William P. Osborne

Coherent and Noncoherent Detection CPFSK

Continuous phase frequency shift keying (CPFSK) is potentially an attractive modulation scheme for use on channels whose performance is limited by thermal noise. In this paper results for the performance available with CPFSK are given for coherent detection and noncoherent detection with arbitrary modulation indices and arbitrary observation intervals. This work serves two purposes. First, it provides interesting, new results for the noncoherent detection of CPFSK which indicate that the performance of such a system can be better than the performance of coherent PSK. Secondly, it provides a complete analysis of the performance of CPFSK at high SNR as well as low SNR and thereby unifies and extends the results previously available.

Uplink-noise limited satellite channels

Systems which transmit data through a nonlinear relay, such as a satellite, must deal with a composite channel that can be separated into two distinct channels-the uplink channel between the user and the relay, and the downlink channel between the relay and the final destination. If the system has a strict power limitation and high data rate demands, such as a small satellite transmitting through NASAs TDRSS Network, the dominant noise is present on the uplink rather than the downlink channel. Such a system is deemed to be uplink-noise limited and presents the designer with a number of problems not encountered in a more typical downlink-noise limited channel. Whereas the transmitted signal constellation can be pre-distorted to take into account the effect of the nonlinearity in the down-link limited channel, no amount of pre-distortion will solve the problems encountered when the majority of the noise is present before the nonlinearity. Instead, the receiver must be modified to reflect the non-Gaussian noise due to the operation of the nonlinearity on Gaussian noise. Under three assumptions-there is no downlink-noise present, the downlink channel is wideband relative to the data, and the filter proceeding the nonlinearity meets both matched filter and Nyquist requirements-such modifications can be made based on the nature of the nonlinearity. By mapping the ideal decision region through the nonlinearity, performance almost identical to that of a linear-wideband AWGN channel can be achieved. This paper develops the theoretical performance of the receiver described for a nonlinearity typical of a satellite channel. Performance curves are presented for QPSK, 8PSK, 16PSK and 16QAM modulation schemes.

An analysis of carrier phase jitter in an M-PSK receiver utilizing MAP estimation

The use of 8 and 16 PSK TCM to support satellite communications in an effort to achieve more bandwidth efficiency in a power-limited channel has been proposed. The authors address the problem of carrier phase jitter in an M-PSK receiver utilizing the high SNR approximation to the maximum a posteriori estimation of carrier phase. In particular, numerical solutions to 8 and 16 PSK self-noise and the amplitude suppression factor in the loop are presented. The effect of changing SNR on the loop noise bandwidth is also discussed. This data is then used to compute variance of phase error as a function of SNR. Simulation data is used to verify these calculations. The results show that there is a threshold in the variance of phase error verse SNR curves that is a strong function of SNR and a weak function of loop bandwidth. The M-PSK variance thresholds occur at SNRs in the range of practical interest for the use of 8 and 16-PSK TCM. This suggests that phase error variance is an important consideration in the design of these systems.<<ETX>>

Synchronization in M-PSK modems

It is economical for a single modem to receive multiple modulation formats. The use of binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 8PSK, and 16PSK modulation is considered. The authors address a technique for achieving carrier synchronization for all four of these PSK schemes in a single modem. This is accomplished by using quadrature channel carrier recovery processing and a version of the maximum a posteriori (MAP) estimation phase detector algorithm. A model for the simulation of this tracking process is derived, and results are presented to confirm its functionality.<<ETX>>

Fast TCM decoding: phase quantization and integer weighting

TCM, combining modulation and coding, achieves coding gains over conventional uncoded multilevel modulation without the attendant bandwidth expansion. Since TCM was proposed Ungerboeck (1982, 1987) substantial work has done in this area. A large portion of the TCM work has been in the area of high-speed data transmission over voice grade modems using quadrature amplitude modulation, QAM. QAM, not having a constant envelope, is unattractive for employing a TWT with its nonlinear behavior as the power stage. Additional work has been done in utilizing M-ary PSK with TCM. Simulations by Taylor and Chan (1981) utilizing a 4-state convolutional code demonstrated the coding gain of a rate 2/3 coded 8-PSK modulation scheme. Wilson et. al. (1984) obtained results for 16-PSK TCM using codes with 4 to 32 states and achieved coding gains of 3.5 to 4.8 dB respectively, over 8-PSK and demonstrated that small memory codes achieved good gains with simple design procedures. >

Pragmatic trellis coded modulation: a simulation using 24-sector quantized 8-PSK

Pragmatic trellis coded modulation (TCM) uses the industry standard, 64-state, binary convolutional code. The authors present the logic design and simulation results for a system which effectively implements pragmatic TCM for rate 2/3 encoded 8 phase shift keying (PSK). This system associates each sector of a quantized phase receiver with a pair of weights to be used as soft decision inputs of the Viterbi decoder. This system approaches 3 dB of coding gain at bit error rates of 10/sup -5/ and less.<<ETX>>

Quantization of signals and metrics in the decoding of 8-PSK TCM

The performance of rate 2/3 8-PSK Trellis Coded Modulation was investigated as a function of the numerical resolution of the metrics and signal vectors used in decoding by the Viterbi algorithm. It was found that a 16 state Ungerboeck code using 5 bit I and Q with 7 bit metrics achieved roughly the same performance as pragmatic TCM. The findings were applied to the digital logic design of a TCM decoder.<<ETX>>

Pragmatic trellis coded modulation: a hardware implementation using 24-sector 8-PSK

The authors present the construction and test results of a system which implements pragmatic trellis coded modulation (TCM) for rate 2/3 encoded 8-PSK (phase shift keying). Pragmatic TCM is described. This system associates each sector of a quantized phase receiver (F. Carden et al., 1988) with a pair of weights to be used as soft decision inputs of the Viterbi decoder. This system approaches 3 dB of coding gain at bit error rates of 10/sup -5/ and less.<<ETX>>

Multi-mode modem/codec designs

The design of an integrated modem/codec unit which can receive coded and uncoded BPSK an QPSK using de facto standard coding schemes as well as 8PSK-TCM and 16PSK-TCM is examined. The design is totally compatible with todays modulation schemes and capable of processing tomorrows TCM codes. This is accomplished in the modem by using quadrature channel carrier recovery processing and a version of the MAP phase detector algorithm. The symbol synchronization is accomplished with a derivative of an early-late gate designed to accommodate multilevel signals.

A comparison of modulation schemes in bandlimited AWGN channels

In recent years, as data rates rise for seemingly decreasing available bandwidths, a great deal of research has been directed toward finding bandwidth efficient modulation schemes. Two such methods are partial-response signaling and trellis-coded modulation. Both of which promise performance gains in a bandlimited channel when compared to uncoded systems. This paper will compare the performance of these schemes, when applied to a QPSK system over various channel bandwidths.