Costas Loop BPSK Demodulation for Spread Spectrum System

Abstract

When the carrier is deliberately suppressed to conserve the transmit power as in PSK, the frequency and phase of the carrier should be estimated and corrected to required accuracy at the receiver to allow for coherent data demodulation. Costas loop which is a mathematical equivalent of the well-known squaring loop has been found to provide optimum demodulation of this type of suppressed carrier signals. In this paper, we present the design and performance of the Costas receiver developed to meet the requirements of 20 Mbps Direct Sequence Spread Spectrum System under development for digital voice communication over a satellite channel utilizing full transponder bandwidth of 36 MHz. The performance of the Costas receiver in terms of bit-error rate as a function of carrier-to-noise ratio and frequency pull-in range for a given loop bandwidth have been determined experimentally and compared with theoretical results. The demodulator performance at data rates of 20 kbps and 32 kbps (assuming ADM speech) for different ratios of IF bandwidth to data rate has also been obtained.

It has been found that this loop provides satisfactory performance with P_E≤1 × 10⁻⁴ at C/N of 7 dB and with sweep aided, a tracking range of more than ± 10 kHz. Further, even though Costas loop is mathematically identical to squaring loop, it has been seen that satisfactory carrier locking is obtained with carrier-to-noise ratios down to 0 dB; it permits less complexities at higher frequencies eliminating the use of band-pass filters allowing the demodulation to be relatively insensitive to centre frequency drifts. This scheme with minor modifications can also be useful for different applications

Indexing terms:

• Costas loop
• BPSK demodulation
• spread spectrum system