基于跳频扩时光正交码的光码分多址系统性能分析

提出了一种基于光正交码（OOC）的跳频方案，与OOC系统相比，采用该跳频码字的光码分多址（OCDMA）系统的可容纳用户数目得到了极大提高，并且多用户干扰（MAI）大大降低，从而系统整体性能得到提高。同时提出了使用该跳频码的多比特传输光码分多址系统方案，该方案利用了跳频码字数目的充裕性，使得系统传输速率成倍提高。     

Performance of coherent OFDM-CDMA for broadband mobile communications

Uncoded orthogonal frequency division multiplexing (OFDM) transmission technique applied in a multipath environment has a bit error rate (BER) comparable with a narrowband radio channel because the fading of each subcarrier is frequency-nonselective. To overcome this behaviour and to reduce the BER, a combination of OFDM and CDMA has been proposed recently. In an OFDM-CDMA system the energy of each information symbol is spread over several subcarriers. Therefore a diversity gain can be obtained in a broadband fading channel.In this paper we discuss the performance of OFDM-CDMA with coherent QPSK signalling over a frequency-selective Rayleigh fading channel. Channel estimation and demodulation are integral parts that determine the performance of the system. The method for channel estimation presented in this paper is based on a two-dimensional array of pilot symbols with second-order regression in the time domain and interpolation in the frequency domain. Quantitative comparison of four different detection algorithms in frequency-selective Rayleigh fading with noisy channel state information (CSI) will be presented in this paper: conventional correlation (equal gain correlation, EGC), orthogonality restoring correlation (ORC), ORC with a threshold in order to suppress subcarriers with low signal strength (TORC), and an iterative improvement based on a maximum likelihood approach. With TORC and iterative improvement a gain of approximately 9 dB over conventional OFDM can be obtained at a BER of 10^–3 in Rayleigh fading.     

OFDM—CDMA检测技术性能分析

传统的CDMA技术在无线通信方面因其诸多优点而备受青睐,但在抗码间串扰和多址干扰等方面存在的问题使其总体性能受到限制。而随着正交频分复用（OFDM）技术的日趋成熟,其强大的抗多径性能正得到越来越多的关注。这里介绍了一种最新的无线通信技术：OFDM－CDMA,对其发射和接收系统模型进行了完整的描述并对接收系统中的检测技术性能进行了比较。分析表明OFDM－CDMA兼备了CDMA和OFDM技术的优点,在频谱利用率、抗码间干扰、抗多径和多址干扰等方面都优于传统的CDMA技术。     

CDMA，OFDM及OFDM—CDMA性能对比

在MATLAB平台上搭建了正交频分多路复用（OFDM）、码分多址（CDMA）以及正交频分多路复用－码分多址（OFDM－CDMA）的仿真系统,分析了用户数、多径及信噪比对系统误码率的影响,并进行了性能比较,得出了有意义的结论。     

A programmable architecture for OFDM-CDMA

Combining multicarrier (OFDM) and CDMA technologies is attractive for future wireless broadband communications and software radio realization. Based on the unified framework known as OFCDMA, we develop a programmable structure for OFDM-CDMA transceivers in spite of three different scenarios to combine OFDM and CDMA. By adjusting system parameters without changing the fundamental hardware and software architecture, various system scenarios can be implemented, which might serve as the foundation to design software radio     

Polyphase codes for uplink OFDM-CDMA systems

We propose a OFDM-CDMA system that employs polyphase codes to support variable spreading factors. A systematic approach for constructing the polyphase code sequences of variable spreading factors is developed. Polyphase codes exhibit better auto- and cross-correlation properties than Hadamard codes. When employed in OFDM-CDMA systems, polyphase codes result in certain structured multiple-access interference (MAI) caused by multipath. Analytical and numerical results show that OFDM-CDMA systems employing polyphase codes have better PAPR performance than those using Hadamard codes. The BER performance of the OFDM-CDMA system using polyphase codes is evaluated by numerical results and compared to that of the OFDM-CDMA system using Hadamard codes with and without clipping.     

Performance analysis of multicarrier CDMA systems with frequency offsets and random spreading under optimum combining

The carrier-frequency offset effect on the performance of asynchronous multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) systems with aperiodic random spreading and correlated Rayleigh fading is studied in this paper. We obtain the optimum combining filter that maximizes the signal-to-interference-plus-noise ratio (SINR) of the combined statistics and exploits correlated information among subchannels. A closed-form expression for the unconditional covariance matrix of the interference-plus-noise vector, which forms the basis of our theoretical analysis of the maximum SINR and the average bit error probability formula, is derived by averaging several random parameters including asynchronous delays, correlated Rayleigh fading, and signature sequences. The analytic results obtained are applicable to MC-CDMA with appropriate modifications. Furthermore, we show that the MC-CDMA system with a common random signature sequence over all subcarriers for a given user outperforms that with distinct sequences over different subcarriers. Finally, the performance of MC-CDMA systems using the optimum combining technique is compared with that of different combining filters in the simulation.     

Performance analysis of a linear MMSE receiver for bandlimited random-CDMA using quadriphase spreading over multipath channels

This paper analyzes the bit-error-rate (P/sub e/) performance of a linear minimum mean-square error (LMMSE) receiver for bandlimited direct-sequence code-division multiple-access systems which use quadriphase spreading with aperiodic pseudonoise (PN) sequences. The analysis is based on the improved Gaussian approximation (IGA) with focus on chip pulse shaping. It shows that the IGA reduces to the standard Gaussian approximation (SGA) if 1) random quadriphase spreading is employed, 2) the spreading factor takes moderate to large values, and 3) the chip pulse excess bandwidth (BW) is zero. Hence, the SGA, known for its inaccuracy in low regions of P/sub e/, remains an accurate approximation even when the number of active users in the system is small as long as the aforementioned conditions are met. The analysis holds for either matched or different transmit and receive filters. Consequently, closed form conditional P/sub e/ expressions are derived for the coherent selective RAKE and the LMMSE receivers and verified with Monte Carlo simulations. Numerical results are presented to illustrate the performance improvement achieved by the LMMSE receiver which, in contrast to the coherent selective RAKE receiver, not only suppresses interference when the excess BW of chip pulse is nonzero, but also harnesses the energy of all paths of the desired user. Under the examined scenarios tailored toward current narrowband system settings, the LMMSE receiver achieves 60% gain in capacity over the selective RAKE receiver. A third of the gain is due to interference suppression capability of the receiver while the rest is credited to its ability to collect the energy of the desired user diversified to many paths. Future wideband systems will yield an ever larger gain.     

Joint detection for OFDM-CDMA communications in multipath fading channels

This paper proposes and compares four block joint detection algorithms for the orthogonal frequency division multiplexing (OFDM)-code division multiple access (CDMA) communication systems. The performance of the proposed OFDM-CDMA joint detectors with orthogonal and non-orthogonal spreading sequences are investigated in mobile radio channels. The proposed OFDM-CDMA detectors are zero forcing (ZF) and minimum mean square error (MMSE) detectors with decision feedback (DF) structures. The proposed joint detectors do not require discrete Fourier transform (DFT) or fast Fourier transform (FFT) operations and can be used to compensate the channel distortion. Hence for hardware implementation, the proposed detectors are more efficient than the conventional methods. The channel sorting method offers approximately 2 dB gain for the DF joint detectors and reduces the impairing effect of error propagation. The bit error rate (BER) performance of orthogonal codes is better than that of non-orthogonal ones in time varying channels. Simulation examples are given to demonstrate the effectiveness of the proposed detectors.     

Performance analysis of joint switched diversity and adaptive modulation

In this paper, a simple and practical system based on a switched diversity scheme with adaptive modulation is presented. This system provides a reduced number of channel estimation while offering the optimum spectral efficiency given by a selection diversity system. In addition, the switching threshold is easily manipulated so as to make an efficient use of the tradeoff between spectral efficiency and channel estimation overhead. An extension of switched diversity into a multiuser scheduling is later also considered. This switch-based multiuser access scheme results in a lower average feedback load than that for the optimal selection-based multiuser scheme. Numerical results show that we can obtain a trade-off between spectral efficiency and the feedback load by choosing the switching threshold appropriately.     

Performance analysis of low processing gain DS/CDMA systems withrandom spreading sequences

The performance analysis of a RAKE receiver for low processing gain direct sequence code-division multiple access (DS/CDMA) systems is presented. The multipath interference effects, that have typically been assumed negligible or approximated as Gaussian noise, become significant for low processing gain systems. A scheme to accurately evaluate the multipath interference effects is proposed     

Performance of wideband CDMA systems with complex spreading andimperfect channel estimation

The coherent RAKE reception of wideband code division multiple access (W-CDMA) signals with complex spreading is considered. A general multipath-fading channel model is assumed. A dedicated pilot channel, which is separate from the data channels, is used for the purpose of channel estimation. Based on a digital implementation, the coherent demodulation scheme is presented. Pilot channel estimation error, due to multiple access and multipath interference, is studied. The system performance is evaluated by means of the bit error rate (BER). The analysis shows that the error of channel estimation significantly degrades system performance and can be effectively suppressed by low pass filters (LPFs). A discussion on the envelope variation of complex spread signals is also included, which illustrates that the complex spread signal has a more stable envelope than the dual-channel spread signal. The power ratio of pilot to data channels should be chosen in the range of 0.2 to 0.4 (or -7 to -4 dB), in order to achieve maximum system capacity     

A MAC protocol for a wireless LAN based on OFDM-CDMA

Local access encompasses requirements of high throughput, transfer performance, flexibility, and ease of user/terminal mobility at a relatively low cost. While some of these targets can often be met satisfactorily with wired infrastructures, it is getting increasingly popular to resort to wireless access (wireless LAN), mainly because of its simpler deployment, continuous coverage, availability of ad hoc connectivity, and user/terminal mobility support. This article outlines the proposal of a broadband quality-oriented WLAN system. The basic system characteristics are described, with details given on a MAC protocol supporting IP and exploiting an OFDM-CDMA technique     

Performance analysis of joint voice-data PRMA over random packeterror channels

We consider a joint voice-data packet reservation multiple-access (PRMA) system with transmission errors. Two analysis methods are presented. The first is a combined Markov and equilibrium point analysis. In this method, equilibrium point analysis is used to obtain the equilibrium number of backlogged data terminals, while Markov analysis is used to compute the stationary state probability distribution, assuming the number of backlogged data terminals is equal to the equilibrium value. The second is a Markov analysis using an approximate marginal distribution of backlogged data terminals. System performance measures, namely, voice packet dropping probability, data packet delay, and throughput, are derived in the presence of transmission errors. Computation complexity and accuracy are compared for the two analysis methods. Simulation results are also presented for comparison with analysis results, and good agreement is observed, especially when the packet header error rate is small     

Performance analysis of linear joint equalization and multiple access interference cancellation for multitone CDMA

In [18] the combination of multitone modulation with direct sequence spectrum spreading has been introduced. The performance of a correlation receiver with diversity has been evaluated for a multipath channel in the presence of a multiple access interference (MAI) using the Gaussian assumption. In the present paper we analyze the problem of joint equalization and cancellation of the multipath and multiple access interferences for these multitone CDMA (Code Division Multiple Access) systems. A linear structure is first investigated for a mean square error (MSE) criterion. In order to understand the potential of the system we study the steady-state behavior of the joint equalizer-canceler. The investigation is carried out for a joint device following a bank of matched filters and assuming perfect carrier phase and timing recovery. An exact expression for the bit error probability of BPSK symbols before and after cancellation, is obtained in the form of an integral The performance of the joint device is investigated for different types of channels. The resistance to the near-far effect is also demonstrated. Then adaptive structures are derived from a least square approach. Their performance is demonstrated by means of computer simulations.This author would like to thank the Belgian NSF for its financial support.This author would like to thank the Belgian IRSIA for its financial support.     

A simple construction of OFDM-CDMA signals with low peak-to-average power ratio

This paper investigates the peak-to-average power ratio (PAR) reduction scheme employing a simple symbol transform in OFDM-CDMA systems. This approach is very simple because of no additional complexity and works without restriction on the allocation of spreading code, maintaining the original transmission efficiency. Simulation results show that the investigated scheme gives the PAR reduction gain of 2 /spl sim/ 3 dB compared to the original OFDM and OFDM-CDMA signals, and can provide the further PAR reduction by combining the selective mapping (SLM) and partial transmit sequence (PTS) schemes, which are less complex compared to the ordinary SLM and PTS approaches, respectively.     

Stability analysis of correction schemes for spreading resistance measurements

It is illustrated how the stability behaviour of a correction scheme for spreading resistance measurements can be analysed on the basis of its stability curves. This technique is applied on some recent schemes which incorporate a resistivity dependent probe radius. Their behaviour is compared with schemes implementing a fixed radius. From this analysis it follows precisely when the variable radius schemes considered will be stable or unstable. The primary cause for eventual instability is indicated and a new iteration approach is proposed which is able to remove this cause resulting in a drastically improved stability behaviour.     

Scanning spreading resistance microscopy for failure analysis of nLDMOS devices with decreased breakdown voltage

Scanning Spreading Resistance Microscopy (SSRM) is successfully applied to investigate failing nLDMOS test devices that exhibit a lowered break down voltage (BVDSS) in electrical test. Cross-sectional, two-dimensional maps of the local sample resistivity from fail and reference (pass) devices reveal significant differences of the dopant concentration in individual, specific regions. This important information enables unambiguous identification of the root cause of the device failure to be dopant related. Furthermore, from a set of hypothesis, which explains the failed electrical test, SSRM results confirm exactly one and rule out the other. These are two important steps towards root cause identification. Since a relative comparison of fail and pass SSRM scans is sufficient for this failure analysis, an extensive data calibration for the absolute dopant concentration by means of additional SSRM measurements on test samples with known dopant concentration is not required. The ability of SSRM to prove or disprove miscellaneous fail hypothesis even without data calibration makes this method a very powerful tool for analysis of dopant related failure types.     

联合多幅多相差分制在衰落信道下的性能分析

本文研究了联合多幅多相差分调制方式－ARDPM在衰落信道下及不同多谱乐频移时的性能，并通过计算QAM系统和ARDPM系统在不同多谱勒频移时的误码率上界，对两个系统的性能进行了比较。在多谱勒频移较小时 ，ARDPM系统的性能优于QAM，但是随着多谱勒频移的增大，ARDPM系统的性能将迅速恶化，此时QAM系统的性能将优于ARDPM。最后本文给出计算机的理论计算曲线和实际的仿真结果。     

Performance of direct-sequence spread-spectrum RAKE receivers withrandom spreading sequences

In this paper, we derive error probability expressions for binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK) spread direct-sequence spread-spectrum (DSSS) systems employing random spreading sequences with RAKE receivers. The derived expressions accurately take into account the effect of interpath interference which usually has been neglected in previous analyses. Selection, equal gain, and maximal ratio techniques are considered for diversity combining. Two possible finger assignment strategies, one based on the instantaneous amplitudes and another based on the average powers of the multipath components, are considered for the assignment of multipath components to the available demodulating fingers in the RAKE receiver. Also, various simple, and in many cases, closed-form approximations for the error probabilities are derived and their accuracies are assessed