Forward Error Control Research Articles

Performance study of hybrid spread-spectrum random-access communications

A numerical study of the performance of hybrid direct-sequence slow-frequency-hopped spread-spectrum random-access schemes using Reed-Solomon forward-error control coding is performed. The performance measures of interest are the maximum numbers of simultaneous transmitters that can be tolerated in the vicinity of a receiver at a desirable level of the bit (or symbol) error probability, the normalized throughput, and the packet error probability. For the case where all simultaneous signals have the same received power levels, a critical comparison of the performance of direct-sequence, frequency-hopped, and hybrid spread-spectrum systems with the same overall bandwidth expansion is conducted with respect to the above performance measures, and ranges of parameters, for which each system outperforms the others, are identified. It is also established that hybrid spread-spectrum random-access schemes have considerably higher throughput than the uncoded nonspread-spectrum ones, for the desired range of values of the packet error probability. >

Performance of an asynchronous hybrid spread spectrum system in the presence of interference, Rician fading and AWGN

An evaluation is made on the performance of an asynchonous hybrid direct sequence/frequency hopped spread spectrum communication communication system in the presence of a combined partial band noise (PBN), multiple access interference (MAI), Rician fading and additive white Gaussian noise (AWGN). Two modulation schemes are considered; the first is phase shift keyed (PSK) modulation with coherent demodulation, the second is M-ary frequency shift keying (MFSK) with noncoherent demodulation. Both schemes employ forward error control. The performance of these two schemes will be investigated against Rician fading and multiple access interference. In all cases the following are shown: the minimum carrier-to-interference ratio (C/I) required to guarantee a desirable bit error rate as a function of the band jammed (ρ) assuming a fixed number of users, the number of users that can be supported in the network as a function of ρ under various jamming and interference conditions, and the vulnerability of the system to jamming in the presence of Rician fading.

Very Small Aperture Terminal (VSAT) Systems for Digital Satellite Communication: An Overview

This paper presents an overview of digital satellite communication systems based on low-cost, very small aperture terminals (VSATs) located directly at subscribers' premises. Over the last few years, such VSAT systems have become feasible because of a combination of recent advances in satellite and earth station technology. These include the deployment of higher power satellites, the use of Ku-band (14/12 GHz) frequencies, the availability of lowcast earth station antennas and microwave electronics, along with the development of cost-effective implementations for modulation, spectrum spreading, forward error control (FEC) and protocol processing. Digital communication services which potentially achieve reasonable combinations of VSAT cost and satellite usage are identified for consideration. Specific applications considered are: one-way data distribution, point-to-point data, two-way interactive-data packet networking, digital voice (including telephony and packet voice messaging), and satellite ISDN. App...

An error control system with multiple-stage forward error corrections

A robust error control coding system is presented. This system is a cascaded FEC (forward error control) scheme supported by parity retransmissions for further error correction in the erroneous data words. The error performance and throughput efficiency of the system are analyzed. Two specific examples of the error control system are studied. The first example does not use an inner code, and the outer code, which is not interleaved, is a shortened code of the NASA standard RS code over GF(2/sup 8/). The second example, as proposed for NASA uses the same shortened RS code as the base outer code C/sub 2/, except that it is interleaved to a depth of 2. It is shown that both examples provide high reliability and throughput efficiency even for high channel bit-error rates in the range of 10/sup -2/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

ATCS: Advanced Train Control System radio data link design considerations

The American Association of Railroads (AAR) and the Association of Canadian Railroads (ACR) jointly initiated an implementation study of the Advanced Train Control System (ATCS) to improve the efficiency and economics of railroad operations. The data link between locomotives and the dispatcher is identified as a vital link within ATCS. The design of this link includes considerations on modulation, data rates, protocol, access scheme, forward error control, and their impact on spectral needs. The steps taken toward specifying ATCS radio-link design parameters to meet operational requirements of railroads are described. Extensive computer simulations to evaluate suitability of the chosen system parameters such as data rate, access scheme, and coding were made under some realistic mobile channel conditions. Results of this design exercise are summarized. >

Further studies on efficient AFEC schemes for Ka-band satellite systems

Adaptive fade margin is required to counter the severe but varying rain attenuation in Ka satellite communications. In searching for a suitable rain countermeasure, the effectiveness of the adaptive forward error control (AFEC) scheme is studied. Two AFEC schemes using convolutional codes and concatenated codes are proposed and their performance analyzed. The schemes can provide a progressively adaptive fade margin of 10.1 and 10.4 dB, respectively, in excess of the systems fixed fade margin. To improve the efficient use of shared resources of the system, an AFEC scheme using double coding is introduced, and its performance is analyzed. In this scheme a single codec is used repeatedly, and as a result the hardware cost is reduced and the utilization increased. The effective usable capacity of a conceptual AFEC resource sharing scheme using Golay double coding is also analyzed. >

Efficient adaptive forward error control schemes for Ka‐band satellite systems

AbstractAdaptive fade margin is required to counter the severe but varying rain attenuation in Ka‐band satellite communications. In a search for a suitable rain countermeasure, the effectiveness of adaptive forward error control (AFEC) schemes is studied. Two AFEC schemes using convolution codes and concatenated codes are proposed and their performances analysed. The schemes can provide progressively adaptive fade margins of 10‐1 and 10‐4 dB, respectively, in excess of the system's fixed fade margin.To improve the efficient use of shared resources of the system, an AFEC scheme using double coding is introduced and its performance analysed. In this scheme a single codec is used repeatedly, resulting in reduced hardware cost and increased use. By using convolutional code and Golay code the above scheme can provide adaptive fade margins of 10°8 dB and 8°1 dB, respectively. Although concatenated codes have slightly better performance than double codes, the hardware implementation and decoding complexity of the latter are significantly less.Implementation complexity of AFEC resource sharing in the light of code complexity, time frame expansion, link condition monitoring and signalling is discussed in detail.

Adaptive Forward Error Control for Digital Satellite Systems

Two hybrid schemes of time-frequency resource sharing to increase the rain margin of Ku-and Ka-band satellite systems are proposed. Scheme 1 requires sharing a small pool of bandwidth for adaptive forward error control coding, sharing a small pool of time frame for rate reduction, and sharing a portion of low frequency time-division multiple access (TDMA) back-up frame for downlink transmission to the rain affected stations. Scheme 2 utilizes variable rate modulation and forward error correction, shares a small pool of time frame for rate reduction, and shares a portion of low frequency TDMA back-up frame. Effective usable capacities of the system using these schemes are calculated. Distribution of resources in order to maximize the effective usable capacity is also analyzed. The results obtained are compared with other adaptive schemes. Preliminary analysis shows that the utilized capacity of scheme 1 exceeds 99 percent of the effective usable capacity possible if it never rains for an outage of 0.05 percent and fade margin of 2.5 dB. For scheme 2 similar performance is achievable at a fade margin of 1.5 dB. For higher outage objectives the loss of effective utilized capacity is higher for scheme 2.

Alternatives in digital communications

The following major interrelated aspects in digital communications are described: source encoding, channel encoding and transmission, and receiver methods. Methods for digitizing speech and video are summarized. Digitized voice can be transmitted at 10 kb/s, while phone video requires 1.544 Mb/s. The major aspects of both forward-error control (FEC) and automatic request for repeats (ARQ) are considered. Power-limited forward channels are distinguished from bandwidth-limited ones. Signal distance concepts and maximum-likelihood detection principles provide the framework for describing both block-coded and convolutional-coded signaling. Convolutional coding with Viterbi decoding appears to achieve the most cost-effective forward-error control. The improved FEC techniques benefit the transmission of real-time digitized voice and video and any communication over severely power-llmited links. ARQ methods continue to be effective and practical for most cases where the delay associated with retransmission can be tolerated.

Errors and error control

In this paper the nature of errors on telephone data communications channels and the basic techniques for dealing with these errors are discussed. Results of measurements recently taken on dialed connections are reviewed, and it is observed that conventional random-error- or burst-error-correcting codes cannot assure reliable communication on these channels. More generally, it is shown qualitatively that automatic-repeat-request (ARQ) systems are inherently better suited to the task than forward-error-control (FEC) systems. The throughput, or effective data rate, of ARQ systems is discussed, and two basic types of ARQ systems, stop-and-wait and continuous, are compared. It is concluded that with the more common stop-and-wait system, the throughput is unsatisfactory in applications involving high transmission rates and/or long propagation delays. A brief summary of error-correcting codes suitable for use on telephone channels (when a return channel is not available for ARQ) is included. Finally, hybrid schemes where FEC systems are embedded within ARQ systems are briefly discussed.

Test results of a time-dispersed forward error control system

The purpose of this correspondence is to describe some results of a recent test of a forward error control system over operational HF radio circuits. This system employed simple algebraic block codes in conjunction with bit interlacing. In the system tested, symbol separation amounted to approximately one second, thereby allowing single bursts of two seconds to be corrected. Of course, many bursts of shorter duration are also correctable. Tests were conducted using two tone channels on a DCS HF link operating between NAVCOM centers located at Rtockton, Calif. and Cheltenham, Md. The basic test configuration is shown.