22,937 research outputs found
Broadband MC DS-CDMA Using Space-Time and Frequency-Domain Spreading
No full textIn this contribution multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS) assisted transmit diversity and frequency-domain (F-domain) spreading is investigated in the context of broadband communications over frequency-selective Rayleigh fading channels. We consider the attainable capacity extension of broadband MC DS-CDMA with the advent of using Time-Frequency-domain (TF-domain) spreading. The BER performance of STS assisted broadband MC DS-CDMA using Binary Phase Shit Keying (BPSK) modulation and TF-domain spreading is investigated by simulation for a range of parameter values. Both the correlation based single-user detector and the decorrelating multiuser detector are considered. Our study shows that the number of users supported by the broadband MC DS-CDMA system is determined by the product of the T-domain spreading factor and the F-domain spreading factor, while it is independent of the frequency diversity order. Furthermore, when multiuser detection assisted F-domain spreading is considered, the broadband MC DS-CDMA system is capable of supporting a substantially increased number of users, while maintaining a similar bit error ratio (BER) performance to that of the broadband MC DS-CDMA system using no F-domain spreading
Ant-Colony-Based Multiuser Detection for MC DS-CDMA Systems
Full text linkIn this contribution we present a novel ant colony optimization (ACO) based multi-user detector (MUD) designed for synchronous multi-carrier direct sequence code division multiple access (MC DSCDMA) systems. The operation of the ACO-based MUD is based on the behaviour of the ant colony in nature. The ACO-based MUD aims for achieving the same bit-error-rate (BER) performance as the optimum maximum likelihood (ML) MUD, without carrying out an exhaustive search of the entire MC DS-CDMA search space constituted by all possible combinations of the received multi-user vectors. We will demonstrate that the system is capable of supporting almost as many users as the number of chips in the spreading sequence, while searching only a small fraction of the entire ML search space. It will also be demonstrated that the number of floating point operations per second is a factor of 108 lower for the proposed ACO-based MUD than that of the ML MUD, when supporting K = 32 users in a MC DS-CDMA system employing 31-chip Gold codes as the T-domain spreading sequence
Ant-colony-based multiuser detection for multifunctional-antenna-array-assisted MC DS-CDMA systems
No full textA novel Ant Colony Optimization (ACO) based Multi-User Detector (MUD) is designed for the synchronous Multi-Functional Antenna Array (MFAA) assisted Multi-Carrier Direct-Sequence Code-Division Multiple-Access (MC DS-CDMA) uplink (UL), which supports both receiver diversity and receiver beamforming. The ACO-based MUD aims for achieving a bit-error-rate (BER) performance approaching that of the optimum maximum likelihood (ML) MUD, without carrying out an exhaustive search of the entire MC DS-CDMA search space constituted by all possible combinations of the received multi-user vectors. We will demonstrate that regardless of the number of the subcarriers or of the MFAA configuration, the system employing the proposed ACO based MUD is capable of supporting 32 users with the aid of 31-chip Gold codes used as the T-domain spreading sequence without any significant performance degradation compared to the single-user system. As a further benefit, the number of floating point operations per second (FLOPS) imposed by the proposed ACO-based MUD is a factor of 108 lower than that of the ML MUD. We will also show that at a given increase of the complexity, the MFAA will allow the ACO based MUD to achieve a higher SNR gain than the Single-Input Single-Output (SISO) MC DS-CDMA system. Index Terms—Ant Colony Optimization, Multi-User Detector, Multi-Functional Antenna Array, Multi-Carrier Direct-Sequence Code-Division Multiple-Access, Uplink, Near-Maximum Likelihood Detection
Spectral-Efficiency of TDD Multiuser Two-Hop MC-CDMA Systems Employing Egocentric-Altruistic Relay Optimization
Full text linkIn this contribution we investigate the spectral-efficiency of a two-hop cooperative network using multicarrier code-division multiple-access (MC-CDMA) transmission scheme. The two-hop network constitutes K source users transmitting signals to K destinations with the aid of N relays. Our focus is on the relay optimization, when assuming that the N relays cooperate or do not cooperate with each other. Specifically, in this contribution the egocentric-altruistic (E-A) optimization is introduced, which constitutes an E-optimization motivating to suppress the multiuser interference (MUI) of the source-relay channels and an A-optimization aiming to pre-mitigate the potential MUI of the relay-destination channels. Both the minimum mean-square error (MMSE) and zero-forcing (ZF) optimization criteria are considered. Furthermore, the spectral-efficiency performance of the two-hop MC-CDMA systems using the proposed E-A relay optimization is investigated by simulations, when assuming communications over frequency-selective fading channels
Near-Optimum Iterative Multiuser Detection in Time-Frequency-Domain Spread Multicarrier DS-CDMA Systems
Full text linkIn this contribution we propose and study a novel iterative time-frequency-domain (TF-domain) multiuser detector (MUD) for the multicarrier direct-sequence code-division multiple-access system employing both T-domain and F-domain spreading, which, for brevity, is referred to as the TF/MC DS-CDMA system. The proposed iterative TF-domain MUD consists of a set of T-domain soft-input soft-output MUDs (SISO-MUDs) and a set of F-domain SISO-MUDs, which exchange information through two multiuser interference (MUI) cancellation units. Both the T-domain and F-domain SISO-MUDs are operated under the maximum {\em a-posteriori} (MAP) principles. In this contribution the complexity and bit error rate (BER) performance of the TF/MC DS-CDMA employing the proposed iterative TF-domain MUD are investigated and also compared with the other existing MUD schemes, including the optimum MUD and the joint/separate minimum mean-square error (MMSE) MUDs. Our study shows that the iterative TF-domain MUD is capable of achieving nearly the same BER performance as the optimum MUD, but at much lower complexity
Performance Comparison of FH/MC DS-CDMA with Single- and Multi-Carrier DS-CDMA
No full textIn this contribution we review the slow frequency hopping multicarrier direct-sequence code-division multiple-access (SFH/MC DS-CDMA). Then, the system's performance is ecalated and compared to that of the conventional single-carrier (SC) DS-CDMA and MC DS-CDMA under the assumptions of constant system bandwidth and of constant transmitted signal power. Both random and 'uni$ormlFH are considered and their advantages as well as disadvantages are investigated. We assume that the system operates in a multipath Nakagami-m fading environment and a maximum ratio combining (URC) assisted RAKE receiver is used for demodulation
Recital on Multicarrier Communications: Space-Time Coded versus Adaptive OFDM/MC-CDMA
No full textThis overview portrays the evolution of multi-carrier systems from their conception and demonstrates that they are capable of supporting the requirements of future wireless systems of the next generation. The a novel multiple access scheme based on slow frequency hopping multicarrier direct-sequence code-division multiple-access (SFHIMC DS-CDMA) is highlighted, which can be rendered compatible with the existing 2nd-generation narrow-band CDMA, and 3rd-generation wideband CDMA mobile communication systems. A RAKE receiver structure with maximum ratio combining is considered and its performance is evaluated over the range of Nakagami multipath fading channels, under the assumption that the receiver has an explicit knowledge of the associated frequency-hopping (FH) patterns invoked. It is argued furthermore that symbol-by-symbol adaptive Orthogonal Frequency Division Multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput in comparison to their fixed-mode counterparts. By contrast, various diversity techniques, such as Rake receivers and space-time coding, mitigate the channel quality variations in their effort to obtain a reduced BER. This paper investigates a combined system constituted by a constant-power adaptive modem employing spacetime coded diversity techniques in the context of both OFDM and MC-CDMA. The combined system is configured to produce a constant uncoded BER and exhibits virtually error free performance, when a turbo convolutional code is concatenated with a space-time block code. It was found that the advantage of the adaptive modem erodes, as the overall diversity-order increases
Multi-Functional Antenna Array Assisted MC DS-CDMA Using Downlink Preprocessing Based on Singular Value Decomposition
No full textIn this contribution we propose and investigate a transmitter preprocessing scheme designed for downlink transmission in multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) systems using multiple base-station antenna arrays, where each antenna array employs multiple array elements. The transmitter preprocessing scheme is derived based on the singular value decomposition (SVD). Our transmitter preprocessing design is capable of supporting a high number of MC DS-CDMA users, while maintaining a high diversity gain at a low detection complexity at the remote mobile stations (MSs). The characteristics of MC DS-CDMA using the proposed transmitter preprocessing scheme are discussed and the achievable bit-error-rate (BER) performance is investigated, when assuming that each subcarrier experiences flat Rayleigh fading. Our simulation results demonstrate that for a SVD-assisted MC DS-CDMA system using M distinct transmit antenna arrays and time (T)-domain spreading sequences of length Ne chips, the number of users supported can be as high as MNe, since Ne and M number of users may be distinguished in the T-domain and spatial-or S-domain, respectively. Furthermore, the SVD-assisted MC DSCDMA system is capable of supporting M ?Ne number of users at a near single-user BER performance
Partial Equalization for MC–CDMA Systems in Non-Ideally Estimated Correlated Fading
No full textMulticarrier code-division multiple access (MC–CDMA) can support high data rates in next-generation multiuser wireless communication systems. Partial equalization (PE) is a low-complexity technique for combining the signals of subcarriers to improve the achievable performance of MC–CDMA systems in terms of their bit error probability (BEP) and bit error outage (BEO) in comparison with maximal ratio combining, orthogonality restoring combining, and equal-gain combining techniques. We analyze the performance of the multiuser MC–CDMA downlink and derive the optimal PE parameter expression, which minimizes the BEP. Realistic imperfect channel estimation and frequency-domain (FD) block-fading channels are considered. More explicitly, the analytical expression of the optimum PE parameter is derived as a function of the number of subcarriers, number of active users (i.e., the system load), mean signal-to-noise ratio (SNR), and variance of the channel-estimation errors for the aforementioned FD block-fading channel. We show that the choice of the optimal PE technique significantly increases the achievable system load for the given target BEP and BEO
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