The WiMax Forum harmonizes IEEE 802.16 and ETSI HIPERMAN into a WiMax standard. The core components of a WiMax system include the subscriber station (SS), also known as the customer premise environment (CPE), and the base station (BS). A BS and one or more CPEs can form a cell with a point-to-multipoint (PTM) structure, in which the BS acts as central control over participating CPEs. The WiMax standard specifies the use of licensed and unlicensed bands within the 2- to 11-GHz range, allowing non-LOS (NLOS) transmission, which is highly desired for wireless service deployment, as NLOS does not require high antennas in order to reach remote receivers, which reduces site interference and the deployment cost of CPE. NLOS raises multipath transmission issues such as signal distortion and interference. WiMax employs a set of technologies to address these issues:
» OFDM : As discussed earlier in this chapter, OFDM uses multiple orthogonal narrowband carriers to transmit symbols in parallel, effectively reducing ISI and frequency-selective fading.
» Subchannelization : The subchannelization of WiMax uses fewer OFDM carriers in the upstream link of a terminal, but each carrier operates at the same level of the base station. Subchannelization extends the reach of upstream signals from a terminal and reduces its power consumption.
» Directional antennas : Directional antennas are advantageous in fi xed wireless systems because they are more powerful in picking up signals than are omnidirectional antennas; hence, a fixed CPE typically uses a directional antenna, while a fixed BS may use directional or omni directional antennas.
» Transmit and receive diversity : WiMax may optionally employ a transmit and receive
diversity algorithm to make use of multipath and refl ection using MIMO radio systems.
» Adaptive modulation : Adaptive modulation allows the transmitter to adjust modulation schemes based on the SNR of the radio links. For example, if the SNR is 20 dB, 64 QAM will be used to achieve high capacity. If the SNR is 16 dB, 16 QAM will be used, and so on. Other NLOS schemes of WiMax, such as directional antenna and error correction, are also used.
» Error-correction techniques : WiMax specifi es the use of several error-correction codes and algorithms to recover frames lost due to frequency-selective fading or burst errors. These codes and algorithms are Reed Solomon FEC, convolutional encoding, interleaving algorithms, and Automatic Repeat Request (ARQ) for frame retransmission.
» Power control : In a WiMax system, a BS is able to control power consumption of CPEs by sending power-control codes to them. The power-control algorithms improve overall performance and minimize power consumption.
» Security : Authentication between a BS and an SS is based on the use of X.509 digital certificates with RSA public key authentication. Traffi c is encrypted using Counter Mode with Cipher Block Chaining Message Authentication Code Protocol (CCMP) which uses Advanced Encryption Standard (AES) for transmission security and data integrity authentication. WiMax also supports Triple Data Encryption Standard (3DES).
Initially, the WiMax Forum has focused on fi xed wireless access for home and business users using outdoor antennas (CPEs), and indoor fi xed access is under development. A base station may serve about 500 subscribers. WiMax vendors have begun to test fi xed wireless broadband access in metropolitan areas such as Seattle. Due to its relatively high cost, the major targets of this technology are business users who want an alternative to T1, rather than residential home users. A second goal of the forum is to address portable wireless access without mobility support, and another is to achieve mobile access with seamless mobility support (802.16e). Recall that a Wi-Fi hotspot offers wireless LAN access within a limited coverage of an AP; the WiMax Forum plans to build MetroZones that allow portable broadband wireless access. A MetroZone comprises base stations connected to each other via LOS wireless links, and 802.16 interfaces for laptop computers or PDAs that connect to the 「 best 」 base station for portable data access. This aspect of WiMax seems more compelling in terms of potential data rate compared with 3G cellular systems.
Like the Wi-Fi forum, the WiMax forum aims at providing certifi cation of WiMax products in order to guarantee interoperability. In March 2005, Alvarion, Airspan, and Redline began to conduct the industry's fi rst WiMAX interoperability test. WiMax chips for fixed CPEs and base stations developed by Intel will be released in the second half of 2005, and WiMax chips for mobile devices will be released in 2007. At the time of this writing, some WiMax systems were expected to go into trial operation in late 2005.
Source of Information : Elsevier Wireless Networking Complete 2010