Performance of a 64 QAM radio using digital filters with small rolloffs

The increasing demand for transmission capacity of public and private networks and the limited availability of radio frequencies is the major driving force behind the search for microwave radio concepts with high spectral efficiency. In the past this has led to the design of systems with increasingly higher modulation levels, with the bulk of modern high capacity digital microwave radios employing a 64 QAM modulation scheme. Based on a commercially available 64 QAM, 155 Mbit/s radio system, investigations were carried out to explore the limits of enhancing spectral efficiency by employing pulse shaping filters with small rolloffs. Comparisons were made to a standard system equipped with surface acoustic wave (SAW) filters with a rolloff factor of 34%. By switching from the SAW filter in the modulator to a digital filter, without any need for further modifications, satisfactory system performance could be sustained even at rolloffs as low as 15%