Suppression of multiuser interference (MUI) and mitigation of time- and frequency-selective (doubly selective) channel effects constitute major challenges in the design of third-generation 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 multiuser detection, and enables separation of the desired user by a simple code-matched channel-independent 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 multiuser transceivers, existence, user efficiency, special cases, backward compatibility with direct-sequence code-division multiple access (DS-CDMA), and error control coding, are briefly discussed.
Bibliographical noteFunding Information:
Manuscript received July 2, 2001; revised January 28, 2003. This work was supported by the FWO–Flanders (Belgium), the ARL/CTA under Grant DAAD19-01-2-011, and the National Science Foundation Wireless Initiative under Grant ECS-9979443. The material in this paper was presented in part at the 35th Asilomar Conference on Signals, Systems, and Computers, Pacific Grove, CA, November 2001 and at IEEE GLOBECOM, San Antonio, TX, November 2001.
- Delay diversity
- Doppler diversity
- Multiple access
- Multiuser detection
- Time- and frequency-selective channels