L8-Linecodes

L8-Linecodes - Transmission line codes In a practical...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Transmission line codes In a practical digital transmission system, it is often necessary to encode or transform the binary source digit stream to conform to the characteristics of the transmission channel and the associated equipment. This signal matching operation is carried out by the process called line coding or transmission coding. Line coding is also referred to baseband modulation. For example, a baseband channel is normally ac-coupled by either capacitors or a transformer (to facilitate power feeding over lines, and to avoid earth currents from flowing due to different earth potentials between distant locations). For a baseband signal to be conveyed over such a channel would require that the signalling pulse sequence does not build up short-term dc offsets, i.e., there is no long runs of “1”s and “0”s. Any long runs of “1”s or “0”s would result in “droop” or “baseline wander” as shown as follow: Baseline or dc wander due to ac coupling . Properties of a line code: 1. Transparency 2. Unique decodability 3. Coding efficiency 4. Favourable spectrum 5. Adequate timing information 6. Error detection or correction capability 7. Framing information 8. Low error extension Transmitted signal time Received signal time Essential Desirable Required for good coding operation
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 1. Transparency: No restriction should be imposed on the content of the transmitted message sequence, i.e., all digits will be transmitted faithfully. 2. Unique decodability: The coded sequence can be unambiguously decoded to retrieve the original input. 3. Coding efficiency: Frequently the input code and the output code do not have the same code length because (a) Additional code bits/symbols are included to achieve the required features (b) M-ary signalling is adopted. Example: Input binary sequence Output coded sequence Binary m bits n bits with n m M- ary j bits k symbols with M k 2 j For a M -ary symbol, the information content in one symbol interval is given by interval l bits/symbo ) ( log 2 M H = . Assuming that all M -levels are equally likely, the probability of occurrence of a given level is M P 1 = , or ). ( log 322 . 3 ) ( log 10 2 P p H = = The system capacity C , defined as the maximum rate of transmitting information, is s s T P T M T H C ) ( log ) ( log 2 2 = = = bits/sec where T s is the symbol period.
Background image of page 2
3 In general, a code consists of n different symbols with the probability of the i th symbol occurring as P i , the rate of transmission of information over one symbol period is ) ( log 1 2 = = n i i i P P H bits/symbol. Now, if the input to the encoder is at a rate of S i symbols per second, then the information capacity is ) ( log 2 1 i M i i i i P P S C = = bits/sec. Similarly, the information capacity of the output of the encoder at a rate
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 05/09/2011 for the course ENGR 601 taught by Professor Kah-chung during the Three '11 term at Curtin.

Page1 / 18

L8-Linecodes - Transmission line codes In a practical...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online