Hello, everyone!
Today, I would like to share with you an article about WiMAX - IEEE 802.16-2004 and IEEE 802.16e-2005 Standard.
WiMAX stands for World Interoperability for Microwave Access. It is a broadband wireless technology that supports fixed, nomadic, portable and mobile access. WiMAX is largely supported by the computer and the telecommunications industry, cost-effective and standard base. It is engineered to deliver the latest type of ubiquitous fixed and mobile services such as Voice 0ver Internet Protocol (VoIP), Information Technology and Video at very low cost.
WiMAX systems are able to cover a large geographical area, up to 50 km and to deliver significant bandwidth to end-users up to 72 Mbps. To meet the requirements of different types of access, two versions of WiMAX have been defined.
The first is based on IEEE 802.16-2004 and is optimised for fixed and nomadic access. The initial WiMAX Forum CERTIFIED products will be based on this version of WiMAX.
The second version is designed to support portability and mobility, and will be based on the IEEE 802.16e amendment to the standard.
1. IEEE 802.16-2004 Standard
The IEEE 802.16 standard which includes Medium Access Control (MAC) and physical (PHY) layer specifications, aims at supporting Internet services over wireless metropolitan area networks (WMAN). It is also an alternative to traditional wired networks, such as Asymmetric Digital Subscriber Line (ADSL) and cable-modem.
There are two modes (two different air-interfaces) defined in WiMAX networks:
* Point-to-Multi-Point (PMP)
* Mesh modes.
In PMP mode, two SSs (Subscriber Stations) can only communicate through BS (Base Station), while in Mesh mode, two SSs can communicate directly.
In the PHY layer, the IEEE 802.16 standard adopts the orthogonal frequency division multiplexing (OFDM), which is a multicarrier modulation scheme. The IEEE 802.16 standard has two OFDM-based modes:
* OFDM
* Orthogonal frequency division multiplexing access (OFDMA).
Both of these technologies allow subcarriers to be adaptively modulated (e.g., QPSK, 16-QAM, and 64-QAM), depending on transmission distance and noise. Moreover, OFDMA has scalability to provide efficient use of bandwidth.
The MAC layer of IEEE 802.16 standard was originally designed for the PMP mode. On the later amendments of the IEEE 802.16a and the IEEE 802.16d, the mesh mode was included.
The IEEE 802.16a adopts OFDM to provide greater spectral efficiency and to mitigate interference. IEEE 802.16d covers most of the quality of service (QoS) aspects.
The IEEE 802.16e introduces scalable OFDMA into the standard, and supports mobile communications. With handover mechanisms, WiMAX is thus able to support mobile communications at vehicular speeds.
2. IEEE 802.16e -2005 Standard
On July 2002, a study group called IEEE 802.16 Mobile WirelessMAN Task Group was initiated to produce an amendment covering the PHY and MAC layers for combined, fixed, and mobile operations in the licensed band range. The amendment was approved in December 2005 and the new standard called IEEE 802.16e-2005 was published in February 2006.
The scope of this standard is to provide mobility enhancement support for SS moving at the vehicular speed, in addition to corrections to 802.16-2004 fixed operation that was developed as IEEE 802.16-2004 and published along with IEEE 802.16e-2005.
802.16e introduces many changes to PHY and MAC layer protocols owing to mobility support, which required addressing new issues that were not required in 802.16-2004, such as handoff and power management.
The mobile WiMAX PHY layer is based on OFDMA technology. The network is an IP end-to-end conventional architecture that provides high-speed broadband. With a modification from fixed WiMAX to mobile WiMAX, the PHY layer also supports BWs from 1.25 to 20 MHs.
The standard is designed to accommodate either TDD or FDD (Time / Frequency Division Duplexing) deployments, allowing for both full- and half-duplex terminals in the FDD case.
The MAC layer was initially designed specifically for the point-to-point wireless access environment. It supports higher layer or transport protocols such as asynchronous transfer mode (ATM), Ethernet, or IP, and is designed to easily accommodate future protocols that have not yet been developed.
MAC layer specification practices considerable departures from 802.16-2004 to provide support for mobility. It adds support for handoff and power management.
QoS Support - 802.16e defines new scheduling mechanisms: the extended real-time polling service (ErtPS), which is based on two services defined in 802.16-2004; the unsolicited grant service (UGS); and the real-time polling service (rtPS).
ErtPS is similar to UGS in providing unicast grants, thus saving the delay incurred for requesting the bandwidth. However, ErtPS allocations are dynamic as rtPS while UGS allocations are fixed. The ErtPS is introduced to support real-time service flows that generate periodical variable sised data packets. Thus, ErtPS is especially important to support VoIP, since it allows for managing traffic rates and improves latency and jitter.Handover Support - 802.16e includes new MAC-level request/grant mechanisms to achieve similar seamless mobility as that provided for cellular users. 802.16e includes fast base station switching and hard handoff mechanisms for inter-cell and inter-sector handover.
In 802.16e, handoff process may be triggered for two reasons. One is due to fading of the signal, interference level, etc. within the current cell or sector. The other is due to the fact that another cell can provide a higher level of QoS for the mobile station (MS).
Furthermore, 802.16e supports macro diversity handovers and inter-technology roaming. Macro-diversity handovers support handoffs between different sised cells, while inter-technology roaming addresses MS handoffs from BS to backhaul or wired network by providing roaming authentication mechanisms.Power management - It is a critical process for mobile applications to enable efficient operation of the MS. 802.16e defines two power management operations, sleep mode and idle mode.
Idle mode operation is carried out by MS when the MS does not intend to register to a specific BS as the MS traverses a region covered by multiple BS. The advantage of idle mode for the BS is to avoid multiple handoffs and other normal operations while the SS is traversing the region, and for the BS and network is to avoid unnecessary handoffs from an inactive MS. When the MS enters the idle mode, it needs to periodically check for broadcast messages sent by the BS to see if new downlink frames have been sent to it.
Sleep mode operation is a state in which MS sends a request to be unavailable to the BS. If the BS responds with approval, the MS is provided with a sleep interval time vector that determines the length of the sleep mode period. The benefit of the sleep mode operation is to minimise MS power usage and utilisation of the air interface resources of the BS. While the MS is in the sleep mode, the MS scans other BSs to collect information required for handover during the sleep mode.
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