ARQ Systems Research Articles

Methods for packet combining in HARQ systems over bursty channels

The performance of ARQ systems can be improved by combining current and prior transmissions at the receiver. Two techniques for combining outputs in a packet-based communication system are presented. In both techniques the fundamental unit of retransmission is a packet, and the fundamental unit of combining is a codeword. The techniques are analyzed for a bursty channel and a system that employs Reed--Solomon coding and bounded-distance errors-and-erasures decoding. Performance results show that the packet-combining schemes provide significant gains in throughput and reductions in error probability when compared with a system that does not employ combining.

Exact performance analysis of linear block codes in frequency-hop spread spectrum communication systems

We present a framework for determining the exact decoder error and failure probabilities for linear block codes in a frequency-hop communication channel with an arbitrary number of conditional symbol error and erasure probabilities. Applications are demonstrated for type-I hybrid ARQ systems by deriving equations for the packet error probability and throughput. Because these quantities are too small to be obtained by simulation, the framework provides exact results which are unobtainable by previous work.

Transmission bandwidth control schemes for direct sequence/spread spectrum systems in the presence of periodic partial‐band interference

AbstractWhen a processing gain of a direct sequence spread spectrum (DS/SS) system is too small to suppress nar rowband interferences and the interference power is much higher than that of the desired signal, an adaptive filter [5‐7] cannot compensate for the uncorrelated interference signals because the filter coefficients cannot be updated. In this case, if the interfered bands are known, a notch or bandpass filtering is useful. However, such frequency domain filtering in the receiver may suppress not only the interfering signals but also the desired one. Prefiltering in the transmitter [8] cannot achieve the same performance as DS/SS systems without a bandpass‐fllter in the absence of interference.This paper proposes and investigates a scheme of controlling the transmission bandwidth and its power spectral density using two spreading sequences in order to improve the performance in the presence or absence of high‐power periodical partial‐band interference. Two concrete systems also are proposed: a spreading sequence serial transmission system using different spreading 81 sequences; and a spreading sequence selectable ARQ system. In the former system, the transmitter does not consider the phase of the periodical partial‐band interfer ence in the receiver. Therefore, the system is realized easily because of the rather loose timing control require ment on the penalty of bit rate and because it does not require backward channels. On the other hand, in the latter system, 75 percent of the transmitted data are not degraded by the interference because of the more careful channel state selection at the transmitter than the former system.

A type I hybrid ARQ system with adaptive code rates

Type I hybrid ARQ systems use an error-control code of fixed rate for error-correction in each transmitted packet. In such systems, the code rate must be carefully chosen to match the channel bit error rate (BER) in order to maximize system throughput. Whenever there is a mismatch between code rate and channel BER, the throughput suffers. In this paper, we proposed a type I hybrid ARQ system which automatically adjusts its code rates to match channel BERs. The proposed system is very simple to implement and yet performs significantly better than type I hybrid ARQ systems of fixed code rate. >

On renewal inner channels and block code error control super channels

We investigate the statistical distributions of three events, namely the block error detection, correction and misdetection events of a block code on a discrete renewal inner channel, when this inner channel can be modeled by means of a partitioned Markov chain. In order to represent the above mentioned statistical distributions, we introduce the concept of a channel model. Analytical procedures to determine some of the parameters of these super channel models are presented. The application of these models in the design of error control systems, such as ARQ systems, is investigated. >

Selective repeat type‐II hybrid fec/arq systems using concatenated codes

AbstractHybrid forward error correction/automatic‐repeat request (FEC/ARQ) systems achieve greater throughput than ARQ systems by combining forward error correction with automatic‐repeat requests. A type‐II hybrid FEC/ARQ system is capable of adapting error‐correction capacity according to varying channel conditions. Although a high throughput can be achieved in such systems by using conventional convolutional or rate compatible punctured convolutional (RCPC) codes, when the channel signal‐to‐noise ratio (SNR) Es/N0 is low, resulting throughput efficiency and mean block delay performances are unsatisfactory.In this paper, selective repeat type‐II hybrid FEC/ARQ systems using concatenated codes are proposed to improve the throughput efficiency and mean block delay performances on channels with low SNRs. Systems using concatenated Reed‐Solomon outer codes and either rate one‐half convolutional or rate compatible convolutional (RCPC) inner codes are evaluated. The throughput efficiency and mean packet waiting and block delay times of these two types of concatenated coding systems are evaluated. A theoretical analysis and computer simulation of throughput efficiency of the proposed systems show increased performance for a wide range of Es/N0 over previous systems that use either stand‐alone rate one‐half convolutional or RCPC codes. It is also shown that the transmitter mean packet queueing delay and mean block delay times are reduced. In particular, by computer simulation, it is shown that when the traffic density is high, mean block delay times are greatly reduced by the use of RCPC inner codes.

A practical reliability metric for block codes used on binary-input channels

A reliability metric (RM) for a block code is defined to be a function that operates on both the decoder input (a block of channel output) and the decoder output (the codeword estimate) and produces a real number as a measure of the reliability of the decoder decision. The best RM has the disadvantage of depending on the codeword probabilities. Thus, the ideal RM is defined as the value that would be computed for equally likely codewords. The implementation of the ideal RM is too costly for most applications. The author proposes an easily implemented RM for binary-input memoryless channels (for state-observable channels with a freely evolving state such as some fading channels) when the codewords consist of n 2/sup m/-ary symbols, each of which is transmitted serially by m uses of the channel. Simulation results for some BCH codes and some Reed-Solomon codes used in a simple ARQ system show that the proposed RM performs nearly as well as the ideal RM and much better than a previously proposed practical RM. >

New mapping rules for combination off m-ary modulation and error-detecting codes in ARQ systems

In some ARQ schemes, the combination of modulation and channel coding improves the performance of a communication system. The paper describes a new method for the integration of an error-detecting code with M-ary modulation. Euclidean distances are obtained among modulated signals using successive transmissions of the same codeword larger than those obtained by other similar schemes. The results of the theoretical analysis show that the described method allows improvement of both the error probability and the throughput of an ARQ protocol with respect to other schemes.

Delay performances of a continuous arq system with copy‐transmissions

AbstractThe characteristics of packet transmission delay in a continuous ARQ (Go‐Back‐N type) system are analyzed here. The copies of each packet stored in the buffer are sent to reduce the delay. The optimum copy size minimizing the delay depends not only on packet error probability and round‐trip propagation delay but also on offered traffic intensity. It is possible to reduce the delay by transmitting many copies and using the residual transmission capacity when offered traffic is light. On the contrary, when offered traffic is heavy, transmission of few copies is preferable in consideration of the number of lost copies for a successful transmission. Finally, considering that packet error probability may be reduced in some practical situations, the relation among packet error probability, propagation delay and traffic density where the optimum copy size changes from 1 to 2 is derived.

Hybrid ARQ system employing soft decoding algorithm

AbstractIt is known that by applying a soft decoding algorithm to the error‐detecting code in the ARQ system, a hybrid ARQ system can be realized in which error detection and correction are possible. This paper discusses the application of the soft decoding algorithm to the ARQ system, where the multivalued digital modulation is employed. A scheme for a practical system is proposed, including the detection of channel measurement information in the multivalued digital modulation system and estimation of error patterns. By the proposed method, the hybrid ARQ system can easily be realized by adding some peripheral circuits to the conventional ARQ receiver. Theoretical study is made on the number of error patterns that can be estimated for a long block length as well as the probability of miss and the probability of retransmission request. The proposed scheme was applied experimentally to the ARQ system employing SG VIII V.29 modem based on CCITT recommendation, and the usefulness of the proposed scheme was verified by experiment.

Simulated throughput efficiency of TD-SAW ARQ system in digital mobile radio

Computer simulation results for the throughput efficiency of stop-and-wait automatic-repeat-request with time diversity reception (TD-SAW ARQ) are presented for digital mobile radio channels. It is shown that the efficiency η in the TD-SAW ARQ system does not decrease as much as in the SAW ARQ system, when the maximum Doppler frequency fD becomes large.

Reliability and throughput analysis of a concatenated coding scheme

The performance of a concatenated coding scheme for error control in ARQ systems is analyzed for both randomerror and burst-error channels. In particular, the probability of undetected error and the system throughput are calculated. In this scheme, the inner code is used for both error correction and error detection, and the outer code is used for error detection only. Interleaving/deinterleaving of the outer code is assumed. A retransmission is requested if either the inner code or the outer code detects the Presence of errors. Various coding examples are considered. The results show that concatenated coding can provide extremely high system reliability (i.e., low probability of undetected error) and high system throughput.

Efficient ARQ with time diversity reception technique—a time diversity ARQ

A technique for increasing the efficiency of an ARQ system using time diversity reception is proposed fo mobile radio use. In this technique, the receiver demodulator output associated with each symbol is weighted and combined every time a new retransmission is made. Theoretical results show that this technique offers a considerable reduction in the average number of transmissions, in comparison with conventional ARQ systems.

An ARQ Scheme with Memory and Soft Error Detectors

This paper describes a technique for increasing the efficiency of an ARQ communication system utilizing error detection codes. In this technique the blocks containing errors are not discarded at the receiver, as in the conventional ARQ systems. Such blocks are retained in order to make use of the information contained in them. A reliability value is associated to each demodulated symbol; this reliability value is updated every time a new retransmission is made, even if the vector received is in error. The process of updating the symbols often eliminates automatically the errors introduced by the channel. The results obtained by means of a simulation show that this method offers a considerable reduction in the average number of retransmissions, in comparison with conventional ARQ systems.

Evaluation of hybrid error control systems

In the paper, the performances of hybrid as well as of ARQ and FEC systems are evaluated by simulation generating error statistics corresponding to a case of data transmission over local switched telephone lines. The channel model was based on finite time renewal processes. The criteria for a hybrid system with convolution codes were specially developed. The results show that, for simulated channel, characterised by dense bursts and many random errors in between, hybrid systems based on convolutional codes can improve the reliability considerably.

A Modified Selective-Repeat Type-II Hybrid ARQ System and Its Performance Analysis

The hybrid ARQ scheme with parity retransmission for error control, recently proposed by Lin and Yu [1], [2], is quite robust. This scheme provides both high system throughput and high system reliability. In this paper, a modified Lin-Yu hybrid ARQ scheme is presented. The modified scheme provides a slightly better throughput performance than the original Lin-Yu scheme; however, it is more flexible in utilizing the error-correction power of a code. The modified scheme can be incorporated with a rate 1/2 convolutional code using Viterbi decoding. Furthermore, the pure selectiverepeat ARQ is a degenerated case of the modified scheme in selective mode. Lin and Yu analyzed their scheme only for a receiver buffer of size <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> is the number of data blocks that can be transmitted in a round-trip delay interval. No analysis for other buffer sizes was given. In this paper, the throughput performance of the modified Lin-Yu scheme is analyzed for any size of receiver buffer. Consequently, the throughput efficiency of the pure selective-repeat ARQ for any receiver buffer size can be obtained. We also show that the modified scheme achieves the same order of reliability as a pure ARQ scheme.

An Analysis of a TDMA Channel Using Stop-and-Wait, Block, and Select-and-Repeat ARQ Error Control

In this paper, the multiaccess broadcast communication channel operating under a TDMA control discipline and employing an ARQ scheme is examined. The generating function of the message delay distribution is derived. The effects of nonzero bit error rates on multipacket messages under several stop-and-wait and block ARQ systems are incorporated. Numerical results for the steady-state mean and variance of the message delay are presented for a stationary transmission error process model in which errors occur as independent events. Results for this channel under a select-and-repeat ARQ system are developed for single packet messages. Stability requirements are stated, useful bounds on the average message delay at steady state are derived, and numerical examples are presented.

An Improved Selective-Repeat ARQ Strategy

A new procedure for handling retransmissions in a selective-repeat ARQ system is proposed. This procedure can operate with a receive buffer of minimal size; in addition it places little computational load on the transmit and receive processors. The procedure is simple enough that its throughput can be calculated exactly. Analysis of this strategy shows that: 1)it yields higher throughput than earlier ARQ techniques; 2) for modest receive buffer size, its throughput differs little from channel capacity; 3) as buffer size increases, throughput approaches channel capacity. The final section of the paper considers the performance of ARQ systems on channels in which errors occur in bursts. It indicates that on reasonably good channels, error burstiness has little effect on throughput.

The application of m.f.s.k. techniques to h.f. telegraphy

The paper briefly describes the history and the principles of m.f.s.k. signalling as applied to h.f. telegraphy. It quotes mathematical relationships for the performance of such systems, and derives equations for the occupied bandwidth. These relationships, together with the results of measurements carried out on different experimental m.f.s.k. systems, and practical limitations suggested by experience, are used to suggest an approach to the selection of optimum parameters.Systems suitable for different telegraphy and telemetry applications are derived and discussed. Practical problems of synchronization and source stability are analysed, and methods of error coding for ARQ and FEC systems are described.It is concluded that for data links in the range of 20 to 200 bits per second, the use of m.f.s.k. techniques can give an improvement in performance equivalent to about 10 dB in signal-to-noise ratio, or a reduction in error rate of the order of 10/1 in the raw data (or more than 100/1 if an error coding system is used).

Improved retransmission logics for bidirectional ARQ systems

Two retransmission logics are described which, when used in conjunction with high-reliability bidirectional datatransmission systems, can improve the throughput by up to 17% compared with systems using Metzner&apos;s ‘newword’ strategy. Only a small amount of additional logic is required at transmitter and receiver.