Rate conversion for arbitrary sampling rates in the transmit path of a digital transceiver

This paper addresses the subject of signal generation in the transmit path of a digital communications system. In order to minimize cost it is desirable to use a free running oscillator instead of a controlled oscillator. The signal generation task can be separated into pulse shaping at a fixed symbol rate and rate conversion to the arbitrary sampling rate by means of interpolation and decimation. As an alternative, we introduce the technique of rate adaptation and modulation, which generates the transmit signal immediately at sample rate and thus saves the interpolator. We compare the complexity of the two approaches both applied to the uplink of a UNITS system, discuss their advantages and disadvantages, and show that the use of an expensive controlled oscillator can be avoided in favor of a free running oscillator.