Flash-OFDM, and SO on). 2002: IEEE 802.11g standard for wireless LAN. 2004: IEEE 802.16-2004 standard for fixed wireless MAN (Fixed WiMAX). 2004: ETSI DVB-H (Digital Video Broadcasting - Handheld) standard. 2005: IEEE 802.16e (Mobile WiMAX) standard. 2007: First demonstration of OFDMA based 3G-LTE air interface. 2007: 3GPP2 specified OFDMA for Ultra Mobile Broadband (UMB) standard. 2008: IEEE 802.20 (MobileFi) standard. 2009: TeliaSonera launched first 3GPP 3G-LTE system in December. The focus of this chapter is on OFDM and OFDMA. In Section 2.2 a technical background on OFDM is presented while Section 2.3 discusses principles of OFDM. In the following two sections we list the advantages and impairments associated with OFDM. Section 2.6 provides a brief overview of OFDMA and Section 2.7 listed the justifications why OFDM is required for packet-based networks like NGMN. The concluding remarks are provided in Section 2.8. Technical Background A single carrier system modulates information onto one carrier by altering frequency, phase or amplitude of the carrier. For digital signals, the information is in the form of bits, or collections of bits called symbols, that are modulated onto the carrier. As higher bandwidths (data rates) are used, the duration of the bits or symbol (collection of bits) of information becomes smaller. The system becomes more susceptible to interference from external sources and losses due to impulse noise, signal reflection, and SO on. The type of interference due to spurious emissions, inter-modulation products, and SO on, is called frequency interference. Frequency Division Multiplexing (FDM) extends the concept of SCM (single carrier modulation) by using multiple sub-carriers within the same single channel (spectrum). FDM allows the division of total data that needs to be sent into various sub-carriers offering various advantages over SCM. First, the data do not have to be divided evenly nor does it have to originate from the same information source. Second, it allows separate modulation/