Channel Estimation Using Time-Multiplexed Pilots in HSUPA Uplink

High date rates of up to 5.76 Mbps with spreading factors as low as 2 are supported in high speed uplink packet access (HSUPA) channels of UMTS, as specified by 3GPP. Receivers for WCDMA are usually implemented as Rake receivers or variants of it. Such receivers require accurate estimation of complex gains of multipaths received. Pilot symbols are provided as part of DPCCH channel and can be used for pilot-aided estimation of multipath complex gains. However the pilot symbols are in time-multiplexed form i.e. not transmitted continuously in time. Also, for channels corresponding to high vehicular speeds, the coherence time of channel is very low. Hence the accurate estimation of channel taps with granularity as fine as 2 chips becomes a complex task. We present here a method of channel estimation based on interference-compensated cubic smoothing splines. This method has been found to be robust to additive noise, while at the same time able to closely track the channel variations with fine granularity. The smoothing splines are a variant of least-squares splines, but with added constraints on smoothness. A comparison of the proposed method with alternative methods of channel estimation is also presented.