共查询到20条相似文献,搜索用时 21 毫秒
1.
This paper presents a multi-rate quasi-synchronous CDMA (QS-CDMA) system with QPSK modulation based on the new signature waveforms,
namely GPZ-chip waveforms which are constructed by the generalized pairwise Z-complementary (GPZ) codes and optimal chip waveforms.
According to the properties of GPZ-chip waveforms, the multiple access interference of different users can be reduced or eliminated.
The interferences among different streams of the same user are handled by an optimal (or suboptimal) multi-stream detector(s).
The performance of the proposed system is simulated and analyzed in a cellular communication environment consisting of multipath
fading. The result shows that the proposed system has similar performance as compared to the single-rate QS-CDMA system employing
GPZ codes with one chip waveform due to the properties of multi-stream detection. Also, the proposed system can achieve a
significant interference reduction as compared to the multi-rate system employing concatenated WH/m sequence sets with one
chip waveform due to the zero correlation zone properties of the GPZ-chip waveforms, thus resulting better BER performance. 相似文献
2.
We consider synchronous code-division multiple access (CDMA) systems over an additive white Gaussian noise (AWGN) channel, where all users are divided into groups of small size. The signature waveforms for users in each group are constructed from the same signature sequence but with different chip waveforms. To minimize the multiple access interference (MAI) at the output of the correlators, Welch-bound-equality (WBE) sequences and chip waveforms having optimal correlation property are employed. The main idea behind the proposed construction is to suppress the inter-group interference from users in different groups as much as possible (even to remove it completely) at the expense of introducing the intra-group interference among the users in the same group. The intra-group interference, however, can be easily handled by a low-complexity, optimal (or suboptimal) multiuser detector(s) if the group size is kept small enough. As special cases, the proposed constructions correspond to the optimal design of the signature waveforms and the conventional system that uses a single chip waveform, respectively. Thus the proposed construction offers a flexibility to trade performance for complexity. In particular, it is demonstrated that, while the conventional system's error performance is very sensitive to even a small amount of overload, the proposed system with two users per group can have up to 100% overload with an excellent error performance. 相似文献
3.
Performance bounds on chip-matched-filter (CMF) receivers for bandlimited direct-sequence spread-spectrum multiple-access (BL-DS/SSMA) systems with aperiodic random spreading sequences are obtained. First, the optimum transmit-receive chip waveform pairs that maximize the conditional signal-to-interference ratio are derived. This leads to performance bounds on CMF receivers when the conditional Gaussian approximation for cyclostationary multiple-access interference (MAI) is exploited. The bounds are used to examine the dependence of the MAI suppression capability of the CMF receivers on the excess bandwidth of the system and the delay profile of multiple-access users. The system employing the flat-spectrum chip waveform pair is shown to have near-optimum average bit-error rate performance among the fixed CMF (FCMF) receiver systems. Numerical results are provided for an adaptive CMF receiver and for FCMF receivers employing several different fixed chip waveforms. 相似文献
4.
Lifang Feng Pingzhi Fan Li Hao Kok-keong Loo Vahid Tarokh 《Wireless Personal Communications》2009,50(4):493-507
In this paper, we propose a new construction of signature waveform sets based on Generalized Loosely Synchronization (GLS)
sets and different chip waveforms. The new signature sets are applied into the multi-rate multi-cell quasi-synchronous CDMA
(QS-CDMA) system where each cell is assigned with a GLS set; different users in the same cell are assigned with different
GLS sequences in the same GLS set; user’s different streams are assigned with the same GLS sequence but different chip waveforms.
According to the properties of GLS sets, the inter-cell interference (ICI) and the multi-user interference (MUI) in the same
cell can be reduced significantly. The interferences among different streams of the same user are handled by an optimal (or
suboptimal) multi-stream detector(s), notice that the multi-stream detector mentioned here is also named as multi-user detector
in other references. We compare the performance of the multi-rate multi-cell QS-CDMA system employing the proposed sets with
that of multi-rate system employing well-known concatenated orthogonal/PN sets and that of single-rate system employing GLS
sets. The results show that the multi-rate system employing the proposed sets can achieve significant interference reduction.
Meanwhile the performance of multi-rate system is similar to that of single-rate system due to the inclusion of multi-user
detection.
相似文献
Vahid TarokhEmail: |
5.
Joon Ho Cho Lehnert J.S. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(5):1172-1185
An optimal signal design for band-limited, asynchronous, direct-sequence code-division multiple-access (DS-CDMA) communications with aperiodic random spreading sequences and a conventional matched filter receiver is considered in an additive white Gaussian noise (AWGN) channel. With bandwidth defined in the strict sense, two optimization problems are solved under finite bandwidth and zero interchip interference constraints. First, a chip waveform optimization is performed given the system bandwidth, the data symbol transmission rate, and the processing gain. A technique to characterize a band-limited chip waveform with a finite number of parameters is developed, and it is used to derive optimum chip waveforms which minimize the effect of multiple-access interference (MAI) for any energy and delay profile of users. Next, a joint optimization of the processing gain and the chip waveform is performed, given the system bandwidth and the data symbol transmission rate. A sufficient condition for a system to have lower average probability of bit error for any energy profile is found, and it is used to derive some design strategies. It is shown that the flat spectrum pulse with the processing gain leading to zero excess bandwidth results in the minimum average probability of bit error. Design examples and numerical results are also provided 相似文献
6.
The principle to suppress multiple access interference (MAI) using double chip waveforms (DCW) in asynchronous DS-CDMA systems is analyzed in the paper. Based on the principle, a new method adopting optimized combinations of chip waveforms (CCW) to reduce MAI is proposed. The energy spectrum extension factor (ESEF) of equivalent chip waveform is introduced to optimally select CCW to reduce MAI, improve the signal to interference plus noise ratio (SINR) and bite error rate (BER) performance of asynchronous DS-CDMA users. The general closed form expression of SINR for asynchronous DS-CDMA users with CCW is obtained. The BER is also derived by improved Gaussian approximation (IGA). The theoretical analysis and numerical simulation results show that the optimized CCW using ESEF can effectively suppress MAI better, achieve higher SINR and BER performance compared with DCW. Moreover, the overlap between the simulation and IGA BER curves verifies the theoretical derivation. 相似文献
7.
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. 相似文献
8.
We extend our investigation of generalized multicarrier direct-sequence code-division multiple access by considering two additional types of chip waveforms, namely, the time-domain half-sine and raised-cosine chip waveforms, in addition to the rectangular chip waveform. A range of closed-form equations are provided for quantifying the effect of both the multipath interference and the multiuser interference, when using partially overlapping subcarriers. These closed-form formulas allow us to evaluate the bit-error rate performance of arbitrary code-division multiple access schemes using overlapping subbands with the aid of the standard Gaussian approximation. 相似文献
9.
This paper studies the effect of chip waveform shaping on the performance of band-limited multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) systems. The performance criterion is the average multiple access interference at the output of a correlation receiver. A criterion based on the elementary density function is introduced for the performance comparison of various chip waveforms. It is demonstrated that the performance of MC-DS-CDMA systems is quite insensitive to the chip waveform shaping. Moreover, the optimum chip waveform for MC-DS-CDMA systems is practically the same as that of a single-carrier DS-CDMA system. 相似文献
10.
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 相似文献
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针对无人机群通信对抗中敌方通信信号生成干扰波形的自适应问题,提出了一种基于频谱调制频谱编码(SMSE)模型进行认知干扰波形设计的方法。通过对SMSE框架中6种不同的波形设计参数进行部署,将具有特定频谱结构的波形在频域生成相应功能的认知干扰波形,来应对通信对抗中不同参数下的通信信号。仿真实验表明,该理论模型能够产生单音干扰、多音干扰、宽带干扰、瞄准式窄带干扰等多种压制式干扰波形,并能够对BPSK,QPSK,8PSK,16QAM等调制信号实现调制欺骗式干扰。通过与理论曲线对比,实验结果验证了SMSE模型产生认知干扰波形的理论可行性,这意味着SMSE模型可以实现对通信信号的压制性干扰和欺骗式干扰的一体化应用。 相似文献
13.
Motivated by the emergence of programmable radios, we seek to understand a new class of communication system where pairs of transmitters and receivers can adapt their modulation/demodulation method in the presence of interference to achieve better performance. Using signal to interference ratio as a metric and a general signal space approach, we present a class of iterative distributed algorithms for synchronous systems which results in an ensemble of optimal waveforms for multiple users connected to a common receiver (or colocated independent receivers). That is, the waveform ensemble meets the Welch (1974) bound with equality and, therefore, achieves minimum average interference over the ensemble of signature waveforms. We derive fixed points for a number of scenarios, provide examples, look at ensemble stability under user addition and deletion as well as provide a simplistic comparison to synchronous code-division multiple-access. We close with suggestions for future work 相似文献
14.
Ertan
ztürk Ouz Kucur Guillermo E. Atkin 《International Journal of Communication Systems》2006,19(1):1-16
In this paper, we search for a better chip waveform based on orthogonal wavelets for direct sequence‐code division multiple access (DS‐CDMA) signals to improve the probability of error (Pe) performance with minimal signal bandwidth variations. First, we derive the Pe expression over a quasi‐synchronous additive white Gaussian noise channel for DS‐CDMA signals, which use various pulse shaping waveforms including orthogonal wavelets as chip waveforms. It is observed that this expression depends on the chip waveform. Then, we design an optimum wavelet by using the relationship between wavelets and filter coefficients to reduce the probability of error. The DS‐CDMA system using the optimum wavelet waveform results in a lower probability of error than those using the conventional chip waveforms such as raised cosine, half‐sine and rectangular waveforms. Especially, the Pe of the optimum wavelet‐based scheme becomes significantly better than those of the conventional chip waveforms‐based schemes under the heavy loading that is the case for commercial wireless systems. When the systems work with full load (i.e. the number of users equals the processing gain), the optimum wavelet‐based system results in 0.5, 2.1 and 4 dB better SNR values than those of the raised cosine, half‐sine and rectangular‐based systems, respectively, for a Pe value of 10?3. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
15.
This paper investigates the effect of chip waveform shaping on the error performance, bandwidth confinement, phase continuity, and envelope uniformity in direct-sequence code-division multiple-access communication systems employing offset quadrature modulation formats. An optimal design methodology is developed for the problem of minimizing the multiple-access interference power under various desirable signal constraints, including limited 99% and 99.9% power bandwidth occupancies, continuous signal phase, and near-constant envelope. The methodology is based on the use of prolate spheroidal wave functions to obtain a reduced-dimension discrete constrained optimization problem formulation. Numerous design examples are discussed to compare the performance achieved by the optimally-designed chip waveforms with other conventional schemes, such as offset quadrature phase-shift keying, minimum-shift keying (MSK), sinusoidal frequency-shift keying (SFSK), and time-domain raised-cosine pulses. In general, it is found that while the optimized chip pulses achieved substantial gains when no envelope constraints were imposed, these gains vanish when a low envelope fluctuation constraint was introduced. In particular, it is also shown that MSK is quasi-optimal with regard to the 99% bandwidth measure, while the raised-cosine pulse is equally good with both the 99% and 99.9% measures, but at the expense of some envelope variation. On the other hand, SFSK is quasi-optimal with regard to the 99.9% bandwidth occupancy, among the class of constant-to-low envelope variation pulses 相似文献
16.
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. 相似文献
17.
Maw-Yang Liu Hen-Wai Tsao 《Lightwave Technology, Journal of》2001,19(2):180-185
Since optical code division multiple access (CDMA) is an interference-limited system, we propose a system employing trellis-coded scheme and double optical hardlimiters (TC-DHLs) to alleviate the adverse impact of multiple access interference. For asynchronous transmission, optical orthogonal code (OOC) is utilized as signature sequence. System performance is evaluated under the chip synchronous case among different users, and thermal noise, avalanche photodiode (APD) noise, and interference are taken into consideration. Numerical results show that our proposed scheme can further reduce the error floor up to several orders over systems that using only double optical hardlimiters 相似文献
18.
Ki Jun Kim Soon Yil Kwon Een Kee Hong Keum Chan Whang 《Communications, IEEE Transactions on》2000,48(6):1029-1036
The effect of tap spacing on the performance of a RAKE receiver is analyzed analytically in a frequency-selective fading channel. A continuous time multipath fading channel model is used for the analysis, and the expression of the correlation between the desired signals, interference signals, and noise signals at the output of each branch of the RAKE receiver is derived for various chip waveforms. Since the noise components of each branch signal are correlated to each other, an optimum combining rule based on the maximum-likelihood criterion is derived to gain utmost performance. It is shown that the performance of the system can be improved by setting the tap spacing of the RARE receiver below the chip duration when the bandwidth of the transmitted signal is larger than the inverse of the chip duration. Also, it is shown that the normalized capacity of the system can be increased by using a chip waveform occupying wider bandwidth, which takes advantage of the increased diversity gain merits of a wide-band code-division multiple-access system at the same chip rate. It is noted that the derived combining rule gives diversity gain against the fading process as well as noise whitening processing gain against multiple-access interference at the same time 相似文献
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