Incoherent wideband communications over multipath channels

This paper analyzes the efficiency of spreading modulations over wideband multipath channels. We bring asymptotic wideband results, and focus on the low SNR regime. In coherent systems low SNR implies efficient communications, but in a non-coherent communications the SNR should be high enough in order to enable decoding the message even though the channel is not known. Too high SNR reduces the efficiency of the modulation. For a channel model where path gains are not known while the L path delays are known to the receiver, and for spread spectrum modulations that use the entire bandwidth W at each transmission, one can communicate efficiently in the wideband limit if SNR = ω (L/W). If both path gains and path delays are unknown, efficient communications using spread-spectrum modulations are possible if SNR = ω (log(W/L)/W), given that the specific modulation is still efficient for such SNR. However, if SNR = 0(log(W/L)/W), communications are not efficient.