WiMAX - Physical Layer
The WiMAX physical layer is based on orthogonal frequency division multiplexing. OFDM is the transmission scheme of choice to enable high-speed data, video, and multimedia communications and is used by a variety of commercial broadband systems, including DSL, Wi-Fi, Digital Video Broadcast-Handheld (DVB-H), and MediaFLO, besides WiMAX.
OFDM is an elegant and efficient scheme for high data rate transmission in a non-line-of-sight or multipath radio environment.
Adaptive Modulation and Coding in WiMAX
WiMAX supports a variety of modulation and coding schemes and allows for the scheme to change on a burst-by-burst basis per link, depending on channel conditions. Using the channel quality feedback indicator, the mobile can provide the base station with feedback on the downlink channel quality. For the uplink, the base station can estimate the channel quality, based on the received signal quality.
The following table provides a list of the various modulation and coding schemes supported by WiMAX −
|
Downlink |
Uplink |
| Modulation |
BPSK, QPSK, 16 QAM, 64 QAM; BPSK optional for OFDMA-PHY |
BPSK, QPSK, 16 QAM; 64 QAM optional |
| Coding |
Mandatory: convolutional codes at rate 1/2, 2/3, 3/4, 5/6
Optional: convolutional turbo codes at rate 1/2, 2/3, 3/4, 5/6; repetition codes at rate 1/2, 1/3, 1/6, LDPC, RS-Codes for OFDM-PHY |
Mandatory: convolutional codes at rate 1/2, 2/3, 3/4, 5/6
Optional: convolutional turbo codes at rate 1/2, 2/3, 3/4, 5/6; repetition codes at rate 1/2, 1/3, 1/6, LDPC |
PHY-Layer Data Rates
Because the physical layer of WiMAX is quite flexible, data rate performance varies based on the operating parameters. Parameters that have a significant impact on the physical-layer data rate are channel bandwidth and the modulation and coding scheme used. Other parameters, such as number of sub-channels, OFDM guard time, and oversampling rate, also have an impact.
Following is the PHY-layer data rate at various channel bandwidths, as well as modulation and coding schemes.
