1,721,047 research outputs found
Reduced-rank adaptive multiuser detection in hybrid direct-sequence time-hopping ultrawide bandwidth systems
No full textIn this paper a range of reduced-rank adaptive multiuser detectors (MUDs) are proposed and investigated for the hybrid direct-sequence time-hopping ultrawide bandwidth (DS-TH UWB) systems. The adaptive MUDs are operated based on the recursive least square (RLS) principles. Three types of reduced-rank techniques are investigated, which are the principal component (PC), cross-spectral metric (CSM) and Taylor polynomial approximation (TPA). These reduced-rank adaptive techniques are beneficial to achieving low-complexity, high spectral-efficiency and robust detection in hybrid DS-TH UWB systems. In this contribution bit error rate (BER) performance of the hybrid DS-TH UWB systems using proposed reduced-rank adaptive MUDs is investigated by simulations, when communicating over UWB channels modelled by the Saleh-Valenzuela (S-V) channel model. Our simulation results show that, given a sufficiently high rank of the detection subspace, the reduced-rank adaptive MUDs are capable of achieving a similar BER performance as that of the full-rank ideal minimum mean-square error MUD (MMSE-MUD) but with significantly lower detection complexity. Furthermore, the TPA-based reduced-rank adaptive MUD is capable of yielding a better BER performance than the PC-or CSM-based reduced-rank adaptive MUD, when the same but relatively low rank detection subspace is assumed
Unified error control procedure for global telecommunications systems using redundant residue number system codes
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Improving the Throughput of DS-CDMA Systems Using Adaptive Rate Transmissions Based on Variable Spreading Factors
No full textIn this contribution adaptive rate transmissions are investigated in the context of DS-CDMA systems using variable spreading factors (VSF). In contrast to conventional adaptive rate transmission schemes, where the transmission rate is usually adapted in response to the channel fading, in this contribution the transmission rate is adapted in response to the multiuser interference (MUI) level. We present the philosophy of the proposed adaptive rate transmission scheme and analyze the effective throughput as well as the resultant bit error rate (BER) performance, when communicating over additive white Gaussian noise (AWGN) channels. Our study shows that when the number of active users in a DS-CDMA system can be modelled with the aid of a Markov chain and when the conventional matched filter based receiver is employed, the VSF-assisted adaptive rate transmission scheme is capable of substantially increasing the system’s effective throughput. Specifically, our results show that the effective throughput may be increased by up to 40%, when compared to that of DS-CDMA systems using constant spreading factors. This increased throughput is achieved without wasting power, without imposing extra interference upon other users and without increasing the BER
Space-Time Spreading Assisted Broadband MC DS-CDMA
No full textIn this contribution Multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS) is investigated in the context of broadband communications over frequency-selective Rayleigh fading channels. We consider a range of design issues as well as the achievable Bit Error Rate (BER) performance for the down-link, by assuming synchronous transmission of the user signals. The BER performance of STS assisted broadband MC DS-CDMA using Binary Phase Shift Keying (BPSK) modulation is investigated by simulation for a range of parameter values. Our study shows that by appropriately selecting the system parameters, STS assisted broadband MC DS-CDMA is capable of supporting ubiquitous communications in various communication environments including indoor, open rural, suburban and urban areas without BER performance degradation. Furthermore, the STS based transmit diversity schemes can be designed for attaining a certain required diversity gain, while maintaining a near-constant BER in various communication environments, provided that frequency-selective Rayleigh fading channels are encountered
Performance of Wideband CDMA Using Adaptive Space-Time Spreading over Multipath Nakagami Fading Channels
No full textIn this contribution an adaptive space-time spreading (ASTS) assisted transmit diversity scheme is proposed for improving the throughput of wideband code-division multiple-access (W-CDMA) systems. The performance of the system is investigated, when frequency-selective Nakagami- fading channels, multiuser interference and background noise are considered. Our numerical results demonstrate that the proposed ASTS-based transmission scheme is capable of significantly improving the effective throughput of W-CDMA systems. Specifically, the studied W-CDMA system’s bitrate can be increased by a factor of three at the modest cost of requiring an extra 0.4dB or 1.2dB transmitted power in the context of the investigated urban or suburban areas, respectively
Performance of Distributed-Antenna DS-CDMA Systems Over Composite Lognormal Shadowing and Nakagami-m-Fading Channels
No full textThis paper investigates the uplink performance of a high-capacity direct-sequence code-division multiple-access (DS-CDMA) wireless communication system, where numerous antennas are distributed in the area covered by the system. Following the description of the system, typical characteristics of the distributed-antenna DS-CDMA (DA/DS-CDMA) system are summarized, in comparison with conventional DS-CDMA systems that employ centralized base station (BS) antennas. Then, the bit error rate (BER) performance of the DA/DS-CDMA system without using power control is investigated when the low-complexity correlation and minimum mean square error (MMSE) detectors are employed and when composite transmission pass loss, log-normal shadowing slow fading, and Nakagami-m fast fading are considered. Our analytical and numerical results suggest that the DA/DS-CDMA system constitutes a high-power-efficiency wireless system. For the same set of system parameters, the DA/DS-CDMA system is capable of providing a much higher capacity than the conventional DS-CDMA system. The DA/DS-CDMA system can be free from power control. Furthermore, the hand-over in the DA/DS-CDMA system is also very simple
Uplink Capacity Investigations of TDD/CDMA
No full textIn this contribution we analyze the uplink capacity of a TDD/CDMA system. In TDD/CDMA the mobiles suffer from interferences inflicted by the other mobile stations (MSs) both in the reference cell the MS is roaming in (intracell interference) as we as due to those in the neighbouring cells (intercell interference). Furthermore, in contrast to FDD/CDMA, where the Base Stations (BSs) transmit in an orthogonal frequency band, in TDD/CDMA there is additional interference imposed by other BSs of the adjacent cells. In return for this disadvantage TDD/CDMA guarantees the flexible utilization of all the available bandwidth, which meets the demand for the support of asymmetric uplink and downlink services, such as high speed data rate download in mobile Internet services etc,. This study quantifies the amount of uplink interference inflicted by the MSs and BSs. With the aid of characterising the interference, a detailed discussion of the uplink capacity of TDD/CDMA is provided and two extreme teletraffic loading scenarios of the UTRA TDD/CDMA system are considered. Our numerical results show that the capacity of TDD/CDMA is significantly poorer, than that of FDD/CDMA
Subspace Tracking Based Blind MIMO Transmit Preprocessing
No full textIn this contribution projection approximation subspace tracking using deflation (PASTD) is investigated in the context of MIMO transmit preprocessing systems by exploiting the specific property of Time Division Duplexing (TDD) techniques that the uplink and downlink channels are similar, since they both use the same carrier frequency. Hence the channel estimated from the received signal can also be used for transmit preprocessing. More explicitly, based on the received signal, the PASTD algorithm is used for tracking both the left and the right singular vectors of the MIMO channel matrix, which are required by eigenmode transmissions, instead of periodically reestimating the MIMO channel matrix and performing the singular value decomposition (SVD), which would impose a high computational complexity. A specific deficiency of the family of subspace tracking algorithms is their phase ambiguity imposed by the non-unique nature of the SVD, which is resolved in this treatise by employing differential encoding. The efficiency of the proposed subspace tracking scheme is demonstrated by our performance results, indicating that the advocated technique preforms within 1dB from the BER curve of the perfect channel estimation aided benchmarker
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