We propose an interference avoidance architecture using uneven spreading as a media access mechanism for optical code division multiple access (OCDMA). While an equal-intensity pulse sequence encoded with the spreading sequence assigned to each node is transmitted for a “1” bit in conventional OCDMA with on-off keying (OOK), the proposed architecture creates an uneven-intensity pulse sequence where one of the pulses has higher intensity than the others. The high-intensity pulse allows source nodes to use increased sensing threshold for channel sensing, which leads to an increase in the number of chip offsets available for collision-free transmission. Our receiver with a hard limiter (HL) allows destination nodes to receive the transmission without false positives. Interference avoidance performance is examined by deriving the collision probability and comparing it with the conventional interference avoidance with equal-intensity spreading. Our numerical results show that our architecture has lower collision probability, shorter time required for channel sensing, higher throughput, higher bit rate, and supports more nodes than the conventional one for a fixed collision probability.

In this letter, the reliabilty of the generalized normal-Laplace (GNL) distribution used for modeling the multiple access interference (MAI) plus noise in time-hopping (TH) binary phase-shift keying (BPSK) ultra-wideband (UWB) systems is evaluated in terms of the probability density function and the BER. The multiple access performance of TH-BPSK UWB systems based on GNL model is analyzed. The average BER performance obtained by using GNL approximation well matches with the exact BER results of TH-BPSK UWB systems. The parameter estimates of GNL distribution based on the moments estimation method is also presented.

This paper presents the performance analysis of a De-correlated Modified Code Tracking Loop (D-MCTL) for synchronous direct-sequence code-division multiple-access (DS-CDMA) systems under multiuser environment. Previous studies have shown that the imbalance of multiple access interference (MAI) in the time lead and time lag portions of the signal causes tracking bias or instability problem in the traditional correlating tracking loop like delay lock loop (DLL) or modified code tracking loop (MCTL). In this paper, we exploit the de-correlating technique to combat the MAI at the on-time code position of the MCTL. Unlike applying the same technique to DLL which requires an extensive search algorithm to compensate the noise imbalance which may introduce small tracking bias under low signal-to-noise ratio (SNR), the proposed D-MCTL has much lower computational complexity and exhibits zero tracking bias for the whole range of SNR, regardless of the number of interfering users. Furthermore, performance analysis and simulations based on Gold codes show that the proposed scheme has better mean square tracking error, mean-time-to-lose-lock and near-far resistance than the other tracking schemes, including traditional DLL (T-DLL), traditional MCTL (T-MCTL) and modified de-correlated DLL (MD-DLL).

In this paper, performance of a novel interference cancellation technique for the single user detection in a direct-sequence code-division multiple access (DS-CDMA) system has been investigated. This new algorithm is based on the Cycle-and-Add property of PN (Pseudorandom Noise) sequences and can be applied for both synchronous and asynchronous systems. The proposed strategy provides a simple method that can delete interference signals one by one in spite of the power levels of interferences. Therefore, it is possible to overcome the near-far problem (NFP) in a successive manner without using transmit power control (TPC) techniques. The validity of the proposed procedure is corroborated by computer simulations in additive white Gaussian noise (AWGN) and frequency-nonselective fading channels. Performance results indicate that the proposed receiver outperforms the conventional receiver and, in many cases, it does so with a considerable gain.

This paper presents a novel interference cancellation (IC) scheme for both synchronous and asynchronous direct-sequence code-division multiple-access (DS-CDMA) wireless channels. In the DS-CDMA system, the multiple access interference (MAI) and the near-far problem (NFP) are the two factors which reduce the capacity of the system. In this paper, we propose a new algorithm that is able to detect all interference signals as an individual MAI signal by maximum correlation detection. It is based on the discovery of all the unknowing spreading codes of the interference signals. Then, all possible MAI patterns so called replicas are generated as a summation of interference signals. And the true MAI pattern is found by taking correlation between the received signal and the replicas. Moreover, the receiver executes MAI cancellation in a successive manner, removing all interference signals by single-stage. Numerical results will show that the proposed IC strategy, which alleviates the detrimental effect of the MAI and the near-far problem, can significantly improve the system performance. Especially, we can obtain almost the same receiving characteristics as in the absense of interference for asynchrnous system when received powers are equal. Also, the same performances can be seen under any received power state for synchronous system.

We design M(≥3)-phase spreading sequences of Markov chains optimal in terms of bit error probabilities in asynchronous SSMA (spread spectrum multiple access) communication systems. To this end, we obtain the distributions of the normalized MAI (multiple access interference) for such systems and find a necessary and sufficient condition that the distributions become independent of the phase shifts.

In this paper, the bit error rate (BER) and the outage probability are presented for a maximal ratio combining (MRC) two-dimensional (2D)-RAKE receiver operating in a correlated frequency-selective Nakagami-m fading environment with multiple access interference. A simple approximated probability distribution function of the signal-to-interference-plus-noise ratio (SINR) is derived for the receiver with multiple correlated antennas and RAKE branches in arbitrary fading environments. The combined effects of spatial and temporal diversity order, average received signal-to-noise ratio, the number of multiple access interference, angular spread, antennae spacing and multi-path Nakagami-m fading environment on the system performance are illustrated. Numerical results indicate that the performance of the 2D-RAKE receiver depends highly on the operating environment and antenna array configuration. The performance can be improved by increasing the spatio-temporal diversity gains and antenna spacing.

This paper presents a novel blind multiple access interference (MAI) suppression filter in DS/CDMA systems. The filter is adaptively updated by parallel projections onto a series of convex sets. These sets are defined based on the received signal as well as a priori knowledge about the desired user's signature. In order to achieve fast convergence and good performance at steady state, the adaptive projected subgradient method (Yamada et al., 2003) is applied. The proposed scheme also jointly estimates the desired signal amplitude and the filter coefficients based on an approximation of an EM type algorithm, following the original idea proposed by Park and Doherty, 1997. Simulation results highlight the fast convergence behavior and good performance at steady state of the proposed scheme.

This letter examines a very simple iterative chip-by-chip multiuser detection strategy for spread spectrum communication systems. An interleaving-based multiple-access transmission technique is employed to facilitate detection. The proposed scheme can achieve near single-user performance in situations with very large numbers of users while maintaining very low receiver complexity.

Multiple access interference and near-far effect cause the performance of the conventional single user detector in DS/CDMA systems to degrade. Due to high complexity of the optimum multiuser detector, suboptimal multiuser detectors with less complexity and reasonable performance have received considerable attention. In this paper we apply the classic and a new modified genetic algorithm for multiuser detection of DS/CDMA signals. It is shown that the classic genetic algorithm (GA) reaches an error floor at high signal to noise ratios (SNR) while the performance of proposed modified GA is much better than the classic one and is comparable to the optimum detector with much less complexity. The results hold true for AWGN and fading channels. We also describe another GA called as meta GA to find the optimum parameters of the modified GA. We compare the performance of proposed method with the other detectors used in CDMA.

Multiple access interferecnce (MAI) is a major factor limiting the performance of direct-sequence code-division multiple access (DS-CDMA) systems. Since the amount of MAI is dependent on the correlation among user signals, one way to reduce it is to reduce such correlation. In mobile multiuser communication, each user experiences a different time-varying channel response. This user-dependent characteristic in channel variation can be exploited to assist the separation of different user signals, in addition to the capability provided by the spreading codes. As the correlation among different user channels are expected to decrease with increase in time span, enhanced decorrelation among different users' signals can be effected by spacing out the chips of one modulated symbol in time. Thus we consider chip-interleaving DS-CDMA (CI-DS-CDMA) in this study. We investigate its performance through theoretical analysis and computer simulation. Employing only a slightly modified rake receiver structure, CI-DS-CDMA is shown to attain significant performance gain over conventional DS-CDMA, in multiple access communication over single- and multi-path fading channels, without complicated multiuser detection. CI-DS-CDMA also has a lower demand for short-term power control than conventional DS-CDMA, especially in one-path Rayleigh fading. Results of the theoretical analysis and the computer simulation agree well with each other.