a broadcast mode. From this received signal, each SO that the free space round trip propagation delay (3.3usec/km) is os extracts a timing reference and the data intended for telephony short enough to enable ranging to be performed automatically by subscribers served by that OS. instructing an os to transmit within the up-link control time slot, for In the up-link direction, the telephony signals at each OS are instance, and estimating its distance from the CS. For rural multi- similarly encoded in PCM format, and subsequently suitably for- point systems, the use of synchronised repeaters enables one to matted for short periodic burst transmission at a second frequency cover much longer distances which results in a large round trip Thus the OSs access the CS in time division multiple access propagation delay. Consequently in some systems it is necessary to (TDMA) mode. Although most systems operate in a TDM/TDMA know the locations of OSs within a few kilometres range from the mode, the use of TDMA technique in both directions has also been CS SO that the OSs can be ranged automatically (Berndt, 1986; Hart, used (Bonnerot, 1987). This allows the system to operate in a power 1988). In some systems multi-carrier narrowband TDMA systems saving mode at the expense of introducing further time delay. have been used (Saunders, 1988; Mullen, 1989) SO that channel For most rural applications, the systems are generally designed to access, still via a control channel, uses a combination of frequency operate over considerably longer distances by employing one or division multiple access (FDMA) and TDMA techniques. more synchronised repeaters in cascade as illustrated in Figure 52.6. The radio hop between a repeater and an outstation (or repeater) 52.3.4 requires a further pair of frequencies in order to minimise co-chan- Traffic capacity/grade of service nel interference. In a demand assigned system some of the calls will be lost or delayed. The probability of this occurring represents the grade of 52.3.2 Additional features 