Network Entry, Initialization & Ranging Process | WiMAX Mobility Management

1. Network Entry and Initialization

A SS needs to successfully complete the network entry process with a desired BS to join the network. The network entry process is composed of four stages. The first stage is capability negotiation. After successful completion of initial ranging, the SS will request the BS to describe its available modulation capability, coding schemes, and duplexing methods. During this stage, the SS shall acquire a DL channel. Once the SS finds a DL channel and synchronizes with the BS at the PHY level, the MAC layer will look for DCD (downlink channel descriptor) and UCD (uplink channel descriptor) to get modulation and other parameters. The SS remains in synchronization with the BS as long as it continues to receive the DL-medium access protocol (MAP) and DCD messages. Finally, the SS will receive a set of transmission parameters from UCD as its UL channel. If no UL channel can be found after a suitable timeout period, the SS shall continue scanning to find another DL channel. Once the UL parameters are obtained, the SS shall perform the ranging process.

The second stage is authentication. In this stage, the BS authenticates and authorizes the SS. Then the BS performs key exchange with the SS, such that the provided keys can enable the ciphering of transmission data. The third stage is registration. To register with the network, the SS and the BS will exchange registration request/response messages. The last stage is to establish IP connectivity. The SS gets its IP address and other parameters to establish IP connectivity. After this step, operational parameters can be transferred and connections can be set up.

2. Ranging Process

Ranging is the process of acquiring the correct timing offset and power adjustments such that the SS's transmission is aligned to the BS's timing. Two types are supported for ranging process in the IEEE 802.16e-2005 specification. One is initial ranging, and the other is periodic ranging.

Initial ranging is performed during network initialization and registration/reregistration to allocate CDMA codes in UL ranging opportunities. Then the SS is allowed to join the network to acquire correct TX parameters (timing offset and TX power level). On the other hand, periodic ranging is performed when transmission is on-going on a periodic basis. It uses regular UL burst to allow SS to adjust TX parameters so that the SS can maintain UL communications with the BS.

IEEE 802.16e-2005 Specification

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.

Figure 1: IEEE 802.16e-2005 protocol layers.

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.

Figure 2: Medium Access Control (MAC) layer functionalities in the IEEE 802.16e-2005 specification.

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.

Overview of the Mobile WiMAX Specification

IEEE 802.16 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 Digital Subscriber Line (DSL) and cable-modem. There are two modes defined in WiMAX networks: point-to-multiple-points (PMP) mode and mesh mode.

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 and 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 802.16b 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. We summarize the history of the evolution of the IEEE 802.16 standard in Figure 1.

Figure 1: Evolution of the IEEE 802.16 standard.

The IEEE 802.16 working groups on broadband access standards developed the IEEE 802.16 WirelessMAN standard for WMANs. On the other hand, the WiMAX forum was formed in June of 2001 to ensure interoperability among 802.16 products from different vendors. These groups and their activities may help popularizing WiMAX networks and systems by bringing vendors together and improving the specifications.

According to the IEEE 802.16 specification, the non-line-of-sight (NLOS) transmission range is 4 miles. With the combination of soft-switch technologies, a WiMAX network can work as a wireless "last mile" and make a viable alternative to the Public switched telephone network (PSTN) for VoIP services. In addition, a WiMAX network can work as a point-to-point backhaul trunk with a transmission capability of 72 Mbps at a transmission distance over 30 miles. With its technological advantages of throughput, power, transmission range, and versatility, WiMAX might be a strong competitor of other technologies, such as WiFi and 3G. Therefore, from both economical and technical points of view, WiMAX could be an appealing choice for broadband wireless services.

WiMAX Network Architecture

WiMAX has an IP-based wireless access architecture, which contains three parts: user terminal devices, access service network (ASN), and core service network (CSN). A user terminal device can be a fixed or portable/mobile terminal device, which supports the fixed/nomadic/mobile usage scenarios. Each device can establish a connection link to a WiMAX Base Station (BS), and perform authentication and registration through an access gateway in the CSN. The system architecture is illustrated in Figure 2.

Figure 2: Internet Protocol (IP)-based wireless access architecture of WiMAX.

A mobile WiMAX network has a similar architecture as a cellular network, where PMP links are between each BS and multiple Subscriber Stations (SSs). Each BS provides frequency and timing reference to SSs for synchronization purpose. The detailed MAC layer protocols and message sequences will be described later.