CDM-Qx Satellite Modem Revision 5

Forward Error Correction Options MN/CDMQx.IOM

6–2

6.2 Viterbi

The combination of convolutional coding and Viterbi decoding has become an almost

universal standard for satellite communications. The Modem complies with the Intelsat

standards for Viterbi decoding with a constraint length of seven. This is a de facto

standard, even in a closed network environment, which means almost guaranteed inter-

operability with other manufacturer’s equipment. It provides very useful levels of coding

gain, and its short decoding delay and error-burst characteristics make it particularly

suitable for low data rate coded voice applications. It has a short constraint length, fixed

at 7, for all code rates. (The constraint length is defined as the number of output symbols

from the encoder that are affected by a single input bit.) By choosing various coding rates

(Rate 1/2, 3/4, or 7/8) the user can trade off coding gain for bandwidth compression. Rate

1/2 coding gives the best improvement in error rate, but doubles the transmitted data rate,

and hence doubles the occupied bandwidth of the signal. Rate 7/8 coding, at the other

extreme, provides the most modest improvement in performance, but only expands the

transmitted bandwidth by 14 %. A major advantage of the Viterbi decoding method is

that the performance is independent of data rate, and does not display a pronounced

threshold effect (i.e., does not fail rapidly below a certain value of Eb/No). Note that in

BPSK mode, the Modem only permits a coding rate of 1/2. Because the method of

convolutional coding used with Viterbi, the encoder does not preserve the original data

intact, and is called non-systematic.

Table 6-1. Viterbi Decoding Summary

FOR AGAINST

Good BER performance - very useful coding gain. Higher coding gain possible with

other methods

Almost universally used, with de facto standards for

constraint length and coding polynomials

Shortest decoding delay (~200 bits) of any FEC scheme -

good for coded voice, VOIP, etc

Short constraint length produce small error bursts - good

for coded voice.

No pronounced threshold effect - fails gracefully.

Coding gain independent of data rate.

1 2 ... 129 130 131 132 133 134 135 136 137 138 139 ... 257 258

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Table des matières

Dear customer

Contents

Important Safety Instructions

Environmental Information

2 Old appliance

Appliance Transport

Before Use

Installation

Appliance Alignment

Electrical Connection

818 39 98-00/7

Changing over the Door Hinges

Appliance Description

Control Panel

Prior to Initial Start-Up

Initial Start-Up

Temperature Adjustment

COOLMATIC

Temperature warning

Switching the Appliance off

Holiday Mode

Interior Accessories

Opening the Door

Correct Storage

Defrosting

Cleaning and Maintenance

Energy Saving Tips

What to do if

28 818 39 98-00/7

Replace Light Bulb

Noises during Operation

Technical Terminology

Customer Service

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34 818 39 98-00/7

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