Orthogonal frequency division multiplexing(OFDM) is a kind of multi-carrier modulation(MCM) schemes. OFDM allows overlapping of adjacent frequency bins called subchannels with orthogonality between them. It results in higher spectral efficiency and a long symbol period compared to single carrier schemes.
In this thesis work, we implement a multi-carrier MODEM in real time for telephone channel voice band data transmission using an ADSP-21020 digital signal processor(DSP) chip. The MODEM algorithm is designed based on the principles of the OFDM scheme. A simple start-up protocol is also designed for bidirectional communication. Important features in this implementation of the OFDM MODEM algorithm include moduation and demodulation using the discrete Fourier transform(DFT), two synchronization tones, and single tap equalization in the frequency domain.
We evaluate the performance and test the operation of the OFDM MODEM over AWGN and telephone channels. For AWGN channels, computer simulation of the MODEM algorithm is done and the simulation results show that the symbol error rate(SER) of the MODEM algorithm in this study is very close to the theorical one. For telephone channels, we test the operation of the MODEM algorithm and communication protocols using a telephone channel simulator(TCS-500), and verify that 4-wire full duflex data transmission can be done successfully
The subjects for further study include optimum bit allocation of each subchannel subject to channel status, echo cancellation for full duflex operation over a telephone channel, communication protocols, and user interface.
