B2P1

Question 1

What is a parity check

Answer 1

The principle of the parity check is that for a given block of bits, you add one further bit – the parity bit – which is chosen to be a 1 or 0

Any single error in the block will now make the parity of the block of bits odd, because an error changes to either a 1 to a 0, or 0 to a 1.

Question 2

How does a cyclic redundancy check CRC work

Answer 2

The idea behind CRCs is that remainder (the digits of which are the check digits) is sent to the decoder together with the message. The decoder the calculates the reminder for itself and compares its own locally

Question 3

What is a check digit

Answer 3

The parity bits (binary digits) or other digits that are added to data for error-control coding.

Question 4

What is a polynomial

Answer 4

a mathematical expression consisting of a sum of power in one or more variables.

Question 5

What are CRCs error detecting capabilities

Answer 5

a CRC with a suitable chosen generator in which the highest power is n has the following features:
• Capable of detecting all odd numbers of errors
• Capable of detecting bursts of length n or shorter
• The probability of any burst longer than n+1 escaping detection is 1/2n
• The probability of a burst of length exactly equal to n + 1 escaping detection is 1/2n-1

Question 6

What is a rectangular code

Answer 6

can not only detect presence of a single error, but also identify which bit is in error and correct it

Question 7

What are the 2 categories of error correcting codes

Answer 7

• Block coding

* Convolutional coding (tree or trellis coding)

Question 8

How do you work out the efficient of an (n, k) block code

Answer 8

Efficiency of an (n, k) block code is the ratio k/n which is called the code rate

Question 9

What is a hamming code

Answer 9

An error-correcting code
parity-check bits are used

Parity check 1 : 1 3 5 7
Parity check 2: 2 3 6 7
Parity check 3: 4 5 6 7

Question 10

What is meant by hamming distance

Answer 10

the measure of closeness between code words

Comparing 0001111 and 0000111 gives a distance of 1

Question 11

Because Hamming codes make use of the all the information available in the syndrome, they are called …codes

Answer 11

perfect

Question 12

What is a golay code

Answer 12

A perfect (23,12) block code with a minimum Hamming distance of 7.

Question 13

What is turbo code

Answer 13

Error-control codes that use soft decisions, concatenated coding and iterative decoding to achieve a performance that approaches the theoretical limit described by Shannon’s formula for the capacity of a noisy communications channel.

Question 14

What is meant by hard decision decoding

Answer 14

Decoding in which the decoder only has access to hard decisions (i.e. whether the data is a 1 or a 0) from the demodulator

Question 15

What is meant by soft decision decoding

Answer 15

Decoding in which the demodulator passes soft information to the decoder. Soft-decision decoding therefore takes account of the demodulator’s confidence in the decisions on whether each received symbol is a 1 or a 0

Question 16

What is interleaving

Answer 16

allow error-correcting codes
protect against bursts
another technique employed by turbo coding systems

Question 17

What is latency

Answer 17

delay

Question 18

What is concatenation

Answer 18

In coding design, the technique of feeding the output of one coding process into the input of another

Question 19

What is an LDPC code

Answer 19

Low density parity check LDPC codes are advanced block codes that based on parity checks
Advanced error-correction codes that, like turbo codes, are able to deliver performance close to the Shannon limit. Also known as Gallagher codes.

Question 20

What is a Reed-solomon code

Answer 20

A class of error-correcting codes that are widely used because they can protect against large numbers of errors and, by shortening, allow for flexible trade-offs between code rate and error-correcting abilities.

Question 21

What is meant by erasure

Answer 21

an error for which the location of the error is already known and the code only needs to determine the corrected value of the errored symbol.

Question 22

How is a trellis code encoded

Answer 22

Coding done left to right
Upper branch if input bit is 0
Lower branch if 1

Question 23

What is verterbi decoding

Answer 23

the decider takes account of both what has gone on before and what comes after in deciding how to decode the current data

Question 24

What is meant by puncturing

Answer 24

the process of deleting code output bits to a predetermined pattern in order to increase the code rate.

Question 25

What is Trellis-coded modulation (TCM)

Answer 25

the use of trellises with modulation

Question 26

What is a hybrid automatic repeat request (HARQ)

Answer 26

A method of error control in which the data is protected by a forward error-correcting code (FEC), but in which data retransmission is requested if the errors exceed the correction capability of the FEC scheme