Sequence Code Division Multiple Access Research Articles

Analysis of Multiuser Detectors and Performance improvement in DS-CDMA system using Multistage Multiuser Detection Techniques

The Direct Sequence Code Division Multiple Access (DS-CDMA) system faces several disruptions, this is most crucial of which is the Multi Access Interference (MAI) caused by its users. The efficiency of the system gradually declines as every quantity rises, and the MAI rises, especially in faded environments. This work proposes a multiple-phase multiuser identification approach called Differencing Partial Parallel Interference Cancellation (DPPIC), which improves the overall efficiency. The methods known as Differencing Parallel Interference Cancellation (DPIC) and Partial Parallel Interference Cancellation (PPIC) are combined in this methodology. Current solutions for Parallel Interference Cancellation (PIC) and PPIC have enhanced overall effectiveness; however, this has come at the expense of increasing the complexity of computation. As the variety of consecutive stages grows, the MAI falls. Using the DPIC approach may reduce the computational burden without improving system functionality. The use of the Partial Differencing Parallel Interference Cancellation (PDPIC) technique can enhance system performance while lowering the level of complexity. According to the simulation findings, Bit Error Rate (BER) vs normalized signal strength (i.e., Eb / N0) performs more effectively for the suggested DPPIC approach than for PIC, the PPIC, and PDPIC.

Performance Improvement of CDMA Wireless Sensor Networks in Low SNR Channels Based on Raptor Codes

Low signal to noise ratio (SNR) channels require careful design of communication systems in order to operate reliably in such extreme channels. Special measures and techniques must be considered when designing communication systems for such environments to counteract the effect of high noise power. Channel coding is a powerful tool to aid against adverse channel conditions including low SNR. Coupling channel coding with a spreading technique can further strengthen the performance of communication systems. Hence, we studied the performance of a system that combines Raptor codes with direct sequence code division multiple access (DS-CDMA) in low SNR scenarios. The studied design can operate in a wide range of low SNR values. Simulation results show that the system can offer satisfactory BER performance at different code rates. Also, combining Raptor codes with DS-CDMA showed an improvement of approximately 6 dB when compared with other schemes using DS-CDMA in low SNR conditions.

Acquisition Time and Probabilities of Detection and False Alarm in Direct Sequence Code Division Multiple Access Systems

Multiple access interference (MAI) is one of the most important issues to consider in the design, implementation and operation of 5G systems using code division multiple access (CDMA) technique based on spread spectrum that offers a more flexible solution, compared to other techniques. Especially, it permits a much higher user bit rate, due to the bandwidth allocated to the emitted signal; thus, allowing very attractive multimedia services. In this paper, we develop the expressions of the detection probability, false alarm probability and the average acquisition time - in a direct sequence CDMA (DS-CDMA) transmission system - using a serial search and taking into consideration MAI. The evolution of these parameters according to the signal- to-noise ratio (SNR) and the decision threshold leads to study performances of the system in terms of detection and average acquisition time. The obtained results reveal that the acquisition time decreases when the SNR increases. Additionally, it is found that the probability of detection increases when the threshold decreases, and increasing SNR it reaches a constant value for an SNR/chip around 0 dB for all thresholds.

Generalized Receiver with Decision-Feedback Equalizer for Multicode Wideband DS-CDMA

In the present paper, a chip-level minimum mean-square-error (MMSE) decision-feedback equalizer for the downlink receiver of multicode wideband direct sequence code-division multiple access (DS-CDMA) wireless communication systems over frequency-selective channels is investigated. Firstly, the MMSE per sym-bol achievable by an optimal decision-feedback equalizer is derived, assuming that all interchip interference (ICI) of the desired user can be eliminated. The MMSE of the decision-feedback equalizer is always less than or at most equal to that of linear equalizers. When all the active codes belong to the desired user, the ideal deci-sion-feedback equalizer is able to eliminate multicode interference and approach the performance of the single-code case at high signal-to-noise ratio (SNR) range. Secondly, we apply the hypothesis-feedback equalizer or tentative-chip decision-feedback equalizer in the multicode scenario. The tentative-chip decision-feedback equ-alizer outperforms the chip-level linear equalizer and the decision-feedback equalizer that only feeds back the symbols already decided. The performance gain increases with SNR, but decreases with the number of active codes owned by the other users. When all the active codes are assigned to the desired user, the tentative-chip decision-feedback equalizer eliminates the multicode interference and achieves single-user performance at the high SNR, similarly, to the ideal decision-feedback equalizer. The asymptotic performance of the decision-feed-back equalizer is confirmed through the bit error rate (BER) simulation over various channels.

Two cross coupled and Madgwicks filter for estimation of multi-channel dividing systems

The estimates of Rayleigh fading channels are rapidly changing in multi carrier direct sequence code division multiple access (MC-DS-CDMA) multiplexing systems. The most widely accepted answer to this issue is the conventional solution least square (LS) or mean square error estimator (MMSE) using the recursive least squares algorithm (RLS) or the least mean squares (LMS) algorithm. In much of the previous work, only one Kalman filter was used for estimation. In this paper, a Kalman filter is used with a Madgwicks filter together to satisfy the fading problems. However, this requires a priori evaluation of auto regressive (AR) parameters. A standard solution involves the first matching of the auto-completion function of the applying the AR method to Jakes' problem and then tackling it (YWE). Even more the results procedure is limited to crowd constraints and is related to an AR+ process of noise, an approximation considered. In fact, depending on simulation findings, high-AR models outperform conventional models on the basis of spectral estimate and bit error margins (BER). Nevertheless, in order to save costs of computing, the 5-D model of AR is a possibility. The proposed process outperforms edge of art competitors in terms of bit error rate as demonstrated by results.

Capacity Estimation for a DS-CDMA System in Nakagami-m Fading

Ιn this paper, a novel closed-form expression of the Shannon average channel capacity per user for direct- sequence code-division multiple access (DS-CDMA) systems, operating in Nakagami-m fading, with optimum RAKE reception, is obtained. Numerical results are also presented to illustrate the proposed mathematical analysis and to point out the effect of the fading severity on the user’s average channel capacity.

Robust or nonrobust: On MC-DS-CDMA acquisition in LEO satellite communications

Low Earth Orbit (LEO) satellite communications can provide global coverage in the sixth generation communication (6G) networks. To combat the strong Partial Band Interferences (PBIs) and multipath fading in LEO satellite communication systems, the Multicarrier Direct Sequence Code Division Multiple Access (MC-DS-CDMA) technique is a promising alternative to the traditional Single-carrier Direct Sequence Code Division Multiple Access (SC-DS-CDMA) system for its advantages of high bandwidth efficiency, superior interference rejection capability, and low complexity of parallel signal processing. However, limited studies have been conducted on the performance analysis of MC-DS-CDMA acquisition systems in the presence of a large Doppler shift, a unique characteristic of LEO satellite communications. To bridge this gap, we investigate the performance of MC-DS-CDMA systems with two-dimensional acquisition and noncoherent equal gain combining over Rician fading channel in the presence of the Doppler shift and PBIs. The performance metrics are detection probability and Mean Square Error (MSE) of the Doppler factor and delay. Specifically, we derive the closed-form expressions for the MSE and the Probability Density Function (PDF) of the acquisition decision variable and obtain the detection probability. We conduct extensive numerical experiments to verify the theoretical analysis and performance gain of MC-DS-CDMA over the SC-DS-CDMA. The results show that MC-DS-CDMA with two-dimensional acquisition is more robust to multipath Rician fading than SC-DS-CDMA. Moreover, MC-DS-CDMA outperforms SC-DS-CDMA regarding the detection probability and MSE when combating the strong PBIs.

HCR-WSN: Hybrid MIMO cognitive radio system for wireless sensor network

Multi Input Multi Output (MIMO) based Cognitive Radio (CR) provisions an efficient utilization of the scarce electromagnetic spectrum. Traditional CR includes Interweave and Underlay techniques. We propose a Hybrid MIMO CR system for Wireless Sensor Network, HCR-WSN, that utilizes both underlay and interweave CR techniques and provides higher spectral efficiency. The wireless sensor network consisting of CR nodes achieves higher throughput using the proposed framework.HCR-WSN improves the spectral efficiency gain of an Uplink Hybrid Cognitive Radio MIMO system in wireless sensor network (WSN), by implementing Narrowband and Wideband (Multiple Carrier Direct Sequence Code Division Multiple Access, i.e., MC-DS-CDMA) technology adhering to the Interference Threshold limit. The WSN is consisting of nodes acting as primary users (PU) and CR secondary users (SU). The proposed Hybrid CR system ensures efficient spectral utilization under circumstances resulting in near zero throughput for SU. In the presence of PU, CR-SU transmits in either Narrowband or MC-DS-CDMA based on the proposed transmission selection algorithm. In the absence of PU, SU uses Interweave technique thereby allowing other competing SUs to transmit in Interference Alignment underlay mode in a multi-user scenario. The achievable throughput is enhanced using the proposed technique in a multi-user hybrid MIMO cognitive radio wireless sensor network.

Non orthogonal multiple access for DS-CDMA systems

In direct sequence code division multiple access (DS-CDMA), the symbols of a single user are transmitted over a given spreading code. In this paper, we propose to increase the throughput of CDMA sy...

Performance Analysis of Sliding Window Multiuser Detectors Used in Asynchronous Ds-cdma Systems (Dept.E)

In this paper, the sliding-window method is applied on both decorrelating and linear minimum mean-squared error (LMMSE) multiuser detectors. The analytical expressions for the error probability of those multiuser detectors (MUDs) are derived. The analysis is presented for asynchronous direct sequence code-division multiple access (DS-CDMA) in additive white Gaussian noise (AWGN) and multipath fading environment. The adopted detectors divide the received signal stream into processing windows and process them window by window. The effects of expanding the processing-window length on the error probabilities are investigated. The analysis shows that the error performance of the decorrelating detector is much better than that of the LMMSE detector when the power of the interfering users is larger than the desired user power. It is also shown that the performance of the presented detectors will be significantly improved with processing-window length up to four symbols.

Cooperative inter-satellite ranging using custom code division multiple access frame for space traffic management

Space traffic is considered a complex contemporary issue originated from the difficulty inherited in the nature of space missions. Satellites and space objects are actively tracked using Earth-based surveillance networks or in-situ sensors like the GPS. The data collected from these Earth-based surveillance networks albeit their public availability, they remain only valuable for trajectory and state verification due to their limited positional accuracy. This is exhibited in the poor accuracy (within 2–5 km) of public two-line elements (TLEs), which worsen when propagated in the future to more than tens of kilometers. Consequently, precision orbit data obtained from onboard GPS systems, are commonly used to alleviate, to a degree, the uncertainty regarding space conjunctions. However, the lack of GPS data standardization and sharing regarding critical satellite conjunctions together with the limited deployment of the GPS system onboard nano-satellites and its susceptibility to Doppler shifts in low Earth orbits, inhibits the full exploitation of the GPS system in space traffic.Targeting instant inter-satellite ranging for improving space situational awareness (SSA), this research explores the benefits of the proposed custom signature sequence to optimize a signal acquisition and detection for coherent and asynchronous synchronisation. This comes with the advantage of accurate propagation delay estimation and bias errors correction. Data sharing also becomes possible using the proposed custom direct sequence code division multiple access (DS-CDMA) and Reed-Solomon forward error correction for a future-proof inter-satellite ranging and data sharing solution. Coherent detection, acquisition and synchronisation are proposed based on data-assisted correlation which reduces the false- and miss-alarms respectively by 27% and 97%. This CDMA frame is based on orthogonal dual-channel separation which offers considerable noise and interference resilience for data recovery, in addition to precise frequency and timing errors correction during synchronisation. Results show that the ranging accuracy approaches meter-level at no expense of increased bandwidth beyond 2 MHz. Occupying the same bandwidth, the ranging resolution scales linearly with the spreading factor contributed by the Kronecker filter length. Additionally, the delay measurement solution is proven unsusceptible to the Doppler frequency errors caused by the high relative velocity between the satellites and Doppler errors within 16 kHz were corrected. Further, the coding gain increases by 16 dB compared to narrow-band communications, using CCSDS Reed-Solomon encoder coupled with data sequence spreading, offering considerable data transfer resilience in low signal-to-noise conditions.

Two-Dimensional Joint Acquisition of Doppler Factor and Delay for MC-DS-CDMA in LEO Satellite System

Low Earth Orbit (LEO) satellite systems have received considerable attentions for its advantages and potential applications in global communication. Yet LEO satellite-ground communication is challenged by the huge Doppler shift. This article is devoted to studying how to deal with this challenge in Multi-carrier Direct Sequence Code Division Multiple Access (MC-DS-CDMA) systems in the presence of multi-path and partial band interferences for LEO satellite-ground link in urban areas. To this end, we design a two-dimensional joint acquisition algorithm. Specifically, we acquire the delay and Doppler factor by solving the Maximum Likelihood (ML) problem with decision variable falling into two-dimensional uncertain region of the estimated parameters. We then use grid-based searching method to access the ML estimates of the delay and Doppler factor at the cost of acceptable complexity. We also derive the detection probability and Mean Square Error (MSE) in both Additive White Gaussian Noise (AWGN) and Rayleigh fading channels. It is shown that the performance of MC-DS-CDMA with the proposed algorithm significantly outperforms its single-carrier counterpart in Rayleigh fading channel, and is proved to be more tolerant to partial band interferences for both AWGN and Rayleigh fading scenarios.

Performance Comparison of Adaptive filtering Algorithms for Parallel Interference Cancellation in Asynchronous MC DS-CDMA Systems

The Bit Error Rate (BER) comparison of Adaptive Parallel Interference Cancellation (APIC) algorithms in asynchronous Multicarrier Direct Sequence Code Division Multiple Access (MC-DS-CDMA) system is introduced. The APIC is used to eliminate the Multiple Access Interference (MAI) in MC-DS-CDMA system. The Adaptive filtering algorithms considered are, Normalized Least Mean Squares (NLMS), Variable Step Size Normalized Least Mean Squares (VSS-NLMS), and Generalized Normalized Gradient Descent (GNGD) algorithms. These algorithms are used in coincidence with APIC system for updating cancellation weights. The performance comparison between these algorithms is performed via extensive computer simulations. Simulation results demonstrate that the VSSNLMS algorithm outperforms all other algorithms, the NLMS is modular, while the worst performance is obtained by GNGD.

A fractional Fourier transform–based channel estimation algorithm in single-carrier direct sequence code division multiple access underwater acoustic communication system

Due to the complexity and variability of the underwater acoustic channel, the communication signal is affected by multi-path, time delay, and Doppler frequency shift. Based on the advantageous characteristics of fractional Fourier transform on chirp signal processing, a fractional Fourier transform–based algorithm using combined linear frequency–modulated signal is proposed, which can estimate parameters of underwater acoustic channel and has a better performance than the existing methods. To distinguish multi-user in underwater acoustic communication system, a single-carrier direct sequence code division multiple access communication system combined with the fractional Fourier transform–based algorithm is proposed. Thus, a preliminary study on underwater multi-target identification is carried out. The simulation and experimental results show that the fractional Fourier transform–based algorithm is simple and effective, and the energy can be focused at the “best” fractional order, which can directly determine the multi-path number and complete the channel estimation. The proposed single-carrier direct sequence code division multiple access communication system has good performance on bit error rate when we use corresponding spreading code to distinguish multi-user.

Discrete time domain analysis of chaos-based wireless communication systems with imperfect sequence synchronization

Abstract Problem of sequence synchronization in chaos-based direct sequence spread-spectrum (DS-SS) systems and code division multiple access systems (CDMA) has been widely investigated. However, no mathematical expressions have been derived in closed form for the bit error probability in these systems when they operate with the imperfect time synchronization (represented by a delay between the received and reference spreading sequence generated in the receiver). Precise derivatives for this bit error probability are necessary to quantify the effect of imperfect synchronization on the overall properties of the system. To implement a random delay between the received and the receiver reference sequence, all signals in this paper are represented in the discrete time domain. To represent finite and random discrete delays between the sequences (which occur in a limited interval) the Gaussian and uniform probability density functions are expressed in discrete form. Furthermore, due to the finite value of possible random discrete delays, the expressions of related truncated density functions are expressed in closed form. Following this approach, the expressions for the bit error probability in closed form for chaotic and random spreading sequences have been derived.

PN sequence blind estimation in multiuser DS‐CDMA systems with multipath channels based on successive subspace scheme

SummaryIn this paper, we proposed a new method based on expanding subspace algorithm and finite alphabet characteristics, for blind estimation of the users' spreading sequences in the multiuser direct sequence code division multiple access system in the presence of the multipath channels. In the proposed scheme, we show that the estimation of the users' overall channels in the direct sequence code division multiple access system is equivalent to the impulse response estimation of the multi‐input multi‐output finite impulse response channels. Our proposed approach is based on the successive estimation of the columns of the equivalent multi‐input multi‐output finite impulse response channels from the lowest degree columns to the highest degree ones. Accordingly, each user's overall channel that is the convolution of the original multipath channel and the spreading sequence is estimated. Then we extract PN sequences from the overall channel using finite alphabet characteristics of the spreading sequence chips for each user. According to simulation results, our proposed scheme outperforms the conventional methods in that it does not require symbol synchronization and does not have channel constraints (for example, AWGN and single user system) in the multipath channels.

New scheme for underwater acoustically wireless transmission using direct sequence code division multiple access in MIMO systems

This paper proposes a new technique based on Direct Sequence Code Division Multiple Access for underwater acoustically wireless transmission with excessive transmission rate. Environment of subsea is challenging for wireless communication because the medium in which waves are propagating is not air. In fact, it is propagated through fractions of water having different densities. Finding out various techniques for multipath access targeting the physical layer of Acoustic Sensor Networks is indeed necessary. The recent approaches have suggested that coded modulation techniques with exploited diversity are highly preferred in order to enhance the dependability of the acoustic link in different multipath channels. The proposed technique divides the channel into sub ones and transmits information via those sub channels. In variety-spectrum, a signal in a bandwidth is unfold within frequency domain and broad bandwidth. Experimental results show that Bit Error Rate (BER) of this method is better than that of channel equalization in the respective systems.

Efficient MAI Cancellation Scheme in MC-DS-CDMA Using SIC

As of late, the group of multi carrier direct sequence code division multiple access (MC-DS-CDMA) transmission technologies has drawn a considerable measure of consideration in giving broadband wireless communication services. It stands as a promising procedure to help multiple clients with secured high speed wireless communications. In any case, the limit of the MC-DS-CDMA is restricted via multiple access interference (MAI) as of different clients and furthermore it influences the CDMA frameworks performance. To beat such disadvantage, a proficient wireless transmission plot that mitigates the MAI for dual tree complex wavelet transform centered MC-DS-CDMA frameworks is proposed by this given paper. In the proposed conspire, total mean square error equalizer is utilized to presents the initial appraisal of clients’ data and the successive interference cancellation plot is after that utilized to regenerate and call off the MAI from the desired client. In this proposed scheme, MAI is revived; subsequently as of the signal that was received it is subtracted in time domain ahead of equalization. The proposed system successfully maintains a deliberate distance from the MAI and also enhances the limit is revealed in experimental outcomes.

MAI statistics estimation and analysis in a DS-CDMA system

A primary limitation of Direct Sequence Code Division Multiple Access DS-CDMA link performance and system capacity is multiple access interference (MAI). To examine the performance of CDMA systems in the presence of MAI, i.e., in a multiuser environment, several works assumed that the interference can be approximated by a Gaussian random variable. In this paper, we first develop a new and simple approach to characterize the MAI in a multiuser system. In addition to statistically quantifying the MAI power, the paper also proposes a statistical model for both variance and mean of the MAI for synchronous and asynchronous CDMA transmission. We show that the MAI probability density function (PDF) is Gaussian for the equal-received-energy case and validate it by computer simulations.

Explicit Full Correlation Distribution of Sequence Families Using Plateaued Functions

The design of code division multiple access sequence families dates back to the Gold sequences from the 1960s. Since then there has been a number of different such designs with good correlation properties, some optimal and some near-optimal. In this paper, we use the concept of plateaued functions with arbitrary degree, in order to compute their full correlation distributions. First, we give an explicit correlation distribution of a sequence family using a non-quadratic function. Then for the quadratic functions, we present a general classification of “Gold-like” sequence families for all possible characteristics $p$ and degrees $n$ of the Galois field $\mathbb {F}_{p^{n}}$ used to define the sequences. We are able to obtain the full correlation distribution of the families we consider. This paper also uses techniques from the theory of algebraic curves in order to obtain some of the results.