|
|
| Line 1: |
Line 1: |
| {{refimprove|date=October 2006}}
| | Oscar is what my wife enjoys to contact me and I completely dig that name. For many years he's been operating as a receptionist. Her family life in Minnesota. Doing ceramics is what adore performing.<br><br>My web blog :: [http://203.250.78.160/zbxe/?document_srl=810524&mid=gallery&sort_index=regdate&order_type=desc home std test kit] |
|
| |
| In [[digital communications]] '''shaping codes''' are a method of [[encoding]] that changes the distribution of signals to improve efficiency.
| |
| | |
| ==Description==
| |
| Typical digital [[communication systems]] uses M-[[Quadrature Amplitude Modulation]]([[QAM]]) to communicate through an analog channel (specifically a [[Channel (communications)|channel]] with [[Gaussian noise]]). For Higher bit rates(M) the minimum [[Signal to Noise ratio]] (SNR) required by a QAM system with [[Error detection and correction#Error-correcting code|Error Correcting Codes]] is about 1.53 [[Decibel|dB]] higher than minimum SNR required by a Gaussian source(>30% more transmitter power) as given in [[Shannon–Hartley theorem]]
| |
| :<math> C = B \log_2 \left( 1+\frac{S}{N} \right) </math>
| |
| | |
| where
| |
| :''C'' is the [[channel capacity]] in [[bits per second]];
| |
| :''B'' is the [[Bandwidth (signal processing)|bandwidth]] of the channel in [[hertz]];
| |
| : ''S'' is the total signal power over the bandwidth and
| |
| : ''N'' is the total noise power over the bandwidth.
| |
| :''S/N'' is the [[signal-to-noise ratio]] of the communication signal to the Gaussian noise interference expressed as a straight power ratio (not as [[decibels]]).
| |
| | |
| This 1.53 dB difference is called the ''shaping gap''. Typically digital system will encode bits with uniform probability to maximize the [[Entropy (information theory)|entropy]]. Shaping code act as buffer between digital sources and modulator communication system. They will receive uniformly distributed data and convert it to Gaussian like distribution before presenting to the modulator. Shaping codes are helpful in reducing transmit power and thus reduce the cost of Power amplifier and the interference caused to other users in the vicinity.
| |
| | |
| ==Application==
| |
| | |
| Some of the methods used for shaping are described in the trellis shaping paper by Dr. [[Dave Forney|G. D. Forney Jr.]]<ref>http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=119687</ref>
| |
| | |
| [[Shell mapping]]<ref>http://books.google.co.in/books?id=jtputBwrGvcC&pg=PA258&lpg=PA258&dq=shell+mapping&source=bl&ots=8tO8iNxr4i&sig=xaOuhKAMMweHYPPHnZuzKZmHYjc&hl=en&ei=oVugTIbZIoe0cMjjpeEJ&sa=X&oi=book_result&ct=result&resnum=8&ved=0CD0Q6AEwBw#v=onepage&q=shell%20mapping&f=false</ref> is used in [[V.34 (recommendation)|V.34]] modems to get a shaping gain of .8 [[Decibel|dB]].
| |
| All the shaping schemes in the literature try to reduce the transmitted signal power. In future this may have find application in [[wireless network]]s where the interference from other nodes are becoming the major issue.
| |
| | |
| == References == | |
| {{reflist}}
| |
| *[http://www.freepatentsonline.com/5150381.html Trellis shaping for modulation systems ]
| |
| | |
| {{DEFAULTSORT:Shaping Codes}}
| |
| [[Category:Digital audio]]
| |
| [[Category:Data transmission]]
| |
| [[Category:Error detection and correction]]
| |
| [[Category:Information theory]]
| |
| [[Category:Telecommunication theory]]
| |
Oscar is what my wife enjoys to contact me and I completely dig that name. For many years he's been operating as a receptionist. Her family life in Minnesota. Doing ceramics is what adore performing.
My web blog :: home std test kit