nsiderations which can influence the Figure 52.5 One example of a frequency re-use pattern choice of modulation methods. At frequencies above about 10GHz using 90 degree sector antennas microwave source stability consideration is important in the selec- tion of a suitable modulation method. For instance, for dielectric resonator stabilised microwave oscillators, stabilities of 10-4 (tem- (Shindo, 1981; Mohamed, 1985). In practice, the locations of the perature range of 5°C to 60°C around 26GHz have been reported systems will inevitably depend on the locations of population (Ogawa, 1984a). In one system employing binary frequency shift centres. Furthermore, the terrain features can have a significant keying (FSK) modulation, the resulting d.c. offset at the CS de- effect on frequency re-use distance. The re-use plan should there- modulator has been used to convey a measure of the frequency fore be used as a guide in determining the frequencies of systems offset information back to the os SO that the transmit oscillator which are widely separated geographically when there is a possi- frequency can be corrected, thus providing a closed loop for fine bility of using similar systems subsequently in the intervening frequency tuning (Hewitt, 1984; Mohamed, 1985). terrain. Burst transmissions arriving at a CS from all the OSs are nomi- Further variations of the frequency plan are possible by conside- nally at the same frequency. At the CS the carrier phase has to be ring the use of both horizontal and vertical polarisations in different recovered from the preamble associated with each burst. However, sectors/cells (Murakami, 1983). Interference calculations must differential demodulation does not require carrier recovery and allow for co-channel interference from geographically distant sys- although it requires a higher C/N ratio, its implementation in the items under differential rainfall fading conditions. up-link direction is often preferred. The source stability consider- An important consideration in the use of mult