Packet-length adaptive CLSP/DS-CDMA: performance in burst-error correlated fading channels

The authors analyze throughput-delay performance of an unslotted channel load sensing protocol (CLSP)/direct sequence (DS)-code division multiple access (CDMA) packet radio network (PRN) with adaptive packet length over burst-error correlated fading channels. CLSP controls the packet access in uplink of unslotted ALOHA/DS-CDMA systems so that contention is avoided and throughput is maximized. However, due to high uncertainty of radio channels, the performance of CLSP/DS-CDMA PRN may suffer from notable degradations. Using theoretical analysis and simulation, the authors show that in highly correlated fading environments adapting the length of radio packets to fading conditions significantly improves system performance and energy efficiency of mobile terminals. In their modeling, they study the relation between the fade statistics and the packet length in correlated Rayleigh fading channels. The effects of reception diversity, imperfect transmit power control (TPC), and user mobility are considered. The results are used to develop simple, energy-efficient, and robust adaptation mechanisms.     

Analysis of unslotted direct-sequence spread spectrum multipleaccess network with packet combining

A packet combining scheme is investigated for an unslotted random access code division multiple access network over a frequency selective Rayleigh fading channel. It is shown that this simple diversity combining mechanism is highly advantageous for systems which can both tolerate a certain delay and operate over highly time-varying channels     

Performance of CDMA random access systems with packet combining in fading channels

We analyze the system performance of code-division multiple-access (CDMA) random access systems with linear receivers and packet combing in multipath fading channels. Both slotted and unslotted CDMA systems with random spreading codes are considered. The analysis is based on large systems in which both the offered load and the processing gain tend to infinity but their ratio is fixed. It is relatively easy to characterize the traffic in such large systems, which enables us to derive the system throughput and average delay. From the analysis results, it is observed that multiuser detection and packet combining substantially improve the system performance.     

ALOHA with Multipacket Messages and ARQ-Type Retransmission Protocols--Throughput Analysis

The throughput of slotted ALOHA systems with multipacket message transmissions is evaluated. The conventional strategy of retransmitting the entire message when collisions occur results in a reduction in throughput when compared with the single packet case. However, it is proved analytically that the use of ARQ-type retransmission protocols can provide significant performance improvement. In particular, it is shown that a simple go-back-N(GBN) protocol can offer a modest throughput increase, while a selective reject (SREJ) strategy provides exactly the same throughput as a single packet slotted ALOHA system, irrespective of message length statistics. These results motivate a new scheme (referred to as unslotted selective reject (SREJ) ALOHA) for increasing the throughput of unsynchronized random access channels. It is demonstrated that unslotted SREJ ALOHA with optimized parameters can achieve typical maximum throughputs (after accounting for all overheads) at least 25-40 percent higher than conventional pure ALOHA.     

Performance analysis of slotted fiber-optic code-divisionmultiple-access (CDMA) packet networks

This paper examines code-division multiple-access (CDMA) techniques used in slotted fiber-optic packet networks. Since the inherent properties and signal processing of the conventional communication channels are different from those of the fiber-optic channels, new code sequences must be constructed for fiber-optic applications. The goal of our research is to analyze the performance of fiber-optic CDMA packet networks using code sequences with given orthogonality properties     

Stability, Throughput, and Delay of Asynchronous Selective Reject ALOHA

An analytical performance evaluation model is presented for selective reject (SREJ) ALOHA, a recently proposed high-capacity protocol for unslotted channels with high propagation delay. SREJALOHA approaches the theoretical asynchronous multiaccess throughput limit of 0.368 for fixed or variable length message traffic, using subpacketization of messages in conjunction with a selective reject retransmission policy. A finite user flow equilibrium-based model for SREJ-ALOHA with variable length messages is derived and used to characterize stability, throughput, and delay. Particular attention is given to the development of a stability analysis methodology for unslotted ALOHA-type protocols with variable length transmissions. Numerical results based on a satellite channel example are presented to demonstrate the significant performance advantages offered by SREJ-ALOHA, even after accounting for realistic subpacket overheads.     

Twelve random access strategies for the fiber optic networks

Twelve random-access strategies that do not constrain the distance or transmission rate of a network, and can use the capabilities of fiber-optic components, are described and compared. The twelve strategies consist of three protocols, each of which can use two timing arrangements and two network-access devices. The three protocols are the standard ALOHA protocol, LCSMA, and LCSMA/CD. The last two protocols operate on linear-unidirectional networks and use local information at the transmitter to increase the throughput of the system. The networks considered have a common point that all transmitted signals pass through before being received. This makes two timing arrangements possible; a slotted system or an unslotted system. The taps on the network can be either passive or active     

Throughput of unslotted direct-sequence spread-spectrummultiple-access channels with block FEC coding

An analysis of unslotted random-access direct-sequence spread-spectrum multiple-access (DS/SSMA) channels with block forward error correction (FEC) coding is presented. Extending a methodology that was introduced in an earlier paper on unslotted packet code-division multiple access (CDMA) without coding, a procedure for calculating the error probability of an L-bit packet in the variable message length, FEC-coded, DS/SSMA environment is described. This procedure is then used in conjunction with appropriate flow equilibrium traffic models to compute channel throughput. Using BCH block coding as an example, the analytical model is exercised to obtain throughput versus channel traffic curves over a range of code rates, leading to an assessment of maximum achievable throughput and the associated optimum FEC code rate. The results show that the use of block FEC coding provides a significant improvement in the bandwidth-normalized channel throughput (utilization), approaching values competitive with those for comparable narrowband ALOHA channels     

A low DC, self-clocking, two-color transmission code for optical communications

A new self-clocking channel code is demonstrated and compared to other codes used in digital communication systems. The new code requires two channels and is best suited for wavelength-multiplexed optical fiber transmission systems. An experimental demonstration of the proposed code in a fiber-optic communication link is reported and the code performance is analyzed.     

Comments on `An efficient demand-assignment multiple-access schemefor satellite mobile radio dispatch networks' [and reply]

The commenter calls the attention of the authors of the above paper (see ibid., vol.38, p.204-10, Nov. 1989) to very closely related earlier work that deals with the essential issues in a more fundamental way. Specifically, these consider traffic capacity comparisons for demand-assigned, multiple-access systems that use slotted or unslotted ALOHA-type request channels. The analysis and results are directly applicable to any arrangement of channels in an FDMA (frequency multiple access)/TDMA (time division multiple access) multiplex and also account for the possibility that requests cannot be served because specific assets are busy although message channels are available. The authors agree that using the blocked-calls-queued service discipline to increase traffic capacity has been analyzed in the previous work cited. They point out that this call-processing discipline is particularly useful in the satellite mobile radio dispatch application examined because of the dispatcher bottleneck. Another important issue in mobile satellite demand-assignment multiple-access for radio dispatch is the large fraction of signaling overhead relative to the very short holding time of individual mobile radio calls. The authors note that their paper offers the techniques of multiple-calls batch-processing and pipeline signaling as effective measures in dealing with this issue     

Performance analysis of DS/SSMA unslotted ALOHA system withvariable length data traffic

We analyze the throughput of a direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA system with variable length data traffic. The system is analyzed for two cases: (1) systems without a channel load sensing protocol (CLSP) and (2) systems with a CLSP. The bit-error probability and the throughput are obtained as a function of the signal-to-noise ratio (SNR) during message transmission, considering the number of overlapped messages and the amount of time overlap. We assume that the generation of data messages is Poisson distributed and that the messages are divided into packets before transmission. The system is modeled as a Markov chain under the assumption that the number of packets in a message is geometrically distributed with a constant packet length. The throughput variance of the DS/SSMA unslotted ALOHA system with variable length data traffic is obtained as the Reed-Solomon code rate varies. Results show that a significant throughput improvement can be obtained by using an error-correcting code     

A novel spread slotted Aloha system with channel load sensingprotocol

This paper presents a novel spread slotted Aloha system with channel load sensing protocol (CLSP). CLSP is an effective scheme to improve the throughput performance in spread unslotted Aloha systems. In spread slotted Aloha systems, however, it does not make sense to utilize CLSP because the slot size is usually the same as the packet size. The slot size of the proposed system is set less than the packet size, thereby enabling the authors to apply CLSP and improving the throughput performance. Another feature of the proposed system is that the system is not likely affected by the time difference between channel load sensing and timing of packet access, which they call the access timing delay. Throughput performance of the proposed system is evaluated in the presence of the access timing delay and a significant increase of the throughput is shown compared with that of spread unslotted Aloha with CLSP     

Fiber-optic subcarrier multiplexed CDMA local-area networks forsubband image transmission

This paper investigates the application of subcarrier multiplexed code-division-multiple-access (CDMA) techniques to image transmission over fiber-optic local-area networks (LANs). In the hybrid scheme, CDMA is used to suppress the interference caused by the laser nonlinearity in the subcarrier multiplexing (SCM) fiber-optic communication systems. Likewise, the SCM scheme is able to increase the channel data rate of CDMA systems. This hybrid system combines the advantages of both schemes and is particularly well suited to subband coding that divides the image information into multiple parallel data streams using an analysis filter bank, each of which is transmitted via a unique subcarrier-code pair, where the spreading code and subcarrier frequency correspond to the image and one of its subbands, respectively. This hybrid scheme also allows more than one image to be transmitted and be accessed simultaneously at the same channel bandwidth, in which each image is assigned a particular spreading code added to its digital data modulating the subcarrier. After transmission, each received signal is independently recovered at a high-Q surface acoustic wave (SAW) receiver with the matching subcarrier-code pair. Other concurrent signals are rejected by the SAW. Then, all the recovered subbands are reassembled by a synthesis filter bank into a close reproduction to the original image. The image quality of subband image transmission via CDMA/SCM fiber-optic channels is evaluated and examined     

非时隙CDMA分组网络吞吐量性能分析:统一的方法

CDMA分组网络性能主要受限于信道中同时传输的其它分组的干扰.与时隙CDMA分组网络相比,非时隙CDMA分组网络所受到的干扰情况更加复杂.本文提出了一种非时隙CDMA分组网络的干扰分析模型,并采用递归方式建立了网络吞吐量性能分析的统一方法.在此基础上,分析了网络节点突发固定长度分组和突发可变长度分组两种情况下的网络吞吐量性能,并讨论了扩频增益和分组传输方式对网络性能的影响,给出了相应的数值结果.文章最后进一步讨论了吞吐量的上下界问题.与前人提出的马尔可夫模型相比,采用本文提出的方法可以得到更为准确的吞吐量性能.     

Unslotted deflection routing: a practical and efficient protocolfor multihop optical networks

This paper is concerned with all-optical networks using deflection routing and time division multiplexing. Slotted networks make use of the synchronous arrival of the packets to the routers to minimize locally the number of deflections. We show that the difference in performance between slotted and unslotted networks is mainly due to the fact that unslotted networks cannot easily perform such local optimization. We also show that minimizing locally the number of deflections in unslotted networks gives rise to an NP-complete problem. To overcome this problem, we have designed a heuristic whose aim is to limit locally the number of deflections. We experimentally demonstrate that this heuristic enhances unslotted routing almost at the same performance level as slotted routing. As a consequence, we have shown that unslotted deflection routing can be implemented is a way which makes it a competitive alternative to slotted deflection routing for optical time division multiplexing deflection networks     

Fiber-optic switch arrays for optical subscriber transmissionsystems

Optical array devices play an important role in subscriber transmission systems because they can efficiently accommodate the large number of optical channels required. The use of two single-mode fiber-optic switch arrays in subscriber transmission systems is discussed. The first is a 16-element micromechanical switch array which is electrostatically driven and is used to separate abnormal subscriber units that transmit disturbing light signals. The second is a 16-element switch array which interconnects a pair of probe fibers and any one of 16-channel subscriber-line fibers to execute fault testing     

SRS效应对DWDM光通信系统质量影响的分析

文章基于在时域分步法存在衰减、色散走离和受激Raman散射(SRS)效应作用条件下的N信道SRS效应计算模型,使用MATLAB软件进行仿真,通过数据仿真分析了SRS耦合效应对密集波分复用(DWDM)光通信系统质量造成的影响以及输入功率、信道数与SRS耦合效应的关系,对进一步研究SRS效应对DWDM光通信系统性能的影响有着十分重要的意义.     

光纤通信中降低LDPC码译码复杂度的方法

伪循环(QC)低密度奇偶校验(LDPC)码可以给光纤通信系统带来更高的编码增益,但译码过程需要的对数似然比(LLR)的计算很复杂.为了降低LDPC译码器硬件设计的复杂度,文章提出一种光纤信道下简化的LLR的计算方法.仿真结果表明,该计算方法与原计算方法相比仅有0.05 dB的译码性能损失.     

WDM光纤通信系统中PMD自动补偿的研究

文章提出采用粒子群优化算法,提取信号的偏振度(DOP)作为反馈信号来补偿多信道光纤通信系统中的偏振模色散(PMD).作为例子,对两信道的波分复用(WDM)系统中的PMD进行了自动补偿,并进行了数值模拟,得到了两个信道补偿前后的DOP以及眼图变化情况.模拟结果表明这种算法对补偿多个信道的PMD是有效的.     

Low-rate super-orthogonal channel coding for fiber-optic CDMAcommunication systems

In this paper, we consider using practical low-rate error correcting codes in fiber-optic code division multiple-access (CDMA) communication systems. To this end, a different method of low-rate channel coding is proposed. As opposed to the conventional coding schemes, this method does not require any further bandwidth expansion for error correction in fiber-optic CDMA communication systems. The low-rate channel codes that are used for demonstrating the capabilities of the proposed method are super-orthogonal codes. These codes are near optimal and have a relatively low complexity. We evaluate the upper bounds on the bit-error probability of the proposed coded fiber-optic CDMA system assuming both on-off keying and binary pulse position modulation schemes. It is shown that the proposed method significantly outperforms the uncoded systems for various receiver structures such as a correlator with and without hard-limiter and chip-level detector. Furthermore, the performance of the proposed coded fiber-optic CDMA system is also evaluated in the presence of different values of dark current