Abstract:
Narrow band channel is a channel in which the bandwidth of a message signal does not exceed
the channel’s coherence bandwidth. As the channel is being narrow its frequency response is
considered flat. Here the common modulation techniques are AM, FM, SSB, BPSK and CW.
The narrowband channel used by NATO uses continuous phase modulation (CPM). So a
loopback system is to be developed which achieves 96k/bits data rate and 24 kHz channel
bandwidth. This whole loopback system is developed on a software defined radio (SDR) kit. The
data rate of 96kbits is not achievable in a bandwidth specification of 24 kHz as the thermal noise
and adaptive white Gaussian noise (AWGN) effects the transmitted message.
At these transmitting situations the continuous phase modulation of the baseband signal are
efficient enough to reduce the effect and to regain the actual form of the transmitted signal. The
achievability of such a data rate depends upon the value of modulation index “h” and the value
of “k” in the process of encoding at transmitted end. Viterbi encoder at transmitting end and
decoder at receiving end reduces the bit error. The whole development of system is done on
small form factor SDR module which consists of TMS320DM6446 digital processing module,
ADS5500 data conversion module and an RF module with selectable bandwidth of 5 MHz – 20
MHz this whole system depends upon the CPM modulation block and decoding block at the
receiving end.
