Patent Publication Number: US-8982997-B2

Title: Signaling and channel estimation for uplink transmit diversity

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/244,126, filed on Sep. 21, 2009, the contents of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to wireless communications and more particularly to methods and systems for providing transmit diversity and channel estimation for uplink transmissions. 
     BACKGROUND 
     Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and other content. These systems may be multiple-access systems capable of simultaneously supporting communication for multiple wireless terminals by sharing the available transmission resources (e.g., frequency channel and/or time interval). Since the transmission resources are shared, efficient allocation of the transmission resources is important as it impacts the utilization of the transmission resources and the quality of service perceived by individual terminal users. One such wireless communications system is the Orthogonal Frequency-Division Multiple Access (OFDMA) system in which multiple wireless terminals perform multiple-access using Orthogonal Frequency-Division Multiplexing (OFDM). 
     OFDM is a multi-carrier modulation technique that partitions the overall system bandwidth into multiple orthogonal frequency subchannels, each of which is associated with a respective subcarrier that may be modulated with data. Because the subchannels are made orthogonal, some spectral overlap between the subchannels is permitted, leading to a high spectral efficiency. In OFDM systems, the user data stream is split into parallel streams of reduced rate, and each obtained substream then modulates a separate subcarrier. 
     In OFDMA, the transmission resource extends over two dimensions: frequency channels and time intervals. The resources of a given frequency channel may involve contiguous and/or non-contiguous groups of subcarriers. 
     Examples of OFDM communication systems include, but are not limited to, wireless protocols such as the wireless local area network (“WLAN”) protocol defined according to the Institute of Electrical and Electronics Engineering (“IEEE”) standards radio 802.11a, b, g, and n (hereinafter “Wi-Fi”), the Wireless MAN/Fixed broadband wireless access (“BWA”) standard defined according to IEEE 802.16 (hereinafter “WiMAX”), the mobile broadband 3GPP Long Term Evolution (“LTE”) protocol having air interface High Speed OFDM Packet Access (“HSOPA”) or Evolved UMTS Terrestrial Radio Access (“E-UTRA”), the 3GPP2 Ultra Mobile Broadband (“UMB”) protocol, digital radio systems Digital Audio Broadcasting (“DAB”) protocol, Hybrid Digital (“HD”) Radio, the terrestrial digital TV system Digital Video Broadcasting-Terrestrial (“DVB-T”), the cellular communication systems Flash-OFDM, etc. Wired protocols using OFDM techniques include Asymmetric Digital Subscriber Line (“ADSL”) and Very High Bitrate Digital Subscriber Line (“VDSL”) broadband access, Power line communication (“PLC”) including Broadband over Power Lines (“BPL”), and Multimedia over Coax Alliance (“MoCA”) home networking. 
     Generally, in OFDMA systems each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink (DL)) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink (UL)) refers to the communication link from the terminals to the base stations. 
     In 3GPP LTE, the following physical channels are defined: 
     Downlink (DL)
         Physical Broadcast Channel (PBCH): This channel carries system information for user equipments (UEs) requiring access to the network.   Physical Downlink Control Channel (PDCCH): The main purpose of this physical channel is to carry scheduling information.   Physical Hybrid ARQ Indicator Channel (PHICH): This channel is used to report the Hybrid ARQ status.   Physical Downlink Shared Channel (PDSCH): This channel is used for unicast and paging functions.   Physical Multicast Channel (PMCH): This physical channel carries system information for multicast purposes.   Physical Control Format Indicator Channel (PCFICH): This channel provides information to enable the UEs to decode the PDSCH.       

     Uplink (UL)
         Physical Uplink Control Channel (PUCCH): This channel is used to transport user signaling data from one or more UE that can transmit on the control channel. The PUCCH transports, for example, acknowledgment responses and retransmission requests, service scheduling requests, and channel quality information measured by the UE to the system.   Physical Uplink Shared Channel (PUSCH): This channel is used to transport user data from one or more mobiles that can transmit on the shared channel.   Physical Random Access Channel (PRACH): This uplink physical channel allows a mobile to randomly transmit access requests when the mobile attempts to access the wireless communication system.       

     The DL and UL communication links in a wireless multiple-access communication system may be established via one antenna at the transmitter and one antenna at the receiver (single-input-single-output, or SISO), via multiple antennas at the transmitter and one antenna at the receiver (multiple-input-single-output, or MISO), via one antenna at the transmitter and multiple antennas at the receiver (single-input-multiple-output, or SIMO), or via multiple antennas at the transmitter and multiple antennas at the receiver (multiple-input-multiple-output, or MIMO). 
     A MIMO system may employ transmit diversity to combat the effect of fast fading by using multiple antennas to transmit a data stream via multiple independently fading channels. Transmit diversity schemes can be divided into open loop transmit diversity (OLTD) and closed-loop transmission diversity (CLTD) schemes. In OLTD, no feedback is required from the receiver. In one type of CLTD, a receiver knows an arrangement of transmission antennas, and computes a phase and amplitude adjustment that should be applied at the transmitter antennas in order to maximize a power of the signal received at the receiver. In another type of CLTD, referred to as selection transmit diversity (STD), the receiver provides feedback information to the transmitter regarding which antenna(s) to be used for transmission. 
     An example OLTD scheme is the Alamouti 2×1 space-time diversity scheme. The Alamouti 2×1 space-time diversity scheme contemplates transmitting a Alamouti 2×2 block code using two transmission antennas using either two time slots (i.e., Space-Time Block Code (STBC) transmit diversity) or two frequency subcarriers (i.e., Space-Frequency Block Code (SFBC) transmit diversity). 
     A major problem with the multi-carrier modulation in general and OFDM communication systems in particular is the high peak-to-average power ratio (PAPR) that is inherent in the transmitted signal. Large signal peaks occur in the transmitted signal when the signals in the subcarriers add constructively in phase. Such large signal peaks may saturate the power amplifier (PA) at the transmitter and thus, cause nonlinear distortion of the transmitted signal, which results in a large degradation of performance, e.g. increase of both the bit error rate (BER) and the out-of-band radiation (spectral spreading). This high PAPR problem may be partially overcome in DL transmission by utilizing more advanced PAs with larger dynamic ranges. However, when it comes to UL transmission, the restrictions of the user equipment (UE), in terms of price and dimensions, precludes the possibility of this solution. 
     In 3GPP Release 8 E-UTRA, in which only one transmit antenna is supported at the UE, two separate methods have been utilized for PUCCH and PUSCH, respectively, to keep the PAPR as low as possible. In PUCCH, where code-division multiple access (CDMA) is the multiple-access method, orthogonal spreading codes are designed such that they provide a relatively low PAPR at the output of the Inverse Discrete Fourier Transform (IDFT) signal processing step of OFDM. In PUSCH, on the other hand, Single Carrier Frequency-Division Multiple Access (SC-FDMA) has been adopted as the multiple-access scheme to decrease the PAPR as compared with OFDMA. SC-FDMA is a modulation and multiple-access scheme which, due to its inherent single carrier structure, has a lower transmit signal PAPR than OFDMA. 
     In Advanced E-UTRA, increased peak data rates (e.g. up to 500 Mbps in the UL) are targeted. A promising technique to fulfil these high data rates is MIMO. In cases where MIMO is used, the UE can utilize an OLTD scheme to support the target data rates at acceptable error rates. However, the OLTD schemes currently proposed suffer either from the high PAPR problem described above or from what is known in the art as the orphan symbol problem. For example, STBC preserves the low PAPR property but requires an even number of symbols per slot, whereas SFBC works for any number of symbols but increases the PAPR. Cyclic Delay Diversity (CDD), another candidate transmit diversity scheme, preserves the low PAPR property and works for any number of symbols, but suffers from poorer performance relative to STBC and SFBC. 
     Another problem that arises due to MIMO relates to UL channel estimation. UL reference signals (RS) in LTE can be classified into three broad types: reference signals for demodulation of PUSCH, reference signals for demodulation of PUCCH, and reference signals for measurement of UL channel quality. Currently, for certain PUCCH formats in LTE, RS symbols are separated from each other within each slot. This RS separation is not beneficial in low SNR, because interpolation accuracy is degraded in low SNR. A straightforward solution is to assign two orthogonal sequences (OS) to each UE so that the channel estimation for each antenna can be performed as in the single antenna case. However, this solution wastes resources (orthogonal sequences) which could otherwise be used to support more UEs. Moreover, this solution entails some signalling overhead to inform the UEs which additional sequence to choose. 
     A need exists for improved transmit diversity schemes for uplink transmission. A need also exists for improved channel estimation schemes. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided a method of transmitting a data stream from a transmitter in a multiple-input-multiple-output (MIMO) wireless communication system, where the transmitter comprises a plurality of transmit antennas. According to the method, a discrete Fourier transform (DFT) is applied to the data stream to generate a plurality of symbol sequences; symbols of a first symbol sequence from the plurality of symbol sequences are paired with symbols of a second symbol sequence from the plurality of symbol sequences to generate a plurality of symbol pairs, wherein the pairing results in an orphan symbol; a space-time block code (STBC) is applied to the symbol pairs to generate a plurality of sets of STBC symbols, each set of STBC symbols being associated with a corresponding one of the plurality of antennas; a cyclic delay diversity (CDD) operation is applied to the orphan symbol to generate a plurality of CDD symbols, each CDD symbol being associated with a corresponding one of the plurality of antennas; and each one of the antennas transmits the corresponding set of STBC symbols and the corresponding CDD symbol. 
     In accordance with a further aspect of the present invention, there is provided a mobile station comprising a controller and a plurality of transmit antennas, the mobile station operable to transmit a data stream in a multiple-input-multiple-output (MIMO) wireless communication system. The controller is operable to: apply a discrete Fourier transform (DFT) to the data stream to generate a plurality of symbol sequences; pair symbols of a first symbol sequence from the plurality of symbol sequences with symbols of a second symbol sequence from the plurality of symbol sequences to generate a plurality of symbol pairs, wherein the pairing results in an orphan symbol; apply a space-time block code (STBC) to the symbol pairs to generate a plurality of sets of STBC symbols, each set of STBC symbols being associated with a corresponding one of the plurality of antennas; apply a cyclic delay diversity (CDD) operation to the orphan symbol to generate a plurality of CDD symbols, each CDD symbol being associated with a corresponding one of the plurality of antennas; and transmit from each one of the antennas the corresponding set of STBC symbols and the corresponding CDD symbol. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures which illustrate embodiments of the invention by example only, 
         FIG. 1  is a block diagram of a cellular communication system; 
         FIG. 2  is a block diagram of an example base station that might be used to implement some embodiments of the present application; 
         FIG. 3  is a block diagram of an example mobile terminal that might be used to implement some embodiments of the present application; 
         FIG. 4  is a block diagram of an example relay station that might be used to implement some embodiments of the present application; 
         FIG. 5  is a block diagram of a logical breakdown of an example OFDM transmitter architecture that might be used to implement some embodiments of the present application; 
         FIG. 6  is a block diagram of a logical breakdown of an example OFDM receiver architecture that might be used to implement some embodiments of the present application; 
         FIG. 7A  is an example SC-FDMA transmitter; 
         FIG. 7B  is an example SC-FDMA receiver; 
         FIG. 8  is an example Space-Time Block Code (STBC) scheme; 
         FIG. 9  is an example reference signal arrangement that may be used to support channel estimation; and 
         FIG. 10  is another example reference signal arrangement that may be used to support channel estimation. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawing figures in which like reference designators refer to like elements,  FIG. 1  shows a base station controller (BSC)  10  which controls wireless communications within multiple cells  12 , which cells are served by corresponding base stations (BS)  14 . In some configurations, each cell is further divided into multiple sectors  13  (not shown). In general, each base station  14  facilitates communications using OFDM with mobile terminals  16 , which are within the cell  12  associated with the corresponding base station  14 . The movement of the mobile terminals  16  in relation to the base stations  14  results in significant fluctuation in channel conditions. As illustrated, the base stations  14  and mobile terminals  16  may include multiple antennas to provide spatial diversity for communications. As described in more detail below, relay stations  15  may assist in communications between base stations  14  and mobile terminals  16 . Mobile terminals  16  can be handed off  18  from any cell  12 , sector  13  (not shown), base station  14  or relay  15  to an other cell  12 , sector  13  (not shown), base station  14  or relay  15 . In some configurations, base stations  14  communicate with each and with another network (such as a core network or the internet, both not shown) over a backhaul network  11 . In some configurations, a base station controller  10  is not needed. 
       FIG. 2  depicts an example of a base station  14 . Base station  14  generally includes a control system  20 , a baseband processor  22 , transmit circuitry  24 , receive circuitry  26 , antennas  28 , 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  16  (illustrated in  FIG. 3 ) and relay stations  15  (illustrated in  FIG. 4 ). A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert 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 wireless network via the network interface  30  or transmitted to another mobile terminal  16  serviced by the base station  14 , either directly or with the assistance of a relay  15 . 
     On the transmit side, 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 one or more carrier signals having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signals to the antennas  28  through a matching network (not shown). Modulation and processing details are described in greater detail below. 
       FIG. 3  illustrates an example of a mobile terminal  16 . Similarly to the base station  14 , the mobile terminal  16  will include a control system  32 , a baseband processor  34 , transmit circuitry  36 , receive circuitry  38 , antennas  40 , and user interface circuitry  42 . The receive circuitry  38  receives radio frequency signals bearing information from one or more base stations  14  and relays  15 . A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     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. The baseband processor  34  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
     For transmission, baseband processor  34  receives digitized data, which may represent voice, video, data, or control information, from the control system  32 , which it encodes for transmission. The encoded data is output to the transmit circuitry  36 , where it is used by a modulator to modulate one or more carrier signals that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver 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 used for signal transmission between the mobile terminal and the base station, either directly or via the relay station. 
     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 utilizes the performance of an Inverse Fast Fourier Transform (IFFT) on the information to be transmitted. For demodulation, the performance of a Fast Fourier Transform (FFT) on the received signal recovers 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 preferably used for at least downlink transmission from the base stations  14  to the mobile terminals  16 . Each base station  14  is equipped with “n” transmit antennas  28  (n&gt;=1), and each mobile terminal  16  is equipped with “m” receive antennas  40  (m&gt;=1). Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labelled only for clarity. 
     When relay stations  15  are used, OFDM is preferably used for downlink transmission from the base stations  14  to the relays  15  and from relay stations  15  to the mobile terminals  16 . 
       FIG. 4  illustrates an example relay station  15 . Similarly to the base station  14 , and the mobile terminal  16 , the relay station  15  includes a control system  132 , a baseband processor  134 , transmit circuitry  136 , receive circuitry  138 , antennas  130 , and relay circuitry  142 . The relay circuitry  142  enables the relay  14  to assist in communications between a base station  16  and mobile terminals  16 . The receive circuitry  138  receives radio frequency signals bearing information from one or more base stations  14  and mobile terminals  16 . A low noise amplifier and a filter (not shown) may cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     Baseband processor  134  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. Baseband processor  134  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
     For transmission, baseband processor  134  receives digitized data, which may represent voice, video, data, or control information, from control system  132 , which it encodes for transmission. The encoded data is output to the transmit circuitry  136 , where it is used by a modulator to modulate one or more carrier signals that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signals to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  130  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are used for signal transmission between the mobile terminal and the base station, either directly or indirectly via a relay station, as described above. 
     With reference to  FIG. 5 , a logical OFDM transmission architecture will be described. Initially, base station controller  10  will send data to be transmitted to various mobile terminals  16  to base station  14 , either directly or with the assistance of a relay station  15 . As described in more detail below, base station  14  uses the channel quality indicators (CQI) values associated with the mobile terminals to schedule the data for transmission as well as select an appropriate modulation and coding scheme (MCS) level for transmitting the scheduled data. The CQI values may be received directly from the mobile terminals  16  or determined at the base station  14  based on information provided by the mobile terminals  16 . In either case, the CQI value associated with each mobile terminal  16  may for example be a function of the signal-to-interference ratio (SIR), as well as of the degree to which the channel amplitude (or response) varies across the OFDM frequency band. 
     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  16 . As described in more detail below, the channel coding for a particular mobile terminal  16  is based on the current CQI value associated with that mobile terminal. In some implementations, the channel encoder logic  50  uses known Turbo encoding techniques. 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. As described in more detail below, the degree of modulation is chosen based on the CQI value 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  16 . The STC encoder logic  60  will process the incoming symbols and provide “n” outputs corresponding to the number of transmit antennas  28  for the base station  14 . The control system  20  and/or baseband processor  22  as described above with reference to  FIG. 5  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  16 . 
     For the present example, assume the base station  14  has two antennas  28  (n=2) and the STC encoder logic  60  provides two output streams of symbols. Accordingly, each of the symbol streams output by the SIC 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. The IFFT processors  62  will preferably operate on the respective symbols to provide an inverse Fourier Transform. The output of the TUFT processors  62  provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by prefix 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 (DUG) and digital-to-analog (DIA) 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  28 . Notably, pilot signals known by the intended mobile terminal  16  are scattered among the sub-carriers. The mobile terminal  16 , which is discussed in detail below, will use the pilot signals for channel estimation. 
     Reference is now made to  FIG. 6  to illustrate reception of the transmitted signals by a mobile terminal  16 , either directly from base station  14  or with the assistance of relay  15 . Upon arrival of the transmitted signals at each of the antennas  40  of the mobile terminal  16 , the respective signals are demodulated and amplified by corresponding RF circuitry  70 . For the sake of conciseness and clarity, only one of the two receive paths is described and illustrated in detail. Analog-to-digital (AID) 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 PET processing provides an accurate conversion from the time domain 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 EFT 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. Continuing with  FIG. 6 , 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 STC decoder  100  sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols. The relay station could act as another base station or as a terminal in the context of this invention. 
     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 dc-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 dc-interleaved bits are then processed by rate dc-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 . 
     In parallel to recovering the data  116 , a CQI value, or at least information sufficient to determine a CQI value at the base station  14 , is determined and transmitted to the base station  14 . As noted above, the CQI value may be a function of the signal-to-interference ratio (SIR), as well as the degree to which the channel response varies across the various sub-carriers in the OFDM frequency band. For this embodiment, the channel gain for each sub-carrier in the OFDM frequency band being used to transmit information is compared relative to one another to determine the degree to which the channel gain varies across the OFDM frequency band. Although numerous techniques are available to measure the degree of variation, one technique is to calculate the standard deviation of the channel gain for each sub-carrier throughout the OFDM frequency band being used to transmit data. 
       FIGS. 1 to 6  provide one specific example of a communication system that could be used to implement embodiments of the application. It is to be understood that embodiments can be implemented with communications systems having architectures that are different than the specific example, but that operate in a manner consistent with the implementation of the embodiments as described herein. 
     In one embodiment, the communication system of  FIGS. 1 to 6  is 3GPP LTE-compliant, and Single Carrier Frequency-Division Multiple Access (SC-FDMA) is utilized as the multiple-access scheme for uplink transmissions from mobile device  16  to either of relay  15  and base station  14 . 
       FIGS. 7A and 7B  illustrate the basic signal processing steps needed at the transmitter and receiver, respectively, for a SC-FDMA channel. As shown, SC-FDMA can be viewed as a Discrete Fourier Transform (DFT) pre-coded OFDMA scheme. There are several similarities in the overall transceiver processing of SC-FDMA and OFDMA. Those common aspects between OFDMA and SC-FDMA are depicted generally as OFDMA transmit circuitry  702  and OFDMA receive circuitry  704 , as they will be obvious to a person having ordinary skill in the art in view of the present specification. SC-FDMA is distinctly different from OFDMA because of the DFT pre-coding  703  of the modulated symbols, and the corresponding IDFT  705  of the demodulated symbols. Because of this pre-coding, the SC-FDMA subcarriers are not independently modulated as in the case of the OFDMA subcarriers. As a result, the PAPR of the SC-FDMA signal is lower than the PAPR of the OFDMA signal. 
     In accordance with some embodiments of the present application, Space-Time Block Code (STBC) transmit diversity is utilized as the OLTD scheme by feeding a specific sequence to the STC encoder block  60  of  FIG. 5  (e.g. for PUSCH and PUCCH). More specifically, for PUSCH, STBC may be used with symbols paired from two consecutive slots. If the symbols have a low PAPR property, then with STBC, the low PAPR is preserved. If after pairing there exists an unpaired symbol (i.e., an orphan symbol), the orphan symbol may be sent using a Cyclic Delay Diversity (CDD) scheme with a large delay. The delay may be, for example, a quarter of a cycle or three quarters of a cycle. The delay provides implementation simplicity without any performance degradation. Also, the delay makes CDD equivalent to a pre-coded MIMO system by providing frequency diversity if the channel conditions change over time. The codebook may be of size 4 as already adopted for downlink closed loop MIMO systems. This approach is equivalent to a pre-coding matrix hoping (PMH) scheme, where the precoding matrix changes from one entry in the codebook to another. 
     For PUCCH, STBC may be used with symbols paired from two orthogonal sequences. In PUCCH, the number of symbols in each subframe is even. However, in some cases the number of symbols in each slot is odd. In such cases, the last symbol of slot  0  may be paired with the first symbol of slot  1 . Because the two slots are in different frequency bands, a minimum mean square error (MMSE) receiver may be used for these symbols. 
       FIG. 8  illustrates STBC coding applied to paired symbols from two consecutive low PAPR sequences such that the low PAPR property is preserved. If a pair consisting of two STBC symbols from different slots is transmitted on different frequency bands due to inter-slot hopping, a minimum mean square error (MMSE) receiver may be used. Otherwise, an Alamouti decoder can be used. For example, PUCCH channel formats  2 / 2   a / 2   b  may have an odd number of symbols in each slot while having an even number of symbols in each subframe. 
     In cases where the number of symbols per subframe is odd, a pairing operation results in an unpaired symbol, also referred to as an orphan symbol. For the orphan symbol, CDD transmit diversity may be used. For example, in PUSCH one symbol of the second slot is dedicated for the sounding reference signal (SRS). 
     The cyclic shift (CS) of CDD is proposed to be a quarter or three quarters of an OFDM or SC-FDMA symbol (CS=N/4 or CS=3N/4) for low complexity implementation and for better performance. As noted above, with CS=N/4 or CS=3N/4, CDD can be considered equivalent to pre-coding matrix hopping (PMH) in the frequency direction and in the order of: 
     
       
         
           
             
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     The above codebooks have been already adopted for downlink closed-loop MIMO. PMH can be implemented with low complexity, and performs as well as (or better than) CDD with other CS values. 
     For channel estimation in PUCCH when multiple antennas are transmitting simultaneously, a two-level code division multiplexing (CDM) scheme for reference signal (RS) transmission may be used, where the first level separates the UEs (same as supporting only one transmit antenna) and the second level separates the antennas. For example, when there are two transmit antennas and two RSs per slot, the same RS is transmitted from both antennas. However, the block of four RSs (across time and space) is Hadamard coded. Advantageously, only one sequence is assigned to each UE at each time. To improve the performance of this scheme, the RSs may be transmitted in the middle of each slot in order to reduce the effect of intra-code interference. 
       FIG. 9  shows an example RS arrangement for two RSs in the same slot (e.g. PUCCH with normal cyclic prefix (CP) in LTE for format  2 ,  2   a  and  2   b ). Hadamard coded RSs may be transmitted, with both antennas using the same orthogonal sequence (OS) at a given slot. The RSs are Hadamard coded across time and space within each slot: at RS 1 , both transmit antennas T×1 and T×2 transmit sequence S 1 ; at RS 2 , transmit antenna T×1 transmits sequence S 1  and transmit antenna T×2 transmits sequence −S 1 . 
       FIG. 10  shows another example RS arrangement for two RSs in the same slot. As previously noted, for a two-layer CDM RS arrangement the RSs could be put next to each other in the middle of the slot, as shown in  FIG. 10 . In one embodiment, for PUCCH with normal CP for formats  2 / 2   a / 2   b , symbols  3  and  4  may be used for RSs in slot  0 , and symbols  2  and  3  may be used for RSs in slot  1 . It will be appreciated that by using this approach intra-code interference is suppressed, and better performance can be achieved. 
     Advantageously, the proposed two-level CDM RS transmission enables efficient CDM MIMO channel estimation. The same number of UEs as the single transmit antenna case may be multiplexed and there is no need to change the existing RS sequence assignment scheme. Also, as there is a one-to-one relation between the UL transmission mode (T×D vs. non-T×D) and the RS sequence generation/mapping scheme, there is no need to introduce an additional signalling bit. As already noted, the proposed RS arrangement enables the two-level CDM RS transmission scheme to achieve better performance by suppressing the intra-code interference. 
     Other modifications will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.