Coded Orthogonal Frequency Division Multiplexing (COFDM) is a modulation form that is ideal for the requirements of terrestrial broadcasting channels. COFDM has the ability to deal with high levels of multi-path propagation. It uses a wide spread of delays between the received signals.
COFDM is an innovative approach with numerous intriguing features used in Digital Video Broadcasting - Terrestrial (DVB-T) and WLAN. It transforms blocks of thousands of modulated signal samples through iFFT into the frequency domain and also inserts guard intervals into the spectrum.
COFDM is a substitute for a single carrier waveform. In addition to the ability to deal with high levels of multi-path propagation, COFDM handles co-channel narrowband interference very well. Therefore, COFDM has been preferred for two new broadcasting standards, namely:
- Digital Audio Broadcasting (DAB): DAB was specifically built to handle the rigors of signal reception in moving cars - particularly the issue of multi-path reception which, in the above case, is time varying.
- Digital Video Broadcasting - Terrestrial (DVB-T): DVB-T requires a higher capacity when compared with DAB. Multi-path tolerance is crucial due to the extensive usage of set-top television antennas.
In order to accommodate the specific requirements of DAB and DVB-T, COFDM is used, with proper adjustments in parameters. The overall performance of COFDM with respect to multi-path propagation and interference can be achieved only by a sensible choice of parameters. A successful implementation also demands attention to detail about how the forward error-correction coding is used.
COFDM involves the modulation of data into a multitude of carriers with the help of the FDM technique. The key features that make it ideal for terrestrial channels include:
- Orthogonality
- Use of guard intervals
- Use of error coding, channel-state information (CSI) and interleaving
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