Abstract:
A method of estimating interference in a received signal is disclosed. The method includes receiving a plurality of subcarriers from a remote transmitter. Each of the subcarriers is multiplied by a control signal. At least two of the subcarriers are compared to produce a differential signal. Interference is estimated in response to the differential signal.

Description:
BACKGROUND OF THE INVENTION 
     The present embodiments relate to wireless communication systems and, more particularly, to a method and apparatus for interference estimation in a Long Term Evolution (LTE) wireless receiver. 
     Conventional cellular communication systems operate in a point-to-point single-cell transmission fashion where a user terminal or equipment (UE) is uniquely connected to and served by a single cellular base station (eNB or eNodeB) at a given time. An example of such a system is the 3GPP Long Term Evolution (LTE Release-8). Advanced cellular systems are intended to further improve the data rate and performance by adopting multi-point-to-point or coordinated multi-point (CoMP) communication where multiple base stations can cooperatively design the downlink transmission to serve a UE at the same time. An example of such a system is the 3GPP LTE-Advanced system (Release-10 and beyond). This greatly improves received signal strength at the UE by transmitting the same signal to each UE from different base stations. This is particularly beneficial for cell edge UEs that observe strong interference from neighboring base stations. 
       FIG. 1  shows an exemplary wireless telecommunications network having cells A and B. The illustrative telecommunications network includes base stations  100  in cell A and  110  in cell B, though in operation, a telecommunications network necessarily includes many more base stations. Base station  100  is synchronized with UEs  102  and  106  and communicates over respective wireless channels  104  and  108 . Likewise, base station  110  is synchronized with UE  112  and communicates over wireless channel  114 . Because each UE is synchronized with its respective base station, intra-cell interference is not a significant problem. For example, UEs  102  and  106  do not significantly interfere with each other or with base station  100 . Base stations  100  and  110 , however, are not synchronized. Therefore, UE  112  is not synchronized with either UE  102  or  106 . This lack of synchronization causes significant inter-cell interference for UEs near a cell boundary. For example, UE  106  primarily communicates with base station  100  over channel  108 . Thus, uplink transmission from UE  106  to base station  100  produces significant inter-cell interference  116  at base station  110 . Likewise, downlink transmission from base station  110  to UE  112  produces significant inter-cell interference  116  at UE  106 . 
     Turning now to  FIG. 2 , there is a diagram of a subframe  200  having a Physical Resource Block (PRB) pair. The eNB may configure 1, 2, 4, or 8 PRB pairs for communication with the UE. However, each PRB pair is a replica, and only one PRB pair is shown for the purpose of explanation. Each column of the diagram of the subframe corresponds to 12 subcarriers or tones in an OFDM symbol. There are 14 OFDM symbols in the subframe with a normal cyclic prefix (CP). The 3 OFDM symbols on the left side of the subframe include resource elements (REs) for transmission of a legacy physical downlink control channel (PDCCH) and legacy cell-specific reference signals (CRS). These 3 OFDM symbols are necessary for backwards compatibility with previous wireless standards. The 11 OFDM symbols on the right include resource elements (REs) for transmission of an enhanced physical downlink control channel (EPDCCH), and demodulation reference signals (DMRS) or pilot signals, as well as cell-specific reference signals (CRS) and orphan or unused REs. Orphan REs may exist because the UE always assumes that 24 REs are reserved for DMRS transmission in a PRB pair configured for EPDCCH transmission. 
     A cause of inter-cell interference is that both base stations use the same subcarriers or tones for each PRB with reuse 1. This means that the base station assumes that all 12 subcarriers are available for each PRB and is especially problematic in areas of dense deployment. If only a portion of the subcarriers were allocated to each base station, inter-cell interference would be reduced at the expense of bandwidth and throughput. Several attempts to reduce inter-cell interference through inter-cell interference coordination (ICIC) technology have been developed. For example, Kimura et al., “Inter-Cell Interference Coordination (ICIC) Technology,” Fujitsu Sci. Tech. J., Vol. 48, No. 1, pp. 89-94 (January 2012), have developed a method of fractional frequency reuse (FFR) to allocate different frequencies to UEs near a cell boundary. Others, such as Xing (U.S. Pub. No. 2014/0078922) employ a spreading code for adjacent cells to identify and remove interfering signals. Other methods rely on channel estimation as determined from known pilot signals. For example, Dua et al. (U.S. Pub. No. 2014/0016689) programs an equalizer by estimating a channel impulse response (CIR) and determining noise and power estimates based on the CIR. Equalizer inputs of a covariance matrix are adjusted based on these noise power estimates. A disadvantage of this method, however, is that errors in channel estimation are considered interference and noise. Moreover, in areas of dense deployment near cell boundaries, signal quality is degraded and channel estimation errors are significant. 
     While the preceding approaches provide steady improvements in wireless communications, the present inventor has recognized that still further improvements in interference detection are possible. Accordingly, the preferred embodiments described below are directed toward this as well as improving upon the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     In a preferred embodiment of the present invention, there is disclosed a method of estimating interference in a received signal. The method includes receiving a plurality of subcarriers from a remote transmitter. Each of the subcarriers is multiplied by a control signal. At least two of the subcarriers are compared to produce a differential signal. Interference is estimated in response to the differential signal. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagram of a wireless communication system of the prior art; 
         FIG. 2  is a diagram of a pair of Physical Resource Blocks of the prior art; 
         FIG. 3  is a block diagram of a wireless receiver of the present invention; and 
         FIG. 4  is a flow diagram showing calculation of equalizer weights based on a differential between adjacent subcarriers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Inter-cell interference is a significant problem and a major source of performance degradation in both uplink and downlink LTE wireless communication systems. This problem is especially significant in cell areas with dense deployment. An accurate estimate of interference information is necessary to effectively suppress inter-cell interference. 
     Referring to  FIG. 3 , there is a block diagram of a Long Term Evolution (LTE) diversity receiver of the present invention. The receiver includes receive antennas  300 - 304 , however, receivers of the present invention may include as few as two receive antennas and as many as N receive antennas, where N is an integer. Circuit  306  is coupled to the receive antennas and extracts pilot signals from a received data stream as is known in the art. The data stream is then applied to receiver equalizer circuit  308 . The equalized data stream is subsequently applied to circuit  310  for demapping, deinterleaving, and decoding. The decoded data stream is then applied to a baseband processor (not shown). 
     Extracted pilot signals are applied to circuit  312  to estimate the wireless channel. Circuit  312  is coupled to equalizer weight calculation circuit  322 . The data stream and extracted pilot signals are also applied to circuit  320  according to the present invention. Circuit  320  may be realized in software, hardware, or a combination of hardware and software. The Long Term Evolution (LTE) data stream comprises a data frame as shown at  FIG. 2 . Circuit  320  includes multiplication circuit  314 , differential circuit  316 , and covariance matrix circuit  318 . Circuit  320  is also coupled to equalizer weight calculation circuit  322 . 
     Turning now to  FIG. 4 , there is a flow diagram that will be used to explain operation of the receiver of  FIG. 3 . A data stream of symbols is initially received by N receive antennas  300 - 304  at step  400 . The LTE data stream for N receive antennas is given by equation [1]. 
     
       
         
           
             
               
                 
                   
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     Here, vector {right arrow over (y)} is the received data or pilot signal from all N receive antennas, s is the transmitted signal or data stream, H is the channel between a remote transmitter and each respective receive antenna, and {right arrow over (H)} are respective interference and noise components associated with each channel. At step  402  the pilot signals are extracted from the data stream by circuit  306 . The pilot signals are applied to circuit  312  at step  404  to estimate the wireless channel between a remote transmitter and the N receive antennas. The channel estimate is then applied to equalizer weight calculation circuit  322 . 
     At step  406 , each subcarrier from the multiple receive antennas of the received signal is multiplied by a corresponding control signal or known pilot signal s* by circuit  314 . The products are stored in vector {right arrow over (z)} n  as in equation [2], where n is the index of each subcarrier.
 
 {right arrow over (z)}   n   ={right arrow over (y)}   n   ×s   n *  [2]
 
     Circuit  316  calculates a differential {right arrow over (q)} n  between any two adjacent subcarriers n and n+1 at step  408  as in equation [3].
 
 {right arrow over (q)}   n   ={right arrow over (z)}   n   −{right arrow over (z)}   n+1   [3]
 
     At step  410 , circuit  318  calculates a covariance matrix R of interference for each subcarrier group as in equation [4]. Here, n 0  and n 1  are preferably lower and upper indices of a column of subcarriers of the data frame of  FIG. 2 , and H denotes a Hermitian transpose. 
     
       
         
           
             
               
                 
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     At step  412 , the channel estimate from step  404  and the covariance matrix R from step  410  are applied to equalizer weight circuit  322 . Equalizer weights W for the data stream are calculated by weight circuit  322  in response to the channel estimate and covariance matrix R. These weights are applied to receiver equalizer circuit  308  at step  414  to correct received data symbols and suppress interference in the received signal. In general, covariance matrix R can be used in any equalizer weight calculation method to suppress interference energy in the received signal. In a preferred embodiment of the present invention, the channel estimate Ĥ from circuit  312  is used together with covariance matrix R in a linear minimum mean squared error (LMMSE) method according to equation [5] to produce equalizer weights W.
 
 W=Ĥ   H ( ĤĤ   H   +R ) −1   [5]
 
The corrected data symbols less interference are then applied to circuit  310  for demapping, deinterleaving, and decoding. The decoded symbols are then applied to a baseband processor.
 
     There are several advantages of the present invention over interference suppression methods of the prior art. First, interference suppression of the present invention does not depend on the channel estimate. Thus, errors in the channel estimate do not negatively impact interference suppression. This is especially important in high density areas where signal quality is degraded. Second, the present invention advantageously employs the LTE wireless characteristic that there is little difference in channels for adjacent or closely spaced subcarriers. Thus, a difference in signals on adjacent subcarriers is primarily due to interference. Third, adjacent LTE subcarriers are typically separated by 15 KHz or 7.5 KHz. This is much less than the coherence bandwidth of the channels. For example, the coherence bandwidth for 0.9 correlation for the extended pedestrian A specification is approximately 460 Hz and for the extended vehicular A specification is approximately 60 Hz. Consequently, it is not strictly necessary to compare signals on adjacent subcarriers as long as the subcarriers are closely spaced. Moreover, multiple comparisons such as with upper and lower adjacent subcarriers are possible for confirmation of the covariance matrix. 
     Still further, while numerous examples have thus been provided, one skilled in the art should recognize that various modifications, substitutions, or alterations may be made to the described embodiments while still falling with the inventive scope as defined by the following claims. Other combinations will be readily apparent to one of ordinary skill in the art having access to the instant specification.