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     

Coherent orthogonal filter using pilot symbol-assisted channelestimation for DS-CDMA

A coherent orthogonal filter (COF) using pilot symbol-assisted channel estimation is proposed for DS-CDMA cellular mobile radio. In the proposed scheme, a complex fading envelope in the multi-path environment is estimated using pilot symbols, and tap coefficients of orthogonal filter are controlled for maximising the signal to interference ratio (SIR) of a RAKE combined signal. Computer simulation results show that the required E_b/N₀ of the proposed COF is reduced by ~10.0 dB compared to conventional matched filter receiver at an average BER of 3×10² when there are 10 users and processing gain is 31     

DS-CDMA系统中连续导频模式下最优信道估计方法

本文建立了RAKE接收机中信道估计器的时域数学模型,以信道估计器输出的估计值与实际信道参数之间的相关系数作为衡量信道估计精度的技术指标,给出了信道估计精度与RAKE接收机误比特率之间关系的数学公式,并指出基于MMSE准则的信道估计方法即为连续导频模式下使得RAKE接收机获得最小误比特率的最优信道估计方法.     

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。     

Robust channel prediction technique for decision directed RAKEreceivers

The authors describe a robust channel prediction technique for a direct sequence spread spectrum (DS-SS) system in a fast fading environment. Coherent diversity reception is employed for improved performance. To estimate the channel response, an FIR type linear prediction filter is employed for each RAKE tap. The stability of the decision directed receiver is achieved through differential encoding of the data bits. It is demonstrated through simulations that the performance of the proposed decision directed receiver is better than a receiver which relies on a pilot signal     

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.     

Near Optimum and Highly Efficient Decision-Feedback Receivers for Direct Sequence Ultra-Wideband Radio

This paper investigates three decision- feedback receivers for Direct-Sequence Ultra-Wideband Radio (DS-UWB) based on two-channel BPSK modulation with ternary spreading code. A compact and insightful vector-matrix signal model is built up for receiver design under UWB channel dispersion over many consecutive symbols. First, we consider the design of a RAKE with Decision Feedback (RAKE-DF) receiver. Using the Gaussian approximation technique, we derive its analytic performance under no error propagation. It is shown that the RAKE-DF receiver suffers from a significant error floor due to the uncancelled pre-cursor ISI (or pre-ISI abbreviated). Next, we design the MMSE-DF receiver to achieve a better performance by suppressing the pre-ISI, too. However, the MMSE receiver requires costly matrix inversion. Then a new receiver is investigated as the third one, which is called the RAKE with Bi-Directional Decision Feedback (RAKE-BDDF) receiver, for efficiently canceling both the post-(cursor) ISI and pre-ISI at symbol rate. It cannot only attain the matched filter bound approximately, but also maintain a similar complexity as that of the RAKE-DF receiver. Simulation and semi-analytic BER curves are included for performance comparison of the three receivers in the presence of the CM2 and CM4 UWB channels.     

A two-dimensional RAKE receiver architecture with an FFT-based matched filtering

This work proposes a two-dimensional (2-D) RAKE receiver, which is a spatial-temporal matched filter implemented in the frequency domain. To form a beam pattern, we calculate the spatial frequency spectra of received signals on the antenna array using fast Fourier transform (FFT). After FFT beamforming, a bank of FFT-based matched filters is used to perform code matching. Afterward, the code-matched signals are summed up with maximal-ratio combining through a spatial-temporal channel-matched filter implemented in the frequency domain. This 2-D RAKE receiver includes a channel sounder that is used to estimate the spatial and temporal channel impulse response parameters, such as delays, directions of arrivals, and complex gains of multipath components. Monte Carlo simulations have been used to evaluate the receiver bit-error rate performance in both static channel and mobile radio channel environments. Simulation results show that the RAKE receiver performs well in both kinds of channels.     

Downlink Specific Linear Equalization for Frequency Selective CDMA Cellular Systems

We derive and compare several linear equalizers for the CDMA downlink under frequency selective multipath conditions: minimum mean-square error (MMSE), zero-forcing (ZF), and RAKE. MMSE and ZF equalizers are designed based on perfect knowledge of the channel. The downlink specific structure involves first inverting the multipath channel to restore the synchronous multi-user signal transmitted from the base-station at the chip-rate, and then correlating with the product of the desired user's channel code times the base-station specific scrambling code once per symbol to decode the symbols. ZF equalization restores orthogonality of the Walsh-Hadamard channel codes on the downlink but often suffers from noise gain because certain channel conditions (no common zeros) are not met; MMSE restores orthogonality only approximately but avoids excessive noise gain. We compare MMSE and ZF to the traditional matched filter (also known as the RAKE receiver). Our formulation generalizes for the multi-channel case as might be derived from multiple antennas and/or over-sampling with respect to the chip-rate. The optimal symbol-level MMSE equalizer is derived and slightly out-performs the chip-level but at greater computational cost. An MMSE soft hand-off receiver is derived and simulated. Average BER for a class of multi-path channels is presented under varying operating conditions of single-cell and edge-of-cell, coded and un-coded BPSK data symbols, and uncoded 16-QAM. These simulations indicate large performance gains compared to the RAKE receiver, especially when the cell is fully loaded with users. Bit error rate (BER) performance for the chip-level equalizers is well predicted by approximate SINR expressions and a Gaussian interference assumption.     

二维RAKE接收机中一种新的信道矢量估计算法

在二维RAKE接收机中，通常仅用导频信道来估计信道矢量，当导频功率较低时，估计误差较大。本文提出一种新的信道矢量估计算法，该算法采用导频信道与业务信道联合估计技术，通过分集组合两个信道的估计结果，使信道矢量的估计精度得到了提高，仿真结果表明，该算法能有效地降低系统的误码率。     

Adaptive MMSE maximum likelihood CDMA multiuser detection

The well-known code division multiple access maximum likelihood receiver (MF-ML) uses a bank of matched filters as a generator of sufficient statistics for maximum likelihood detection of users transmitted symbols. In this paper, the bank of matched filters is replaced by a bank of adaptive minimum mean squared error (MMSE) filters as the generator of sufficient statistics. This formal replacement of the MF bank by the adaptive MMSE filter bank has significant conceptual consequences and provides improvement by several performance measures. The adaptive MMSE-ML receiver's digital implementation is significantly computationally simplified. The advantages of the proposed adaptive MMSE-ML receiver over the MF-ML receiver are: (1) ability to perform joint synchronization, channel parameter estimation, and signal detection where the signal is sent over an unknown, slowly time-varying, frequency-selective multipath fading channel; (2) increased information capacity in a multicellular environment; and (3) significantly improved bit error rate (BER) performance in a multicellular mobile communications environment. The information capacity and the BER of the proposed MMSE-ML receiver are analyzed. Numerical results showing the BER performance of the MMSE-ML receiver in a multipath channel environment are presented     

Turbo decoder SNR estimation with RAKE reception

The joint performance of a turbo decoder and RAKE receiver using the MAP algorithm depends on the accuracy of the channel reliability factor. In a high data rate/low processing gain environment, inherent interference that results from non-idealities of the RAKE receiver complicate the estimation of the channel reliability factor. The combined performance of a turbo decoder and RAKE receiver is analyzed in a timedispersive and time-varying channel with distinct multipath components. Approaches are examined for estimating the channel reliability factor using the limited information that is known by the RAKE receiver. The sensitivity of performance to SNR mismatches is computed. The impact of the processing gain and the number of multipath components on BER performance is analyzed along with the effect of the channel time coherence. By accounting for the non-ideal RAKE interference effects, improvements in the channel reliability factor calculation result in BER performance improvements on the order of 0.5-2 dB.     

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.     

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     

Comparison of pilot symbol-assisted and differentially detectedBPSK for DS-CDMA systems employing RAKE receivers in Rayleigh fadingchannels

The capacity of a code-division multiple-access (CDMA) system is a function of the bit error rate (BER) performance of individual users. Therefore, it is important to optimize the individual links before proceeding to system level analysis. This is particularly true for operating in a fading channel where the performance without diversity reception is rather poor. This paper compares the BER performance of differential detection and pilot symbol-assisted coherent detection of a direct-sequence (DS) spread-spectrum (SS) signal on a frequency-selective Rayleigh fading channel using RAKE reception. Both equal gain and maximal ratio combining are considered, and the effect of convolutional coding with interleaving is studied. It is shown that in the particular cases considered in this paper, rate 1/8 convolutionally encoded pilot symbol-assisted BPSK performs better than coded differential detection, thus providing a higher system capacity     

相关Nakagami信道下信道估计器对RAKE接收机性能影响研究

在实际环境中RAKE接收机在接收信号时都要进行信道系数估计,以便进行相干合并。文章研究了不采用导频信号与采用导频信号两种估计器,具有估计误差情况下,在相关Nakagami信道下的RAKE接收机的性能,并且通过特征函数法得到了误码率公式的闭合表达式。比较了两种估计器对系统性能的影响,最后给出了数值结果,可以看出,当导频信号能量与信号能量相等,且导频数目比较多时（大于5）,采用导频信号的系统性能优于直接估计信道系数的系统。直接估计信道系数的系统性能在信噪比大于5dB后,其性能远远优于导频信号的估计系统。     

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.     

Adaptive parameter estimation using parallel Kalman filtering forspread spectrum code and Doppler tracking

We present a new digital direct-sequence (DS) receiver with joint estimation of code delay, multipath gains and Doppler shift. A parameter estimator consisting of a parallel bank of extended Kalman filters (EKF's) extracts estimates of the timing, τ and the multipath coefficients, f_l distorting the received signal. A “detected” estimate of the Doppler shift, v_r distorting the received signal is also provided by the estimator. We compute the bit error rate that results when a RAKE matched filter uses the estimated parameters to detect the DPSK encoded binary data in the received signal. The bit-error rate (BER) is evaluated, and successful performance of the proposed receiver in the presence of Doppler shift distortion is observed in many cases. We demonstrate that the receiver can operate when the multipath coefficients vary in time (Doppler spread)     

Experiments on pilot symbol-assisted coherent multistageinterference canceller for DS-CDMA mobile radio

A three-stage coherent multistage interference canceller (COMSIC) employing pilot symbol-assisted (PSA) channel estimation for replica generation of multiple access interference (MAI) is implemented and its performance in the presence of frequency selective multipath fading is experimentally evaluated by a multipath fading simulator. A fast transmission power control (TPC) method suitable for COMSIC is also proposed, in which the signal-to-interference plus background noise power ratio (SIR) at the matched filter (MF) based RAKE receiver is measured to achieve a short TPC delay and the target signal-to-interference ratio value is compensated by an outer loop so that the measured block error rate (BLER) is equal to the prescribed target value. The experimental results show that as expected the COMSIC satisfactorily reduces the MAI even when the number of active users is equal to the spreading factor in a multipath fading environment, and thus, improves the bit error rate (BER) performance in a multiuser environment. The results also show that the proposed fast TPC method with a two-slot delay associated with COMSIC works satisfactorily and the combination of COMSIC and fast TPC significantly decreases the transmission power of a mobile station (required transmission power of a mobile station with COMSIC at the average BER of 10^-3 is decreased by approximately 2.0 (3.0) dB compared with the MF-based RAKE receiver with (without) antenna diversity reception). This extends the cell coverage, battery life, and increases the system capacity in the reverse link     

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.