isi - Inter Symbol Interference(ISI and Root-raised...

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Inter Symbol Interference(ISI) and Root-raised Cosine (RRC) filtering 1 Intuitive Guide to Principles of Communications www.complextoreal.com Inter Symbol Interference (ISI) and raised cosine filtering Inter-symbol interference (ISI) is an unavoidable consequence of both wired and wireless communication systems. Morse first noticed it on the transatlantic telegraph cables transmitting messages using dots and dashes and it has not gone way since. He handled it by just slowing down the transmission. Time Amplitude Symbol Time (a) Time Amplitude Delay Spread (b) Fig 1 - (a) What was sent, (b) What was received. During these early attempts at transmission, it was noticed is that the received signals tended to get elongated and smeared into each other. A short pulse to represent a dot was received as a much-smeared version of the same thing. The problem appeared to be related to the properties of the medium used and the distance of signal travel. To counter this undesired effect, intermediate repeating stations were established and ways had to be devised to reduce this smearing. How does this actually affect us now? Fig 2 shows a data sequence, 1,0,1,1,0, which we wish to send. This sequence is in form of square pulses. Square pulses are nice as an abstraction but in practice they are hard to create and also require far too much bandwidth. So we shape them as shown in the dotted line. The shaped version looks essentially like a square pulse and we can quickly tell what was sent even visually. Advantage of (an arbitrary) shaping at this point is that it reduces bandwidth requirements and can actually be created by the hardware. Copyright 2002 Charan Langton www.complextoreal.com
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