Improving detection and estimation in pilot-aided frequency selective CDMA channels

The IS-95 downlink channel incorporates a pilot signal to aid the mobile receiver in obtaining coherent channel estimates for use in the RAKE receiver. At the transmitter, the traffic signals are orthogonal to the pilot signal; however, in frequency selective channels where more than one resolvable multipath component is received, the received traffic channels of one multipath component are no longer orthogonal to the replicate pilot signals of another multipath component. This nonorthogonal multipath noise contribution will cause degradation in the channel estimate and resulting RAKE output. To address this problem, we propose a multipath noise canceller which provides improved capacity and/or signal quality with little additional complexity compared to conventional receivers. This technique is recursive in that it uses the estimated channel coefficients from the previous symbol interval to reconstruct and subtract the multipath noise from the RAKE finger inputs on the current symbol interval. With no changes required to the IS-95 standard and only minimal changes needed at the handset, this technique increases the downlink capacity by 8% and 11% for, respectively, 3-path and 4-path environments