Multi-access Interference Research Articles

Gaussian Approximation for the Wireless Multi-Access Interference Distribution

The problem of Gaussian approximation for the wireless multi-access interference distribution in large spatial wireless networks is addressed. First, a principled methodology is presented to establish rates of convergence of the multi-access interference distribution to a Gaussian distribution for general bounded and power-law decaying path-loss functions. The model is general enough to also include various random wireless channel dynamics such as fading and shadowing arising from multi-path propagation and obstacles existing in the communication environment. It is shown that the wireless multi-access interference distribution converges to the Gaussian distribution with the same mean and variance at a rate 1/√λ, where λ >; 0 is a parameter controlling the intensity of the planar (possibly non-stationary) Poisson point process generating node locations. An explicit expression for the scaling coefficient is obtained as a function of fading statistics and the path-loss function. Second, an extensive numerical and simulation study is performed to illustrate the accuracy of the derived Gaussian approximation bounds. A good statistical fit between the interference distribution and its Gaussian approximation is observed for moderate to high values of λ.

Performance improvement of hybrid SCM SAC-OCDMA networks using multi-diagonal csode

Hybrid Subcarrier Multiplexing (SCM) incorporating spectral amplitude coding optical code division multiple access (SAC-OCDMA) technology was investigated both mathematically and by simulation. The hybrid SCM/SAC-OCDMA system combined two schemes in such as a way that the resulting hybrid system is robust against the Multi- Access Interference (MAI) and much enhanced the channel data rate. In this paper, we describe the improved performance of the hybrid SCM/SAC-OCDMA system based on the use of spectral amplitude coding to suppress impairments and to improve the overall system improve the overall sensitivity of system performance at a BER of 10-9. As a result, the proposed system based on Multi-Diagonal (MD) code shows better performance in comparison to the conventional code that employs the SAC-OCDMA system. We also evaluate the different design parameters under the maximum number of subcarriers (18 subcarriers in each optical channel) multiplexed for integrating OCDMA technology onto the existing fiber-wireless infrastructure at 155 M bit/s data rate for different fiber lengths. Further, a BER of 4.4 × 10-10 was achieved based on the proposed system for 45 km at 1.21 GHz. Key words: Subcarrier multiplexing (SCM), spectral amplitude coding optical code division multiple access (SAC-OCDMA), multi diagonal (MD) code.

A Hybrid Interference Cancellation technique for overloaded CDMA with timing offset error

A novel method of Hybrid Parallel/Successive Interference Cancellation (HIC) for Overloaded Direct Sequence Code Division Multiple Access (OCDMA) system involving two groups of users is proposed in this paper. The impact of timing synchronization error on acceptable channel load in a synchronous binary spread CDMA system is studied by mitigating the effect of multi-access interference (MAI). A small timing synchronization error is introduced in the DS-CDMA system under study to disturb the orthogonality of group-1 users and group-2 users. This orthogonality loss leads to Multi User Interference (MUI) in group-1 users and it is cancelled using Parallel Interference Cancellation (PIC). The interference of group-2 users on group-1 users is cancelled using Successive Interference Cancellation (SIC). The analytical expressions of the Bit Error Rate (BER) are derived. The simulation results prove that the BER performance of the proposed HIC method is better than the conventional PIC and SIC.

On Optimal Front-End Filter for Single-User Detection in IR-UWB Systems

We consider a K user Gaussian multiple access system with impulse radio ultra-wideband signaling. The receiver is assumed to treat the Multi-Access Interference (MAI) as additive noise and does not enjoy multiuser detection. It is well known that for such a system the MAI is neither Gaussian nor white. Although, the non-Gaussianity of the MAI has been extensively studied, little attention has been directed to the non-whiteness of it. In this paper, we show that by employing a new front-end filter at the receiver, the non-whiteness of the MAI can be exploited to improve the performance of the single-user detector. We use the signal-to-interference-plus-noise ratio (SINR) as a merit to evaluate the performance improvement brought to the system. The SINR improvement depends on the interference level as well as pulse waveform. The bit error rate of the system is also simulated and compared with that of conventional matched filter receiver.

OFDMA Uplink Frequency Offset Estimation with Multi-Access Interference Mitigation

In this paper, we consider the frequency offset estimation for Orthogonal Frequency-Division Multiple Access (OF-DMA) uplink (UL) transmissions. We first analyze the negative effect of Multi-Access-Interference (MAI) on OFDMA UL, and then propose two interference reduction/elimination methods, i.e., the Reduced-Rank-Projector (RRP) and Shift-Sampling-Projector (SSP) methods, to eliminate/reduce the heavy MAI due to the frequency offsets. Finally, we propose a new training sequence group named the Round-Robin Training Sequence Group (RRTSG), which has a high interference mitigation capabilities for OFDMA UL transmission. The Cramer-Rao Lower Bound (CRLB) for an unbiased frequency offset estimator in a Multiple Access (MA) system is also derived. Numerical results show that the proposed methods are suitable to eliminate/mitigate the effect of the frequency offset on OFDMA UL transmission.

Calculating Outage Probability of Block Fading Channels Based on Moment Generating Functions

Block fading channels present a realistic model in many communication scenarios. We propose a method by which the outage probability of block fading channels can be evaluated in a straighforward and computationally efficient manner by utilizing the moment generating function of a single block's mutual information. The flexibility of moment generating functions helps obtain outage probabilities in various cases such as wideband noise jamming and multi-access interference. The effect of discrete input alphabets on outage is also investigated and turns out to be vital especially under interference.

Preamble-based improved channel estimation for multiband UWB system in presence of interferences

Interferences reduce the performance of a correct data signal detection and decoding. This problem becomes severe when interferences exist during the period of channel estimation. This will destroy the accuracy of channel estimation and will eventually result severe degradation in the performance of signal detection and decoding in the entire data packet/frame. In this article, we propose an improved channel estimation technique for multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband (UWB) system in presence of interferences. In particular, we work towards a preamble-based channel estimation technique in multi-access and narrowband interfering environments. We construct two preamble symbols with opposite, both in amplitude and phase, sequences in frequency-domain. Time-domain redundancy (TR) is introduced into these preamble symbols before transmission. Based on this time-domain redundancy property, we eliminate multi-access interference (MAI) through an adaptive select and replace (ASR) scheme. We show that the use of opposite sequences in preambles leads a simple addition, subtraction and compare (ASC) scheme to cancel narrowband interference (NBI). In addition, we apply a frequency-domain filter, driven by channel's power delay profile (PDP), to get a further enhancement in the estimation accuracy. Simulation results urge that our proposed technique outperforms the conventional channel estimation methods.

Code Hopping Communications for Anti-Interception with Real-Valued QZCZ Sequences

With the real-valued sequences, a new kind of Code Hopping Code Division Multiple Access (CH-CDMA) communication systems is proposed for anti-interception. While the sequence parameters are the same, the number of the real-valued sequence sets are much more than those of the traditional integer sequence, therefore, the codes for hopping are no longer in serious deficiency as they used to be, which makes the performance of anti-interception improved greatly. Moreover, using real-valued sequences with Quasi-Zero Correlation Zone (QZCZ) may eliminate the Inter-Symbol Interference (ISI) and Multi-Access Interference (MAI) so as to overcome the far-near effect and improve the performance of anti-interference.

Multi-User Detection Techniques for DS-CDMA UWB System

Ultra-wideband(UWB) is currently one of three new technologies of wireless communication. Multi-user detection technique is mainly used to eliminate multiple access interference（MAI）in DS-CDMA UWB communication systems. Linear multi-user detection algorithm for multi-access interference suppression or even elimination has certain results. But for a fixed decision threshold is set to 0, the detection performance of linear multi-user detection algorithm will be decreased significantly. Therefore a kind of multi-user detection with pre-processing has been researched. Through the pre-processing, we set dynamic threshold to direct decision the user's data bit information so as to solve the problem the linear multi-user detector’s decision threshold which is fixed impact the detection performance. We may get the global optimal solution though pre-processing of using the thought of the branch and bound algorithm and forcing rules. Then the original problem changes into a smaller multi-user detection problem, in order to get the aim of reducing MAI, increasing the probability of a correct decision and reducing the bit error rate. From the simulation experiment results, it is shown that it has a significant effect in system detection, because after pre-processing the probability of a correct decision has increased, and the bit error rate has been reduced.

CDMA Multiuser Detection Based on Improved Particle Swarm Optimization Algorithm

CDMA multiuser detection (MUD) is a crucial technique to mobile communication. We adopt improved particle swarm optimization (PSO) algorithm in MUD which incorporates factor and utilizes function to discrete PSO. Comparison of BER and near-far effect has verified its effectiveness on multi-access interference (MAI). The algorithm accelerates the convergent speed meanwhile it also displays feasibility and superiority in case simulation.

Spectral shaping of spreading sequences as a mean to address the trade-off between narrowband and multi-access interferences in UWB systems

This paper presents a way to cope with the need of simultaneously rejecting narrowband interference and multi-access interference in a UWB system based on direct-sequence CDMA. With this aim in mind, we rely on a closed-form expression of the system bit error probability in presence of both effects. By means of such a formula, we evaluate the effect of spectrum shaping techniques applied to the spreading sequences. The availability of a certain number of degrees of freedom in deciding the spectral profile allows us to cope with different configurations depending on the relative interfering power but also on the relative position of the signal center frequency and the narrowband interferer.

A New Technique for Reduction the Phase Induced Intensity Noise in SAC-OCDMA Systems

A new code for reduction the phase induced intensity noise has been presented. The new code is proposed for Spectral Amplitude-Coding Optical Code Division Multiple Accesses (SAC-OCDMA). This new code family we call it Dynamic Cyclic Shift (DCS) code. The DCS code reduced the effect of Multi Access Interference (MAI) due to it is the property of variable cross correlation. We find that the performance of the DCS code is a batter than other SAC-OCDMA codes such as; Random Diagonal (RD) code, Modified Quadratic Congruence (MQC) code and Modified Frequency Hopping (MFH) code. Through the mathematical calculation and simulation analysis, for the bit-error rate of DCS code is significantly better than other SAC-OCDMA codes, the effect of Phase Induced Intensity Noise is reduced. In addition, proofof-principle simulations of 10 Gb/s for 20 km have been successfully demonstrated and achieved low BER compared to the other codes.

Interference Cancellation in Cooperative CDMA Networks

The wireless communication networks are subjected to multi access interference and multipath fading. To minimize the interference cancellation in CDMA networks, multiple user detection schemes and cooperative communication networks are used. consider the uplink of a cooperative CDMA network, where users cooperate by relaying each other’s messages to the base station. When spreading waveforms are not orthogonal, multiple access interference (MAI) exists at the relays and the destination, causing cooperative diversity gains to diminish. To overcome this problem, we integrate various multiuser detection (MUD) schemes to mitigate MAI in achieving the full advantages of cooperation. Specifically, the relay-assisted decorrelating multiuser detector (RADMUD) is proposed to separate interfering signals at the destination with the help of precoding at the relays along with pre-whitening at the destination. In this paper we examined the BER performance of various MUD schemes are analyzed and compared with cooperative system. The advantages of RAD-MUD with co-operative communication shows better BER performance compared with non co-operative wireless communication system and other existing cooperative MUD schemes are also shown through MATLAB Simulations.

BER Performance Evaluation of two Types of Antenna Array-Based Receivers in a Multipath Channel

Smart antennasystems have received much attention in the last few years because they can increase system capacity by dynamically tuning out interference while focusing on the intended user. In this paper, we focused our research on the performance of two kinds of smart antenna receivers. An analytical model is proposed for evaluating the BER performance using a closed-form expression. Also, for the adaptive array, a simple way to account the multi-access interference can be exploited to evaluate the average probability of error when the users are randomly distributed within an angular sector. The proposed model confirms the benefits of adaptive antennas in reducing the overall interference level (intercell/intracell) and to find an accurate approximation of the error probability. In the two kinds of receivers, we assessed the impact of smart antenna systems and we considered the case of conventional single antenna receiver model as reference (single user/single antenna).

Logical Topology Design for Eliminating Cycle Attacks in Optical Code Path Networks

In optical code path networks a cycle attack phenomenon has been reported. Cycle attack is caused by multiaccess interference (MAI) among different optical code (OC) paths carried by the same wavelength. A previous study proposed a heuristic wavelength assignment scheme to avoid this unintended cycle attack problem. However, the wavelength assignment retry in the previous proposal may impose an overhead on OC path establishment latency, and furthermore it was not a complete solution to eliminate cycle attacks. In this paper, we propose a cycle-attack-free logical topology design to eliminate the unintended cycle attack problem in the network planning phase instead of the wavelength assignment. The basic idea is to convert the physical network topology into some tree-based logical topologies in which OC paths are routed. Simulation results show that (a) using the tree-based topologies with smaller average hop count helps to obtain better blocking performance and (b) in distributed optical path establishment our proposed approach performs better in terms of blocking and delay performance under the relative dynamic traffic condition.

Quasi-Synchronous CDMA Using Properly Scrambled Walsh Codes as User-Spreading Sequences

An orthogonal spreading code family based on scrambled Walsh-Hadamard (WH) sequences is proposed for quasi-synchronous (QS) code-division multiple-access (CDMA) systems. With 1 chip offset, either the top or the bottom half rows of the WH matrix remain orthogonal if they are scrambled using the proposed scrambling patterns. With quadriphase scrambling applied on a quarter row of the WH matrix, the zero-correlation zone (ZCZ) is extended to 3 chips. The proposed spreading codes satisfy the theoretical limit of code size for a given code length and ZCZ. Compared with existing spreading codes for QS-CDMA, the proposed spreading codes have several advantages: 1) A special procedure for code family construction is not needed; it only requires a simple modification to the scrambling sequence of the conventional CDMA, which employs WH sequences as user signatures. 2) Flexible code family size; the size can easily be extended for multicell applications without having to consider existing codes used. Moreover, if synchronization is guaranteed to be within 1 chip, the code size can be doubled by allocating another quarter member of the WH sequence set. 3) The benefits of long code scrambling from the conventional cellular CDMA systems are inherited, such as robustness against interception and multiaccess interference randomization.

A Low-Complexity Optimum Multiuser Receiver for DS-CDMA Wireless Systems

Multiuser detection is an important technology in wireless CDMA systems for improving both data rate as well as user capacity. However, the computational complexity of multiuser detection prevents the widespread use of this technique. Most of the CDMA systems today and in the near future will continue to use the conventional matched filter with its comparatively low user capacity and a slow data rate. However, if we could lower the computational complexity of multiuser detectors, CDMA systems would offer an increased system capacity with a better data rate. In this paper, a new scheme for reducing the computational complexity of multiuser receivers is proposed. It utilizes the transformation matrix algorithm to improve the performance of multiuser receivers by effectively reducing the bit error rate (BER). In addition to the transformation matrix algorithm, a quantitative analysis of the processing gain for a multiuser DS-CDMA system is presented. The quantitative analysis of the processing gain demonstrates that how the reduced BER could be used to achieve reasonable values of processing gain by which unwanted signals or interference can be suppressed relative to the desired signal at the receiving end. We present that the proposed scheme can reduce the asymptotic computational complexity of multiuser receivers while at the same time effectively eliminates the unwanted signals. The proposed algorithms not only are shown to substantially improve the performance of the multiuser detectors by means of reduced BER but also have a much lower multi-access interference. The performance measure adopted in this paper is the achievable bit rate for a fixed probability of error (10 -7 ) and consistent values of SNR.

Suppression of ICI and MAI in SC-FDMA communication system with carrier frequency offsets

Similar to other orthogonal frequency division multiplexing (OFDM)-based systems, carrier frequency offset (CFO) is a challenging problem in uplink communications of single carrier frequency division multiple access (SC-FDMA) system. It must be noticed that CFO, which is mainly due to oscillator instability and/or Doppler shift, would generate inter carrier interference (ICI) as well as multi-access interference (MAI) to disturb the received signals and seriously degrade system performance. Frequency synchronization in uplink communications is difficult because different users always experience different CFOs and one user's CFO correction would misalign other users. In this paper, we proposed a suppression method to overcome the multi CFOs problem. To implement this algorithm, block type pilots would be exploited, which is also utilized in LTE uplink standard. The proposed algorithm is based on the following two assumptions. Firstly the proposed algorithm is applied in this scenario, where different users start to communicate with the base station at different symbol periods, and secondly, during the pilot block and the following data blocks, the CFO of each user is quasi static, which is feasible since CFO is slow varying. Compared with other interference suppression methods, the proposed method could directly estimate the interference components from the inverse pilot matrix, thus it does not need to do the CFO estimation. Further more, since block type pilots is a common pilot pattern in wireless communication system, this algorithm can be easily extended to other communication system. Simulation results show that the proposed suppression algorithm can significantly improved system performance.

Multipath delay acquisition in asynchronous doubly-selective DS/CDMA fading channels

In this letter, we investigate the problem of estimating the multipath signal delays in an asynchronous direct-sequence/code-division-multiple-access (DS/CDMA) system operating over a doubly-selective fading channel. A novel blind estimation procedure is proposed which only requires knowledge of the spreading code of the user of interest, but no prior information as to the transmitted data symbols and the structure of the multiaccess interference. Numerical examples are provided to assess the performance of the proposed algorithm.

Asynchronous detection of optical code division multiple access signals using a bandwidth-efficient and wavelength-aware receiver

We experimentally demonstrate what we believe to be a novel detection scheme for interfacing asynchronous optical code division multiple access (CDMA) signals with an electronic clock and data recovery system that operates only at the baseband bandwidth. This allows using a large optical bandwidth expansion factor in which the optical chip rate is much larger than the bandwidth of the optoelectronic receiver. The received optical CDMA signal is launched into a four-wave-mixing-based wavelength-aware all-optical front end that rejects multiaccess interference, followed by an amplitude-noise suppression stage comprised of a semiconductor optical amplifier. The clean signal is then converted into a non-return-to-zero-like signal by a baseband receiver. Using the proposed detection scheme, asynchronous transmission and detection of optical CDMA signals is implemented. With the novel detection scheme, the classic CDMA near-far problem is mitigated, and error-free detection is easily obtained.