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This paper presents a unified mathematical performance analysis of the physical layer in orthogonal frequency division multiple access (OFDMA) wireless ad hoc networks, where several independent transmitter-receiver pairs share a common wideband channel in a local area environment. Multiuser interference (MUI) occurs when the signals from different users arrive at a given receiver with arbitrary timing misalignments, leading to the destruction of the orthogonality between subcarriers. Precise interference analysis in white Gaussian noise and Rayleigh multipath fading is developed in a partially loaded OFDMA network. New exact expressions of symbol and bit error rates are given in the case of interleaved subcarrier assignment schemes. On the other hand, tight upper bounds and accurate improved Gaussian approximations are developed for arbitrary subcarrier assignment schemes. Furthermore, expressions of the cutoff rates are derived and employed to estimate the spectral efficiency in bits/sec./Hz. These are used to quantify the improvement in the spectral efficiency that can be achieved by a common MUI mitigating technique based on the extension of guard intervals and dynamic positioning of FFT windows.