In broadband communications, OFDM is an effective means to capture multipath energy, mitigate the intersymbol interference, and offer high spectral efficiency. OFDM is used in many communications systems, e.g., wireless local area networks (WLANs) and wireless personal area networks (WPANs). Recently, OFDM together with time–frequency interleaving across subbands, the so-called multiband OFDM [9], has been adopted in the ultra-wideband (UWB) standard for wireless personal area networks (WPANs).

To improve the performance of OFDM systems, the fundamental concept of cooperative diversity can be applied. Nevertheless, special modulations/cooperation strategies are needed to efficiently exploit the available multiple carriers.

In this chapter, we study an OFDM cooperative protocol that improves spectral efficiency over those based on fixed relaying protocols while achieving the same performance of full diversity. By exploiting limited feedback from the destination node, the described protocol allows each relay to help forward information of multiple sources in one OFDM symbol. We also describe a practical relay assignment scheme for implementing this cooperative protocol in OFDM networks.

System model

In this section, we describe the system model of a wireless network, in which we take into consideration the random users’ spatial distribution. The channel model, the signal model, and the performance measure in term of outage probability are discussed.

We consider an OFDM wireless network such as a WLAN or a WPAN with a circular cell of radius ρ. The cell contains one central node and multiple users, each communicating with the central node.