Case study on handoff strategies for wireless overlay networks

https://doi.org/10.1016/j.csi.2012.06.002Get rights and content

Abstract

One of the most challenging topics for next‐generation wireless networks is the process of vertical handoff since many of wireless technologies overlap each other and build a heterogeneous topology. Several parameters, pertaining to user/application requirements and network conditions, such as data rate, service cost, network latency, speed of mobile, and etc. must be considered in the handoff process of heterogeneous networks along with RSSI information. In this paper, adaptive fuzzy logic‐based vertical handoff decision‐making algorithms are presented for wireless overlay networks which consist of GSM/GPRS/Wi-Fi/UMTS/WiMAX technologies. The parameters data rate, monetary cost, speed of mobile and RSSI information are processed as inputs of the proposed fuzzy‐based systems. According to these parameters, an output value, which varies between 1 and 10, is produced. This output value is utilized to determine whether a handoff process is necessary or not and to select the best candidate access point in the vicinity. The results show that, compared to the traditional RSSI‐based algorithm significantly enhanced outcomes can be achieved for both user and network as a consequence of the proposed fuzzy‐based handoff systems. The simulation results are also compared with those of classical MADM (Multiple Attribute Decision Making) method, i.e. SAW (Simple Additive Weighting). According to the results obtained, the proposed vertical handoff decision algorithms are able to determine whether a handoff is necessary or not, properly, and select the best candidate access network considering the aforementioned parameters. Moreover, fuzzy‐based algorithm noticeably reduces the number of handoffs compared to SAW‐based algorithm.

Highlights

► Adaptive fuzzy‐based vertical handoff decision‐making algorithms are presented. ► A cognitive radio that senses GSM/GPRS/Wi-Fi/UMTS/WiMAX technologies is designed. ► The simulation results are compared with a classical MADM method, i.e. SAW. ► Data rate, cost, mobile speed and RSSI information are processed as inputs. ► Fuzzy‐based algorithm reduces the number of handoffs compared to SAW based.

Introduction

In recent years, the exponential growth of wireless communication technologies has forced the emerging diverse technologies, e.g.; wireless cellular networks, WLANs (Wireless Local Area Networks), WWANs (Wireless Wide Area Networks) etc. to coexist. This new trend, also referred as the next‐generation networks, aims that all the wireless technologies work together in order to provide QoS‐supported and cost‐efficient services for mobile users at anywhere and anytime. Wireless terminals deployed in such a heterogeneous network structure might be aware of all the networks around. Considering this requirement, it is obvious that, the recent wireless technology called cognitive radio (CR) that is capable of connecting different wireless technologies with perception and adaptation attributes will be remedied.

Although mobile users expect to obtain both real-time services (voice transfer, video conferencing etc.) and non-real time services (video transfer, simple message service, etc.) with minimum cost and optimum quality, a single wireless access point cannot satisfy the requirements of all current and upcoming services. Therefore, the coexistence of heterogeneous wireless networks to accomplish application requirements and user expectations is an imperious trend. The next‐generation wireless networks should be integrated to form a heterogeneous network which ensures user mobility and service continuity by seamless switching between different technologies at anywhere and anytime. In order to provide seamless mobility between various wireless technologies overlapped, vertical handoff algorithms need to be developed for next‐generation wireless structures.

Handoff is described as a process of transferring an ongoing call or data session from one access point to another in homogeneous or heterogeneous wireless networks. Traditional handoff process, referred also as horizontal handoff, is taken place between two adjacent cells which have the same wireless technologies. Horizontal handoff process, commonly, considers the link quality condition parameters such as RSSI (received signal strength indicator) or SNR (signal to noise ratio) to manage handoff procedure. If these parameters drop below a specified threshold then handoff is decided. Since the next‐generation networks have a heterogeneous structure, it is evident that the traditional handoff mechanisms will not be sufficient when triggering handoff and choosing the new serving station.

Contrast to horizontal handoff, vertical handoff decision requires a tradeoff among many handoff metrics including application requirements, user requirements and network conditions. When dealing with the vertical handoff process, these requirements involve the evaluation of such parameters as QoS, monetary cost, RSSI, SNR, and etc. which may have different levels of importance.

As stated earlier, networks in heterogeneous structure have to act together with various working conditions for dynamically adapting to the environment. In order to manage seamless transmissions between existence diverse networks, cognitive radio concept which can be described as a self-aware communication system that efficiently uses available spectrum in an intelligent way [1] would be assistance for next‐generation networks.

In this study, a Smart Mobile Terminal (SMT) is proposed that scans the environment for available Radio Access Technologies (RATs).1 It evaluates the working conditions of RATs using its fuzzy logic‐based algorithm, triggers handoff process if necessary, and chooses the best Access Point (AP) to camp on. The SMT has an adaptive multi-criteria handoff decision system which has the ability to adapt its structure according to the user/application requirements and network conditions. Together with the other functions, the proposed SMT is capable of sensing the environment for available APs and changing its working parameters such as frequency band, bandwidth, modulation scheme, MAC protocol, etc. to be able to camp on the more appropriate AP.

The proposed SMT has been modeled and simulated using OPNET Modeler Software for more realistic performance evaluation. The fuzzy logic‐based handoff algorithms incorporated in SMT have been implemented in MATLAB Software as well. In the study, the results of fuzzy‐based handoff algorithm are compared with those of SAW algorithm.

The remainder of the paper is organized as follows. The next section presents related works about the vertical handoff in the literature. In Section 3, a brief introduction on handoff procedure is given. Overall properties and design stages of the proposed smart terminal together with related algorithms are described comprehensively in Section 4. Section 5 includes an example heterogeneous network scenario that has overlapping RATs with different working parameters as well as proposed SMT, followed by performance evaluation. The last section gives the summary about the proposed scheme and simulation results with final remarks.

Section snippets

Related works

A number of proposals have been found for vertical handoff decision algorithms in the literature. A fuzzy logic‐based algorithm that adopts the received signal strength (RSS) threshold, bandwidth and cost values as input parameters was proposed in Xia et al. [2]. And the weight of each QoS metric is adjusted along with the networks changing to trace the network condition. In Ling et al. [3], a fuzzy‐based vertical handoff decision algorithm is aimed between GSM, GPRS and WLAN networks with

Handoff process

Handoff is a process of switching a user call or data session between cells each has the same or a different type of network technologies seamlessly provided that the connection would never be dropped. One of the handoff processes, horizontal handoff, occurs in homogenous cellular systems when the user moves out from the coverage area of serving cell to a new cell [8]. Unlike homogenous cellular systems, in wireless overlay networks, mobiles need to switch their connections both horizontally

Smart mobile terminal architecture

The smart terminal proposed in this study is in complete charge of managing the handoff process as well as its other functions. Smart Mobile Terminal (SMT) senses the wireless medium by scanning the frequency spectrum periodically in order to get the available APs’ working parameters. These parameters extracted from the handoff broadcast packets are used in the proposed fuzzy logic‐based handoff algorithms which have the capability of deciding handoff process and choosing the best candidate AP

Assumptions

In our simulation scenario that is modeled using OPNET Modeler and illustrated in Fig. 8, there are four wireless networks; i.e., GSM, Wi-Fi, UMTS and WiMAX networks, each has specific working parameters. The proposed SMT explained in former sections is also modeled in OPNET. The implementation of our fuzzy handoff strategies which are incorporated into the SMT is completed with MATLAB FIS editor.

Simulation results and discussion

SMT is capable of generating time sensitive voice traffic (13 Kb/s), GSM data traffic (9.6 Kb/s),

Conclusions

Decision making is one of the major challenges in Vertical Handoff issue since there are plenty of parameters that must be considered. In this work, we proposed an adaptive fuzzy‐based handoff decision system that is capable of switching between Wi-Fi, GSM, UMTS, and WiMAX technologies. The developed system is able to combine parameters such as data rate, cost, speed of mobile and RSSI in order to initiate handover process and select the best candidate AP. The simulation results of the proposed

Ali Çalhan received the M.Sc. and Ph.D. degrees from University of Kocaeli, Turkey in 2006 and 2011, respectively. His research interests are cognitive radio, wireless heterogeneous networks, wireless communications and artificial intelligence.

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Ali Çalhan received the M.Sc. and Ph.D. degrees from University of Kocaeli, Turkey in 2006 and 2011, respectively. His research interests are cognitive radio, wireless heterogeneous networks, wireless communications and artificial intelligence.

Celal Çeken received the M.Sc. and Ph.D. degrees from University of Kocaeli, Turkey in 2001 and 2004, respectively. His active research interests include wireless communications, broadband networks, ATM networks, high‐speed communication protocols, Wireless Sensor Networks and cognitive radio.

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