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									                                       (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 5, May 2011

    An Overview on Radio Access Technology (RAT)
    Selection Algorithms for Heterogeneous Wireless
                           J.Preethi                                                       Dr.S.Palaniswami
                   Assistant Professor,                                                          Professor,
    Department of Computer Science and Engineering                         Department of Electrical and Electronics Engineering
       Anna University of Technology, Coimbatore                                    Government College of Technology
                          India                                                                     India
Abstract—Next generation wireless networks (NGWN) will                  accurately as possible into its original wave form by the base
be heterogeneous in nature where different radio access                 station.
technology coexist in the same coverage area. The                          The second generation was implemented to improve
coexistence of different RATs require a need for Joint                  transmission quantity system capacity and network coverage.
Radio Resource Management (JRRM) to support the                         In second generation systems (e.g., Personal communication
provision of quality of service and efficient utilization of            systems (PCS), Global System for Mobile Communication
radio resources. The Joint Radio Resource Management                    (GSM), Code Division Multiple Access (CDMA), Time
(JRRM) manages dynamically the allocation and                           Division Multiple Access (TDMA) and Orthogonal Frequency
deallocation of radio resources between different Radio                 Division Multiplexing (OFDM)) are based on Digital
Access Technology (RAT). The homogenous Call                            Transmission.
Admission Control (CAC) algorithms do not provide a                        In digital systems, more efficient use of the available
single solution to address the heterogeneous architectures              spectrum is achieved by digital encoding of the speech data.
which characterize next generation wireless networks. This              Due to the transition from 2G to 3G, a number of standards
limitation of homogeneous CAC algorithms necessitates                   have been developed, which are categorized as 2.5G. These
the development of RAT selection algorithms for                         are add-ons to the 2G standards and mainly focus on
heterogeneous wireless network. The goal is to select the               deployment of efficient IP connectivity within the mobile
most suitable RAT for each user. This paper investigates                networks.
ten different approaches for selecting the most appropriate                2.5G is a stepping stone between 2G and 3G cellular
Radio Access Technology (RAT) for incoming calls among                  wireless technologies, invented for marketing purposes only.
the Heterogeneous Wireless Networks. This RAT selection                 2.5G implements a packet switched domain which includes
works in two steps; the first step is to select a suitable              GPRS (General Packet Radio Service), EDGE (Enhanced Data
combination of cells among the different RATs. The                      rates for GSM).
second step chooses the most appropriate RAT to which                      The objective of the third generation (3G) is to provide
the users can be attached and to choose the suitable                    fairly high speed wireless communications to support
bandwidth to allocate for the users.                                    multimedia, data and video in addition to voice. 3G includes
                                                                        Universal Mobile Telecommunications systems (UMTS) [1],
                                                                        CDMA2000 based on W-CDMA technologies which provides
Keywords- Radio Access Technology (RAT) selection, Joint Radio          services like wireless access to the Internet and high data rate
Resource Management (JRRM), Heterogeneous Wireless Networks.            applications like real time video transmission. To cope with
                                                                        these, high bandwidth services and the enormous increase in
                      I.      INTRODUCTION                              the number of users, a more efficient use of radio spectrum is
   In the recent years, mobile communication systems have               required [2].
experienced a significant change leading to the introduction of            In turn, the perspective of beyond 3G systems is that of
the heterogeneous wireless networks. The first generation               heterogeneous networks, which provides wireless services
mobile communications systems (e.g. Nordic Mobile                       independently of its location in a completely transparent way
Telephony (NMT) and Advanced Mobile Phone System                        [3]. The user terminal should be able to pick the best access
(AMPS)) are based on analog transmission techniques. Analog             technology such as Wireless Local Area Network (WLAN),
signals are radio transmissions sent in a wave-like form. A             the Universal Mobile Telecommunication Systems (UMTS)
mobile device sends the waves to a base station where they are          and the Global System for Mobile Telecommunication
processed to determine the signals next destination (i.e.               (GSM)/Enhanced Data rate for GSM Evolution (EDGE) Radio
another base station, mobile phone, land line phone etc.,) Once         Access Network (GERAN) at its current location and use the
the destination is determined, the signal is reconstructed as           access technology seamlessly for the provision of desired
                                                                        service. This leads to the introduction of new Radio Resource

                                     (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 5, May 2011

Management (RRM) techniques referred to as (JRRM) Joint                  III. BENEFITS OF HETEROGENEOUS JOINT RADIO
Radio Resource Management algorithms which manages                      RESOURCE MANAGEMENT ALGORITHMS
dynamically the allocation and deallocation of radio resources              Each Radio Access Network (RAN) differs from the others
between different Radio Access Technology (RAT). That is,               by the air interface technology, cell size, services supported,
instead of performing the management of radio resources                 bit rate capabilities, coverage, mobility support etc., therefore
independently for each RAT, some form of overall and global             the heterogeneous characteristics offered by the network is
management of the pool of radio resources can be envisaged.             considered. As a result, RAT provide further flexibility in the
   The coexistence of different RATs require a need for Joint           way how radio resources can be managed. This lead to the
Radio Resource Management (JRRM) to support the provision               introduction of RRM. The basic function of Call Admission
of quality of service and efficient utilization of radio                Control (CAC) algorithm is to decide whether a new handoff
resources. In heterogeneous wireless networks, different RATs           call can be accepted into a RAT without violating service
coexist in the same coverage area. The goal is to select the            commitments [5]. CAC has been used in wired networks and
most suitable RAT for each user. In this paper, a                       in homogenous wireless networks such as GSM, UMTS,
comprehensive survey of different RAT selection algorithms              WLAN, Satellite network etc., However, homogenous CAC
for a heterogeneous wireless network is proposed. Section II            algorithms do not provide a single solution to address the
explains about the architecture of heterogeneous wireless               heterogeneous architecture. This limitation of homogenous
network. Section III presents the benefits of Joint Radio               CAC algorithm necessitates the development of RAT selection
Resource Management algorithms and Section IV presents the              algorithm for heterogeneous wireless network.
comprehensive survey of RAT selection algorithms Section V                  Joint Call Admission Control algorithm is one of the JRRM
gives the proposed work and lastly the conclusions are                  algorithms. Within the JRRM, the initial RAT selection, i.e the
presented in Section VI.                                                allocation of connections to specific RANs at session initiation
    II.   HETEROGENEOUS WIRELESS NETWORK BEYOND 3G                      and the Vertical Handover (VHO) i.e the capability to switch
                                                                        ongoing connections from one RAN to another. These are the
   Next generation networks will be heterogeneous where                 key enablers to properly manage the heterogeneous radio
different radio access networks such as WLAN, UMTS,                     access network and become the key for the JRRM functions.
WiMax and satellite networks which is illustrated in the                The benefits of Heterogeneous Joint Radio Resource
Figure 1. In order to provide the mobile users with the                 Management Algorithms are
requested multimedia services and corresponding quality of                    Efficient utilization of radio resources,
service (QoS) requirements[4], these radio access technologies                Consistent provisioning of QoS across different
will be integrated to form a heterogeneous wireless access                        RATs,
network. Such a network will consist of a number of wireless
                                                                              Overall stability of network,
network and will form the fourth generation (4G) or next
                                                                              Increase in Operator‟s revenue and
generation of wireless networks. However, each access
network provides different levels of QoS, in terms of                         Enhancement of users satisfaction.
bandwidth, mobility, coverage area and cost to the mobile



                           AP                                                   Node

                                                                           Cellular / GSM/ GPRS/

                                                                           User Equipment /Mobile

                                     Fig.1. Integration of Heterogeneous Wireless Access Network

                                      (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 5, May 2011

 IV.    AN OVERVIEW ON RAT SELECTION ALGORITHMS                         namely WLAN and WWAN. Four Fuzzy logic controllers are
The section describes different RAT selection algorithms for            used separately for selecting the best access network and
initial RAT selection and Vertical Handover. References                 Particle swarm optimization is used to optimize the fuzzy
[6],[7] presents the good review of RAT selection algorithms.           logic controllers and Genetic algorithm is used to optimize the
In Random based RAT selection, when a new call or vertical              weight coefficients.
handover arrives, any of the available RATs will be selected
randomly. In Load balancing based RAT selection, the main                   D) Soft Computing techniques based RAT Selection
objective is to uniformly distribute the load among all the
available RATs in heterogeneous wireless network                            Soft computing techniques based RAT selection admits an
[8],[9],[10]. In Policy based RAT selection, it allocates users         incoming call based on applying soft computing techniques for
to the RAT based on some specific rules specified by the                the RAT selection. The soft computing techniques applied for
network. A simple policy has been proposed in [11], which               RAT selection are discussed
includes Voice GERAN (VG) policy, Voice UTRAN (UV)                            Fuzzy logic [17], focuses on the issues related to
policy and Indoor (IN) policy. Service-class based RAT                           mobility       management        in    future    mobile
admits calls into a particular RAT based on class of service                     communication scenario where a multi segment
such as voice, video streaming, real-time video, web browsing                    access network is integrated into an IP core network.
etc., [12]. Service cost based RAT selection admits incoming                     [17] Proposes a new approach to handover
call into the least expensive RAT in order to reduce the service                 management by applying a fuzzy logic concept to a
cost incurred by the users. Path loss based RAT selection                        heterogeneous environment. For handover initiation,
algorithm makes call admission algorithm based on path-loss                      parameters considered are network coverage,
measurements taken in the cells of each RAT[13].                                 perceived QoS and Signal Strength (SS).
                                                                              The framework for JRRM algorithm [18-19] based on
Some of the other RAT selection algorithms are discussed in                      fuzzy neural mechanism as explained in Fig.2
this section.                                                                    consists of three main blocks namely fuzzy based
                                                                                 decision, reinforcement learning and multiple
    A) Network layer based RAT selection algorithm                               objective decision making. The inputs for the fuzzy
                                                                                 based decision block are signal strength of each RAT,
     Network layer based RAT selection algorithm admits calls                    resource availability of each RAT and mobile speed
into a particular layer. If the layer cannot accommodate the                     of the user. The Fuzzy based decision consists of
call, this algorithm tries to admit the call into the next layer.                three parts namely fuzzifier, inference engine and
These algorithms are very simple but can lead to highly                          defuzzifier. The fuzzifier allocates a value from 0 to 1
unbalanced network load. Network layer based RAT selection                       for each input. In the inference engine, for each of the
algorithm is explained in [14]. The objective of this algorithm                  fuzzy subset defined in the fuzzifer, fuzzy rules are
is to minimize new call blocking probability while                               associated to indicate if it is suitable to be selected.
guaranteeing a hard constraint on handoff call dropping                          The output of the inference engine is a value that
probability.                                                                     varies between Y(yes), N (no), PY(probably yes) and
                                                                                 PN (probably no). The defuzzifer will convert the
    B) Utility/cost function based RAT selection                                 output of the inference engine into fuzzy selected
                                                                                 decision (FSD). The subjective and techno-economic
     Utility/Cost function based RAT selection admits the                        criteria in the form of user preferences (UP) and
incoming calls into a particular RAT based on some utility or                    operator preferences (OP) are inputs to the multiple
cost function derived from a number of criteria. It considers                    objective decision making. The outputs of the fuzzy
user time constraints, and estimates the complete file delivery                  neural algorithm are cell/RAT selection and amount
time for each available access network and then selects the                      of bit rate allocated for the selected RAT.
most promising access network. These algorithms are very                      Fuzzy MADM (Multiple Attribute Decision Making)
efficient but are very complex and incur high computational                      method [20-22] operates in two steps. The first step is
overhead. [15] present the utility based RAT selection                           to convert the imprecise fuzzy variables to crisp
algorithms for selecting the RAT.                                                numbers. The second step is to use classical MADM
                                                                                 technique to determine the ranking order of the
    C) Mobile based RAT selection algorithm                                      candidate networks. The highest ranking RAT is then
                                                                                 selected for the call.
     This algorithm uses mobile terminal measurements from                    Using Fuzzy logic controllers, genetic algorithms and
different radio access technology for the initial RAT selection                  particle swarm optimization for decision making of
[16]. The inputs to this algorithm are speed of the mobile user,                 radio access technology selection for the next
signal strength, quality of service and service cost. This                       generation wireless networks under given input
algorithm uses fuzzy logic controllers, genetic algorithms and                   criteria on user velocity, type of service and service
particle swarm optimization for decision making under given                      parameters, Quality of service and service costs of the
input criteria. In this, two access networks are considered                      mobile user [23].
                                         (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 5, May 2011

                                               REINFORCEMENT LEARNING


Signal strength                                                                                                            MULTIPLE        Allocated
                                                        INFERENCE                                             Fuzzy
   Resource                  FUZZIFIER                                            DEFUZZIFIER                              OBJECTIVE       Bandwidth
  availability                                            ENGINE                                                           DECISION
Mobile speed                                                                                                                                  RAT
                                                                                                               User                         Selected
                                                     Fuzzy based Decision                                   Preferences

                                          Fig.2. Block Diagram of the Fuzzy Neural System
                                                       (Figure taken from [18])

           This algorithm uses mobile terminal measurements                     G) Network Controlled Cell Breathing RAT selection
           from different radio access technologies within a
           given time interval, with aim to obtain information                         The Network controlled cell breathing RAT
           for multi criteria decision making between different                      selection algorithm is applied for the Initial RAT
           access networks available to the terminal.                                selection     and     Vertical   handover    between
                                                                                     CDMA/TDMA             based    systems    [26].   In
     E) Hopfield Neural network RAT selection                                        FDMA/TDMA systems like GSM/GPRS, there is no
                                                                                     Intra cell interference, but there will be inter cell
             Here the RAT selection mechanism is based on the                        interference for a single user exchanging the
           Hopfield neural networks model [24]. The input                            information between the cells. In case of CDMA
           parameters to the HRM model are gathered from                             based systems like UMTS, there are both Intra cell
           different layers of the protocol stack. This approach                     and Inter cell interferences. CDMA systems are more
           allows improvement in terms of service retainability,                     sensitive for multi user interference than
           decreased packet loss, jitter. The main advantage of                      FDMA/TDMA systems. By effectively controlling
           HRM is smaller complexity. The smaller complexity                         the cell radius of the CDMA RAT, the capacity of the
           results in smaller CPU load.                                              CDMA systems is increased and target coverage area
                                                                                     is assured.
     F)    Operator RAT selection based on the Fittingness
           Factor                                                               H) Performance Analysis of Radio Access Technology
                                                                                   Selection Algorithms using 2-Dimensional Markov
              The initial RAT selection and vertical handover is                   Model
           done based on measuring the fittingness factor
           depending on network and terminal capabilities for                          The performance analysis of Radio Access
           each RAT [25]. The fittingness factor is calculated by                    Technology selection is made using Markov model
           three factors namely the capabilities of the terminal,                    [27]. To evaluate the performances metrics, Mobility
           User-centric suitability and network centric suitability                  based algorithm and Always WLAN algorithm are
           of the overall RAT perspective. For Initial RAT                           considered. Authors Porjazoski and Borislav
           selection, fittingness factor is calculated for each                      Popovski considered the RAT selection algorithms
           RAT. The RAT with the highest fittingness factor is                       performances, like user mobility, call arrival rate, call
           selected first and if admission is not possible, select                   duration, size of RAT cells. The results explained that
           the next RAT in the decreasing order with respect to                      Always WLAN algorithm have higher rate of vertical
           fittingness factor.      For Vertical handover, the                       handover than Mobility based algorithm. Mobility
           fittingness factor of the serving RAT and the new                         based RAT selection algorithm shows better
           RAT are measured and the RAT with the highest                             performances in the means of blocking probability for
           fittingness factor is given the call.                                     higher percentage of vehicular users. For smaller
                                                                                     percentage of vehicular users and for very high traffic
                                                                                     loads, Always WLAN algorithm performs better then
                                                                                     mobility based algorithm.

                                            (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 5, May 2011

                         V PROPOSED WORK                                         [12] W.Zhang, “Performan ce of Real-time and Data traffic in Heterogeneous
                                                                                      Overlay Wireless Networks”, 19th International Teletraffic Congress
   Applying optimizing methodologies like Fuzzy logic,                                (ITC 19), Beijing, China, August 29- September 2, 2005.
Genetic algorithm, Neural Networks, Particle Swarm                               [13] J.Perez-Romero, O.Sallent, R.Agusti, “Enhanced Radio Access
optimization and Simulated Annealing for the RAT selection                            Technology Selection exploiting Path loss information”, 17th annual
                                                                                      IEEE Symposium on Personal, Indoor and Mobile Radio
of the heterogeneous wireless network and comparing these                             Communication (PIMRC’06).
algorithms and finding out the best access network.                              [14] R.B.Ali, S.Pierre, “An Efficient predictive admission control policy for
                                                                                      heterogeneous wireless bandwidth allocation in next generation mobile
                                                                                      networks”, International Conference on Communications and Mobile
                         VI CONCLUSION                                                computing (IWCMC’06), Vancouver, Canada, July3-6, 2006.
                                                                                 [15] O.Ormond, J.Murphy, G.Muntean, “Utility –based Intelligent network
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the same coverage area. The goal is to select the most suitable                       communications (ICC 2006), Istanbul, Turkey, June 11-15.
                                                                                 [16] Aleksandar Tudzarov and Toni Janevski, “Efficient Radio Access
RAT for each user. The coexistence of different RATs
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requires a need for Joint Radio Resource Management                                   International Journal of Research and Reviews in Next Generation
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                                              (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 5, May 2011

                           AUTHORS PROFILE

                               S. Palaniswami received the B.E. degree in
                               electrical and electronics engineering from
                               the Govt., college of Technology,
                               Coimbatore, University of Madras, Madras,
                               India, in 1981, the M.E. degree in electronics
                               and communication engineering (Applied
                               Electronics) from the Govt., college of
                               Technology,        Bharathiar      University,
                               Coimbatore, India, in 1986 and the Ph.D.
                               degree in electrical engineering from the
                               PSG Technology, Bharathiar University,
                               Coimbatore, India, in 2003.
Coimbatore, India, in 2003. He was the Registrar of Anna University
Coimbatore, Coimbatore, India, from May 2007 to May 2010. Currently he is
heading the Department of Electrical and Electronics Engineering, His
research interests include Control systems, Communication and Networks,
Fuzzy logic and Networks, AI, Sensor Networks. . He has about 25 years of
teaching experience, since 1982. He has served as lecturer, Associate
Professor, Professor, Registrar and the life Member of ISTE, India.

J.Preethi received the B.E. degree in Computer Science and Engineering
from Sri Ramakrishna Engineering College, Coimbatore, Anna University,
Chennai, India, in 2003, the M.E. degree in Computer Science and
Engineering from the Govt. college of Technology, Anna University, Chennai,
India, in 2007 and she is currently pursuing the part time Ph.D. degree in the
Department of Computer Science and Engineering from the Anna University
Coimbatore, Coimbatore, India. Currently, she works as a Assistant Professor
in the Department of Computer Science and Engineering, Anna University
Coimbatore. Her research interests include Mobile adhoc networks, Mobile
Communication systems especially in Radio Access Technology selection,
Fuzzy logic and Neural Networks, Genetic Algorithms and AI.


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