Handover Process | WiMAX Mobility Management

According to the scope of node movement, mobility can be divided into micro-mobility and macro-mobility. On the link layer, most access networks provide mobility by having an access router keep track of the specific AP to which a MS is attached. The localized mobility between pico-cells (probably heterogeneous cells) in the same subnet and the mobility between subnets in one domain is called micro-mobility, whereas the mobility between domains in wide-area wireless networks is called macro-mobility. The mobility solutions like Mobile IP are classified as macro-mobility. But Mobile IP is not suitable for micro-mobility due to its signaling overhead, handover latency, and transient packet loss.

1 Hard Handover and Soft Handover

Hard handover is mandatory to be supported in mobile WiMAX networks. Hence, break-before-make operations may happen during the handover process. In other words, link disconnection may occur and throughput may degrade. Therefore, various levels of optimization are demanded to reduce association and connection establishment with the target BS. These optimization methods are not clearly defined in the IEEE 802.16e specification, so they should be supported on specific WiMAX systems and products.

On the contrary, soft handover is optional in mobile WiMAX networks. Two schemes, macro-diversity handover (MDHO) and fast Base Station switching (FBSS) are supported. In case of MDHO, MS receives from multiple BSs simultaneously during handover, and chooses one as its target BS. As for FBSS, the MS receives from/transmits to one of several BSs (determined on a frame-by-frame basis) during handover, such that the MS can omit the decision process of selecting the target BS to shorten the latency of handover.

2 MAC Layer Handover Procedure

The handover procedure in IEEE 802.16e-2005 is divided into MAC- and PHY-layer handover. Looking at the MAC-layer handover procedure, it is divided into the network topology acquisition phase and the handover process phase according to its performing sequence.

In the network topology acquisition phase, as illustrated in Figure 1, three functions are performed, namely network topology advertisement, MS scanning for neighboring BSs, and association procedure. After receiving a neighbor advertisement message broadcast from the serving BS, the MS gets all the neighboring BSs of its current serving BS. The MS can then perform synchronization with each neighboring BS, and then continue to the handover process phase.

Figure 1: Network topology acquisition phase for handover.

Figure 1: Network topology acquisition phase for handover.

During the handover procedure, the process includes handover decision, handover initiation, and ranging procedures, followed by authorization and registration procedures. These procedures include cell reselection, handover decision and handover initiation, synchronization with new DL, acquisition of UL parameters, ranging, MS reauthorization, reregistration, and termination with the serving BS. These are shown in Figure 2 and Figure 3.

Figure 2: Handover decision, handover initiation, and ranging procedures.

Figure 3: Authorization and registration procedure.

When the MS migrates from its serving BS to its target BS, the following process is executed. First, the MS conducts cell reselection based on the information obtained from the network topology acquisition stage. The handover decision and the handover initiation can be originated by both MS and BS using the MOB_MSHO-REQ/MOB_BSHO-REQ message. When the target BS is decided, the MS sends a MOB_HO-IND message to the serving BS and the actual handover process begins as illustrated in Figure 2.

In the ranging process, the MS can synchronize to the DL of the target BS and obtain DL and UL parameters using the DCD/UCD message. Then RNG_REG/RNG_RSP messages are exchanged to complete the initial ranging process. It may be done in a contention-based or non-contention-based manner.

If the RNG_REG contains the serving BSID, the target BS can obtain the MS information from the serving BS through the backbone network. If the MS is already associated with the target BS at the previous stage, some steps may be omitted. Therefore, the neighboring BS scanning and association should be done right after the handover initiation by utilizing preobtained information before the channel condition changes.

If all physical parameter adjustments are done successfully, the network re-entry process is initiated. Figure 3 shows this procedure. It includes MS authorization and new BS registration. The target BS requests MS authorization information via its backbone network. The new BS registration is performed by REG_REQ and REG_RSP messages. This includes capabilities negotiation, MS authorization, key exchange, and registration. After successful registration with the target BS, the MS can send a MOB_HO-IND message to the serving BS to indicate that handover is completed.

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.