Coherent detection of QAM signals in land mobile radio

This thesis forms part of a joint universities project in which it is required to design and build a digital modem for the transmission of speech or data over a 900MHz land mobile radio channel. The main objectives being to try to maximize the bandwidth efficiency and attain near-optimum system performance. The theoretical modem design is presented here. The other parts of the system, that is the error control coding, speech coding, RF design qnd the actual hardware implementation are described elsewhere. All the systems described here have been designed to satisfy the overall system requirements. In particular, it must be possible to build this modem with existing technology without undue equipment complexity. All aspects of the digital'modem design are addressed, namely the choices of modulation scheme, pulse shaping filtering, packet structure and timing and synchronization methods suitable for the transmission of a digital signal over the fading radio channel. The important problems of channel estimation and data detection are examined in more detail. The first system described is one in which only one digital signal is transmitted in a narrowband channel. In the second system a novel technique of transmitting two signals in the same frequency band from two different mobiles to a single base station is described, which makes use of the fact that these two transmission paths are fading independently. The third system describes a method for transmitting back to these mobiles from the base station, again in the same frequency band. Although these systems have been designed specifically for use over 900MHz cellular land mobile radio channels, the ,techniques described are directly applicable to digital signal transmission over any flat fading channel.