Clock synchronization for wavelet-based multirate transmissions

This paper deals with symbol timing recovery for multirate transmission systems employing wavelets as signaling pulses either at baseband or with linear bandpass modulation. Our analysis is twofold. First, we investigate the impact of clock errors on the bit-error rate of the optimum receiver for a wavelet-based multirate signal. Second, we consider a nondata-aided maximum-likelihood clock synchronization scheme implemented as a multiple delay-locked loop. We derive a linearized loop model and assess its performance in terms of tracking jitter variance. The analytical results are contrasted with those obtained by simulation and with the relevant modified Cramer-Rao bound     

Joint synchronization and demodulation of trellis codes

This paper examines the joint synchronization and detection of Ungerboeck coded modulation. Estimation theory is used to derive a synchronization structure that is efficient in estimating carrier phase and symbol timing. The maximum likelihood receiver generates estimates of carrier phase and timing that are free of data-dependent jitter for any continuous pulse shape. Various feedback schemes to be used with Ungerboeck codes are presented and simulated. Simulations show that efficient estimates of carrier and clock can be found when joint data and parameter estimation of an Ungerboeck coded signal is performed     

DVB-T接收端同步系统设计与仿真

提出了一种DVB-T接收端的同步方案,包括符号粗同步-精载波频偏联合同步、符号精同步-采样钟相位联合估计、粗载波频偏估计3个部分,完成DVB-T接收端同步,并给出了该同步方案在高斯信道、莱斯信道下的仿真结果.     

Timing recovery for OFDM transmission

Orthogonal frequency division multiplexing (OFDM) is an effective modulation technique for high-rate and high-speed transmission over frequency selective fading channels. However, OFDM systems can be extremely sensitive and vulnerable to synchronization errors. In this paper, we present a scheme for performing timing recovery that includes symbol synchronization and sampling clock synchronization in OFDM systems. The scheme is based on pilot subcarriers. In the scheme, we use a path time delay estimation method to improve the accuracy of the correlation-based symbol synchronization methods, and use a delay-locked loop (DLL) to do the sampling clock synchronization. It is shown that by using this scheme, the mean square values of the symbol timing estimation error can be decreased by several orders of magnitude compared to the common correlation methods in both the AWGN and multipath fading channels. In addition, the scheme can track the symbol timing drift caused by the sampling clock frequency offsets     

Carrier and Clock Recovery for 8 PSK Synchronous Demodulation

Synchronous demodulation of Phase-shift-keyed transmissions requires the recovery of carrier and clock timing signals. Envelope detection of the 70 MHz IF signal is used to recover clock, and baseband processing is used to recover carrier for an 8 PSK digital radio with a data rate of 90 Mbits/s in the 11 GHz band. A sequential phase detector is used in a high noise application. Two versions of the carrier recovery circuit are presented. The first one employs hot carrier diode quads to form a compound Costas loop. The second one employs ECL exclusive-OR gates. Emphasis is placed on providing high performance while conserving energy, space, and manufacturing cost.     

Clock-aided carrier recovery in trellis-coded PSK

The effects of imperfect carrier phase recovery on uncoded and trellis-encoded PSK (phase-shift keying) are compared. A recently proposed clock-aided carrier recovery scheme is analyzed and compared to the standard frequency multiplication circuit. Results obtained through a combination of analysis and computer simulation compare uncoded 4-PSK with trellis-encoded 8-PSK carrying two bits per channel symbol. The quality of the recovered carrier phase is evaluated using the ratio of tone power to background. The system error probability is evaluated and used to determine how accurate the time clock recovery must be in a clock-aided carrier recovery circuit in order to outperform classical recovery systems. It is shown that if the clock recovery system is accurate enough, clock-aided carrier recovery used with coded 8-PSK can compensate for the increased sensitivity of the latter to phase errors     

Digital frequency estimation in burst mode QPSK transmission

In burst digital transmission using PSK (phase shift keying) modulation with coherent detection, the recovery of the carrier reference phase and the symbol clock is a key aspect. If all users have a common clock synchronization, symbol timing needs not to be recovered in each burst. A digital processor for carrier recovery without preambles, in the presence of frequency offset, is considered. As an example, a 2 Mb/s QPSK transmission system is considered in which E _b/N_o=10 dB, and the burst and estimation interval length L=15. Using the algorithm described and averaging eight successive estimated frequency offsets, in order to eliminate anomalous errors, the BER (bit error rate) degradation is equal to 0.14 dB when Δf=20 kHz     

On the synchronization techniques for wireless OFDM systems

The latest research works on the synchronization scheme for either continuous transmission mode or burst packet transmission mode for the wireless OFDM communications are overviewed in this paper. The typical algorithms dealing with the symbol timing synchronization, the carrier frequency synchronization as well as the sampling clock synchronization are briefly introduced and analyzed. Three improved methods for the fine symbol timing synchronization in frequency domain are also proposed, with several key issues on the synchronization for the OFDM systems discussed.     

Analysis of self-noise in a fourth-power clock regenerator

A method for computing the power spectra of the in-phase and quadrature components of self-noise in a fourth-power clock recovery scheme is presented. This method is shown to be effective if the symbol waveform is time-limited or can be so approximated. The method applies to baseband signaling schemes such as pulse-amplitude modulation and to modulated carrier schemes such as quadrature-amplitude modulation. Comparison is made with the squaring-type recovery technique using cosine rolloff Nyquist pulses on the binary-phase-shift-keyed mode, at several excess bandwidth factors (α). It is shown that when α is less than 0.4, the fourth-power circuit gives less time jitter than the squarer for a noiseless channel. The method for self-noise spectrum calculation is verified by computer simulation. A prefilter that completely eliminates time jitter in the fourth-power clock recovery circuit is described     

Residual Synchronization Error Elimination in OFDM Baseband Receivers

It is well known that an OFDM receiver is vulnerable to synchronization errors. Despite fine estimations used in the initial acquisition, there are still residual synchronization errors. Though these errors are very small, they severely degrade the bit error rate (BER) performance. In this paper, we propose a residual error elimination scheme for the digital OFDM baseband receiver aiming to improve the overall BER performance. Three improvements on existing schemes are made: a pilot‐aided recursive algorithm for joint estimation of the residual carrier frequency and sampling time offsets; a delay‐based timing error correction technique, which smoothly adjusts the incoming data stream without resampling disturbance; and a decision‐directed channel gain update algorithm based on recursive least‐squares criterion, which offers faster convergence and smaller error than the least‐mean‐squares algorithms. Simulation results show that the proposed scheme works well in the multipath channel, and its performance is close to that of an OFDM system with perfect synchronization parameters.     

Joint linear timing and carrier phase estimation of DS-CDMAmultiuser communications

Previous work on multiuser synchronization assumed that either timing recovery or carrier phase recovery schemes were available. Therefore, a generalized joint linear timing and carrier-phase estimation scheme to effectively estimate both the received timings and carrier phases at the same time is of paramount interest. Based on the idea to linearly combine the sufficient statistics of timings and carrier phases, we introduce the generalized joint linear timing and carrier phase estimation scheme with commensurate complexity of the generalized linear timing estimator. To avoid the difficulty of the design by minimizing the mean-square error (MSE), a simple and novel design criterion for this generalized joint linear estimation is proposed. With a discrete timing assumption, we not only mathematically prove that the MSE of the proposed estimator converges to zero as noise vanishes for an arbitrary number of users, but also conduct simulations to verify it. On the other hand, for the general case with a continuous timing assumption, arguments and simulations are provided to confirm the effectiveness of our proposed estimator for an arbitrary number of users. We further show that such an efficient linear-complexity joint estimation scheme is effective in the near-far environment. Consequently, the nonpolynomial (NP) hard multiuser synchronization over additive white Gaussian noise (AWGN) channel is successfully solved     

OFDM系统中同步问题的研究

OFDM系统的实现还存在着一些方面问题,如：同步问题尤其是频偏,峰值功率与平均功率比值过高,器件的非线性化等。OFDM系统对同步偏差敏感是本文讨论的重点,尤其是频偏。因此怎样获得准确的符号定时和载波频偏估计对OFDM系统性能至关重要。本文围绕OFDM同步过程中的符号定时和载波频率偏差问题进行描述,在分别介绍同步过程中捕获和跟踪两个阶段各自一些典型算法基础上,对一种基于保护间隔／循环前缀（PI／CP）的联合实现符号定时和载波同步最大似然估计（MLE）算法进行叙述并仿真。     

Power suppression at the Nyquist frequency for pilot-aided PAM andQAM systems

Two low-redundancy codes for in-band pilot insertion are introduced. KF10 suppresses the baseband PAM and the modulated QAM signal power at the Nyquist frequency so that the dual-pilot tone calibration technique of recent interest can be arranged to convey both a symbol timing and a carrier reference to the receiver. OF00 suppresses the signal power at zero as well as the Nyquist frequency and allows the utilization of a triple-pilot scheme which provides, in addition to a symbol timing, a carrier reference with no 180° phase ambiguity     

Efficient non-data-aided carrier and clock recovery for satelliteDVB at very low signal-to-noise ratios

This paper deals with the analysis and design of efficient non-data-aided clock and carrier (frequency/phase) synchronization algorithms intended for use in satellite digital video broadcasting systems employing turbo-coding techniques to enhance the power efficiency. The above issue appears quite challenging in view of both the extremely low signal-to-noise ratio, typical of turbo-codes operation, and the very short time allocated to the acquisition of synchronization parameters. These constraints rule out most conventional clock/carrier recovery schemes and demand a careful search for specific, highly efficient algorithms. We propose and analyze a clock/carrier synchronization scheme capable of operating at values of E_b/N _o as low as 1 dB with lock-in delay not exceeding 50 ms     

基于FPGA的数字基带传输位同步恢复

提出了一种基于全数字锁相环提取数字基带传输位同步时钟的设计方案,该方案采用环路鉴相器产生误差信号控制本地位同步电路的添加/扣除门在时钟输出的脉冲序列中附加或扣除1个或几个脉冲实现同步。给出了该方案的整体电路,并经VHDL程序设计,在MaxplusⅡ环境下做了时序仿真,从仿真结果分析了设计方法可实现数字基带传输位同步时钟的提取。     

一种全数字位同步检测算法

位同步检测是实现全数字接收机开环定时恢复的关键技术。本文推导了基带PAM(脉冲幅度调制)信号的互相关函数,指出基带PAM信号的互相关函数中含有位同步信息,以此为依据得到了一种位同步检测算法。该算法可采用递归结构实现,且与载波相位误差无关。     

Efficient modulation techniques for satellite telemetry—receiver design and performance evaluation

In this paper we address the design of the data demodulator for high-efficiency satellite communications suited for telemetry applications. We choose Gaussian minimum shift keying as the modulation format and analyse its relative merits with respect to the more traditional filtered offset quadrature phase-shift keying scheme, when applied to medium-to-high data rate telemetry. Our study encompasses both geostationary orbits and the more challenging low earth orbit scenarios, and investigates the acquisition and tracking performance of the receiver with the aid of computer simulations. In particular, we present design criteria leading to the development of efficient, low-complexity schemes for symbol timing and carrier phase/frequency recovery and we analyse the impact of these synchronization algorithms on the system performance. Next, we discuss the demodulator performance degradation when the transmitted signal is passed through on-board solid-state or travelling-wave-tube power amplifiers. The paper is concluded by a comparative analysis of the above modulation schemes, with special emphasis on their power and spectral efficiency.     

基于软件无线电原理导航接收机信号解算

简要介绍了导航系统的功能及信号特性,应用软件无线电理论进行了接收机设计,特别对信号解算过程做了重点论述,其中包括数字中频采样、数字下变频、载波恢复与跟踪、帧同步信号的提取和幅度解调等。根据理论分析进行了计算机仿真,对具有一定频偏及加有噪声的中频信号采样,解调出基带信号,得到所需的数据信息和角度信息,同时对影响角度解调精度的因素做了简要分析,为接收机的设计和参数的选取提供了依据。     

一种快速FH-OFDM通信帧格式及其验证

针对通信中跳频与正交频分复用两种技术结合存在同步困难的技术难点,提出一种快速跳频OFDM无线宽带通信帧格式,并设计了一种占用带宽为10 MHz、跳频速率为2 kHz、基带数据传输速率为14.324 Mbit/s的快速跳频OFDM通信收发系统.该帧格式使用加长循环前缀的方式提供高精度载波同步依据,将伪随机序列隐藏到频域数据中以便利用其完成精确定时同步、采样时钟同步和整数倍频偏估计,并且插入了梳状导频以便接收端进行信道估计.实验结果表明,接收机星座图可以很好地收敛,在信噪比为10 dB情况下,误码率低于10-8.本帧格式可较好地满足视频等数据的无线通信要求.     

Noncoherent Data Transition Tracking Loops for Symbol Synchronization in Digital Communications Receivers

Starting with the maximum a posteriori (MAP) estimation approach, this paper derives the optimum (in the MAP estimation sense) means for performing symbol timing recovery in the absence of carrier phase information (i.e., prior to carrier phase recovery). Specifically, we examine the necessary modification of a well-known form of coherent symbol synchronizer, namely, the data transition tracking loop (DTTL) to allow its operation in the absence of carrier phase information, i.e., as a so-called noncoherent symbol sync loop. By employing such a noncoherent scheme, one can eliminate the need for iteration between the carrier and symbol sync functions, as typically takes place in receivers that more commonly perform carrier tracking and acquisition prior to symbol timing. The performance of both the linear and nonlinear versions of this noncoherent DTTL is obtained by a combination of analysis and simulation, and compared with that of the corresponding coherent DDTLs.