Abstract:
An exemplary method of communicating includes determining a first user channel estimate from a communication comprising first and second user reference sequences. The first user reference sequence is reconstructed using the determined first user channel estimate. The communication is revised based upon the reconstructed first user reference sequence. Then determining a second user channel estimate is based on the revised communication.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 60/997,115 which was filed on Oct. 1, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention generally relates to communication. More particularly, this invention relates to multiple input multiple output (MIMO) wireless communications. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Wireless communication systems are well known and in widespread use. Typical cellular communication arrangements include a plurality of base station transceivers strategically positioned to provide wireless communication coverage over selected geographic areas. A mobile station (e.g., notebook computer or cellular phone) communicates with a base station transceiver using an over-the-air interface. 
         [0004]    There are various techniques that have been developed to increase system capacity and user features. For example, multiple input multiple output (MIMO) techniques allow a plurality of users to be communicating over one or more antennas simultaneously. 
         [0005]    MIMO technology increases the data rate through transmission of parallel data streams on each antenna. A MIMO receiver needs to estimate the channel response from each transmitting antenna reliably to demodulate the mutual interfere signals. A reference sequence (e.g., pilot) allows for deriving channel estimation of the MIMO channel. The principle of the pilot sequence design for the MIMO technology is to minimize mutual interference. Orthogonal pilot sequences between the transmitted antennas minimize the mutual interference and enable reliable channel estimation from each transmitted antenna. 
         [0006]    The possible orthogonal sequences are time division multiplexing (TDM), frequency division multiplexing (FDM), and code division multiplexing (CDM). The TDM and FDM pilot sequence techniques are limited to having a pilot sequence transmitted from one antenna and no transmission from the other antenna at the instance in time or frequency, respectively. The CDM technique allows for simultaneous transmission of the pilot from all antennas with orthogonal code sequence covering. The CDM pilot receiver therefore must separate the orthogonal code sequence for individual channel estimation of each transmitting antenna. 
         [0007]    The 3GPP long term evolution (LTE) adopts the CDM type pilot for the uplink SC-FDMA radio technology. LTE MIMO receivers therefore must be able to individually estimate each received channel. This is complicated by the interference introduced by the channel effects, which effectively render the orthogonal codes non-orthogonal. 
         [0008]    Zadoff-Chu sequences are used as the orthogonal sequences of the CDM reference signal design in the LTE uplink. The root Zadoff-Chu sequences are orthogonal sequences. The shifts or offsets of any root Zadoff-Chu sequences are also a set of the orthogonal sequences. The offsets of the Zadoff-Chu sequences are considered in the LTE for PUCCH, multi-user MIMO (MU-MIMO) technologies and sounding reference signal (SRS). 
         [0009]    The generation of the base Zadoff-Chu sequence is as follows, 
         [0010]    For the number of subcarriers M sc   RS ≧36, the sequence r(0), . . . , r(M sc   RS −1) is given by 
         [0000]        r ( n )= e   jαn   x   u (( n +θ)mod  N   ZC   RS ), 0≦n&lt;M sc   RS    
         [0011]    where θ is an offset and the u th  root Zadoff-Chu sequence is defined by 
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         [0000]    and the length N ZC   RS  of the Zadoff-Chu sequence is given by the largest prime number such that N ZC   RS &lt;M sc   RS . The factor e jαn  corresponds to a cyclic shift in the time domain. For the number of subcarriers M sc   RS &lt;36, the computer generating sequences are supported with properties similar to the short Zadoff-Chu sequences. 
         [0012]    The allocation of the reference sequence (RS) in the LTE UL frame is at all subcarriers of the 4 th  and 11 th  symbols in each resource block (RB) for PUSCH. The Zadoff-Chu sequences are mapped to each subcarriers in the frequency domain. For MU-MIMO, each user has one Zadoff-Chu sequence in the same RS location in the resource block. 
         [0013]    The UL PUCCH, MU-MIMO and SRS receives the superposition of multi-user reference signals (RSs or Pilots), which are a pair of Zadoff-Chu sequences with relative offset. The Zadoff-Chu sequence has the shifting property of two offset sequences superposition in the frequency domain resulting in a time domain shift. The shifting property also applies at the multi-path fading channel condition. The generic algorithms for the MIMO pilot separation are to filter the unwanted shifting sequence. This could be achieved by transforming the pilot signals to the time domain to filter out the other reference sequences and transforming back to the frequency domain as known. The time domain filter is a simple multiplication of zero filling to cut off the undesired reference signals. The frequency and time domain transformation and the time domain filtering could be combined as the frequency domain filtering. The frequency domain filter coefficients are derived from the combination of DFT, time domain filtering and IDFT. The frequency domain filtering is a convolution of data and FIR filter. The frequency domain filtering technique could reduce the processing power by computing the frequency domain filter coefficients in advance and storing it for lookup. However, these filter coefficients will change subjective to the number of input data (length of Zadoff-Chu Sequence). The computational complexity of both methods is similar. 
         [0014]    Such a filtering technique provides sufficient performance of CDM pilot separation for channel estimation of an individual transmitting antenna. However, the filtering effect introduces performance degradation at the edge of the RB since the data to convolve with the frequency domain filter is cut off at the edge. The performance degradation of the pilot separation is very sensitive when the reference sequence is short (e.g. 1 RB). In addition to the reduced quality when the number or received RBs is low, the known filtering technique is computationally complex. 
       SUMMARY 
       [0015]    An exemplary method of communicating includes determining a first user channel estimate from a communication comprising first and second user reference sequences. The first user reference sequence is reconstructed using the determined first user channel estimate. The communication is revised based upon the reconstructed first user reference sequence. Then determining a second user channel estimate is based on the revised communication. 
         [0016]    The reconstructed reference sequence is used as feedback for estimating the channel of the other user in a channel estimation technique that resembles single input single output channel estimation. 
         [0017]    The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  schematically illustrates selected portions of a communication system designed according to an embodiment of this invention. 
           [0019]      FIG. 2  schematically illustrates an example embodiment of a channel estimation module. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  shows selected portions of a wireless communication system  20 . A base station  22  communicates with a plurality of mobile stations  24 ,  26  and  28 . The communications between the base station  22  and the mobile stations  24 ,  26  and  28  may include multiple user, multiple input multiple output (MU MIMO) or SRS communications. MIMO communications involve multiple transmitting antennas transmitting simultaneously. Orthogonal constant amplitude zero autocorrelation (CAZAC) sequences (e.g., Zadoff-Chu sequences) are assigned to the mobile stations, respectively. Such sequences allow for individually detecting the communication from each mobile station. 
         [0021]    The base station  22  includes a channel estimation module for detecting each received channel from each user of a MIMO communication. The channel estimation module uses an iterative pilot interference cancellation (IPIC) scheme. The principle of the IPIC scheme for MU MIMO or SRS includes performing a noisy Single Input Single Output (SISO) frequency domain channel estimation for one user first with the other user as the interference source. The SISO frequency domain channel estimation is based on an average filtering process in one example. The output of the channel estimate for the one user is used to reconstruct the reference sequence of that one user by multiplying the channel estimate by the associated Zadoff-Chu sequence. The reconstructed reference sequence is then used as feedback for interference cancellation in the received communication. The output of the interference cancellation is used to perform the SISO frequency domain channel estimation for another user. 
         [0022]    The process of estimating the channel, reconstructing the reference sequence and using that as feedback for interference cancellation is iteratively repeated until a desired number of iterations are complete or a desired accuracy is achieved. The channel estimation performance for MIMO or SRS improves each time when the other user&#39;s pilot interference is cancelled. 
         [0023]      FIG. 2  schematically illustrates one example channel estimation module  30 . This example includes a frequency domain averaging filter  32  that operates much like a SISO channel estimation filter. In one example, the filter  32  uses a simple averaging technique to provide a channel estimate. 
         [0024]    A communication (RS) is received that includes a plurality of user reference sequences. For purposes of discussion a first user (k=1) and a second user (k=2) will be considered. A multiplier  34  multiplies the received signal by the complex conjugate of the CAZAC sequence assigned to the first user. The filter  32  filters the product from the multiplier  34  to determine a first user channel estimate (H 1,est ). 
         [0025]    Another multiplier  36  multiplies the just determined first user channel estimate by the CAZAC sequence of the first user to reconstruct the reference sequence of the first user. A delay is added to the reconstructed first user reference sequence at  38 . The reconstructed first user reference sequence is feedback for interference cancellation at  40 . In this example, the reconstructed first user reference sequence is subtracted from the received communication to revise the communication. 
         [0026]    The revised communication is then provided to the multiplier  34  where it is multiplied by the complex conjugate of the second user CAZAC sequence. The product is then filtered by the filter  32  to determine a second user channel estimate. 
         [0027]    The second user reference sequence is then reconstructed by the multiplier  36  and a delay is added at  38 . The reconstructed second user reference sequence is then used as feedback for interference cancellation for revising the first user channel estimate. In this example, the reconstructed second user reference sequence is subtracted from the received communication to revise the communication. The revised communication is then multiplied by the complex conjugate of the first user CAZAC sequence in the multiplier  34 . The product is then filtered to provide a revised first user channel estimate. 
         [0028]    A revised reconstructed first user reference sequence is then determined by the multiplier  36  using the first user CAZAC sequence. The revised reconstructed first user reference sequence is then used as feedback for interference cancellation for revising the second user channel estimate. The process of revising each user channel estimate proceeds in this manner as many times as desired. 
         [0029]    In one example, a preselected number of iterations is used. In some examples as few as two or three iterations are satisfactory. In another example, each revised channel estimate is compared to a previous channel estimate for the corresponding user and the iterative process is concluded when a selected amount of convergence is detected. 
         [0030]    The computational complexity of the IPIC scheme is much lower than that of time or frequency domain filtering, which required FFT/DFT for time domain filtering or convolution for frequency domain filtering. The complexity of the IPIC scheme for the length N subcarriers corresponds to the linear multiplication of length N multiplied by the number of iterations. The time domain filtering, by comparison, requires a complexity on the order of N log N. The frequency filtering scheme, by comparison, has a computation complexity on the order of N 2 . 
         [0031]    The disclosed example IPIC scheme is useful for joint pilot separation and channel estimation for LTE CDM type reference signals, for example. The IPIC of the disclosed example provides better performance in channel estimation and no performance degradation of the edge effects that were otherwise present with other techniques. The example IPIC scheme also has less computational complexity in implementation, which increases the efficiencies and economies of a receiver. 
         [0032]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this invention. The scope of legal protection given to this invention can only be determined by studying the following claims.