Optimal signature sets for oversaturated quasi-scalable direct-sequence spread-spectrum systems

Signature sets that maximize the sum capacity of an oversaturated code-division multiple-access (CDMA) system are derived, under the constraint that the signature sets have to be quasi-scalable. It turns out that the optimal signature set is composed of orthogonal signature sets, with at most one incomplete orthogonal set. As compared to the Gaussian multiple-access channel (GMAC), the loss in spectral efficiency for this multiple-orthogonal CDMA (OCDMA) system remains very low.     

Design techniques for high-resolution current-mode sigma-deltamodulators

This paper describes techniques for the design of high-resolution oversampling analog-to-digital converters based on current memories. A key point is the reduction of nonlinearities, in particular those introduced by the current switches. A current-memory cell with very high precision and linearity has been designed and used in an experimental third-order Σ-δ modulator in a 0.8-μm digital CMOS process. A linearity of better than 14 b and a maximum signal-to-noise+distortion ratio (SNDR) of 80 dB has been measured for an oversampling ratio (OSR) of 64     

Sensitivity of multiple-access techniques to narrow-bandinterference

This paper investigates the sensitivity of several multiple-access techniques to narrow-band interference. The analysis covers time-division multiple access (TDMA), code-division multiple access (CDMA), and orthogonal frequency-division multiple access (OFDMA). The study is carried out under the assumption that all the considered multiple-access systems occupy the same total bandwidth, and the bit rates of all active users are identical. A major finding of this study is that CDMA with pseudonoise spreading sequences is more sensitive to narrow-band interference than TDMA. We point out that the signal-to-jammer power ratio at the decision device input is in fact identical for both multiple-access techniques, but the amplitude distribution of the jammer term at the threshold detector input is more favorable to TDMA, which turns out to be more robust in terms of bit-error rate. Another finding is that in terms of sensitivity to narrow-band interference, orthogonal CDMA (OCDMA) is closer to TDMA than to CDMA with pseudonoise sequences, because the degradation is not the same for all users. Finally, we discuss the relationship of OCDMA and TDMA and highlight the superiority, in terms of capacity over the narrow-band interference channel, of TDMA to the other multiple-access techniques considered in this paper     

Timing Recovery in the Presence of a Residual Echo Signal

This paper deals with the problem of symbol timing recovery at the subscriber end of a full duplex two wire digital subscriber loop employing echo cancellation. Both nondecision-aided and decisionaided synchronization are considered. It is demonstrated that the presence of a residual echo signal can substantially degrade the synchronizer performance. This degradation is shown to be larger for nondecisionaided synchronizers than for decision-aided synchronizers. The smaller the synchronizer bandwidth and the signal-to-echo ratio, the more the decision-aided synchronizer outperforms the nondecision-aided synchronizer.     

Cycle-slip analysis of the Mth-power NDA feedforward carriersynchronizer for MPSK

We analyze the cycle-slipping behavior of the Mth-power nondecision-aided (NDA) feedforward carrier synchronizer for M-ary phase-shift keying (MPSK). The averaging filter is a block window accumulator, and postprocessing is used to eliminate equivocation. An asymptotic (high E_s/N_o) expression for the cycle-slip probability is derived and verified by computer simulation. A “hybrid” approach, combining analytical results with importance sampling simulation techniques, allows for the fast and accurate determination of the cycle-slip probability for all E_s /N_o values of practical interest     

An improved OCDMA/OCDMA scheme based on displaced orthogonal user sets

We introduce a new type of OCDMA/OCDMA for oversaturated channels, by displacing in time the orthogonal signature sets of the users. A displacement by an integer multiple of a chip period considerably improves the performance of iterative detection of the user data, as compared to quasi-orthogonal sequences (QOS) and conventional random O/O sequences. An additional displacement by half a chip period reduces the variance of the cross correlation between the users of the two sets by up to 50%, and results in an additional performance improvement for square root cosine rolloff chip pulses. This improved O/O system can accommodate a number of users equal to twice the spreading gain N, when N/spl ges/32. For a practical rolloff of 25% and N=128, the acceptable channel overload can almost be tripled with improved O/O as compared to conventional O/O.     

A Class of Phase Detector Characteristics for Symbol Synchronizers Yielding Unbiased Estimates

Most practical synchronizers operating on a PAM waveform corrupted by additive noise can be analyzed by means of the theory of the phase-locked loop (PLL). In this paper, we present a class of synchronizers for which the equivalent phase detector characteristic is such that the timing instants generated by the voltage controlled clock (VCC) are unbiased with respect to the ideal sampling instants to be used in the data reconstruction path. As a consequence of this, the VCC output signal can directly activate the sampler in the data reconstruction path, without being properly delayed for bias compensation.     

The Optimum Closed-Loop Transfer Function of a Phase-Locked Loop Used for Synchronization Purposes

This paper presents an expression for the optimum closed-loop transfer function of a phase-locked loop (PLL) which estimates the phase of an unmodulated sinuosoid corrupted by an additive random disturbance. This matter has been extensively treated (e.g., [1, ch. 4]) in the conventional PLL theory, where the disturbance is a stationary noise process. However, for an important class of synchronizers operating on a PAM waveform corrupted by stationary noise, the disturbance at the input of the PLL cannot be treated as a stationary process. In this case we show that the PLL transfer function which is optimized according to the conventional PLL theory is far from optimum. Therefore, we derive the closed-loop transfer function which is optimum for a more general disturbance. This extension of the conventional PLL theory applies to most synchronizers.     

Linear Precoders for Bit-Interleaved Coded Modulation on AWGN Channels: Analysis and Design Criteria

This paper investigates linear precoding for bit-interleaved coded modulation (BICM) on additive white Gaussian noise (AWGN) channels. Concatenating a linear precoder as an inner encoder with an outer (convolutional) encoder produces a powerful code with a limited decoding complexity. Linear precoders are examined and optimized for two scenarios: using (i) a noniterative decoding strategy and (ii) an iterative decoding strategy under a perfect feedback assumption. The precoder design is based on an information-theoretical approach, on the one hand, and a pair-wise error probability (PEP) analysis, on the other hand. Both approaches render convenient precoder design rules. For a binary phase-shift keying (BPSK) signal set, the optimal precoders that result from these rules are also derived. Numerical results confirm the analytical findings and simulations illustrate the effectiveness of the approach.     

Turbo Estimation and Equalization for Asynchronous Uplink MC-CDMA

In this contribution, we propose a new code-aided synchronization and channel estimation algorithm for uplink MC-CDMA. The space alternating generalized expectation- maximization (SAGE) algorithm is used to estimate the channel impulse responses, propagation delays and carrier frequency offsets of the different users. The estimator, multi-user detector, equalizer, demapper and channel decoder exchange soft information in an iterative way. The performance of the proposed algorithm is evaluated through Monte Carlo simulations. Impressive performance gains are visible as compared to a conventional data-aided estimation scheme.