WiMAX

A broadband wireless access (BWA) solution based on the standards recommendations from the IEEE 802.16 Working Group and the European Telecommunications Standards Institute (ETSI) HiperMAN group. WiMAX is promoted by the WiMAX Forum, a special interest group with members from the manufacturing, carrier, service provider, and consulting communities.Where line of sight (LOS) can be achieved, the WiMAX cell radius is as much as 50 kilometers (31 miles). Under non-line of sight (NLOS) conditions, the maximum cell radius is approximately 9 kilometers (5.6 miles). WiMAX standards provide for aggregate raw bandwidth up to about 70 Mbps per base station (BS), although the throughput is much less due to overhead, as well as issues of LOS, link distance, air quality, electromagnetic interference (EMI), radio frequency interference (RFI), and other signal impairments. Mobile network deployments described in 802.16e are expected to provide up to 15 Mbps of aggregate raw bandwidth within a cell radius of up to 3 kilometers. WiMAX supports a maximum signaling rate of 70 Mbps and the maximum throughput of approximately 40 Mbps over the shortest distance between the BS and the user antenna under LOS conditions. Over the maximum distance of 50 kilometers under LOS conditions, or the maximum distance of 9 kilometers under NLOS conditions, throughput drops considerably. The transmission rate is symmetrical, i.e., the same for the uplink (upstream), i.e., the link from the remote terminal back to the BS, as for the downlink (downstream). The sole exception to this symmetry is in the case of full-featured CPE at the cell edge, where uplink transmission rates are constrained by power limitations. WiMAX employs orthogonal frequency division multiplexing (OFDM), which subdivides the spectrum into a number of independent, narrowband subcarriers, across which it sends the signal in parallel fashion.Through sub-channelization on the uplink,WiMAX concentrates signal power into fewer OFDM subcarriers, thereby extending the reach of the system, mitigating the effects of physical obstructions in an NLOS environment and reducing CPE power consumption. Multiple-input multiple-output (MIMO) antennas employ space/time coding to compensate for multipath fading over long loops. At the customer premises is an adaptive, passive array antenna known as a pizza box, as it is about the size and shape of a pizza box. Rate-adaptive modulation dynamically adjusts the signal modulation technique of each carrier to compensate for variations in signal quality at that carrier frequency. Reed-Solomon forward error correction (FEC) is employed to deal with issues of signal quality and automatic repeat request (ARQ) is employed to request retransmission of any remaining errored frames. 802.16 specifications include several multiplexing options. Frequency division duplex (FDD) supports both half-duplex (HDX) and full duplex (FDX) communications, and time division duplex (TDD) supports half-duplex (HDX), only.The 802.16 security protocol is built on enhancements to the privacy key management (PKM) developed for cable modem communications.The protocol uses X.509 digital certificates with Rivest-Shamir-Adleman (RSA) encryption for authentication and key exchange. Traffic encryption options are data encryption standard (DES) and advanced encryption standard (AES). 802.16 specifications include convergence sublayers designed for mapping services to and from 802.16 connections. The ATM convergence sublayer is for ATM services and the packet convergence sublayer is for packet services such as IPv4, IPv6, Ethernet, and Virtual LAN (VLAN). WiMAX offers differential quality of service (QoS) based on four polling schedules: