Capacity enhancement of band-limited DS-CDMA system using weighteddespreading function

This paper addresses capacity enhancement of a band-limited direct-sequence code-division multiple-access system by using a weighted despreading function (WDF) in the receiver. An ideal Gaussian channel with perfect power control is assumed. The system performance is measured by the signal-to-interference-plus-noise ratio of the decision variable derived in the frequency domain, the bandwidth efficiency factor, the capacity enhancement factor, and the bit-error rate. It is shown that tuning a parameter of the WDF employed helps to partially flatten the in-band cross-spectrum of a pair of spreading and despreading functions. Numerical results show that the capacity of the proposed system improves over the conventional system using rectangular despreading function. To assess practical implications of the WDF receiver, the sensitivity to timing error is also analyzed     

An improved design of chip waveforms for band-limited DS-CDMA systems

This paper introduces an efficient and improved design of chip waveforms to minimize the multiple-access interference in band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. For ease of implementation, the DS-CDMA system employs a time-limited chip waveform, whereas its band limitation is ensured by the low-pass filters at both the transmitter and receiver ends. The design uses sinusoids to synthesize the time-limited chip waveform so that the portion of its spectrum across the specified bandwidth is as flat as possible. It is shown that by using a simple series expansion (with only a few terms) the synthesized chip waveforms significantly outperform the spreading/despreading waveforms previously proposed, particularly for large values of the chip duration-bandwidth product.     

Interference-Resilient Block-Spreading CDMA With Minimum-MAI Sequence Design

Code-division multiple-access (CDMA) schemes based on block spreading implement the spreading of entire data blocks rather than single symbols, thus achieving a higher robustness against the frequency selectivity of the channel and allowing the use of efficient modulation/equalization schemes operating in the frequency domain (FD). In this paper, we present a new block CDMA (B-CDMA) system where a single cyclic prefix (CP) is used at the end of each spread block. This provides a higher spectral efficiency with respect to existing schemes. By observing that complete orthogonality among users is achievable only for half-loaded systems on dispersive channels, we introduce new criteria for the design of spreading and despreading sequences, which aim at minimizing the mean-square error at the output of the despreader. For the equalization of the received signal, we propose an iterative block decision feedback equalizer, which iterates between equalization and decoding. Equalization filters are designed to minimize the mean-square error and take into account the residual interference due to the nonorthogonality of the spreading sequences. The performance of B-CDMA is evaluated in an uplink wireless scenario and compared to existing CDMA schemes.     

Low complexity frequency-domain despreading for cyclic-prefix CDMA systems

In this letter, a frequency-domain despreading method for the cyclic-prefix code-division multiple-access (CP-CDMA) system is introduced. Using the orthogonality transformation property of the discrete Fourier transform (DFT), we can despread the received CP-CDMA signals in the frequency domain. Moreover, we propose an efficient architecture for the proposed frequency-domain despreader. Comparison with the conventional time-domain despreading approach shows that the proposed architecture can save a large amount of computation. The proposed scheme is therefore suitable for efficient implementation of CP-CDMA receivers that adopt frequency-domain equalization and despreading.     

Direct-sequence spread spectrum narrowband interference rejectionusing property restoration

A new method of rejecting narrowband interference in direct-sequence spread spectrum (DS/SS) communications is presented. A typical approach is to reject the interference using a filter with large attenuation at the interference frequencies before despreading. The interference rejection method presented incorporates vector space projection techniques to suppress the correlated interference. Several signal characteristics are formulated which lead to constraint surfaces in the vector space of possible solutions. These constraint surfaces describe interference rejection solutions which introduce minimal distortion to the spread spectrum signal and simultaneously remove the interference. The constraint surfaces essentially correspond to spread spectrum signal estimates which, after interference rejection, conform to known characteristics of the transmitted spread spectrum signal. The formulation of the surfaces relies on prior knowledge about the spread spectrum signal correlation and spectral properties     

基于内积运算和正交矩阵的并行扩频传输新方案

为了提高并行解扩的频谱效率和功率效率,提出了以内积运算为数学理论基础,以线性算子扩频、结合律并行扩频传输、分配律并行解扩为特征的IOR并行扩频传输方案,较经典并行扩频相关解扩和经典并行扩频匹配滤波解扩的频谱效率和功率效率分别提高了2096128倍和1048576倍。在8×randn(1,length(b))干扰下,并行扩频传输64比特,其100000次大样本检验结果表明统计平均误码率和标准差分别为0.0000和0.0007。     

Orthogonalizing matched filtering (OMF) detector for DS-CDMA mobilecommunication systems

This paper proposes a new blind interference suppression scheme for direct-sequence code-division multiple-access (DS-CDMA) mobile communication systems. The proposed detector, the orthogonalizing matched filtering (OMF) detector, consists of a bank of despreading filters and a signal combiner. The received composite signal is first despread by the filters in the bank. The despreading filter outputs are then weighted and combined. The weight vector is adaptively determined and updated so that the average combined signal power is minimized while keeping the combiner's response to the desired signal constant. The proposed algorithm for OMF weight updating requires neither knowledge about other users' spreading sequences nor desired user's signal reference. The similarity between the proposed OMF detector and Applebaum's sidelobe canceller adaptive array is discussed in detail. Results of computer simulations are shown to demonstrate the OMF performance in various mobile communications environments     

Optical code-division multiple access using broad-band parametrically generated light

A novel approach for an optical direct-sequence spread spectrum is presented. It is based on the complementary processes of broad-band parametric down-conversion and up-conversion. With parametric down-conversion, a narrow-band continuous-wave (CW) optical field is transformed into two CW broad-band white-noise fields that are complex conjugates of each other. These noise fields are exploited as the key and conjugate key in optical direct-sequence spread spectrum. The inverse process of parametric up-conversion is then used for multiplying the key by the conjugate key at the receiver in order to extract the transmitted data. A complete scheme for optical code-division multiple access (OCDMA) based on this approach is presented. The salient feature of the approach presented in this paper is that an ideal white-noise key is automatically generated, leading to high-capacity versatile code-division multiple-access configurations.     

Performance evaluation of a generalized linear SIC for DS/CDMA signals

The linear successive interference canceler (LSIC) is a multiuser detector that separates code-division multiple-access (CDMA) signals in a multistage manner. In each stage, a user is detected, and its contribution is regenerated and canceled from the input of that stage. A user's spreading sequence is employed for despreading and respreading, and the magnitude of the despreader output is used as the amplitude estimate to reconstruct that user's signal. This paper describes a generalized version of the LSIC (GLSIC) that employs various types of linear filters for the despreading and respreading operations. We analyze the bit error rate (BER), asymptotic multiuser efficiency (AME), and the mean and variance of the amplitude estimates of the GLSIC. It is shown that for certain choices of linear filters, complete cancellation of a user can be achieved, irrespective of the reliability of the symbol estimates. We also demonstrate that, from a BER and AME viewpoint, it is not beneficial to use a linear canceler with a decorrelator or a minimum mean-square-error (MMSE) receiver.     

A DS-CDMA system using despreading sequences weighted by adjustablechip waveforms

This paper evaluates the performance of a direct-sequence code-division multiple-access system using coherent receivers in which the despreading sequences are weighted by adjustable chip waveforms. The chip weighting waveforms under consideration are designed for multiple-access interference (MAI) rejection. Assuming that the received chip waveforms are rectangular, new expressions for the signal-to-interference-plus-noise ratio (SINR) of the decision variable are derived when different weighted despreading sequences (WDSs) are used in the receiver. The novelty of the derived expressions is that each of the expressions, when the system parameters are given, is determined only by one parameter of the adjustable chip waveforms employed. As a result, we can simply tune the parameter to its optimal value in real-time for MAI rejection without knowing the other users' spreading codes, timing, and phase. The criterion for tuning the parameter is to maximize the SINR of the decision variable based on the relative strength between the additive Gaussian white noise and the MAI. Numerical results show that when the multiple-access interference is significant, the receivers using WDSs outperform significantly the conventional receiver using a rectangular despreading sequence. Brief analysis for bandlimited spreading signals is also provided to reveal the practical implications of the proposed technique     

Interference suppression for DS/CDMA

A direct-sequence spread spectrum (DS/SS) receiver for suppressing multiple-access interference in direct-sequence code-division multiple-access (DS/CDMA) communication systems is introduced. The proposed receiver does not require knowledge of other users' spreading codes, timing, or phase information. Moreover, the receiver allows the number of taps to be chosen independently of the processing gain and, hence, is easily applicable to CDMA systems employing either a small or a large processing gain. Performance analysis, including average probability of error and signal-to-noise ratio, is provided, and results are presented for systems varying from lightly loaded (for example, eight user/255 chip) to heavily loaded (for example, 50 user/200 chip). Performance results indicate that the proposed receiver outperforms the linear correlation receiver and, in many cases, it does so by a considerable margin     

Experimental spreading and despreading of the optical spectrum

An experimental realization of direct-sequence spreading and despreading of the optical spectrum using off-the-shelf electrooptic modulators is reported. This capability is required to implement a fiber-optic network originally devised by G. Foschini and G. Vannucci (1983). The network utilizes spread-spectrum techniques on the optical spectrum, together with coherent signal detection to accommodate a very large number of simultaneous users, for a total throughput measured in hundreds of Gb/s. The results obtained provide confidence that a network built around this idea would perform according to theoretical calculations. Coherent spectral spreading and despreading is thus established as a promising communication technique for fiber-optic systems     

Dual-phase continuous phase modulation for spread-spectrum multiple-access communication

A new class of direct-sequence spread-spectrum multiple-access (DS-SSMA) systems with continuous phase modulation (CPM) is defined. The signals are unique in that the spreading is done by adding an extra phase term to the information phase while maintaining phase continuity, constant envelope, and efficient bandwidth usage. The spreading phase is formed from the spreading code and is independent of the information phase, which allows despreading to be done separately before data detection, which, in turn, allows a simple CPM detector to be employed for data detection. The information phase is considered in the minimum-shift keying (MSK) format, and a serial-MSK-type spread-spectrum receiver is considered for performance analysis. Expressions for the signal-to-noise ratio, the power spectral density, and the probability of bit error are developed, along with methods for computing their values to an arbitrarily close approximation. Numerical results show that the proposed system is an attractive alternative to the conventional DS-SSMA systems.     

Chip waveform design for DS/SSMA systems with aperiodic randomspreading sequences

The dependence of the error performance and spectral efficiency of direct-sequence spread-spectrum multiple-access (DS/SSMA) systems with matched filter receivers on the chip waveform is examined. The actual shape of the chip waveform, as well as its energy, is found to influence the statistical properties of the multiple-access interference (MAI). An approach to design waveforms that may result in interchip interference (ICI) is proposed and a criterion for design based on the conditional Gaussian approximation of the MAI for systems with aperiodic random spreading sequences is derived. For a simplified system, a closed-form solution for optimal band-limited waveforms is obtained for excess bandwidth less than or equal to one by using a performance metric that includes the effect of ICI. Numerical results, based on an analytical method, as well as Monte Carlo simulations, are provided to evaluate the performance of the proposed waveforms in general systems with conventional matched filter receivers     

Linear MMSE Space–Time Equalizer for MIMO Multicode CDMA Systems

We propose an enhanced chip-level linear space–time (ST) equalizer for multiple-input multiple-output (MIMO) multicode code-division multiple-access (CDMA) systems. In the MIMO multicode CDMA systems, the reuse of the same spreading codes in different transmit antennas significantly degrades the equalization performance if the ST equalizer uses a minimum mean-squared error (MMSE) weighting vector that minimizes the mean-squared error of the equalizer output chip sequence. As the CDMA despreader concatenated to an ST equalizer distorts interstream interference components differently from multipath interference and background noise components, the chip-level MMSE weighting vector usually steers in suboptimal directions in the signal space. Therefore, we propose a new MMSE weighting vector that takes the despreading effect into account in this paper. Simulation results show a substantial performance improvement through the new weighting vector.     

Quadriphase DS-CDMA with pulse shaping and the accuracy of the Gaussian approximation for matched filter receiver performance analysis

Probability of bit-error (P/sub e/) performance of asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems is analyzed. In particular, the effects of pulse shaping, quadriphase (or direct-sequence quadriphase shift keying (DS-QPSK)) spreading, aperiodic spreading sequences and the coherent correlator or, equivalently, the matched filter (MF) receiver are considered. An exact P/sub e/ expression and several approximations: one using the characteristic function (CF) method, a simplified expression for the improved Gaussian approximation (IGA) and the simplified improved Gaussian approximation (SIGA) are derived. Two main results are presented. Under conditions typically satisfied in practice and even with a small number of interferers, the standard Gaussian approximation (SGA) for the multiple-access interference component of the MF statistic and for MF P/sub e/ performance is shown to be accurate. Moreover, the IGA is shown to reduce to the SGA for pulses with zero excess bandwidth. Second, the P/sub e/ performance of quadriphase DS-CDMA is shown to be superior or equivalent to that of biphase DS-CDMA. Numerical examples with Monte Carlo simulation are presented to illustrate P/sub e/ performance for square-root raised-cosine (Sqrt-RC) pulses and spreading factors of moderate to large values.     

Spread-time code-division multiple access

An alternative code-division multiple-access (CDMA) scheme to spread spectrum (SS), called spread time (ST) is proposed for bandlimited multiple-access channels. ST-CDMA can be considered the time-frequency dual of SS-CDMA. In ST-CDMA pseudorandom (PN) sequences are assigned to each user, and the Fourier transform of the transmitted pulse for a given user is determined by modulating the phase of the desired transmitted spectrum by the user's PN-sequence. The transmitted data for a particular user can be recovered by sampling the output of a filter matched to the user's pulse. Implementations are described in which surface acoustic wave devices are used to perform the matched filtering or Fourier transformation. Averaged signal-to-interference plus noise ratio (SIR) and spectral efficiency are computed for both asynchronous ST and direct-sequence SS-CDMA systems, assuming an arbitrary channel transfer function H(f), which is the same between all pairs of users. The results are the same for SS and ST provided that the magnitude of the Fourier transform of the chip shape in the SS system is the same as the magnitude of the Fourier transform of the ST pulse shape. The main advantage of the ST technique Is the flexibility with which the transmitted spectrum can be selected. We derive the transmitted spectrum that maximizes the SIR subject to an average power constraint     

Comparison of two interleaving techniques for coded hybridDS/SFH-SSMA

The author presents a performance comparison of two interleaving techniques for coded hybrid direct-sequence/slow frequency-hopped spread spectrum multiple-access (DS/SFH-SSMA) systems with noncoherent DPSK demodulation, employing predetection diversity (equal gain combining (EGC)) over indoor radio channels     

High-Rate Direct-Sequence Spread Spectrum With Error-Control Coding

High-rate direct-sequence (DS) spread spectrum is a modulation technique in which most or all of the spreading is provided by nonbinary data modulation. For applications to mobile ad hoc wireless networks, the limited processing gain of high-rate DS spread spectrum gives only modest protection against multiple-access or multipath interference, which limits the applicability of the modulation technique to fairly benign channels. In this paper, we explore the increased interference-rejection capability that can be obtained from convolutional coding with Viterbi decoding, Reed–Solomon coding with errors-and-erasures decoding, and block product coding with iterative decoding. For channels with multiple-access or multipath interference, performance results are given for several soft-decision decoding metrics, the benefits of adaptive-rate coding are illustrated, and the accuracy and utility of the Gaussian approximation are described. We also show how to use the bit-error probability for a system without error-control coding to determine which modulation method will give the best packet-error probability in a system with error-control coding.     

Rate-Distortion Optimized Video Transmission Over DS-CDMA Channels with Auxiliary Vector Filter Single-User Multirate Detection

In this paper, we consider the rate-distortion optimized resource allocation for video transmission over multi-rate wireless direct-sequence code-division-multiple-access (DS-CDMA) channels. We consider the performance of transmitting scalable video over a multipath Rayleigh fading channel via a combination of multi-code multirate CDMA and variable sequence length multirate CDMA channel system. At the receiver, despreading is done using adaptive space-time auxiliary-vector (AV) filters. We propose a new interference cancelling design that uses just a single AV filter for single-user mutirate despreading. Our experimental results show that the proposed interference cancelling design has excellent performance in scalable video transmission over DS-CDMA systems that use a combination of multicode multirate and variable processing gain multirate CDMA. The proposed design takes advantage of the fact that single user's video data is transmitted using two spreading codes, one for the base layer and one for the enhancement layers, and of the fact that these spreading codes can have different processing gains. The proposed interference cancelling design is compared with two conventional single-user multirate CDMA receiver configurations, however now we use an AV filter rather than a simple matched filter. We also propose a resource allocation algorithm for the optimal determination of source coding rate, channel coding rate and processing gain for each scalable layer, in order to minimize the expected distortion at the receiver.