Narrowband digital modems for land mobile radio

摘要

The modems operate in the 900 MHz band and with a channel spacing of 25 kHz. Each transmitted signal from a mobile is a four-level QAM signal, with an element rate of 12,000 bauds and a total bandwidth of 24 kHz. 20% redundancy is allowed for retraining and synchronization purposes, giving a useful transmission rate of 19,200 bit/s for each QAM signal. Coherent demodulation together with a new technique of combined signal-detection and channel-estimation are used at the receiver, for more accurate tracking of the rapidly fading signal. Two four-level QAM signals can be transmitted simultaneously over each separate frequency band, giving a resultant bandwidth efficiency of just over 1·5 bit/s per Hz for useful data. The independent fading of the two signals, together with the particular detection and estimation processes employed at the receiver, prevent any undue interference between the two signals. The signals fed from the base station to the mobiles are 16-level QAM signals, that are frequency division multiplexed, with a channel spacing of 25 kHz. Each signal has an element rate of 12,000 bauds and a total bandwidth of 24 kHz. Two mobiles are fed from any one 16-level QAM signal, and the bandwidth efficiency for useful data is again just over 1·5 bit/s per Hz. The base station/mobile receiver uses two antennas, in an arrangement of space diversity, together with the maximum-likelihood combining of the two signals. The channel estimator is regularly retrained, so that it is not subject to catastrophic failure, and the detector achieves near-maximum-likelihood detection of the received data signal.