A Low-PAPR Differential Frequency Shift Orthogonal Keying Transceiver for Multi-Carrier Spread Spectrum System over High Mobility Multipath Channels

A new differential transceiver with a frequency-shift orthogonal keying (FSOK) technique is proposed for the multi-carrier spread spectrum (MC-SS) system over high mobility multipath fading channels. The design of the transceiver involves the following stages. First, the data stream is mapped into MPSK-FSOK symbols and spreaded by the frequency-shift orthogonal sequences. Second, the differential block encoder is exploited to combat the mobile channels. The Chu sequence is adapted for initial differential encoding, making the post-IFFT transmit signals with a low peak-to-average power ratio. Next, for the receiver, the maximum ratio combining technique is used for the block-based differential frequency-domain equalizer, which can overcome the multipath fading channel effect without requiring channel estimation. Finally, an efficient maximum likelihood despreading and demapping scheme is used to detect the modulation symbols. Furthermore, the differential MC-SS transceiver can be easily re-configured for a MISO differential MC-SS system with high link quality. Simulation results show that, under high mobility multipath channels, the proposed SISO differential MC-SS system can outperform the conventional MC-SS system. The proposed MISO differential MC-SS system with space-time diversity gain and M-ary modulation gain also exhibits excellent performance.     

Delay-time=code division multi-access in frequency-selective channel

A new hybrid multi-access technique, called delay-time/code division multi-access (DT/CDMA), is proposed. Multiplexing is accomplished in both the delay-time domain and code domain. Delay-time division multiplexing is achieved by assigning different cyclic time delays to the same spreading sequence. At a receiver, frequency-domain equalisation, despreading, and demultiplexing are performed simultaneously in the frequency domain. The bit error rate performance when using the proposed DT/CDMA in a frequency-selective Rayleigh fading channel is evaluated by computer simulation.     

On the Design of Comb Spectrum Code for Multiple Access Scheme

A new methodology that applies frequency orthogonality to the spreading code design has been presented. The main purpose is to reduce the receiver complexity of the cyclic prefix code division multiple access (CP-CDMA) over the downlink channel and to enable parallel transmission over the uplink channel. The receiver complexity has been found for requiring full-band FFT and IFFT calculations in the channel equalizer. To handle this problem, we developed the comb spectrum codes organized in frequency orthogonal groups, in each of which the codes have time correlation orthogonality. For the downlink channel, the reduction of complexity in signal processing of a CSCDMA receiver is due to that the separation of groups is simple and each separated group is in the form of a narrower bandwidth CP-CDMA. Thus, the full-band FFT and IFFT can be replaced by the partial FFT and partial IFFT as shown in this proposed approach. For the uplink channel, we assign the code groups to different users to form a group division multiple access (GDMA) for enabling an independent channel equalization of each user at base station. The theoretical analysis and the simulation results agree well with each other and both confirm the effectiveness of this approach.     

无线体域网低复杂度重复码解扩频方法研究

为了提高数据传输的可靠性,降低窄带接收机的复杂度,无线体域网标准IEEE 802.15.6采用了基于重复码的扩频方法。通过设计一种根据动态阈值选择性进行量度运算的解扩频结构,并结合硬判决阈值特性利用简化的最大比合并以及广义最小距离原理,提出了硬阈值—合并法和硬阈值—删除法,计算复杂度较小,同时解扩频性能近似最优。理论分析及仿真结果表明,与基于最大似然方法的解扩频相比,其扩频增益接近理论值,在高信噪比环境下,计算复杂度可降低86%以上。     

海面的红外被动成像

本文在研究海面的红外成像特性中,建立了基于刚体的双尺度成像模型,从被动成像方面讨论了海面辐射特性的变化规律.该模型仿真结果很好地说明了风向对海面红外被动成像特性的影响.     

TDD-CDMA系统中一种新的多用户干扰抑制方案

传统的多用户处理是在上行进行的,本文提出了应用于TDD-CDMA系统中下行方向的一种多用户处理方案.在TDD-CDMA系统中,可以利用上行信道参数对下行信道参数做出估计.本文提出了一种实时利用信道参数来降低CDMA中的多用户干扰的方法.在该方案中,其多用户处理的基本思路是通过优化系统的多用户扩频码字和解扩码字来降低多用户干扰.为了达到最大的信干比,本文提出了系统扩频码选择的优化准则,它能够充分利用信道的特性.本文根据一定的迭代算法给出优化问题的解.数值分析结果表明,采用上述方案的CDMA系统相对于采用Gold码的系统能够极大地降低多用户干扰,提高信干比.     

A low complexity reduced-rank MMSE receiver for DS/CDMA communications

The minimum mean squared error (MMSE) receiver is a linear filter which can achieve optimal near-far resistance in direct-sequence code-division multiple-access communications. However, one of the main problems of this receiver is the required number of filter taps, which is typically large. This is especially true in systems with a large processing gain in which case the receiver's computation burden becomes very high. As a result, methods for reducing the complexity of the MMSE receiver have been of great interest in recent years. We propose an efficient partitioned MMSE receiver based on a classification algorithm. It is shown that the computational complexity (in terms of the filter taps) of the proposed receiver can be reduced significantly while good performance is maintained. Based on the special structure of our proposed receiver, we also propose a release-merge adaptive partition algorithm which can update the partition and the receiver's coefficients simultaneously. In particular, it is demonstrated that the proposed receiver can perform much better than previously proposed reduced-rank MMSE receivers, such as the partial despreading MMSE receiver and the cyclically shifted filter bank receiver, with even a smaller number of taps.     

A DS-CDMA system using despreading sequences weighted by adjustablechip waveforms

This paper evaluates the performance of a direct-sequence code-division multiple-access system using coherent receivers in which the despreading sequences are weighted by adjustable chip waveforms. The chip weighting waveforms under consideration are designed for multiple-access interference (MAI) rejection. Assuming that the received chip waveforms are rectangular, new expressions for the signal-to-interference-plus-noise ratio (SINR) of the decision variable are derived when different weighted despreading sequences (WDSs) are used in the receiver. The novelty of the derived expressions is that each of the expressions, when the system parameters are given, is determined only by one parameter of the adjustable chip waveforms employed. As a result, we can simply tune the parameter to its optimal value in real-time for MAI rejection without knowing the other users' spreading codes, timing, and phase. The criterion for tuning the parameter is to maximize the SINR of the decision variable based on the relative strength between the additive Gaussian white noise and the MAI. Numerical results show that when the multiple-access interference is significant, the receivers using WDSs outperform significantly the conventional receiver using a rectangular despreading sequence. Brief analysis for bandlimited spreading signals is also provided to reveal the practical implications of the proposed technique     

Spacecraft on-board SAR image generation for Eos-type missions

An assessment of feasibility is provided for onboard SAR (synthetic-aperture radar) image generation for Eos (Earth Observing System)-type missions. Specifically, a processor architecture is proposed using a frequency-domain processor for range correlation and a modular fault-tolerant time-domain processor for azimuth correlation. It is shown that using available space qualifiable technology to implement the proposed architecture, an onboard SAR processor having acceptable power, mass, and fault-tolerant characteristics should be feasible for Eos-type applications     

Performance of coherent receiver with weighted despreading sequencefor DS-CDMA

The performance of a DS-CDMA system using coherent receivers with the despreading sequences weighted by adjustable chip waveforms is presented. A new expression for the signal to interference plus noise ratio (SINR) of the decision variable is derived for the proposed receiver. When the system parameters are given, the derived SINR expression is determined only by one parameter of the weighted despreading sequence. As a result, the parameter can be simply tuned in practice for multiple access interference (MAI) rejection     

一种适用于在FPGA中实现频域均衡的高精度FFT结构

本文简单介绍了频域均衡基本原理 ,并针对其在 FPGA中的实现提出了一种基于频率抽取算法的 FFT实现结构。此结构具体服务于 2 5 6点基二 FFT变换 ,为充分满足在FPGA中实现频域均衡的要求 ,具有占用硬件资源合理 ,计算精度高的特点     

On reducing computational complexity of codebook search in CELPcoding

A computationally efficient codebook search method in code-excited linear prediction is proposed. The method can reduce the computational complexity by almost one half compared to the frequency-domain codebook search method that is currently regarded as the fastest search method, while giving almost the same quality of speech. This reduction is possible as a result of the simultaneous use of frequency-domain search and code vector sparsity     

Rate-Distortion Optimized Video Transmission Over DS-CDMA Channels with Auxiliary Vector Filter Single-User Multirate Detection

In this paper, we consider the rate-distortion optimized resource allocation for video transmission over multi-rate wireless direct-sequence code-division-multiple-access (DS-CDMA) channels. We consider the performance of transmitting scalable video over a multipath Rayleigh fading channel via a combination of multi-code multirate CDMA and variable sequence length multirate CDMA channel system. At the receiver, despreading is done using adaptive space-time auxiliary-vector (AV) filters. We propose a new interference cancelling design that uses just a single AV filter for single-user mutirate despreading. Our experimental results show that the proposed interference cancelling design has excellent performance in scalable video transmission over DS-CDMA systems that use a combination of multicode multirate and variable processing gain multirate CDMA. The proposed design takes advantage of the fact that single user's video data is transmitted using two spreading codes, one for the base layer and one for the enhancement layers, and of the fact that these spreading codes can have different processing gains. The proposed interference cancelling design is compared with two conventional single-user multirate CDMA receiver configurations, however now we use an AV filter rather than a simple matched filter. We also propose a resource allocation algorithm for the optimal determination of source coding rate, channel coding rate and processing gain for each scalable layer, in order to minimize the expected distortion at the receiver.     

A unified analysis for coded DS-CDMA with equal-gain chip combining in the downlink of OFDM systems

This paper presents a novel unified analysis for the bit-error rate in the downlink of convolutionally coded and single-user detected multicarrier code-division multiple access (CDMA), multicarrier direct-sequence CDMA, time and frequency-domain spreading CDMA, and orthogonal frequency-division multiple access. Performance is analyzed under the assumption of Hadamard spreading codes, equal-gain chip combining, and a highly correlated frequency-selective Rayleigh-fading channel. Closed-form expressions are presented for the cumulative distribution function, probability distribution function, and moment-generating function for the signal-to-noise plus interference ratio after despreading. The presented results assume error-free channel estimates, a perfectly synchronized receiver, and are found to reasonably well agree with simulation results.     

Fast unbiased echo canceller update during ADSL transmission

This paper presents an efficient method for updating echo canceller (EC) coefficients in asymmetric digital subscriber line transceivers during transmission that removes the dependency of updates on the far-end signal. The new method improves on the frequency-domain EC architecture by Ho et al. (1996), in that it converges faster, improves the stability to perturbation, and is not prone to bias from pilot tones. The new EC updating system architecture is presented for both the central office and remote terminal systems and is fully compliant with the ITU-T G.DMT standard.     

Efficient FPGA-Based Implementations of MIMO-OFDM Physical Layer

Recently, Multiple-input Multiple-output and Orthogonal Frequency Division Multiplexing (MIMO-OFDM)-based technologies have been deployed in advanced wireless communication systems. In this paper, we present a prototype Field Programmable Gate Array (FPGA) design for an efficient physical layer implementation of a MIMO-OFDM technique. We propose a pipelined architecture using a Fast Fourier Transform that is shared across modulations for the MIMO-OFDM system. As more channels are used, more resources can be saved by using our proposed architecture. Experimental results show that the proposed implementation saves at least 40 percent of the hardware resources, while achieving the same data rate as known baseline MIMO-OFDM implementations. In addition, we propose efficient hardware implementation of the Almouti encoder and decoder. They are also designed to use minimum hardware resource by using the proposed architecture.     

Iterative frequency-domain channel estimation and equalization for single-carrier transmissions without cyclic-prefix

Compared to conventional time-domain equalization, frequency-domain equalization (FDE) presents a computationally efficient alternative for the reception of single carrier (SC) transmissions. In this paper, we consider iterative FDE (IFDE) with explicit frequency-domain channel estimation (FDCE) for non-cyclic-prefixed SC systems. First, an improved IFDE algorithm is presented based on soft iterative interferencecancellation. Second, a new adaptive FDCE (AFDCE) algorithm based on per-tone Kalman filtering is proposed to track and predict the frequency-domain channel coefficients. The AFDCE algorithm employs across-tone noise reduction, exploits temporal correlation between successive blocks, and adaptively updates the auto-regressive model coefficients, bypassing the need for prior knowledge of channel statistics. Finally, block-overlapping is used to facilitate the joint operation of IFDE and AFDCE. Simulation results show that, compared to related IFDE and adaptive channel estimation schemes, the proposed schemes offer lower mean-square error (MSE) in channel prediction, lower bit error rate (BER) after decoding, and robustness to non-stationary channels.     

Band-limited DS-CDMA using exponentially weighted despreadingsequence

The signal-to-interference ratio (SIR) performance of a band-limited DS-CDMA system using an exponentially-weighted despreading sequence (EWDS) in the receiver is addressed. It is shown that tuning a parameter of the EWDS helps to flatten the in-band spectrum of the received coloured signal, leading to a better performance when the transmission bandwidth is limited. With the constraint of the system transmission bandwidth equal to and twice the chip rate, numerical results show that the SIR of the proposed system can be increased up to 28 and 94%, respectively, when compared with a conventional receiver using a rectangular despreading sequence     

DVB-CID信号解调算法与性能研究

载波标签(CID, Carrier Identification)技术是近年来提出的从技术与标准上根本解决卫星通信无线频率干扰问题的创新技术。本文针对DVB-CID信号的解扩及信息提取进行了深入研究,提出了一种基于主载波消除处理模式下的抗频偏解扩算法,此算法无需载波波形重构,复杂度低,抗噪性能好,可广泛应用于此类扩频隐藏信号的解调。仿真表明,此算法解扩性能接近理论限,应用于CID信号的解调接收工程实践可获得良好效果。      

Frequency-Domain Set-Membership Filtering and Its Applications

Frequency-domain adaptive filtering is appealing in many applications, particularly channel equalization. This paper presents frequency-domain set-membership filtering (F-SMF) and derives adaptive algorithms for F-SMF. The F-SMF is employed to design single-carrier frequency-domain equalizer (SC-FDE). With an unconventional parameter-dependent error-bound specification, an F-SMF algorithm is derived and shown to provide superior performance with sparse updates of parameter estimates. Exploring the feature of sparse updates, we present an innovative parallel adaptive architecture that shares the updating processors and that finds natural appeal in frequency-domain diversity combining and equalization for very dispersive fading channels like those found in broadband wireless communications