Design and Monte Carlo Simulations of a WAAS GPS Receiver Channel with Decision Feedback

Phillip W. Ward and Kenneth S. Barron

Abstract:	The Federal Aviation Agency (FAA) Wide Area Augmentation System (WAAS) geostationary satellites will transmit L 1 C/A-codes compatible with the code division multiple access and carrier frequency characteristics of the Standard Positioning Service (SPS) Global Positioning System (GPS) antennas and recei\.ers. However, the designs of the GPS receiver code generators need to be changed to replicate the FAA WAAS C/A-codes. Other differences in the WAAS signals also require GPS receiver design changes. The WAAS receiver channels will each require a Viterbi decoder. The broadcast data containing the GPS integrity message, the differential corrections and the reconstructed ionospheric model for the FAA WAAS avionics user segment require a 250 bits per second data rate (500 symbols per second modulation rate) to meet the response time requirements, The data modulation rate on the conventional GPS satellite signals is only 50 bits per second. The higher data modulation rate on the WAAS Ll C/A-code signals increases the squaring loss, which increases the bit error rate in the data demodulation and decreases the tracking threshold of the receiver channel for this signal, Also, the received WAAS signal is weaker than the received GPS C/A-code signal, further compounding the problem. Convolutional coding of the WAAS data message in the satellite and Viterbi decoding in the receiver compensate for the increase in the symbol error rate due to the increased squaring loss and the weaker signal. However, the convolutional coding technique provides no compensation for the loss in tracking threshold. Thus, the WAAS channel becomes the weak link of the modified avionics SPS GPS receiver. /ur earlier paper Mirignrittg the Squathg Loss in the II:&45 CL4-Cocle describes a tracking loop technique which utilizes the WAAS convolutional coding technique to mitigate the increased squaring loss and the weaker signal effects on the tracking thresholds of the WAAS receiver channels. The proposed design implements decision feedback from the re-encoded output of the Viterbi decoder to perfoim data wipeoff in the cairier tracking loop so that a pure phase lock loop (PLL) can be used instead of a Costas PLL. The analytical results showed up to 4 dB improvement in the WAAS tracking threshold, thereby making the WAAS channel the strongest link of the modified avionics SPS GPS receiver. A key design issue addressed in this paper is the amount of delay that the Viterbi decoder introduces into the carrier tracking loop. The paper shows that this delay (108 ms minimum) can result in carrier loop instability for the wideband third order PLL used in the Monte Carlo simulations. Stability is achieved with the same PLL design using a masimum- likelihood decision feedback design with a 20 ms delay. Monte Carlo simulations for the WAAS maximum- likelihood decision feedback (pure PLL), the conventional WAAS Costas PLL and the SPS Costas PLL receiver designs are presented and compared. The’ simulations determine the probability of phase lock (under periodic maximum expected dynamic stress) as a fiction of C/N,.
Published in:	Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996) September 17 - 20, 1996 Kansas City, MO
Pages:	1735 - 1743
Cite this article:	Ward, Phillip W., Barron, Kenneth S., "Design and Monte Carlo Simulations of a WAAS GPS Receiver Channel with Decision Feedback," Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996), Kansas City, MO, September 1996, pp. 1735-1743.
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