We present two multilevel constant-envelope continuous-phase modulation (CPM) schemes with four-dimensional (4-D) trellis coding. The receiver is composed of a simple quadrature demodulator, followed by a symbol-rate sampler and a Viterbi decoder matched to the code trellis. The first modulation is a quaternary CPM scheme whose phase transitions over a symbol interval are those of /spl pi//4-shift quaternary phase-shift keying (QPSK). The demodulator filter is optimized so as to minimize the combined effect of intersymbol interference (ISI) and noise at the decision instants. We use Wei's (1987) 16-state 4-D trellis code, and redefine the set partitioning tree so as to maintain the same minimum distance between parallel transitions as in quadrature amplitude modulation (QAM) signal sets. The resulting modulation outperforms minimum-shift keying (MSK) by as much as 3.5 dB, in addition to reducing the 30-dB signal bandwidth by 20%. Next, we introduce an octonary (8-level) CPM scheme whose phase transitions are those of /spl pi//8-shift 8PSK. The same trellis code and receive filter optimization are also applied to this modulation which is shown to achieve better error rate performance than MSK, while saving some 60% of the transmitted signal bandwidth.

In addition to the conventional matched filter, implementation of Gardner's (1990) frequency detector (GFD) based on the maximum-likelihood principle also involves a frequency-matched filter. the present authors give a reduced-complexity frequency detector (RCFD) derived from Gardner's detector through a simple approximation to the frequency-matched filter. They analyze its steady-state jitter properties and frequency acquisition performance and compare it to the original GFD as well as to the well-known balanced quadricorrelator. The results show that performance of the RCFD is close to that of the original GFD and that this detector significantly outperforms the balanced quadricorrelator which involves a similar hardware complexity. Another contribution of the paper is the analysis of GFD and RCFD when their outputs are computed at twice the symbol rate.>

Using static and time-varying channel impulse responses, the authors analyze the performance of orthogonal frequency-division multiplexing (OFDM) on multipath fading channels. They show that, provided it employs a frequency domain equalizer, single-carrier transmission substantially outperforms OFDM signalling, a result that contradicts the usual claims that OFDM signalling is more resistant to multipath fading than is standard single-carrier transmission. This finding is supported by computer simulation results and analytical arguments related to the decision process. The implication of the results is that not only OFDM signalling increases the system sensitivity to nonlinear distortion and carrier synchronization errors, but it also turns out to offer no performance advantage on fading radio channels.>

The timing jitter induced by pointer adjustments in the basic STM-1 frame represents a serious technical problem in SDH-based networks. This paper describes two jitter reduction techniques to cope with this phenomenon. The first technique is based on digital phase-lock loop (PLL) theory, and obtained through two structural modifications of a previously proposed desynchronizer. The second technique is entirely novel, and it avoids the generation of the random noise with uniform probability density, which is required in PLL-type desynchronizers to smooth the 1-b phase steps at the output of the first stage. We describe two different methods to adapt the speed of this desynchronizer to the incoming pointer adjustment statistics. The performance of both jitter reduction techniques is investigated in both the normal mode and the degraded mode of operation of the network. Using a design example, it is shown that the peak-to-peak jitter in the presence of isolated pointer adjustments that characterize the normal operation mode is kept below 0.1 b. It is also shown that with frequency offsets up to 4.6 ppm in the degraded mode, the peak-to-peak jitter does not exceed 0.6 b. Lower jitter values are achievable if the complexity and memory requirements of the desynchronizer are allowed to increase.>

The authors present an efficient data predistortion technique with memory for compensation of high-power amplifier (HPA) nonlinearities in digital microwave radio systems employing quadrature amplitude modulation (QAM) signal formats. A practical implementation method is described which trades off performance against complexity and which makes it possible to implement this kind of predistorter in 256-QAM, and higher-level QAM systems. Using the 16-, 64-, and 256-QAM signal constellations, it is shown that the proposed technique achieves a considerably higher performance than that of conventional memoryless data predistortion of the predistortion technique with memory based on finite-order inverses of nonlinear systems. Specifically, numerical results show that the proposed technique achieves a gain that is in excess of 2 dB over conventional memoryless data predistortion.>

Two data predistortion techniques are presented that compensate for high-power amplifier (HPA) nonlinearities in digital microwave radio systems by employing quadrature amplitude-modulation (QAM) signal formats. The first one is a T/2-spaced predistortion technique that ensures distortion-free HPA output at two points per symbol interval T. The second is a T/3-spaced predistortion technique which cancels nonlinear distortion at the HPA output at three points per symbol interval. As opposed to conventional data predistortion, which can only compensate for warping of the signal constellation, the new techniques are effective against intersymbol interference. Using the 64- and 256-QAM signal constellations, it is shown that the proposed techniques lead to a very efficient utilization of the available HPA power. It is shown that, of the two techniques, the T/3-spaced data predistortion employs narrower transmit-pulse shaping and achieves higher protection against adjacent-channel interference at the expense of some additional complexity.>

An analysis made of the performance of predistortion, equalization, and intersymbol interference (ISI) cancellation techniques in compensating for the transmit amplifier nonlinearity in digital microwave radio systems. The study is carried out using the 64 QAM and 256 QAM signal formats and two values of the roll-off factor in the Nyquist pulse shaping. The simulated compensation techniques include three types of predistortion, two ISI cancellers, and several nonlinear equalizers with or without decision feedback. A basic result is that decision-feedback equalizers do not offer any significant advantage over nonrecursive equalizers. It is also shown that ISI cancelers with a memoryless equalizer as first-stage decision device do not perform any better than nonlinear equalizers of similar complexity. Another contribution an improved fifth-order analog signal predistortion technique is analyzed. The gain that can be achieved using a modified 256 QAM signal constellation that is more robust to nonlinear distortion is quantified.>

For asymmetric channels, the performance of symmetric and asymmetric baseband equalizers is analyzed. For both structures, the frequency response and the output mean-square error (MSE) of the optimum equalizer of infinite length are given. Several specific modem imperfections are considered, and the associated CNR (carrier-to-noise ratio) degradation is computed for 256-QAM systems. The results show that while asymmetric equalizers lead to a very limited compensation of modem imperfections, asymmetric equalizers lead to a dramatic improvement and, in the range of practical interest, reduce the CNR degradation almost to zero.>