Interference and noise are major factors limiting the performance of cellular wireless communication networks. Most modern mobile terminals, or User Equipment (UE) include some demodulation capability to mitigate interference, thus improving the signal to interference plus noise ratio (SINR) measured at the output of the receiver. Improvements in SINR result in better performance. This capability can be used, for example, in detecting transmitted data information or in computing some measure of channel quality information (CQI) that is used for link adaptation, power control and user scheduling. The same is true of base stations.
Canceling or suppressing interference typically relies on identification of a set of channel resources that experience a particular form of interference as well as the estimation of interferer parameters for that set. The process of identification estimates the presence of an interference source that affects multiple channel resources. The process of interference parameter estimation is more easily accomplished when the interferer does not vary significantly in time or frequency due to fading, dispersion or intermittent transmission of the interferer. Since the estimation process typically is performed in the presence of random (i.e., thermal) noise, a sufficient number of estimation samples are required to minimize the effects of the noise. Further, estimation degrades when those samples are used where the interference varies abruptly.
Orthogonal frequency-division multiplexing (OFDM) is a spread spectrum multiplexing technique that distributes transmitted data over a large number of sub-carriers that are spaced apart at precise frequencies. Data is divided into a plurality of parallel data streams, one for each sub-carrier. Each sub-carrier is modulated at a low symbol rate, maintaining total data rates similar to conventional, single-carrier modulation schemes in the same bandwidth. Additionally, pre-coding may be applied across the transmitted symbols prior to mapping to the OFDM sub-carriers, for example, to obtain transmit diversity or to obtain better peak to average power in the transmitted signal.
In OFDM systems such as the Long Term Evolution (LTE) of the Universal Mobile Telecommunications System (UMTS), discontinuities can exist in the interference due to a number of reasons: 1) interferer signals transmitted from other cells can be packet-based, so interferer transmissions may be intermittent; 2) in addition to scheduling in time, transmissions can be scheduled in different frequency sub-bands; 3) multiple users can exist, for example from multiple base stations in the downlink and from multiple users in the uplink; 4) different transmission ranks can be used which present different interferer profiles; and 5) different pre-coding schemes can be used, which also present different interferer profiles. Similar situations exist in other OFDM systems, such as WiMAX.
For LTE, since scheduling is performed on contiguous groups of OFDM sub-carriers and symbols, called resource blocks, one approach to mitigating the impact of interference discontinuities is to restrict the estimation of the interferer parameters to use only received data corresponding to a single resource block. This avoids the problem of having to use an adjacent resource block, which may contain a different interferer profile. However, this potentially limits the estimation accuracy in the case where the same interferer profile exists in one or more adjacent resource blocks, and could be used to improve interference estimation.