数字CATV系统中激光器削波对QAM信号影响的研究

目前的CATV网络不仅要传输一般的模拟信号,而且要能够传输QAM调制的数字信号。重点分析了数字CATV网络中数字频道和模拟频道的相互影响以及QAM调制的原理,讨论了在发射机端激光器削波对数字信号的影响,并给出了仿真结果。分析结果表明,QAM信号的误码率和QAM信号的光调制度、削波失真系数以及削波失真与高斯噪声之间的比例等都有密切的关系,激光器削波将严重恶化数字QAM信号的误码率。     

一种用于DVB-C的全数字QAM解调器结构

该文提出了一种适用于数字有线电视的全数字QAM解调器结构,对解调器的载波恢复、符号同步、自动增益控制和自适应均衡等进行研究分析,通过仿真给出了相应的参数,最后给出整个解调器在AWGN信道条件下的误码性能,从仿具结果可以看出,该文提出的解调器具有较好的性能,且易于全数字实现。     

The multitone channel

The maximum bit rate of multitone QAM (quadrature amplitude modulation) over a general linear channel is found. First, the overall bit rate for an AWGN channel with a two-level transfer function is maximized, using a multitone QAM system. The power distribution between the tones and the number of bits/symbol per tone is optimized for a given symbol error rate. Extending these results to the general channel, it is shown that the optimum power division for multitone signals is similar to the water-pouring solution of information theory. Furthermore, multitone QAM performance is about 9 dB worse than the channel capacity, independent of the channel characteristics. The multitone results throughout are compared to those of an equivalent single-tone linearly equalized system. The comparison shows that the multitone system is useful for some channels, e.g. those with deep nulls in the transfer function. The maximum bit error rate over a twisted-pair channel which is performance dominated by near-end crosstalk (NEXT) is also found     

Pseudobinary and Pseudoquaternary Detection Processes for Linearly Distorted Multilevel QAM Signals

The paper describes two new detection processes suitable for QAM signals that have 16 or more levels (possible data symbol values) and that have been subjected to linear distortion, causing severe intersymbol interference in the received signal. The detectors are developments of the pseudobinary systems previously described, and are designed to operate in the presence of severe intersymbol interference. Results of computer simulation tests are presented, comparing the tolerances to additive white Gaussian noise of various arrangements of each detector with those of more conventional detectors. The tests have been carried out on a model of a 19 200 bit/s synchronous serial data transmission system, operating with a 64-level QAM signal that is transmitted, in turn, over two different telephone circuits.     

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     

On differentially encoded star 16QAM with differential detectionand diversity

Star 16QAM is a modulation method that transmits 4 bits per symbol and has the advantage that it may be differentially encoded and detected. It is very robust to fast multiplicative Rayleigh fading and is suitable for mobile telephone systems and personal communication networks. The main contribution of this paper is the derivation and bit error probability simulation of the maximum likelihood differential detector using phase differences and amplitude ratios from L diversity branches for bit decisions. As a comparison, much simpler previously known post detection combining techniques are generalized for star 16QAM and optimized. The bit error probability is simulated for both diversity detectors on a multiplicative Rayleigh fading channel with additive white Gaussian noise. It is found that the bit error probability of the ML detector may also be obtained by the simple combining detector. This is also true for the error floor due to the maximum Doppler frequency. The diversity gain is almost 8 dB, measured in signal to noise ratio per diversity branch, at a bit error probability of 1 percent. The diversity detector can sustain an almost 3 times larger Doppler frequency again at a bit error probability of 1 percent. We also show that star 16QAM offers, at most, 3 subchannels with different bit error probabilities     

Quotient coding for fading channels

Multiplicative Rayleigh fading is a frequent problem in wireless communications. If the channel is relatively benign and fading is not severe, one may obtain higher bit rates for an equivalent bandwidth by using M-ary QAM modulation (MQAM). A variation, used to combat channel fading while still retaining MQAM, is differential MQAM (i.e., DQAM). The term differential refers to the phase which is coded exactly as in DPSK, however, the amplitude is still subject to distortion by the fading channel. In this paper, we propose a technique called quotient coding, which is designed to remove channel effects from the symbol amplitude as well as its phase. In particular, we shall apply it to MQAM resulting in modulation which we term QQAM. In contrast to DQAM, QQAM is just as effective at suppressing the effects of channel fading with respect to the entire symbol as DPSK is for the phase alone. In fact, the scaling of the amplitude at the receiver is entirely irrelevant to QQAM     

Performance of MMSE Receiver Based CDMA System with Higher Order Modulation Formats in a Fading Channel

This paper studies the effect of using higher order modulation formats on the performance of minimum mean-squared error (MMSE) receiver based direct-sequence (DS) code-division multiple access (CDMA) systems at different loading levels in additive white Gaussian noise (AWGN) and slow fading channels. The performance of BPSK, QPSK, and 16QAM modulation formats are compared and analytical and simulation results are presented in terms of the bit error rates (BER) for these different modulation formats. A comparison of the rejection of the near-far effects for each modulation scheme is also presented. The main contribution of this paper is in showing that user capacity may be increased by using higher order modulation schemes to cause the MMSE receiver to operate away from the interference limiting region. In particular it is shown that under high loading levels, 16QAM outperforms QPSK and BPSK for identical bandwidth and information rate, while at moderate loading levels, QPSK represents the best option. A combination of pilot symbol assisted modulation (PSAM) and linear prediction are used to estimate the fading process. A general structure of the MMSE receiver capable of demodulating a wide range of digital modulation formats in this type of environment is presented.     

SER Computation in M-QAM Systems with Phase Noise

This paper presents an exact closed-form expression for the symbol error rate of the square and rectangular quadrature amplitude modulation (QAM) constellations, under the assumption that the transmitted and/or received signals are corrupted by the phase noise fluctuation. Phase noise is one of the most important radio frequency (RF) imperfections which usually comes from the local oscillator (LO) at the transmitter and/or receiver. In this paper, the additive white Gaussian noise (AWGN) channel is assumed. Although AWGN is a simple channel, but our exact analysis of the symbol error rate in this paper can be led to the precise study of the real communication systems in the fading channel. Hence, an exact closed-form solution for the symbol error rate is derived here as a finite summation of the two-dimensional Q-function and verified by the simulation.     

比特谱相关改进循环谱的单通道混合信号参数估计快速算法

为了应对复杂环境下非合作通信、电磁频谱监管等宽带接收中存在的先验信息缺失,针对单通道混合信号的参数盲估计问题,提出了一种基于比特谱相关算法改进循环谱估计的快速算法。针对信号集{BPSK、QPSK、OQPSK、8PSK、8QAM、16QAM、16APSK、32APSK、CPM},系统给出整套参数估计算法流程,通过高阶累积量以及功率中心方法确定信号个数以及载波频率,然后利用改进循环谱提取符号速率谱线,完成单通道混合信号参数估计。论文对改进算法的统计特性进行了理论推导。理论和实验表明:算法不需要同步码等先验信息,适用于高斯噪声信道和多径平坦衰落信道,同时降低了计算复杂度、存储空间、估计方差以及数据量的需求,当混合信号等功率混合时,且信噪比分别为-4 dB和-1 dB时,算法对载波频率和符号速率的估计正确率可以达到90%,验证了算法的有效性和可行性。      

功放非线性对QAM解调误码率的影响

QAM调制由于高频谱效率得到了广泛应用,但由于QAM信号的功率峰均比较高,功率放大器的非线性会导致解调性能的下降。针对功放非线性对QAM解调性能的影响,传统方法采用仿真实验的方法来获得,而文中推导了高斯信道下功率放大器的非线性对QAM解调误码率影响的闭合表达式,可求出非线性功放在不同静态工作点对应的误码率。仿真结果验证了理论分析的正确性。     

Maximum likelihood sequence detection using a pilot tone

This paper derives, analyzes, and simulates a maximum likelihood (ML) sequence detector (MLSD) for a linearly modulated signal transmitted with a pilot tone (PT-MLSD). The transmitted signal is distorted by a time-varying frequency-selective Rayleigh fading channel and corrupted by additive Gaussian noise. The received signal is unsynchronized in that there are residual carrier frequency, carrier phase, and symbol timing offsets. The PT-MLSD treats the channel as a stochastic process, and so symbol sequences are distinguished by their autocovariances. Coherent communication is possible even in overspread channels. As the sequences' autocovariances explicitly account for the channel's time variation, the PT-MLSD's bit error rate (BER) floor is normally lower than the BER floor suffered by receivers that estimate the channel impulse response conventionally. Both the data-bearing signal and pilot tone are used together for synchronization, equalization, and detection. The pilot tone is only needed to remove the constellation's phase ambiguity and provide a stable amplitude reference for QAM constellations. It is not needed for estimating the channel impulse response. The pilot tone does not require a spectral null for its insertion, and it does not significantly degrade the peak-to-average or maximum-to-minimum power ratios. Thus, many of the disadvantages of other pilot tone systems are avoided, as there is no bandwidth expansion, and linear amplification is not made appreciably more difficult     

Pilot symbol-assisted detection of CPM schemes operating in fastfading channels

We present a coherent detection technique for continuous phase modulation (CPM) operating in the Rayleigh flat fading channel. The technique is based on the idea of inserting periodically data dependent pilot symbols that force the CPM signal to pass through known phase states. This transmission format enables the receiver to extract from the received signal the channel fading gains at regularly spaced instants. When coupled with proper channel estimation filters, very accurate channel state information (CSI) can be estimated at the receiver for fading compensation. Moreover, the accuracy of the CSI can be further refined by adopting a multiple-pass decoding approach. The paper discusses (a) the pilot symbol encoding technique required to force a M-level CPM scheme with a modulation index of p/M, p is an integer, to return periodically to a set of known phase states, (b) the optimal channel estimation filters, (c) a trellis-based precoding technique that can reduce the bit error rate in M-level CPM systems by close to 50%, and (d) a multiple-pass channel estimator/demodulator. Analytical and simulation results are presented for minimum shift keying (MSK), Gaussian MSK, and four-level continuous phase frequency shift keying with a modulation index of 1/4. It is observed that our pilot symbol-assisted CPM schemes exhibit no irreducible error floor even at a channel fade rate of three percent the symbol rate. The implicit phase coding in CPM and the accurate CSI provided by the pilot symbols lead to a diversity effect in the bit error rate curves of these modulation schemes     

Improved auxiliary particle filter‐based synchronization of chaotic Colpitts circuit and its application to secure communication

In this paper, we propose a synchronization scheme based on an improved auxiliary particle filter (IAPF) for chaotic Colpitts circuit and conduct an experimental study on the synchronization performance with application to secure communications. Specifically, with the synchronization scheme, when the chaotic signals generated by an analog Colpitts circuit are transmitted through a nonideal channel, the distorted signals are processed digitally by the novelly designed IAPF at the receiver, in order to obtain the synchronized signals of the transmitter circuit. Experimental results indicate that synchronization can be achieved over both the additive white Gaussian noise channel and the multipath fading channel with low signal‐to‐noise ratio, even if there exist severe circuit parameter mismatches between the transmitter and the receiver. Furthermore, a chaos‐masking secure communication system is constructed and verified over both the additive white Gaussian noise channel and the multipath fading channel, and the bit error rate is evaluated versus different signal‐to‐noise ratios and symbol periods. It is shown that the achievable bit error rate can reach the order of magnitude of 10 ^{− 4} without error correction coding techniques. In addition, security analysis demonstrates that the proposed chaotic secure communication system is resistant to the brute‐force attack. Copyright © 2013 John Wiley & Sons, Ltd.     

On the Implementation of a Quasi-Generic Synchronization Architecture for Linear Digital Modulations

With increasing availability of software-defined radio platforms, users are no longer tied to receiving or transmitting only one type of signal. In applications, such as signal intelligence, the user may not know ahead of time the characteristics of the signal to be received. This uncertainty results in a need for more flexible receiver architectures that can be easily modified to work for multiple signal types. In this paper the FPGA implementation of a quasi-generic synchronization architecture is presented that is easily adaptable, at implementation time, to most linear modulation schemes. The implementation is shown to work for QPSK, 8QAM, 16QAM and 32QAM with timing frequency errors of up to 4% of the symbol rate.     

针对线性群时延畸变的加权多模盲均衡算法

群时延失真是影响卫星通信系统误码性能的重要因素之一。本文分析了高速卫星信道的线性群时延模型,依此模型提出了改进的加权多模盲均衡算法(MWMMA)。该算法引入Sigmoid函数的变形形式,构造了加权幂指数随迭代误差自适应变化的函数关系,在不需要通过大量仿真实验预先确定模式转变门限值的前提下,利用迭代误差的递减实现算法从MCMA模式通过多模自适应切换为DD模式。通过对多种算法的仿真比较,得到该算法不仅能使通过群时延信道的16QAM信号收敛集中,而且具有更小的稳态剩余码间干扰;同时,基于该算法对64QAM和256QAM传输信号的均衡仿真结果,验证了该算法对高阶QAM信号的均衡具有一定的适用性。      

Symbol mapping diversity design for multiple packet transmissions

In this paper, we present a simple, but effective method of enhancing and exploiting diversity from multiple packet transmissions in systems that employ nonbinary linear modulations such as phase-shift keying (PSK) and quadrature amplitude modulation (QAM). This diversity improvement results from redesigning the symbol mapping for each packet transmission. By developing a general framework for evaluating the upper bound of the bit error rate (BER) with multiple transmissions, a criterion to obtain optimal symbol mappings is attained. The optimal adaptation scheme reduces to solutions of the well known quadratic assignment problem (QAP). Symbol mapping adaptation only requires a small increase in receiver complexity but provides very substantial BER gains when applied to additive white Gaussian noise (AWGN) and flat-fading channels.     

8-QAM Software Defined Radio Based Approach for Channel Encoding and Decoding Using Forward Error Correction

The aim of this paper is to use a software defined radio (SDR) based approach in order to select channel encoding and decoding method accordingly using 8-QAM (Quadrature Amplitude Modulation) in terms of bit error rate (BER). By selecting a higher order format of QAM, we are able to carry more bits of information per symbol; also the data rate can be increased thus achieving greater distance between adjacent points in the I–Q plane by distributing the points more evenly. Hence the constellation points are more distinct and data errors are reduced. In the present work 8-QAM is chosen as modulation scheme so that balance can be maintained between higher data rates while maintaining an acceptable bit error rate for SDR. Channel coding schemes forward error correction are used where the re-transmission of the data is not feasible, thus redundant bits are added along with the message bits and transmitted through the channel. On the receiver side, this channel coded signal is decoded in order to get back the original data even if the channel coded signal undergoes some interference from the noise in the transmission medium. The performance is then analyzed in terms of BER for Hamming and convolution coding algorithms at a particular value of SNR in LabVIEW graphical programming. With the help of LabVIEW we were able to design the systems in a block-based manner in shorter time as compared to the commonly used text-based programming languages.     

Adaptive predistortion of COFDM signals for a mobile satellite channel

In this paper, we consider the optimization of the performance of QPSK and 16‐QAM coded orthogonal frequency division multiplexing (COFDM) signals over the non‐linear and mobile satellite channel. A high power amplifier and Rician flat fading channel produces non‐linear and linear distortions; an adaptive predistortion technique combined with turbo codes will reduce both types of distortion. The predistorter is based on a feedforward neural network, with the coefficients being derived using an extended Kalman filter (EKF). The conventional turbo code is used to mitigate Rician flat fading distortion and Gaussian noise. The performance over a non‐linear satellite channel indicates that QPSK COFDM followed by a predistorter provides a gain of about 1.7 dB at a BER of 3×10^?3 when compared to QPSK COFDM without the predistortion scheme and 16‐QAM COFDM provides a gain of 0.5 dB output back‐off and 1.2 dB signal to noise ratio at a BER of 3×10^?5 when compared with an adaptive predistorter based on the Harmmerstein model. We also investigate the influence of the guard time interval and Doppler frequency effect on the BER performance. When the guard interval increases from 0 to 0.125T samples and the normalized Doppler frequency is 0.001, there is a gain of 0.7 and 1 dB signal to noise ratio at a BER of 6×10^?4 for QPSK and 16‐QAM COFDM, respectively. Copyright © 2003 John Wiley & Sons, Ltd.     

16‐QAM‐Based Highly Spectral‐Efficient E‐band Communication System with Bit Rate up to 10 Gbps

This paper presents a novel 16‐quadrature‐amplitude‐modulation (QAM) E‐band communication system. The system can deliver 10 Gbps through eight channels with a bandwidth of 5 GHz (71‐76 GHz/81‐86 GHz). Each channel occupies 390 MHz and delivers 1.25 Gbps using a 16‐QAM. Thus, this system can achieve a bandwidth efficiency of 3.2 bit/s/Hz. To implement the system, a driver amplifier and an RF up‐/down‐conversion mixer are implemented using a 0.1 µm gallium arsenide pseudomorphic high‐electron‐mobility transistor (GaAs pHEMT) process. A single‐IF architecture is chosen for the RF receiver. In the digital modem, 24 square root raised cosine filters and four (255, 239) Reed‐Solomon forward error correction codecs are used in parallel. The modem can compensate for a carrier‐frequency offset of up to 50 ppm and a symbol rate offset of up to 1 ppm. Experiment results show that the system can achieve a bit error rate of 10^?5 at a signal‐to‐noise ratio of about 21.5 dB.