6
OFDM and All-IP
6.1 Introduction to OFDM
OFDM stands for Orthogonal Frequency Division Multiplexing. A special case of Frequency Division Multiplexing or FDM, OFDM is both a modulation and multiplexing technique. The concept of OFDM has existed since around 1966; however, it has really found its importance beyond the research laboratories only recently in cellular communications. OFDM is used as a modulation technique in many systems, such as DAB (Digital Audio Broadcasting), DVB (Digital Video Broadcasting), WLAN (Wireless Local Area Network), PCS (Personal Communication System), etc. Some major developmental milestones include the first OFDM schemes, including the orthogonal QAM proposed in 1996. Although the first patents were issued in 1970 it was not until 1987 that OFDM was employed for digital broadcasting. OFDM was used for ADSL, HDSL, DAB, DVB, etc. in 1991–1997, along with the 802.11a/g standards for WLAN (1999–2002). It is expected that OFDM will become a key technology for 4G systems.
In OFDM, the signal is split into several narrow band channels at different frequencies, modulated (e.g. PSK modulation) by data and then re-multiplexed by the OFDM carrier to create a sub-carrier. There have been many ways to explain this but a ‘truck-load’ analogy might suit the best. Consider a truck that is full of a load, for example 400 kg, that needs to be transported from point A to point B. A large truck can carry the load of 400 kg or four smaller trucks can carry 100 kg each, ...
