Enhancement of Australian VHF combat net radios

Conventional VHF combat net radios (CNR) employ noncoherent FSK to provide 16 kbps in a 25 kHz channel. The poor bit error rate (BER) performance, and relatively low data rate prevent high throughput for the digital networks and C3I systems built on them. A low cost enhancement has been proposed as a near to medium term measure. We report the initial efforts for this proposal, including design consideration, simulation and test results. A demonstration system has been constructed which retains the existing RF components of the radios while replacing the hardware FSK modem by a software modem based on an external PC. A number of forward error control (FEC) and modulation schemes have been proposed and a subset of them have been simulated and implemented in real time. Initial measurements show promising performance in terms of remarkably reduced BER, or increased data rate. Iterative decoding is used which can combat signal distortion and impulsive interference that is often encountered in tactical deployment. Coded QPSK at the current data rate yields 14.5 dB E_b/N_o improvement over the FSK modem. A data rate of 64 kbps is achieved with coded 64 QAM. The selected scheme(s) can be implemented with FPGA or ASIC technology on a replacement module for the current radio.