With increasing congestion in civil land mobile radio networks, communication authorities are looking to data transmission as an aid to spectrum conservation. It is often difficult, however, to establish a satisfactory communication channel, particularly for data, because of the variability of the received signal and ignition interference. The object of this work is to investigate how quasi-synchronous operation of two or more transmitters can improve the quality of the received signal. A major problem with quasi-synchronous operation is the interactions between the transmissions which cause periodic nulls in the resultant when equal or near equal signals are received. Sideband Diversity is a technique which utilises the redundancy of double sideband amplitude modulated signals to overcome interactions between the transmissions thereby allowing the diversity advantage offered by multi-transmitter operation to be realised. Theoretical improvements in the distribution of the received signal in the presence of multipath propagation are presented for both long and short base line transmitters. The maximum improvement occurs when equal mean signals are received from the various transmitters, but a significant improvement is still evident when the mean transmissions differ by 20dB. Additionally, in a long base line scheme the slow, or long term, fading of the various transmissions will be uncorrelated and hence a further improvement in signal distribution is achieved. Extensive field trials to study the effectiveness of sideband diversity in improving the bit error rate of data transmission to vehicles have been carried out for both FSK and DPSK data modulation methods. Improvements in error performance of between one and two orders of magnitude have been measured. The residual errors after sideband diversity has been applied are heavily dependent upon ignition interference.
|Date of Award||1978|