Development of Narrow-Band Digital Land Mobile Radio

A narrow-band digital land mobile system has been developed that operates in the frequency bands of 150 and 400MHz, which are commonly used by transportation-related companies, local government, and public-sector organizations— and are therefore very congested. The number of users that can be accommodated in these bands is almost doubled by reducing the channel separation to 6.25 kHz, about half that of a conventional FM system. A carrier bit rate of 9.6 kbps is achieved by using π/4 shift QPSK modulation with a roll-off factor of 0.2. Laboratory and field testing showed that: •Without propagation delay spread, a BER of 10−2 was obtained without using space diversity. • With a propagation delay spread of 10μs, a BER of 6 × 10−3 was obtained without space diversity. These measurements confirmed the technical feasibility of this narrow-band system. Its widespread implementation will help mitigate the congestion in private radio systems. key words: mobile communication, FDMA, digital modulation, π/4 shift QPSK modulation