lecture18 - ECE 5670 Digital Communications Lecture 18...

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Unformatted text preview: ECE 5670 : Digital Communications Lecture 18: Passband Wireless Communication 1 3/29/2011 Instructor: Salman Avestimehr Introduction Beginning with this lecture, we will study wireless communication. The focus of these lectures will be on point to point communication. Communication on a wireless channel is inherently different from that on a wireline channel. The main difference is that unlike wireline channel, wireless is a shared medium. The medium is considered as a federal resource and is federally regulated. The entire spectrum is split into many licensed and unlicensed bands. An example of the the point to point communication in the licensed band is the cellular phone communication, whereas wi-fi, cordless phones and blue tooth are some of the examples of communication in the unlicensed band. The transmission over a wireless channel is restricted to a range of frequencies bracketleftbig f c − W 2 , f c + W 2 bracketrightbig around the central carrier frequency f c . Typically f c ≫ W ; (1) for example, W ≈ 1 MHz and f c ≈ 1 GHz for cellular communication. On the other hand, the wireline channel is quite a contrast: the carrier frequency f c = 0. Further more, the same wireless system (say cell phones) use different carrier frequencies in different cities. Clearly, it is not practical to tailor the communication strategies to the different carrier frequencies. It would be a lot better if we could design the system for a fixed carrier frequency and then have a simple mechanism to translate this design to suit the actual carrier frequency of operation. This is indeed possible, and the fixed carrier frequency might as well be zero. This has the added advantage that it lets us borrow our earlier understanding of communication strategies on the wireline channel. In other words, the plan is to design for “baseband” even though we are looking to communicate in passband. The focus of this lecture is on this conversion. Finally we also see how the effects of the wireless channel (which is in passband, after all) translate to the baseband: in other words, we derive a “baseband equivalent” of the passband wireless channel. Baseband Representation of Passband Signals Let’s begin with a baseband signal x b ( t ) (of double sided bandwidth W ) that we want to transmit over the wireless channel in a band centered around f c . From our discussion in the 1 Based on lecture notes of Professor Pramod Viswanath at UIUC....
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This note was uploaded on 10/02/2011 for the course ECE 5670 taught by Professor Scaglione during the Spring '11 term at Cornell.

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lecture18 - ECE 5670 Digital Communications Lecture 18...

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