Abstract:
Inter-carrier interference (ICI) cancellation in an OFDMA receiving signals from two transmitters is performed by identifying the transmitted sub-carriers that cause the largest ICI to sub-carriers received from other transmitters, and removing the ICI contribution from these sub-carriers. This may be accomplished by calculating the ICI terms only based on the interfering sub-carrier and the frequency offset. Alternatively, the transmissions causing the ICI are demodulated, the ICI on other signals is then determined and subtracted, and other signals are then demodulated. Which transmissions cause the largest ICI on others depends on the relative strength of the corresponding sub-carriers and how much orthogonality is lost. The latter might be due to frequency error, Doppler spread, or a combination of both.

Description:
This application claims priority to provisional patent application Ser. No. 60/870,685, entitled ICI CANCELLATION FOR OFDMA SYSTEMS filed Dec. 19, 2006. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to wireless communications systems and in particular to a system and method for inter-carrier interference cancellation in an OFDMA uplink. 
     BACKGROUND 
     Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation scheme utilizing multiple closely-spaced, orthogonal sub-carriers. Each sub-carrier is modulated with a conventional modulation scheme (e.g., quadrature amplitude modulation) at a low symbol rate, maintaining data rates similar to conventional single-carrier modulation schemes in the same bandwidth. OFDM modulation provides economical, robust communications under poor channel conditions, such as narrowband interference and frequency-selective fading due to multipath propagation. The low symbol rate allows for the use of a guard interval between symbols, reducing inter-symbol interference. OFDM is deployed or planned for a variety of wireless litigation systems, including IEEE 802.16 (WiMAX), some IEEE 802.11a/g wireless LANs (Wi-Fi), IEEE 802.20 Mobile Broadband Wireless Access (MBWA), and the like. 
     One proposal for a new flexible wireless cellular communication system, which can be seen as an evolution of the 3G WCDMA standard, is 3G Long Term Evolution (3G LTE). This system will use OFDM as multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to, and even exceeding, 100 Mb/s will be supported for the largest bandwidth. For the uplink, a kind of pre-coded OFDM is employed, where the primary purpose of the pre-coding is to reduce the large peak-to-average (PAR) ratio commonly known to be one of the drawbacks with OFDM. 
     OFDM is uniquely suited for LTE for a number of reasons. Relatively low-complexity receivers, as compared to other access techniques, can be used in case of highly time-dispersive channels. Additionally, at least in theory, OFDM allows for very efficient usage of the available bandwidth. For example, in the case of only one user transmitting, it is possible to exploit the fact that the channel quality typically is very different at different frequencies (that is, the channel is said to be frequency selective). Also, since the information in OFDM is transmitted on a large number of sub-carriers, different modulation and coding can be applied on different sub-carriers, rather than using the same modulation and coding on all sub-carriers. 
     One of the main challenges of OFDM is to ensure that the sub-carriers are orthogonal to one another. This implies that, for example, frequency offset and phase noise must be maintained at a sufficiently low level. If the orthogonality is lost, information on one sub-carrier is leaked to other sub-carriers, primarily to the closest ones. This leakage is referred to as inter-carrier interference (ICI). 
     OFDMA allows several users to share the available bandwidth by allocating different sub-carriers to the different users, making the users orthogonal to one another. The allocation of sub-carriers may be dynamic, such as allocating a larger number of sub-carriers to users that have a larger amount of data to transmit. Unlike to the situation with a single user in OFDM, loss of orthogonality of the sub-carriers may be significant if the different users&#39; signals are received with very different power, which may occur in the uplink or the downlink. 
     Two of the major factors giving rise to ICI are frequency error and Doppler spread. A frequency error is due to a mismatch between the transmitter and the receiver in generating the carrier frequency. A frequency error will also be manifest when the transmitter and the receiver would have identical frequency generators, but where one of the receiver or transmitter is moving relative to the other. For a multi-path channel, different paths will experience different Doppler frequency shifts, giving rise to a spread in the experienced Doppler frequency at the receiver side. 
     For OFDM, the ICI caused by a frequency error can be accurately modeled as: 
                 I   ⁡     (     δ   ⁢           ⁢   f     )       =         π   2     3     ⁢       (       δ   ⁢           ⁢   f       Δ   ⁢           ⁢   f       )     2         ,         
where δf is the frequency error and Δf is the carrier spacing between the sub-carriers. Since all the sub-carriers are affected by the same frequency offset, the frequency error may be removed prior to applying the FFT, to eliminate the ICI.
 
     If instead the ICI is caused by Doppler spread, then if the paths are assumed to arrive from all directions with a uniform distribution (referred to as Jakes&#39; model), the ICI can be accurately modeled as: 
                 I   ⁡     (     f   D     )       =         π   2     3     ⁢       (               ⁢     f   D         Δ   ⁢           ⁢   f       )     2         ,         
where f  D  is the maximum Doppler frequency and Δf is the carrier spacing between the sub-carriers.
 
     If the ICI caused by a frequency error or Doppler spread is assumed to have the same effect as additive white Gaussian noise (AWGN), then the total noise experienced by a receiver is simply calculated as N+I, where N is power of the AWGN and I is the ICI power. Consequently, the effective signal-to-noise ratio (SNR) experienced by the system can be expressed as 
               SNR   eff     =       S     N   +   I       .           
Using the effective SNR as defined above, it is easy to determine if ICI is an issue of not. It is also easily seen that the larger effective SNR that is required, the harder requirements there will be on keeping the ICI at a low level.
 
     From these formulas, it is clear that a straightforward way to reduce the ICI is to increase the carrier spacing Δf . A known feature of OFDM is redundancy in the form of a cyclic prefix (CP) prepended to the useful part of each OFDM symbol of duration T u . The minimum duration of the CP should be at least as long as the (expected) maximum delay spread of the channel where the system is supposed to operate. Since the carrier spacing is the reciprocal of T u , increasing Δf means that T u  will be decreased, but the CP duration must be maintained. Accordingly, increasing Δf results in reduced spectrum efficiency. 
     Another strategy to reduce ICI is to estimate the ICI and then remove its impact on the received signal. In general, ICI cancellation is a complex operation that adds cost and increases power consumption in an OFDM receiver. There are two major reasons for the complexity of ICI cancellation. First, from a mathematical perspective, removing the impact of ICI involves computing the inverse to a very large matrix, which is a computationally intensive task. Second, to estimate the ICI, both the channel and the channel derivative must be estimated. Since ICI reduces the effective SNR, accurate channel estimation cannot be performed, resulting in poor estimates of the ICI. An iterative approach to ICI cancellation has been suggested in the art, beginning with initial channel estimation and ICI cancellation. Following the initial ICI cancellation, improved channel estimates are obtained from the signals from which the initial ICI estimate has been removed. An improved ICI estimate is then obtained using the improved channel estimates. This iterative procedure may be repeated to obtain the desired performance improvement. Such iterative ICI estimation is computationally complex, and introduces delay. 
     One known scheme for ICI cancellation relies on subtracting the ICI from different sub-carriers, rather than attempting to invert a matrix. While this approach yields a significant gain improvement, especially if used together with windowing, it has been shown that the gain remains far from that ideally possible if the ICI could be fully removed, primarily because the channel estimate, and in particular the channel change, are difficult to estimate with sufficient accuracy. ICI cancellation schemes known in the art are complex, and although some yield considerable improvement, in general the improvement is far below what is theoretically possible. 
     Prior art OFDM ICI cancellation has only been considered when all the sub-carriers are transmitted by the same user. That is, a signal is sent from one transmitter, over a plurality of sub-carriers, and is received by a single receiver. 
     SUMMARY 
     According to one or more embodiments disclosed and claimed herein, a system and method is presented for ICI cancellation when a total received signal comprises signals transmitted by a plurality of transmitters. This methodology allows for very efficient solutions with low computational complexity, but that achieve ICI cancellation performance much closer to the ideal case than prior art solutions. ICI cancellation is performed by identifying the transmitted signals that cause the largest ICI to received signals from other transmitters, and removing the ICI contribution from these transmissions. This may be accomplished by calculating the ICI terms only based on the received signal and the frequency offset. Alternatively, the transmissions causing the ICI are demodulated, the ICI on other signals is then determined and subtracted, and other signals are then demodulated. Which transmissions cause the largest ICI on others depends on the relative strength of the corresponding signals and how much orthogonality is lost. The latter might be due to frequency error, Doppler spread, or a combination of both. 
     One embodiment relates to a method of cancelling ICI in an OFDMA wireless communication system receiver receiving signals from at least a first transmitter on a first set of sub-carriers and second transmitter on a second set of sub-carriers. A frequency offset in the sub-carrier received from the first transmitter is estimated. The ICI in the set of sub-carriers received from the second transmitter caused by the first transmitter is calculated based on the estimated frequency offset in the set of sub-carriers received from the first transmitter. The calculated ICI is subtracted from the set of sub-carriers received from the second transmitter. 
     Another embodiment relates to a method of receiving signals from two or more transmitters, each transmitting on one or more unique sub-carriers in an OFDMA wireless communication system. The received power level and the relative frequency offset of each received signal is estimated. The ICI each received sub-carrier causes on other received sub-carriers is estimated in response to its relative received power and frequency offset. The sub-carriers are serially demodulated in response to the ICI they cause other sub-carriers. 
     Still another embodiment relates to a receiver in an OFDMA wireless communication system. The receiver includes a receiver operative to receive signals from a plurality of transmitters, the signals carried on a plurality of sub-carriers, and to measure the received signal power levels. The receiver also includes a frequency estimation unit operative to estimate frequency offsets in received signals. The receiver further includes an ICI cancellation unit operative to estimate the ICI in a sub-carrier received from a second transmitter caused by a first transmitter in response to the frequency offset and relative power level of a sub-carrier received from a second transmitter, and further operative to cancel the estimated ICI from the sub-carrier received from the second transmitter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of an OFDM receiver. 
         FIG. 2  is a graph depicting the received signal power of OFDM sub-carriers transmitted by two users. 
         FIGS. 3 and 4  are graphs depicting the simulated effective SNR as a function of frequency error for different power offsets, with and without ICI cancellation. 
         FIGS. 5 and 6  are graphs depicting the simulated effective SNR as a function of the power offset between users&#39; signals for different frequency errors, with and without ICI cancellation. 
         FIGS. 7 and 8  are graphs depicting the simulated effective SNR as a function of error in the estimation of frequency error, for different frequency errors. 
         FIGS. 9 and 10  are graphs depicting the calculated effective SNR as a function of frequency error when ICI is cancelled from increasing number of sub-carriers, for different received signal powers. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a functional block diagram of the relevant portion of an OFDM receiver  10 . The receiver  10  includes a Fast Fourier Transform (FFT)  12 , ICI Cancellation function  14 , Channel Estimation function  16  providing channel estimates to the ICI Cancellation function  14 , Frequency Estimation function  18  providing frequency estimates to the ICI Cancellation block  14 , Demodulator function  20 , and Further Processing  22  (such as soft value generation, FEC decoding, and the like). In some embodiments, the receiver  10  further includes a Symbol Decision function  24 , which further aids ICI Cancellation  14  by providing decoded symbol information. 
     To simplify the description, the present invention is described for the up-link transmission in an OFDM system having 15 kHz sub-carrier spacing. Only two users transmitting to the base station are considered, with each user transmitting on a single resource block of 12 sub-carriers, corresponding to a bandwidth of 180 kHz. Those of skill in the art will readily recognize that the present invention is not limited to this specific configuration, but rather may be advantageously applied to ICI cancellation for any multi-user transmissions in an OFDM wireless communication system. 
       FIG. 2  depicts the receipt of transmissions on sub-carriers from two users—user  1  and user  2 . As depicted, the transmissions from user  1  are received at a considerably higher power level than those of user  2 . Due to a relatively large frequency error in the signal transmitted from user  1 , user  1 &#39;s signals cause interference in the signals received from user  2 . Since in general the received signal comes from different users, and therefore different sub-carriers of the signal may experience different frequency errors, no attempt is made in the base station to estimate and compensate for the frequency error prior to processing the signal by the FFT. 
     One potential source for a large frequency error in user  1 &#39;s signal is that user  1  may be traveling at a high speed towards the base station. When user  1 &#39;s mobile terminal is receiving, it will experience a positive frequency error due to the Doppler effect. Consequently, the mobile terminal will adjust its frequency so that it matches the true carrier frequency plus the Doppler frequency, and will demodulate received signals properly. Then, when the mobile terminal transmits, it will transmit at a carrier frequency that equals the correct carrier frequency plus the Doppler shift. Since the signal received at the base station (carrier frequency+Doppler) also will experience a positive Doppler shift due to user  1 &#39;s relative speed, the frequency error experienced at the base station for user  1  will be twice the Doppler frequency. 
     Because the frequency error in the signal received from user  1  is twice the Doppler shift, it might cause a significant leakage in the FFT, wherein information on one sub-carrier leaks over to another sub-carrier. This leakage will degrade the performance for user  1 , and in addition it may completely ruin reception performance for user  2  if the signal from user  1  is received at the base station at much higher power than the signal from user  2 , as depicted in  FIG. 2 . 
     Numerically, suppose that user  1  is moving at 100 km/h and the carrier frequency is 2.6 GHz. This corresponds to a Doppler frequency shift of 240 Hz. The effective frequency error experienced at the base station will therefore be 480 Hz. Considering how this affects the performance for user  1 , an upper bound on the ICI that user  1  causes to itself can be obtained by assuming an infinite number of sub-carriers being used, rather than just 12. The ICI bound obtained in this way becomes 
               I   ⁡     (   480   )       =           π   2     3     ⁢       (     480   15000     )     2       =     0.0034   =       -   25     ⁢           ⁢     dB   .                 
Thus, if for instance the required SNR for user  1  is 15 dB, there would be a margin of 10 dB to the “noise-floor” caused by ICI, and the effect of ICI can safely be neglected.
 
     Next, consider the ICI that is caused from user  1  to user  2 . Suppose that the signal from user  1  is received at higher power than the signal from user  2 , as depicted in  FIG. 2 . This may occur, for example, if user  1  is much closer to the base station than user  2 , and no power control is applied.  FIG. 3  depicts the effective SNR for various frequency errors, where the S/N=30 dB and the signal from user  1  is received with 10 dB higher power.  FIG. 4  depicts the effective SNR for various frequency errors, where the S/N=40 dB and the signal from user  1  is received with 20 dB higher power.  FIG. 5  depicts the effective SNR for various power offsets between the received signals, where the S/N=30 dB and the signal from user  1  is received with a frequency error of 500 Hz.  FIG. 6  depicts the effective SNR for various power offsets between the received signals, where the S/N=30 dB and the signal from user  1  is received with a frequency error of 1000 Hz. As shown, the degradation for user  2 , with no ICI cancellation, is substantial. 
       FIGS. 3-6  also depict that ICI cancellation can drastically improve the effective SNR experienced by user  2 , according to the following methodology. Suppose that user  1  is transmitting symbol S K+L  on sub-carrier K+L, and let H K+L  and H′ K+L , denote the (average) channel transfer function for sub-carrier K+L, and the change of H K+L  during the information-carrying part of the OFDM symbol, respectively. 
     The corresponding received signal on sub-carrier K+L can be written R K+L =S K+L H K+L , and the ICI that falls into sub-carrier K is approximately given by 
     
       
         
           
             
               R 
               
                 K 
                 , 
                 
                   K 
                   + 
                   L 
                 
               
             
             = 
             
               
                 S 
                 
                   K 
                   + 
                   L 
                 
               
               ⁢ 
               
                 H 
                 
                   K 
                   + 
                   L 
                 
                 ′ 
               
               ⁢ 
               
                 
                   1 
                   
                     j 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     L 
                   
                 
                 . 
               
             
           
         
       
     
     Thus, to determine the ICI, the transmitted symbol as well as the channel&#39;s derivative must be estimated, which usually is very difficult. However, in the case that the experienced channel change is due to a frequency error we note that
 
 H′   K+L   ≈j 2 πδfH   K+L   /Δf  
 
where the approximation comes from the fact that the channel change is assumed to be linear in the direction of the tangent, i.e., the approximation that is used is
 
exp( j 2 πδf/Δf )≈1 +j 2 πδf/Δf , when δf is small.
 
     Since R K+L =S K+L H K+L , it follows that 
                     R     K   ,     K   +   L         =       ⁢       S     K   +   L       ⁢     H     K   +   L     ′     ⁢     1     j   ⁢           ⁢   2   ⁢           ⁢   π   ⁢           ⁢   L                     ≈       ⁢         R     K   +   L         H     K   +   L         ⁢   j2π   ⁢       δ   ⁢           ⁢   f       Δ   ⁢           ⁢   f       ⁢     H     K   +   L       ⁢     1     j2π   ⁢           ⁢   L                     =       ⁢       R     K   +   L       ⁢         δ   ⁢           ⁢   f       Δ   ⁢           ⁢   fL       .                   
Since R K+L  is just the received symbol prior to equalization, and δf is the frequency offset, which can be estimated with rather high accuracy, the ICI term can also be accurately estimated. This frequency estimation function is depicted as block  18  in the receiver  10  block diagram of  FIG. 1 . Note that the ICI is estimated by an estimate of the frequency error—neither the channel nor the derivative of the channel need to be estimated, as is usually the case in conventional approaches to ICI cancellation.
 
     In the graphs of  FIGS. 3-6 , one ICI cancellation algorithm, denoted “Full non-DD ICI cancellation,” uses the above expression for estimating the ICI component and then subtracts it from a received signal. The other algorithm, denoted “Full DD ICI cancellation,” uses an actually transmitted signal and the actual channel experienced, thus reducing the noise term somewhat. DD stand for Decision Directed, and refers to the fact that in an actual implementation, the transmitted signal is not known, but must be determined. This is depicted by the dashed-line function Symbol Decision  24  in  FIG. 2 , which provides the ICI Cancellation  14  with what the receiver determines the transmitted symbol to have been. “Full” ICI cancellation refers to the fact that ICI from all sub-carriers transmitted by user  1  are subtracted from the signal from user  2 . 
     The results in  FIGS. 3-6  are obtained under the assumption that the frequency error in the signal received from user  1  has been perfectly estimated. Of course, this is not the case in practice.  FIGS. 7 and 8  depict the effective SNR of a received signal as function of estimation error for the frequency used to estimate the ICI.  FIG. 7  depicts a 250 Hz frequency error;  FIG. 8  depicts a 500 Hz error. In both cases, the S/N=30 dB and the signal from user  1  is received with 20 dB higher power. As expected, the effective SNR is degraded when the frequency error is not correctly estimated. The graphs additionally demonstrate that even when the frequency estimation error is relatively large, the gain is still significant compared to the case where no ICI cancellation is performed. 
       FIGS. 9 and 10  graph the calculated effective SNR as a function of frequency error for ICI cancellation from different numbers of sub-carriers, and depict how the receiver performance varies depending on the number of sub-carriers transmitted by user  1  for which the corresponding ICI in user  2 &#39;s signal is cancelled. Data graphed in the figures was obtained analytically. In  FIG. 9 , the signal from user  1  is received with 10 dB higher power than the signal from user  2 ; in  FIG. 10 , the user  1  signal is 20 dB higher. S/N=30 dB in both cases. The lower curve corresponds to L=0, meaning that no ICI cancellation is performed. The next curve graphs L=1, wherein only ICI from the user  1  sub-carrier closest (in frequency) to user  2 &#39;s signal is cancelled. L=2 means that ICI from the two closest user  1  sub-carriers are cancelled, and so on. For L=12, full cancellation is performed, meaning that the ICI from all user  1  sub-carriers are cancelled from the signal from user  2 . In  FIGS. 9 and 10 , this curve is hard to see since it is perfectly horizontal—indicating no SNR degradation due to ICI over 1500 Hz of frequency error in user  1 &#39;s received signal. 
     As  FIGS. 9 and 10  demonstrate, the ICI cancellation methodology of the present invention is scalable. For relatively slight interference, only ICI contributed by the closest interfering sub-carriers from user  1  may be removed from a sub-carrier received from user  2  to achieve an acceptable SNR. For more severe interference, ICI contributed by most or all of the interfering sub-carriers may need to be removed. Additionally, ICI from a variable number of the interfering sub-carriers from user  1  may need to be cancelled from other, further (in frequency) sub-carriers from user  2 . That is, while ICI from most or all user  1  sub-carriers may need to be calculated and removed from adjacent user  2  sub-carriers, user  2  sub-carriers further removed may require ICI cancellation from fewer of user  1 &#39;s sub-carriers (e.g., only the closest few). 
     Given the teachings herein, those of skill in the art may readily perform the tradeoffs between computational complexity, power consumption for ICI cancellation calculations, receiver delay, and achievable SNR improvement for any given situation. Such determination may, for example, be based on the degree of frequency error in an interfering signal and the relative received power between interfering and interfered signals. In any event, calculating and removing ICI caused by one or more individual sub-carriers transmitted by a first transmitter on a received signal transmitted from a second transmitter may achieve greater ICI cancellation than prior art methods, at reduced computational complexity. 
     As those of skill in the art will readily recognize, any or all of the functional blocks depicted in FIG.  1 −including the FFT  12 , ICI Cancellation  14 , Channel Estimation  16 , Demodulator  20 , Further Processing  22 , Frequency Estimation  18 , and Symbol Decision  24 —may, in any receiver  10 , be implemented as hardware circuits, as programmable logic, as firmware or software executing on a microprocessor or Digital Signal Processor (DSP), or any combination thereof. Although the present invention has been explicated herein in terms of two users transmitting via mobile terminals to a base station, the invention is not limited to this system implementation, and may be advantageously applied to any OFDMA receiver that receives signals from two or more transmitters on two or more sub-carriers. 
     The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.