Characterisation of narrowband modulation schemes over a measured fading FM channel

British Gas has for some time been developing a private mobile data network for its field staff as part of its Company Field System (CFS) strategy. Whilst the network is currently using 1200 bit/s FFSK under predominately static conditions, early application successes quickly demonstrated the wide ranging benefits of the system and stimulated the need for a higher throughput system. As a result, the Engineering Research Station (ERS) has been investigated, techniques to increase the systems data throughput. The authors detail part of the investigation, the live characterisation of the RF channel under static and mobile conditions and the development and characterisation of two high speed baseband modulation schemes. The two high speed subcarrier modulation schemes evaluated were 16 APSK and 8 PSK, both operating at 4.8 kbit/s over a 12.5 kHz VHF FM channel. To reference the performance of the higher speed schemes, a 1200 bit/s FFSK system was characterised. Several test runs were performed in the field and the k-factor measured by fast received signal level sampling. The authors detail the measurement technique and the burst error statistics recorded on each type of run. The authors describe the approach being used to identify suitable coding schemes to accommodate the measured burst error statistics