On the cyclostationarity of orthogonal frequency division multiplexing and single carrier linear digital modulations: theoretical developments and applications

Punchihewa, Anjana (2008) On the cyclostationarity of orthogonal frequency division multiplexing and single carrier linear digital modulations: theoretical developments and applications. Masters thesis, Memorial University of Newfoundland.

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Abstract

In recent years, new technologies for wireless communications have emerged. The wireless industry has shown great interest in orthogonal frequency division multiplexing (OFDM) technology, due to the efficiency of OFDM schemes to convey information in a frequency selective fading channel without requiring complex equalizers. On the other hand, the emerging OFDM wireless communication technology raises new challenges for the designers of intelligent radios, such as discriminating between OFDM and single-carrier modulations. To achieve this objective we study the cyclostationarity of OFDM and single carrier linear digital (SCLD) modulated signals. -- In this thesis, we first investigate the nth-order cyclostationarity of OFDM and SCLD modulated signals embedded in additive white Gaussian noise (AWGN) and subject to phase, frequency and timing offsets. We derive the analytical closed-form expressions for the nth-order (q-conjugate) cyclic cumulants (CCs) and cycle frequencies (CFs), and the nth-order (q-conjugate) cyclic cumulant polyspectra (CCPs) of OFDM signal, and obtain a necessary and sufficient condition on the oversampling factor (per subcarrier) to avoid cycle aliasing. An algorithm based on a second-order CC is proposed to recognize OFDM against SCLD modulations in AWGN channel, as an application of signal cyclostationarity to modulation recognition problem. -- We further study the nth-order cyclostationarity of OFDM and SCLD modulated signals, affected by a time dispersive channel, AWGN, carrier phase, and frequency and timing offsets. The analytical closed-form expressions for the nth-order (q-conjugate) CCs and CFs, the nth-order (q-conjugate) CCPs of such signals are derived, and a necessary and sufficient condition on the oversampling factor (per subcarrier) is obtained to eliminate cycle aliasing for both OFDM and SCLD signals. We extend the applicability of the proposed algorithm in AWGN channel to time dispersive channels to recognize OFDM against SCLD modulations. The proposed algorithm obviates the preprocessing tasks; such as symbol timing, carrier and waveform recovery, and signal and noise power estimation. This is of practical significance, as algorithms that rely less on preprocessing are of crucial interest for receivers that operate with no prior information in a non-cooperative environment. It is shown that the recognition performance of the proposed algorithm in time dispersive channel is close to that in AWGN channel. In addition, we have noticed that the performance of recognizing both OFDM and SCLD signals does not depend on the modulation format used on each subcarrier for OFDM and for SCLD signals respectively.

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