Article Preview
TopIntroduction
The proliferation of communication devices coupled with the exponential demands for high data rates motivates multiuser communications. For multiuser communications, wireless technology is the most active technology that will drive solutions for high data rates. However, wireless technologies are encumbered with problems of which random channel fading is dominant. Given this, there are considerable research aimed at providing resiliency by deploying intelligent wireless networks (Giannakis et. al, 2000) (Liu, 2000). A key aspect is the development of signal processing and receiver techniques that provides optimum capacity for the large number of users. Since the multiuser access will support high data rates without requiring the need for increased bandwidth, a blend of signal processing with physical layer techniques will drive capacity demands while countering the effects of fading. A metric capturing the system/receiver performance is diversity (Diggavi & Tse, 2005). Diversity can either be a function of the signal to noise ratio, number of times a signal is transmitted. In CDMA systems, diversity is represented by length of orthogonal spreading sequences (Hochwald, Marzetta, & Papadias, 2001). In pure fading systems, diversity is given as a function of the Signal to Noise Ratio (SNR) (Diggavi & Tse, 2005; Xiao & Skoglund, 2005).
A major driver for countering the effects of fading is cooperative diversity. This is the process of using alternate transmission paths under the assumption that all the signal paths to the destination fade independently. Cooperative diversity will have a wide range of applications such as vehicle-to-vehicle communications and wireless sensor networks (Dohler & Li, 2010). Wireless sensor networks are increasingly deployed for numerous purposes such as data logging, crime control, unsupervised monitoring of environmental conditions such as humidity. The communicating nodes are highly dependent on limited battery power. A major design criterion in sensor network deployment is optimizing the networks lifetime without degrading communication between sensor nodes. Therefore, limiting transmission power save battery life and coverage is a predominant problem. Therefore, cooperative techniques are beneficial to close up coverage gaps which helps in preserving network integrity (Dohler & Li, 2010). Cooperative diversity makes use of various cooperative relay strategies to achieve spatial diversity gains. For a properly designed system, the diversity can scale up to the number of nodes known as a full diversity system (Wang, Cano, Giannakis, & Laneman, 2007). Therefore, the performance will depend on the number of cooperative relay nodes, the operations at each node and the channel model in operation. There are numerous references on cooperative communications (Lozana, Heath, & Andrews, 2013; Laneman, Tse, & Wornell, 2004).