1.3 OFDM Modulation
1.3.1 OFDM as a Multicarrier Modulation
Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation increasingly used in communication systems (WiFi, WiMAX (Worldwide Interoperability for Microwave Access), 4G/LTE, power line communication (PLC)) because it presents several advantages compared with single carrier modulation and classical frequency division multiplexing (Bingham, 1990; Van Nee and Prasad, 2000; Prasad, 2004; Li and Stuber, 2006; Armstrong, 2007):
- Efficient use of the spectrum because subcarrier orthogonality allows overlap.
- Less sensitive to channel fading (multipath).
- Channel estimation and equalization in the frequency domain carries low complexity.
- In frequency-selective fading possibility to avoid the affected subcarriers or to adapt their modulation as a function of their SNR.
- Possibility to avoid inter-symbol interference (ISI) with a cyclic prefix.
- Narrow band interferers will only affect few subcarriers.
- Coexistence with other systems: subcarriers can be turned on/off.
However, the use of OFDM modulation presents some drawbacks which have to be taken into account during the system design and specification, such as:
- Very sensitive to frequency and phase offsets and timing error:
- Break the orthogonality between subcarriers.
- OFDM temporal signal has a high peak to average power ratio (PAPR):
- Poor efficiency of the PAs.
- Signal clipping and distortion degrades the SNR and generates out-of-band emissions.