Multiple access methods

Communication satellites are designed to relay several, or more usually many, signals simultaneously. In some cases there may be a separate transponder for each carrier; this is typical of broadcasting satellites and of satellites used for distributing television signals to terrestrial broadcasting stations. More usually, each transponder will relay, not one carrier, but several or many. This is called 'multiple access'. There are three basic techniques for achieving multiple access without unacceptable interference between the various signals involved.

In frequency division multiple access (FDMA) the carriers that will be relayed by a transponder are assigned carrier frequencies within the transmission band of the transponder, the frequency separation between assigned frequencies being sufficient to avoid overlap of emission spectra. The travelling wave tube (TWT) and solid state power amplifiers which are used in transponders have relatively constant gain characteristics within a certain range of drive levels, but they become non-linear, then saturate, as an upper limit is approached. Therefore the output of the transponder will contain the input carriers, amplified, plus distortion products, such as harmonics of the carriers and the products of intermodulation between them, the level of which will be high if the input carrier aggregate is powerful enough to drive the amplifier close to satura­tion (Westcott, 1972; Chitre and Fuenzalida, 1972).

For a transponder operating in the FDMA mode, the power level of each up-link carrier reaching the satellite must be set with two objectives. The first is to obtain at the output of the amplifier the optimum ratio between useful carrier power and noise due to the distortion products in the vicinity of the carriers. This involves backing-off the aggregate input level from the point where the amplifier would be driven to maximum total output, in order to obtain a larger reduction in distortion products. The output backoff necessary for TWTs is typically in the range 6dB to lOdB, although the available useful power output can be increased above that level by optimising the assignment of frequencies to carriers and by the use of TWT linearising networks. The second objective is to divide the available output carrier power between the carriers in accord­ance with their down-link transmission needs.

Figure 51.11 Frame and burst format of the INTELSAT TDMA system. RB1 and RB2 are the reference bursts from reference stations 1 and 2 respectively. The drawing is not to scale

FDMA may be used for groups of carriers which have been modulated in any way, analogue or digital. Some of the carriers assigned frequencies in an FDMA system may themselves be multiple access systems, using time division multiple access (TDMA). Furthermore, if the C/N ratio in the output of the trans­ponder is not too high, it may be feasible to overlay the FDMA signals with spread spectrum signals, forming, in effect, a code division multiple access (CDMA) system.

A time division multiple access (TDMA) system, operating alone in a transponder, allows the full power to the transponder to be used, that is, no back off is required. This is because only one carrier is present in the transponder at any instant in time. Each earth station in the system transmits its signals in turn, in bursts, in assigned time slots, typically using PSK modulation, a brief guard time being assigned between each pair of burst slots to ensure that the bursts do not overlap even if small timing errors arise. Figure 51.11 illustrates the frame structure of a high capacity TDMA system.

Signals which are to be transmitted over a TDMA system must be digital. Bits within a frame are stored at the transmitting earth station, then assembled into a burst with the necessary preamble bits and transmitted at high speed at the appropriate time. At the receiver the reverse process puts the signal bits into store, then reads them out at the appropriate lower speed, frame by frame. The charac­teristics of TDMA systems vary over a wide range because the principle can be applied in many different circumstances, ranging from the transmission of low information rate monitoring or control signals with an aggregate bit rate of a few kbit/s, probably trans­mitted on a frequency assigned within a FDMA system, to the high capacity international telecommunications network TDMAsystems operating at 120Mbit/s in the INTELSAT and EUTELSAT systems (INTELSAT, 1972; Eutelsat, 1981; Hills and Evans, 1973).

On board switched TDMA has become feasible in multi-beam satellites like INTELSAT VI, using switch matrices which can operate within the TDMA frame to route one burst to down-link beam A and the next burst to another down-link beam, B.

The functioning of TDMA systems which make efficient use of the time dimension demands precise timing and complex control of access. Such systems may be costly. Where the traffic flowing through the system is light, much simpler systems which use prin­ciples first explored within the ALOHA system may provide adequ­ate availability. In these, the transmission path is normally open and an earth station with information to send verifies that no down-link burst from another earth station is in progress; it then transmits its burst. However, several hundreds of milliseconds elapse before the start of a signal from an earth station, sent via a geostationary satellite, can be received at another earth station. Two earth stations may therefore inadvertently transmit overlapping bursts, causing both messages to be mutilated. If this happens, they are both retrans­mitted automatically.

CDMA systems do not structure their use of transponders either in frequency or time. Earth stations transmit spread spectrum sig­nals which can be identified, after re-transmission by the satellite, by the coding which the signal elements carry.

These various multiple access systems differ in the effectiveness with which they use the facilities provided by a transponder. Figure 51.12 provides a measure of the capacity of a transponder having a bandwidth of 36MHz, using various multiple access and modulation techniques, as a function of the C/N ratio at the earth stations. Methods for calculating transponder performance are given in Hills and Evaas, 1973, and in Bargellini, 1972.

Figure 51.12 Telephone channel capacity in 36 MHz channel

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