Hidden Markov models for burst error characterization in indoorradio channels

Many digital communication channels exhibit statistical dependencies among errors. The design of error control schemes for such channels and their performance evaluation is simplified if appropriate generative models of the overall communication link are available. This paper presents a new class of generative models based on the interconnection of hidden Markov submodels parameterized by the Baum-Welch algorithm. The method has some resemblance to the well-studied problem of speech recognition of isolated words; however, in our approach, instead of dealing with words, one deals with error bursts, and the final goal is to generate bursts rather than to recognize words. The proposed model is particularly suitable for simulating error profiles with long bursts, as is often the case in indoor radio channels, where the error-free gaps inside a burst are heavily nonrenewal. The merits of the method are corroborated by applying the technique to two particular examples of indoor code-division multiple-access (CDMA) radio links     

Burst error characterization of FEC coded digital channels

This paper reports on laboratory measurements to determine the characteristics of error bursts encountered on INTELSAT IDR and IBS links for data rates between 64 and 44,736 kb/s. In addition to burst length distributions, several other burst statistics are presented: number of errors per burst, average BER within bursts, and the distribution of error-free block lengths. These results are intended for use in system design and evaluation of video/speech/data codec performance which are employed on such links.     

Safe and effective error rate monitors for SS7 signaling links

This paper describes SS7 error monitor characteristics, discusses the existing SUERM (signal unit error rate monitor), and develops the recently proposed EIM (error interval monitor) for higher speed SS7 links. A SS7 error monitor is considered safe if it ensures acceptable link quality and is considered effective if it is tolerant to short-term phenomena. Formal criteria for safe and effective error monitors are formulated. This paper develops models of changeover transients, the unstable component of queue length resulting from errors. These models are in the form of recursive digital filters. Time is divided into sequential intervals. The filter's input is the number of errors which have occurred in each interval. The output is the corresponding change in transmit queue length. Engineered EIMs are constructed by comparing an estimated changeover transient with a threshold T using a transient model modified to enforce SS7 standards. When this estimate exceeds T, a changeover will be initiated and the link will be removed from service. EIMs can be differentiated from SUERM by the fact that EIMs monitor errors over an interval while SUERMs count errored messages. EIM offer several advantages over SUERMs, including the fact that they are safe and effective, impose uniform standards in link quality, are easily implemented, and make minimal use of real-time resources     

A Broadcast Protocol for File Transfers to Multiple Sites

A retransmission Protocol for a broadcast connection (point-to-multipoint) is proposed and its performance characteristics are considered. The protocol is designed for transfers of large files Over a satellite channel that is time-shared to carry both the data from the broadcasting transmitter and the set of acknowledgments from the multiple receiver sites. A mathematical model of the transmission system that includes separate error processes for uplink and downlink errors on data transmissions, and similar processes for errors on the acknowledgment frames as well, is used to analyze the performance of the scheme. Exact analytical expressions for the relative throughput of the channel are obtained for two special cases: 1) the uplink is error-free; and 2) the acknowledgments are error-free. For the general case, upper and lower bounds are derived and are shown to be virtually indistinguishable for many practical sets of system parameters. The results demonstrate that the broadcasting of large files to multiple receivers can be done both efficiently and reliably.     

High altitude platform networks: A feedback suppression algorithm for reliable multicast/broadcast services

High Altitude Platforms (HAP) are currently considered as an alternative solution for providing reliable multicast and broadcast services. The major problem limiting the scalability of such systems is Feedback Implosion, a problem arising whenever a large number of users transmit their feedback messages. In the present Letter, a novel scheme is proposed which takes into account rain attenuation being the dominant factor impairing link performance. The proposed scheme is based on the selection of an Area Representative (AR) which is responsible for providing quick feedback messages to the HAP, thus suppressing feedback messages sent by other users. The proposed scheme is assessed both through analytical modeling and simulation with very satisfactory results.     

An evaluation of error performance estimation schemes for DS1transmission systems carrying live traffic

Under the memoryless binary symmetric channel assumption, the author evaluates performance estimation schemes for DS1 transmission systems carrying live traffic. Bipolar violations, framing bit errors, and code-detected errors are commonly used to estimate bit error ratios and the respective numbers of errored seconds and severely errored seconds that are fundamental parameters in characterizing the performance of DS1 transmission systems. A basic framework based on the coefficient of variation is proposed to evaluate several estimation schemes. Serious drawbacks of the existing estimation schemes based on the superframe (D4) format are identified. A new method for estimating the number of errored seconds is proposed. A computer simulation shows that this proposed method performs much better than the conventional counting method. The performance of the cyclic redundancy check (CRC) code of the extended superframe (ESF) format is also evaluated through the use of a computer simulation model. The simulation results show that all the errored seconds are detected by the CRC code. This is a welcome feature of the code for real-time performance monitoring. Furthermore, the results suggest a new threshold of 326 CRC errors per second for determining severely errored seconds     

Automatic PMD compensator in a 160-Gb/s OTDM transmission over deployed fiber using RZ-DPSK modulation format

We report on a magneto- and electrooptically tuned dynamic polarization-mode dispersion (PMD) compensator that enabled error-free single-channel 160-Gb/s RZ-DPSK transmission over a 75-km SMF link with a mean differential group delay (DGD) of more than 30% of the bit period. Polarization scrambling and DOP measurements were utilized to automatically adapt the compensator. We present bit error rate (BER) measurements and describe the operation of the compensator in detail.     

Detection of baseband signals using a delta modulator

The performance of a digital delta modulator when used as a repeater for binary non-return-to-zero signals between a noisy channel and an error-free link is analyzed. A Markov chain is used to model the accumulator output (observation process) of the device, and a Bayes test with a probability of error criterion is formulated to evaluate its performance in additive band-limited Gaussian channel noise. The key design parameters of sampling rate and accumulator range are varied for computation of performance degradation relative to the optional matched filter. Numerical results are presented that demonstrate the feasibility of this technique     

Link failure detection for maintaining session continuity in packet data networks

A link failure in the path of a virtual circuit in a packet data network will lead to premature disconnection of the circuit by the end-points. A soft failure will result in degraded throughout over the virtual circuit. If these failures can be detected quickly and reliably, then appropriate rerouteing strategies can automatically reroute the virtual circuits that use the failed facility. In this paper, we develop a methodology for analysing and designing failure detection schemes for digital facilites. Based on errored second data, we develop a Markov model for the error and failure behaviour of a T1

1 T1 carrier, the lowest level in the plesiochronous digital carrier hierarchy in the United States. AT1 carrier has a payload of 24 64Kbps PCM channels.

trunk. The performance of a detection scheme is characterized by its false alarm probability and the detection delay. Using the Markov model, we analyse the performance of detection schemes that use physical layer or link layer information. The schemes basically rely upon detecting the occurrence of severely errored seconds (SESs). A failure is declared when a counter, that is driven by the occurrence of SESs, reaches a certain threshold. For hard failures, the design problem reduces to a proper choice of the threshold at which failure is declared, and on the connection. reattempt parameters of the virtual circuit end-point session recovery procedures. For soft failures, the performance of a detection scheme depends, in addition, on how long and how frequent the error bursts are in a given failure mode. We also propose and analyse a novel Level 2 detection scheme that relies only upon anomalies observable at Level 2, i.e. CRC failures and idle-fill flag errors. Our results suggest that Level 2 schemes that perform as well as Level 1 schemes are possible.     

Impact of Hot-Potato Routing Changes in IP Networks

Despite the architectural separation between intradomain and interdomain routing in the Internet, intradomain protocols do influence the path-selection process in the Border Gateway Protocol (BGP). When choosing between multiple equally-good BGP routes, a router selects the one with the closest egress point, based on the intradomain path cost. Under such hot-potato routing, an intradomain event can trigger BGP routing changes. To characterize the influence of hot-potato routing, we propose a technique for associating BGP routing changes with events visible in the intradomain protocol, and apply our algorithm to a tier-1 ISP backbone network. We show that (i) BGP updates can lag 60 seconds or more behind the intradomain event; (ii) the number of BGP path changes triggered by hot-potato routing has a nearly uniform distribution across destination prefixes; and (iii) the fraction of BGP messages triggered by intradomain changes varies significantly across time and router locations. We show that hot-potato routing changes lead to longer delays in forwarding-plane convergence, shifts in the flow of traffic to neighboring domains, extra externally-visible BGP update messages, and inaccuracies in Internet performance measurements.      

Characterization of error sequences in UHF digital mobile radiochannels

A statistical characterization of transmission errors for digital mobile radio links at 910 MHz in different types of urban areas is presented. Error sequences for messages of 2-s duration at a nominal rate of 20-kb/s for binary-phase-shift-keying (BPSK) demodulated transmissions have been obtained and characterized by Fritchman's channel model, including one error state and two to four error-free states. In the data analyzed, no trend has been observed in the Fritchman parameter as a function of the type of area or of vehicle speed. Typical Fritchman's parameters are established for low and intermediate error rate channels (on the order of 10^-3 and 10 ^-2 error rates) and for higher error rate channels. Results are also presented for a limited sample of quadrature-phase-shift-keying (QPSK) demodulated 40-kb/s transmissions     

Gigabit/s indoor wireless systems with directional antennas

The design of a high speed (>150 Mb/s) wireless local area network (WLAN), requires that many factors be considered, including technical, economic, and regulatory. A major technical factor is the channel response behavior (multipath) in the indoor environment as a function of the frequency band, building type and radio system architecture. The consequences of designing indoor wireless systems with directional antennas at one or both ends of a line-of-sight (LOS) link are investigated. We determine how narrow the beamwidth must be so that the maximum data rate is not limited by multipath effects. For such beamwidths, simple unequalized two-level frequency shift keying (FSK) or phase shift keying (PSK) modems can be used in place of the more costly and complex “anti-multipath” modems, and data rates above 1 Gb/s may be achieved. The channel impulse response in an empty room is estimated using geometrical optics, observing that with directional antennas, multipath rays must arrive from the same direction as the LOS ray. The link outage probability is then estimated as a function of the antenna beamwidth, and guidelines are established for the selection of the frequency band and antenna placement. Experiments using a 19-GHz 622-Mb/s binary phase shift keying (BPSK) link and 15° beamwidth horn antennas in an office building with plaster walls and large metallized windows have demonstrated error-free performance on both LOS and non-LOS (NLOS) links     

SS/TDMA Steady-State Synchronization: Analysis and Operation During Deep Signal Fades

An analysis of SS/TDMA steady-state synchronization is presented, in which the need for reliable sync maintenance during periods of deep signal fades, beyond the point of communications outage, is considered. The time required for sync-maintenance measurements, together with impact on guard time requirements, will lead to degradation of SS/TDMA transmission efficiency. The need to ensure a sufficiently low rate of occurence of errors in the syncmaintenance measurements also impacts transmission efficiency. A generalized formulation relating the degree of immunity to signal fading, degradation in transmission efficiency, and rate of occurrence of measurement errors is developed. Numerical results are obtained for a typical satellite communications system of current interest. It is shown that sync-maintenance systems can be designed to operate reliably during severe signal fades, orders of magnitude beyond the point of communication outage, at the expense of minimal degradation of transmission efficiency. For this level of performance it is shown that, with proper choice of parameters, virtually error-free syncmaintenance operation can be achieved. Numerical values of typical design parameters are presented.     

Performance of a 46-Gbps Dual-Polarization QPSK Transceiver With Real-Time Coherent Equalization Over High PMD Fiber

We report experimental results on the transmission performance of a coherent dual-polarization quadrature phase-shift-keyed (DP-QPSK) transceiver with real-time electronic equalization over fiber having 50-ps mean polarization mode dispersion (PMD). Both single-channel, single-span and multi-channel, multi-span measurements were performed over the high PMD fiber, which has near Maxwellian statistics for the differential group delay (DGD) as well as higher order PMD as expected for a long, mode-coupled fiber. Transmission of eighty channels was achieved over 8$, times ,$100 km of TrueWave Reduced Slope fiber plus distributed high PMD fiber, where individual channels had instantaneous DGD as high as 127 ps. The dependence of the OSNR penalty on the launch state of polarization was evaluated and found to be minimal, and the contribution of fiber nonlinearities to the transmission penalty was evaluated for two different per-channel launch powers. Finally, using a transceiver equipped with forward-error-correction, error-free transmission over the 800-km link was demonstrated over a 10-day period; during this time the DGD of the measured channel varied from 13 to 116 ps.      

Link Error Analysis and Modeling for Video Streaming Cross‐Layer Design in Mobile Communication Networks

Particularly in wireless communications, link errors severely affect the quality of the services due to the high error probability and the specific error characteristics (burst errors) in the radio access part of the network. In this work, we show that thorough analysis and appropriate modeling of radio‐link error behavior are essential to evaluate and optimize higher layer protocols and services. They are also the basis for finding network‐aware cross‐layer processing algorithms which are capable of exploiting the specific properties of the link error statistics, such as predictability. This document presents the analysis of the radio link errors based on measurements in live Universal Mobile Telecommunication System (UMTS) radio access networks as well as new link error models originating from that analysis. It is shown that the knowledge of the specific link error characteristics leads to significant improvements in the quality of streamed video by applying the proposed novel network‐ and content‐aware cross‐layer scheduling algorithms. Although based on live UMTS network experience, many of the conclusions in this work are of general validity and are not limited to UMTS only.     

Metric anticipation to manage mobility in mobile mesh and ad hoc wireless networks

In mobile ad hoc networks (MANETs), node mobility management is performed by the routing protocol. It may use metrics to reflect link state/quality. But, the delay between measures of the link quality and its integration in the route computation is very detrimental to the mobility management. Consequently, routing protocols may use lossy links for a few seconds leading to a significant performance deterioration. In this paper, we propose a new routing metric technique calculation which aims at anticipating link quality. Basically, the idea is to predict metric values a few seconds in advance, in order to compensate the delay involved by the link quality measurement and their dissemination by the routing protocol. Our technique is based on measurements of signal strength and is integrated in two classical routing metrics: ETX (expected transmission count) and ETT (expected transmission time). Validations are performed through both simulations and a testbed experimentation with OLSR as routing protocol. NS-3 simulations show that our metric may lead to a perfect mobility management with a packet delivery ratio of 100%. Experiments on a testbed prove the feasibility of our approach and show that this technique reduces the packet error rate by a factor of 3 in an indoor environment compared to the classical metrics calculation.     

Stable Throughput of Cognitive Radios With and Without Relaying Capability

A scenario with two single-user links, one licensed to use the spectral resource (primary) and one unlicensed (secondary or cognitive), is considered. According to the cognitive radio principle, the activity of the secondary link is required not to interfere with the performance of the primary. Therefore, in this paper, it is assumed that the cognitive link accesses the channel only when sensed idle. Moreover, the analysis includes: (1) random packet arrivals; (2) sensing errors due to fading at the secondary link; (3) power allocation at the secondary transmitter based on long-term measurements. In this framework, the maximum stable throughput of the cognitive link (in packets/slot) is derived for a fixed throughput selected by the primary link. The model is modified so as to allow the secondary transmitter to act as a ldquotransparentrdquo relay for the primary link. In particular, packets that are not received correctly by the intended destination might be decoded successfully by the secondary transmitter. The latter can, then, queue and forward these packets to the intended receiver. A stable throughput of the secondary link with relaying is derived under the same conditions as before. Results show that benefits of relaying strongly depend on the topology (i.e., average channel powers) of the network.     

Enhanced Low Complex Double Error Correction Coding with Crosstalk Avoidance for Reliable On-Chip Interconnection Link

As the technology scales down, shrinking geometry and layout dimension, on- chip interconnects are exposed to different noise sources such as crosstalk coupling, supply voltage fluctuation and temperature variation that cause random and burst errors. These errors affect the reliability of the on-chip interconnects. Hence, error correction codes integrated with noise reduction techniques are incorporated to make the on-chip interconnects robust against errors. The proposed error correction code uses triplication error correction scheme as crosstalk avoidance code (CAC) and a parity bit is added to it to enhance the error correction capability. The proposed error correction code corrects all the error patterns of one bit error, two bit errors. The proposed code also corrects 7 out of 10 possible three bit error patterns and detects burst errors of three. Hybrid Automatic Repeat Request (HARQ) system is employed when burst errors of three occurs. The performance of the proposed codec is evaluated for residual flit error rate, codec area, power, delay, average flit latency and link energy consumption. The proposed codec achieves four magnitude order of low residual flit error rate and link energy minimization of over 53 % compared to other existing error correction schemes. Besides the low residual flit error rate, and link energy minimization, the proposed codec also achieves up to 4.2 % less area and up to 6 % less codec power consumption compared to other error correction codes. The less codec area, codec power consumption, low link energy and low residual flit error rate make the proposed code appropriate for on chip interconnection link.     

一种低时延的短波自组网优化链路状态路由协议

优化链路状态路由(Optimized Link State Routing,OLSR)协议是一种先验式路由协议,网络中的所有节点通过周期性地发送控制消息来计算全网路由信息。在短波自组织网络中,节点周期性地发送控制消息会占据大量的信道资源,大幅增加网络的控制开销,浪费短波有限的带宽资源,导致网络通信性能急剧下降。其次,受到地形地貌、天线方向和接收性能的个体差异等影响,造成无线链路不稳定,导致网络中存在非对称链路,增加了通信端到端时延。为此,提出了一种低时延的短波自组网OLSR协议。该协议在执行MPR(Multipoint Relay)选择算法时综合考虑了节点的连接度和链路可靠性,在优化MPR节点个数的同时选择链路可靠性较大的节点作为MPR节点,在进行路由选择时能够利用网络中的非对称链路。仿真结果表明,该协议能优化数据包投递成功率、吞吐量、端到端时延和网络控制开销等性能指标。