IEEE 802.16 standard, the so-called Worldwide Interoperability for Microwave Access (WiMAX), is one of the latest technologies to provide broadband wireless access. The main advantage of WiMAX is high bandwidth over long transmission range. The standard provides specifications for the air interface, including the medium access control (MAC) and physical (PHY) layers.
1.1 IEEE 802.16 Standard
In December 2001, IEEE 802.16-2001 standard was defined for using the frequency range of 10-66 GHz, with maximum transmission rate up to 70 Mbps or even 100 Mbps and transmission range of 20 km. However, the initial standard only supports line-of-sight (LOS) transmission. In 2003, IEEE 208.16a-2003 that can support Non-LOS (NLOS) environment and 2-11 GHz range was approved. Until 2004, IEEE 802.16 standard has revised and consolidated previous standards and evolved to the IEEE 802.16-2004 standard [1]. The standard specifies the PHY and MAC layers for fixed applications, also known as fixed WiMAX.
However, after IEEE 802.16-2004 publication, it still needs an upgrade. The main problem for IEEE 802.16-2004 is the lack of mobility features, because mobility support is considered as one of the key features in wireless network. Other features were needed and some errors had to be corrected. This gave way to IEEE 802.16e [2] published in December 2005. It is an amendment with the support of mobility and changes to the IEEE 802.16-2004 standard. This is generally known as mobile WiMAX. The main differences are mobile station (MS), MAC layer handover procedure, Orthogonal Frequency Division
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Multiplexing (OFDMA) PHY layer, power saving, security, Adaptive Antenna System (AAS), Multiple Input Multiple Output (MIMO), Multicast and Broadcast service (MBS) feature, and Quality of Service (QoS).
In March 2006, IEEE 802.16 Relay Task Group [3] was established, which developed multihop relay amendment called IEEE 802.16j. The seventh draft, IEEE 802.16j-D7 [4]
was released in October 2008. It provides specifications for multihop relay features in order to gain the promising advantages including coverage extension and throughput enhancement to WiMAX system. The solutions can operate with legacy IEEE 802.16e end terminal devices, new base station (BS) and relay station (RS) to be realized and provide services by legacy MS. Table 1.1 shows the main differences between the two technologies [5].
Table 1.1 Comparison of 802.16j and 802.16e capabilities
802.16e 802.16j
Topology PMP only Tree structure
Hops Single hop Multihop
Traffic aggregation No Yes, over multihop path
System Lower Higher within BS coverage area
Coverage Lower Higher
Cost Higher Lower
Legacy 802.16e stations － Backward compatible
Mobility support Yes Yes
PHY support OFDMA OFDMA extension
Examples of the most important usage scenarios for relay stations are shown in Figure
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1.1 [6]. In the fixed infrastructure usage model, fixed RSs (FRS) are deployed in order to provide coverage extension at cell edge, to provide coverage for indoor users, to provide coverage for users in coverage holes that in shadow or valleys between buildings, and to provide access for clusters of users outside the coverage area of the BS. Nomadic RSs (NRS) are deployed temporarily to provide additional coverage or capacity in some areas where BSs and/or fixed RSs do not provide sufficient coverage or capacity. Examples of temporary coverage can be required of emergency, disaster recovery situations, or special event or fairs which require the coverage be provided for the duration of the event.
BS
Figure 1.1 Examples of usage scenarios for relay stations
A mobile RS (MRS) can work in either moving RS mode or moving BS mode. In moving RS mode, MRS can be mounted on a vehicle, such as a bus or a train, connected to a BS via a mobile link. In this case, the RS provides a fixed access link to end terminals
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riding on the vehicle. Different from moving RS mode, in moving BS mode, MRS has network access capability completely. Working as a mobile router (MR), MRS in moving BS mode can assign IP address and connection identifier (CID) to each MS which connected to it, therefore only the MRS has to perform both MAC layer (layer 2) and network layer (layer 3) handover procedure, MS is not aware that there is a handover event.
However, moving BS mode actually simplifies the handover procedure, but that will add much more hardware costs to MRS. IEEE 802.16j will basically perform moving RS mode.
In this thesis, we will focus on MRS in moving RS mode.
1.2 Motivation and Purpose
In a network layer handover, the MRS involved in different IP subnets or in different networks. Thus, in a network layer handover, a MS needs to establish a new care of address (CoA) configuration and registration to maintain connectivity. In moving RS mode, MS must wait for MRS finished MAC layer handover procedure, then MS can perform CoA confirmation which takes at least 1 second using Mobile IPv6 [7]. The handover causes interruption latency too long to support real-time applications such as voice over Internet Protocol (VoIP) or video streaming, etc. The handover process timing using conventional handover scheme is shown in Figure 1.2.
To overcome the problem described above, in this thesis, we propose a new mobility scheme for MRS in moving RS mode and corresponding MAC management messages. We focus on MRS moving between different IP subnets. Our goal is to make link layer and network layer handover procedures be performed concurrently to reduce the service disruption time so that users can get satisfactory Quality of Experience while using real-time application services over WiMAX.
5 Layer 2
handover latency
Layer 3 handover latency Decision and
Initiation Execution Movement Registration
detection
CoA configuration
Figure 1.2 Conventional handover process timing
1.3 Thesis Organization
The rest of this thesis is organized as follows. Chapter 2 presents an overview of IEEE 802.16j system architecture and the background about handover schemes in WiMAX. Next, Chapter 3 discusses the mobility scheme we proposed in details. In Chapter 4, we evaluate our scheme through numerical analysis. The simulation and results are presented in Chapter 5.
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