Simple and robust noncoherent detection circuit for DSSS MSK signal in a large frequency offset environment

Abstract

In this thesis, a simple symbol detection circuit which is robust to frequency offset for DSSS MSK signal is presented. The complexity of the detection circuit is significantly reduced by implementing the correlators using multiplexers instead of complex floating point multipliers. This is because the coefficients of correlators are one of [1, -1, i, -i] in complex form using double correlation algorithm. It is independent of the symbol and the lag of double correlation, when the over sampling ratio is 2 and the set of sample values is $[0, \frac{\sqrt{2}}{2}, 1]$ in a half-sine which is the pulse shape of offset QPSK to be equivalent to MSK with precoding of input data. Therefore, it reduces not only the number of multipliers, but also the number of operations by 50%. This is also applicable to the coefficients of correlators in a timing synchronization circuit. In this case, the multipliers for double correlations of input data are able to be used in both the symbol detection circuit and the timing synchronization circuit in common. The performance of this detection circuit is comparable to the optimal value obtained by double correlation in offset QPSK and nearly reaches that of noncoherent maximum likelihood detection with 10ppm frequency offset in AWGN channel. And this detection is available for 80ppm frequency offset, while the noncoherent maximum likelihood detection with zero frequency offset is not available for over 1/2 chip time offset in both AWGN and multipath fading channels. The detection circuit is more robust to frequency offset than the noncoherent maximum likelihood in particular when the channel experiences multipath fading. For an application, the demodulator for IEEE 802.15.4 LR-WPANs was implemented in system on chip. The chip contains about 75,000 transistors in $5mm^2$ area fabricated in 0.18um 1P6M CMOS. The chip operates at 16MHz clock frequency, and the power consumption of the chip including RF circuits is 50mW when it operates as ...