水下网络突发分组前导码检测的自适应阈值算法

本文针对水声信道的特点,提出了一种适于水下传感器网络突发通信分组前导码检测的自适应阈值算法。该算法根据估计的信号瞬时功率自适应地调整阈值,以取代固定阈值算法中的预设常数阈值。进行了理论分析和仿真,证明该算法具有恒虚警的特点,检测概率比固定阈值算法有明显的提高。针对检测到相关函数旁瓣的情况,加入了滑动窗口。该算法经过了湖试验证,性能良好,具有实用价值。     

一种混沌扩频序列的产生方法及其优选算法

提出一种基于组合映射模型产生混沌扩频序列的方法. 根据扩频序列的特性要求和多址干扰性能指标,给出了一种混沌扩频序列的优选算法;将得到的优选序列应用于直扩码分多址系统,在不同信道条件下进行仿真,并与优选的Logistic混沌扩频序列进行性能比较,结果表明本方法产生的混沌扩频序列具有和Logistic混沌扩频序列相近的良好性能,而且保密性更好. 关键词：码分多址优选算法多径信道误码率     

Static hybrid amplify and forward (AF) and decode and forward (DF) relaying for cooperative systems

In this paper, we propose a static hybrid amplify and forward (AF) and decode and forward (DF) relaying protocol for cooperative systems. In such a scheme, relays close to the source amplify the received signal whereas the remaining relays transmit only if they decode correctly. We consider two subclasses of the proposed hybrid AF–DF relaying protocol. In the first one, all AF relays and DF relays that have decoded correctly transmit using orthogonal channels. The second protocol, called opportunistic hybrid AF–DF relaying, consists in activating only the relay offering the highest instantaneous signal to noise ratio (SNR) among AF relays and the relays that have decoded correctly. The outage SNR probability, and the exact and asymptotic bit error probability (BEP) values of both all-participating and opportunistic hybrid AF–DF relaying protocols are derived and compared to conventional AF and DF relaying. The proposed protocol offers better performance than AF relaying and similar performance to DF relaying with a lower computational complexity. Simulation results are also provided to verify the tightness of the derived results.     

接近非相干水声通信信道容量的信号处理算法

恶劣情况下的非相干水声通信信道模型为随机相位Rayleigh衰落,推导了该模型的信道容量曲线。为实现接近非相干信道容量的可靠通信,提出多进制低密度校验码(LDPC)和恒重码级联码的多进制非相干概率域迭代处理算法。在信道幅度和相位完全未知的情况下,根据矩估计得到信号和噪声频点幅度的统计参量,进而得到恒重码的码字后验概率,再对多进制LDPC码进行因子图迭代译码。仿真证明本算法与现有的最大能量检测非迭代译码算法相比,与信道容量曲线的差距从4.5dB缩小至1.5dB。给出了实际海试湖试通信效果,频段为6~10kHz,数据速率为357bps,海试时近似垂直通信距离为5km,湖试时水平通信距离近3km、多径超过50mS,两种情况下无差错通信的信噪比门限为2dB,验证了本算法的优势。     

On signal space diversity: An adaptive interleaver for enhancing physical layer security in frequency selective fading channels

Signal space diversity is a powerful technique that increases reliability of detection over fading channels. In this paper, we explore the ability of this technique to provide secure communication. We enhance the security of OFDM systems in frequency selective fading channels by providing more diversity gain to the legitimate user compared to an eavesdropper. This is done by adapting the interleaving pattern to the channel of the legitimate user in rich multipath environments, where spatially separated channels are typically independent to each other. This ensures secrecy in a time division duplex system, where the eavesdropper has no information regarding the channel of the legitimate user. The scheme can also be used in the conventional frequency division duplex system, which is more challenging in terms of security aspects because of the channel state information leakage. A theoretical analysis is presented on the bit-error probability in Rayleigh fading environment. The numerical results support the conclusion that adapting the interleaving pattern to the CSI of the legitimate user provides a gain in the bit-error rate performance over the eavesdropper.     

Performance of two-hop infrastructure based multi-antenna regenerative relaying in Rayleigh fading channel

Cooperative relaying is considered as an effective technique to enlarge the coverage area and enhance the system capacity for the future wireless systems. In this paper, an infrastructure based multi-antenna cooperative relay network has been investigated. Closed form expressions of outage probability and average error rate have been derived, when the relay and the destination perform selection combining of the signals. The relay is assumed to operate in the adaptive decode and forward mode. The effect of number of antennas installed on the relay and their placement has also been studied.     

一种DCSK通信系统的延迟合并接收方案

提出一种多径衰落信道下DCSK通信系统的延迟合并接收方案. 理论分析和仿真结果证实了该方案的误码性能. 结果表明,和原DCSK通信系统相比,使用了延迟合并接收方案的DCSK通信系统误码性能有所提高;不同接收路径延迟对应的误码性能基本相同;各接收路径采用等增益合并方式时误码性能最好;其理论上限随着接收路径数增加而逐渐降低.     

Performance evaluation of improved double-threshold energy detector over Rayleigh-faded sensing and imperfect reporting channels

Cognitive radio (CR) has been viewed as a promising solution to spectrum scarcity. In order to design a reliable CR system, many improvements have been proposed to enhance spectrum sensing performance of secondary users (SUs) in a CR network (CRN). Sensing reliability and transmission throughput of SUs are two important performance criteria, which should be optimized to enhance signal protection of primary user (PU) as well as spectrum utilization rate. In this paper, we consider Rayleigh-faded sensing channels and SUs use improved energy detector (IED) to make their local decisions. The final decision is made in a fusion center (FC) through the cooperative spectrum sensing (CSS) scheme with erroneous reporting channels. We show that the improved double-threshold energy detector (IDED) outperforms the conventional energy detector (CED) in terms of the total error rate. Furthermore, we evaluate the transmission throughput of the CRN through various ED schemes with detection constraints over both perfect and imperfect reporting channels. We show that the IDED has the highest achievable throughput among different ED schemes over imperfect reporting channels.     

Signal processing for noncoherent underwater acoustic communication approaching channel capacity

Noncoherent underwater acoustic communication channel in adverse conditions is modeled as a phase-random Rayleigh fading channel,and its capacity curve is derived.To approach the channel capacity curve,the concatenated code of the nonbinary LDPC code and the constant weight code is proposed for noncoherent communication which can late be iteratively decoded in the probability domain.Without information of channel amplitude or phase in the receiver,statistic parameters of the respective signal and noise bins were estimated based on the moment estimation method,the posterior probabilities of the constant weight code words were further calculated,and the nonbinary LDPC code was decoded with the nonbinary factor graph algorithm.It is verified by simulations that by utilizing the proposed concatenated code and its processing algorithm,gap to channel capacity curve is reduced by 3 dB when compared to the existing method.Underwater communication experiments were carried out in both deep ocean（vertical communication,5 km）and shallow lake（horizontal communication,near 3 km,delay spread larger than 50 ms）,in which the signal frequency band was 6 kHz to10 kHz,and the data transmission rate Was 357 bps.The proposed scheme can work properly in both experiments with a signal-to-noise ratio threshold of 2 dB.The performance of the proposed algorithm Was well verified by the experiments.