Thursday, September 3, 2009

Orthogonal Frequency Division Multiplexing (OFDM)

Some of the key technologies used in WiMAX systems include orthogonal frequency division multiplexing, frequency reuse, adaptive modulation, diversity transmission and adaptive antennas.

Orthogonal Frequency Division Multiplexing (OFDM)

OFDM is a process of transmitting several high speed communication channels through a single communication channel using separate sub-carriers ( frequencies) for each radio channel. The use of OFDM reduces the effects of multi-path and delay spread, which is especially important for lower frequencies and near line of sight (NLOS) transmission.

Multi-path propagation is the transmission of a radio signal which travels over two or more paths from a transmitter to a receiver. Multi-path transmission can cause changes in the received signal level as delayed signals can either add or subtract from the received signal level. Multi-path is not usually a challenge on systems that use higher frequencies as these systems tend to use highly directional (high-gain) antennas for direct line of sight transmission.

Multi-path propagation is frequency dependent meaning that the multiple paths radio signals travel will vary depending on its’ frequency.

Figure 1illustrates how a transmitted signal may travel through multiple paths before reaching its destination. In this example, the same signal is reflected off an office building where it is received by the subscriber device. The reflected signal is delayed (travels a longer path) and subtracts from the direct signal resulting in a dead spot (fade) at the receiver. Furthermore, mutli-path propagation is sensitive to frequency and that distortion occurs at different points when other frequencies are used. When a different frequency is used, the reflected signal is redirected and it does not subtract from the direct signal.

Figure 1: Multi-path Propagation

For a wide radio channel that is divided into several sub-carriers, each subcarrier channel operates at a different frequency and can have different transmission characteristics than other sub-carriers. Because multiple sub-carriers are typically combined for a single subscriber, this can reduce the effects of multi-path fading.

The use of multiple sub-carriers also has the effect of reducing the symbol rate, which can reduce the effects of delay spread. Delay spread is a product of multi-path propagation where symbols become distorted and eventually overlap due to the same signal being received at a different time. It becomes a significant problem in mountainous areas where signals are reflected at great distances. Delay spread can be minimized by either using an equalizer to adjust for the multi-path distortions or to divide a communication channel into sub-carriers (e.g. OFDM) where each sub-carrier transfers data at a much slower data transmission rate thereby reducing the effects of delay spread.

Figure 2 demonstrates how OFDM divides a single radio channel into multiple coded sub-channels. A high-speed digital signal is divided into multiple lower-speed sub channels that are independently from each other and can be individually controlled. The OFDM process allows bits to be sent on multiple sub channels. The channels selected can be varied based on the quality of the sub channel. In this figure, a portion of a sub channel is lost due to a frequency fade. As a result of the OFDM encoding process, the missing bits from one channel can be transmitted on other channels.

Image from book Figure 2: Orthogonal Frequency Division Multiplexing (OFDM)
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