Overview

One of the distinctive features of CDMA is that it is both a physical layer modulation and a medium access control (MAC) layer multiple-access scheme. Traditional design paradigms for wireless networks are mostly layered, allowing little or no information exchange between the link layer and the physical layer. As a result, the role of signal processing has been primarily limited to the physical layer, and solving problems typically concerns signal enhancement, noise/interference reduction, and parameter estimation. As wireless services evolve to meet increasing end-user expectations, the interactions between layers that have not been sufficiently explored become increasingly important. Some of the cross-layer problems, ranging from architectural principles for multimedia communications to scalable multirate CDMA, can be approached from signal processing perspectives.

In the last chapter of this book, we will discuss an entirely different type of application of signal processing in wireless communication. In particular, we will describe a packet radio network that exploits the spatial diversity provided by antenna arrays at both the physical and MAC layers. The access mode considered combines CDMA (including the extreme cases of TDMA and FDMA; see Chapter 1) with the so-called space-division multiple-access (SDMA). The MAC protocol is "channel-aware" in the sense that it adapts to the channel conditionsof an antenna array system to achieve throughput multiplication and reduction of packet delays. Such gain, provided by a unique cross-layer design, cannot be obtained by physical layer processing alone. We will show that the bottom-up design has the potential to make a substantial...