While cellular systems such as time division multiple access (TDMA) use time to segment a communication channel and CDMA segments according to spreading codes, OFDM segments according to frequency.

It divides the spectrum into a number of equally spaced carriers and allocates a portion of a system's information on each tone or subcarrier. OFDM can be viewed as a form of frequency division multiplexing (FDM). It allows the bundling of data over narrowband carriers transmitted in parallel over different frequencies.

High bandwidth is achieved by using these parallel subchannels that are spaced apart at precise frequencies while being as close as possible without overlapping or interfering. Thus, they're orthogonal, as made possible by the FFT.

To be relatively immune to effects like selective channel-fading, OFDM systems incorporate error correction. The design of coding for an OFDM system is central to making OFDM an effective method. However, building OFDM-based systems has multiple considerations, which depend on the operating environment and application requirements.

Figure 1: Each subcarrier is modulated at a low data rate, making symbol duration longer. A guard interval or cyclic prefix is appended to each symbol.

Mobile systems typically operate under challenging and unpredictable channel conditions. The wireless channel is variable due to factors such as multipath and shadow fading, time dispersion, and Doppler or delay spread. These factors are all related to variability, which is introduced by the user's mobility and the various environments that may be encountered by a receiver.

Signal reflections
Multipath occurs as a transmitted signal, which is reflected by objects in the environment between the transmitter and receiver. These objects include buildings, trees, hills or even automobiles. The transmitted signal arrives at the receiver in various paths of different length.

Time dispersion represents distortion to the signal. This is manifested by the spreading in time of the modulation symbols. It occurs when the coherence bandwidth of the channel is smaller than the modulation bandwidth. Time dispersion leads to intersymbol interference, where the energy from one symbol spills over into another symbol, thus increasing the BER.

Doppler spread represents the random changes in the channel introduced as a result of a user's mobility and a relative motion of objects in the channel. Doppler has the effect of shifting or spreading the frequency components of a signal.

The coherence time of the channel is the inverse of the Doppler spread and is a measure of the speed at which the channel characteristics change. In effect, this determines the rate at which fading occurs. When the channel's rate of change is higher than the modulated symbol rate, fast fading occurs. Slow fading, on the other hand, occurs when the channel changes are slower than the symbol rate.

OFDM can be used to facilitate single frequency networks (SFN). In this configuration, the channel bandwidth is divided into many narrow subcarriers that are transmitted concurrently. Each subcarrier is modulated at a low data rate, making symbol duration longer. A guard interval or cyclic prefix is appended to each symbol (Figure 1, above).

This period of repeated data allows the multiple arrivals (due to multipath) to combine constructively while allowing the orthogonality of the subcarriers to be maintained. The guard interval's duration is typically set to capture the most important arrivals. The performance limits are partially determined by interference from signals with long delays that exceed the maximum delay difference for constructive combining outside the guard interval window.

Figure 2: FLO-transmitted signals are organized into super frames.

Wireline, wireless
OFDM is used in various wireline and wireless technologies including ADSL, 802.11a/g/n, digital audio broadcasting, DTV and UWB. It is also used in mobile broadcast technologies such as forward link only (FLO), which offers mobile TV services via the MediaFLO system.

Many design trade-offs must be considered when developing an OFDM-based system. The most fundamental trade-off is the number of subcarriers (transform size) and the guard interval duration.

The FLO PHY layer uses a transform size of 4,096 subcarriers (4K mode) and a guard interval defined as one-eighth of the nominal FLO OFDM symbol duration. The 4K mode provides improved mobile performance compared with an 8K mode, while retaining a suf- ficiently long guard interval that is useful in fairly large SFN cells. Robust performance can then be maintained to greater than 200kph, with graceful degradation beyond. This is supported by the FLO pilot structure (used for channel estimation), which enables receivers to handle delay spreads greater than the guard interval.

In OFDM, information is impressed on a tone by phase and amplitude modulation. Each subcarrier is typically modulated with QPSK or QAM. The FLO air interface supports the use of QPSK, 16QAM and layered modulation techniques. It also incorporates error correction and coding techniques, including turbo inner and Reed-Solomon outer codes.

Rapid TV channel change is achieved through an optimized pilot and interleaver structure design, which also assures time diversity. The pilot structure and interleaver designs optimize channel utilization while ensuring fast channel change.

FLO-transmitted signals are organized into super frames (Figure 2, above). Each super frame consists of four frames of data, including the TDM pilots, overhead information symbols (OIS), and frames containing wide- and local-area data.

The TDM pilots are provided to allow acquisition of the OIS, which describes the location of the data for each media service carried in the super frame.

Frequency diversity
Each super frame consists of 200 OFDM symbols per megahertz of allocated bandwidth (1,200 symbols for 6MHz). Each symbol contains seven interlaces of data-bearing subcarriers.

Each interlace is uniformly distributed in frequency to achieve the full frequency diversity within the available bandwidth. These interlaces are assigned to logical channels that vary in duration and number of interlaces used.

This provides flexibility in the time diversity achieved by a given data source. Lower data rate channels can be assigned fewer interlaces to improve time diversity, while higher data rate channels may use more interlaces to minimize the radio's on-time. The acquisition time for both low and high data rate channels is the same. Frequency and time diversity are maintained without compromising acquisition time.

Multicast logical channels (MLCs) are used to carry realtime content at variable rates to obtain statistical multiplexing gains that are possible with variable rate codecs. Each MLC has a specific, independent coding rate and modulation, which provides support for various reliability and QoS depending on the application requirements.

To minimize power consumption, the FLO multiplexing scheme enables device receivers to just demodulate the content of one or more logical channels that it is interested in.

The principal driving force of OFDM's increased popularity is the desire for faster wireless technologies and increase in multimedia applications, which require higher speeds and spectral efficiency. This is particularly illustrated in the use of OFDM in a modern technology such as FLO, which was designed from OFDM's fundamental principles to support mobile TV. This enabled it to deliver optimal channel change time and performance for mobile devices without compromising power consumption.