Deorthogonalization of ODS-CDMA QPSK by AM/PM Nonlinearity |
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Authors: | Pawlowski Peter R. |
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Affiliation: | (1) LinCom Corporation, 5, 5110 W. Goldleaf Circle, Suite 250, Los Angeles, CA-, 90056 |
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Abstract: | When using a conventional demodulator, a sufficient condition to maintain ODS-CDMA codeword orthogonality is constant relative channel amplitude over the codeword duration. When transmitted over a memoryless, AM/PM nonlinearity channel, the chip-to-chip fluctuating amplitude of a composite ODS-CDMA QPSK waveform produces a chip-to-chip fluctuating phase that deorthogonalizes the ODS-CDMA codewords, resulting in an additional multiple access self-interference similar to that found in asynchronous DS-CDMA. For ODS-CDMA QPSK in an AM/PM nonlinearity channel, we utilize the Central Limit Theorem, derive, and evaluate: (i) an expression for the signal-to-distortion ratio at the demodulator output, (ii) an expression for the uncoded bit error probability, and (iii) an upper bound on the convolutionally-coded bit error probability. We find that the degradation to BER depends on both the AM/PM nonlinearity slope and the ODS-CDMA channel loading. |
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Keywords: | spread spectrum code division multiple access AM/PM nonlinearity |
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