Vertical Handover in 4G networks

SINR BASED VERTICAL HANDOFF STRATEGY

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Maximum achievable data rate for the given carrier bandwidth and SINR can be determined with the help of Shannon capacity formula, the maximum achievable data rate R is given by:

Where:

• W is the carrier bandwidth

• γ is SINR received at user end when associated with WLAN or WCDMA

• Г is the dB gap between uncoded QAM and channel capacity, minus the coding gain.

Let RAP and RBS be the maximum achievable downlink data rate while user connected with WLAN and WCDMA.

From Shannon capacity, we have:

Where, γAP and γBS are the receiving SINR from WLAN and WCDMA respectively. We are interested in the relationship between required γAP and γBS while offering the same downlink data rate to the user by WLAN and WCDMA.

Letting RAP = RBS , we can solve the equation and get the relationship between γAP and γBS as:

The parameters in (4) are:

• The carrier bandwidth for WLAN WAP is 1MHz , and 5MHz for WCDMA WBS .

• ГAP equals to 3dB for WLAN , and ГBS equals to 16dB for WCDMA.

Having the relationship between the maximum achievable data rate and the receiving SINR from both WLAN and WCDMA makes the SINR based vertical handoff method applicable, in which the receiving SINR from WCDMA γBS is being converted to the equivalent γAP required to achieve the same data rate in WLAN, and compared with the actual receiving SINR from WLAN.

With the combined effects of both SINR being considered, handoff is triggered while the user is getting higher equivalent SINR from another access network. It means that given the receiver end SINR measurements of both WLAN and WCDMA channel, the handoff mechanism now has the knowledge of the estimated maximum possible receiving data rates a user can get from either WLAN or WCDMA at the same time within the handover zone, where both WLAN and WCDMA signal are available. This gives the vertical handoff mechanism the ability to make handoff decision with multimedia QoS consideration, such as offer the user maximum downlink throughput from the integrated network, or guarantee the minimum user required data rate during vertical handoff.

A handover Scenario

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A scenario in which a user travels through an area with overlapping coverage of a 3G network, WLAN cell and WiMAX is used to explain the three main components of the VHD and their contributions.

Performance evaluation metrics for VHD algorithms

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VHD algorithms can be quantitatively compared under various usage scenarios by measuring the mean and maximum handover delays, the number of handovers, the number of failed handovers due to incorrect decisions, and the overall throughput of a session maintained over

a typical mobility pattern. These metrics are further explained below:

Handover delay refers to the duration between the initiation and completion of the handover process. Handover delay is related to the complexity of the VHD process, and reduction of the handover delay is especially important for delay-sensitive voice or multimedia sessions.

Number of handovers: Reducing the number of handovers is usually preferred as frequent handovers would cause wastage of network resources. A handover is considered to be superfluous when a handover back to the original point of attachment is needed within a certain time duration, and such handovers should be minimized.

Handover failure probability: A handover failure occurs when the handover is initiated but the target network does not have sufficient resources to complete it, or when the mobile terminal moves out of the coverage of the target network before the process is finalized. In the former case, the handover failure probability is related to the channel availability of the target network, while in the latter case it is related to the mobility of the user.

Throughput: The throughput refers to the data rate delivered to the mobile terminals on the network. Handover to a network candidate with higher throughput is usually desirable.

Classification of VHD algorithms

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There are various ways to classify VHD algorithms. I have chosen to divide VHD algorithms into four groups based on the handover decision criteria used and the methods used to process these.

RSS based algorithms: RSS is used as the main handover decision criterion in this group. Various strategies have been developed to compare the RSS of the current point of attachment with that of the candidate point of attachment. In RSS based horizontal handover

decision strategies are classified into the following six subcategories: relative RSS, relative RSS with threshold, relative RSS with hysteresis, relative RSS with hysteresis and threshold, and prediction techniques. For VHD, relative RSS is not applicable, since the RSS from different types of networks can not be compared directly due to the disparity of the technologies involved. For example, separate thresholds for each network. Furthermore, other network parameters such as bandwidth are usually combined with RSS in the VHD process.

Bandwidth based algorithms: Available bandwidth for a mobile terminal is the main criterion in this group. In some algorithms, both bandwidth and RSS information are used in the decision process. Depending on whether RSS or bandwidth is the main criterion considered, an algorithm is classified either as RSS based or bandwidth based.

Cost function based algorithms: This class of algorithms combine metrics such as monetary cost, security, bandwidth and power consumption in a cost function, and the handover decision is made by comparing the result of this function for the candidate networks. Different weights are assigned to different input metrics depending on the network conditions and user preferences.

Combination algorithms: These VHD algorithms attempt to use a richer set of inputs than the others for making handover decisions. When a large number of inputs are used, it is usually very difficult or impossible to develop analytical formulations of handover decision processes. Due to this reason, researchers apply machine learning techniques to formulate the processes. A literature survey reveals that fuzzy logic and artificial neural networks based techniques are popular choices. Fuzzy logic systems allow human experts’ qualitative thinking to be encoded as algorithms to improve the overall efficiency. Examples of applying this approach into VHD can be found. If there is a comprehensive set of input-desired output patterns available, artificial neural networks can be trained to create handover decision algorithms. It is also possible to create adaptive versions of these algorithms. By using continuous and real-time learning processes, the systems can monitor their performance and modify their own structure to create highly effective handover decision algorithms.

VHD criteria

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Several parameters as shown in Fig. 1 have been proposed for use in the VHD algorithms.

We briefly explain each of them below.

Received signal strength (RSS) is the most widely used criterion because it is easy to measure and is directly related to the service quality. There is a close relationship between the RSS readings and the distance from the mobile terminal to its point of attachment. Majority of existing horizontal handover algorithms use RSS as the main decision criterion, and RSS is an important criterion for VHD algorithms as well.

Network connection time refers to the duration that a mobile terminal remains connected to a point of attachment. Determining the network connection time is very important for choosing the right moment to trigger a handover so that the service quality could be maintained at a satisfactory level. For example, a handover done too early from a WLAN to a cellular network would waste network resources while being too late would result in a handover failure. Determining the network connection time is also important for reducing the number of superfluous handovers, as handing over to a target network with potentially short connection time should be discouraged. The network connection time is related to a mobile terminal’s location and velocity. Both the distance from the mobile terminal to its point of attachment and the velocity of the mobile terminal affect the RSS at the mobile terminal. The variation of the RSS then determines the time for which the mobile terminal stays connected to a particular network. Network connection time is especially important for VHD algorithms because heterogeneous networks usually have different sizes of network coverage.

Available bandwidth is a measure of available data communication resources expressed in bit/s. It is a good indicator of traffic conditions in the access network and is especially important for delay-sensitive applications.

Power consumption becomes a critical issue especially if a mobile terminal’s battery is low. In such situations, it would be preferable to handover to a point of attachment which would help extending valuable battery life

Monetary cost: For different networks, there would be different charging policies, therefore, in some situations the cost of a network service should be taken into consideration in making handover decisions.

Security: For some applications, confidentiality or integrity of the transmitted data can be critical. For this reason, a network with higher security level may be chosen over another one which would provide lower level of data security.

User preferences: A user’s personal preference towards an access network could lead to the selection of one type of network over the other candidates.

Handover Processes

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Handover is the process of maintaining a user’s active sessions when a mobile terminal changes its connection point to the access network (called ‘‘point of attachment”), for example, a base station or an access point. Depending on the access network that each point of attachment belongs to, the handover can be either horizontal or vertical.

A horizontal handover takes place between points of attachment supporting the same network technology,for example, between two neighbouring base stations of a cellular network. On the other hand, a vertical handover occurs between points of attachment supporting different network technologies, for example, between an IEEE 802.11 access point and a cellular network base station.

A handover process can be split into three stages: handover decision, radio link transfer and channel assignment. Handover decision involves the selection of the target point of attachment and the time of the handover. Radio link transfer is the task of forming links to the new point of attachment, and channel assignment deals with the allocation of channel resources. VHD algorithms help mobile terminals to choose the best network to connect to among all the available candidates. Here, we only focus on the research efforts and recent developments on improving the efficiency of VHD process. In contrast to horizontal handover decision algorithms which mainly consider RSS as the only decision criterion, for VHD algorithms, criteria such as cost of services, power consumption and velocity of the mobile terminal may need to be taken into consideration to maximize user satisfaction

Introduction

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Growing consumer demand for access to communication services anywhere and anytime is driving an accelerated technological development towards the integration of various wireless access technologies. Such integration combines islands of access networks into a seamless system, referred to as Fourth Generation (4G) wireless systems. 4G wireless systems will provide significantly higher data rates, offer a variety of services and applications previously not possible due to bandwidth limitations, and allow global roaming among a diverse range of mobile access networks.

In a typical 4G networking scenario, handsets or Mobile Terminals (MTs) with multiple interfaces will be able to choose the most appropriate access link among the available alternatives. These access links include IEEE 802.11 Wireless Local Area Network (WLAN) access, IEEE 802.16 Worldwide Interoperability for Microwave Access (WiMAX) , satellite systems and Bluetooth, in addition to the traditional cellular telephony networks. For a satisfactory user experience, MTs must be able to seamlessly transfer to the “best” access link among all available candidates with no perceivable interruption to any ongoing voice or video conversation. Such ability to handover between heterogeneous networks is referred to as vertical hand overs . As an important step towards achieving this objective, the emerging IEEE 802.21 standard creates a framework to support protocols for enabling seamless vertical handovers. IEEE 802.21 provides only the overall framework, leaving the implementation of the actual algorithms to the engineers designing the system. Therefore, it is essential to develop efficient vertical handover decision (VHD) algorithms to ensure the success of this new framework.