PATENT DOCUMENT

Publication Number: US-9088443-B2
Application Number: US-201414242358-A
Country: US
Kind Code: B2

Title: Channel estimation and interference cancellation for virtual MIMO demodulation

Abstract:
A method and system for wireless communication in a wireless communication network. The wireless communication network has a first mobile terminal and a second mobile terminal arranged in virtual multiple input, multiple output (“V-MIMO”) communication with a base station. A first wireless communication uplink channel corresponding to the first mobile terminal is estimated. The estimate is based on a first reference symbol signal and the cancellation of interference from a second reference symbol signal received from the second mobile terminal. A second wireless communication uplink channel corresponding to the second mobile terminal is estimated. The estimate is based on the second reference symbol signal and the cancellation of interference from the first reference symbol signal received from the first mobile terminal. The estimated first wireless communication uplink channel is used to demodulate a first data signal received from the first wireless device, and the estimated second wireless communication uplink channel is used to demodulate a second data signal received from the second wireless device.

Claims:
The invention claimed is: 
     
       1. A method for operating a base station as part of a wireless communication network, the wireless communication network having a plurality of mobile terminals arranged in virtual multiple input, multiple output (“V-MIMO”) communication with the base station, the method comprising:
 receiving a composite signal in response to transmission of uplink reference signals respectively by the mobile terminals, wherein the received composite signal includes a superposition of signal components corresponding respectively to the uplink reference signals the mobile terminals, wherein the mobile terminals are configured to transmit respective sounding reference signals that are different from the uplink reference signals; 
 for each of the plurality of mobile terminals, determining a corresponding first channel estimate based at least on a sounding reference signal (SRS) transmitted by the mobile terminal; 
 for each of the plurality of mobile terminals, generating an initial estimate for the corresponding signal component based on the corresponding uplink reference signal and the corresponding first channel estimate; 
 for each of the plurality of mobile terminals, computing an improved estimate for the corresponding signal component by removing the initial estimate for one or more other mobile terminals from the received composite signal; and 
 for each of the plurality of mobile terminals, determining a corresponding second channel estimate based on the corresponding improved estimate and the corresponding uplink reference signal. 
 
     
     
       2. The method of  claim 1 , wherein each second channel estimate is determined using a least squares estimation method. 
     
     
       3. The method of  claim 2 , wherein each second channel estimate is determined in a frequency domain. 
     
     
       4. The method of  claim 1 , further comprising, for each of the plurality of mobile terminals:
 computing a corresponding channel response based on the corresponding second channel estimate and a corresponding correlation matrix, wherein said computing the corresponding channel response uses a minimum mean squared error (MMSE) method; and 
 demodulating a received data signal using the corresponding channel response to obtain corresponding demodulated user data for the mobile terminal. 
 
     
     
       5. The method of  claim 4 , further comprising:
 for each of the plurality of mobile terminals, performing a first error checking of the corresponding demodulated user data. 
 
     
     
       6. The method of  claim 5 , further comprising, in a case where there are no errors in the demodulated user data for the plurality of the mobile terminals:
 asserting that the demodulated user data from the plurality of the mobile terminals are valid; and 
 processing the demodulated user data as V-MIMO data. 
 
     
     
       7. The method of  claim 5 , further comprising, in a case where the first error checking of the demodulated data for a first of the plurality of mobile terminals yields a failure:
 generate an interference signal corresponding to at least one of the mobile terminals other than the first mobile terminal, wherein said generating the interference signal uses the demodulated user data and the channel response corresponding to the at least one of the mobile terminals; 
 cancel the interference signal from the received data signal to obtain an interference canceled signal corresponding to the first mobile terminal; and 
 regenerate the user data for the first mobile terminal based on the interference canceled signal corresponding to the first mobile terminal. 
 
     
     
       8. The method of  claim 7 , wherein said cancelling the interference signal from the received data signal includes:
 (a) multiplying the channel response corresponding to said at least one of the mobile terminals by the demodulated user data corresponding to the same at least one of the mobile terminals; and 
 (b) subtracting the multiplication result of step (a) from the received data signal. 
 
     
     
       9. The method of  claim 7 , further comprising performing a second error checking on the regenerated user data for the first mobile terminal. 
     
     
       10. The method of  claim 9 , further comprising signaling a next hybrid automatic repeat request (HARM) if the second error checking yields a failure. 
     
     
       11. A base station for use in a wireless communication system, the base station capable of engaging in wireless communication with a plurality of mobile terminals arranged in virtual multiple input multiple output (“V-MIMO”) communication with the base station, the base station arranged to:
 receive a composite signal in response to transmission of uplink reference signals respectively by the mobile terminals, wherein the received composite signal includes a superposition of signal components corresponding respectively to the uplink reference signals, wherein the mobile terminals are configured to transmit respective sounding reference signals that are different from the uplink reference signals; 
 for each of the plurality of mobile terminals, determine a corresponding first channel estimate based at least on a sounding reference signal (SRS) transmitted by the mobile terminal; 
 for each of the plurality of mobile terminals, generate an initial estimate for the corresponding signal component based on the corresponding uplink reference signal and the corresponding first channel estimate; 
 for each of the plurality of mobile terminals, compute an improved estimate for the corresponding signal component by removing the initial estimate for one or more other mobile terminals from the received composite signal; and 
 for each of the plurality of mobile terminals, determine a corresponding second channel estimate based on the corresponding improved estimate and the corresponding uplink reference signal. 
 
     
     
       12. The base station of  claim 11 , wherein the base station is further arranged to, for each of the plurality of mobile terminals:
 compute a corresponding channel response based on the corresponding second channel estimate and a corresponding correlation matrix, wherein said computing the corresponding channel response uses a minimum mean squared error (MMSE) method; and 
 demodulate a received data signal using the corresponding channel response to obtain corresponding demodulated user data for the mobile terminal. 
 
     
     
       13. The base station of  claim 12 , wherein the base station is further arranged to, for each of the plurality of mobile terminals, perform a first error checking of the corresponding demodulated user data. 
     
     
       14. The base station of  claim 13 , wherein, in a case where the first error checking of the demodulated data for a first of the plurality of mobile terminals yields a failure, the base station further arranged to:
 generate an interference signal corresponding to at least one of the mobile terminals other than the first mobile terminal, wherein said generating the interference signal uses the demodulated user data and the channel response corresponding to the at least one of the mobile terminals; 
 cancel the interference signal from the received data signal to obtain an interference canceled signal corresponding to the first mobile terminal; and 
 regenerate the user data for the first mobile terminal based on the interference canceled signal corresponding to the first mobile terminal. 
 
     
     
       15. The base station of  claim 14 , wherein said cancelling the interference signal from the received data signal includes:
 (a) multiplying the channel response corresponding to said at least one of the mobile terminals by the demodulated user data corresponding to the same at least one of the mobile terminals; and 
 (b) subtracting the multiplication result of step (a) from the received data signal. 
 
     
     
       16. A non-transitory memory medium storing program instructions for operating a base station as part of a wireless communication network, the wireless communication network having a plurality of mobile terminals arranged in virtual multiple input, multiple output (“V-MIMO”) communication with the base station, wherein the program instructions, when executed by a processor, cause the processor to implement:
 receiving a composite signal in response to transmission of uplink reference signals respectively by the mobile terminals, wherein the received composite signal includes a superposition of signal components corresponding respectively to the uplink reference signals, wherein the mobile terminals are configured to transmit respective sounding reference signals that are different from the uplink reference signals; 
 for a first of the mobile terminals, computing an improved estimate for the signal component corresponding to the first mobile terminal by removing at least an initial estimate for the signal component of a second of the mobile terminals from the received composite signal, wherein the initial estimate for the signal component of the second mobile terminal is determined based on the uplink reference signal from the second mobile terminal and an initial channel estimate corresponding to the second mobile terminal, wherein the initial channel estimate is determined based at least on a sounding reference signal (SRS) transmitted by the second mobile terminal; 
 for the first mobile terminal, determining an updated channel estimate based on (a) the improved estimate for the signal component corresponding to the first mobile terminal and (b) the uplink reference signal transmitted by the first mobile terminal. 
 
     
     
       17. The non-transitory memory medium of  claim 16 , wherein the updated channel estimate for the first mobile terminal is determined in the frequency domain. 
     
     
       18. The non-transitory memory medium of  claim 16 , wherein the program instructions, when executed by the processor, further cause the processor to implement:
 for the first mobile terminal, computing a corresponding channel response based on the updated channel estimate for the first mobile terminal and a corresponding correlation matrix. 
 
     
     
       19. The non-transitory memory medium of  claim 18 , wherein the program instructions, when executed by the processor, further cause the processor to implement:
 for the first mobile terminal, demodulating a received data signal using the channel response for the first mobile terminal to obtain demodulated user data for the first mobile terminal. 
 
     
     
       20. The non-transitory memory medium of  claim 19 , wherein the program instructions, when executed by the processor, further cause the processor to implement:
 for the first mobile terminal, asserting that the demodulated user data for the first mobile terminals is valid in response to determining that there are no errors in the demodulated user data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/129,316, filed on May 13, 2011, entitled “Method and System for Reduced Complexity Channel Estimation and Interference Cancellation for V-MIMO Demodulation”, invented by John Panicker et al., which is a National Stage Entry of International Application No. PCT/CA2009/001627, filed Nov. 12, 2009, which claims the benefit of priority to U.S. Provisional Application No. 61/114,351, filed Nov. 13, 2008. All of the above-identified Applications are hereby incorporated by reference in their entireties as though fully and completely set forth herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to wireless communications and in particular to a method and system for wireless communication channel estimation and interference cancellation used to demodulate virtual multiple input, multiple output (“V-MIMO”) signals. 
     BACKGROUND OF THE INVENTION 
     Demand for high speed wireless communications is increasing at a fast pace. This is driven both by the sheer number of wireless communication terminals being activated as well as the increasing bandwidth demand. The latter is in turn driven by the increasing number of applications consuming the bandwidth, e.g., streaming multimedia, web browsing, GPS functionality, etc. As the computation capacity of the wireless communication terminals increases, so too do the terminals&#39; ability to execute complex, bandwidth consuming applications. 
     Wireless communication networks, such as cellular networks, operate by sharing resources among the mobile terminals operating in the communication network. As part of the sharing process, resources relating to assigned channels, codes, etc. are allocated by one or more controlling devices within the system. Certain types of wireless communication networks, e.g., orthogonal frequency division multiplexed (“OFDM”) networks, are used to support cell-based high speed services such as those under certain standards such as the 3rd Generation Partnership Project (“3GPP”) e.g., Long Term Evolution (“LTE”), 3GPP2, e.g., Ultra-Mobile Broadband (“UMB”) and the IEEE 802.16 broadband wireless standards. The IEEE 802.16 standards are often referred to as WiMAX or less commonly as WirelessMAN or the Air Interface Standard. 
     OFDM technology uses a channelized approach and divides a wireless communication channel into many sub-channels which can be used by multiple mobile terminals at the same time. These sub-channels and hence the mobile terminals can be subject to interference from adjacent cells and other mobile terminals because neighboring base stations and mobile terminals can use the same time and frequency resource blocks. The result is that spectral efficiency is reduced, thereby reducing both communication throughput as well as the quantity of mobile terminals that can be supported in the network. 
     This problem is further exacerbated in multiple input, multiple output (“MIMO”) environments. Multiple Input, Multiple Output Orthogonal Frequency Division Multiplexing (“MIMO-OFDM”) is an OFDM technology that uses multiple antennas to transmit and receive radio signals. MIMO-OFDM allows service providers to deploy wireless broadband systems that take advantage of the multi-path properties of environments using base station antennas that do not necessarily have line of sight communications with the mobile terminal. 
     MIMO systems use multiple transmit and receive antennas to simultaneously transmit data, in small pieces to the receiver, which processes the separate data transmissions and puts them back together. This process, called spatial multiplexing, can be used to proportionally boost the data-transmission speed by a factor equal to the smaller of the number of transmitting and receiving antennas. In addition, since all data is transmitted both in the same frequency band and with separate spatial signatures, this technique utilizes spectrum very efficiently. 
     MIMO operation implements a channel matrix (N×M) where N is the number of transmit antennas and M is the number of receive antennas to define the coding and modulation matrix for the wireless communication channel as a whole. The less correlated each column in the matrix is, the less interference experienced in each channel (as a result of the multiple antennas). In the case where there is a totally uncorrelated arrangement, i.e., the dot product between columns is zero, the channels are considered orthogonal to one another. Orthogonality provides the least antenna-to-antenna interference, thereby maximizing channel capacity, and data rate due to the higher post-processing signal to interference and noise ratio (“PP-SINR”). PP-SINR is the SINR after the MIMO decoding stage. 
     Virtual MIMO (“V-MIMO”), also referred to as Multi-User MIMO (“MU-MIMO”) implements the MIMO technique described above by using multiple simultaneously transmitting mobile terminals each having one or more antennas. The serving base station includes multiple antennas. Although the base station can treat virtual MIMO operation as traditional MIMO in which a single mobile terminal has multiple antennas and can separate and decode the transmissions from the multiple simultaneously transmitting mobile terminals, channel correlation among mobile terminals as discussed above results in channel capacity loss due to inter-mobile terminal interference. 
     Because wireless communication channels are subject to interference and distortion, techniques have been developed to estimate certain properties of the channel so that the receiver, e.g., base station, can take these properties into account when decoding the received data. For example, multipath distortion and fading can alter the amplitude and phase of the transmitted wireless signal. The result is that, if the wireless communication channel is not accurately estimated, the decoded data will likely be improperly decoded. For example, a 16QAM or 64QAM (quadrature amplitude modulation) signal modulates a plurality of bits. Decoding of those bits is based on the amplitude and phase of the received signal as applied to a modulation constellation. If the amplitude and/or phase of the transmitted signal changes by the time it is received at the receiver, the mapping to the constellation will be errant, resulting in improper decoding. If the channel can be estimated by the receiver, the changes in amplitude and phase can be considered by the receiver during the mapping and decoding process. 
     The problem is made even more complex in V-MIMO environments. V-MIMO relies on spatial multiplexing. In order to properly recover the signal, the receiver also must decorrelate the signals and remove interference. These tasks have traditionally been done in the time domain. These tasks are quite processing and time intensive when 2, 4 or more mobile terminals are part of the V-MIMO arrangement. The result is that the cost of equipment at the receiver becomes exorbitant, if it even can be implemented all. 
     Also, while techniques for channel estimation based on least squares algorithms are known, these techniques are insufficient for V-MIMO implementations, such as where two or more mobile terminal signals are superimposed in a set of resource blocks. Even the use of known minimum mean square error (“MMSE”) techniques fall short for V-MIMO applications. 
     Therefore what is needed is a cost effective, scalable and processing efficient system and method for estimating a wireless communication channel and cancelling interference that can be used in a V-MIMO environment such as on the base station uplink receiver in an LTE network. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides a method and system for demodulating uplink data (from mobile terminal to base station) in a virtual multiple input, multiple output (“V-MIMO”) wireless communication network. Reference symbol signals are used to estimate the wireless channels and the mutual interference between multiple mobile terminal or base station transmissions by using the estimates to cancel interference from the other mobile terminals or base stations participating in the V-MIMO session. These estimates are then used to demodulate the uplink user, i.e., mobile device, data signals. Error, e.g., CRC, checking is performed on the demodulated user data. In the case where the error checking from one of the mobile terminals fails and the error checking from the other mobile terminal passes (yielding valid user data), the correctly demodulated data from the passing mobile terminal is used to cancel the interference from the failing mobile terminal data signal. The user data signal is regenerated post-cancellation and the re-checked for errors. 
     In accordance with one aspect, the present invention provides a method for wireless communication in a wireless communication network in which the wireless communication network has a plurality of mobile terminals arranged in virtual multiple input, multiple output (“V-MIMO”) communication with a base station. Of note, although the term V-MIMO is used herein to describe the present invention, it is understood that this term is not intended to limit the invention in any way and that this term as used herein is interchangeable with multi-user MIMO (“MU-MIMO”) and co-operative MIMO. An uplink reference signal is received from each of the plurality of mobile terminals. A first reference signal channel estimate is determined for each of the plurality of mobile terminals based on the corresponding received reference signal. An interference cancelled estimate is received for each of the plurality of mobile terminals using the corresponding first reference signal channel estimate. A corresponding second reference signal channel estimate is determined for each of the plurality of mobile terminals based on the corresponding interference cancelled estimate. 
     In accordance with another aspect, the present invention provides a base station for use in wireless communication system in which the base station is capable of engaging in wireless communication with a plurality of mobile terminals arranged in virtual multiple input multiple output (“V-MIMO”) communication with the base station. The base station receives an uplink reference signal from each of the plurality of mobile terminals, determines a first reference signal channel estimate for each of the plurality of mobile terminals based on the corresponding received reference signal, determines an interference cancelled estimate for each of the plurality of mobile terminals using the corresponding first reference signal channel estimate and determines a corresponding second reference signal channel estimate for each of the plurality of mobile terminals based on the corresponding interference cancelled estimate. 
     In accordance with yet another aspect, the present invention provides a method system for wireless communication in a wireless communication network. The wireless communication network has a first mobile terminal and a second mobile terminal arranged in virtual multiple input, multiple output (“V-MIMO”) communication with a base station. A first wireless communication uplink channel corresponding to the first mobile terminal is estimated. The estimate is based on a first reference symbol signal received from the first mobile terminal and is based on a second reference symbol received from the second mobile terminal. The second reference symbol signal is used to estimate and cancel the interference of a third reference symbol signal received from the second mobile terminal from the first reference symbol signal. A second wireless communication uplink channel corresponding to the second mobile terminal is estimated. The estimate for the second wireless communication uplink channel is based on the third reference symbol signal received from the second mobile terminal and the cancellation of interference by the first reference symbol signal received from the first mobile terminal based on a fourth reference signal received from the first mobile terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a diagram of an embodiment of a system constructed in accordance with the principles of the present invention; 
         FIG. 2  is a block diagram of an exemplary base station constructed in accordance with the principles of the present invention; 
         FIG. 3  is a block diagram of an exemplary mobile terminal constructed in accordance with the principles of the present invention; 
         FIG. 4  is a block diagram of an exemplary OFDM architecture constructed in accordance with the principles of the present invention; 
         FIG. 5  is a block diagram of the flow of received signal processing in accordance with the principles of the present invention; 
         FIG. 6  is a diagram of an exemplary scattering of pilot symbols among available sub-carriers; 
         FIG. 7  is a block diagram of an exemplary channel estimation process of the present invention; 
         FIGS. 8A and 8B  are a flow chart of an exemplary uplink data demodulation and interference cancellation process of the present invention; 
         FIG. 9  is a flow chart detailing the regeneration and the first mobile terminal to second mobile terminal interference cancellation process of  FIGS. 8A and 8B ; 
         FIG. 10  is a flow chart detailing the regeneration and the second mobile terminal to first mobile terminal interference cancellation process of  FIGS. 8A and 8B ; and 
         FIG. 11  is a graph of signal to noise ratio vs. symbol error rate for a number of exemplary wireless uplink communications processes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As an initial matter, while certain embodiments are discussed in the context of wireless networks operating in accordance with the 3rd Generation Partnership Project (“3GPP”) evolution, e.g., Long Term Evolution (“LTE”) standard, etc., the invention is not limited in this regard and may be applicable to other broadband networks including those operating in accordance with other orthogonal frequency division multiplexing (“OFDM”)-based systems including WiMAX (IEEE 802.16) and Ultra-Mobile Broadband (“UMB”), etc. Similarly, the present invention is not limited solely to OFDM-based systems and can be implemented in accordance with other system technologies, e.g., code division multiple access (“CDMA”), single carrier frequency division multiple access (“SC-FDMA”), etc. 
     Before describing in detail exemplary embodiments that are in accordance with the present invention, it is noted that the embodiments reside primarily in combinations of system components and processing steps related to improving wireless communication channel estimation and interference cancellation for virtual multiple input, multiple output (“V-MIMO”) demodulation, such as in an LTE uplink receiver. 
     Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     Referring now to the drawing figures in which like reference designators refer to like elements, there is shown in  FIG. 1 , a system constructed in accordance with the principles of the present invention and designated generally as “ 6 .” System  6  includes one or more base stations  8  (known as eNodeB in LTE systems) and one or more mobile terminals  10  (shown as mobile terminals  10   a  and  10   b  in  FIG. 1 ). Of note, although the term “base stations” is used herein, it is understood that these devices are referred to as “eNodeB” devices in LTE environments. Accordingly, the use of the term “base station” herein is not intended to limit the present invention to a particular technology implementation. Rather, the term “base station” is used for ease of understanding, it being intended to be interchangeable with the term “eNodeB” within the context of the present invention. Although not shown, mobile terminals  10  can communicate with base stations  8  via one or more relay nodes. Base stations  8  communicate with one another and with external networks, such as the Internet (not shown), via carrier network  12 . Base stations  8  engage in wireless communication with mobile terminals  10  directly or via one or more relay nodes. Similarly, mobile terminals  10  engage in wireless communication with base stations  8  directly or via one or more relay nodes. 
     Base station  8  can be any base station arranged to wirelessly communicate with mobile terminals  10 . Base stations  8  include the hardware and software used to implement the functions described herein to support V-MIMO uplink channel estimation and interference cancellation in accordance with the present invention. Base stations  8  include a central processing unit, transmitter, receiver, I/O devices and storage such as volatile and nonvolatile memory as may be needed to implement the functions described herein. Base stations  8  are described in additional detail below. 
     According to one embodiment, mobile terminals  10  may include a wide range of portable electronic devices, including but not limited to mobile phones, wireless data terminals, and similar devices, which use the various communication technologies such as LTE, advanced mobile phone system (“AMPS”), time division multiple access (“TDMA”), CDMA, global system for mobile communications (“GSM”), general packet radio service (“GPRS”), 1x evolution-data optimized (abbreviated as “EV-DO” or “1xEV-DO”) and universal mobile telecommunications system (“UMTS”). Mobile terminals  10  also include the hardware and software suitable to support the functions used to engage in wireless V-MIMO communication with base station  8 . Such hardware can include a receiver, transmitter, central processing unit, storage in the form of volatile and nonvolatile memory, input/output devices, etc. 
     Relay nodes (not shown) are optionally used to facilitate wireless communication between mobile terminal  10  and base station  8  in the uplink (mobile terminal  10  to base station  8 ) and/or the downlink (base station  8  to mobile terminal  10 ). A relay node configured in accordance with the principles of the present invention includes a central processing unit, storage in the form of volatile and/or nonvolatile memory, transmitter, receiver, input/output devices and the like. Relay nodes also include software to implement the MAC control functions described herein. Of note, the arrangement shown in  FIG. 1  is general in nature and other specific communication embodiments constructed in accordance with the principles of the present invention are contemplated. 
     Although not shown, system  6  can include a base station controller (“BSC”) that controls wireless communications within multiple cells, which are served by corresponding base stations (“BS”)  8 . It is understood that some implementations, such as LTE and WiMAX, do not make use of a BSC. In general, each base station  8  facilitates communications using V-MIMO OFDM with mobile terminals  10 , which are illustrated as being within the geographic confines of the cell  14  associated with the corresponding base station. Movement of mobile terminals  10  in relation to the base stations  8  can result in significant fluctuation in channel conditions as a consequence of multipath distortion, terrain variation, reflection and/or interference caused by man-made objects (such as buildings and other structures), and so on. 
     Multiple mobile terminals  10  may be logically grouped together to form a V-MIMO group  16 . Of note, although  FIG. 1  shows two mobile terminals  10  grouped to form V-MIMO group  16 , the invention is not limited to such. It is contemplated that more than two mobile terminals can exist in a V-MIMO group  16 . It is also contemplated that a mobile terminal can have more than one antenna to operate using traditional MIMO for wireless communications as well as participate as part of a V-MIMO group  16 . Even using diversity channels, where orthogonality-based scheduling is ineffective and mobile terminals  10  therefore interfere with each other, mobile terminals  10  can still be paired in accordance with the present invention to take advantage of the multi-user gain associated with MIMO wireless communication. 
     Base station  8  is also shown in  FIG. 1  as including two antennas  18  to support V-MIMO operation. It is understood that the present invention is not limited to such and that base station  8  can include more than two antennas  18  or even a single antenna  18  in support of multiple mobile terminals  10 .  FIG. 1  shows each mobile terminal  10  engaged in wireless communication with each antenna  18  of base station  8 . As is discussed below in detail, base station  8  includes receiver software and/or hardware to estimate the wireless channel using the minimum mean square error (“MMSE”) approach discussed below in detail in accordance with the present invention. Base station  8  also includes receiver software and/or hardware to cancel interference on the V-MIMO uplink 
     A high level overview of the mobile terminals  10  and base stations  8  of the present invention is provided prior to delving into the structural and functional details of the preferred embodiments. It is understood that relay nodes can incorporate those structural and functional aspects described herein with respect to base stations  8  and mobile terminals  10  as may be needed to perform the functions described herein. 
     With reference to  FIG. 2 , a base station  8  configured according to one embodiment of the present invention is illustrated. The base station  8  generally includes a control system  20 , a baseband processor  22 , transmit circuitry  24 , receive circuitry  26 , one or more antennas  18 , and a network interface  30 . The receive circuitry  26  receives radio frequency signals bearing information from one or more remote transmitters provided by mobile terminals  10  (illustrated in  FIG. 3 ). Preferably, a low noise amplifier and a filter (not shown) cooperate to amplify and remove out-of-band interference from the signal for processing. Down conversion and digitization circuitry (not shown) then down converts the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  22  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor  22  is generally implemented in one or more digital signal processors (“DSPs”) or application-specific integrated circuits (“ASICs”). The received information is then sent across a wireline or wireless network via the network interface  30  or transmitted to another mobile terminal  10  serviced by the base station  8 . 
     On the transmit side, the baseband processor  22  receives digitized data, which may represent voice, data, or control information, from the network interface  30  under the control of control system  20 , and encodes the data for transmission. The encoded data is output to the transmit circuitry  24 , where it is modulated by a carrier signal having a desired transmit frequency or frequencies. A power amplifier (not shown) amplifies the modulated carrier signal to a level appropriate for transmission, and delivers the modulated carrier signal to the antennas  18  through a matching network (not shown). Modulation and processing details are described in greater detail below. 
     With reference to  FIG. 3 , a mobile terminal  10  configured according to one embodiment of the present invention is described. Similar to base station  8 , a mobile terminal  10  constructed in accordance with the principles of the present invention includes a control system  32 , a baseband processor  34 , transmit circuitry  36 , receive circuitry  38 , one or more antennas  40 , and user interface circuitry  42 . The receive circuitry  38  receives radio frequency signals bearing information from one or more base stations  8 . Preferably, a low noise amplifier and a filter (not shown) cooperate to amplify and remove out-of-band interference from the signal for processing. Down conversion and digitization circuitry (not shown) then down convert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  34  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations, as will be discussed in greater detail below. The baseband processor  34  is generally implemented in one or more digital signal processors (“DSPs”) and application specific integrated circuits (“ASICs”). 
     With respect to transmission, the baseband processor  34  receives digitized data, which may represent voice, data, or control information, from the control system  32 , which the baseband processor  34  encodes for transmission. The encoded data is output to the transmit circuitry  36 , where it is used by a modulator to modulate a carrier signal that is at a desired transmit frequency or frequencies. A power amplifier (not shown) amplifies the modulated carrier signal to a level appropriate for transmission, and delivers the modulated carrier signal to the antennas  40  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are applicable to the present invention. 
     In OFDM modulation, the transmission band is divided into multiple, orthogonal carrier waves. Each carrier wave is modulated according to the digital data to be transmitted. Because OFDM divides the transmission band into multiple carriers, the bandwidth per carrier decreases and the modulation time per carrier increases. Since the multiple carriers are transmitted in parallel, the transmission rate for the digital data, or symbols, on any given carrier is lower than when a single carrier is used. 
     OFDM modulation is implemented, for example, through the performance of an Inverse Fast Fourier Transform (“IFFT”) on the information to be transmitted. For demodulation, a Fast Fourier Transform (“FFT”) on the received signal is performed to recover the transmitted information. In practice, the IFFT and FFT are provided by digital signal processing carrying out an Inverse Discrete Fourier Transform (“IDFT”) and Discrete Fourier Transform (“DFT”), respectively. Accordingly, the characterizing feature of OFDM modulation is that orthogonal carrier waves are generated for multiple bands within a transmission channel. The modulated signals are digital signals having a relatively low transmission rate and capable of staying within their respective bands. The individual carrier waves are not modulated directly by the digital signals. Instead, all carrier waves are modulated at once by IFFT processing. 
     In one embodiment, OFDM is used for at least the downlink transmission from the base stations  8  to the mobile terminals  10 . Each base station  8  is equipped with n transmit antennas  18 , and each mobile terminal  10  is equipped with one or more receive antennas  40 , the total of which is referred to as m. Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labeled only for clarity.  FIG. 1  shows n=2 and m=2. 
     With reference to  FIG. 4 , a logical OFDM transmission architecture is described according to one embodiment. Initially, the base station controller sends data to be transmitted to various mobile terminals  10  to the base station  8 . The base station  8  may use the channel quality indicators (“CQIs”) associated with the mobile terminals to schedule the data for transmission as well as select appropriate coding and modulation for transmitting the scheduled data. The CQIs may be provided directly by the mobile terminals  10  or determined at the base station  8  based on information provided by the mobile terminals  10 . In either case, the CQI for each mobile terminal  10  is a function of the degree to which the channel amplitude (or response) varies across the OFDM frequency band and the strength of the transmitted signal. 
     The scheduled data  44 , which is a stream of bits, is scrambled in a manner reducing the peak-to-average power ratio associated with the data using data scrambling logic  46 . A cyclic redundancy check (“CRC”) for the scrambled data is determined and appended to the scrambled data using CRC adding logic  48 . Next, channel coding is performed using channel encoder logic  50  to effectively add redundancy to the data to facilitate recovery and error correction at the mobile terminal  10 . Again, the channel coding for a particular mobile terminal  10  is based on the CQI. The channel encoder logic  50  uses known Turbo encoding techniques in one embodiment. The encoded data is then processed by rate matching logic  52  to compensate for the data expansion associated with encoding. 
     Bit interleaver logic  54  systematically reorders the bits in the encoded data to minimize the loss of consecutive data bits. The resultant data bits are systematically mapped into corresponding symbols depending on the chosen baseband modulation by mapping logic  56 . Preferably, Quadrature Amplitude Modulation (“QAM”) or Quadrature Phase Shift Key (“QPSK”) modulation is used. The degree of modulation is preferably chosen based on the CQI for the particular mobile terminal. The symbols may be systematically reordered to further bolster the immunity of the transmitted signal to periodic data loss caused by frequency selective fading using symbol interleaver logic  58 . 
     At this point, groups of bits have been mapped into symbols representing locations in an amplitude and phase constellation. When spatial diversity is desired, blocks of symbols are then processed by space-time block code (“STC”) encoder logic  60 , which modifies the symbols in a fashion making the transmitted signals more resistant to interference and more readily decoded at a mobile terminal  10 . The STC encoder logic  60  will process the incoming symbols and provide  17  outputs corresponding to the number of transmit antennas  18  for the base station  8 . The control system  20  and/or baseband processor  22  will provide a mapping control signal to control STC encoding. At this point, assume the symbols for the n outputs are representative of the data to be transmitted and capable of being recovered by the mobile terminal  10 . 
     For the present example, assume the base station  8  has two antennas  18  (n=2) and the STC encoder logic  60  provides two output streams of symbols. Accordingly, each of the symbol streams output by the STC encoder logic  60  is sent to a corresponding IFFT processor  62 , illustrated separately for ease of understanding. Those skilled in the art will recognize that one or more processors may be used to provide such digital signal processing, alone or in combination with other processing described herein. It is further envisioned that processing functionality can likewise be consolidated into a lesser number of processors than referenced herein. The IFFT processors  62  will preferably operate on the respective symbols to provide an inverse Fourier Transform. The output of the IFFT processors  62  provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by like insertion logic  64 . Each of the resultant signals is up-converted in the digital domain to an intermediate frequency and converted to an analog signal via the corresponding digital up-conversion (DUC) and digital-to-analog (D/A) conversion circuitry  66 . The resultant (analog) signals are then simultaneously modulated at the desired RF frequency, amplified, and transmitted via the RF circuitry  68  and antennas  18 . Notably, pilot signals known by the intended mobile terminals  10  are scattered among the sub-carriers. The mobile terminals  10 , which are discussed in detail below, will use the pilot signals for channel estimation. 
     Reference is now made to  FIG. 5  to illustrate reception of the transmitted signals by a mobile terminal  10 . Upon arrival of the transmitted signals at each of the antennas  40  of the mobile terminal  10 , the respective signals are demodulated and amplified by corresponding RF circuitry  70 . For the sake of conciseness and clarity, only one of the receive paths is described and illustrated in detail, it being understood that a receive path exists for each antenna  40 . Analog-to-digital (“A/D”) converter and down-conversion circuitry  72  digitizes and downconverts the analog signal for digital processing. The resultant digitized signal may be used by automatic gain control circuitry (“AGC”)  74  to control the gain of the amplifiers in the RF circuitry  70  based on the received signal level. 
     Initially, the digitized signal is provided to synchronization logic  76 , which includes coarse synchronization logic  78 , which buffers several OFDM symbols and calculates an auto-correlation between the two successive OFDM symbols. A resultant time index corresponding to the maximum of the correlation result determines a fine synchronization search window, which is used by fine synchronization logic  80  to determine a precise framing starting position based on the headers. The output of the fine synchronization logic  80  facilitates frame acquisition by frame alignment logic  84 . Proper framing alignment is important so that subsequent FFT processing provides an accurate conversion from the time to the frequency domain. The fine synchronization algorithm is based on the correlation between the received pilot signals carried by the headers and a local copy of the known pilot data. Once frame alignment acquisition occurs, the prefix of the OFDM symbol is removed with prefix removal logic  86  and resultant samples are sent to frequency offset correction logic  88 , which compensates for the system frequency offset caused by the unmatched local oscillators in the transmitter and the receiver. Preferably, the synchronization logic  76  includes frequency offset and clock estimation logic  82 , which is based on the headers to help estimate such effects on the transmitted signal and provide those estimations to the correction logic  88  to properly process OFDM symbols. 
     At this point, the OFDM symbols in the time domain are ready for conversion to the frequency domain using FFT processing logic  90 . The results are frequency domain symbols, which are sent to processing logic  92 . The processing logic  92  extracts the scattered pilot signal using scattered pilot extraction logic  94 , determines a channel estimate based on the extracted pilot signal using channel estimation logic  96 , and provides channel responses for all sub-carriers using channel reconstruction logic  98 . In order to determine a channel response for each of the sub-carriers, the pilot signal is essentially multiple pilot symbols that are scattered among the data symbols throughout the OFDM sub-carriers in a known pattern in both time and frequency.  FIG. 6  illustrates an exemplary scattering of pilot symbols among available sub-carriers over a given time and frequency plot in an OFDM environment. Referring again to  FIG. 5 , the processing logic compares the received pilot symbols with the pilot symbols that are expected in certain sub-carriers at certain times to determine a channel response for the sub-carriers in which pilot symbols were transmitted. The results are interpolated to estimate a channel response for most, if not all, of the remaining sub-carriers for which pilot symbols were not provided. The actual and interpolated channel responses are used to estimate an overall channel response, which includes the channel responses for most, if not all, of the sub-carriers in the OFDM channel. 
     The frequency domain symbols and channel reconstruction information, which are derived from the channel responses for each receive path are provided to an STC decoder  100 , which provides STC decoding on both received paths to recover the transmitted symbols. The channel reconstruction information provides equalization information to the STC decoder  100  sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols 
     The recovered symbols are placed back in order using symbol de-interleaver logic  102 , which corresponds to the symbol interleaver logic  58  of the transmitter. The de-interleaved symbols are then demodulated or de-mapped to a corresponding bitstream using de-mapping logic  104 . The bits are then de-interleaved using bit de-interleaver logic  106 , which corresponds to the bit interleaver logic  54  of the transmitter architecture. The de-interleaved bits are then processed by rate de-matching logic  108  and presented to channel decoder logic  110  to recover the initially scrambled data and the CRC checksum. Accordingly, CRC logic  112  removes the CRC checksum, checks the scrambled data in traditional fashion, and provides it to the de-scrambling logic  114  for de-scrambling using the known base station de-scrambling code to recover the originally transmitted data  116 . 
     Although  FIGS. 4 and 5  are shown and described with respect to communication from base station  8  to mobile terminal  10 , it is understood that the same concepts apply to uplink communications from mobile terminal  10  to base station  8 . 
     The present invention provides a two part solution to the aforementioned problems regarding channel estimation and demodulation. The first aspect provides a reduced complexity minimum mean squared error (“MMSE”) channel estimation which allows reference signal (“RS”) channel estimation of V-MIMO mobile terminals  10 . The second aspect provides for demodulation of the data segments of the V-MIMO mobile terminals  10 . Frequency domain data signal interference regeneration and cancellation is used for the weaker mobile terminal  10  signal in the V-MIMO. 
     Although described below in detail, the channel estimation is performed using mutual interference cancellation from the uplink reference signal that is jointly shared by mobile terminals  10  in the V-MIMO. This is accomplished using sounding reference signal (“SRS”) based channel estimates. A reduced complexity MMSE method is used for RS channel estimation. With respect to data demodulation and in particular the data signal demodulation of the weaker mobile terminal  10  in the V-MIMO, the successfully decoded stronger mobile terminal  10  data signal is canceled from the weaker mobile terminal  10  signal and the weaker signal regenerated. This arrangement provides V-MIMO channel estimation and demodulation performance comparable to other channel estimation techniques, but advantageously does so at one quarter of the computational complexity. In other words, the present invention advantageously reduces the data demodulation complexity by a factor of 4 compared with other demodulation methods. 
     The reduced complexity MMSE RS channel estimation method suitable for use in V-MIMO RS channel estimation is described. In accordance with this aspect of the present invention an uplink reference signal is received from each mobile terminal  10 . Using the received uplink referenced signals, a first reference signal channel estimate for each of the mobile terminals  10  is determined. For example, such a first reference signal can be a sounding reference signal (“SRS”). The SRS for each mobile terminal can be used to determine an interference cancellation estimate for each mobile terminal  10 . With the interference cancellation estimate having been determined, a second reference signal channel estimate can be determined for each of the mobile terminals  10 . For example, this second reference signal channel estimate can be an MMSE channel estimate. 
     Of note, although the present invention is described in  FIGS. 7-10  with reference to two mobile terminals  10 , e.g., mobile terminal  10   a  and mobile terminal  10   b , it is understood that the use of two mobile terminals is for ease of explanation and understanding only, the V-MIMO used in actual operation need not be limited to two mobile terminals  10 . 
     The reduced complexity MMSE RS channel estimation method suitable for use in V-MIMO RS channel estimation is described with reference to  FIG. 7 . Initially, base station  8  receives a composite signal on the physical uplink shared channel (“PUSCH”) from the plurality of mobile terminals  10 . The signal contains the received PUSCH demodulation reference symbol (“DMRS”) signal. A 1024-point Fast Fourier Transform (“FFT”) is performed on the reference signal by base station  8  and the gain normalized (step S 100 ). After the FFT, and in the case of a two mobile terminal  10  V-MIMO, the signal can be described as y=χ 1 h 1 +χ 2 h 2 +n. From the resulting frequency domain signal, the resource blocks (“RB”) can be isolated in the V-MIMO. The signal y is then normalized in step S 100 . With respect to this equation (y), χ 1  and χ 2  refer to the demodulation reference signal vectors for mobile terminal  10   a  and mobile terminal  10   b , respectively. h 1  and h 2  refer to the frequency domain complex channel response vectors of mobile terminal  10   a  and mobile terminal  10   b , respectively, and n is the additive white Gaussian noise with a variance σ n   2 . 
     Because the reference symbol sequences of mobile terminals  10   a  and  10   b  are known to the base station  8  receiver (by virtue of being “reference signals”), the buffered sounding reference signals (“SRS”) based channel estimates for the two mobile terminals  10  in the V-MIMO can be determined (steps S 102  and steps S 104 , respectively). These estimates are referred to herein as
 
{ĥ 1 } and {ĥ 2 }, respectively.
 
     The demodulation reference signal (“DMRS”) for mobile terminals  10   a  (step S 106 ) and mobile terminal  10   b  (step S 108 ), which as discussed are known to base station  8  are used along with the SRS estimates to regenerate an estimate of the received DMRS signal {X 1 ĥ 1 } for mobile terminal  10   a  (step S 110 ) and an estimate of the DMRS signal {X 2 ĥ 2 } for mobile terminal  10   b  (step S 112 ). 
     Regarding mobile terminal  10   a , equation {X 1 ĥ 1 } is subtracted from (y) (step S 114 ) to create the interference canceled estimate of the DMRS signal received from mobile terminal  10   b  given by:
 
 y   u1     —     canceled   =y−X   1   ĥ   1 .
 
The RS gain is normalized for the RS signal corresponding to mobile terminal  10   b  (step S 116 ) and a least squared (“LS”) estimation is performed for mobile terminal  10   b  (step S 118 ) in which the LS channel estimate for mobile terminal  10   b  is given by:
 
 Ĥ   LS2   =X   2   −1   y   u1     —     canceled .
 
     A similar process is performed with respect to mobile terminal  10   a . The regenerated signal for mobile terminal  10   b  given by
 
{X 2 ĥ 2 }
 
is subtracted from (y) (step S 120 ) to provide an interference canceled estimate of the DMRS signal received from mobile terminal  10   a , given by the equation
 
 y   u2     —     canceled   =y−X   2   ĥ   2 .
 
The reference signal gain for mobile terminal  10   a  is normalized (step S 122 ) and estimation for mobile terminal  10   a  is determined (step S 124 ). This determination is represented by:
 
 Ĥ   LS1   =X   1   −1   y   u2     —     canceled  
 
     With the LS channel estimates having been determined, these LS estimates can be used to determine the reduced complexity MMSE channel response to extract the DMRS channel response estimates for mobile terminals  10   a  and  10   b . With respect to mobile terminal  10   a , the signal to noise ratio (“SNR”), the β, and the resource block (“RB”) length are used to generate a correlation matrix (step S 126 ). Channel response estimation for mobile terminal  10   a  is determined using an MMSE method (step S 128 ). An exemplary MMSE method for step S 128  is given by the following equation: 
     
       
         
           
             
               
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     With respect to mobile terminal  10   b , a correlation matrix is determined for mobile terminal  10   b  (step S 130 ) using the SNR of mobile terminal  10   b , the β for mobile terminal  10   b  and the resource block length. The MMSE channel response estimate for mobile terminal  10   b  is determined using the LS estimation from step S 118  and the MMSE correlation matrix from step S 130  (step S 132 ). An exemplary MMSE method for step S 132  is given by the following equation: 
     
       
         
           
             
               
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     The result of the process shown and described with reference to  FIG. 7  provides uplink channel estimation for mobile terminals  10  involved in V-MIMO communication with base station  8 . This is done using best effort processing to create the effect of mobile terminal  10   a  RS signal on mobile terminal  10   b  and vice versa. Because base station  8  has advance information and knowledge of the RS sequences of mobile terminals  10   a  and  10   b , the use of frequency domain channel estimates for mobile terminals  10   a  and  10   b  can be used by base station  8  to approximately generate each mobile terminal signal which is canceled prior to LS estimates of the other user is determined. As discussed above, the present invention proposes using SRS estimates. Mobile terminals in idle mode and traffic mode periodically transmit these SRS signals. Therefore, it is possible to use the SRS base channel estimates for regeneration as discussed above. For low mobility devices, e.g., mobile terminals traveling at less than 60 km/hr, the previous SRS based channel estimates are sufficiently accurate to generate the approximate interference experienced from a mobile terminal  10  with respect to the other mobile terminals  10 . 
     With the channel response estimates in hand, these estimates can be used by base station  10  to demodulate and extract actual uplink data received from mobile terminals  10   a  and  10   b . The data demodulation/extraction process is discussed with reference to  FIGS. 8   a ,  8   b ,  9  and  10 . 
     Initially base station  8  receives a PUSCH data signal from mobile terminals  10   a  and  10   b . Base station  8  normalizes the data gain (step S 134 ) and the user data signals from mobile terminals  10   a  and  10   b  are demodulated and equalized using the MMSE channel estimates from steps S 128  and S 132  from  FIG. 7  (steps S 136  and S 138 , respectively). Once equalized and demodulated, the resultant data is checked for errors, such as using a cyclic redundancy check (“CRC”). This is shown in  FIG. 8   a  as step S 140  for mobile terminal  10   a  and step S 142  for mobile terminal  10   b . The CRC&#39;s are then evaluated (step S 144 ). If the CRCs for both mobile terminal  10   a  and mobile terminal  10   b  pass, the demodulated data for mobile terminals  10   a  and  10   b  are considered to have been properly demodulated and the user data are output by the process and used by base station  8  as V-MIMO mobile terminal data (step S 146 ). As shown in  FIG. 8   a  output M refers to the demodulated data from mobile terminal  10   a  in step S 136  and output N corresponds to the demodulated data for mobile terminal  10   b  from step S 138 . 
     If the CRC for mobile terminal  10   a  passes and the CRC for mobile terminal  10   b  fails (step S 148 ), the data estimates and channel estimates of mobile terminal  10   a  are used to regenerate and cancel its interference from mobile terminal  10   b  (step S 150 ). Step S 150  is discussed in detail below. Because the CRC with respect to mobile terminal  10   a  in step S 148  passed, the user data corresponding to mobile terminal  10   a  is valid and the demodulated data for mobile terminal  10   a  is output by the inventive process for use by base station  8  (step S 152 ). This output is shown as output M. The CRC for the mobile terminal  10   b  user data is checked again after the interference cancellation and data signal regeneration are applied to the signal for mobile terminal  10   b  (step S 154 ). If the CRC for mobile terminal  10   b  passes, the demodulated data corresponding to mobile terminal  10   b  is considered properly demodulated and valid and is output by the process for use by base station  8  (step S 156 ). The demodulated data for output mobile terminal  10   b  is shown in  FIG. 8   a  as data U. If the CRC for mobile terminal  10   b  does not pass (step S 154 ), the next HARQ attempt for mobile terminal  10   b  is transmitted by base station  8  (step S 158 ). 
     The case where step S 148  fails, e.g., mobile terminal  10   a  does not pass its CRC or mobile terminal  10   b  fails its CRC, a determination is made as to whether the CRC for mobile terminal  10   a  fails and the CRC mobile for terminal  10   b  passes (step S 160 ). 
     In the case where the CRC has not failed for mobile terminal  10   a  or the CRC terminal  10   b  does not pass, e.g. where the CRC&#39;s have failed for both mobile terminal  10   a  and mobile terminal  10   b , the base station  8  signals the next HARQ attempt for both mobile terminal  10   a  and mobile terminal  10   b  (step S 162 ). 
     In the case where the CRC fails for mobile terminal  10   a  and passes for mobile terminal  10   b  (step S 160 ), the process of the present invention continues on  FIG. 8   b  where the data signal and channel estimates for mobile terminal  10   b  are used to regenerate and cancel its interference from the mobile terminal  10   a  signal (step S 164 ). Step S 164  is discussed in detail below. Because the CRC corresponding to the data received from mobile device  10   b  is valid, the corresponding data is valid and the process outputs the demodulated data wireless terminal  10   b  for V-MIMO use by base station  8  (step S 166 ). This is shown as output N in  FIG. 8B . 
     CRC for mobile terminal  10   a  is checked again after the regeneration and interference cancellation (step S 168 ). If the CRC for mobile terminal  10   a  passes, valid data received from mobile terminal  10   a  is indicated and the process outputs the demodulated data (step S 170 ) for use by base station  8  to further process the V-MIMO data. The demodulated data output at step S 170  is shown as data V. In the case where the CRC from mobile terminal  10   a  does not pass (step S 168 ), base station  8  signals the next HARQ attempt for mobile terminal  10   a  (step S 172 ) and the process ends. 
     Regeneration and interference cancellation of mobile terminal  10   a  with respect to mobile  10   b  of step S 150  is described with reference to  FIG. 9 . The MMSE channel estimates for mobile device  10   a  (step S 128 ) is multiplied with the successfully demodulated data for mobile device  10   a  (step S 136 ), the resultant output of which is shown as the value M′ (step S 174 ). M′ is then subtracted from the PUSCH data for mobile terminals  10   a  and  10   b  (step S 176 ). The result is the interference cancelled and regenerated data signal for mobile terminal  10   b , shown as output Q. 
     The regeneration and cancellation of interference corresponding to mobile terminal  10   b  with respect to mobile terminal  10   a  of step S 164  is described and discussed in detail with reference to  FIG. 10 . To remove the interference of mobile device  10   b  from the data corresponding to mobile device  10   a , the MMSE channel estimates for mobile terminal  10   b  from step S 132  are multiplied with the successfully demodulated data for mobile device  10   b  from step S 138  to form an estimate of the channel output based on mobile device  10   b  (step S 178 ). The resultant output is shown as N′. Of note, N′ (as well as M′) are based on the equation discussed above given by:
 
 y=χ   1   h   1 +χ 2   h   2   +n  
 
     The estimate derived as N′ is subtracted from the PUSCH data for mobile terminal  10   a  and mobile terminal  10   b  (step S 180 ) to produce an estimate of the signal received from mobile device  10   a  (step S 180 ). The result is the interference cancelled and regenerated data signal for mobile terminal  10   a , shown as output T. 
       FIG. 11  shows the relationship between symbol error rate and SNR for mobile terminal  10   a  and  10   b  using quadrature phase-shift keying (“QPSK”) for mobile terminals  10  traveling at 30 km/hr and 60 km/hr. These relationships are shown with reference to the ideal channel. The ideal channel estimate shown and described with reference to  FIG. 11  is defined as having perfect knowledge of the channel, i.e., that there is no error in the estimate. 
     As is shown in  FIG. 11  mobile terminals  10  traveling at 30 km/hr yield symbol error rate versus SNR curves that are less than an order of magnitude from the ideal estimate at low SNRs and approach the ideal estimate as the SNR increases. For the faster traveling mobile terminals, the curve follows a similar path but does diverge somewhat at higher SNRs. 
     Of note, the graph shown in  FIG. 11  is the result of a simulation assuming SRSs are transmitted once every one or two milliseconds. This helps to provide SNR based channel estimates that can be reliably used for the mutual interference cancellation in the RS signals for the mobile terminals in the V-MIMO. The simulation also assumed that one mobile terminal signal is sufficiently stronger compared with the other mobile terminal signal so that the stronger mobile terminal signal data can be reliably demodulated. This also allows the stronger mobile terminal interference to be accurately regenerated and canceled from the weaker signal and so that the interference canceled weak signal can be demodulated. It is also assumed that the base station  8  uses a single antenna. It is contemplated that in an embodiment using two antennas at base station  8 , turbo coding and interleaving, an additional 10 dB of approximate performance gain is expected for a given error rate versus SNR curve. 
     The present invention advantageously provides a method and system which allows channel estimation for V-MIMO mobile terminals to be determined in a non-computationally complex manner and also allows these channel estimates to be used to regenerate and recover mobile device uplink data through the interference cancellation and regeneration process discussed above. 
     In implementing the present invention, it is preferable to have a high SNR mobile terminal  10  and another mobile terminal  10  with a low signal to noise ratio (“SNR”). In such case, it is likely that the high SNR mobile terminal  10  will pass its CRC, thereby providing a good source to use for the channel estimate. Pairing the high SNR and low SNR mobile terminals  10  can be done using power control. In other words, the performance of the present invention can be enhanced by pairing up mobile terminals  10  in the V-MIMO using power control to pair up a high SNR mobile terminal  10  with a low SNR mobile terminal  10 . 
     There are a number of other aspects of the present invention that can be implemented to further enhance performance. For example, accommodations of modulation coding sets (“MCS”) can be reserved for use only in V-MIMO cases. Also, in the case where the first HARQ attempt fails, subsequent HARQ attempts may be paired with other mobile terminals  10  (as compared with the mobile terminals  10  in the current V-MIMO set). It is also contemplated that uplink power control parameters of the mobile terminals  10  can be set to ensure that there is approximately a 5-10 dB difference in their received signals. Control parameters in this case would refer to base station  8  received signal power Po and path loss compensation factor α. It is further contemplated that V-MIMO operation can be considered only in cases where all physical resource blocks (“PRB”) are in use and there is an incoming traffic request. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.

Metadata:
Filing Date: 20140401
Publication Date: 20150721
Grant Date: 20150721
Priority Date: 20081113
Inventors: PANICKER JOHN P.
BOUDREAU GARY D.
PETRAS MICHAEL
ILLIDGE ED
Assignee: APPLE INC
CPC Classifications: [{"code": "H04W24/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W24/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0059", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0056", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0036", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L1/0048", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W24/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0036", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04B7/0413", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0059", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L25/0204", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B1/1081", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L25/0224", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0056", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L25/0202", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04J11/0056", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L25/0204", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0059", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0056", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W24/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L25/0204", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04J11/0059", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/005", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0036", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0036", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W24/00", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0059", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/0036", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04J11/0056", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J11/005", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 42169565