This document summarizes key aspects of satellite communication including error detection and correction. It discusses the elements of a digital communication system including source and channel encoders/decoders. It defines different types of errors that can occur like single bit, multiple bits, and burst errors. It then explains various error detection techniques like parity check and cyclic redundancy check (CRC). It also discusses forward error correction (FEC) where redundant bits are added to allow errors to be corrected at the receiver without retransmission. Specific error correction coding schemes like linear block codes are also summarized.

1. Satellite Communication
Satellite Error Detection And
Correction

2. Objectives
Types of Errors
Overview Of Digital Communication Elements
Source Encoder and Channel Encoder and Their Decoders
Definition of Error
Error Detection
Error Detecting Codes
Forward Error Correction
Error Correction
Error Correction Methods

3. Elements of Digital Communication
 The elements which form a digital communication system is represented by the following block diagram for the ease of
understanding

4. Source Encoder and Channel Decoder
.
Source Encoder
The source encoder compresses the data into minimum number of bits. This process helps in effective
utilization of the bandwidth. It removes the unnecessary excess bits.
Channel Encoder
The channel encoder, does the coding for error correction. During the transmission of the signal, due to the noise
in the channel, the signal may get altered and hence to avoid this, the channel encoder adds some redundant bits
to the transmitted data. These are the error correcting bits.
Channel Decoder
The channel decoder, after detecting the sequence, does some error corrections. The distortions which might occur
during the transmission, are corrected by adding some redundant bits. This addition of bits helps in the complete
recovery of the original signal.
Source Decoder
The resultant signal is once again digitized by sampling and quantizing so that the pure digital output is obtained
without the loss of information. The source decoder recreates the source output.

5. What is Error ?
Error is a condition when the output information does not match with the input information. During transmission,
digital signals suffer from noise that can introduce errors in the binary bits travelling from one system to other or
from during propagation of the signals. That means a 0 bit may change to 1 or a 1 bit may change to 0.
Satellite
Downlink
Earth Station
Signal Noise
Uplink
User User
Earth Station
Error is unpredictable change of data or bits
from 0 to 1 or from 1 to 0

6. Types of Errors
Error
Single Bit
Error
Multiple
Bits Error
Burst
Error
 There may be three types of errors
Single Bit Error occurs when
there is only one bit which is
corrupt from either 1 to 0 or
0 to 1
Single Bit Error
Multiple bits error occur when there is two or
more bits which is corrupt but not
consecutive bits. For example
Multiple Bits Error
Burst error occur when there
is two or more bits which is
corrupt but they should be
consecutive bits. For example
Burst Error

7. Error Detection
•
Error detection is the process of detecting errors between sender and receiver.
Error Detection involves Looking only to see if there is an error occurred.
Error detection is the detection of errors caused by noise or other impairments during
transmission from the transmitter to the receiver. Errors in the received data are
detected by means
 Parity Check and
 Two dimensional parity check
 Checksum
 Cyclic Redundancy Check (CRC).
In these cases, few extra bits are sent along with actual data to confirm that bits received
at other end are same as they were sent. If the counter-check at receiver end fails, the
bits are considered corrupted.
Error detection means to decide whether the received data is
correct or not without having a copy of the original message.

8. Parity Check ( Error Detection )
Parity Check
Cyclic
Redundacy
Check
(CRC)
It is the simplest technique for detecting and correcting errors. The MSB of an
8-bits word is used as the parity bit and the remaining 7 bits are used as data
or message bits. The parity of 8-bits transmitted word can be either even
parity or odd parity.
Even parity -- Even parity means the number of 1's in the given word including
the parity bit should be even number (2, 4, 6,....).
Odd parity -- Odd parity means the number of 1's in the given word including
the parity bit should be odd number(1, 3, 5,....).

9. • Parity checking at the receiver can detect the presence of an error if the parity of the receiver
signal is different from the expected parity. That means, if it is known that the parity of the
transmitted signal is always going to be "even" and if the received signal has an odd parity, then
the receiver can conclude that the received signal is not correct.
If an error is detected, then the receiver will ignore the received byte and request for
retransmission of the same byte to the transmitter or it can use FEC(Forward Error Correction)
How does Error Detection take place using Parity Check method

10. Cyclic Redundancy Check (CRC)
• At the other end, the receiver performs division operation
on codewords using the same CRC divisor. If the remainder
contains all zeros the data bits are accepted, otherwise it is
considered as there is some data corruption occurred in
transit.
• CRC is a different approach to detect if
the received frame contains valid data.
This technique involves binary division
of the data bits being sent. The divisor
is generated using polynomials. The
sender performs a division operation
on the bits being sent and calculates
the remainder.
• Before sending the actual bits, the sender adds
the remainder at the end of the actual bits. Actual
data bits plus the remainder is called a codeword.
The sender transmits data bits as codewords.

11. Error Correction
• In the digital world, error correction can be done in two ways:
Error Correction
Forward Error Correction
( FEC )
Backward Error Correction
( BEC )
• When the receiver detects an error in the data
received, it requests back the sender to retransmit
the data unit.
• Backward Error Correction, is simple and can only
be efficiently used where retransmitting is not
expensive. For example, fiber optics. But in case of
wireless transmission retransmitting may cost too
much, So Forward Error Correction is used
• When the receiver detects some error in the data
received, it executes error-correcting code, which
helps it to auto-recover and to correct some kinds of
errors.
• FEC is used for wireless communication such as
satellite communication which our objective now

12. Forward Error Correction
• In telecommunication and information theory, forward error correction (FEC) is a system of error
control for data transmission, whereby the sender adds redundant data to its messages, also known as
an error correction code.
• FEC devices are often located close to the receiver of an analog signal, in the first stage of digital
processing after a signal has been received.
• Noise or Error is the main problem in the signal,
which disturbs the reliability of the
communication system.
• Error control coding(FEC) is the coding
procedure done to control the occurrences of
errors and their correction if they occur. These
techniques help in Error Detection and Error
Correction.
• these codes have been classified into
 Linear block codes and
 Convolution codes.

13. Linear Block Coding -----Cont------
• In linear block codes, the encoder splits up the incoming data stream into blocks of k digits and
processes each block individually by adding redundancy (extra digits) as a parity check according to a
predefined algorithm.
• The output of the encoder is a codeword with n digits, where n > k.
• The general formula of a linear block code word is
.
Y = GX
Where
• G is the Generator Matrix that creates
the check bits from data bits
• X is the input Message or Input Data
• Y is the Output Code or the Code word

14. Linear Block Coding -----Cont------
• Example: Consider message data X to be a 2-bit data word and the generator matrix is
G =
1 0
0 1
1
0
Solution Y =
1 0
0 1
1
0
1 0
X = 1 0 is the data word or input data
G =
1 0
0 1
1
0
is the Generator Matrix
Y = GX is the formula using this is the code word
to found Code word thus Y is a (3,2) code word having
two data bits and one parity bit
= [(1*1) + (0*0)][(1*0) + (0*1)][(1*1) + (0*1)]
= ( 1 + 0 ) ( 0 + 0 )( 1 + 0)
= 101

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‫تم‬‫بحمد‬‫هللا‬
Group 2 Members
1. Ahmed Muhumed Mohamed ID = 10513
2. Abdirahman Adam Ibraahim ID = 13399
3. Hamze Ismael Ali ID =
4. Shucayb Abdi Ismael ID =
5. Mohamoud Abdilahi Jaamac ID = 10258
6. Abdikarem Mubarak Mohamed ID =

16. END -------------------------
QUESTIONS