Concepts of Orthogonal Frequency Domain Modulation

A number of new technologies are being combined by the telecommunications industry as it prepares for the next generation of mobile services. One of the key changes is the choice of OFDM for the air interface. This paper will describe OFDM and show why it has the ability to improve the spectral efficiency of digital radio links. It will also discuss the use of Multiple Input/Multiple Output (MIMO) antennas to improve signal reception and combat the effects of multipath fading.

Orthogonal Frequency Domain Modulation (OFDM) has been around for the last 40 years; wireline ADSL (Asymmetric Digital Subscriber Line – aka “broadband”), DAB and DVB-T (Digital Audio Broadcast and Digital Video Broadcast – Terrestrial used in Europe and elsewhere, and several types of WiFi already use this technology.

As a technology suitable for use in mobile handsets, however, it had to wait for the development of today’s small, low-cost, low power chipsets that can support the complex mathematics involved in creation and demodulation of OFDM transmission.

Three variants are on the horizon: the 3rd Generation Partnership Project (3GPP) standards body, which promotes the GSM family of technologies, is developing a system called Long Term Evolution (LTE).

3GPP2, which deals with cdma2000 and associated technologies, is working on Universal Mobile Broadband (UMB), while the IEEE has its 802.16 standard with its WiBro and WiMAX™ variants. All of these use OFDMA (Orthogonal Frequency Division Multiple Access) as the access methodology, bringing to OFDM support for multiple independent users and the ability to perform handovers while maintaining the link.