
forward a replication of the data stream to each base station; and
a set of antenna signal processing components at each base station to perform antenna signal processing operations on the data stream it receives to generate antenna signals,
wherein the antenna signals are transmitted over selected antenna elements at each of the base station to form a cooperative MIMO transmission.

29. The multicell wireless network of claim 28, wherein the cooperative MIMO transmissions comprises orthogonal frequency division multiple access (OFDMA) MIMO transmissions.

30. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism encodes the cooperative MIMO transmissions using space-time coding.

31. The multicell wireless network of claim 30, wherein the space-time coding comprises space-time trellis coding.

32. The multicell wireless network of claim 30, wherein the space-time coding comprises space-time block coding.

33. The multicell wireless network of claim 30, wherein the cooperative MIMO transmission mechanism encodes the cooperative MIMO transmissions using space-time coding with delay diversity.

34. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism encodes the cooperative MIMO transmissions using space-frequency coding.

35. The multicell wireless network of claim 30, wherein the cooperative MIMO transmission mechanism encodes the cooperative MIMO transmissions using space-time-frequency coding.

36. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism performs spatial multiplexing on the cooperative MIMO transmissions.

37. The multicell wireless network of claim 28, wherein the cooperative MIMO transmissions include downlink transmissions from the plurality of base stations to terminals and uplink transmissions from the terminals to the plurality of base stations.

38. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism performs spatial beamforming for selected MIMO transmissions.

39. The multicell wireless network of claim 38, wherein the cooperative MIMO transmission mechanism performs spatial beamforming in combination with one of space-time, space-frequency and space-time-frequency coding of the selected MIMO transmissions.

40. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism performs transmitting a cooperative MIMO transmission to at least two terminals simultaneously.

41. The multicell wireless network of claim 40, wherein the cooperative MIMO transmission mechanism further performs spatial beamforming on the cooperative MIMO transmissions such that MIMO transmissions are directed toward intended users while spatial nulling is effected toward unintended users.

42. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism further performs jointly decoding uplink MIMO transmissions received from multiple terminals.

43. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism further performs separately decoding uplink MIMO transmissions received from multiple terminals.

44. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism further performs jointly encoding downlink MIMO transmissions sent to multiple terminals.

45. The multicell wireless network of claim 28, further comprising:
a cooperative MIMO master encoder to:
generate antenna drive signals; and
send respective portions of the antenna drive signals to each of the base stations;
wherein cooperative MIMO transmissions are performed by driving selected antenna elements at the base stations with the respective portion of the antenna drive signals sent to those base stations.

46. The multicell wireless network of claim 45, wherein the cooperative MIMO master encoder is co-located with one of the base stations.

47. The multicell wireless network of claim 46, further comprising a synchronizing mechanism to synchronize transmission of the respective portions of antenna drive signals sent to the base stations.

48. The multicell wireless network of claim 28, further comprising a synchronizing mechanism to synchronize the antenna signal processing operations at the base stations such that the generated antenna signals are transmitted over the selected antenna elements at different base stations in synchrony.

49. The multicell wireless network of claim 28, further comprising:
a master encoder to:
perform MIMO encoding on a data stream to be transmitted over a corresponding cooperative MIMO channel, the MIMO encoding producing respective sets of encoded data sequences on which antenna signal processing is to be performed at respective base stations;
send each respective set of encoded data sequences to its corresponding base station; and
a set of antenna signal processing components at each base station to perform antenna signal processing operations on the set of encoded data sequences it receives to generate antenna signals,
wherein the antenna signals are transmitted over selected antenna elements at each of the base station to form a cooperative MIMO transmission over the cooperative MIMO channel.

50. The multicell wireless network of claim 49, further comprising a synchronizing mechanism to synchronize the antenna signal processing operations at the base stations such that the generated antenna signals are transmitted over the selected antenna elements at different base stations in synchrony.

51. The multicell wireless network of claim 28, wherein the cooperative MIMO transmission mechanism performs cooperative MIMO transmissions to facilitate terminal handoff between wireless network cells or sectors.