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Abstract

Contents

Introduction

Among the variety of infocommunication fields, wireless mobile communication is developing most rapidly: the amount of mobile data traffic increases exponentially, and it’s expected to achieve a mark of about 8,000 petabytes per month around the world in 2017, according to the «Ericsson» company forecast [1]. Wide variety of wireless technologies of different scale are rapidly evolving, such as:

- Wireless personal area networks (WPAN): BlueTooth, ZigBee;

- Wireless Local Area Network (WLAN): IEEE 802.11 standards;

- Wireless Metropolitan Area Network (WMAN): IEEE 802.16 standards;

- Wireless Wide Area Network (WWAN): different generations of cellular technologies – 2G (GSM, GPRS, EDGE), 3G (UMTS, CDMA2000, HSDPA), 4G (LTE Advanced).

Users' demands towards the set and quality of infocommunication services increase, and access has to be provided continuously and independently of geographical location, according to the Always Best Connected (ABC) concept [2]. Variety of radio access technologies (RATs) and the increasing number of multistandard user terminals allows integrating different technologies into a single network to meet users demands, i.e. forming a heterogeneous wireless network (see Fig. 1). Such a network will consist of overlapping segments of different RATs. This will increase the capacity of the network and expand its coverage, and will help providing services at a lower price and with higher quality.

Heterogeneous wireless network

Figure 1 — Heterogeneous wireless network [3]

1. Theme significance

In a heterogeneous wireless environment transparent user mobility management is a key task, which is implemented by the procedure of transferring (switching) of connection from one access point (AP) or base station (BS) to another, which is called handover (handoff). Handover between BSs / APs within the same technology is called horizontal handover, between different types of networks is called vertical handover. The traditional mechanism for horizontal handover is based on the received signal strength (RSS): handover triggers when the RSS level of the serving BS falls below the set threshold. To eliminate the "ping-pong" effect on the border of a cell, additional mechanisms are applied, such as hysteresis loop and delay timers.

However, in a heterogeneous network environment parameters of different types of networks lay in different boundaries, thus RSS is not a sufficient criteria for effective and intelligent handover: for multiservice traffic quality of service parameters, user preferences, applications requirements, cost should also be considered. That is why the task of development of such a criteria and an algorithm that takes into account the maximum number of parameters and allows to perform intelligent handover is important.

2. Goal and objectives of the research

The aim of this study is to improve the quality of mobile services through the use of heterogeneous wireless network resources and intelligent vertical handover algorithm.

To achieve this goal a number of issues should be addressed:

  1. Analyzing vertical handover procedure and its classification.
  2. Considering the parameters of the destination network selection criteria in vertical handover.
  3. Development of a vertical handover algorithm.
  4. Simulation of the developed algorithm use.
  5. The simulation results estimation and further recommendations.

3. Main contributions of the research

Within the master's research it is planned to develop an intelligent vertical handover algorithm using fuzzy logic to process parameters and to use in the decision phase of handover. This algorithm will allow operators to use resources effectively and provide services at higher quality (meaning, higher bandwidth and network coverage).

Prospective scientific novelty of the paper is development of a vertical handover algorithm that uses

  • fuzzy logic to process the input parameters and calculate the handover trigger criteria;
  • algorithm of RSS prediction based on “gray analysis” theory (GPT) to reduce the call dropping probability;
  • method of analytic hierarchal process (AHP) to calculate the weights of each criteria;
  • ranking technique for order preference by similarity to ideal solution (TOPSIS) to solve the problem of the destination network selection.

    • 4. Handover classification and phases

    Handover is a key mechanism to seamless mobility. As mentioned above, the handover can be horizontal (intrasystem) and vertical (intersystem), which takes place only in heterogeneous networks.

    There is also another classification, which divides handover into soft and hard. If the transfer of ongoing session is carried out without losing the connection, handover called soft (or make-before-break type): the mobile station (MS) can communicate with multiple network access devices simultaneously. This type of handover is shown in the animation (see Figure 2). In another case the MS can only be associated with one BS or AP at the same time, the handover is called hard (or break-before-make type): at first the connection to the current network is dropped and only then a new connection is established.

    Illustration of soft handover between the WiMAX base station and WLAN access point. Animation consists of 3 frames with a delay of 1 second between frames, the number of cycles - 7. The total size - 42 KB

    Figure 2 - Illustration of soft handover between the WiMAX base station and WLAN access point

    (Animation consists of 3 frames with a delay of 1 second between frames, the number of cycles - 7. The total size - 42 KB)

    Handover can also be classified depending on the initiator into mobile-controlled, network-controlled and mobile-assisted handover. It also can be upward and downward: upward handover occurs from the smaller network with higher capacity to wider network with smaller capacity and is performed while going out of the coverage of smaller network. The downward handover is otherwise and occurs with the purpose of improving the network performance.

    Basic requirements for handover are: low latency, enhanced reliability, reduction of signaling and processing overhead, minimum handover ratio (since frequent switching leads to significant energy costs and degradation of bandwidth utilization), to provide a transparent mobility, load balancing on the radio access networks. The system of vertical handover should choose the most appropriate time of initiation and the most appropriate radio access network among all available.

    The VHO process consists of three main steps: system discovery (handover iformation gathering, or handover initiation phase), handover decision (also called network or system selection), and handover execution [5]. Handover information gethering is used to collect all the information required to trigger handover. Handover decision phase is used to determine whether and how to perform the handover by selecting the most suitable access network based on some criteria using infromation gathered on the previous step, and by giving instructions to the execution phase. And finally handover execution step is used to actually switch channels to the newly selected access network, it cooperates with control signaling, and the IP management protocols. 

    Desicion-making is the most crutual step of handover proccess in terms of quality and continuity of service requirements fulfilment. Therefore our study is manely focused on the development of appropriate criteria or desicion scheme.

    Conclution

    In heterogeneous networks it’s very important to ensure the efficient use of the resources and transparent mobility of users. This is implemented through the intelligent vertical handover procedure since ineffective handover can cause loss of connection, QoS degradation to an unsatisfactory level, or significant user expenses.

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