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Frequency Division Multiple Access (FDMA) divides the assigned frequency range into multiple carrier frequencies to support multiple conversations as shown in Figure 12.2. Multiple narrowband frequency channels are derived from a wider band of radio spectrum, much as Frequency Division Multiplexers (FDMs) operate in the wired world (Chapter 2). Transmission #1 takes place on one frequency, while transmission #2 takes place on another. The station equipment must be frequency agile, in order to search and seize an available frequency channel, especially as the mobile transmitter/receiver moves from one cell to another in a cellular network.
Figure 12.2 Frequency Division Multiple Access (FDMA).
Analog cellular systems employ FDMA, which also can be used in digital systems. In either scenario, each conversation is supported by a single channel; a single base station or cell site can support multiple channels, which are subsets of a wide band of radio spectrum. For instance, the U.S. AMPS (Advanced Mobile Phone System) system provides for a total allocation of 40 MHz, which is divided into 666 frequency pairs, each of which provides 60 kHz (30 kHz for the forward channel and 30 kHz for the reverse channel). The two channels supporting a single conversation are widely separated in order to avoid confusion on the part of the terminal equipment; the AMPS systems provides for 45 MHz separation. On average, U.S. AMPS cell sites support 57 frequency channels [12-6]. Table 12.1 provides a comparison of AMPS and GSM, a digital standard discussed later in this chapter.
Transmission Mode | Multiplexing Technique | Duplexing Technique | Frequency Band | |
---|---|---|---|---|
AMPS | Analog | FDMA | FDD | 800 MHz |
GSM | Duplex | FDMA/TDMA | TDD | 800 MHz & 900 MHz |
Frequency Division Duplex (FDD) is a means of providing duplex (bidirectional) communications. Forward and backward channels make use of separate frequencies. FDD is used with both analog and digital wireless technologies, including cordless telephony and cellular.
Time Division Multiple Access (TDMA) is a digital technique which divides each frequency channel into multiple time slots, each of which supports an individual conversation (Figure 12.3). This concept is exactly the same as in the wired world where TDMs perform the same function. The total available bandwidth, the bandwidth of the individual channels, and the number of time slots per channel vary according to the particular standard in place, as well as the specific coding technique employed. For instance, GSM involves a carrier channel of 200 kHz, with a channel rate of approximately 200 Kbps. The channel is divided into 8 time slots of 25 Kbps each, easily supporting low-bit-rate digitized voice of 9.6 Kbps, plus overhead for framing and signaling. Each conversation makes use of 2 time slots, one for the forward channel and one for the reverse channel. Services based on TDMA offer roughly 3 times the traffic capacity of FDMA services [12-1]. A comparison of GSM and AMPS is provided in Table 12.1.
Figure 12.3 Time Division Multiple Access (TDMA).
TDMA terminal equipment is simpler and less expensive. This largely is due to the fact that the transmit and receive functions are not operative at the same point in time, eliminating the full-duplex (FDX) requirement. A typical TDMA system, such as GSM, actually is both FDMA and TDMA. Multiple carrier channels are provided, divided by frequency. Within each carrier frequency channel, multiple time slots (digital channels) are supported.
E-TDMA (Enhanced TDMA), developed by Hughes Network Systems, is an improvement over TDMA, employing Digital Speech Interpolation (DSI) compression and half-rate vocoders (voice coders), operating at 4.8 Kbps, in order to improve on bandwidth utilization. E-TDMA is said to provide 16:1 improvement over analog technology.
Time Division Duplex (TDD) is a digital means of providing bidirectional communications. The forward and backward channels make use of separate time slots. TDD can be employed with both channels using the same frequency (ping-pong transmission), or it may be used in conjunction with FDD.
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