On the architecture and performance of an FFT-based spread-spectrumdownlink RAKE receiver

This paper describes a spread-spectrum downlink RAKE receiver that computes data detection in the frequency domain. We assume a pilot signal is transmitted with data signals for channel sounding. The pilot signal does not degrade the receiver bit error rate (BER) performance because the receiver estimates the pilot signal and subtracts the estimated pilot signal from the received signal before data detection. A spreading code matched filter, a channel matched filter, and a sounding receiver are implemented by fast Fourier transform (FFT)-based matched filtering and integrated in a unified architecture. Monte Carlo simulation is used to evaluate the receiver BER performance in both a static channel and a mobile radio channel. Simulation results show that the RAKE receiver performs well in both kinds of channels     

2-D rake receiver based on space-time channel parameter estimation

2-D RAKE receiver is an efficient way to realize the space-time processing for CDMA systems with aperiodic spreading codes. The Direction Of Arrival (DOA) and the relative time delay of every user's multipath must be known to realize the 2-D RAKE receiver. In the third generation CDMA mobile communication system, auxiliary pilot channel is used in the uplink channels. The different user's Vector Channel Impulse Response (VCIR) can be estimated from the pilot channel easily. The VCIR contains spatial and temporal information. In this paper, by utilizing the known pulse shape function, a parameter matrix method used to estimate the Spatial Signature Vector (SSV) and the relative time delay is proposed in frequency domain. The DOA can be estimated from the SSV. By reconstructing the SSV and utilizing approximate Capon space filter, the performance of the 2-D RAKE receiver with uniform circular array can be improved with a little additional computation work.     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

A receiver of simple structure for antenna array CDMA systems

In code division multiple access systems, the two-dimensional (2-D) RAKE structure had been proposed using multiple antennas to increase the capacity. It consists of multiple fingers of a beamformer and a correlator. In this paper, we consider a simpler receiver structure. We only use one finger, which consists of one pair of spatial and temporal filters to combine signals. It is shown that the performance of the receiver system is comparable with that of the 2-D RAKE receiver. Furthermore, it is observed that the proposed receiver and the 2-D RAKE receiver have limited near-far resistance by the generation of the space domain due to antenna arrays     

Low-complexity space-time processor for DS-CDMA communications

A novel wideband beamforming technique for cellular CDMA systems is presented in this paper. The proposed algorithm asymptotically provides the maximum SINR estimate of the signal with the desired code (SDC) by optimally combining desired signals from different paths and canceling strong multiuser access interference (MUAI). A two-dimensional (2-D) matched filter structure is used where not only different temporal samples of the matched filter output are processed but where those from matched filters connected to different antennas are processed as well. In contrast to previously proposed techniques, an exact code synchronization for the SDC is not required. The algorithm presented herein asymptotically provides the arrival time of the multipaths within a bit period and the optimum beamformers for extracting each of them. Space-time filters for combining the fingers across both space and time while canceling the MUAIs are constructed correspondingly. The instrumental property exploited by this technique is the fact that although the respective spectra of the SDC and MUAI components at the matched filter output are statistically identical, the respective spectra of their squared values differ. A simplified RAKE structure-based receiver is also proposed. The 2-D RAKE receiver considerably decreases the computations but requires a coarse SDC code synchronization. A technique to achieve coarse SDC code synchronization is also proposed     

Bit error probability of a matched filter in a Rayleigh fadingmultipath channel in the presence of interpath and intersymbolinterference

The exact bit error probability of a matched filter receiver in a known discrete Rayleigh fading multipath channel is determined for a binary antipodal system. The matched filter receiver or the RAKE is one form of a diversity receiver. Traditionally, both the interpath interference (IPI) and the intersymbol interference (ISI) caused by the non-ideal autocorrelation function of the bit waveform are neglected in the analysis of the RAKE. Previously, the exact matched filter bounds; which include IPI, were derived. This bound gives the performance of the matched filter if only one symbol is transmitted. In our analysis, a symbol sequence is transmitted instead of only one symbol. Thus, our analysis includes both IPI and ISI as well as the correlation between the paths, The analysis permits direct comparisons between different kinds of bit waveforms     

Performance of a Multicarrier CDMA System with Rayleigh Fading Multipath Channels

This paper examines the performance of a multicarrier CDMA system in a Rayleigh fading channel without a RAKE receiver. Since the multicarrier technique is a diversity reception technique used in the frequency domain, the signal is transmitted at different frequency bands and undergoes different fading. The superimposed signal of the multipath components after going through the mobile channel will be processed by an Fourier Transform to enter the frequency domain, where the time delays in various paths are converted to the phase shifts in the compound signal. In this paper, we use sounding bits to extract the characteristics of the mobile channel and to partly remove the multipath effect. From the simulation, it is found that the multicarrier technique is simpler than the technique used in the single carrier system with a RAKE receiver while producing a better performance than that of the RAKE receiver.     

Small delay multipath diversity in spread spectrum communication systems

A new double-filter receiver for spread spectrum wireless systems is presented. The receiver exploits the inherent diversity due to the small delay difference between the multipaths which may not be exploited by the conventional RAKE receiver. The bit error rate performances of both the coherent and the noncoherent detectors are analyzed and compared with the ideal RAKE receiver and the matched filter bound. We also examine the best and the worst cases of a three-path fading channel for the proposed receiver. Optimum pulse shapes under various channel conditions are designed. Implementation issues in code-division multiple access systems are discussed. The numerical results show that the proposed receiver achieves significant gains for a given spreading factor under a complexity constraint.     

智能天线中一种迭代的联合信道估计 与符号检测算法

在传统的二维RAKE接收机中,通常使用导频信道来估计信道矢量,进而求取加权系数,用于业务信道符号序列的检测.当导频功率较低时,信道估计误差较大,系统性能较差.本文提出了一种新的算法,先利用导频进行信道矢量的初始估计,然后用带投影的迭代最小二乘法,进行信道和符号序列的联合估计,该算法具有计算复杂度低,收敛速度快的特点.仿真结果表明:与传统二维RAKE接收机相比,该算法使系统的误码性能得到了有效改善.     

Signal Processing for Passive Radar Using OFDM Waveforms

Passive radar is a concept where illuminators of opportunity are used in a multistatic radar setup. New digital signals, like Digital Audio/Video Broadcast (DAB/DVB), are excellent candidates for this scheme, as they are widely available, can be easily decoded to acquire the noise-free signal, and employ orthogonal frequency division multiplex (OFDM). Multicarrier transmission schemes like OFDM use block channel equalization in the frequency domain, efficiently implemented as a fast Fourier transform, and these channel estimates can directly be used to identify targets based on Fourier analysis across subsequent blocks. In this paper, we derive the exact matched filter formulation for passive radar using OFDM waveforms. We then show that the current approach using Fourier analysis across block channel estimates is equivalent to the matched filter, based on a piecewise constant assumption on the Doppler-induced phase rotation in the time domain. We next present high-resolution algorithms based on the same assumption: first we implement MUSIC as a 2-D spectral estimator using spatial smoothing; then we use the new concept of compressed sensing to identify targets. We compare the new algorithms and the current approach using numerical simulation and experimental data recorded from a DAB network in Germany.      

DS-CDMA的一种时频二维联合解扩方法

多径环境中传统的DS-CDMA信号接收,一般采用RAKE接收技术.本文提出一种不使用RAKE接收机的DS-CDMA接收算法:基于时频二维扩频的思想,考虑DS-CDMA发射机发射的一个扩频符号对应时间内的信号,在接收机中先将接收到的信号做一个比扩频增益N小的DFT变换,联合对多个DFT变换后的信号进行扩频解扩处理,从而恢复出发射的原始信息比特.本文对比了DS-CDMA传统的RAKE接收算法、基于时频二维扩频的DS-CDMA接收算法(2DSS)、以及Wang等提出的具有信道选择的频域解扩算法的复杂度,并就性能进行了计算机仿真验证.结果表明,在根均方时延较小信道中的性能优于传统RAKE接收算法和具有信道选择的频域解扩算法的性能.     

60 GHz芯片间无线互连系统的MMSE-RAKE接收机误码性能分析

针对60 GHz芯片间无线互连信道中存在的多径衰落问题,将匹配滤波器和最小均方误差算法应用到60 GHz脉冲通信系统,重点分析多径信道下采用最小均方误差合并算法的RAKE接收机的误码性能。在IEEE 802.15.3c信道模型的基础上,对采用不同合并方式、不同干扰用户数目下的RAKE接收机误码性能进行了研究。仿真结果表明,随着干扰芯片数量的增加,引入匹配滤波器和最小均方误差算法的RAKE接收机不仅降低了接收机的采样率,而且有效提高了系统抗多用户干扰的能力,为芯片间无线互连系统的RAKE接收机设计提供了技术参考。     

A 123-mW W-CDMA uplink baseband receiver IC with beamforming capability

In this paper, architecture and circuit design of a beamforming baseband receiver IC for uplink W-CDMA communication systems is presented. In the proposed receiver, a four-antenna-element beamformer and a four-finger RAKE combiner are adopted to exploit both spatial diversity and path diversity receiving. To minimize the size and power consumption of the receiver, a latch-based 1024-tap complex delay line is custom designed for the matched filter in the channel estimation circuit. The receiver chip was fabricated in a 0.35-/spl mu/m n-well CMOS single-poly quadruple-metal technology. The minimum supply voltage with the chip running at the nominal 15.36-MHz clock rate is measured at 2.15 V. The chip has an area of 6 mm by 6.3 mm and a power consumption of about 123 mW.     

Performance of DS-CDMA receivers with MRC in nakagami-m fading channels with arbitrary fading parameters

The receivers that combine spatial antenna diversity with temporal multipath diversity are known as two-dimensional (2-D) RAKE receivers. In this paper, we consider the outage probability and the bit error rate performance of a coherent binary phase shift keying 2-D RAKE receiver in the context of an asynchronous direct sequence (DS)-code division multiple access (CDMA) system operating in a Nakagami-m fading channel with real and arbitrary fading parameters. The closed-form expressions derived for the two wireless performance measures are easily evaluated numerically and enable the link designer to examine the effects of system parameters, such as the number of receive antennas, RAKE fingers per antenna, and asynchronous CDMA users in the cell, as well as channel conditions, such as the amount of fading in the combined paths and the multipath intensity profile of the channel on the link performance. In addition, the diversity loss due to correlated fading among the spatially separated RAKE fingers is quantified.     

Equalization for DS-UWB Systems—Part I: BPSK Modulation

Ultra-wideband wireless transmission has attracted considerable attention both in academia and industry. For high-rate and short-range transmission, direct sequence based ultra-wideband (DS-UWB) systems are a strong contender for consumer market applications. Due to the large transmission bandwidth, the UWB channel is characterized by a long root-mean-square delay spread and the RAKE receiver cannot always overcome the resulting intersymbol interference. We therefore study equalization for DS-UWB systems. This paper is comprised of two parts. In this first part, we consider DS-UWB with binary phase-shift keying (BPSK) modulation, which is the mandatory transmission mode for DS-UWB systems promoted by the UWB Forum industry alliance. We derive matched filter bounds for optimum equalization taking into account practical constraints like receiver filtering, sampling, and the number of RAKE fingers when RAKE preprocessing is applied at the receiver. Our results show that chip-rate sampling is sufficient for close-to-optimum performance. For analysis of suboptimum equalization strategies we further study the distribution of the zeros of the channel transfer function including RAKE combining. Our findings suggest that linear equalization is well suited for the lower data rate modes of DS-UWB systems, whereas nonlinear equalization is preferable for high-data rate modes. Moreover, we devise equalization schemes with widely linear processing, which improve performance while not increasing equalizer complexity. Simulation and numerical results confirm the significance of our analysis and equalizer designs and show that low-complexity (widely) linear and nonlinear equalizers perform close to the pertinent matched filter bound limit.     

Bidirectional iterative ISI canceller for high-rate DSSS/CCK communications

This paper proposes a new RAKE receiver incorporated with a bidirectional iterative intersymbol interference (ISI) canceller in order to reinforce multipath robustness of high-rate direct-sequence spread-spectrum complementary code keying (DSSS/CCK) systems. The proposed RAKE receiver first combines multipath signal components through a channel matched filter (CMF) and removes postcursor-ISI by employing a codeword decision feedback equalizer (DFE). Then, a CCK codeword detector tentatively determines the current CCK codeword symbol and reuses it to subtract precursor-ISI from the previous symbol. Therefore, the ultimate symbol decision is made using the delayed signal with both postcursor-ISI and precursor-ISI cancelled. The detection performance can be more improved through an iterative refinement processing between the postcursor and the precursor components. Simulation results exhibit a significantly improved error rate performance of the proposed receiver compared with that of the legacy RAKE receiver employing only a postcursor DFE. The additional cost for realization of the proposed receiver is one symbol decision delay and reuse complexity of the DFE and the codeword detector.     

时变衰落信道中同步时间扩展信号的分集接收技术

扩频通信可以区分多径信号进行RAKE分集接收,是一种有效处理多径效应的手段。与此对应,时间扩展通信也可以在频域上进行类似RAKE的分集接收,有效处理时变衰落带来的影响。时间扩展会带来严重的符号间干扰,同步时间扩展可以有效控制和处理符号间干扰。基于信道分解的分集接收技术保证了同步时间扩展在时变衰落信道下的性能。获得10-4误码率,本文分集算法(扩展长度1024)在时变衰落信道中所需的信噪比与AWGN信道不分集处理所需的信噪比只相差1.4dB。     

短波低速率数据传输研究

以第三代短波通信网络标准中的低速数据传输规程为基础,设计了一种低信噪比且存在多径衰落的信道中数据传输的调制解调方案.方案中利用相关搜索、FFT捕获的方法捕获信号,利用滑动相关完成位同步和信道估计,利用RAKE接收机抗多径衰落,并用DSP实现了数据的低误码率传输.     

DS-CDMA系统新的空时并联结构 及二维自适应DBF算法

针对直接序列扩频码分多址(DS-CDMA)移动通信接收系统,本文提出了一种新的空时二维信号处理结构,这是一种"智能天线与瑞克(RAKE)接收机的并联连接"(PI-SA&RR)结构.基于PI-SA&RR结构,提出了空时二维自适应数字波束形成(ST2-DADBF)算法.与"二维RAKE接收机"(2-DRR)相比,ST2-DADBF算法不需要对延时-波达方向(DOA)进行估计就能够获得最优空-时域联合输出.与"导频符号辅助相干自适应天线阵列分集接收机"(PSA-CAAADR)相比,ST2-DADBF算法能够使智能天线和RAKE接收机联合参与自适应算法,不需要对RAKE分支信道参数进行估计,是一种真正的二维自适应算法.针对多用户移动通信系统,同时考虑到信道的时延扩展和空间角度弥散的影响,本文给出了一种接收信号的通用模型,基于该模型对ST2-DADBF算法进行了计算仿真.仿真结果表明,ST2-DADBF算法能够在空时二维域中捕获感兴趣用户的各个多径分量,并将这些多径分量同步相干合成,同时抑制其它用户在时域和空域形成的干扰,因而可获得良好的误码率(BER)性能.     

Combination performance of truncated decorrelator and coherent RAKE receiver for DS-CDMA

The combination performance of a truncated decorrelator and a coherent RAKE receiver using pilot symbol-aided channel estimation is evaluated by computer simulation for DS-CDMA in several multipath fading environments. Computer simulation results show that the proposed truncated decorrelator can within a window of several symbols, improve the bit error rate (BER) performances compared to the conventional matched filter receiver in fast Rayleigh fading environments