W I MAX   O VERVIEW

Following the success of the Internet technology, broadband data communication services have been provisioned to the expert communities for decades, which for the wired and fiber connections have been achieved with the turn of the century. For wireless it is due any time within the next decade where superior mass production of quality wireless components to extend the frequency range and overcome shadowing and multipath fading issues using super sensitive receivers [38]. Now, with the industry capable of providing the WiMAX technology for superiority of virtually nil infra-structure costs, we are able to offer a data-enabled very low cost wireless metropolitan area network (WMAN) style wireless broadband access (WBA) solutions which in long run may overshadow competitive solutions [39] due to the fact that WiMAX is able to provide broadband wireless access with wide service coverage, high data throughput, high mobility and greater service flexibility [40, 41]. Figure 4.2 shows a simplified WiMAX network architecture, which consists of the access service networks (ASNs; see Figure 4.2 (a)) and the connectivity service networks (CSNs; see Figure 4.2 (b)).

An ASN provides radio access (such as radio resource management, paging and location management) to the WiMAX mobile station (MS; Figure 4.2 (e)). The ASN comprises ASN gateways (ASN-GWs; see Figure 4.2 (c)) and WiMAX BSs (see Figure 4.2 (d)). Every ASN-GW connects to several BSs. The ASN-GWs are also connected to each other to coordinate MS mobility. A CSN consists of network nodes such as the mobile IP (MIP) home agent (HA; see Figure 4.2 (f)) [3], the authentication authorization, and accounting (AAA) server (see Figure 4.2 (g)) and the dynamic host configuration protocol (DHCP) server (see Figure 4.2 (h)). The CSN provides IP connectivity (such as Internet access and IP address allocation) to a WiMAX MS and interworks with the ASNs to support capabilities such as AAA and mobility management. Before an MS is allowed to access WiMAX services, it must

be authenticated by the ASN-GW (which serves as the authenticator) and the AAA server in

Figure 4.2 Simplifiied WiMAX Network Architecture

The WiMAX Physical (PHY) and Media Access Control (MAC) layers are defined in IEEE 802.16 standard to support multiple services with point-to-multipoint and mesh broadband wireless access [35]. The point-to-multipoint mode defines one-hop communication between a BS and an MS, while the mesh mode allows traffic to be directly exchanged and forwarded among neighboring BSs. IEEE 802.16 is initially designed as an access technology for WMAN. The first specification IEEE 802.16-2004 targets on fixed and nomadic accesses. In IEEE 802.16e-2005 amendment, the IEEE 802.16e system (Mobile WiMAX) further provides functions to facilitate mobile accesses. We introduce the functions of MAC and PHY layers in the following subsections. Details of WiMAX technology can be found in [38]. Figure 4.3 illustrates the IEEE 802.16 protocol stack. The functions of the WiMAX PHY and MAC layers are described in the following subsections.

4.3.1 The Media Access Control Layer 

There are three sublayers in IEEE 802.16 MAC layer: service-specific convergence sublayer (CS; see Figure 4.3 (a)), the MAC common part sublayer (see Figure 4.3 (b)), and the security sublayer (see Figure 4.3 (c)).

Figure 4.3 IEEE 802.16 Protocol Stack

The service-specific CS performs packet classification, header suppression, and converts packets between the upper layer and the MAC layer. The IEEE 802.16 currently supports packet CS and ATM CS to interface with IP and ATM protocol layers, respectively. In IEEE 802.16, the connections between the MSs and the BSs can be identified with unique connection identifications (CIDs). The packet CS may check the IP or TCP/UDP header of a packet to determine its CID. Besides the CID mapping, the CS may perform the optional payload header suppression to eliminate the redundant parts of the packets during the transmission over the air interface.

The MAC common part sublayer provides the medium access, connection management, and QoS functions that are independent of specific CSs. After the packets are processed by the CS, the MAC common part may perform automatic repeat request (ARQ) for retransmitting lost packets. ARQ is optional in IEEE 802.16 but is mandatory for IEEE 802.16e.

In IEEE 802.16, QoS functions are implemented in the MAC common part sublayer. Several service classes are defined to satisfy various QoS requirements. For example, a VoIP connection is often associated with unsolicited grant service (UGS) to support constant bit-rate (CBR) or CBR-like flows with constant bandwidth allocation. According to the QoS associated, the BS schedules radio resources with various scheduling disciplines, such as round-robin and first-in-first-out.

The security sublayer provides privacy and protections through encryption, decryption, and authentication. In IEEE 802.16, an MS is requested to perform the authentication and authorization before attaching to a WiMAX network. During the authorization procedure, the MS negotiates with the BS to generate the session key. To perform packet encryption and decryption, each connection is linked with a security association (SA), which contains the security information and settings such as encryption keys. Packet encryption and decryption are exercised based on the information in the SA.

Before accessing the WiMAX network, an MS should perform a complete spectrum search and synchronize the time and frequency with a BS through the ranging procedure. Then the MS starts the network entry procedure to negotiate the capabilities with the BS and performs authorization process to generate the keys used between the MS and the BS. Finally, the MS obtains an IP address from the BS, and establishes data connections with the BS.

4.3.2 The Physical Layer 

In the PHY layer (see Figure 4.3 (d)) IEEE 802.16 defines several specifications for different frequency ranges and applications. For example, orthogonal frequency division modulation (OFDM) is used for non-line-of-sight operations in the frequency bands below 11 GHz. By

extending the OFDM technology, orthogonal frequency division multiple access (OFDMA) allows one channel to be shared by multiple users. The IEEE 802.16 standard defines a set of adaptive modulation and coding rate configurations that can be used to trade off data rate against system robustness under various wireless propagation and interference conditions. The allowed modulation types are binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16QAM), and 64QAM [35].

Several duplexing technologies are provided in IEEE 802.16. In time division duplex (TDD), a WiMAX frame consists of a downlink subframe and an uplink subframe and a short transition gap is placed between the downlink and uplink subframes for receive and transmission transitions in the radio. The gap between the downlink burst and the subsequent uplink burst is called transmit/receive transition gap (TTG). The gap between the uplink and the subsequent downlink is called receive/transmit transition gap (RTG).

The duration of an OFDM symbol includes the useful symbol time and a prefix. In OFDM, all users within the same cell or sector use orthogonal subcarriers to carry the OFDM symbols.

The OFDM symbol uses a fixed-length cyclic prefix (CP) to counteract the intersymbol interference. The ratio of the CP length to the useful symbol time is defined as the guard interval, which is used by the receiver to collect signals from multiple paths and improve system performance.