莱斯衰落信道中混合FDMA/CDMA性能分析

分析混合频分多址／码分多址系统在莱斯多径衰落信道的性能,并与占据相同带宽的宽带ＣＤＭＡ系统进行比较。系统采用二进制ＤＰＳＫ调制和瑞克接收机。结果表明,直达 径分量较强或信噪比较大时,混合系统的性能优于宽带ＣＤＭＡ系统,否则,宽带ＣＤＭＡ系统最优。     

改进的移动音频带宽扩展算法

由于3G移动信道环境严酷,带宽资源有限,为了实现低码率、高质量的音频信号传 输和重建,在研究现有语音/音频带宽扩展技术的基础上,提出了一种基于EAAC+编码标准的 改进移动音频编码算法。该算法通过检测并去除短暂态信号带来的正弦分量,从而降低计算 的复杂度,实现传输信道带宽的扩展,并保证重建音频信号的音质。     

Blind compensation of nonlinear distortion for bandlimited signals

Nonlinear distortion of bandlimited signals results in spectral spreading. This paper develops a blind nonlinear compensation method for bandlimited signals by suppressing the spectral content of the distorted signal above the original signal bandwidth by means of adaptive nonlinear filtering. The nonlinear compensator is constructed using a power series filter with adaptive coefficients. The adaptive coefficients are identified blindly by applying a least-squares criterion to the out-of-band spectral content of the nonlinear compensator output. The extraction of the out-of-band signal is efficiently performed by the discrete cosine transform. The effectiveness of the blind nonlinear compensation method is demonstrated by way of simulation examples involving periodic, colored noise, and bandlimited speech signals.     

Spread spectrum fiber-optic local area network using optical processing

Spread spectrum code division multiple access (CDMA) allows asynchronous multiple access to a local area network (LAN) with no waiting. The additional bandwidth required by spread spectrum can be accommodated by using a fiber-optic channel and incoherent optical signal processing. New CDMA sequences are designed specifically for optical processing. It is shown that increasing the number of chips per bit, by using optical processing, allows an increase in capacity of a CDMA LAN. An experiment is performed demonstrating the performance of an optical CDMA LAN, operating at 100 Mbd with three users.     

Spread spectrum multiple access for mobile meteor burstcommunications

The paper considers the application of a direct sequence code division multiple access (CDMA) scheme to a mobile meteor burst communication network. It is shown that a CDMA scheme solves the significant multiple-access problem caused by low channel diversity when remote station nodes are closely spaced. The CDMA scheme is shown to provide low waiting times and high throughput for networks of many thousands of nodes. A comparison of the CDMA scheme to a random frequency division multiple access scheme is provided where the offered traffic load, network size, system bandwidth, and interference levels are varied     

Blind code-timing estimation for CDMA systems with bandlimited chip waveforms in multipath fading channels

In this paper, we present a blind code-timing estimator for asynchronous code-division multiple-access (CDMA) systems that use bandlimited chip waveforms. The proposed estimator first converts the received signal to the frequency domain, followed by a frequency deconvolution to remove the convolving chip waveform, and then calculates the code-timing estimate from the output of a narrowband filter with a sweeping center frequency, which is designed to suppress the overall interference in the frequency domain. The proposed estimator is near-far resistant, and can deal with time- and frequency-selective channel fading. It uses only the spreading code of the desired user, and can be adaptively implemented for both code acquisition and tracking. We also derive an unconditional Crame/spl acute/r-Rao bound (CRB) that is not conditioned on the fading coefficients or the information symbols. It is a more suitable lower bound than a conditional CRB for blind code-timing estimators which do not assume knowledge of the channel or information symbols. We present numerical examples to evaluate and compare the proposed and several other code-timing estimators for bandlimited CDMA systems.     

Cellular CDMA capacity with out-of-band multihop relaying

In this paper, we consider the capacity of cellular code division multiple access (CDMA) when there is out-of-band ad hoc traffic relaying. The mobile stations (MSs) are dual-mode, having both ad hoc and cellular CDMA radios. An active MS is free to choose any available relay station (RS) within its ad hoc radio coverage area for dual-hop communication with the CDMA base station (BS). Communications between the RSs and the MSs use bandwidth which is available to the ad hoc radio and does not consume the CDMA capacity. Using this mechanism, CDIVIA interference can be reduced by dynamically selecting RSs which have more favorable CDMA link characteristics. Several relay station selection criteria are considered, namely, ad hoc relaying with low relative interference (ARRI), with best link gain (ARLG), and with shortest distance (ARSD). The relay station selection protocols are compatible with existing wireless local area network (WLAN) standards such as IEEE 802.11. An analytic model is used to compute the effects on uplink and downlink CDMA capacities when out-of-band relaying is added. The results show that very significant capacity improvements are possible by using these criteria compared with conventional CDMA with hard or soft handoff. Ad hoc relaying which dynamically tracks CDMA link quality can achieve greater capacity improvements than that using a distance-based relay station selection. Relaying, which considers both signal and interference conditions, achieves better capacity than that based on signal link quality alone.     

Performance Analysis of Subspace Based Downlink Channel Estimation for W-CDMA Systems Using Chaotic Codes

This paper presents blind channel estimation for downlink W-CDMA system that employs chaotic codes and Walsh codes for spreading information bits of the multiple users. In a W-CDMA system, while transmitting over multipath channels, both intersymbol interference (ISI) as a result of Inter Chip Interference and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users’ signature waveform. Earlier work focused on a subspace based channel estimation algorithm for asynchronous CDMA systems to estimate the multiple users’ symbols, where only AWGN channel was considered. In our work, we study a similar subspace-based signature waveform estimation algorithm for downlink W-CDMA systems, which use chaotic codes instead of pseudo random codes, that provide estimates of the multiuser channel by exploiting structural information of the data output at the base station. In particular, we show that the subspace of the (data+noise) matrix contains sufficient information for unique determination of channels, and hence, the signature waveforms and signal constellation. We consider Rayleigh and Rician fading channel model to quantify the multipath channel effects. Performance measures like bit error rate and root mean square error are plotted for both chaotic codes and Walsh codes under Rayleigh and Rician fading channels.     

Efficiently structured CDMA receiver with near-far immunity

A Gaussian code division multiple access (CDMA) channel is shared by K active users who transmit asynchronously with BPSK modulation by independent binary data streams. A conventional direct sequence CDMA receiver has only limited capability for suppressing cochannel multiple user interference (MUI) from the K-1 other CDMA signals while attempting to receive and demodulate each CDMA signal. A new and efficient feedback receiver structure is described for coherent demodulation of the K asynchronous CDMA signals. By incorporating adaptive cancellation of the cochannel MUI in a feedback structure, the new design offers the following advantages: protection of the receiver's synchronization loops as well as its data demodulators against MUI; lower bit error rate (BER) than the conventional receiver for equal-strength as well as near-far MUI; and implementation simplicity close to that of the conventional CDMA receiver. It is shown that all the CDMA signals are canceled to a common level determined by N₀, the one-sided power spectral density of the input AWGN; the bandwidth spreading ratio N; and K. The BER for any signal may be computed as if the N₀ matched filter input were multiplied by the factor (6N/π)(1.5 N+1-K) for K⩽1.5 N. Computer simulations corroborate this result. A straightforward enhancement to the receiver structure is shown to allow it to overcome the deleterious affects of multipath propagation     

Analysis of new methods for broadcasting digital data to mobileterminals over an FM-channel

Two new methods using an FM-radio channel for transmission of digital data to mobile terminals are examined: 1. A modification of the radio data system (RDS). In RDS, additional digital information is multiplexed with a stereo sound signal. A new system is suggested where the data signal can be multiplexed with a mono audio signal. This causes extension to the bandwidth available for the data signal, and therefore the RDS bitrate can be increased. Error calculations are performed both for the original RDS system and for the new system. 2. Orthogonal frequency division multiplexing (OFDM). OFDM is used in the digital audio broadcasting system (DAB), which is designed to transmit digital audio in the FM band. In OFDM a signal is divided over a large number of 2- or 4-PSK modulated orthogonal subcarriers. The subcarriers of 6 different programmes are multiplexed in one beam to reduce the effects of frequency selectivity of the transmission channel. A new system based on OFDM is proposed, in which the carriers of each programme are transmitted in one FM-channel with a bandwidth of 200 kHz instead of multiplexed with the carriers of other programmes. Error calculations are performed for the subcarriers used in the OFDM modulation method     

Performance comparison of asynchronous and synchronouscode-division multiple-access techniques for fiber-optic local areanetworks

Synchronous code-division multiple access (S/CDMA) is investigated for fiber-optic local area networks. It is shown that the large bandwidth expansion required by spread-spectrum techniques, such as CDMA, can be accommodated by using a fiber-optic channel for transmission and incoherent optical signal processing for code generation and correlation. Prime sequence codes, previously developed for a fiber-optic network using (asynchronous) CDMA, are modified to fit a synchronous transmission format. A performance comparison of CDMA and S/CDMA systems reveals that S/CDMA can accommodate a larger number of subscribers and more simultaneous users than CDMA. An environment for which S/CDMA would be suited is discussed     

Efficient Pulse Shaping Using MSK or PSK Modulation

The increasing requirement for high data rate, bandwidth efficient digital radio systems has led to the development of MSK-type modulation methods designed to achieve a compact signal spectrum. These modulation methods include sinusoidal frequency shift keying (SFSK), special MSK-type pulse shapes, and multiamplitude minimum shift keying (MAMSK). When more compact signal spectra are required, bandlimited filtering must be introduced. This note considers the use of conventional MSK or PSK modulators followed by newly developed bandlimited pulse shaping filters. With this approach, it is shown that MSK and offset QPSK modulators yield identical signals on the channel when filtered by properly designed bandlimited pulse shaping networks.     

Bandlimited quasi-synchronous CDMA: a novel satellite accesstechnique for mobile and personal communication systems

Recent trends in digital communications are opening commercial applications to code division multiple access (CDMA). A novel access technique based on bandlimited quasi-synchronous CDMA (BLQS-CDMA) is described, showing all the advantages of synchronizing conventional direct sequence CDMA to drastically reduce the effect of self-noise. Bandlimitation is achieved with no detection loss by means of Nyquist chip shaping, leading to a simple all-digital demodulator structure. Detection losses due to imperfect carrier frequency and chip timing synchronization are analytically derived and numerical results are checked by computer simulations. Impairments due to satellite transponder distortions are evaluated. The full digital modem structure is presented, together with possible applications to mobile and very small aperture terminal (VSAT) satellite communications     

On chip-matched filtering and discrete sufficient statistics forasynchronous band-limited CDMA systems

The problem of generating discrete sufficient statistics for signal processing in code-division multiple-access (CDMA) systems is considered in the context of underlying channel bandwidth restrictions. Discretization schemes are identified for (approximately) bandlimited CDMA systems, and a notion of approximate sufficiency is introduced. The role of chip-matched filtering in generating accurate discrete statistics is explored. The impact of approximate sufficiency on performance is studied in three cases: conventional matched filter (MF) detection, minimum mean-squared-error detection, and delay acquisition. It is shown that for waveforms limited to a chip interval, sampling the chip-MF output at the chip rate can lead to a significant degradation in performance. Then, with equal bandwidth and equal rate constraints, the performance with different chip waveforms is compared. In all three cases above, it is demonstrated that multichip waveforms that approximate Nyquist sine pulses achieve the best performance, with the commonly used rectangular chip pulse being severely inferior. However, the results also indicate that it is possible to approach the best performance with well-designed chip waveforms limited to a chip interval, as long as the chip-MF output is sampled above the Nyquist rate     

Non-Symmetric Decompanding for Improved Performance of Companded OFDM Systems

Recent work in literature suggests the use of companding transforms to reduce the peak to average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) systems. However, previous performance evaluations assume the availability of infinite bandwidth. In bandlimited conditions, the performance deteriorates as a result of filtering out the out-of-band radiation (OBR). In this letter, we first study the effect of filtering on the performance of various companding techniques. We further propose a simple yet very effective method to overcome this performance degradation. The essential idea is to use companding and decompanding transforms which are not exact inverse of each other. Simulation results show that the proposed scheme significantly outperforms the previous schemes under more realistic channel conditions.     

Efficient two-stage frequency-domain equalisation for mitigating MAI in CDMA systems

An efficient two-stage frequency-domain equalisation (TS-FDE) is proposed, in order to mitigate the effect of multiple access interference (MAI) in code division multiple access (CDMA) systems having no cyclic-prefix (CP). In TS-FDE, the first stage performs channel equalisation and signal detection. Utilising the detected signal and a channel response, the second stage constructs the circularity of a channel matrix for a received signal, and then it performs once more the channel equalisation like an FDE using the CP. The MAI mitigating capability of the TS-FDE is evaluated in a mobile channel.     

ZCZ‐CDMA and OFDMA using M‐QAM for broadband wireless communications

This paper considers direct‐sequence code‐division multiple‐access with zero‐correlation zone sequences (ZCZ‐CDMA) and orthogonal frequency‐division multiple‐access (OFDMA) schemes using M‐ary QAM signaling for broadband wireless communications. Their system structures, complexities and performances in both AWGN and multipath frequency‐selective fading channels are evaluated and compared. For ZCZ‐CDMA, joint suppression of the multipath fading interference and multiple‐access interference can be achieved with a reduced family‐size of the spreading sequences. For OFDMA, analytical and simulation results indicate that it has the same performance as ZCZ‐CDMA in fast time‐varying multipath fading channels. In time‐invariant or slowly time‐varying channels, where the channel information can be made available to transmitters, OFDMA outperforms ZCZ‐CDMA, offers a higher capacity and is more flexible for system reconfiguration with a comparable computational complexity. Copyright © 2004 John Wiley & Sons, Ltd.     

Analysis of CDMA signal spectral regrowth and waveform quality

Closed-form expressions for the spectral regrowth and waveform quality of a reverse-link code divison multiple access (CDMA) signal passed through a weakly nonlinear circuit are derived using the power series and statistical methods. The third-order nonlinearity is expressed in terms of IP₃ to include the memory effects of the circuit in-band and out-of-band reactances. The analysis is based on a time-domain model of the signal that gives more accurate distortion estimates than the widely used Gaussian approximation. The model is used to derive the probability density function and other statistical properties of the CDMA signal to compare them to the Gaussian noise properties. Differences in statistics and distortions of OQPSK and QPSK modulated signals are discussed     

Receiver Design for Uplink Multiuser Code Division Multiple Access Communication System Based on Neural Network

In this paper, we consider the receiver design problem for the uplink multiuser code division multiple access (CDMA) communication system based on the neural network technique. The uplink multiuser CDMA communication system model is described in the form of space–time domain through antenna array and multipath fading expression. Novel suitable neural network technique is proposed as an effective signal processing method for the receiver of such an uplink multiuser CDMA system. By the appropriate choice of the channel state information for the neural network parameters, the neural network can collectively resolve the effects of both the inter-symbol interference due to the multipath fading channel and the multiple access interference in the receiver of the uplink multiuser CDMA communication system. The dynamics of the proposed neural network receiver for the uplink multiuser CDMA communication system is also studied.     

CDMA cellular engineering issues

The frequency reuse efficiency for the proposed code division multiple access (CDMA) cellular system is analytically derived, and it is shown that the cell capacity of a fully loaded multiple cell system is about 75% of what would be available for a single cell system. In addition, an engineering issue in transitioning from analog advanced mobile phone system (AMPS) to CDMA system is discussed. Calculating excessive interference power due to a CDMA channel, it is shown that when a CDMA channel is introduced in a cell, there should be at least one ring of buffer cells, in which the analog channels falling into the CDMA band are not reused