Article written by John Scourias, with comments in maroon by Tom Farley

Pages: Table of Contents (1) (2) (3) (3A) (4) (5) (5A) (6) (7) (8) (9) (10) (11) (12) (13) (14)

Article written by John Scourias, with comments in maroon by Tom Farley

4.1 Multiple access and channel structure

Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.

This is the correct, complete view of GSM. It's not enough to say, as I have too many times, that GSM and conventional cellular (IS-136) are TDMA based. While that it is true, it is more true to say such systems are TDMA and FDM based. First, we have a number of radio frequencies, each separated by 200khz. This is the frequency division multiplexing part. (Or the FDMA part, a minor semantic difference.) Secondly, we have the transmission technology, TDMA, by which we put several calls on a single frequency. These calls are broken into many pieces, each piece of each call sent one after another. Each call separated by slight differences in time. GSM is a TDMA/FDMA system.

Weick calls a burst "a sequence of signals counted as a unit in accordance with some specific criterion or measure." Bits are single pulses of electrical energy. Much like the single dash of a Morse Code key. With Morse code we use long and short pulses of energy to stand for letters. Although of uniform length, the pulses we use in digital radio do the same thing. Bits grouped in patterns represent voice and data. We also use bits, as shown in the diagram below, for signaling. In the channel depicted a burst of bits is a marker, an indicator, a signal within a signal. It's what the mobile first looks for in the digital stream flowing from the base station. More on this on the next page.

Channels are defined by the number and position of their corresponding burst periods. All these definitions are cyclic, and the entire pattern repeats approximately every 3 hours. Channels can be divided into dedicated channels, which are allocated to a mobile station, and common channels, which are used by mobile stations in idle mode.

Terminology alert! Cellular radio uses the word channel in many ways. It is a pair of radio frequencies. And channels are part of the digital stream that flows back and forth from the mobile to the base station. Channels, therefore, can be carried on a channel. Confusing, isn't it? The discussion below focuses on data channels, not radio channels.

4.1.1. Traffic channels

A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26-frame multiframe, or group of 26 TDMA frames. The length of a 26-frame multiframe is 120 ms, which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow Associated Control Channel (SACCH) and 1 is currently unused (see Figure 2). TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously, thus simplifying the electronics.

We've seen these characters before. Reading the Channels page might help you understand what follows. We'll discuss them individually as they come up later in the article.

In addition to these full-rate TCHs, there are also half-rate TCHs defined, although they are not yet implemented. Half-rate TCHs will effectively double the capacity of a system once half-rate speech coders are specified (i.e., speech coding at around 7 kbps, instead of 13 kbps). Eighth-rate TCHs are also specified, and are used for signalling. In the recommendations, they are called Stand-alone Dedicated Control Channels (SDCCH).