RF Analog Impairments Modeling for Communication Systems Simulation: Application to OFDM-based Transceivers

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.

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