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.     

Spectrum Sensing Based on Deep Learning Classification for Cognitive Radios

Spectrum sensing is a key technology for cognitive radios.We present spectrum sensing as a classification problem and propose a sensing method based on deep learning classification.We normalize the received signal power to overcome the effects of noise power uncertainty.We train the model with as many types of signals as possible as well as noise data to enable the trained network model to adapt to untrained new signals.We also use transfer learning strategies to improve the performance for real-world signals.Extensive experiments are conducted to evaluate the performance of this method.The simulation results show that the proposed method performs better than two traditional spectrum sensing methods,i.e.,maximum-minimum eigenvalue ratio-based method and frequency domain entropy-based method.In addition,the experimental results of the new untrained signal types show that our method can adapt to the detection of these new signals.Furthermore,the real-world signal detection experiment results show that the detection performance can be further improved by transfer learning.Finally,experiments under colored noise show that our proposed method has superior detection performance under colored noise,while the traditional methods have a significant performance degradation,which further validate the superiority of our method.     

Receiver-Initiated Collision Avoidance in Wireless Networks

Many medium-access control (MAC) protocols for wireless networks proposed or implemented to date are based on collision-avoidance handshakes between sender and receiver. In the vast majority of these protocols, including the IEEE 802.11 standard, the handshake is sender initiated, in that the sender asks the receiver for permission to transmit using a short control packet, and transmits only after the receiver sends a short clear-to-send notification. We analyze the effect of making the collision-avoidance handshake, receiver initiated and compare the performance of a number of receiver-initiated protocols with the performance of sender-initiated collision avoidance protocols. Analytical and simulation results show that the best-performing collision avoidance MAC protocol based on receiver-initiated or sender-initiated collision avoidance is one in which a node with data to send transmits a dual-purpose small control packet inviting a given neighbor to transmit and asking the same neighbor for permission to transmit. The receiver-initiated protocols we present make use of carrier sensing, and are applicable to either baseband or slow frequency-hopping radios in which an entire packet can be sent within the same frequency hop (which is the case of frequency hopping spread spectrum (FHSS) commercial radios).     

Broadband multicarrier communication receiver based on analog to digital conversion in the frequency domain

This paper introduces a multicarrier communication receiver for broadband applications based on analog to digital conversion (ADC) of the received signal in the frequency domain. The samples of the spectrum of the received signal are used in the digital receiver to estimate the transmitted symbols through a matched filter operation in the discrete frequency domain. The proposed receiver is aimed at the reception of high information rates in a multicarrier signal with very large bandwidth. Thus, the receiver architecture provides a solution to some of the challenging problems found in the implementation of conventional wideband multicarrier receivers based on time-domain ADC, since It efficiently parallelizes the A/D conversion reducing the sampling speed requirements. We show that the sampling rate requirements are relaxed as the number of frequency samples is increased, which introduces a trade-off between complexity and sampling rate. The new receiver possesses additional advantages, including scalability with increasing frequency samples, the possibility of optimally allocating the available number of bits for the ATD conversion across the frequency domain samples which potentially reduces the distortion introduced by the high-speed ADC, narrowband interference suppression that can be directly carried out in the frequency domain, and inherent robustness to frequency offset which makes it an attractive solution when compared with traditional multicarrier receivers. We also investigate how the proposed receiver responds to common multicarrier communication receiver problems such as phase noise and channel frequency selectivity.     

Performance of orthogonal multicarrier CDMA in a multipath fadingchannel

A new multicarrier direct sequence code-division multiple-access (DS-CDMA) system is proposed. Transmitted data bits are serial to parallel converted to a number of parallel branches. On each branch each bit is direct-sequence spread-spectrum (DS-SS) modulated and transmitted using orthogonal carriers. This procedure provides the following advantages: the transmission bandwidth is more efficiently utilized, the effect of frequency selective multipath interference can be mitigated, and frequency/time diversity is achieved. The system is analyzed with both a conventional matched-filter (MF) receiver and a RAKE receiver for each carrier. The performance is compared to that of the conventional single carrier system with RAKE receiver. It is shown that the multicarrier system is able to out-perform the RAKE receiver when the system parameters are selected properly     

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 fully digital noncoherent and coherent GMSK receiver architecturewith joint symbol timing error and frequency offset estimation

We propose a fully digital noncoherent and coherent Gaussian minimum shift keying (GMSK) receiver architecture with joint frequency offset compensation and symbol timing recovery. Carrier phase offset can be estimated if the coherent demodulation mode is adopted. The converted base-band complex signal is first frequency discriminated and then passed through a digital filter which performs a fast Fourier transform (FFT). The frequency offset can be estimated from the DC component of the FFT, and the symbol timing error can be estimated from the phase angle of the FFT at a specified frequency which is equal to an integral multiple of half the bit rate. These two estimated parameters are then used for frequency offset compensation and symbol timing recovery during a preamble period. Coarse carrier phase can be estimated by averaging sampled in-phase and quadrature-phase signals and finding its phase angle within the preamble period after carrier frequency offset is estimated and compensated. The bit error rate (BER) performance of this GMSK receiver architecture is assessed for an additive white Gaussian noise (AWGN) channel by computer simulation     

联合两级滤波与压缩感知的测距仪干扰抑制方法

针对测距仪(Distance Measure Equipment , DME)脉冲信号对L频段数字航空通信系统1(L-band Digital Aeronautical Communication System 1 , L-DACS1)接收机产生干扰的问题,提出一种联合两级滤波与压缩感知的DME干扰抑制方法。首先,接收机通过高通滤波器估计出DME干扰,并在时域将其从接收信号中消除;然后,通过低通滤波器进一步滤除残留的DME干扰和噪声;最后,针对经过两级滤波后仍残留的DME干扰,基于其在时域中的稀疏特性应用压缩感知算法实现重构并消除。数值仿真表明,所提方法可获得的DME干扰功率衰减值约41dB,有效克服了DME脉冲干扰对L-DACS1系统接收机的影响。      

Cyclic prefixed single carrier and multicarrier transmission: bit rate comparison

We consider cyclic prefixed single carrier and adaptive multicarrier transmission over a frequency selective channel. We compare the achievable bit rate for a target bit error rate. We analytically prove that the bit rate achieved with multicarrier transmission with adaptive modulation is always higher than that obtained with single carrier transmission when a one-tap frequency domain equalizer is used. We also show that the same adaptively loaded multicarrier scheme reaches the performance of single carrier transmission with a block decision-feedback equalizer.     

Performance of Convolutionally Coded Multicarrier DS-CDMA System

A new convolutionally coded direct sequence （DS） CDMA system is proposed. The outputs of a convolutional encoder modulate multiple band-limited DS-CDMA waveforms. The receiver detects and combines signals for the desired user and feeds a soft-decision Viterbi decoder. The performance of this system is compared to that of a convolutionally coded single carrier DS CDMA system with a Rake receiver. At roughly equivalent receiver complexity, results will demonstrate superior performance of the coded multicarrier system.     

Iterative detection of turbo-coded offset QPSK in the presence of frequency and clock offsets and AWGN

The key contribution of this paper is to develop transmitter and receiver algorithms in discrete-time for turbo-coded offset QPSK signals. The procedure for simulating a clock offset between the transmitter and receiver is described. Due to the use of up-sampling, matched filtering and a differential correlation approach at the receiver, the time required for detecting the start of frame (SoF) is just around 500 symbols, which is also the length of the preamble. The initial estimate of the SoF and the frequency offset, obtained using the differential correlation approach, is improved using an iterative process. A novel two-step maximum likelihood (ML) frequency offset estimation is proposed, which significantly reduces the complexity over the conventional ML estimation. The decision-directed carrier and timing recovery algorithms use simple first-order IIR filters to track the carrier phase and clock slip. The proposed synchronization and detection techniques perform effectively at an SNR per bit close to 1.5?dB, in the presence of a frequency offset as large as 30% of the symbol-rate and a clock offset of 25?ppm (parts per million). It is shown via simulations that the performance loss with respect to the bare turbo code is only about 0.5?dB, for a preamble length of 500 and a BER of 10^?7. The proposed techniques are well suited for software implementation.     

一种软件无线电GMSK非相干接收机

本文给出了一种适用于软件无线电的突发模式非相干GMSK接收机。该接收机可以在很短的前导字序列内完成载波和符号同步，采用线性近似的非相干检测算法来对非线性的GMSK。信号实现解调。该接收机采用前馈结构实现快速同步，同时具有较低的计算复杂度和相对优越的性能，适合于在线性软件无线电接收机平台上实现。通过仿真给出了该接收机与几种典型GMSK。接收机的性能比较，并且在实际软件无线电平台上得到验证。     

Performance of a New Premulticoded Multicarrier DS-CDMA System in Nakagami Fading

In this paper, we proposed a new multicarrier, direct sequence code division multiple access, the so-called premulticoded MC-DS-CDMA system. In this scheme, the basic MC-DS-CDMA system is augmented with a precoder and a multicode encoder at the transmitter. The multicode encoder is an inverse Walsh-Hadamard transform (IWHT) unit, while the precoder is a recently suggested ldquoconstant amplituderdquo encoder that guarantees that the IWHT output stream has binary level. While IWHT, multicarrier (MC) transmission, and direct sequence spreading provide us with frequency diversity, the constant amplitude precoder ensures that the transmitted waveform is spread BPSK signal, minimizing peak to average-power ratio (PAPR) problem. The multicode and (MC) combination also addresses the need for flexible high data rate services in present day wireless communications. In the receiver, after matched filtering or correlation demodulation for each subcarrier and WHT operation, a detection strategy based on minimum-mean-square-error (MMSE) criterion is used. We have derived an upper bound for the bit error rate (BER) performance in Nakagami fading channel. Numerical results are compared with those obtained from a single carrier DS-CDMA system (SC-DS-CDMA) and a MC-DS-CDMA scheme without the precoding and IWHT units. We show that our proposed MC-DS-CDMA scheme has a much better BER performance compared with both of these schemes.     

基于参差重复导言的频率同步算法

具有重复信号的导言是突发通信系统中常用的同步方法。本文设计了一种新颖的参差重复导言,并给出最小均方误差的频率同步算法。与传统算法相比,该算法不仅提高了频率捕获精度,捕获范围也有效扩大且倍数可调。虽然本文仅研究了该算法在单载波频域均衡系统中的应用,但其基本原理同样适用于多载波系统。     

Multicarrier CDMA systems using time-domain and frequency-domain spreading codes

For wideband code-division multiple-access systems, the paper introduces a multicarrier modulation scheme that performs the spreading simultaneously in the time and frequency domains. This scheme attains higher flexibility and spectrum efficiency because system parameters can be selected at will. The performance is compared with that of a single carrier RAKE system by calculating the probability of error over a frequency-selective Rayleigh fading channel. The proposed scheme outperforms the single carrier RAKE system if the system parameters are selected properly for given conditions, such as bandwidth and delay spread.     

一种基于机器学习的卫星重叠隐蔽通信方法

针对传统卫星重叠通信中单个掩护信号带宽以及功率容限不够的问题,利用卫星转发器频谱环境中多个掩护信号提出了一种频域分割-子谱功率控制联合优化的多掩护信号重叠通信方法,建立了隐蔽通信信号传输性能和隐蔽性能的双目标优化问题,信关站侧采用感知的历史频谱数据训练生成支持向量机回归预测模型,用来预测不同转发器频谱环境下隐蔽信号的通信性能和隐蔽性能,并将训练好的预测模型下载到通信终端;终端侧利用双目标背包算法将支持向量机回归预测模型预测的隐蔽信号的通信性能和隐蔽性能作为价值因素、掩护信号个数作为背包重量来选择转发器频谱环境中的掩护信号,并且求解出隐蔽信号的频域分割和子频谱的功率控制参数,从而实现终端通信信号隐藏在卫星转发器的频谱环境中的目的。     

Multiple-Access Interference Suppression for MC-CDMA by Frequency-Domain Oversampling

Multiple-Access Interference Suppression for MC-CDMA by Frequency-Domain Oversampling A technique for interference suppression in multicarrier code-division multiple-access (MC-CDMA) systems is proposed which exploits the structural differences in signals that arrive at the receiver with Doppler shifts or carrier offsets. Oversampling the received signal in the frequency domain and properly combining the samples provides the interference suppression. Frequency-domain oversampling is accomplished by using a time extension of the conventional MC-CDMA signal or unconsumed portions of the cyclic prefix. Furthermore, a receiver structure is introduced that despreads and combines groups of samples so that a linear minimum mean-squared error solution for combining the groups is easily found. This combining scheme increases the signal-to-interference ratio experienced by the desired user in the MC-CDMA system. In addition, the receiver performs as well in severe near–far scenarios when there is sufficient frequency separation between the signals of the desired user and an interferer. Numerical results show that the proposed receiver significantly outperforms the conventional MC-CDMA receiver in many channel realizations.     

基于信号波形协同提高无线通信系统容量的研究

针对有限频谱资源与迅速增长业务需求的矛盾,提出了一种基于信号波形协同的无线通信技术。在传统正弦载波通信系统的基础上,在相同的频谱范围内利用正弦信号和线性调频信号在频域和分数域能量积聚特性的对偶关系在正弦载波通信信号上叠加线性调频信号波形来传输不同的用户信息,并在接收端分别通过频域和分数域滤波处理实现两类用户信号波形的分别提取,从而达到提高频谱效率和增加系统容量的目的。建立了系统模型,给出了理论分析,并进行了数值验证。研究结果表明,在传统正弦载波通信体制的基础上,通过信号波形协同和变换域滤波处理可以有效地提高频谱效率,理论上能够将系统容量提升近一倍。     

A suboptimal receiver for joint spatial-temporal filtering andmultiuser detection in mobile radio communications

This paper proposes a suboptimal receiver for joint spatial-temporal filtering and multiuser detection in mobile radio communications using single carrier signaling. The proposed receiver is a reasonable approximation of the maximum likelihood (ML) based optimal receiver described in the present paper. A cascaded connection of an adaptive array antenna and an ML multiuser sequence estimator is the basis of the proposed receiver. The major advantages of the proposed receiver over conventional adaptive array antennas are: (1) delayed path components of desired signals can be effectively combined; (2) interference signals exceeding the degree of freedom; and (3) those having the same incident angle as that of desired path components can both be suppressed. The proposed receiver does not require prohibitively large computational complexity. Results of computer simulations presented in this paper show that the proposed receiver exhibits excellent performance even in severe multipath fading environments     

Scheme of Cooperative Spectrum Sensing Based on Adaptive Decision Fusion Algorithm

Spectrum sensing is one of the core technologies for cognitive radios(CR),where reliable detection of the signals of primary users(PUs) is precondition for implementing the CR systems.A cooperative spectrum sensing scheme based on an adaptive decision fusion algorithm for spectrum sensing in CR is proposed in this paper.This scheme can estimate the PU prior probability and the miss detection and false alarm probabilities of various secondary users(SU),and make the local decision with the Chair-Varshney rule so that the decisions fusion can be done for the global decision.Simulation results show that the false alarm and miss detection probabilities resulted from the proposed algorithm are significantly lower than those of the single SU,and the performance of the scheme outperforms that of the cooperative detection by using the conventional decision fusion algorithms.