A Performance Study of NLA 64-State QAM

The performance of a modulation technique which is both power and bandwidth efficient, termed nonlinearly amplified 64-state quadrature amplitude modulation (NLA 64-state QAM), is studied. To assess the effects of selective fading and/or system filter imperfections, computer simulated results are presented which show the sensitivity of the modulation technique to typical group delay and amplitude distortions. In addition to the above-mentioned results, the effects of modulator imperfections in the power level combining on the Peperformance are evaluated.     

Linearisation of RF PA by even-order nonlinear baseband signal processed in digital domain

This paper represents, for the first time, the linearisation method of RF power amplifiers, which is based on using the second- and fourth-order nonlinear signals shaped by digital signal processing in a baseband domain. The linearisation signals are generated by the second- and fourth-order nonlinear modifications of the baseband signal in the proposed manners. The composite second- and fourth-order linearisation signals modulate carrier second harmonics, and the modulated signals are then driven to the gate and drain of the amplifier transistor. The linearisation effects of the proposed method are evaluated on a single stage power amplifier for the simulated QAM and OFDM digitally modulated signals at diverse input power levels up to the 1 dB compression point. Moreover, the FPGA implementation of a system for the generation and processing of the fundamental signal, and the second-order linearisation signals is presented in the paper, and the proposed technique is verified for the generated QAM signal. In addition, the linearisation of the two-way asymmetrical Doherty amplifier is performed in the experiment by using software defined radio platforms for synthesising the fundamental signal and the second-order linearisation signals.     

Weighting Factor Estimation Method for Peak Power Reduction Based on Adaptive Flipping of Parity Bits in Turbo-Coded OFDM Systems

In this paper, we propose a weighting factor (WF) estimation method for peak power reduction (PPR) based on adaptive flipping of parity carriers in a turbo-coded orthogonal frequency-division multiplexing (OFDM) system. In this PPR scheme, the peak-to-average power ratio of a turbo-coded OFDM signal is reduced with adaptive flipping of the phase of the parity carriers corresponding to the WFs. At the receiver, the WFs are estimated at a turbo decoder by exploiting the redundancy of an error-correcting code using no extra side information. The proposed WF estimation method is based on an iterative decoding of the turbo code, i.e., the turbo decoder provides not only error correction capability but the WF estimation function as well. When the proposed WF estimation method is used for the system using a turbo code with constraint length $K = 4$ and a code rate of $R = 1/2$, the instantaneous power of the OFDM signal at the complementary cumulative distribution function of $10^{-4}$ can be reduced by about 2.1 dB through the application of the PPR scheme. When the bit error rate (BER) performance is evaluated as a function of the peak signal-to-noise power ratio (PSNR), the proposed method achieves better BER performance than the case without the PPR in an attenuated 12-path Rayleigh fading condition. The improvements in BER performance as a function of PSNR are about 1.1, 2.0, and 2.1 dB at $hbox{BER} = 10^{-4}$ for turbo-coded OFDM signals using QPSK, 16-state quadrature amplitude modulation (QAM), and 64-state QAM schemes, respectively.      

卫星通信中不同调制信号的预失真研究

卫星通信技术近年来发展较快,承载的通信业务也较多,导致频谱资源紧张,功率资源受限,迫切需要采用高阶调制、多载波传输等方式来缓解通信资源的紧张,QPSK、8PSK、16QAM等更高阶调制方式在卫星通信中将会获得越来越广泛的应用。这类信号直接通过功放后,信号产生失真,影响接收端信号的正确解调,预失真技术是近年来补偿功放非线性的最有效手段之一。在研究这几种高阶调制信号特点的基础上,对几种调制方式的信号分别进行预失真处理,并分析各信号的抗噪声性能,总结出16QAM信号在卫星通信中的应用会缓解目前通信资源紧缺问题。     

Analysis of different sub-carrier allocation of M-ary QAM-OFDM downlink in RoF system

In this paper, the performance of a 60 GHz radio over fiber (RoF) system with 4/16/64 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) downstream signals is studied. Delivery of 10 Gbit/s M-ary QAM (MQAM) OFDM signals through the 20-km-long single-mode fiber (SMF) is complicated in terms of intensity modulation and direct detection (IM/DD). Using self-homodyne method, the beating of two independent light waves generating the millimeter-wave at the photodetector can be down-converted to baseband in the electrical domain. Meanwhile, three kinds of sub-carrier arrangement schemes are compared and discussed, and the simulation results show that lower peak-to-average power ratio (PAPR) can be obtained adopting the adjacent scheme. At bit error rate (BER) of 10-3, the receiver sensitivity using 4QAM-OFDM sub-carrier signal is almost enhanced by 4 dB and 9 dB compared with those of 16QAM-OFDM signal and 64QAM-OFDM signal.     

Six-dimensional trellis-coding with QAM signal sets

A family of 6-D trellis-coded modulation (TCM) schemes which involve a 2-step partitioning of the constituent QAM signal alphabet is presented. With infinite constellations without shaping, the asymptotic coding gain is 3 dB for the 2-state code, 4 dB for the 4- and 8-state codes, and 5 dB for the 16- and 32-state codes which involve a smaller alphabet expansion. The authors also describe a rotationally invariant 16-state code that achieves the same asymptotic gain as its linear counterpart. Practical signal constellations are described for 6-D TCM with the spectral efficiency of uncoded 64-QAM, and the performance of these schemes is studied by means of computer simulations. It was found that they achieve an additional coding gain of 0.2-0.3 dB over infinite hypercube-type constellations. The performance of the presented schemes at practical signal-to-noise ratio values is evaluated using transfer function techniques     

Ka-band analog front-end for software-defined direct conversion receiver

A six-port Ka-band front-end architecture based on direct conversion for a software-defined radio application is proposed in this paper. The direct conversion is accomplished using six-port technology. In order to demodulate various phase-shift-keying/quadrature-amplitude-modulation (PSK/QAM) modulated signals at a high bit rate, a new analog baseband circuit was specially designed according to the I/Q equations presented in the theoretical part. An experimental prototype has been fabricated and measured. Simulation and measurement results for binary PSK, quaternary PSK (QPSK), 8 PSK, 16 PSK, and 16 QAM modulated signals at a bit rate up to 40 Mb/s are presented to validate the proposed approach. A software-defined radio can be designed using the new front-end and only two analog-to-digital converters (ADCs) because the I/Q output signals are generated by analog means. Previous six-port receivers make use of four ADCs to read the six-port dc levels and require digital computations to generate the I/Q output signals. With the proposed approach, the load of the signal processor will therefore be reduced and the modulation speed can be significantly increased using the same digital signal processor.     

Error probabilities for QAM systems on partially coherent channelswith intersymbol interference and crosstalk

We describe an efficient procedure to calculate the probability of error P_e for a quadrature amplitude modulation (QAM) communications system operating over a channel that introduces distortion, interference, and noise. The method is an extension of the saddlepoint integration technique introduced by Helstrom (1986) to efficiently evaluate P_e for one-dimensional pulse-amplitude modulation (PAM) systems with intersymbol interference (ISI) and crosstalk. We consider the effects of noise, random carrier phase offset, ISI, and crosstalk between the I and Q channels. The error probability is written as a double Laplace inversion integral and can be easily applied to any rectangular constellation. This integral is calculated by extending the saddlepoint integration technique to two complex dimensions. Results are presented for QAM systems with 16, 64, and 256 symbols. The technique can be directly extended to environments with cochannel interference consisting of other QAM signals     

A MSE optimized polynomial equalizer for 16QAM and 64QAM constellation

Recently, a Maximum Entropy (MaxEnt) algorithm and its derivation called WNEW algorithm were presented by the same author. It was shown that the MaxEnt and WNEW algorithm have improved equalization performance compared with Godard’s, reduced constellation algorithm and the sign reduced constellation algorithm. In this paper, a new equalization method is proposed for the 16QAM and 64QAM input constellation based on the WNEW algorithm which is extended with some polynomials of the equalized output and optimized with the mean square error criteria. According to simulation results, the new equalization method leads to over 15 dB advantage in the residual Intersymbol Interference compared to the results presented by Godard, 10 dB advantage compared with the WNEW algorithm and 5 dB advantage compared with the MaxEnt algorithm.     

Data predistortion techniques using intersymbol interpolation

Two data predistortion techniques are presented that compensate for high-power amplifier (HPA) nonlinearities in digital microwave radio systems by employing quadrature amplitude-modulation (QAM) signal formats. The first one is a T/2-spaced predistortion technique that ensures distortion-free HPA output at two points per symbol interval T. The second is a T/3-spaced predistortion technique which cancels nonlinear distortion at the HPA output at three points per symbol interval. As opposed to conventional data predistortion, which can only compensate for warping of the signal constellation, the new techniques are effective against intersymbol interference. Using the 64- and 256-QAM signal constellations, it is shown that the proposed techniques lead to a very efficient utilization of the available HPA power. It is shown that, of the two techniques, the T/3-spaced data predistortion employs narrower transmit-pulse shaping and achieves higher protection against adjacent-channel interference at the expense of some additional complexity     

Iterative demodulation, demapping, and decoding of coded non-square QAM

It is shown that a non-square (NS) 2/sup 2n+1/-ary quadrature amplitude modulation (QAM) can be decomposed into a single parity-check (SPC) block encoder and a memoryless modulator in such a way that the inherent block encoder has a recursive nature. When concatenated with a forward-error-correcting (FEC) code, iterative demodulation, demapping, and decoding of the FEC code and the inherent SPC code of NS-2/sup 2n+1/-QAM is then possible. Simulation results show that coded NS-8QAM performs nearly 2 dB better than standard 8QAM and star-8QAM, and nearly 1 dB better than 8-ary phase-shift keying when the FEC code is a rate-1/2, 16-state convolutional code, while coded NS-32QAM performs 0.75 dB better than standard 32QAM.     

Differential space time block codes using nonconstant modulus constellations

We propose differential space time block codes (STBC) using nonconstant modulus constellations, e.g., quadrature amplitude modulation (QAM), which cannot be utilized in the conventional differential STBC. Since QAM constellations have a larger minimum distance compared with the phase shift keying (PSK), the proposed method has the advantage of signal-to-noise ratio (SNR) gain compared with conventional differential STBC. The QAM signals are encoded in a manner similar to that of the conventional differential STBC. To decode nonconstant modulus signals, the received signals are normalized by the channel power estimated forgoing training symbols and then decoded with a conventional QAM decoder. Assuming the knowledge of the channel power at the receiver, the symbol error rate (SER) bound of the proposed method under independent Rayleigh fading assumption is derived, which shows better SER performance than the conventional differential STBC. When the transmission rate is more than 3 bits/channel use in time-varying channels, the simulation results demonstrate that the proposed method with the channel power estimation outperforms the conventional differential STBC. Specifically, the posed method using the channel power estimation obtains a 7.3 dB SNR gain at a transmission rate of 6 bits/channel use in slow fading channels. Although the performance gap between the proposed method and the conventional one decreases as the Doppler frequency increases, the proposed method still exhibits lower SER than the conventional one, provided the estimation interval L is chosen carefully.     

A radio-over-fiber system with photonic generated 16QAM OFDM signals and wavelength reuse for upstream data connection

We have experimentally demonstrated a wavelength reuse scheme for up-link connection in a radio-over-fiber (ROF) system with photonics generated 2.5 Gbit/s 16QAM OFDM signals. In this architecture, 2.5 Gbit/s 16QAM OFDM signals are carried by the optical millimeter-wave (mm-wave) carriers which are generated with four times frequency of the local oscillator (LO) signal. The power penalties for both down- and up-stream signal delivery over 20 km fiber are less than 1 dB.     

一种基于折线逼近的对数似然比简化算法

针对16QAM信号的对数似然比计算,该文提出一种新颖的折线逼近简化算法,采用分段的折线逼近16QAM信号的对数似然比曲线,进而使用简单的线性运算替代原标准LLR算法中复杂的非线性运算,仿真结果表明,该算法可以理想地逼近标准LLR算法的计算结果,在BITCM系统中应用不会带来系统的性能折损。     

基于决策论的数字调制信号识别方法

以决策论为基础提出了一种改进的数字调制信号识别方法,该方法仅需4个相对简单的特征参数,就能识别2ASK、4ASK、2FSK、4FSK、2PSK、4PSK和16QAM这7种数字调制信号。仿真结果表明,该方法复杂度较低,识别正确率有较大提高,尤其对于2ASK、4ASK、MPSK/MFSK及16QAM的识别,在信噪比较低的情况下,具有较好的识别效果。     

高阶累积量和分形理论在信号调制识别中的应用研究

提出了将信号高阶累积量和分形盒维数相结合的特征提取方法。信号高阶累积量特征具有良好的抗噪性能,被广泛应用于调制识别。2ASK和BPSK的高阶累积量、以及2FSK,4FSK,8FSK的高阶累积量相等,使得只提取信号高阶累积量不足以区分信号。针对这一问题,引入信号的分形盒维数,提取信号的高阶累积量和分形盒维数构成联合特征参数,构建级联神经网络分类器,对信号进一步进行分类。对2ASK, 4ASK, BPSK, 4PSK, 2FSK, 4FSK, 16QAM七种信号进行了仿真,结果表明,该方法提取的特征参数计算复杂度低,具有较好的抗噪性能。在信噪比不低于5dB、测试样本数不少于200的条件下,正确识别率达到了85%以上。      

Transmission of SDH signals through future satellite channels usinghigh-level modulation techniques

The authors discuss the possibility of transmitting synchronous digital hierarchy (SDH) signals through two-link nonlinear satellite channels. Transmitting such high bit rate signals through a standard 54 MHz or 36 MHz transponder bandwidth requires the use of high-level modulation schemes. The techniques and technologies needed to make the use of 16-ary quadrature amplitude modulation (QAM) and 64-ary QAM transmissions feasible for future satellite communication systems are examined. It is shown that it is possible to transmit a synchronous transport module-level 1 (STM-1) signal through a standard 54 or 36 MHz transponder bandwidth using 16-ary QAM or 64-ary QAM transmission, respectively, for the 6/4 GHz band. However, for higher frequency bands, due to high fade margins needed to achieve the high availability and performance for SDH systems, is not practical to transmit the STM-1 signal through such standard transponder bandwidths     

A new analog predistortion criterion with application to highefficiency digital radio links

Describes a new approach for designing analog cubic predistorters in view of minimizing nonlinear distortion introduced by the high power amplifier (HPA) in microwave radio links. The approach is based on a global mean squared error minimization between the input to the predistorter and the output of the power amplifier, rather than on the elimination of the cubic term. In this manner, two advantages are obtained. Firstly, the off-band distortion terms are reduced so that it is possible to eliminate the HF transmission filter. Secondly, the interference produced by the nonlinear distortion can be uniformly distributed over the entire constellation so that it is possible to reduce the back-off value of the HPA. Results of the application of the method proposed to some high spectrally efficient QAM systems, such as the 512-QAM system, are presented     

Pilot-Symbol-Aided 16PSK and 16QAM for Digital Land Mobile Radio Systems

The paper proposes a novel pilot-symbol-aided (PSA) technique for fading compensation of digital signals in the mobile environments. In a PSA system, the data sequence at the transmitter is divided into frames of data. A pilot symbol from a known pseudoradom-symbol sequence is inserted periodically into a frame of data symbol for transmission. In a conventional PSA-receiver, these pilot symbols are extracted from the received signal and used to estimate the effects of signal distortion introduced in the fading channel. The resultant estimates are then used to correct the distortion effects in the received data frames. In the paper, a novel estimation technique that uses the data symbols as well as the pilot symbols is proposed. The technique has the major advantages of simple implementation and short storage-delay time. Results are presented in a series of computer-simulation tests. These assess the effectiveness of the estimation technique on the BER performances of a 16-ary phase-shift keyed (16PSK) and a 16-ary quadrature-amplitude modulated (16QAM) signals in the frequency-selective and frequency-nonselective Rayleigh fading channels. The channels are corrupted by co-channel interference or additive white Gaussian noise. Results of differential-detected 16PSK and star-16QAM signals are also presented for comparison. It has been shown that, the use of PSA technique can significantly improve the bit-error-rate performances of the systems, relative to those using differential detection.     

Direct 16 QAM six-port modulator

A direct 16 quadrature amplitude modulation (QAM) modulator based on the six-port technique is proposed. It uses a six-port passive microwave network to implement the modulation scheme with suitable terminations. A microwave prototype was built to validate the 16 QAM modulation up to 200 Mbit/s data rate at 4.2 GHz carrier frequency. The error vector magnitude and local oscillator (LO) power testing show that this six-port 16 QAM modulator has wide dynamic range capability to overcome the LO power variation. It has potentially low cost and low power consumption for RF communications applications.