Symbol rate simulation of Doppler spectrum effects for WCDMA receivers

Monotonic chip rate increase of wireless mobile CDMA systems had been observed in the last years (e.g. WCDMA, CDMA2000). One of the most demanding tasks when simulating a CDMA system is to pass a transmitted signal through a multipath-fading channel. Each channel tap is modeled as a time varying filter that represents the fading process of a single path. A conventional way of simulating the effect of the multipath fading channel is at a fractional chip rate. In this paper, we present an analysis and a method to simulate Doppler spread filtering in terms of the receiver symbol rate rather than chip rate; thus enabling complete fading channel simulation at the symbol rate. The analysis begins by employing a de-spreading element on a chip rate received signal. This operation translates the incoming signal of chip rate resolution into symbol rate resolution. The presented analysis leads to a closed form of the Doppler spectrum shaping in terms of the receivers symbol rate. The new form of the Doppler spectrum is given with respect to the conventional Doppler spectrum form. Furthermore, although the channel model is formulated in terms of chip rate, it is seen that the Doppler spreading can be applied in the receiver symbol rate. This results with a potential of substantial reduction in the simulation calculation effort.