Mohamed Adnan Landolsi

DS-CDMA chip waveform design for minimal interference under bandwidth, phase, and envelope constraints

This paper investigates the effect of chip waveform shaping on the error performance, bandwidth confinement, phase continuity, and envelope uniformity in direct-sequence code-division multiple-access communication systems employing offset quadrature modulation formats. An optimal design methodology is developed for the problem of minimizing the multiple-access interference power under various desirable signal constraints, including limited 99% and 99.9% power bandwidth occupancies, continuous signal phase, and near-constant envelope. The methodology is based on the use of prolate spheroidal wave functions to obtain a reduced-dimension discrete constrained optimization problem formulation. Numerous design examples are discussed to compare the performance achieved by the optimally-designed chip waveforms with other conventional schemes, such as offset quadrature phase-shift keying, minimum-shift keying (MSK), sinusoidal frequency-shift keying (SFSK), and time-domain raised-cosine pulses. In general, it is found that while the optimized chip pulses achieved substantial gains when no envelope constraints were imposed, these gains vanish when a low envelope fluctuation constraint was introduced. In particular, it is also shown that MSK is quasi-optimal with regard to the 99% bandwidth measure, while the raised-cosine pulse is equally good with both the 99% and 99.9% measures, but at the expense of some envelope variation. On the other hand, SFSK is quasi-optimal with regard to the 99.9% bandwidth occupancy, among the class of constant-to-low envelope variation pulses.

On the accuracy of Gaussian approximations in the error analysis of DS-CDMA with OQPSK modulation

This paper considers the bit error probability analysis of direct-sequence code-division multiple access with offset quadrature phase-shift keying (OQPSK) modulation and random signature sequences. A statistical characterization of the decision variable at the output of the correlation receiver is obtained, and several techniques for evaluating the error performance, previously derived for binary phase-shift keying (BPSK)-modulated systems, are then extended to the case of OQPSK. This includes exact bit error evaluation, as well as various approximation methods based on Gaussian modeling of the multiple-access interference terms. These techniques, known as the standard and improved Gaussian approximations, have increasing complexity depending on the level of accuracy required. For BPSK, the standard approximation is typically found to be inaccurate, unless the number of users becomes large, and only the improved approximation yields sufficient accuracy in general. But for the case of OQPSK, the relative performance comparison developed in this paper shows that, for all practical purposes, the simple standard approximation is surprisingly very accurate even with a small number of users.

Hybrid TOA/AOA approximate maximum likelihood mobile localization

This letter deals with a hybrid time-of-arrival/angle-of-arrival (TOA/AOA) approximate maximum likelihood (AML) wireless location algorithm. Thanks to the use of both TOA/AOA measurements, the proposed technique can rely on two base stations (BS) only and achieves better performance compared to the original approximate maximum likelihood (AML) method. The use of two BSs is an important advantage in wireless cellular communication systems because it avoids hearability problems and reduces network signaling burden. Simulation results show that, for certain scenarios, the proposed hybrid TOA/AOA AML with two BSs can outperform the AML with up to six BSs.

Optimized CDMA forward link power allocation during soft handoff

In CDMA cellular systems, soft handoff often yields significant gains in performance in terms of increased capacity and coverage, and reduced call drop rates. This is particularly true for the reverse link in CDMA cellular systems. On the other hand, increasing the number of base stations that a mobile is simultaneously using can cause degradations in RF capacity and call dropping probability in the forward link. This paper proposes a new optimized scheme for allocating fractions of power transmitted to a particular mobile from the different base stations involved in soft handoff. The resulting scheme will minimize the overall transmitted power, reduce unnecessary interference to mobiles in other cells, and increase capacity; while maintaining the required qualify of service.

New results on the reverse link capacity of CDMA cellular networks

The reverse link Erlang capacity of an IS-95 based CDMA system is studied under non-uniform traffic loading conditions. In particular, the variation in the capacity of a center cell surrounded by lightly loaded cells is assessed analytically, and its sensitivity to various factors is discussed. The statistics of the other cell interference factors are obtained, and some numerical results using typical system parameter values are given.

Improved regular and semi-random rate-compatible low-density parity-check codes with short block lengths

Powerful rate-compatible codes are essential for achieving high throughput in hybrid automatic repeat request (ARQ) systems for networks utilising packet data transmission. The paper focuses on the construction of efficient rate-compatible low-density parity-check (RC-LDPC) codes over a wide range of rates. Two LDPC code families are considered; namely, regular LDPC codes which are known for good performance and low error floor, and semi-random LDPC codes which offer performance similar to regular LDPC codes with the additional property of linear-time encoding. An algorithm for the design of punctured regular RC-LDPC codes that have low error floor is presented. Furthermore, systematic algorithms for the construction of semi-random RC-LDPC codes are proposed based on puncturing and extending. The performance of a type-II hybrid ARQ system employing the proposed RC-LDPC codes is investigated. Compared with existing hybrid ARQ systems based on regular LDPC codes, the proposed ARQ system based on semi-random LDPC codes offers the advantages of linear-time encoding and higher throughput.

Improved weighting algorithm for NLOS radiolocation

Non-Line-of-Sight (NLOS) propagation is a major problem facing accurate radiolocation using Time-of-Arrival (TOA) measurements in wireless communication systems. Weighting is among the important NLOS mitigation approaches whereby the system attempts to localize with both Line-of-Sight (LOS) and NLOS measurements, but provides scaling to minimize the effects of the NLOS contributions. In this paper, we present an improved weighting location algorithm that utilizes the geometrical feature of cell layout and TOA range measurements from three base stations (BSs). The MS location is estimated using weighted ranges that produce special points, called potential points. Simulations studying the performance of the proposed algorithm for different NLOS error models show that the proposed algorithm performance is notably better than traditional algorithms, even under highly NLOS conditions.

New technique for improving performance of LDPC codes in the presence of trapping sets

Trapping sets are considered the primary factor for degrading the performance of low-density parity-check (LDPC) codes in the error-floor region. The effect of trapping sets on the performance of an LDPC code becomes worse as the code size decreases. One approach to tackle this problem is to minimize trapping sets during LDPC code design. However, while trapping sets can be reduced, their complete elimination is infeasible due to the presence of cycles in the underlying LDPC code bipartite graph. In this work, we introduce a new technique based on trapping sets neutralization to minimize the negative effect of trapping sets under belief propagation (BP) decoding. Simulation results for random, progressive edge growth (PEG) and MacKay LDPC codes demonstrate the effectiveness of the proposed technique. The hardware cost of the proposed technique is also shown to be minimal.

DS-CDMA chip waveform design for optimal power-bandwidth performance

The effect of chip waveform shaping on the power efficiency and spectral occupancy of DS-CDMA communication systems is investigated assuming random sequences and coherent reception. A method for designing optimum chip waveforms that minimise the average power of the multiple access interference at a given bandwidth occupancy is derived. The performance improvements of the optimally designed chip waveforms are illustrated by comparing the achieved SIR gain and capacity increase with the figures of some known examples of modulation schemes (such as OQPSK and MSK) that employ conventional chip shaping pulses.

Signal Design for Improved Multiple Access Capacity in DS-UWB Communication

The paper addresses the signal design and performance analysis of direct-sequence ultra-wideband (DS-UWB) multiuser communication employing offset quadrature-spread phase shit keying (OQPSK) modulation with novel chip waveforms. Analytical bit error rate (BER) performance analysis is derived for different receiver structures, emphasizing the impact of the chip mean-squared auto-correlation function (MS-ACF) on BER performance in multiuser and multipath environments. A low-complexity flexible algorithm for designing chip pulses that maximize multiuser system capacity by minimizing the chip MS-ACF is proposed. The optimized waveforms are based on the use of prolate spheroidal wave function (PSWF) expansions. The proposed schemes also provide increased spectrum placement flexibility and minimized phase jumps compared to carrier-less pulse amplitude modulation (PAM) UWB schemes. Several design examples and numerical results are presented to illustrate the relative performance of the novel pulses compared to UWB schemes employing Gaussian monocycle derivatives and modified Hermite polynomials, and it is shown that a clear improvement in multiple access capacity can be achieved by the proposed PSWF-based pulses, particularly with maximum ratio combining receivers.