qpr - Tutorial 16 By Charan Langton Signal Processing and...

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Tutorial 17 & Partial response signaling and Quadrature partial response modulation (QPR) www.complextoreal.com 1 Tutorial 16 By Charan Langton Signal Processing and Simulation Newsletter Partial Response signaling and Quadrature Partial Response (QPR) modulation Nyquist limit states that the number of symbols you can safely transmit in a channel of bandwidth 1 Hz that has inter-symbol interference is 2 symbols per second. But to achieve this bandwidth efficiency you need a brick wall filter, which is not build able. Raised cosine signaling helps counter ISI but at the cost of increased bandwidth. Fact 1 : A channel with no ISI using a raised cosine signaling with alpha = 0, allows us to successfully transmit a symbol rate equal to twice the bandwidth. Fact 2 : Since practical alpha is around .2 to .3, the realistic symbol rate possible is only (1 + α )W, with W as the excess bandwidth over the theoretical limit. One of the main reasons we can not transmit 2 symbols per Hz is inter-symbol interference. Raised cosine signaling is one way to counter ISI. But it limits the symbol rate that can be safely transmitted. Is there some other way that also counters ISI but allows us to achieve 2 symbols per Hz of bandwidth? Partial Response signaling, also called Quadrature Partial Response (QPR) a concept that is used both for pulse shaping and as a way to modulate information was proposed by Adam Lender in 1964. The technique also goes by the general name of correlative coding . There is an alphabet soup here that is often confusing. Partial response, correlative coding, duobinary, modified duobinary, Class I coding , are all names for the same thing.
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Tutorial 17 & Partial response signaling and Quadrature partial response modulation (QPR) www.complextoreal.com 2 Inter-symbol interference can be defined as spreading of symbol energy into adjacent symbol. The result of which is that the adjacent symbol shape is corrupted. However, with raised cosine signaling, the interference can be controlled such that if it is zero at the slicer timing pulse, then its effect is negated. It does not matter how much ISI distorts the signal as long as we know at the time of the sampling instant what the amount of that interference is. This amount is zero for raised cosine signaling because the signal shape (a sinc function) is forced to pass through zero as shown in Fig. 1. Although the interference is present, it is zero just at the instant we sample the signal to make decision about if it±s a 0 or a 1. This means that at the sampling instant, the sampled voltage belongs only to the pertinent symbol, so the decision based on this value is likely to be correct. And that±s the concept of controlled introduction of ISI . 1 2 3 4 Interference into Symbol 4 from symbols 1, 2 and 3 are all zero.
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qpr - Tutorial 16 By Charan Langton Signal Processing and...

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