Abstract-An adaptive subcarrier allocation and an adaptive modulation for multiuser orthogonal frequency-division multiplexing (OFDM) are considered. The optimal subcarrier and bit allocation problems, which are previously formulated as nonlinear optimizations, are reformulated into and solved by integer programming (IP). A suboptimal approach that performs subcarrier allocation and bit loading separately is proposed. It is shown that the subcarrier allocation in this approach can be optimized by the linear-programming (LP) relaxation of IP, while the bit loading can be performed in a manner similar to a single-user OFDM. In addition, a heuristic method for solving the LP problem is presented. The LP-based suboptimal and heuristic algorithms are considerably simpler to implement than the optimal IP, plus their performances are close to those of the optimal approach. Index Terms-Integer programming (IP), linear programming (LP), multiuser orthogonal frequency-division multiplexing (OFDM), subcarrier and bit allocation.

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A data-aided (DA) timing recovery method for general continuous phase modulation (CPM) signaling is introduced. This method requires only symbol rate sampling and is independent of the carrier phase offset, owing to the use a differential operator. By examining the phase of the data after the differential operation, we derive an expression of the phase error caused by the timing offset and obtain an efficient timing error function.Then a feedback timing recovery algorithm is developed by exploiting this timing error function. The characteristics of the algorithm are examined through computer simulation.

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