![]() ![]() If the message is sent at a slower rate of 10 seconds per bit (the message now takes 100 seconds) SNR will improve by 10 dB. Assume a 10 bit message is send at a rate of 1 second per bit (thus the message takes 10 seconds). There are many so called FEC coding schemes, but the basic principles are the same for all. An excellent article that demonstrates and explains the fundamental principles behind Forward Error Correction codes, without diving into math, is ' The principle of forward error correction' by Paul Nicholson.Īs seen before in ' Dot length and SNR', SNR can be improved by slowing down the transmission rate. But there are far more efficient FEC methods.įEC is used in several modes that are popular on 630 m, such as WSPR, JT9 and Opera. report) several times to ensure that it is copied correctly. This FEC technique is very intuitive and is also spontaneously used in phone and CW: if conditions are poor or QRM is high we tend do repeat a piece of information (eg. The most straightforward FEC method is the repetition code, where each bit of the message is repeated a number of times. This longer code allows error correction at the RX side, without the need of backward communication. FEC ( Forward Error Correction): with this technique we do not transmit the message itself, but a much longer 'code'.Note that CRC requires a 2-way communication. The checksum (last 3 bits) can be used to check if the data (first 10 bits) is received well.ĬRC is used in the Packet radio ( AX25 protocol). This reminder is added to the packet, that becomes ''1010010110 110'. In this case the integer devision 662/8=82, the reminder is 6 (662-8♸2) or binary '110'. Now we divide this value by 8 and take the reminder. If this is not the case a request to repeat the package is send back.Ī simple example: assume we split the transmitted data in 10 bit packets, thus each packet can have a value between (2 10-1). This checksum can be used to verify if the packet is received without error. To each packet some additional bits, called the checksum, are added. CRC ( Cyclic Redundancy Check): with this method the transmitted data is split into packets.Another route to improve SNR is to go digital and use a error correction method. The 2 main error correction methods are: QRSS and DFCW are analog modes where reduction of the bandwidth is used to improve SNR. Weak signal modes Forward error correction (FEC)
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