OFDMA Tutorial - Theory, principles, design considerations and applications

OFDMA is a new modulation method for the 2 generation of Broadband Wireless Access (BWA) systems that combines both upstream access and modulation together. OFDMA is based on modulating multiple orthogonal sub-carriers. Unlike traditional multi-carrier modulation schemes, several transmitters of the multiple access system modulate the sub-carriers simultaneously. OFDMA offers up to an 18dB gain in the upstream link budget and up to a 12dB gain in the downstream link budget relative to traditional multiple access technologies. These gains are complemented by the capability to operate in severe non-line-of-sight conditions. Armed with these advantages, OFDMA enables deployment of integrated indoor BWA subscriber units (SU). This article discusses the principals and design considerations for the downlink and uplink operation of OFDMA based systems. The topics discussed are: SU synchronization, media access control (MAC) layer aspects and handling of real-world problems such as phase noise and power-amplifier (PA) backoff.