OFDM: Orthogonal frequency division multiplexing

Industrial Wireless Tutorials: OFDM is not actually a spread spectrum technology; it is a specialized form of multiplexing. OFDM multiplexes data onto 52 orthogonal subcarriers using one of several modulation techniques.

By Daniel E. Capano December 5, 2014

Orthogonal frequency division multiplexing (OFDM) is a modulation and multiplexing technique. Modulation is the process by which data is encoded onto a carrier signal, which is then amplified and applied to an antenna. Multiplexing is the process by which one transmission channel is used for multiple signals. Most people are familiar with amplitude and frequency modulation (AM and FM), which deals primarily with analog data like voice and music. For example, digital data modulation is achieved much more efficiently using several different methods. Multiplexing is a technique where multiple signals are combined into a single or shared medium. Multiplexing/demultiplexing is referred to as mux/demux.

The first spread spectrum technique developed was frequency hopping spread spectrum (FHSS) and was used as early as World War II. FHSS was one of the original modulation methods specified in the IEEE 802.11 prime standard. Briefly, FHSS channel hops within a given frequency space, each hop typically being 1 Mhz. The hop sequence is broadcast in the AP’s beacon management frame, allowing the receiver to follow the data through the hop sequence. FHSS was replaced by faster methods in 802.11 but is still used in 802.15 devices, such as Bluetooth.

To effectively and reliably transfer data in the wireless medium, Wi-Fi uses a broad category of modulation and coding methods known as spread spectrum. In spread spectrum modulation, data is "spread" over the channel spectrum by encoding a single bit into a series of bits, or "chips." The addition of this information is called processing gain and has the effect of improving signal-to-noise ratio (SNR). AM and FM are narrowband techniques using a narrow channel spectrum and high power transmission. Spread spectrum is a broadband technique operating over a wide channel spectrum, requiring low power and slower data rates to transmit information. This makes communication more robust and reliable.

Direct sequence spread spectrum (DSSS) is a spread spectrum technique that was first used with the rollout of 802.11. DSSS allows data to be spread over a broad band of frequency spectrum, which is resistant to interference while requiring lower power. The original standard provided for a throughput of 1-2 Mbps; 802.11b introduced high rate-DSSS (HR-DSSS), which provided throughputs of 5.5 and 11 Mbps. Data encoding is called "chipping." Each bit is encoded using Barker code; this code chips 1 bit into 11 bits, which are spread across the channel frequency spectrum. Up to 9 of the 11 bits can be corrupted and data can still be recovered.

OFDM is not actually a spread spectrum technology; it is a specialized form of multiplexing. OFDM multiplexes data onto 52 orthogonal subcarriers using one of several modulation techniques. These subcarriers are then multiplexed into the OFDM carrier. The signal is then demultiplexed at the receiving end and demodulated.

Binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) are used to modulate the signal onto the individual subcarriers, at low data rates; at higher rates, quadrature amplitude modulation (QAM) is used. QAM is a hybrid of phase and amplitude modulation and has resolutions between 16-QAM and 256-QAM.

Each subcarrier is 90 deg apart, orthogonally. This eliminates co- and adjacent channel interference and crosstalk. It is resistant to narrowband interference. Each subcarrier is 312.5kHz wide, numbered -26 to +26 around the channel center frequency. Forty-eight of the subcarriers are used for data, while four are used as "pilot" carriers that carry phase and amplitude references to compensate for any distortion in the transmission. Pilots are used to track the residual phase error, if any, after frequency correction.

802.11a was the first standard to use OFDM in the 5 GHz spectrum, followed by 802.11g, in the 2.4 GHz spectrum. Both provide throughput up to 54 Mbps. Standard 802.11n uses high-throughput OFDM (HT-OFDM) to provide up to 600 Mbps throughput. HT-OFDM uses 56 20MHz subcarriers channel, of which 52 carry data; and 114 40 MHz subcarriers, 108 of which are used for data transmission. Standard 802.11n also uses MIMO and spatial multiplexing to increase throughput.

In addition to resistance interference and data corruption, convolutional coding, an error correcting method, is used to increase resistance to narrowband interference. Convolutional coding is a method of forward error correction (FEC) that allows the receiver to detect and repair damaged bits, and avoid retransmissions. Convolutional coding uses a ratio between bits transmitted and bits encoded to determine the strength of the resistance to interference. Lower ratios indicate less resistance to interference, but higher throughput.

OFDM is used in both wired and wireless communication and is a widely used option for digital communication. Aside from 802.11 WLANs, OFDM is used in mobile communications, digital television, and audio broadcasting.

– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Chris Vavra, production editor, CFE Media, Control Engineering, cvavra@cfemedia.com.

ONLINE extras

www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:

MIMO and spatial multiplexing

Propagation revisited: Multipath

The management, control, and data planes

www.controleng.com/webcasts has wireless webcasts, some for PDH credit.

Control Engineering has a wireless page