Orthogonal frequency division multiplexing (OFDM) is a kind of multicarrier modulation (MCM) that allows spectral overlapping between subchannels maintaining orthogonality. It can achieve high spectral efficiency and has a long symbol period compared with the single carrier modulation. OFDM can be efficiently implemented with DFT.
In this work, we implement an OFDM modem in real time for telephone channels using an ADSP-21020 digital signal processor(DSP) chip. The modem algorithm is designed based on the principles of OFDM. The features implemented include a simple start-up protocol for bidirectional communication, modulation and demodulation using the discrete Fourier transform(DFT), synchronization, single tap equalization, channel sounding, optimum bit allocation, and an energy loading algorithm.
Specifically, the data rate approaching to the channel capacity can be achieved by using the optimum bit allocation and the energy loading algorithms. We evaluate the performance of the OFDM modem over telephone channels by using a telephone channel simulator. The modem performs 4-wire full duplex data transmission successfully.
