Abstract
This chapter provides a detailed overview of orthogonal frequency-division multiplexing (OFDM). OFDM is the transmission scheme used for 3GPP LTE and is also used for several other radio-access technologies. Transmission by means of OFDM can be seen as a kind of multicarrier transmission. The basic characteristics of OFDM transmission, which distinguish it from a straightforward multicarrier extension of a more narrowband transmission scheme is the use of a typically very large number of relatively narrowband subcarriers. In contrast, a straightforward multicarrier extension typically consists of only a few subcarriers, each with a relatively wide bandwidth. The number of OFDM subcarriers can range from less than hundred to several thousand, with the subcarrier spacing ranging from several hundred kHz down to a few kHz. The subcarrier spacing used depends on types of environments the system is to operate in, including aspects such as the maximum expected radio-channel frequency selectivity (maximum expected time dispersion) and the maximum expected rate of channel variations (maximum expected Doppler spread). Once the subcarrier spacing has been selected, the number of subcarriers can be decided based on the assumed overall transmission bandwidth, taking into account acceptable out-of-band emission, and other factors.
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