ock diagram of a system that will estimate the phase offset and briefly describe its operation. Chapter Multiple Access Schemes Teaching Telecommunications Systems to Share chapter explores the different ways in which a communication channel can be shared by multiple users. We looked at this briefly in Chapter 2, where we intro- duced time division multiplexing and frequency division multiplexing. In this chapter, we provide more details SO that you can understand how to build a communication system that shares the channel among the many users who want to use it. 11.1 What It Is Let's say you have been given a license by the FCC to build a wireless communication system operating in a frequency band of 1.8 GHz to 1.805 GHz. You decide that you would like many users to be able to use your system. You also decide that each user will communicate only digital signals (that is, they will use a source coder to make all the information they send digital). You need to find a way to allocate portions of the communication channel to each user. This chapter is about how to do that. There are two general methods that allow many users to share a single communi- cation channel: 1. Multiplexing schemes. Multiplexing schemes are channel-sharing schemes where portions of the channel are assigned to each user by a system controller at a central location. The system controller assigns, in advance, the channel por- tions for each user, and controls each user's access to the channel. 2. Multiple access schemes. Multiple access schemes refer to channel-sharing schemes where a system controller assigns portions of the channel to each user based on current availability. The system controller can update the sharing of portions of the channel based on changes in system demand. Once the system controller tells the user what portion he can use, the user is in charge of making sure he uses the portion requested. Most modern communication systems use multiple access schemes, SO we'll spend our time on them. Chapter Eleven 11.2 The Underlying I