Robust spread spectrum type communication with M-ary quasi orthogonal signaling for wireless fading channels

In this paper, a receiver architecture with reduced complexity is proposed for spread spectrum type communication with M-ary quasi orthogonal signaling. The proposed structure comprises of the channel and code matched filtering (MF) concatenated with a reduced state maximum likelihood sequence estimation (MLSE) type processing at symbol rate. MLSE, which takes the interference caused by nonideal cross- and autocorrelations of the spreading codes and multi-path channel into account, is directly applied to the unwhitenedmatched filter outputs. The hypothetical matched filter bound (MFB) is achieved by the proposed structure with the use of a reasonable number of states. By using the proposed reduced state Ungerboeck type MLSE based receiver, it is seen that M-ary quasi orthogonal signaling is an effective spread spectrum type modulation technique for multi-path channels with large delay spread at low power.