10 - ECE 562 Advanced Digital Communication Lecture 10...

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Unformatted text preview: ECE 562: Advanced Digital Communication Lecture 10: Modeling the Wireline Channel: Intersymbol Interference Introduction We are now ready to begin communicating reliably over our first physical medium: the wireline channel. Wireline channels (telephone and cable lines) are readily modeled as linear time invariant (LTI) systems (their impulse response changes very slowly – usually across different seasons of the year). In this lecture, we will arrive at a simple discrete-time model of the wireline channel, taking into account both the sampling operation and the LTI waveform of the channel itself. The main feature of this model is that the previously transmitted symbols affect the current received symbol. This feature is called inter-symbol interference (ISI) and is the main new challenge that has to be dealt with in wireline channels, apart from the (by now familiar) additive Gaussian noise. Wireline Media A wire is a single, usually cylindrical, elongated strand of drawn metal. The primary metals used in the conducting wire are aluminium, copper, nickel and steel and various alloys therein. We are used to several wireline media in our day-to-day life. We enumerate below a few of the common ones, along with a brief description of their physical composition and, more importantly, their impulse response characteristics. 1. Telephone wire : This connects houses and local telephone exchange, typically using a pair of copper conducting wires. They were designed to carry human voice which are all well contained in under 10 kHz. Depending on the condition of the wire and the length (distance between the house and the local telephone exchange) the telephone wire can be thought of as a low pass filter with bandwidth of about 1 or 2 MHz. 2. Ethernet wire : This is typically a collection of twisted pairs of wires: a form of wiring in which two conductors are wound together for the purposes of canceling out elec- tromagnetic interference from external sources and crosstalk from neighboring wires. Twisting wires decreases interference because the loop area between the wires (which determines the magnetic coupling into the signal) is reduced. The twist rate (usually defined in twists per meter) makes up part of the specification for a given type of cable. The greater the number of twists, the greater the attenuation of crosstalk. Further, the length of the wire decides how much the transmitted signal is attenuated; the longer the distance, the more the attenuation. There has been a standardization of these cable qualities and are typically denoted as “Cat x ” cables: where x stands for the different versions. For instance, the state-of-the-art technology is the Cat6 cable ( x = 6) with four pairs of copper wires twisted together. The maximum allowed length is about 90 meters and the impulse response can be thought of as a low pass filter with the bandwidth of about 250 MHz....
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10 - ECE 562 Advanced Digital Communication Lecture 10...

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