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UBC Theses and Dissertations

OFDM/FM frame synchronization for mobile radio data communication Casas, Eduardo F.


The use of Orthogonal Frequency Division Multiplexing (OFDM) for digital communications over Rayleigh-fading mobile radio channels was proposed by Cimini [1]. OFDM transmits blocks of bits in parallel and reduces the bit error rate (BER) by averaging the effects of fading over the bits in the block. This thesis studies the performance of OFDM/FM, a new modulation technique in which the OFDM baseband signal is used to modulate an FM transmitter. OFDM/FM can be implemented simply and inexpensively by retrofitting existing FM communication systems. Expressions are derived for the BER and word error rate (WER) within a block when each subchannel is QAM-modulated. Several numerical methods are developed to evaluate the overall BER and WER. An experimental OFDM/FM system was implemented using unmodified VHF FM radio equipment and a fading channel simulator. The BER and WER results obtained from the hardware measurements agree closely with the numerical results. The effects of forward error correction (FEC), switching diversity, automatic gain control (AGO, and squelch were tested. A new technique, decision feedback correction (DFC), was developed to reduce the crosstalk interference between the OFDM subchannels. This method significantly improves the BER performance of OFDM/FM. At BERs below 10⁻² the experimental OFDM/FM system has better power efficiency than the serial modulation techniques conventionally used for mobile radio (NCFSK, GTFM, or GMSK). At a BER of 10⁻³ and a normalized block duration (T[sub fd]) of 2.6, the experimental OFDM/FM system is 5 dB more power efficient than serial techniques. The use of DFC can significantly increase this advantage. However, current GTFM and GMSK systems have better spectral efficiency than the OFDM/FM prototype.

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