Orthogonal Frequency Division Multiplexing (OFDM)
The principle of OFDM is transmitting data by dividing the data stream into multiple parallel bit streams that have a much lower bit rate and using these sub-streams to modulate several carriers. OFDM is more resistant to frequency selective fading than single carrier systems are.
- Orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission technique which is based on frequency division multiplexing (FDM).
- In conventional FDM multiple-frequency signals are transmitted simultaneously in parallel where the data contained in each signal is modulated onto subcarriers and therefore the subcarrier multiplexed signal typically contains a wide range of frequencies.
- Each subcarrier is separated by a guard band to avoid signal overlapping.
- The subcarriers are then demodulated at the receiver by using filters to separate the frequency bands.
- OFDM employs several subcarrier frequencies orthogonal to each other (i.e. perpendicular) and therefore they do not overlap.
- Hence this technique can squeeze multiple modulated carriers tightly together at a reduced bandwidth without the requirement for guard bands while at the same time keeping the modulated signals orthogonal so that they do not interfere with each other, as illustrated in Figure 12.53.
- In the upper spectral diagram 10 non-overlapping subcarrier frequency signals arranged in parallel depicting conventional FDM are shown, each being separated by a finite guard band.
- OFDM is displayed in the bottom spectral diagram where the peak of one signal coincides with the trough of another signal.
- Each subcarrier must maintain the Nyquist criterion separation with the minimum time period of T for each subcarrier OFDM uses the inverse fast Fourier transform (IFFT) for the purpose of modulation and the fast Fourier transform (FFT) for demodulation.
- This is a consequence of the FFT operation by which subcarriers are positioned perpendicularly and hence the reason why the technique is referred to as orthogonal FDM.
- It may be observed that a large bandwidth saving in comparison with conventional FDM is identified in Figure 12.53 resulting from the orthogonal placement of the subcarriers.
- Since the orthogonal feature allows high spectral efficiency near the Nyquist rate where efficient bandwidth use can be obtained, OFDM generally exhibits a nearly white frequency spectrum.
- OFDM, also being tolerant to signal dispersion, thus enables high-speed data transmission across a dispersive channel and it has been widely used in high-bit-rate cable and wireless communication systems
- For applications within optical fiber communications it is necessary to incorporate an optical source to convert the OFDM signals into an optical signal format before coupling onto an optical fiber, while at the receiving end the intensity modulated signal can be recovered to as optical OFDM (OOFDM).
- Although the multiplexing approach is similar to optical SCM, the orthogonal nature of the subcarriers is unique to OOFDM.