Orthogonal Frequency Division Multiplexing (OFDM) is special form of multi-carrier modulation, initially proposed in the 1970s. It is particularly suited for transmission over a dispersive (i.e., frequency selective) channel. In 1993 Linnertz et al. proposed MC-CDMA. This is a combination of OFDM with Code Division Multiple Access (CDMA).
In a multipath channel, most conventional modulation techniques are sensitive to intersymbol interference unless the channel symbol rate is small compared to the delay spread of the channel. OFDM is significantly less sensitive to intersymbol interference, because a special set of signals is used to build the composite transmitted signal. The basic idea is that each bit occupies a frequency-time window which ensures little or no distortion of the waveform. In practice, it means that bits are transmitted in parallel over a number of frequency-nonselective channels. Applications of OFDM are found in
- Digital Audio Broadcasting (DAB) and
- Digital Video Broadcasting over the terrestrial network: Digital Terrestrial Television Broadcasting (DTTB). In the DTTB OFDM transmission standard, about 2,000 to 8,000 subcarriers are used.
- Wireless LANs. OFDM is used for HIPERLAN Phase II, supporting 20 Mbit/s in propagation environments with delay spreads up to 1 msecond.
Jean-Paul
Linnartz presents OFDM, MC-CDMA and some recent work on OFDM reception
over rapidly time-varying channels.
J.P.M.G. Linnartz and A. Gorokhov, "New equalization approach for OFDM over dispersive and rapidly time varying channel.", PIMRC '00, London, Sept. 2000, CD-ROM proceedings. PDF (55k)
N. Yee, J.P.M.G. Linnartz and G. Fettweis, "Multi-Carrier CDMA in indoor wireless Radio Networks", IEEE Personal Indoor and Mobile Radio Communications (PIMRC) Int. Conference, Sept. 1993, Yokohama, Japan, pp. 109-113.
N. Yee and J.P.M.G. Linnartz, "Wiener filtering for Multi-Carrier CDMA", IEEE / ICCC conference on Personal Indoor Mobile Radio Communications (PIMRC) and Wireless Computer Networks (WCN), The Hague, September 19-23, 1994, Vol. 4, pp. 1344-1347. PDF
J.P.M.G. Linnartz and A. Gorokhov, "Doppler-resistant OFDM Receivers for Mobile Multimedia Communications", Second Int. Symp. on Mobile Multimedia Systems and Applications, Nov. 9-10, Delft, The Netherlands, pp. 87-92.
