Pourmina and MirMotahhary EURASIP Journal on Wireless Communications and Networking 2012, 2012:14 http://jwcn.eurasipjournals.com/content/2012/1/14 RESEARCH Open Access Load balancing algorithm by vertical handover for integrated heterogeneous wireless networks Mohammad Ali Pourmina* and Navid MirMotahhary Abstract In this paper, an adaptive resource management scheme for hybrid WWAN/WLAN is proposed. Based on proposed joint velocity and average received power (ARP) estimation algorithms, a novel vertical handoff (VHO) for efficient load balancing in multitier network is developed. Simulation results show that proposed scheme achieves significant improvements over conventional schemes. Keywords: mobility aware vertical handover, load balancing heterogeneous networks, power estimation 1. Introduction based on network selection strategies. An active multi- Future wireless communication systems can be visualized mode MT can also change its connections among different as the integration of different radio access network (RAN) types of IHWN. Such a process is called the vertical hand- technologies, to provide always best connected. Heteroge- off (VHO). Traditional handoff algorithms are based on neous wireless networks (HWN) will give the service pro- link quality or estimate of ARP. However, this measure is vider, a chance to provide sufficient capacity, needed to not sufficient for VHO, and other factors like mobile user support the temporally and spatially fluctuating traffic velocity, network condition, and user preferences should demands generated by mobile users. A practical benefit is be considered. Also because of complex structure of next- that users can be served at lower cost and with the better generation networks, more precise and sophisticated quality of service (QoS). To support freedom of movement method for link measurements is required. In mul-tipath between HWNs and seamless roaming, several VHO man- channel, received signal strength (RSS) is consisted of agement architectures and decision-making algorithms three different phenomena (path loss, shadow fading, and have already been proposed [1]. These will allow full fast fading). Because of MT mobility, multipath effect, and exploitation of flexible HWN infrastructure, resources, shadowing, the RSS has fluctuations, which make raw sig- and services. Although individual radio resource manage- nal strength an unstable criterion for triggering vertical ment (RRM) schemes can be tuned to optimally perform and horizontal handoffs (HHOs). Shadowing, large-scale within their respective RANs, they may not efficiently per- variation in path loss, is caused by obstacles in the propa- form in an HWN if the different RRM schemes are not gation path between the MT and the base station (BS). properly managed. Hence, a major issue is how to jointly The small-scale variation is due to the Doppler shift along utilize the resources of the different RANs in an efficient the different signal paths and the time dispersion caused manner while simultaneously achieving the desired QoS by the multipath propagation delays. As one primary indi- and minimizing the service cost from both user and ser- cator of channel quality, the power of the slowly varying vice provider perspectives. In an integrated heterogeneous shadow component is important for handoff decisions and wireless network (IHWN), a mobile terminal (MT) is power control. Most existing handoff algorithms assume equipped with heterogeneous network interface, which is that multipath fluctuations can be adequately filtered and called the multimode terminal. When a multimode MT base their handoff decisions on local mean power estimates generates or originates a new call in an IHWN, it can [2,3]. Although these variations bring back uncertainty in select connections among different types of the IHWN the act of VHO decision making, they can be utilized to extract precious information about propagation environ- ment and mobility behavior of mobile user [4]. In order to * Correspondence: [email protected] Department of Electrical Engineering, Science and Research Branch, Islamic mitigate these variations in RSS, efficient smoothing Azad University, Tehran, Iran © 2012 Pourmina and MirMotahhary; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Pourmina and MirMotahhary EURASIP Journal on Wireless Communications and Networking 2012, 2012:14 Page 2 of 17 http://jwcn.eurasipjournals.com/content/2012/1/14 techniques must be considered. If the averaging interval is multimodal wireless terminals in a heterogeneous environ- too short, fluctuations may not be effectively removed, or if ment; some preliminary measurement works were done the interval is too long, it may cause delay in handoff pro- mainly between WLANs, general packet radio service cedure, or in non-line of site (NLOS) scenarios, it can aver- (GPRS), and digital video broadcasting for terrestrials age out useful information of corner’s positions. To fully (DVB-T), but the described algorithms were very simple, exploit the capacity of the wireless channel, and to over- and it did not present how such redistribution would be come ping-pong effect, an efficient power estimation performed in detailed steps. Efforts on standardization of method is required [5,6]. The ping-pong effect occurs if the VHO operation can be review on [13-15]. Although factors for VHO decision are changing rapidly and an MT the above network selection strategies have their own performs the handoff as soon as it detects a more suitable advantages, they did not put much attention on system BS. Because of heterogeneity, PHY and MAC layer of dif- performance, such as the blocking probability of originat- ferent IHWN are different, so a unified approach must be ing calls and the forced termination probabilities of hori- taken into consideration for collection of specific measures zontal and VHO calls. They fail to address any ARP and from different networks. As a result, more sophisticated also velocity estimation method in order to perform an VHO algorithm is required to extend the throughput of accurate and seamless VHO. Also these have not consid- multilayer network and to increase efficiency of resource ered corner effect and effect of low SNR in cell bound- management for next generation of HWNs. In this paper, aries. In a homogenous environment, the ping-pong effect utilizing an accurate joint velocity and ARP estimation is a phenomenon that rapidly repeats HHOs between two algorithms, a novel VHO algorithm is proposed which can BSs and can be mitigated by means of dwell timer (DT) or effectively be used for load balancing and internetwork hysteresis margin [16]. In a heterogeneous environment, ping-pong effect reduction in HWNs. Also based on Mar- the ping-pong effect occurs if factors for the VHO deci- kov model, an analytical model for performance evaluation sion are changing rapidly and an MT performs handoff as of the VHO algorithm is proposed. This paper is organized soon as the MT detects the better BS [17]. The DT as follows. Section 2 reviews related work on VHO algo- scheme has been used to avoid such ping-pong effects due rithms. In Section 3, propagation model for an IHWN is to the fact that RSS from HWNs is not comparable to introduced. In Section 4, proposed load balancing algo- each other [5,6]. The ping-pong effect can also occur if the rithm is discussed. VHO algorithm analysis framework is MT’s speed is high or its moving direction is irregular. described in Section 5. The performance of the proposed Therefore, the proposed VHO algorithm balances the traf- VHO algorithm is analyzed through a theoretical model fic load in each network based on efficient mitigation of and simulations based on probability of blocking and prob- inter-network ping-pong effect and also based on MT ability of false network layer assignment in Section 6. mobility behavior. Finally, paper is concluded in Section 7. 3. Propagation and noise model 2. Previous works The propagation model discussed here takes into Early works on VHO considered multitier homogeneous account correlated multipath fading, correlated lognor- networks and used the RSS as the main factor of the mal shadowing, and a distance dependant trend [18]. A hand-off decision-making process [7]. However, the VHO discrete model (with the sampling rate of 1/2BW)for needs to be triggered considering a few more factors [8]. the received signal (RS), g[n], is given by In [9], a VHO algorithm is proposed based on a assump- tion that a data call is kept in the higher bandwidth net- γ [n]= s[n].r[n]+η[n] (1) work as long as possible and voice calls are vertically handed over as soon as possible to avoid handoff delay. In where r[n] is the complex envelop due to multipath [10], a network selection strategy that only considers propagation and user mobility, which contains the ’ mobile users’ power consumption is introduced. To maxi- mobile s Doppler amplitude information and s[n] is ARP mize the battery life, the mobile user selects the uplink or (local mean) at the MT and h[n]isAWGnoisewith σ 2 downlink that has the lowest power consumption from all zero mean and variance of n which is added to the RS. of the available networks. In [11], a policy-enabled net- r[n], s[n], and h[n] are mutually independent. r[n]is