The IEEE 802.16e-2005 specification defines the mobile WiMAX network protocols and the related message sequences. The specification consists of the PHY and the MAC layers, as shown in Figure 1. The MAC layer is composed of a security sublayer, a MAC common part sublayer, and a convergence sublayer.
Data transmission between a BS and a SS at the PHY layer relies on the resource allocation of data burst through the OFDMA scheme. A BS can transmit to multiple SSs concurrently in the downlink (DL) direction in separate subchannels or separate symbols; similarly, multiple SSs can transmit to the same BS concurrently in the uplink (UL) direction in separate subchannels or separate symbols. Each channel width is from 1.25 to 20 MHz, which spreads to different frequency bands. Therefore, the PHY layer supports orthogonal subchannels for multiple accesses. This design can reduce interference and improve capacity. Moreover, it has the advantages of flexible subchannelization and bandwidth allocation.
The security sublayer is to ensure the privacy of subscribers across the WiMAX network by encrypting connections between a SS and a BS. In addition, a BS can protect against unauthorized access to data transport services by enforcing encryption of the associated service flows across the network. Privacy employs an authenticated client/server key management protocol in which a BS, the server, controls distribution of keying material to a SS, the client. Additionally, the basic privacy mechanisms are strengthened by adding digital-certificate-based SS authentication to its key management protocol.
Over the security sublayer, there are the MAC common part sublayer and the convergence sublayer. The MAC common part sublayer utilizes a shared medium mechanism to efficiently handle the resource of communication links. On top of the MAC common part sublayer is the convergence sublayer, which includes MAC service access points (APs). The MAC layer functionalities in IEEE 802.16e-2005 specification are illustrated in Figure 2.
In summary, the IEEE 802.16e-2005 specification offers improvements over the technology specified by the original fixed WiMAX standard. These significant improvements can cost-effectively deliver broadband services to end-users, offering increased performance in NLOS environments for mobility and fixed indoor applications. These improvements can be categorized as follows.
§ Mobility: The support for mobility is the major feature of mobile WiMAX, which introduces new MAC for handover and allows a SS to maintain a connection when moving from one BS to another. Mobile WiMAX is designed to support mobility applications up to 160 km/h.
§ High availability: High connection availability in NLOS environments can be supported in mobile WiMAX by using advanced antenna, channel coding, subchannelization, and dynamic modulation technologies to increase link budget.
§ NLOS performance: New technologies have been introduced in mobile WiMAX. These include support for intelligent antenna technology, such as Multiple-Input Multiple-Output (MIMO) and adaptive antenna system (AAS), high-performance coding, such as turbo coding (TC), and a Hybrid Automatic Repeat reQuest (HARQ) mechanism for increasing NLOS performance.
§ Security: Based on the security features of the fixed WiMAX standard, the mobile WiMAX specification introduces a number of enhancements. For example, the AES as well as 3DES are now a mandatory feature. New high-performance coding schemes, such as TC and low-density parity check (LDPC), are included. These features enhance the security of the mobile WiMAX air interface.
§ QoS: Both the connection and service-type-based QoS are designed to meet the requirements of mobile broadband services. These two QoS mechanisms manage both UL and DL directions and support two-way traffic, such as VoIP. The mobile WiMAX QoS has the features of service multiplexing, low data latency, and varying granularity to support real-time broadband multimedia applications.
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