Multiple FM/FDM Carriers Through Nonlinear Amplifiers

For long haul transmissions using frequency-division multiple-access methods, several frequency-modulated carriers are transmitted over a common communications channel consisting of saturating amplifiers operated near saturation to maximize the system power efficiency. One of the problems caused by saturating high power amplifiers is intermodulation distortion, which is the subject of this paper. A simple computation method is introduced which is potentially useful in calculating intermodulation distortion effects of FDM/FM equal power carriers. Based on the results of the study, new and simple empirical expressions for the amplitudes of the single third-order intermodulation products are then presented and compared to previous theoretical and experimental results.     

Spectral efficiency of direct-sequence code-division multiple-access in cellular mobile radio

In this paper, the spectral efficiency achieved by a direct-sequence code-division multiple-access (DS-CDMA) system operating in a cellular mobile radio environment is discussed. In contrast to previous works, here, spectral efficiency is examined in terms of channel capacity per user, estimated in an average sense, and the system's inherent diversity potential is considered. The analysis covers only the forward link, assuming a static model of operation for the considered DS-CDMA system. In addition, it is shown that the application of a power control scheme leads to spectral efficiency enhancement due to cochannel interference reduction. Hence, the derived expressions provide a means for quantitative evaluation of cellular DS-CDMA spectral efficiency with respect to multiple-access and cochannel interference. © 1997 John Wiley & Sons, Ltd.     

Long codes for generalized FH-OFDMA through unknown multipathchannels

A generalized frequency-hopping (GFH) orthogonal frequency-division multiple-access (OFDMA) system is developed in this paper as a structured long code direct-sequence code-division multiple-access (DS-CDMA) system in order to bridge frequency-hopped multicarrier transmissions with long code DS-CDMA. Through judicious code design, multiuser interference is eliminated deterministically in the presence of unknown frequency-selective multipath channels. Thanks to frequency-hopping, no single user suffers from consistent fading effects and constellation-irrespective channel identifiability is guaranteed regardless of channel nulls. A host of blind channel estimation algorithms are developed trading off complexity with performance. Two important variants, corresponding to slow- and fast-hopping, are also addressed with the latter offering symbol recovery guarantees. Performance analysis and simulation results illustrate the merits of GFH-OFDMA relative to conventional OFDMA and long code DS-CDMA with pseudorandom noise codes and RAKE reception     

Multistage decision-directed channel estimation scheme for DS-CDMAsystem with M-ary orthogonal signaling

A multistage decision-directed channel estimation scheme is proposed for a coherent direct-sequence code-division multiple-access (DS-CDMA) system with M-ary orthogonal signaling. The receiver with a multistage decision-directed channel estimator makes alternate use of symbol detection and channel estimation in an iterative manner, resulting in a multistage structure where each stage consists of an estimator and a detector. The estimator obtains the estimates of the channel by using the decisions from the detector in the previous stage, and the detector uses these estimates for coherent detection of the transmitted symbols. Modifications to the process of generating the channel estimates and the decision variables are made to mitigate the error propagation in successive stages. It is shown that the performance of the proposed estimation scheme is better than the conventional scheme, especially for channels with high diversity order. The gain is mostly achieved by two stages for channels with low diversity order and by five stages for channels with high diversity order. The proposed scheme can be used to increase the capacity of DS-CDMA systems currently operating in such environments     

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     

Multicarrier DS/SFH-CDMA systems

Multicarrier direct-sequence/slow-frequency-hopping (MC DS/SFH) code-division multiple-access (CDMA) systems are proposed, in which multiple carriers are modulated by the same DS waveform and hopped in frequency according to a random hopping pattern. The receiver dehops the received signal with the same pattern, provides RAKE receivers for each carrier, and combines the outputs with a maximal ratio combiner (MRC). The performance of the proposed system is investigated over a frequency-selective Rayleigh-fading channel and compared to that of the MC DS-CDMA systems. It is shown that for the same diversity order, the MC DS/SFH-CDMA systems are superior in reducing multiple-access interference (MAI) while preserving the good capability of narrowband interference suppression, when the system parameters are selected properly.     

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。     

Improved channel estimation and SIR measurement in WCDMA downlink systems

The rake receiver performance of a direct-sequence code-division multiple-access (DS-CDMA) system can significantly be degraded unless the channel condition is properly estimated. Since the channel condition is time-varying and location-dependent, it is desirable to employ an adaptive channel estimator (ACE) that can properly adjust the bandwidth of the channel estimation filter (CEF) according to the channel condition such as the maximum Doppler frequency. To this end, we propose low-complexity ACE and signal-to-interference power ratio (SIR) estimator for wide-band CDMA downlink systems with and without the use of transmit antenna diversity. The proposed schemes adjust the bandwidth of the CEF by exploiting the correlation characteristics of the received pilot signal. The analytic design is verified by computer simulation. Simulation results show that the proposed ACE and SIR estimator can significantly improve the link performance for a wide range of channel condition, particularly when the channel condition is poor.     

Analysis and optimization of pilot-channel-assisted BPSK forDS-CDMA systems

Pilot-channel-assisted binary phase-shift keying (PCA-BPSK) in direct-sequence code-division multiple-access (DS-CDMA) systems is investigated. The loss due to imperfect channel estimation is calculated, and the pilot-to-data channel power ratio is optimized analytically. The results show the effects of the various parameters and provide a suitable tool for system design     

采用QPSK调制的导频辅助CDMA系统的分析和优化

该文分析了有导频辅助的采用QPSK调制方式的直扩(DS-CDMA)系统,计算出由于信道估测的不精确而造成的系统性能的损失,并由此得到了数据信道功率和导频信道功率的最佳比及相应的系统误码率。最后给出仿真结果。     

Precise analysis of bit-error probability for asynchronous multicode DS-CDMA systems

A precise bit-error probability (BEP) analysis method is derived for a multicode direct-sequence code-division multiple-access (DS-CDMA) system in an additive white Gaussian noise channel. The method is applicable to a multicode DS-CDMA system with an arbitrary number of multiple code sequences, and any selection of multiple code sequences. The precise method gives results that discriminate the effect of the selection of different multiple code sequences on the BEP, whereas Gaussian approximations (GAs) do not. Thus, the new method can be used to select the best multicode set for a given system, a task that cannot be achieved using GAs. A two-step analytical procedure enables deriving an explicit, compact form for the CF of the receiver decision statistic in a DS-CDMA system with an arbitrary number of multiple code sequences, and for any selection of multiple code sequences.     

On the analytical evaluation of closed-loop power-control errorstatistics in DS-CDMA cellular systems

This paper proposes an analytical study that aims at evaluating the power-control error statistics in wireless direct-sequence code-division multiple-access (DS-CDMA) cellular systems based on an ideal variable step closed-loop power-control scheme. In particular, the cumulative distribution function and the correlation coefficient of the power-control error are derived through a first-order Taylor expansion of the received signal envelope. A novel power-control scheme that exploits the autocorrelation properties of the fading is also proposed, and its performance is analyzed in terms of power-control error statistics. Rayleigh and Rice frequency-selective channel models, which involve the use of a diversity RAKE receiver at the base station, have been taken into account. The proposed analytical approach specifically applies to CDMA systems. A method that aims at estimating the capacity of a DS-CDMA cellular network is also given     

Performance analysis of coherent DS-CDMA systems in a Nakagamifading channel with arbitrary parameters

A new expression for the bit error rate (BER) of an asynchronous direct-sequence code-division multiple-access (DS-CDMA) system with coherent BPSK demodulation in a multipath Nakagami (1960) fading channel is derived. The analysis assumes an arbitrary number of independent, but nonidentical resolvable paths combined by the RAKE receiver. The results obtained show that in such systems, the effect of nonidentical fading on system performance cannot be ignored     

Performance comparison of transmit diversity and beamforming for the downlink of DS-CDMA system

A theoretical analysis has been made to compare the capacity performances of transmit diversity and beamforming in the downlink of direct-sequence code-division multiple-access (DS-CDMA) system. The performances of selection transmit diversity (STD), beamforming (BF), and beam selection transmit diversity (BSTD), which is a hybrid scheme of STD and BF, have been studied at system level with capacity as the performance index. Both fading and interference are jointly considered in multipath propagation environment. It was found that the comparison result strongly relates to the availability of path diversity. The system capacity is maximized by the BF in the presence of path diversity, otherwise it is the BSTD that achieves the best overall capacity performance.     

Received signal statistics in DS-CDMA channels with flat Rayleigh fading and fast closed-loop power control

The statistics of received signal power at the receiver in a cellular direct-sequence code-division multiple-access (DS-CDMA) system with flat Rayleigh fading and fast closed-loop power control mechanism are studied. A simple model for the fast closed-loop power control mechanism is proposed to ease the analysis. The probability density function (pdf) of the received signal power is derived by taking into consideration the updating frequency of power control mechanism, maximum allowable transmitted power, and Doppler frequency of the fading channel. The pdf derived can be used to study the performance of a reverse-link DS-CDMA system with noncoherent M-ary orthogonal modulation. The result shows that when optimum maximum allowable transmitted power is used, system capacity can be substantially increased for typical normalized Doppler frequency in cellular communication.     

Performance of broadband multicarrier DS-CDMA using space-time spreading-assisted transmit diversity

In this contribution multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS)-assisted transmit diversity is investigated in the context of broadband communications over frequency-selective Rayleigh-fading channels. We consider the issue of parameter design for the sake of achieving high-efficiency communications in various dispersive environments. Furthermore, in contrast to conventional MC DS-CDMA schemes employing time (T)-domain spreading only, in this contribution we also investigate broadband MC DS-CDMA schemes employing both T-domain and frequency (F)-domain spreading, i.e., employing TF-domain spreading. The bit-error rate (BER) performance of STS-assisted broadband MC DS-CDMA is investigated for downlink transmissions associated with the correlation based single-user detector and the decorrelating multiuser detector. Our study demonstrated that when appropriately selecting the system parameters, broadband MC DS-CDMA using STS-assisted transmit diversity constitutes a promising downlink transmission scheme. This scheme is capable of supporting ubiquitous communications over diverse communication environments without BER performance degradation.     

Efficient performance analysis of asynchronous cellular CDMA overRayleigh-fading channels

The bit error rate (BER) analysis of a direct-sequence code-division multiple-access (DS-CDMA) cellular system over a Rayleigh-fading channel often results in complicated expressions even though the Gaussian approximation is applied. A combined probability density function (pdf) approach for the forward link and a mean-method technique for the reverse link are proposed to significantly reduce the computational complexity. The simplified BER expressions are derived and yield accurate results     

Performance of the decorrelator receiver for DS-CDMA mobile radio system employing RAKE and diversity through Nakagami fading channel

Multiple-access interference (MAI) and multipath fading are two of the most significant factors limiting the capacity and performance of direct-sequence code-division multiple-access (DS-CDMA) systems. In this paper, synchronous multiuser receivers that combine antenna diversity, RAKE reception, and a multipath decorrelator for MAI cancellation are analyzed in a Nakagami faded environment using a maximal ratio combiner or a selection combiner. A coherent binary phase-shift keying employing DS-CDMA is considered. Arbitrary branch correlation is also considered for any diversity order in the case of identical severity fading on the branches.     

Performance of DS-CDMA Downlink Systems With Orthogonal UCHT Complex Sequences

This letter investigates a transmitted signaling technique using orthogonal unified complex Hadamard transform (UCHT) spreading sequences and the coherent RAKE receiver in direct-sequence code-division multiple-access (DS-CDMA) downlinks to maintain the orthogonality between users and reduce the effect of multipath fading and interference from other users. A general multipath-fading channel model is assumed. System performance is evaluated by means of signal-to-interference-plus-noise ratio (SINR) at the RAKE receiver. It is shown that the SINR of the system employing UCHT complex sequences is independent of the phase offsets between different paths, while the SINR of the system using Walsh-Hadamard (WH) sequences is related to the squared cosine of path phase offsets. As a result, the bit-error ratio performance of the DS-CDMA downlink system employing UCHT complex sequences is better than that of the system with WH sequences at high SINRs     

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。