The Basics Of OFDM
This article is second in a series of quarterly guest columns by Mark Elo, RF Marketing Director, Keithley Instruments, Inc.
As the demand for fast mobile data rates increases, engineers are constantly looking for new transmission methods. Until recently, this was achieved by simply increasing the symbol rate. However, the advent of OFDM (orthogonal frequency division multiplexing) provides an innovative way to provide broadband data at a relatively low symbol rate, making it a much more robust modulation technique for multipath environments such as homes, offices, or cars.
In this article, we'll explore the basics of OFDM and the new demands placed on RF test instrumentation that will need to test the next generation of wireless communication devices.
How Does OFDM Work?
Broadband data at a slow symbol rate sounds contradictory at first; however, the trick to OFDM is to transmit multiple symbols in parallel by using many carriers. Thus, we can keep the symbol rate low on each individual carrier and achieve high bandwidth by having many thousands of carriers. For example, mobile WiMAX (Worldwide Interoperability for Microwave Access) can have in excess of 2,000 carriers. This explains the "frequency division multiplex" part of OFDM. However, the "orthogonal" part is the real key to how the system works. If the carrier spacing is made equal to the symbol rate, this can significantly reduce the cross-carrier interface and allow for the modulation of many carriers, called sub-carriers, in a relatively small bandwidth. For example, WLAN 802.11g has 52 sub-carriers, spaced at 312.5 kHz, with an overall bandwidth of 16.25 MHz.