Channel-Matched Spreading Codes for the Downlink of MC-CDMA

This paper concerns the multiple access performance of spreading codes used in the downlink of multicar- rier code-division multiple-access (MC-CDMA). Complex-valued spreading codes, particularly polyphase codes, are investigated based on a system model in which modulation formats, equalization/combining schemes, and fading statistics are general. Different from previous approaches, we pay special attention to the influence of channel correlation on the performance of spreading codes and try to take advantage of the channel correlation in the design of spreading codes. We begin by considering a worst-case frequency-selective fading channel characterized by independent and identically distributed (i.i.d.) fading for all subcarriers. Unitary polyphase spreading codes are proven to be optimal among all polyphase codes for the i.i.d. fading channel. Next, optimal spreading codes for a two-user case are derived analytically. We proceed to consider a block-wise correlated fading channel model. We propose a class of channel-correlation- matched Kronecker-product unitary spreading codes, which can exploit the channel correlation, thereby reducing multiple access interference (MAI) significantly. Finally, we extend the Welch lower bound to the downlink of MC-CDMA over correlated fading channels and investigate the performance of Welch bound equality codes.