14 - Opportunistic random access: any network topology

Summary

In this chapter, we consider schemes for distributed cross-layer optimization of multi-channel random access by exploiting local channel state and traffic information. We consider the most generic network setting, where users are not necessarily within the transmission ranges of all others; therefore, when a user is transmitting, it may only interfere with some users. One example is an ad hoc wireless network. Later we will also show that it is possible to apply this technology in cellular networks to improve the throughput of cell-edge users. We also consider generic traffic distribution in the network and each user may choose to send packets to or receive packets from different users simultaneously. An example is illustrated in Figure 14.1, where arrows indicate traffic flows and circles transmission ranges of different users. To begin with, in this chapter, we will introduce the optimal channel-aware Aloha for this type of wireless network. A discussion of the optimal distributed channel-aware medium access control (MAC) will be given in the next chapter.

Network model

Consider multi-channel wireless networks. The whole band is divided into K subchannels. All channels between pairs of users are assumed to be reciprocal, i.e. when no interference exists, user A can receive a signal from user B if and only if user B can receive a signal from user A with the same channel gain. However, the interference environments at user A and user B may be different since they are at different locations.