2-14-2002

Forward Error Correction DemystifiedPresented by

Sunrise Telecom Broadband …a step ahead

2-14-2002

Introduction

Forward Error Correction (FEC) is a system that adds additional information to the data stream to correct any errors that are caused by the transmission system.

Data transmission systems, such as cable modems are not real time and can resend the data if it is not received correctly.

In real time applications like video it isn’t possible to resend the data since it’s a constant stream. If the received data can’t be corrected, artifacts will show up in the picture.

2-14-2002

SCTE DVS-031 FEC Layers

The SCTE has specified a system for transmitting digital television over a cable system and this system is known as DVS-031.

General Instrument DCT set tops, Scientific Atlanta Explorer Set Tops and DOCSIS Cable Modems all use DVS-031.

2-14-2002

Trellis Encoder

Trellis DecoderRan-domizer

Inter-leaver

De-randomizer

De-interleaver

Reed Solomon Encoder

Reed Solomon Decoder

Trellis Layer

Randomization Layer

Interleaving Layer

Reed Solomon Layer

FEC Encoding FEC Decoding

Channel

SCTE DVS-031 FEC Layers

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Parity

By adding an additional bit to a group of bits, errors can be detected within the group. This is known as a parity bit.

Even parity means that when the parity bit is added the group of bits including the parity always has an even number of ones. Odd parity means the group would have an odd number of ones.

If after transmission the number of ones is no longer even (for even parity), then there must be an error.

101110001 1

010111011 0

Parity Bit

Always Even Number of Ones

(Even Parity)

101100001 1

010111011 0

Error

Odd Number of Ones Indicates

Error

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How FEC Works

FEC works by the addition of additional data bits to the data stream to determine if errors exist and to try and correct them.

1011100010110100

Data Stream1011 11000 11011 10100 1

1100 0

1=odd

0=Even

Stream With FEC Added

1011100010110100 1111 1100 0

2-14-2002

How FEC Works

1011 11000 11001 10100 1

1100 0

1011 11000 11011 10100 1

1100 0

Error 1=odd

0=Even

Before Transmission

After Transmission the Bit in Error is

Detected and Corrected

Once you know a bit is wrong, correcting it is easy, if you know its wrong and its a zero, then it has to be a one.

2-14-2002

Reed-Solomon Error Correction

Trellis Encoder

Trellis Decoder

Ran-domizerInter-leaver De-randomizer

De-interleaver

Reed Solomon Encoder

Reed Solomon Decoder

Trellis Layer

Randomization Layer

Interleaving Layer

Reed Solomon Layer

FEC Encoding FEC Decoding

Channel

2-14-2002

Reed-Solomon Error Correction

Digital Video transmission uses Reed-Soloman Error Correction using a 128,122 code.

Out of 128 symbols, 122 are used for the video and 6 are used for error correction.

Out of the 122 symbols, up to 3 can be corrected using this code, any more errors than 3 and there will be picture artifacts or system failure.

Data is arranged in tables of 7 bit symbols, not individual bits as in the previous example.

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Calculated Bit Error Rate

Using the FEC overhead you can calculate the symbol error rate, which is very close to the bit error rate.

For all practical purposes symbol error rate, while not exactly the same as BER, is very close for all practical purposes.

Using the FEC to determine the BER allows BER to be measured without removing the service which is usually required for most BER testing.

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Pre and Post FEC BER

To get an accurate idea of the BER performance you need to know both the pre and post FEC bit error rate.

The MPEG decoder needs a BER of better than 1 E-6 in order to operate.

A post FEC BER close to 1 E-6 does not have significant margin and is very close to failing.

You should look at both the Pre and Post FEC BER to determine if the FEC is working to correct errors and if so how hard.

FEC Decoder

Pre FEC BER

Post FEC BERMust be better than 1E-6 for

video to operate

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Interleaving

Trellis Encoder

Trellis Decoder

Ran-domizerInter-leaver De-randomizer

De-interleaver

Reed Solomon Encoder

Reed Solomon Decoder

Trellis Layer

Randomization Layer

Interleaving Layer

Reed Solomon Layer

FEC Encoding FEC Decoding

Channel

2-14-2002

Interleaving

Burst errors can cause more problems with the picture than errors that are spread out.

In an attempt to minimize the affect of burst errors, a technique known as interleaving is used.

Interleaving effectively spreads out burst errors so they have less effect on the picture.

2-14-2002

Interleaving

The DVS-031 specification calls for data to be ordered into packets of 128 symbols.

Data is interleaved by sending 128 symbols from 128 different packets. The next 128 symbols sent are the second symbol from each of the 128 packets and so on.

128 Packets Packets of 128 Symbols Are Loaded

One Symbol From Each of the 128 Packets is Sent

Until all Data is sent

Data is Put Back Into Its Original Order

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Randomization

Trellis Encoder

Trellis Decoder

Ran-domizerInter-leaver De-randomizer

De-interleaver

Reed Solomon Encoder

Reed Solomon Decoder

Trellis Layer

Randomization Layer

Interleaving Layer

Reed Solomon Layer

FEC Encoding FEC Decoding

Channel

2-14-2002

Randomization

Digital Demodulators operate most effectively when there is big difference between symbols and no long strings of the same symbol.

In order to minimize the probability of a long stream of the same symbol a Randomizer on the Encoder and a De-randomizer on the decoder is used.

These blocks help ensure that the data stream has plenty of transitions prior to being applied to the modulator.

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Trellis Coding

Trellis Encoder

Trellis Decoder

Ran-domizerInter-leaverDe-

randomizerDe-

interleaverReed

Solomon Encoder

Reed Solomon Decoder

Trellis Layer

Randomization Layer

Interleaving Layer

Reed Solomon Layer

FEC Encoding FEC Decoding

Channel

2-14-2002

Trellis Coding

An FEC technique known as Trellis Coding is also specified in DVS-031. This FEC is added to the Reed-Solomon FEC to give additional signal-noise improvement.

Unlike Reed-Solomon FEC that works in blocks, Trellis is known as a convolutional code since it operates on a continuous stream.

The Trellis coding used in DVS-031 produces 15 bits output for 14 input.

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Conclusions

FEC works by the addition of additional data bits to the data stream to determine if errors exist and to try and correct them.

DVS-031 FEC uses four processing layers to detect and correct errors.

FEC error statistics can be used to very accurately estimate the BER of a live signal.

CM1000 Cable Modem System

Analyzer

AT2000 Spectrum Analyzer