物理层的超高频RFID标签信号分离

提出一种分离冲突信号算法,先用动尺度变换将冲突信号变换至时—尺域并得到变换矩阵,然后由最小二乘和最小均方误差准则将冲突信号分离以检测标签信号。仿真结果表明,相比传统标签分离方法,所提方法仅采用单接收天线就可分离2个以上冲突标签信号,并在无捕获效应下也具有良好分离性能。     

广播发射射频系统中的信号冲突消除方法分析

信号冲突是影响广播发射射频系统运行的主要因素,对广播信号接收和发送具有很大影响,影响广播发射频系统发射信号的稳定,严重时还会造成系统运行瘫痪。因此必须对广播发射射频系统产生的信号冲突原因进行分析,然后提出新型射频系统冲突算法,减少空间时隙产生,促进广播射频发射系统中信号的传输,保证信号纯熟的稳定性和连续性。     

广播发射射频系统中的信号冲突消除方法研究

本文就在对广播发射机射频系统中的信号冲突进行简单分析的基础上,提出一种射频标签防冲突算法,并应用相关的算法对其进行仿真验证,这对于消除广播发射射频系统中的信号冲突具有积极的作用。     

物理层超高频射频识别标签信号分离中的信道估计

在被动式的超高频(UHF)射频识别(RFID)系统中,当多个标签选择同一个时隙向阅读器发送信息,标签间冲突就会产生,该冲突通常只在介质访问控制(MAC)层上解决。其实,若冲突信号在物理层上被分离,识别效率将能得到很大的提高。在物理层冲突信号分离中,信道估计是一项关键技术,因为好的信道估计有助于准确地恢复冲突信号。传统的信道估计方法在两个标签冲突的情况下具有较好的估计性能,但当冲突标签数超过2时,却会产生较高的误差。该文针对物理层的UHF RFID信号分离问题,提出一种新的信道估计方法。该方法利用已知的前缀信号,采用最小二乘方法对信道进行估计。从实验结果看,当标签冲突数超过2时,该文提出的信道估计方法的误差要小于传统的估计方法,而且估计的信道得到的分离效率也高于传统方法。     

广播发射射频系统中的信号冲突消除方法探讨

当广播的发射系统开始运行时,所产生的信号冲突能够影响广播信号的发射、接收,进而使系统的运行无法达到稳定的状态,也促使了广播在传播信息方面无法维持其正常性.因此,要想消除信号冲突,就应先找到产生信号冲突的原因并作出可行性的分析.通过根据信号理论的相关计算方法,以便调节系统中的信号缝隙,以实现对信号冲突的消除目标.经过探讨广播的发射系统使信号产生冲突的消除方法,使信号的传播具有完整性、连续性.     

大型DCS设备中上位机通信误差消除方法

针对大型DCS设备上位机与下位机同时进行通信时,DCS上位机可能同时接收多个下位机的控制器发来的指令,在时域指令信号方面形成冲突,造成DCS系统的上位机与终端之间通信成功率降低。为解决这一问题,提出一种大型DCS设备中上位机通信误差消除方法,算法利用一种时间空闲间隔的有效填充法、信号间的通信机制,解决上位机与控制器之间时域信号冲突。并利用信号预测检测技术,提前对下个时隙进行调整,减少冲突的可能性。解决时域内的信号冲突问题。实验结果表明,该方法在DCS系统较大的情况下,能较好地协调上位机与控制终端之间的联系,提高了多机器人的通信成功率。     

移动网络安全策略冲突检测方法的改进研究

针对移动网络路由冲突导致安全性低的问题,提出基于Hilbert-Huang变换时频分析的移动网络安全冲突检测算法,构建路由冲突下的网络攻击信号模型,对移动网络的入侵信号进行经验模态分解实现谱特征分析和提取,采用HilbertHuang变换对移动网络入侵信号进行高低频组分分解,结合时频分析方法实现路由冲突下的入侵检测,提高网络安全性能。实验分析表明,采用该检测算法进行网络入侵检测的准确性较好,干扰对抗能力较强,是一种有效的移动网络安全策略。     

短距离无线通信系统的冲突信号分离与解码

短距离通信是物联网中常用的通信方式,多是一传感节点对多物理节点的情形,由于其采用无线共享信道,因此多个信源一起传输必导致冲突发生。通常,该冲突采用随机多址方法解决,但通信效率并不高。采用信号分离和编解码技术可直接从冲突信号中恢复出信源,因此可显著提高通信效率,但该方法需要预知或估计信号衰落系数才能保证良好的性能。本文提出一种有限符号分离的维特比解码方法,该方法通过估计字典矩阵,无需预知衰落系数就能准确提高恢复冲突信号。实验中,我们分别用仿真和软件无线电构建了一个基于FM0码的超高频短距离无线通信系统,实验结果表明本文提出的方法吞吐量达到了约0.61,相比传统的ALOHA系统吞吐量提高了约0.25。      

减轻移动式RFID网络冲突问题研究

RFID被广泛地应用,如物流管理、物体跟踪和商业供应链等等。当从一个读写器发射的信号和另一个读写器发射的信号相干扰时就产生了读写器冲突。除了误操作,读写器冲突同时使射频识别系统总的阅读速率减慢,而且这些问题在移动或手持式读写器中更加严重。提出的读写器抗冲突算法,它通过在控制通道上周期地发送信标以达到抗冲突的目的。该算法不仅适用固定式读写器,同时还适合移动或手持式阅读器的射频网络。     

基于CRC校验的星载AIS系统接收机

为了提升星载船舶自动识别系统(Automatic Identified System)的接收性能,文中提出了一种改进的星载AIS接收机方案。针对多路信号冲突的情况,研究了采用联合维特比解调(JMLSE)算法的AIS接收机误比特特点,选取合适的2比特CRC纠错算法进行纠错,改善信号解调性能。文中针对两路信号冲突情况,在不同功率比、相对时延和频偏条件下进行方案性能仿真。结果表明,使用2比特CRC纠错算法的星载AIS接收方案能够有效降低接收机误包率。     

Semi-blind collision resolution in random access wireless ad hocnetworks

A new signal processing based collision resolution technique for random access wireless ad hoc networks is proposed. Without assuming the knowledge of the propagation channels and signal waveforms, the proposed algorithm is capable of separating colliding packets by exploiting channel diversities and known symbols embedded in data packets. Compared with training-based methods, the proposed algorithm requires considerably fewer known symbols. This algorithm can be applied to various spread spectrum and narrowband systems along with existing medium access control protocols     

Hierarchical constellation for multi-resolution data transmission over block fading channels

We propose a new technique for multi-resolution video/image data transmission over block fading channels. The proposed scheme uses an adaptive scheduling protocol employing a retransmission strategy in conjunction with a hierarchical signal constellation (known also as nonuniform, asymmetric, multi-resolution constellation) to give different transmission priorities to different resolution levels. Transmission priorities are given in terms of average packet loss rate as well as average throughput. Basically, according to the transmission scheduling and channel state (acknowledgment signal) of the previous transmission, it dynamically selects packets from different resolution levels to transmit for the current transmission. The bits from the selected packets are assigned to different hierarchies of a hierarchical 4/16-quadrature amplitude modulation to transmit them with different error protections. The selection of packets for transmission and the assignment of these selected packets to different hierarchies of the hierarchical constellation are referred to as the scheduling protocol in our proposed scheme. We model this protocol by a finite state first order Markov chain and obtain the packet loss rate and the packet transmission rate over Nakagami-m block fading channel in closed-form. Some selected numerical results show that the proposed scheme can control the relative packet loss rate and the packet transmission rate of different resolution levels by varying the priority parameter (or equivalently, the asymmetry) of the hierarchical constellation and the maximum number of allowed retransmissions.     

Contention resolution in random-access wireless networks based on orthogonal complementary codes

This paper proposes a new method for contention resolution in random-access wireless networks. Using orthogonal complementary codes to design access-request packets, users can reserve channel access successfully, even in severe contentions. Collisions among access-request packets can be resolved and exploited, whereas collisions among data packets are avoided. System throughput and delay performance can be enhanced, because random-access contention becomes transparent. Specifically, system throughput approaches the offered load up to the maximum value one with improved average packet delay performance. A joint layer design approach is proposed with both the physical layer signal-detection algorithm and the medium access-control layer random-access protocol. The performance is analyzed with the consideration of signal detection errors. Simulations are performed to demonstrate its superior performance.     

A new wireless network medium access protocol based on cooperation

In this paper, we propose a new media access protocol for wireless networks, that due to its ability to resolve collisions can achieve high throughput. We view the wireless network as a spatially distributed antenna with antenna elements linked via the wireless channel. When there is a collision, the collided packets are saved in a buffer. In the slots following the collision, a set of nodes designated as nonregenerative relays retransmit the signal that they received during the collision slot. By processing the originally collided packets and the signals forwarded by the relays, the destination node can recover the original packets. The proposed scheme maintains the benefits of ALOHA systems, i.e., needs no scheduling overhead and is suitable for bursty sources, such as multimedia sources. It also offers the benefits of multi-antenna systems, i.e., spatial diversity while employing a single transmit/receive antenna at each node. Spatial diversity enables it to be robust to the wireless channel. The proposed approach achieves higher throughput and energy savings than existing techniques that allow for multiple packet reception.     

Packet rescheduling in WDM star networks with real-time servicedifferentiation

In this paper, we propose a novel scheduling scheme, namely priority-differentiated scheduling (PDS), which is designed to handle real-time (high-priority) packets in wavelength division multiplexing (WDM) star networks. PDS allows high-priority packets to preempt the prescheduled low-priority (nonreal time) packets. By scheduling the high-priority packets first, and then having the preempted packets rescheduled, PDS guarantees that the high-priority packets can always achieve the earlier transmission than the others in order to meet the quality of service (QoS) requirements. In addition, it does not sacrifice the performance of nonreal-time packets. As a matter of fact, low-priority packets can also benefit from PDS algorithms. This scheme has the capability of handling channel collision and destination conflict, and it supports variable-length packets. This paper also provides an insight into the data channel selection technique along with PDS. With the proposed algorithm that selects a data channel with minimum scheduling latency (MSL), the channel throughput is improved. The performance of the PDS scheme has been extensively studied by means of numerical simulations     

A prefix-based approach for joint Doppler and channel estimation in underwater communication

Developing a reliable and robust underwater acoustic communication system is a difficult task due to the complicated nature of the underwater channel, non-stationary noise, and several other factors. Indeed, channel estimation or equalization presents numerous challenges in this non-stationary, highly Doppler, multipath environment; as a result, traditional equalizers and PLL-based methods have limited performance. Generally, communication over such time-varying channels is accomplished via packets that contain a prefix/preamble signal for training, a payload containing the actual data, and a silent period for proper alignment. The prefix signal must be designed properly because it is used to estimate the channel and determine the start of the packet. In this paper, we propose a new prefix signal based on the hyperbolic chirp signal, which has Doppler invariance properties. These properties enable the extraction of the entire packet even under severe multipath and Doppler. Our new proposed prefix signal can accurately and efficiently characterize an underwater channel by estimating the multipath delay, amplitude, and Doppler scales. Our new method has been validated through extensive simulations using various channel models for its robustness and effectiveness under various conditions. It has also been tested on a real-world channel.     

GSM/GPRS分裂信道高效重分配策略

现有GSM/GPRS系统各种信道分裂策略都仅限于话音呼叫,即仅允许分裂信道分配给话音呼叫而不允许分配给包呼叫.因而,当系统没有空信道时,即使可以提供分裂信道,一个新到达的包呼叫也将被阻塞.本文提出的分裂信道重分配策略允许话音呼叫和包呼叫均可获得分裂信道,论文还研究了包呼叫得到分裂信道的条件.研究结果表明,允许将分裂信道分配给新到达包呼叫可以获得更优越的性能;在本文的参数设置下,在门限参数θ＝2/3时,新策略的话音阻塞率和平均包传输时间没有明显恶化,而包阻塞率和平均信道利用率却得以显著改善.     

Robust JPEG image transmission using unequal error protection and code combining

JPEG image transmission over noisy channels is highly problematic due to the sensitivity of the JPEG bit stream to error propagation. The use of resynchronization markers and channel coding do not alleviate the problem completely thus making retransmissions inevitable. In packetized image transmission, image packets are repeated n times, to ensure reliable transmission. This paper proposes a new unequal error protection (UEP) scheme which jointly optimizes the allocation of channel code rates and number of repeats to image packets, subject to a constraint on the maximum overall transmission rate. The coding scheme used is the rate compatible punctured convolutional code coupled with the code‐combining technique. An unequal allocation of headers to the image packets is also performed in order to reduce the overall distortion due to error propagation. Simulation results show that the proposed UEP scheme provides a gain of more than 8 dB in peak‐to‐peak signal‐to‐noise ratio over a tandem scheme. The flexibility of the proposed scheme, and the major performance gains obtained, make the scheme appealing for applications like, web‐based image browsing, multi‐hop networks, and wireless image transmission. Copyright © 2007 John Wiley & Sons, Ltd.     

A hybrid beaconless geographic routing for different packets in WSN

The localized operation and stateless features of geographic routing make it become an attractive routing scheme for wireless sensor network (WSN). In this paper, we proposed a novel routing protocol, hybrid beaconless geographic routing (HBGR), which provides different mechanisms for different packets. Based on the requirement of application on latency, we divide the packets of WSN into delay sensitive packets and normal packets. HBGR uses two kinds of Request-To-Send/Clear-To-Send handshaking mechanisms for delay sensitive packets and normal packets, and assigns them different priority to obtain the channel. The simplified analysis is given, which proves that delay sensitive packets have lower latency and higher priority to obtain the channel than normal packets. Moreover, forwarding area division scheme is proposed to optimize the forwarder selection. Simulation results show that HBGR achieves higher packet delivery ratio, lower End-to-End latency and lower energy consumption than existing protocols under different packet generation rates in stationary and mobility scenario. Besides, compared with normal packets, delay sensitive packets have at least 10 % (9 %) improvement in terms of End-to-End latency. The improvement increases with the increasing of packet generation rate, and achieves 58 % (73 %) when the packet generation rate is 24 packets per second in stationary (mobility) scenario.     

A Pseudo-Bayesian ALOHA Algorithm with Mixed Priorities

In reservation-based multiple access protocols, before obtaining a contention-free access to the channel, a mobile terminal must wait for its request packet to be successfully sent to the base station. A pseudo-Bayesian ALOHA algorithm with multiple priorities is proposed in this paper to reduce the waiting time of delay sensitive request packets in a multimedia environment. Packets are transmitted in each slot according to a transmission probability based on the channel history and a priority parameter assigned to their respective priority class. An adaptation of the slotted protocol to the framed environment proposed for wireless ATM is also described. Simulation results are presented to show that the protocol offers a significant delay improvement for high priority packets with both Poisson and self-similar traffic while low priority packets only experience a slight performance degradation.