Orthogonal multiple access over time- and frequency-selective channels

Suppression of Multi-User Interference (MUI) and mitigation of timeand frequency-selective (doubly-selective) channel effects constitute major challenges in the design of thirdgeneration wireless mobile systems. Relying on a Basis Expansion Model (BEM) for doubly-selective channels, we develop a channel-independent block spreading scheme that preserves mutual orthogonality among single-cell users at the receiver. This alleviates the need for complex multi-user detection, and enables separation of the desired user by a simple code-matched channelindependent block despreading scheme that is Maximum Likelihood (ML)-optimal under the BEM plus white Gaussian noise assumption on the channel. In addition, each user achieves the maximum delay-Doppler diversity for Gaussian distributed BEM coefficients. Issues like links with existing multi-user transceivers, existence, user efficiency, special cases, backward compatibility with DS-CDMA, and error control coding, are briefly discussed.