Chip-delay locked matched filter for DS-CDMA systems using longsequence spreading

This paper considers an improved single-user detection technique for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems using long sequence spreading (random-CDMA) Most of the known detection schemes for DS-CDMA suffer from either poor performance under power-imbalance (near-far like) conditions, excessive complexity, or incompatibility with systems employing long sequence spreading. To address these problems, this paper considers a signal-to-noise ratio maximizing linear time-invariant filter for one-shot bit symbol detection exploiting some information about the interferers. This filter, referred to as the chip-delay locked matched filter (CLMF), exploits the cyclostationarity in multiple-access interference, and it can offer good near-far resistance while remaining suitable for systems with long sequence spreading. The CLMF requires knowledge of interferers chip delays and signal powers; however, knowledge of their pseudonoise sequences is unnecessary. This paper also demonstrates the improvement in performances offered by the CLMF over other single-user receivers such as the conventional matched filter and noise-whitening matched filter performance is evaluated in terms of probability of outage for single-rate and dual-rate DS-CDMA systems using bandwidth-efficient chip pulses, over a single-path additive white Gaussian noise channel. Errors in the interferer chip delay estimates degrade the CLMF performance. However, if the root-mean-square value of these errors is less than 5% of the chip interval, then this degradation is small     

三角波扫频的可调谐毫米波RoF下行链路性能分析

理论提出了一种获得高倍频因子的毫米波产生方案并对产生毫米波的RoF下行链路性能进行分析。方案中利用三角波替代正弦波来驱动双驱动马赫增德尔调制器。通过三角波扫频,可以产生更多的频率分量。均匀光纤布拉格光栅可以选择出所需的频率分量进行拍频产生毫米波信号。倍频因子的调谐可以通过调节两个均匀光纤布拉格光栅的中心波长来实现。方案可获得倍频因子分别为4,6,8,10,12 mm的波信号,并对各种倍频因子下的RoF下行链路性能进行了分析。结果表明,在各种倍频因子下,经过10 km光纤的传输,解调后眼图均能保持一定的张开度,满足通信需求。该方案为未来的无线电通信系统提供了一定的支持。     

多芯微结构光纤产生超连续谱

报道了一种多芯微结构光纤(MF)产生超连续谱的现象,利用钛宝石飞秒激光器产生的20 fs光脉冲序列,通过一段长约40 cm的多芯微结构光纤,获得了展宽超过600 nm的光谱(400~1000 nm),而且当耦合位置不同时,可以得到不同颜色的光谱成分。实验结果表明,由非规则填充气孔组成的多芯微结构光纤可以出现超强的非线性和超连续谱展宽。     

Finite memory-length linear multiuser detection for asynchronousCDMA communications

Decorrelating, linear, minimum mean-squared error (LMMSE), and noise-whitening multiuser detectors for code-division multiple-access systems (CDMA) are ideally infinite memory-length (referred to as IIR) detectors. To obtain practical detectors, which have low implementation complexity and are suitable for CDMA systems with time-variant system parameters (e.g., the number of users, the delays of users, and the signature waveforms), linear finite-memory-length (referred to as FIR) multiuser detectors are studied in this paper. They are obtained by truncating the IIR detectors or by finding optimal FIR detectors. The signature waveforms are not restricted to be time-invariant (periodic over symbol interval). Thus, linear multiuser detection is generalized to systems with spreading sequences longer than the symbol interval. Conditions for the stability of the truncated detectors are discussed. Stable truncated detectors are shown to be near-far resistant if the received powers are upper bounded, and if the memory length is large enough (but finite). Numerical examples demonstrate that moderate memory lengths are sufficient to obtain the performance of the IIR detectors even with a severe near-far problem     

Threshold discrimination and blanking for large near-far power ratios in UWB networks

A simple chip-discrimination technique is presented for use with ultra-wideband (UWB) impulse radio (IR) that improves performance for large near-far interference power ratios. A typical spread-spectrum IR that employs a matched-filter sum for bit decisions is susceptible to small numbers of large power pulses that can dominate the bit decision-threshold statistics. This letter describes a technique for chip discrimination prior to the spreading summation, that can greatly reduce the effects of large near-far power ratios among interferers. The technique exploits the very narrow pulsewidth and resulting low-duty-cycle characteristic only achievable with ultra-wide bandwidth. A statistical model is developed that predicts bit-error performance for binary offset pulse position modulation as a function of near-far density and power for varying discrimination thresholds. An analytic solution for perfect chip blanking is developed, and is in good agreement with chip discrimination for large near-far power ratios. We find that even a small number of very near interferers can greatly reduce the performance of a system without blanking or discrimination. Results show substantial improvement using this method for near interferers with near-far power ratios greater than 20 dB.     

Matched-filter template generation via spatial filtering: application to fetal biomagnetic recordings

We have developed a two-step procedure for signal processing of fetal biomagnetic recordings that removes cardiac interference and noise. First, a modified matched filter (MF) is applied to remove maternal cardiac interference; then, a simple signal space projection (SSP) is applied to remove noise. The key difference between our MF and a conventional one is that the interference template and the template scaling are derived from a signal that has been spatially filtered to isolate the interference, rather than from the raw signal. Unlike conventional MFs, ours is able to separate maternal and fetal cardiac complexes, even when they have similar morphology and overlap strongly. When followed by a SSP that preserves only the signal subspace, the noise is reduced to a low level.     

Adaptive blind retrieval techniques for multiuser DS-CDMA signals

In a multiuser direct sequence code division multiple access system, retrieval of the transmitted signals with a conventional receiver is difficult when there is chip asynchronism, multipath propagation and the associated near-far problem. It is shown that despite these problems, an adaptive linear receiver based on a mixed cross-correlation and constant modulus algorithm has the potential to retrieve all users simultaneously. Moreover, for the assumed transmission model, a necessary condition for global convergence of the proposed algorithm is also provided     

Blind and training-assisted subspace code-timing estimation for CDMA with bandlimited chip waveforms

In this paper, we present a group of subspace code-timing estimation algorithms for asynchronous code-division multiple-access (CDMA) systems with bandlimited chip waveforms. The proposed schemes are frequency-domain based techniques that exploit a unique structure of the received signal in the frequency domain. They can be implemented either blindly or in a training-assisted manner. The proposed blind code-timing estimators require only the spreading code of the desired user, whereas the training-assisted schemes assume the additional knowledge of the transmitted symbols of the desired user. Through a design parameter of user choice, the proposed schemes offer flexible tradeoffs between performance, user capacity, and complexity. They can deal with both time- and frequency-selective fading channels. Numerical simulations show that the proposed schemes are near-far resistant, and compare favorably to an earlier subspace code-timing estimation scheme that is implemented in the time domain.     

All digital fast lock DLL-based frequency multiplier

One of the most important parameters in the design of synthesizers is lock time. A new fast lock delay locked loop (DLL) based frequency multiplier is proposed in this paper. Phase detector, charge pump and loop filter in conventional DLLs are replaced by a digital signal processor in the proposed structure. This leads to have better lock time, higher speed and smaller chip aria. The proposed structure can be implemented easily in a real system by means of a suitable powerful digital signal processor. Simulation has been done for 11 delay cells as a delay chain and input frequency equal with 300 MHz. The output frequency is multiplied by 11 (f_OUT = 3.3 GHz), and lock time is obtained about 13 ns which is equal to 4 clock cycles of reference clock.     

A new construction of signature waveforms for synchronous CDMA systems

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.     

A robust PN code tracking algorithm for frequency selective Rayleigh-fading channels

In this letter, we propose a new tracking scheme that is robust against multipath fading for pseudonoise (PN) code tracking in direct-sequence-spread spectrum systems. The proposed scheme employs an adaptive filter whose taps are adapted using a block least-mean square algorithm and it results in minimizing the effect of multipath interference on the tracking performance. We show that the mean-squared tracking error performance of the proposed scheme is not affected by the presence of closely spaced paths (e.g., one to three chips), unlike that of conventional delay locked loops. We also show that the tap-weight distribution of the filter provides accurate estimates of the multipath delays. For example, at E/sub b//N/sub 0/=5 dB, 98% of the time the path estimates lie within one sample (1/5 of a chip) from the actual delays. Furthermore, simulation results suggest that multipath delays over a wide range of terminal speeds can be tracked successfully. The proposed scheme is well suited for wideband code-division multiple-access systems where a large number of closely-spaced multipath components need to be tracked and used in RAKE combining.     

SIR-optimized weighted linear parallel interference canceller on fading channels

In this letter, we present a weighted linear parallel interference canceller (LPIC) where the multiple access interference (MAI) estimate in a stage is weighted by a factor before cancellation on Rayleigh fading and diversity channels. We obtain exact expressions for the average signal-to-interference ratio (SIR) at the output of the cancellation stages which we maximize to obtain the optimum weights for different stages. We also obtain closed-form expressions for the optimum weights for the different stages. We show that this SIR-optimized weighted LPIC scheme clearly outperforms both the matched filter (MF) detector as well as the conventional LPIC (where the weight is taken to be unity for all stages), in both near-far as well as non-near-far conditions on Rayleigh fading and diversity channels.     

Blind adaptive multiuser detection for asynchronous dual-rateDS/CDMA systems

In this paper, the authors consider an asynchronous direct-sequence code division multiple access (DS/CDMA) system wherein users are allowed to transmit their symbols at one out of two available data rates. Three possible access schemes are considered, namely, the variable spreading length (VSL), the variable chip rate (VCR), and the variable chip rate with frequency shift (VCRFS) formats. Their performance is compared for the case that a linear one-shot multiuser receiver is employed. It is also shown that detection of the users transmitting at the higher rate requires a periodically time-varying processing of the observables. Moreover, the problem of blind adaptive receiver implementation is studied, and a cyclic blind recursive-least-squares (RLS) algorithm is provided which is capable of converging to the periodically time-varying high-rate users detection structure. Numerical results show that the proposed receivers are near-far resistant, and that the VCRFS access technique achieves the best performance. Finally as to the adaptive blind receiver implementation, computer simulations have revealed that the cyclic RLS algorithm for blind adaptive high-rate users demodulation outperforms the conventional RLS algorithm in most cases of primary importance     

Time-delay estimation of multiple targets with wavelet based phase-only matched filter

As a well-known variation of the conventional matched filter (MF), the phase-only matched filter (POMF) shows superior range resolution over the MF and thus is widely used in image and radar signal processing. However, performance of the POMF degrades considerably if the input signal-to-noise ratio falls below a certain threshold value. In this paper, a novel time-delay estimator, wavelet based phase-only matched filter (WPOMF) is proposed to provide accurate time-delay estimation for narrow-band radar systems in the presence of multiple targets and high-level noise. Both the theoretical analysis and simulation results illustrate that the proposed method yields superior performance in terms of the estimation accuracy of multiple targets over the conventional or modified MF based time-delay estimators. The simulation results also illustrate that the proposed method has a low computational cost and low SNR threshold close to the estimation accuracy of the maximum-likelihood method.     

Block-Shanno multi-user constant modulus algorithm for use inmulti-user CDMA environment

In a multi-user direct sequence code division multiple access (DS-CDMA) system, retrieval of transmitted signals with a conventional receiver is difficult when there is chip asynchronism, multipath propagation and the associated near-far problem. Results presented show that despite these problems an adaptive linear receiver based on a mixed cross correlation and constant modulus algorithm (CC-CMA) has the potential to retrieve all users simultaneously. Compared with the conventional CC-CMA algorithm, the proposed algorithm achieves faster convergence. Simulations support the improved convergence properties of the algorithm     

自适应部分干扰抵消多用户接收机

在DS/CDMA系统中,采用多用户检测技术可以有效地克服多址干扰(MAI,MultipleaccessInterference)及远近效应。本文提出了一种自适应部分干扰抵消(APPIC,AdaptivePartialParallelInterferencecancellation)算法,根据匹配滤波器输出的判决统计对多址干扰进行选通抵消,以提高系统性能。同时本文给出了自适应部分干扰抵消接收机的非相干实现方案,并给出了仿真结果。     

Multiuser detection of synchronous code-division multiple-accesssignals by perfect sampling

Code-division multiple-access (CDMA) is a multiplexing technique that shows significant advantages over analog and conventional time-division multiple access (TDMA) systems. This technology has become a driving force behind the rapidly advancing communications industry. In order to recover the transmitted signal at the receiver in a CDMA system, demodulating techniques are engaged, where a prominent role is played by the multiuser detector. We introduce a class of novel Bayesian multiuser detectors that are constructed by employing perfect sampling algorithms: the sandwiched CFTP and the Gibbs coupler. We show that the detector based on the sandwiched CFTP can be applied to systems with negative cross-correlations, whereas the Gibbs coupler detector can be used without restrictions. A salient feature of the proposed detectors is the use of exact (perfect) samples from posterior distributions. This feature provides them with several advantages over detectors based on the Gibbs sampler. Simulation results on systems with and without near-far resistance demonstrate improved performance of the proposed detectors over some other popular detectors. We also discuss some important computational issues of the proposed detectors     

宽带低相噪频率综合器设计与实现

为满足某雷达信号设计要求,文中基于国产小数锁相环芯片GM4704产生7.12～9.12 GHz的信号,采用传统的PLL方式产生,低相位噪声、低杂散的频率综合器。同时,给出了设计过程并对相关的设计参数进行分析,应用相关的PLL仿真软件对环路滤波器进行仿真设计,通过实际电路测试,相位噪声达到-97 dBc/Hz@1 kHz与理论计算较接近,杂散达到-70 dB。     

An Adaptive MMOE-PIC Detector for Asynchronous DS-CDMA Communications

We propose a modified linear parallel interference cancelation (PIC) structure using the adaptive minimum mean output-energy (MMOE) algorithm for direct-sequence code-division multiple-access (DS-CDMA) systems. The complexity of the proposed receiver structure is shown to be linear in the number of users and hence, lower complexity than the centralized minimum mean-squared error (MMSE) multiuser detector. It is demonstrated that the proposed receiver structure can significantly reduce the long training period required by the standard adaptive MMOE receiver in near-far environments. Both numerical and theoretical results show that the proposed receiver performs close to the optimum MMSE receiver whereas the conventional adaptive MMOE detector suffers from high BER’s due to the imperfect filter coefficients. Also our results show a three fold increase in the number of users when the MMOE-PIC is used relative to the conventional MMOE receiver. Furthermore, the transient behavior of the proposed MMOE-PIC receiver due to abrupt changes in the interference level is examined. It is shown that the proposed adaptive receiver offers much faster self recovery, with less signal-to-interference ratio (SIR) degradation, than the standard MMOE in sever near-far scenarios.     

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.