16-QAM相干光纤通信系统星座图的优化与选择

基于16-QAM的相干光纤通信系统其抗噪声能力与调制格式的星座图特征密切相关。文章分析了Star 16-QAM的星座图在各种圈比下受噪声影响的特性,通过数值计算获得了在不同噪声条件下星座图的最佳圈比。研究了Square和Star两种16-QAM对激光相位噪声的要求,分析结果表明,获得相同的BER性能时,Star 16-QAM由激光相位噪声引起的光信噪比代价更小。     

改进的16进制正交幅度调制码的相位估计算法

相干光通信系统中的相位恢复是克服信号收发端和传输过程中产生的相位噪声的必要步骤。为获得低的运算复杂度和良好的相位估计效果,本文对传统的四次方相位估计算法进行了改进,使之具有自适应的数据块长度控制功能。通过理论分析以及Matlab数值仿真方法验证改进的效果。对于16进制正交幅度调制码（16QAM）,改进后的算法在几乎不增...     

Square m-QAM相干光通信载波相位估计性能研究

基于前馈载波相位估计算法,研究了square 16-QAM调制相干光纤通信系统的载波相位估计性能,进一步证明了当存在相位噪声时square m-QAM调制并非QAM调制最佳星座图的原因。仿真计算结果显示,由于square 16-QAM调制星座图中星点之间相位距离分布不均匀,引起载波相位估计中出现连续相位跳变,导致载波估计性能劣化,系统误码率增大约2～3倍。     

Carrier synchronization for 3- and 4-bit-per-symbol optical transmission

We investigate carrier synchronization for coherent detection of optical signals encoding 3 and 4 bits/symbol. We consider the effects of laser phase noise and of additive white Gaussian noise (AWGN), which can arise from local oscillator (LO) shot noise or LO-spontaneous beat noise. We identify 8- and 16-ary quadrature amplitude modulation (QAM) schemes that perform well when the receiver phase-locked loop (PLL) tracks the instantaneous signal phase with moderate phase error. We propose implementations of 8- and 16-QAM transmitters using Mach-Zehnder (MZ) modulators. We outline a numerical method for computing the bit error rate (BER) of 8- and 16-QAM in the presence of AWGN and phase error. It is found that these schemes can tolerate phase-error standard deviations of 2.48/spl deg/ and 1.24/spl deg/, respectively, for a power penalty of 0.5 dB at a BER of 10/sup -9/. We propose a suitable PLL design and analyze its performance, taking account of laser phase noise, AWGN, and propagation delay within the PLL. Our analysis shows that the phase error depends on the constellation penalty, which is the mean power of constellation symbols times the mean inverse power. We establish a procedure for finding the optimal PLL natural frequency, and determine tolerable laser linewidths and PLL propagation delays. For zero propagation delay, 8- and 16-QAM can tolerate linewidth-to-bit-rate ratios of 1.8/spl times/10/sup -5/ and 1.4/spl times/10/sup -6/, respectively, assuming a total penalty of 1.0 dB.     

Carrier-phase and frequency-estimation bounds for transmissions with embedded reference symbols

The true Crame/spl acute/r-Rao lower bound (CRLB) for the joint estimation of carrier phase and frequency is derived for transmission bursts with interleaved reference and phase-shift keying/quadrature amplitude modulated data symbols. Results are presented for the special cases of midamble, and preamble and postamble, pilot-symbol insertion. The derivation reveals that the CRLB is a function of the location of the reference symbols in the burst, the number of reference symbols, the number of data symbols, the signal-to-noise ratio and the data-modulation scheme. By distributing the reference symbols symmetrically about the center of the burst and analyzing relative to the middle of the signal vector, the joint frequency and phase estimation can be decoupled, and the optimal phase estimation is achieved. In the decoupled case, the phase CRLB is independent of the location of reference symbols in the burst. In a symmetrical burst, the use of a preamble and postamble is found to provide a lower frequency-estimation CRLB than that with a midamble. It appears that the frequency CRLB is reduced as the reference symbols are symmetrically distributed closer to the ends of the burst.     

基于数据辅助的MPSK信号频域信噪比估计

为在载波频率精确恢复前提高多进制数字相位调制（MPSK）信号在低信噪比下的估计精度,提出了一种数据辅助的MPSK信号频域信噪比估计算法。算法在符号定时恢复和帧同步后提取同步段符号,相关运算后在频域进行信噪比估计。仿真结果表明,算法估计均值无偏,不受载波频率误差的影响,在符号长度为512、信噪比为-10 dB时,均方误差与克拉美罗界只有0.15 dB的偏差,特别适合于接收信号包含载波频率误差且要求低信噪比下具有较高信噪比估计性能的应用。     

Carrier phase recovery of rotated 8/8-QAM signals

A low-complexity algorithm is introduced such that the Cramer-Rao lower bound for non-data-aided carrier phase recovery can be computed most efficiently. In the sequel, this result is used to develop a phase estimator for rotated 8/8-QAM as a powerful alternative to star 16-QAM modulated signals.     

Multilevel QAM Modulation Techniques for Digital Microwave Radios

A pilot carrier injection method is described together with feedback balance coding which reduces spectral power near the carrier. Robustness of carrier recovery using the pilot carrier injection method is theoretically estimated. The estimation suggests that recovered carrier SNR higher than 40 dB can be expected even under muitipath fading with notch depth of 45 dB located just at the carrier frequency. Signatures for multipath fading are estimated for a 64-QAM system with transversal equalizers as a countermeasure. Measured signatures agree reasonably well with the calculated ones. Dependences of signatures on modulation level, transversal equalizer tap number, and rolloff rate are also shown.     

利用DFT和迭代校正的正弦信号频率估计算法

对电磁信号的频率估计广泛应用于通信、雷达、导航和电子对抗等领域。针对正弦信号的频率精确估计,本文提出了一种利用DFT和迭代校正的频率估计算法,并与现有三种基于DFT的频率估计算法进行了性能仿真对比,分析结果表明,新算法的频率估计性能明显优于其他三种算法,可以得到非常逼近CRLB的频率估计值,迭代校正5次时,频率估计的RMSE距离CRLB不到0.07dB,而且没有估计误差平层,算法估计精度高,对频率的取值范围不敏感,性能稳定,迭代校正的复杂度较低,具有很好的应用价值。      

Spectrally efficient digital broadcast systems operated in a phasenoise environment

High spectral efficiency combined with power efficiency, is a requirement for high speed digital broadcast satellite systems. The effect of the recovered carrier phase noise and the nonlinearity on the performance of 16-QAM (quadrature amplitude modulation) and staggered 16-QAM is investigated and compared. It is found that for the phase noise, 16-QAM is more degraded for a roll-off factor of less than 0.4. For roll-off factor of more than 0.8, staggered 16-QAM has superior performance. For the nonlinearity staggered 16-QAM is more sensitive for a roll-off factor of less than 0.4. It is concluded that coincident 16-QAM is suitable for spectrally efficient digital broadcast systems     

Reduced complexity in-phase/quadrature-phase M-QAM turbo equalization using iterative channel estimation

A reduced complexity trellis-based turbo equalizer known as the in-phase (I)/quadrature-phase (Q) turbo equalizer (TEQ-IQ) invoking iterative channel impulse response (CIR) estimation is proposed. The underlying principle of TEQ-IQ is based on equalizing the I and Q component of the transmitted signal independently. This requires the equalization of a reduced set of separate I and Q signal components in comparison to all of the possible I/Q phasor combinations considered by the conventional trellis-based equalizer. It was observed that the TEQ-IQ operating in conjunction with iterative CIR estimation was capable of achieving the same performance as the full-complexity conventional turbo equalizer (TEQ-CT) benefiting from perfect CIR information for both 4- and 16-quadrature amplitude modulation (QAM) transmissions, while attaining a complexity reduction factor of 1.1 and 12.2, respectively. For 64-QAM, the TEQ-CT receiver was too complex to be investigated by simulation. However, by assuming that only two turbo equalization iterations were required, which is the lowest possible number of iterations, the complexity of the TEQ-IQ was estimated to be a factor of 51.5 lower than that of the TEQ-CT. Furthermore, at BER = 10/sup -3/ the performance of the TEQ-IQ 64-QAM receiver using iterative CIR estimation was only 1.5 dB away from the associated decoding performance curve of the nondispersive Gaussian channel.     

A code-aided carrier synchronization algorithm based on improved nonbinary low-density parity-check codes

In order to further improve the carrier synchronization estimation range and accuracy at low signal-to-noise ratio (SNR), this paper proposes a code-aided carrier synchronization algorithm based on improved nonbinary low-density parity-check (NB-LDPC) codes to study the polarization-division-multiplexing coherent optical orthogonal frequency division multiplexing (PDM-CO-OFDM) system performance in the cases of quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) modes. The simulation results indicate that this algorithm can enlarge frequency and phase offset estimation ranges and enhance accuracy of the system greatly, and the bit error rate (BER) performance of the system is improved effectively compared with that of the system employing traditional NB-LDPC code-aided carrier synchronization algorithm.     

Effects of laser phase noise on the performance of optical coherent receivers

Laser phase noise (LPN) plays an important role in optical coherent systems. Based on the algorithm of Viterbi-Viterbi carrier phase estimation (CPE), the effects of LPN imposed on the coherent receivers are investigated for quadrature phase shift keying (QPSK), 8 phase shift keying (8PSK) and 16-quadrature amplitude modulation (16-QAM) optical coherent systems, respectively. The simulation results show that the optimal block length in the phase estimation algorithm is a tradeoff between LPN and additive white Gaussian noise (AWGN), and depends on the level of modulation formats. The resolution requirements of analog to digital converter (ADC) in the coherent receivers are independent of LPN or the level of modulation formats. For the bit error rate (BER) of 10-3, the required bit number of ADC is 6, and the gain is marginal for the higher resolution.     

Cramer-Rao Lower Bound for Non-Data-Aided SNR Estimation of Linear Modulation Schemes

Powerful parameter estimators exhibit a jitter variance which is fairly close to the Cramer-Rao lower bound (CRLB) as the theoretical limit. In contrast to symbol timing and carrier frequency/phase, not very much information is available from the open literature with respect to the signal-to-noise ratio (SNR), i. e., the CRLB has been reported only for the data- aided case and some simple M-PSK examples for non-data-aided estimation of the SNR. Motivated by this background, an efficient algorithm is presented which applies to any M-ary one/two- dimensional modulation scheme with axis/halfplane symmetry and a channel distorted by additive white Gaussian noise. Finally, the performance of different SNR estimators is compared to the derived bound.     

Two-stage carrier synchronization techniques for nonselectivefading

Transmitted reference carrier synchronization techniques can potentially provide near-coherent performance in the presence of time-varying fading. Such techniques are spectrally not as efficient as nonreference-based architectures and can have performance several dB worse than ideal coherent in higher order modulations (e.g., 64-QAM). This paper presents a two-stage demodulator that facilitates improved spectral efficiency or provides better performance (or sometimes both). The two-stage architecture results from examining the optimal detector for M-QAM. The first stage produces tentative decisions and removes the modulation from the received signal. The second stage implements a smoothing filter for the resulting signal, providing an improved carrier estimate. Monte Carlo simulation results demonstrate the improved performance For 4-QAM, 16-QAM, and 64-QAM     

An intelligent multimode voice communications system for indoorcommunications

A novel high-quality, low-complexity dual-rate 4.7 and 6.5 kbits/s algebraic code excited linear predictive codec is proposed for adaptive multi-mode speech communicators, which can drop their source rate and speech quality under network control in order to invoke a more error resilient modem amongst less favorable channel conditions. Source-matched binary Bose-Chaudhuri-Hocquenghem (BCH) codecs combined with unequal protection diversity- and pilot-assisted 16and 64-level quadrature amplitude modulation (16-QAM, 64-QAM) are employed in order to accommodate both the 4.7 and the 6.5 kbits/s coded speech bits at a signaling rate of 3.1 kBd. Assuming an excess bandwidth of 100%, in a bandwidth of 200 kHz 32 time slots can be created, which allows us to support in excess of 50 users, when employing packet reservation multiple access (PRMA). Good communications quality speech is delivered in an equivalent bandwidth of 4 kHz, if the channel signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) of the benign indoors cordless channel are in excess of about 15 and 25 dB for the lower and higher speech quality 16-QAM and 64-QAM systems, respectively, and the PRMA time-slots are sufficiently uninterfered due to using time-slot classification algorithms and due to the attenuation of partitioning walls and ceilings     

空间预警系统中主动段弹道估计误差分析

空间红外预警系统仅可获得弹道导弹主动段尾焰红外辐射信号到达角信息,采用非线性最小二乘法可获得目标运动状态的迭代解.本文即针对其导弹主动段估计误差进行研究,通过理论推导得出状态估计的CRLB,并结合仿真结果给出其估计误差受预警星座分布、目标相对位置等因素影响的分析.     

Ultimate capacity of a laser diode in transporting multichannelM-QAM signals

In this paper, we investigate the ultimate M-ary quadrature amplitude modulated (M-QAM) channel capacity of a laser diode which is limited by the laser clipping induced nonlinear distortions. Our study includes a spectral analysis, a complete system simulation, and an experiment which used up to 70 channels of vector arbitrary waveform synthesizer generated quadrature phase shift keyed (QPSK) or 16-QAM signals to modulate an isolated/cooled distributed-feedback (DFB) laser and two unisolated/uncooled Fabry-Perot (FP) lasers, respectively. Our analytical results show that for an upstream laser diode, over 1000 QPSK channels or 170 16-QAM channels can be delivered, even in the presence of a high relative intensity noise (RIN) of -115 dB/Hz. However, these high capacities are reduced significantly when we consider the effect of collision-based medium access control (MAC) protocols. We found that, in the worst case condition (collisions occur in all but one channels), the ultimate QPSK channel capacity of an upstream laser diode is dramatically reduced from over 1000 to 125 for eight collisions/channel. These results have important implications to systems transporting frequency-stacked return-path bands with or without collision-based MAC channels. As regard to the ultimate capacity of a down-stream laser diode with a RIN level of -135 dB/Hz, we found that as high as 600 and 128 channels of 64-QAM and 256-QAM signals (equivalent to 3600 and 1152 channels of MPEG-II live video signals) can be transported, respectively     

Minimum entropy approach for carrier frequency recovery

This paper introduces a new carrier frequency recovery approach. It can be applied before modulation classification and/or demodulation of the M-ary PSK signals. It relies on the entropy of the instantaneous phase probability density function, and uses the fact that it reaches minimum when the receiver is fine-tuned to the unknown carrier frequency. This estimator is applicable to algorithms requiring high accuracy without any a priori knowledge concerning modulation scheme, signal contents (bit-stream), or its timing parameters. Simulation results have proved the robustness of the algorithm: for low carrier to noise ratios (CNR), corresponding variances are proportional and close to Cramer-Rao lower bounds (CRLB). For CNR greater than 20 dB, they are constant and limited by the resolution of the algorithm.