In
MIMO wireless communications employing LMMSE
receiver, the symbols transmitted through a transmit antenna are estimated at the
receiver in the presence of interference consisting of two main components: one due to the additive
noise and the other due to (interfering) symbols transmitted via the remaining antennas. This has been shown to hamper the performance of a communication
system resulting in incorrect symbol decisions, particularly at low SNR. IMMSE has been devised as a solution to cope with this problem; In IMMSE
processing, the symbols sent via each transmit antenna are decoded iteratively. In each stage of
processing, the received
signal is updated by removing the contribution of symbols detected in the previous iterations. In principle, this reduces the additive interference in which the desired symbols are embedded in. Therefore, the interference level should reduce monotonically as one goes down in
processing order. In a noisy environment, however, any incorrect decision made on a symbol in an iteration leaves its contribution in the updated received
signal available for processing in the following iterations. Fortunately, if the level of interference is estimated and the soft bits are scaled appropriately by the estimated interference power, the performance of IMMSE
receiver can be greatly improved. Preferred embodiments estimate the interference by computing the
probability of error in decoding the symbols of the previous stage(s). The computation of
decision error probability depends on the
constellation size of transmitted symbols and introduces very little processing overhead.