As more signals are sent within a fixed amount of RF spectrum, there are more occasions when signals interfere with another signal that is intentionally transmitted along the same channel, rather than being random noise. When frequency spectrum is available on a limited basis as in cellular or PCS systems, the same frequency is reused in geographically distinct regions (or cells) to maximize the spectrum efficiency. The overall system capacity is generally limited by the interference caused by the reused frequencies.
Interference cancellation (IC) is traditionally achieved by the use of multiple antennas at the receiver terminal. However addition of antennas at the receiver significantly adds to the receiver cost. On the other hand IC using a single antenna is an attractive way to increase system capacity since it eliminates the need for additional RF hardware cost since the algorithm can be implemented digitally in base band.
The recently discovered single antenna interference cancellation (SAIC) algorithms are specifically tailored for GMSK signal detection in the presence of GMSK interferers. The ICUPC 1998 paper “Cochannel Interference Cancellation within current GSM Standard” written by Trigui and Slock exploits the special properties of GMSK modulation for SAIC.
A number of other SAIC approaches have also been proposed in the literature. Examples include: Ottersen, Kristensson, Astely, “A receiver”, International Publication Number WO 01/93439; Arslan, Khayrallah, “Method and Apparatus for Canceling Co-Channel Interference in a Receiving System Using Spatio-Temporal Whitening” International Publication Number WO 03/030478 A1; Meyer, Schober, Gerstacker, “Method for Interference Suppression for TDMA- and/or FDMA Transmission”, filed Dec. 19, 2001. Also of interest are B. Picinbono and P. Chevalier, “Widely Linear Estimation with Complex Data,” IEEE Trans. On. Signal Proc, vol. 43, pp. 2030-2033, August, 1995; W. H. Gerstacker et al, “Equalization with Widely Linear Filtering,” ISIT2001; W. A. Gardner, S. V. Schell, “GMSK Signal Processors For Improved Communications Capacity and Quality, U.S. Pat. No. 5,848,105, Dec. 8, 1998;
The above-mentioned prior art is relevant for SAIC for GMSK signals or any other real modulation signals. However the literature has not dealt with IC techniques involving both real and complex modulation alphabets such as GMSK and 8PSK signals, which are encountered in GSM/EDGE systems. If SAIC gains are to be maximized in EDGE deployments it is necessary to develop a mobile receiver that has interference cancellation capability irrespective of the interference modulation type.
IC methods are generally categorized into two classes namely blind IC, and Joint detection (JD). By “blind”, we mean that the channel parameters of the interfering signal are not known at the receiver and that the interfering signal is not detected together with the desired signal as in JD. The paper titled “The Finite-Length Multi-Input Multi-Output MMSE-DFE written by Al-Dhahir and Sayed may be a useful approach for JD, though it does not handle GMSK-8PSK IC. Significant improvement would be required to extend this technique to the GMSK-8PSK IC problem. So far no prior art is reported on IC involving 8PSK and GMSK signals.
The art would benefit from a system and method that is able to suppress interference between 8PSK and GMSK signals.