B2P1 Flashcards
What is a parity check
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.
How does a cyclic redundancy check CRC work
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
What is a check digit
The parity bits (binary digits) or other digits that are added to data for error-control coding.
What is a polynomial
a mathematical expression consisting of a sum of power in one or more variables.
What are CRCs error detecting capabilities
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
What is a rectangular code
can not only detect presence of a single error, but also identify which bit is in error and correct it
What are the 2 categories of error correcting codes
- Block coding
* Convolutional coding (tree or trellis coding)
How do you work out the efficient of an (n, k) block code
Efficiency of an (n, k) block code is the ratio k/n which is called the code rate
What is a hamming code
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
What is meant by hamming distance
the measure of closeness between code words
Comparing 0001111 and 0000111 gives a distance of 1
Because Hamming codes make use of the all the information available in the syndrome, they are called …codes
perfect
What is a golay code
A perfect (23,12) block code with a minimum Hamming distance of 7.
What is turbo code
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.
What is meant by hard decision decoding
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
What is meant by soft decision decoding
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
What is interleaving
allow error-correcting codes
protect against bursts
another technique employed by turbo coding systems
What is latency
delay
What is concatenation
In coding design, the technique of feeding the output of one coding process into the input of another
What is an LDPC code
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.
What is a Reed-solomon code
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.
What is meant by erasure
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.
How is a trellis code encoded
Coding done left to right
Upper branch if input bit is 0
Lower branch if 1
What is verterbi decoding
the decider takes account of both what has gone on before and what comes after in deciding how to decode the current data
What is meant by puncturing
the process of deleting code output bits to a predetermined pattern in order to increase the code rate.
What is Trellis-coded modulation (TCM)
the use of trellises with modulation
What is a hybrid automatic repeat request (HARQ)
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
