Introduction

In this chapter, we discuss multiple access using CS in the context of wireless communications. Essentially, multiple access means that multiple users send their data to a single receiver that needs to distinguish and reconstruct the data from these users, as illustrated in Figure 9.1. It exists in many wireless communication systems, such as

• Cellular systems: In cellular systems, the mobile users within a cell are served by a base station. The users may need to transmit their data during the same time period; e.g., multiple cellular phone users make phone calls simultaneously.

• Ad hoc networks: In such systems, there is no centralized base station. However, one node may need to receive data from multiple neighbors simultaneously; e.g., a relay node in a sensor network receives the reports from multiple sensors and forwards them to a data sink.

The key challenge of multiple access is how to distinguish the information from different transmitters. Hence, there are basically two types of multiple-access schemes:

• Orthogonal multiple access: This type of multiple access includes time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA) and carrier sense multiple access (CSMA). In TDMA, each transmitter is assigned a timeslot and it can only transmit over the assigned time-slot. Hence, the signals from different transmitters are distinguished in the time. OFDMA has a similar scheme to TDMA. The only difference is that OFDMA allocates different frequency channels to different transmitters and thus separates the transmitters in the frequency domain.