Capacity improvement of multicarrier DS-CDMA system with antenna array

An antenna array based base station receiver for multicarrier Direct Sequence Code Division Multiple Access （DS-CDMA） system is proposed. The main advantage of the receiver is that the spatial diversity is achieved by combining signals of array elements. Based on the detailed analysis of multiuser interference and noise characteristics, the performance of the proposed receiver is analyzed. Theoretical analysis shows significant performance improvement in terms of system capacity due to the use of antenna arrays compared with the conventional single antenna multicarrier DS-CDMA approach. Simulation results confirm the theoretical analysis.     

On the performance of multicarrier RAKE systems

In this paper, we integrate the multicarrier signaling technique and the RAKE receiver to design a DS-CDMA system with the capability to increase the data rate, mitigate the effect of correlation among the various subcarriers, and suppress partial-band interference. In the proposed system, a data sequence is serial-to-parallel converted, and multicarrier DS-CDMA is used on each of the parallel data streams. The receiver provides a RAKE for each subcarrier, and the outputs of the RAKEs are combined by a maximal-ratio combiner. We employ a correlated subcarrier and frequency-selective fading channel model to derive the average probability of error of the system and compare the results with those of both a single carrier RAKE system and a multicarrier DS-CDMA system in a frequency selective Rayleigh fading channel with an exponential multipath intensity profile, when multiple access interference and partial-band interference are present     

Rayleigh衰落信道下多载波DS-CDMA系统性能分析

本文基于多载波传输技术，提出了一种多载波码分多址模型。并在Rayleigh衰落信道下对其误码性能进行了分析。分析及仿真结果表明，系统具有降低多址干扰的能力，同传统的单载波直接序列码分多址系统相比，能支持更大的用户容量。本系统能满足第三代移动通信系统对高速数据传输的要求。     

多载波DS-CDMA系统抗干扰能力分析

结合并行转输和多载波调制技术，提出了一种多载波直接序列码分多址模型，对系统在存在窄带干扰的Rayleigh衰落信道下的性能进行了理论分析，并同Rake接收的单载波DS CDMA系统进行了比较。分析和仿真表明：该系统具有较强的抑制窄带及多址干扰的能力，同单载波DS CDMA系统相比，能支持更多的用户数。     

MMSE multiuser detection for array multicarrier DS-CDMA in fading channels

Reception of asynchronous, multicarrier direct-sequence-code division multiple access (DS-CDMA) in time-varying, multipath radio channels with use of a receiving antenna array is investigated. Interference reducing minimum mean squared error (MMSE) receivers are discussed, and by considering the time-variation of the channel, a modified structure is derived which is efficient for channels experiencing small-scale fading. A blind implementation of this receiver is then proposed. Subspace concepts are applied to formulate a tracking, composite channel vector estimator which operates effectively in fading situations, even when high levels of interference are present. Both the modified MMSE weight matrix and diversity combining weights are generated from these channel estimates. Simulations of the proposed receiver show it to have superior performance over a standard MMSE receiver which is periodically re-evaluated to permit it to follow the channel variations due to small-scale fading. Furthermore, a hybrid MMSE receiver is proposed which applies different processing methods depending on each transmitters mobility, resulting in improved performance.     

Capacity analysis of an uplink synchronized multicarrier DS-CDMAsystem

An uplink synchronous transmission scheme has been proposed for DS-CDMA systems to increase the uplink capacity through transmission timing control and a proper code usage. Since the scheme suppresses the interference along the first paths of other users, it can achieve a significant capacity increase when combined with a multicarrier (MC) technique. This article mathematically compares the proposed uplink synchronized MC DS-CDMA system with conventional DS-CDMA and MC DS-CDMA systems in terms of the maximum number of users     

Performance Analysis of Multicarrier DS-CDMA with Antenna Array in a Fading Channel

In this paper, an antenna array based base station receiver structure for multicarrier direct sequence code-division-multiple-access (Multicarrier DS-CDMA) system with binary phase shift keying (BPSK) modulation is proposed. One of the main advantages of the receiver structure is that the spatial diversity is obtained by combining signals at different array elements. Based on the detailed analysis of multiple access interference (MAI) and additive white Gaussian noise (AWGN) characteristics, the uplink bit error rate (BER) performance of the proposed antenna array Multicarrier DS-CDMA is provided. With regard to spatial domain combining, the optimum spatial combining (OSC) and suboptimum spatial combination (SOSC) weights is derived while the suboptimum set of weights is simplified in the sense that only the knowledge of array vector of desired user alone is sufficient for the combining. Simulation results verify the analysis, and it is shown that MAI is mitigated and subsequently the system performance is improved significantly by incorporating antenna array at the base station.     

Convolutionally coded multicarrier DS-CDMA systems in a multipathfading channel. II. Narrow-band interference suppression

For pt.I see ibid., vol.47, p.1570-92 (1999). In Rowitch and Milstein (1999), a convolutionally encoded multicarrier asynchronous direct-sequence code-division multiple-access (DS-CDMA) system was proposed and compared to a classical single-carrier DS-CDMA system employing a RAKE receiver and the same convolutional code, in the presence of additive white Gaussian noise and multiple-access interference. This paper considers the additional impact of various forms of narrow-band interference on the performance of these two systems and the ability of the multicarrier system to effectively suppress the interference using the innate structure of its receiver. At roughly equivalent receiver complexity, results demonstrate superior performance of the multicarrier system in the presence of such interference, without requiring the addition of a front-end interference suppression filter     

Convolutionally coded multicarrier DS-CDMA systems in a multipathfading channel. I. Performance analysis

This paper presents a multicarrier asynchronous direct-sequence code-division multiple-access (DS-CDMA) system wherein the output of a convolutional encoder modulates multiple band-limited DS-CDMA waveforms, which are transmitted in parallel at different carrier frequencies. The receiver detects and combines signals for the desired user and feeds a soft-decision Viterbi decoder. The performance of this system is compared to that of a conventional single-carrier DS-CDMA system with a RAKE receiver, assuming a slowly varying frequency-selective Rayleigh fading channel and assuming the presence of additive white Gaussian noise and multiple-access interference. Results will demonstrate similar performance at roughly equal receiver complexity     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

Performance of orthogonal multicarrier CDMA in a multipath fadingchannel

A new multicarrier direct sequence code-division multiple-access (DS-CDMA) system is proposed. Transmitted data bits are serial to parallel converted to a number of parallel branches. On each branch each bit is direct-sequence spread-spectrum (DS-SS) modulated and transmitted using orthogonal carriers. This procedure provides the following advantages: the transmission bandwidth is more efficiently utilized, the effect of frequency selective multipath interference can be mitigated, and frequency/time diversity is achieved. The system is analyzed with both a conventional matched-filter (MF) receiver and a RAKE receiver for each carrier. The performance is compared to that of the conventional single carrier system with RAKE receiver. It is shown that the multicarrier system is able to out-perform the RAKE receiver when the system parameters are selected properly     

On the use of interference suppression to reduce intermodulationdistortion in multicarrier CDMA systems

This paper presents a coded multicarrier direct-sequence code-division multiple-access (DS-CDMA) system that, by the use of a minimum mean-squared-error receiver, achieves frequency diversity (instead of path diversity as in a conventional single-carrier RAKE DS-CDMA) and has the ability to suppress the intermodulation distortion and partially compensate for the signal distortion introduced by a nonlinear amplifier at the transmitter. A frequency-selective Rayleigh fading channel is decomposed into M frequency-nonselective channels, based on the channel coherence bandwidth. A rate 1/M convolutional code, after being interleaved, is used to modulate M different DS-CDMA waveforms. The new system is shown to effectively combat intermodulation distortion in the presence of multiple-access interference     

Multicarrier CDMA overlay for ultra-wideband communications

In this letter, the performance of multicarrier code-division multiple-access (CDMA) systems is studied in the presence of narrowband interference for future ultra-wideband (UWB) communications. A Nakagami fading channel is assumed, and notch filters along with diversity techniques are used in the multicarrier CDMA receiver. A complete performance analysis of error probability is given. It is shown that when the number of subcarriers jammed by narrowband interference is small, the multicarrier receiver without notch filters can work well, due to the gain of frequency diversity from nonjammed subcarriers. On the other hand, when the number of subcarriers jammed by the narrowband interference is large, using notch filters can improve the multicarrier system performance significantly.     

Coherent and Differential Downlink Space-Time Steering Aided Generalised Multicarrier DS-CDMA

This paper presents a generalised multicarrier direct sequence code division multiple access (MC DS-CDMA) system invoking smart antennas for improving the achievable performance in the downlink. In this contribution, the MC DS-CDMA transmitter employs an antenna array (AA) and steered space-time spreading (SSTS). Furthermore, the proposed system employs both time and frequency (TF) domain spreading for extending the capacity of the system, which is combined with a user-grouping technique for reducing the effects of multi-user interference (MUI). Moreover, to eliminate the high- complexity multiple input multiple output (MIMO) channel estimation required for coherent detection, we also propose a Differential SSTS (DSSTS) scheme. More explicitly, for coherent SSTS detection MVN_r number of channel estimates have to be generated, where M is the number of transmit AAs, V is the number of subcarriers and N_r is the number of receive antennas. This is a challenging task, which renders the low-complexity DSSTS scheme attractive.     

Downlink beamforming for DS-CDMA mobile radio with multimediaservices

Downlink beamforming is a promising technique for direct-sequence code-division multiple-access (DS-CDMA) systems with multimedia services to effectively reduce strong interference induced by high data rate users. A new downlink beamforming technique is proposed that converts the downlink beamforming problem into a virtual uplink one and takes into account the data rate information of all users. Since the main complexity of this method is due to the existence of multidelay paths, two simplified algorithms are suggested using an equivalent one-path channel vector to replace multipath channel vectors. Computer simulation results are given to evaluate the downlink capacity of DS-CDMA systems using a base station antenna array and the new algorithms proposed     

Comparison of multicarrier DS-CDMA broadcast systems in a multipathfading channel

The performance of a multicarrier direct sequence code-division multiple-access (CDMA) system employed in the forward link of a cellular system operating over a Rayleigh fading channel is analyzed and compared to the performance of both single-carrier CDMA and hybrid multicarrier CDMA/frequency division multiplexing systems. A RAKE receiver is provided for each subcarrier. We compare the performance of all three systems for various multipath intensity profiles. It is found that for a service requiring high quality and a small number of users, the multicarrier system is the best, but for a service requiring low quality and a large number of users, the hybrid system can support more users than the others. Also, for the case when nonorthogonal codes are used, the multiple-access interference in different resolvable paths are correlated. In that case, to maximize the signal-to-noise ratio in a correlated interference environment; maximal-ratio combining (MRC) is not optimal. However, we found that there is not much difference between the optimum combining and the conventional MRC     

Zero intermediate frequency receiver for multicarrier mode of cdma2000

This paper proposes a novel zero Intermediate Frequency (IF) receiver for MultiCarrier (MC) Direct-Sequence Code-Division Multiple Access (DS-CDMA) based on multicartier mode of cdma2000 system.The proposed receiver employs direct RF-to-baseband downconversion,sampling at a rate equal to the bandwidth of the baseband multicarrier signal and complex-valued coefficient FIR filters.This receiver avoids the need for arrays of high Q analog bandpass filters,has a much lower sampling rate half of that of conventional receiver and offers significant savings in hardware implementation.Analytic and simulation results show that the presented receiver performs well for both the AWGN channel and Rayleigh fading channel.     

Broadband multicarrier communication receiver based on analog to digital conversion in the frequency domain

This paper introduces a multicarrier communication receiver for broadband applications based on analog to digital conversion (ADC) of the received signal in the frequency domain. The samples of the spectrum of the received signal are used in the digital receiver to estimate the transmitted symbols through a matched filter operation in the discrete frequency domain. The proposed receiver is aimed at the reception of high information rates in a multicarrier signal with very large bandwidth. Thus, the receiver architecture provides a solution to some of the challenging problems found in the implementation of conventional wideband multicarrier receivers based on time-domain ADC, since It efficiently parallelizes the A/D conversion reducing the sampling speed requirements. We show that the sampling rate requirements are relaxed as the number of frequency samples is increased, which introduces a trade-off between complexity and sampling rate. The new receiver possesses additional advantages, including scalability with increasing frequency samples, the possibility of optimally allocating the available number of bits for the ATD conversion across the frequency domain samples which potentially reduces the distortion introduced by the high-speed ADC, narrowband interference suppression that can be directly carried out in the frequency domain, and inherent robustness to frequency offset which makes it an attractive solution when compared with traditional multicarrier receivers. We also investigate how the proposed receiver responds to common multicarrier communication receiver problems such as phase noise and channel frequency selectivity.     

Quasi-synchronous multicarrier DS-CDMA using Z-Complementary sequences

In this paper, a multicarrier DS-CDMA system which employs Z-Complementary Se- quences (ZCS) as spreading code is investigated. The new system can not only eliminate multipath interference and Multiple Access Interference (MAI), but also support flexible number of users com- pared with system using Orthogonal Complementary (OC) codes. Multicarrier DS-CDMA using OC codes can be regarded as a special case of our proposed system. Zero Correlation Zone (ZCZ) of ZCS can be flexibly adjusted to meet the requireme...     

Pilot symbol-assisted coherent multistage interference cancellerusing recursive channel estimation for DS-CDMA mobile radio

A pilot symbol-assisted coherent multistage interference canceller using recursive channel estimation is proposed for DS-CDMA mobile radio cellular system. Since the channel variation caused by fading is recursively estimated at each interference cancelling stage, the accuracy of channel estimation is improved successively. Computer simulation results show that the required E_b/N₀ at the average BER of 3×10^-2 is improved by ~3.5 dB compared to the matched filter receiver for 10 users and two paths with equal power, and where f_dT=10^-3 (f_d: fading maximum Doppler frequency, T: data symbol duration)