A cross-layer perspective in an uncharted path - Signal processing in random access - Signal Processing Magazine, IEEE

he design of medium access control (MAC) protocols has traditionally been separated from that of the physical (PHY) layer. To a MAC protocol designer, the PHY layer is a black box satisfying the so-called collision model: when only one user transmits, the packet arrives at the receiving node error free. But when transmissions are simultaneous, packets are lost due to collision. Until recently, the theory of random access was based on such an idealized model, and random access protocols were viewed as collision resolution or collision avoidance techniques. In practice, the collision model is both optimistic and pessimistic: optimistic, for it ignores channel effects such as fading and noise on reception, and pessimistic, because it does not accommodate the possibility that packets may be successfully decoded in the presence of simultaneous transmissions. Given the advances in multiuser communications at the PHY layer, the collision model no longer represents all the characteristics of the PHY layer, missing some of its most important properties. Is there a need to go beyond the collision model for wireless networks? Should the MAC layer assume a multiuser PHY layer and be designed with a cross-layer principle in mind? Is the gain of a cross-layer design significant enough to justify replacing a well-tested protocol with a more sophisticated one? Will the cross-layer design be too complicated to implement, and too sensitive to channel changes to be useful? The idea of cross-layer design has been brought to the fore by the phenomenal growth in wireless applications and a continuing push for broadband access. The fundamental challenge, as noted by Gallager in 1985 [1] and more recently by Ephremides T