Collision-Tolerant Media Access Control for Asynchronous Users over Frequency-Selective Channels

In this paper, a frequency-domain cross-layer collision-tolerant (CT) media access control (MAC) scheme is proposed for the up-links of broadband wireless networks with asynchronous users. The collision tolerance is achieved with a frequency-domain on-off accumulative transmission (FD-OOAT) scheme, where the spectrum is divided into a large number of orthogonal sub-channels, and each symbol is transmitted over a small subset of the sub-channels to reduce collisions. Such a radio resource management scheme renders a special signal structure that enables multi-user detection (MUD) in the physical layer to resolve the collisions at the MAC layer. Most existing MUDs require precise symbol level synchronization among users. The proposed scheme, however, can operate with asynchronous users. A new theoretical framework is provided to study the impacts of time-domain user delays on system performance. Both analytical and simulation results demonstrate that the proposed FD-OOAT structure with time-domain oversampling is robust to user delays and the timing phase offset caused by the sampling clock difference between the transmitter and the receiver. It is shown that the proposed scheme can achieve significant performance gains, in terms of both the number of users supported and the normalized throughput.