Combining of signals in a geographical diversity system

Mobile stations have a number of disadvantages over fixed terminals. One solution to these problems is the use of multiple base stations, spread across a wide area, to receive transmission from a single mobile station. Such a system is termed `geographical diversity´. Transmissions from the mobile station can be received by up to five base stations. Since the base stations are separated by relatively large distances effects such as multipath and fading are likely to be uncorrelated so diversity combining will be effective. To perform combining the different versions of the received mobile station transmission must be brought to a central combining site. Since the system was originally designed for use in the HF band which offers bandwidths of only 3 kHz, sending baseband versions to a combining site is extremely difficult. If demodulation occurs at each of the base stations then the data rate for the interconnects is reduced. If data about channel SNR is available for the paths from the mobile station to each of the base stations then a confidence level or weight can be assigned to the demodulated data. A new combining method called `optimum weight combining´ is described which gives optimum performance when only demodulated data and SNR data are available