Open-loop tanlock carrier recovery structure for BPSK and its performance

Abstract

The tanlock carrier recovery structure has been shown to be the low SNR approximation to the optimum ML carrier phase estimator for a carrier with linear data modulations such as MPSK. It has long been known to have better performance than a Costas' loop or a squarer/PLL for BPSK, because its tangent-function phase detector characteristic has an extended linear range compared to the sine-function phase detector characteristic of the latter loops. Its implementation in conventional closed-loop form, however, requires the use of a divider which suffers from the problem of output overflow when the input divisor signal becomes very small. We suggest here an open-loop implementation which does not require the use of a divider and a VCO. The operational and performance advantages of this open-loop implementation over the conventional closed-loop implementation are pointed out. Some sample BER results for a coherent receiver employing this open-loop tanlock carrier recovery system are obtained via computer simulations. The results show that the receiver achieves significant performance gains over differential detection.