chapter 12 Matthew C. Valenti and Jian Sun

chapter 12 Matthew C. Valenti and Jian Sun - CHAPTER 12...

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[20:22 2003/9/25 DOWLA-CH12.tex] DOWLA: Handbook of RF and Wireless Technologies Page: 375 375–400 C HAPTER 12 T URBO C ODES Matthew C. Valenti and Jian Sun This chapter concerns turbo codes, one of the most powerful types of forward-error-correcting channel codes. Included is not only a discus- sion of the underlying concepts, but also a description and comparison of the turbo codes used by the Universal Mobile Telecommunications System (UMTS) and cdma2000 third-generation cellular systems. Channel Coding Forward-error-correcting (FEC) channel codes are commonly used to improve the energy ef±ciency of wireless communication systems. On the transmitter side, an FEC encoder adds redundancy to the data in the form of parity information. Then at the receiver, a FEC decoder is able to exploit the redundancy in such a way that a reasonable number of channel errors can be corrected. Because more channel errors can be tolerated with than without an FEC code, coded systems can afford to operate with a lower transmit power, transmit over longer distances, tolerate more interference, use smaller antennas, and transmit at a higher data rate. Abinary FEC encoder takes in k bits at a time and produces an output (or code word )of n bits, where n>k . While there are 2 n possible sequences of n bits, only a small subset of them, 2 k to be exact, will be valid code words. The ratio k/n is called the code rate and is denoted by r . 375
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[20:22 2003/9/25 DOWLA-CH12.tex] DOWLA: Handbook of RF and Wireless Technologies Page: 376 375–400 376 Handbook of RF and Wireless Technologies Lower rate codes, characterized by small values of r, can generally cor- rect more channel errors than higher rate codes and are thus more energy efFcient. However, higher rate codes are more bandwidth efFcient than lower rate codes because the amount of overhead (in the form of parity bits) is lower. Thus the selection of the code rate involves a tradeoff between energy efFciency and bandwidth efFciency. ±or every combination of code rate (r), code word length (n), modulation format, channel type, and received noise power, there is a theoretical lower limit on the amount of energy that must be expended to con- vey one bit of information. This limit is called the channel capacity or Shannon capacity, named after Claude Shannon, whose 1948 deriva- tion of channel capacity [1] is considered to have started the applied mathematical Feld that has come to be known as information theory. Since the dawn of information theory, engineers and mathematicians have tried to construct codes that achieve performance close to Shannon capacity. Although each new generation of ±EC code would perform incrementally closer to the Shannon capacity than the previous genera- tion, as recently as the early 1990s the gap between theory and practice for binary modulation was still about 3 dB in the most benign chan- nels, those dominated by additive white Gaussian noise (AWGN). In other words, the practical codes found in cell phones, satellite systems, and other applications required about twice as much energy (i.e., 3 dB
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This note was uploaded on 04/20/2008 for the course COMM 125563 taught by Professor Anwar during the Spring '08 term at Air Force Institute of Technology, Ohio.

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chapter 12 Matthew C. Valenti and Jian Sun - CHAPTER 12...

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