Wireless Channel Multiple Access Techniques for Mobile Phones
5.1 Introduction to Multiple Access Techniques
As discussed in Chapter 1, for wireless communication we use air or free-space as the medium and an EM wave as the information carrier. However, there is a problem with this – the air channel can be considered as a single line or one communication link, so how can so many users be served using a single line? The answer is – we need to multiplex the same air channel between many different simultaneous channel users, so we will investigate the various technologies that are available for multiplexing the same air medium between different users. These are mainly: time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), and orthogonal frequency division multiple access (OFDMA). We will discuss these access technologies in detail, and in later chapters we will explain how these are used in various mobile communication standards.
5.1.1 Time Division Multiplexing
In this technique, the same channel is shared by various users using different time slots. Each user has a specific time slot number, and when that particular time slot arrives, it uses the slot to send or receive its data. This is depicted in Figure 5.1.
In Figure 5.1, the same channel is time multiplexed between 8 users. When user-1 on the left-hand side (called-party) is connected to the channel, then at the same time user-1 (the calling party) on the right-hand ...