escribed in turn in the following sections. Time Division Multiple Access Time division multiple access (TDMA) allows multiple users to access a single channel without interference by allocating specific time slots to each user. As shown in Figure 4-13, the time axis is divided into time slots that are assigned to users according to a slot allocation algorithm. A simple form of TDMA is time division duplex (TDD), where alternate transmit periods are used for uplink and downlink in a duplex communication system. TDD is used in cordless phone systems to accommodate two-way communication in a single frequency band. Chapter Four TDMA time slot allocation Station 1 Station 2 Station 3 Station 4 Time slot; Ts TDD time slot allocation Uplink Downlink Figure 4-13: Time Division Multiple Access (TDMA) and Duplexing (TDD) TDMA is used in Bluetooth piconets (see Chapter 10). The master device provides the system clock that determines the timing of slots and, within each time slot, the master first polls slave devices to see which devices need to transmit and then allocates transmission time slots to devices that are ready to transmit. Frequency Division Multiple Access In contrast to TDMA, frequency division multiple access (FDMA) provides each user with a continuous channel that is restricted to a fraction of the total available bandwidth. This is done by dividing the available bandwidth into a number of channels that are then allocated to individual users as shown in Figure 4-14. Frequency division duplex (FDD) is simple form of FDMA in which the available bandwidth is divided into two channels to provide continuous duplex communication. Cellular phone systems such as GSM (2G) and UMTS (3G) use FDD to provide separate uplink and downlink channels, while 1G cellular phone systems used FDMA to allocate bandwidth to multiple callers. Radio Communication Basics Channel allocation Channel allocation Frequency User A User B Downlink User C User D Uplink Figure 4-14: Frequency Division Multiple Access (FDMA) and Duplexing (FDD) 