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In the early days of DMT development, bit loading was studied only in single-user case, where only one pair of modems (transmitter and receiver) was considered. Compared with traditional telephone service, DSL systems always work on high frequency range, which causes the crosstalk interference among the twisted pairs in the same cable noticeable. The SNR of a subchannel is related not only with the PSD of its own transmitter, but also with the PSD of all other transmitters in the same cable that act as disturbers. The bit-loading algorithms need to be extended to multi-user scenario to obtain the global optimum performance among all users. The optimal algorithm for discrete multi-user bit loading is a natural extension of single-user greedy algorithm. A matrix of cost is calculated, with elements that represent the power increment to transmit additional bits for each subchannel. Then, the subchannel with minimum cost is found, and additional bits are assigned to it. The process continues until all subchannels are filled. A drawback of the multi-user greedy bit loading is the computation complexity. For a single iteration of the algorithm, only one subchannel on one user who has the minimum cost is selected to get additional bits. The objective of this chapter is to propose an efficient greedy bit-loading algorithm for multi-user DMT systems. An improved parallel bit-loading algorithm for multi-user DMT will be discussed. The new algorithm is based on multi-user greedy bit loading. In a single Open Access Database www.intechweb.org

Efficient Multi-User Parallel Greedy Bit-Loading Algorithm with Fairness Control For DMT Systems