In conventional energy detection (ED) pulse position modulation (PPM) systems, the optimal decision threshold is 0. However, when cross-modulation interference (CMI) or synchronisation errors occur, the optimal threshold deviates from 0 and the bit error rate (BER) performance is degraded greatly. An adaptive threshold is developed to replace 0 threshold to find the optimal threshold. The research results show that this adaptive threshold effectively mitigates the effect of cross modulation interference (CMI) or synchronisation errors and achieves BER improvement.

A noncoherent, bandwidth-efficient modulation scheme is proposed for frequency-hopping multiple-access (FH-MA) networks. The proposed scheme is a combination of noncoherent M-ary amplitude-shift keying (NMASK) and orthogonal frequency-division multiplexing (OFDM). Using this scheme minimizes the required data bandwidth. The number of frequency slots available to the users increases significantly for a fixed spread-spectrum bandwidth (BW/sub SS/). The effect of the multiple-access interference is reduced. Simple and accurate bit error rate expressions have been derived for FH-OFDM-MASK in additive white Gaussian noise channels and for FH-OFDM-ASK in Rayleigh fading channels.

This article presents a novel universal all-digital clock-smoothing technique - counting-prognostication. Operation principles, performance analysis, and comparisons are given. Analysis and measurement results show that this technique can efficiently smooth jitter and wander for a wide pull-in range and pull-out range, and jitter accumulation is small. A cycle-varying counting-prognostication method, which decreases pull-in time, is also suggested.

High-speed broadband digital communication networks rely on digital multiplexing technology where clock synchronization, including processing, transmission, and recovery of the clock, is the critical technique. This paper interprets the process of clock synchronization in multiplexing systems as quantizing and coding the information of clock synchronization, interprets clock justification as timing sigma-delta modulation (TΔ-ΣM), and interprets the jitter of justification as quantization error. As a result, decreasing the quantization error is equivalent to decreasing the jitter of justification. Using this theory, the paper studies the existing jitter-reducing techniques in transmitters and receivers, presents some techniques that can decrease the quantization error (justification jitter) in digital multiplexing systems, and presents a new method of clock recovery.

Coherent M-ary amplitude-shift keying (MASK) is proposed for use in orthogonal frequency-division multiplexing (OFDM) systems. The frequency separation between subcarriers is only 1/2T instead of 1/T. With a slightly wider bandwidth, an /spl radic/M-ary ASK OFDM can achieve the same bit-error rate (BER) of M-ary quadrature amplitude modulation (QAM) OFDM and a better BER than that of M-ary phase-shift keying (MPSK) OFDM. The /spl radic/M-ary ASK OFDM has the same peak-to-average-power ratio as that of the M-ary QAM OFDM. The MASK OFDM can be implemented digitally and efficiently by fast cosine transform and demodulated by inverse fast cosine transform. Comparisons show that implementation complexity is reduced for additive white Gaussian noise channels with the use of the new scheme.

This paper evaluates the error probability of the rectangular frequency pulse multi-h modulation (MHPM) scheme in slowly-fading, frequency-nonselective or frequency-selective Rician and Rayleigh channels. The evaluation is performed with a method combining analysis and computer simulation. Performance degradations are evaluated for various direct-to-reflected signal ratio, Doppler shifts, and relative time delays in Rician fading channels. Compared with minimum shift keying (MSK), MHPM schemes appear to have retained their coding gains.

The effect of adjacent channel interference (ACI) on the error probability of the rectangular frequency pulse multi-h phase-coded modulation (1REC-MHPM) is studied. Evaluations through simulation were done for different values of the absolute adjacent channel frequency spacing |/spl Delta/f|, different time delay /spl tau/ between the main channel and the interfering channel, and schemes with different number of modulation indexes. It is found that ACI effects can only be eliminated when |/spl Delta/f| approaches 2.0. Comparison with minimum shift keying (MSK) is also made through simulation.

Sequential sequence estimation for communications channels with convolutionally coded input sequences and intersymbol interference (ISI) is proposed. The receiver consists of a whitened matched filter (WMF) and a vector sequential decoder. The metric of the vector sequential algorithm (SA) is derived. Analytical results for error probability and computational complexity of the sequential decoder are presented. The error probability is found to be dominated by the minimum Euclidean distance between the possible output sequences and the computational distribution is Pareto, typical of a sequential decoder. The computational cut-off code rate R/sub comp/ and its lower bound R/sub comp/' are also derived. Numerical simulation results of the error probability and the computational complexity are presented. Comparing with the Viterbi algorithm (VA) reveals that the SA essentially has the same error performance as that of the VA while its average computational complexity is one to two orders of magnitude less than that of the VA.>

The computational complexity of a sequential algorithm (SA) developed for intersymbol interference (ISI) channels is analyzed. To determine the computational complexity, the finite-state machine that models the channel and white matched filter system, of which the SA is a part, is interpreted as a special convolutional encoder followed by a binary symbol to Q-ary symbol mapping. It follows that the computational distribution is Pareto, and that there exists a computational cutoff rate R/sub comp/. For the uncoded data considered, the rate is fixed and the R/sub comp/ criterion translates into a signal-to-noise ratio (SNR) criterion. An upper bound on SNR/sub comp/ is found analytically by assuming a uniform input distribution. Iteration equations developed by S. Arimoto (1976) are adapted to find the true SNR/sub comp/ numerically.>

A vector sequential sequence estimator is proposed for multiple-channel systems with both intersymbol interference (ISI) and interchannel interference (ICI). Both finite ISI-ICI and infinite ISI-ICI are considered. The estimator consists of a multiple-dimensional whitened matched filter and a vector sequential decoder. The metric of the sequential algorithm is derived, and the algorithm's performance is analyzed. Computer simulation results for a two-dimensional finite ISI-ICI channel and a two-dimensional infinite ISI-ICI channel are presented. Analysis and simulation show that the symbol error probability of the vector sequential algorithm is essentially the same as for maximum-likelihood sequence estimation using the vector Viterbi algorithm, while its average computational complexity is much less, although computation per symbol is a random variable with the Pareto distribution. There exists a signal-to-noise ratio above which the ensemble average computation is bounded. An upper bound on this ratio is found.>

A sequential sequence estimator for channels with finite or infinite intersymbol interference (ISI) is proposed. It consists of a whitened matched filter followed by a sequential algorithm. A key component of a sequential estimator is a suitable metric that accounts for the unequal search depth. This metric is derived for the ISI channel. Computer simulations for three different channels show that the sequential sequence estimator has considerably less computational complexity than a maximum-likelihood sequence estimator using the Viterbi algorithm (MLSE-VA), while still maintaining the optimum estimation performance of the MLSE-VA, a performance not achievable with any reduced-state Viterbi algorithm.>