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This paper presents an enhanced version of the previously proposed blind anchored interference-mitigating detector (BAID) for code-division multiple-access transmission. Such a detector, named extended complex BAID (EC-BAID), features invariance to a phase error on the useful channel's carrier and reveals resistance to a large frequency shift (e.g., Doppler shift) on interfering carriers. The EC-BAID is also shown to bear a sensitivity to residual carrier frequency errors on the desired channel which is lower by three orders of magnitude with respect to the case of a data-aided minimum mean-square-error receiver. The performance of the EC-BAID is computed theoretically, and is validated by computer simulations under a variety of system configurations.

A pilot-aided coherent uplink transmission scheme is proposed for mobile satellite communications at L-band. The scheme is designed to enhance link performance as compared to coded noncoherent M-ary orthogonal modulation, while maintaining small envelope fluctuations and low complexity. A rigorous analysis is performed for the proposed technique, when used in conjunction with a direct-sequence code-division multiple-access convolutionally encoded binary phase shift keying link, in a Rice-fading channel with satellite diversity. The pilot-aided scheme has been extensively simulated, accounting for finite interleaving in slow fading and nonideal carrier frequency recovery. The results show a significant performance gain with respect to noncoherent schemes designed for similar links, which goes up to 1.8 dB for 9.6 kb/s services with slow moving users exploiting dual-satellite diversity.

The paper analyzes possible transmission schemes for satellite personal communication systems adopting low/medium-Earth orbit (LEO/MEO) constellations and direct-sequence code division multiple access (DS-CDMA) in the presence of fading and shadowing, the statistics of which are functions of the satellite elevation angle. In particular, the performance of M-ary Walsh-Hadamard orthogonal (MWHO) convolutionally coded DS-CDMA over a Rice-log-normal fading channel is analyzed in depth, and compared to the simpler coded differential BPSK (DBPSK) scheme. Optimization of the performance/complexity tradeoff in the metric computation for soft inputs to the Viterbi (1979) decoder is addressed. Upper bounds on the error probability without or with satellite diversity and equal gain combining are evaluated considering different coding rates. The effect of correlated fading is also taken into account via simulation. Among the various results, it is shown that the optimal coding rate is a function of the satellite elevation angle and of the diversity order, that the loss for using DBPSK instead of MWHO is in the range 1-1.4 dB, and that interleaving depth is a critical parameter.

Code-division multiple-access (CDMA) with direct-sequence/spread-spectrum (DS/SS) modulated signals has emerged as a strong candidate for the air interface of the universal wireless personal communication network planned for the end of the century. In this paper, we take into consideration a low-complexity blind adaptive interference-mitigating detector (BAID) scheme that minimizes the detrimental effect of the multiple-access interference (MAI) on the bit-error-rate (BER) performance of a CDMA data demodulator. We describe a few modifications to the original algorithm that make it more suitable to a practical implementation with dual-binary phase-shift keying (D-BPSK)-modulated DS/SS signals. Specifically, we show how to make the BAID invariant to a possible carrier phase offset introduced in the front-end of the data demodulator, and we also suggest how to increase the robustness of such detector to asynchronous MAI. The uncoded BER performance of the resulting "extended" detector is then evaluated theoretically and by computer simulation in the case of coherent and differential signal detection.

There is an urgent need for substantially enhancing random access schemes to be able to support native massive machine-to-machine communications in the next generation wireless systems. Lately, contention resolution diversity slotted ALOHA (CRDSA) and its variants have contributed to a drastic improvement in throughput compared with the traditional slotted ALOHA protocols. In this paper, we further enhance the CRDSA flexibility and performance along different physical layer dimensions. Based on simulation results, we show that there is still room for improvement in terms of throughput and, using heuristic methods, we find an optimized distribution of the packets power that is entering the demodulator for an additive white Gaussian noise channel. Applying this power distribution, which is easy to implement at the terminal in open loop and in a decentralized manner, is shown to achieve significantly enhanced performance. By introducing multi-frequency CRDSA, a drastic reduction in the terminal’s peak power requirement is attained. Furthermore, we investigate spread spectrum CRDSA, which improves the robustness of the scheme against irresolvable packet collisions allowing for enhanced energy efficiency. To support the work presented herein, we have developed a computationally efficient yet accurate methodology for estimating CRDSA performance, a novel approximate centrally coordinated CRDSA achievable sum rate analysis accounting for modulation and forward error correction constraints, as well as a more accurate loop probability analysis.

We investigate performance of decision-directed maximum-likelihood carrier phase synchronizers for M-PSK signalling (M/spl les/8) on the AWGN channel. Decision-directed feedback (DD-FB) and feedforward decision feedback (FF-DFB) structures are analyzed and compared in detail. A new, accurate model for the FF-DFB synchronizer is presented. Open loop characteristics and closed loop performance, in terms of steady-state jitter and cycle slip rate, are presented in the same framework for both the DD-FB and FF-DFB synchronizers. Performance comparison between the two phase estimator architectures is finally made for each modulation scheme investigated.

Migration towards a full-digital implementation of modems is currently one of the main trends in transmission systems design. The authors describe a noncoherent all-digital delay lock loop (DDLL) suited for chip timing synchronization in band-limited direct sequence spread spectrum (DS/SS) systems, and they thoroughly analyze its performance. The key features of this novel scheme are represented by its low-complexity processing section together with its good tracking capability. Analytical expressions for the DDLL S-curve and steady-state timing jitter are derived and confirmed by a time-domain computer simulation. Furthermore, the Mean Time to Lose Lock (MTLL) of the loop is evaluated and some numerical results are reported. The proposed chip timing synchronization scheme reveals also an improved tracking performance when compared to the traditional analog DLL for rectangular chip DS/SS signals. >

The key synchronization aspects in the system design of a QQPSK (quadrature-quadrature phase-shift-keying) modem are addressed. The sensitivity of the data demodulator to synchronization errors is discussed, and contextually the performances of some IF and baseband carrier phase and symbol timing recovery schemes are evaluated both theoretically and by computer simulations. In particular, a fourth-power IF carrier/clock regenerator and two baseband clock recovery schemes, with and without the aid of data decisions, respectively, are taken into account. The analysis shows on the one hand the substantial robustness of QQPSK to carrier phase errors and the adequacy of the examined carrier extraction scheme. On the other hand, the remarkable sensitivity of QQPSK to symbol timing inaccuracy is stressed and the need to resort to the newly proposed decision-aided baseband clock recovery scheme is pointed out. >

The authors present a nonconventional joint data demodulation-pseudo-noise (PN) code tracking scheme for direct sequence (DS) spread-spectrum (SS) signals which solves problems of component imbalance and sensitivity with hardware simplicity and no performance degradation. An integrate-and-dump Costas loop is used for carrier recovery and data demodulation of the SS signal. Both data and carrier are then used to derive the baseband error signal of the code tracking loop. Moreover, a single passband correlator is used to perform the early-late correlation, leading to a hardware complexity equivalent to that of the tau-dither scheme, but without its loss in performance. Results of a thorough theoretical analysis of the system in an additive Gaussian noise (AWGN) environment are reported. They provide performance curves in terms of steady-state jitter and mean time to first lock loss. A superior jitter performance for low values of E/sub b//N/sub 0/ with respect to a traditional noncoherent delay lock loop (DLL) is shown, along with the potential gain of Manchester coding upon the more usual NRZ format. >