In this study, a closed-form equation is derived which relates the mean-squared error (MSE) to the frequency response of the interpolation filter. Effects of additive white Gaussian noise and timing jitter on the MSE are accounted for in the equation. By using properties of the Nyquist signalling pulse, a simple approximation to the MSE equation is obtained, which is found to be accurate over the range of excess bandwidths commonly used in quadrature amplitude modulation receivers.

There are applications in spread spectrum channels where spatial diversity is required to mitigate fading. The spreading gain in such channels is often limited to reduce the chances of a nearby interferer being in-band. With modest spreading gains the signal bandwidth may be less than the coherence bandwidth. In such cases receiver-based multi-path diversity is not feasible, and spatial diversity is used to combat fading. This study improves and extends the work presented in US patent #6389085 B1, which exploits the structure of spread spectrum signals to implement Nth order spatial diversity with simple incoherent radio frequency combining. An improved combining scheme is proposed here together with a special pseudo-random noise (PN) sequence to reduce intra symbol interference. A unique coherent all-digital demodulator is also provided. The signal-to-noise ratio (SNR) of the new system is about 3.5 dB higher than that of the original system described in the patent. The new receiver is comparable in cost to selection diversity, but does not switch antennas and therefore, does not suffer the momentary signal drop-outs. However, the penalty for eliminating the drop outs is a 1.5 dB lower output SNR.

Recently, non-data aided synchronizers for QAM with a feedforward structure have been significantly improved by incorporating the prefllter originally proposed by Franks to mitigate the self-noise in analog clock regenerators. Prior to this, D'Andrea proposed a synchronizer with a feedback structure that used Franks' prefllter. Although not reported in the literature, the earlier D'Andrea's synchronizer outperforms the more recent feedforward synchronizers, and consequently is believed by the authors to be the optimum non-data aided synchronizer for QAM. An all digital version of D'Andrea's synchronizer is presented in this paper. The steady state timing jitter is thoroughly analyzed by deriving a closed form expression for the noise power spectrum, as well as a simple expression to estimate the variance of the jitter. Performance comparisons are made.

This paper deals with timing jitter reduction in the timing recovery loop of a digital QAM receiver. The main contribution, which is derived analytically, is an economical prefilter to reduce the timing jitter in timing recovery loops containing either the early–late or the Gardner timing-error detector (TED). The proposed prefilter has the advantage of being an infinite impulse response filter that is placed inside the TED and runs at the symbol rate. For small roll off factors, it is shown with a computer simulation that a single-pole filter placed inside either the early–late or the Gardner detector is quite effective in reducing the timing jitter.