Abstract-Seamless vertical handoff between different access networks in the next generation wireless networks remains a challenging problem. A recent vertical handoff scheme that is based on Signal to Interference and Noise Ratio (SINR) may not be the best scheme for selecting the service access point or base station. Although this SINRbased scheme has higher system throughput and lower disconnection probability as compared with other vertical handoff schemes, we presume that the distance is a good criterion for decreasing service disconnection probability and increasing system throughput. The Distance-based Scheme for Vertical Handoff (DSVH) that we propose in this paper for heterogeneous wireless networks is a reactive vertical handoff scheme. We suggest that vertical handoff be based on the Received Signal Strength (RSS) and the distances to access points or base stations, where the main goal of this scheme is to enhance system performance in terms of reducing service disconnection probability and increasing system throughput. The simulation experiments show that our proposed scheme, DSVH, significantly outperforms the SINR-based scheme. It reduces the number of dropped users by 20%. However, the throughput improvrmrnt is insignificant; it is about 1%. I INTRODUCTION Nowadays, mobile users demand to be connected with the Internet while they move freely, and Always Best Connected (ABC) has become a very important service for mobile users so as to get high quality services at high data rates [2] . A state-of-the-art Fourth Generation (4G) wireless network is composed of different wireless subnetworks that complement each other [3] . The integration of such heterogeneous subnetworks should allow mobile stations (MSs) to choose the most appropriate access subnetwork among the available alternatives (these include IEEE 802.11 Wireless Local Area Network (WLAN) and IEEE 802.16 Worldwide interoperability for Microwave Access (WiMAX)), in addition to the traditional cellular networks. One of the important issues in mobility is mobility management. This is concerned with location management and handoff management [6] . Location management enables the system to track the locations of mobile stations continuously as they move from one location to another; this might be in the same system or to a different one. On the other hand, handoff management aims to maintain an active connection with high quality between the MS and the network during the movement of the MS. This process requires keeping track of the state of the MS, either when it is linked with some Base Station (BS) [7] or when it is moving from one BS to another [8] . Handoff can be either horizontal or vertical [1] . A horizontal handoff takes place when the MS switches between points of attachment supporting the same network technology. For example, between two neighboring BSs of a cellular network. On the other hand, a vertical handoff occurs when the MS switches between points of attachment supporting different network technologies, for example, between a cellular network BS and an IEEE 802.11 AP [9] [10]. A handoff process can be divided into three stages: initiation process (radio link transfer), decision process and execution process (channel assignment) [11] [12]. In this paper, we are interested in vertical handoff. Several criteria have been proposed in the literature for use in vertical handoff schemes. The main criteria used are discussed below:  Received Signal Strength (RSS): is the most widely used criterion to decide which network to use for the handoff from a candidate list of networks. It is easy to measure and it is directly related to service quality. Obviously, RSS depends on the velocity of the MS, and on the distance between the MS and its point of attachmen. When a MS notices that the RSS is gradually decreasing, it can be assumed that the MS is moving away from the AP. When the RSS is increasing, then the MS is moving towards an AP. The speed and direction of a mobile node indicate the length of time the current connection can be maintained. Another factor is the coverage area of the network. A small coverage area would cause excessive handoffs between access points within the same network, which can lead to high packet loss.  Signal to Interference plus Noise Ratio (SINR): is the  Radio wave propagation deals with the properties and behavior of radio waves as they propagate from the sender to the receiver. There are three key factors that may impede the propagation of waves. They are reflection, diffraction and scattering. Reflection occurs when radio waves collide with a very large object, such as a mountain, a hill and a tower. Diffraction occurs due to collision with an aliasing object (i.e., an object that contains wavy edges and many protrusions surfaces). Scattering happens when the radio waves pass through paths that contain a large number of objects with small dimensions compared with the wavelength, such as foliage, herbs and street signs [14] . Path loss indicates the decline in the power of the wave during transmission from the sender to the receiver. In general, the path loss depends on the properties of the environment, the topography of the earth and the propagation medium, and the distance between the sender, the receiver, and the height of the BS/AP [13] . The propagation or path loss from the sender antenna to the receiver antenna is computed using the equation: P L = 10 log [ P t / P r ] Where P L is the path loss in decibels (db), P t is the transmitted power in watts, and P r is the received power in watts. Several models have been proposed for computing the path loss in various environments. These models are based on experiments in real environments, where all objects that are present in a particular experimental environment are taken into account, whether they are mountains, towers or buildings … etc [15] . Okumura's Model [16] is a well-known model for predicting the value of the path loss in an urban environment. This model is applicable when the frequency of waves is within the 150-1920 MHz range [17] , the distance between the sender and receiver is from 1-100 km, and the height of the antenna of base station is from 30-1000 m. To determine the median path loss between the sender and receiver, Okumura developed a set of curves that give the median attenuation (A mu ) relative to free space in an urban area with BS antenna height of 200 m and mobile station (MS) antenna height of 3 m. [15] . Another model, the Hata model, is an extension of the Okumura model [18] . This model is applicable for the 150-1500 MHz frequency range, the sender-to-receiver distance range of 1-20 km, and base station heights from 30 to 200 meters and MS heights from 1 to 10 meters. A recent SINR-based vertical handoff scheme [28] [29] has higher system throughput and lower disconnection probability as compared with other vertical handoff schemes. However, we presume that the distance is a better criterion for decreasing. service disconnection probability and enhancing system throughput when selecting the best AP or BS. We propose that wave propagation models use the distance to the AP/BS as a main parameter, and propose a reactive distance-based vertical handoff scheme that aims to improve performance as compared with the recent SINR-based vertical handoff scheme. The proposed scheme has been designed and simulated using MATLAB. In order to evaluate the performance of our proposed scheme, we have compared our results to those of the SINR scheme. The reason behind choosing the SINR scheme is that it has high system throughput and low disconnection probability as compared with other vertical handoff schemes. II LITERATURE REVIEW Vertical handoff schemes are essential components of the structural design of the forthcoming 4G heterogeneous wireless networks. These schemes need to be designed to provide the required QoS to a wide range of applications while allowing seamless roaming among a multitude of access network technologies [1] . There are many proposed schemes for vertical handoff in heterogeneous networks, which can be grouped into four categories: RSS-based, bandwidth-based, area-based and fuzzy logic-based vertical handoff scheme. The idea of the RSS based vertical handoff schemes is to calculate and compare the RSS of the current point of attachment against the others to make handoff decision. A lot of previous work and studies have been conducted in this topic [23] [24] [25] [26] . In this section we discuss three representative RSS based vertical handoff schemes. Zahran et al [23] [27] proposed an adaptive lifetimebased vertical handoff (ALIVE-HO) scheme which takes into consideration the RSS, handoff latency, application QoS and delay tolerance, by presenting an applicationbased signal strength threshold (ASST) tuning mechanism to study the performance of vertical handoff between Third Generation (3G) cellular network and WLAN. The ASST have a significant role in future generation wireless networks where access technologies with different characteristics are expected to seamlessly co-exist and efficiently inter-operate. Therefore, the ASST can be optimally tuned for any access network based on practical system characteristics and requirements. In this scheme, the vertical handoff between 3G cellular network and WLAN can be described through two cases; in the first case, when the MS moves towards a WLAN cell. The handoff to the WLAN is trigger if the average RSS measurement of the ) , and the expected lifetime is less than or equal to the handoff delay. An analytical framework has been proposed to evaluate the performance of adaptive lifetime-based vertical handoff (ALIVE-HO) scheme, which is validated by computer simulation. This analytical framework proved that by introducing the lifetime metric the algorithm adapts to the application requirements and the MS mobility reducing the number of superfluous handoffs, and there is an improvement on the average throughput that provides for the MS because of the MS's ability to remain connected to the WLAN cell as long as possible. Another scheme was designed to minimize the probability of unnecessary handoffs and to improve the overall network utilization. Yan et al [25] [26] proposed a vertical handoff decision scheme based on the distance of traveling distance within a WLAN cell (i.e. the time that MS is expected to spend within a WLAN area). The proposed scheme uses two thresholds which are calculated by the MS as it enters the WLAN area: Distance traveling threshold which based on RSS change rate (i.e. time that the MS is expected to spend within the WLAN area) and distance threshold which is calculated based on various network parameters such as handoff failure, handoff probability, radius of the WLAN area and handoff delays. A handoff to a WLAN is initiated if the WLAN coverage area is available and the estimated traveling distance inside the WLAN area is larger than the distance threshold. While a handoff to the cellular network is initiated if the WLAN RSS is continuously, fading and the MS reaches a handoff commencement boundary area based on its speed. The performance analysis showed that the main improvement of this scheme is that it minimizes the probability of handoff failures, unnecessary handoffs and connection breakdowns whenever the predicted traveling distance inside the WLAN cell is smaller than the distance threshold value. Bandwidth based vertical handoff schemes considers the available bandwidth for MS as the main criterion to make handoff decision. A lot of previous work and studies have been conducted in this topic [22] [27] [28] [29] . In this section, three representative bandwidth based vertical handoff schemes are discussed. Yang et al [22] proposed a bandwidth-based vertical handoff scheme between WLAN and Wideband Code Division Multiple Access (WCDMA) network, using the received Signal to Interference plus Noise Ratio as the handoff criteria. This scheme consider the combined effects of SINR from different access networks; where the SINR value from one network being converted to equivalent SINR value to the target network. A handoff to the network with larger SINR is performed, so the handoff algorithm can provide the knowledge of the achievable bandwidth from both access networks to make handoff decisions with QoS consideration. In addition to that, Yang et al [27] recently propose a Multidimensional Adaptive SINR based Vertical Handoff scheme (MASVH) for next generation heterogeneous wireless networks. This scheme uses the combined effects of SINR, MS required bandwidth, MS traffic cost and utilization from participating access networks to provide seamless vertical handoff with multi-attribute QoS support. Simulation results confirm that the new MASVH scheme improves the system performance in terms of higher throughput and lower dropping probability, as well as reduces the MS traffic cost for accessing the integrated wireless networks. Ayyappan et al [28] [29] proposed SINR based vertical handoff scheme for QoS in heterogeneous wireless networks. In order to provide QoS inside the heterogeneous network, the vertical handoff scheme needs to be QoS aware, which can be achieved by gives the SINR based handoff better than RSS based handoff. This scheme considers the received SINR as a handoff criterion, which can be calculated using the Shannon's capacity theorem as R = W log2 (1 +  / Γ) Where, R is the maximum throughput, W is the carrier bandwidth,  is SINR received at MS, Γ is the gap between uncoded quadrature amplitude modulation and channel capacity. The handoff is initiated when the MS receives higher equivalent SINR from another network. In such cases, the MS tries to switch to another network that will satisfy the service QoS attributes. Simulation results prove that the proposed SINR based vertical handoff scheme provides higher overall system throughput as well as minimum number of dropped MS. Rafiq et al [30] proposed a vertical handoff scheme that takes into account end-to-end QoS in addition to other common parameters. The scheme present an architecture involving an external host based light-weight server, called Access Link Utilization Monitor (ACUM) that disseminates the available end-to-end bandwidth to the mobile node to assist it in making a decision to maintain end-to-end service quality. The authors also describe a fuzzy logic based algorithm that is used in the handoff decision. Area-based schemes make use of geographical information that is gathered by either GPS devices or a physical layer support. Such additional coverage area information is exploited in making proper vertical handoff decision. In this section, two representative areabased vertical handoff schemes are discussed. The mobility models of MS are used as input data for predicting the next served AP. Zhang et al [31] predict a handoff based on the movement of MS using its current location, direction and velocity, to predict the next location L [x, y] after certain period. It finds a serving AP JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 4, NO. 1, FEBRUARY 2012 69 of the location L and if it different from the current AP, it initiates the handoff to that AP. Distance from AP is another scheme to predict a handoff that based on the current position of the MS. The MS compares the distance from the current associated AP with the distance from the APs of neighbor cells. When MS is moving away from the current AP, it calculates the time when it will get out of the cell. If it determines that it will be out of cell in several scans later, it decides the handoff and searches for the next nearest AP. If there is a nearer AP than the current associated AP, the MS determines the handoff to the nearest AP [31] . proposes the Fuzzy NormalizationHandOver Decision strategy algorithm (FUN-HODS) , to obtain system-loading balance and to avoid failures caused by mobile node handovers to network access points with lower velocity capabilities and weaker RSS requirements. The characteristics of fuzzy normalization were applied to handover decisions to make vertical handover decisions as simple as the horizontal one. The simulation experiments proved that the handover fail probability in the FUN-HODS algorithm is lower than the handover fail probability for a traditional fuzzy algorithm when each mobile node has random velocity. D. Fuzzy Logic-basedVvertical Handoff scheme III DISTANCE-BASED SCHEME FOR VERTICAL HANDOFF (DSVH) IN HETEROGENEOUS WIRELESS NETWORKS As discussed before in section two, the radio wave propagation model has been determined for hotspot communication in wireless access technologies like WLAN and WCDMA network. This model is based on extensive experimental data and statistical analysis to compute RSS for WLAN and WCDMA network. The first idea in our scheme is to rely on this radio wave propagation model to calculate the RSS in both WLAN and WCDMA network as well as to study and evaluate the handoff process between WLAN and WCDMA network and vice versa. Our distance-based algorithm is based on RSS, WLAN and WCDMA network vertical handoff threshold value and minimum distance between mobile user and corresponding APs or BSs. The second idea in our scheme is to calculate the mean throughput value for WCDMA and WLAN based on Shannon capacity theorem [22] [28]. We consider WCDMA as an example of 3G networks. Shannon's theorem is based on the average RSS value, available bandwidth and the total noise or interference power over the bandwidth. It uses these parameters in computing channel throughput. In this paper we compare mean throughput values for WLAN and WCDMA network in both SINR and DSVH schemes, this is to see whether the proposed scheme improves the throughput. The Shannon theorem states that the throughput, R, is an upper bound of data rate that can be sent with a given average RSS through an analog communication channel subject to an additive white Gaussian noise of power Г. The expression below represents the maximum possible rate of information transmission through a given channel or system. The channel bandwidth, the received signal level, and the noise level set the maximum throughput rate. It is computed as follows: R = W log2 (1+ Ɣ / Г) (1) Where R is the channel capacity (throughput) in bits per second. W is the bandwidth of the channel in hertz. Ɣ is the total received signal power. Г is the total noise or interference power over the bandwidth, measured in watt or volt. The RSS and throughput for both SINR and DSVH schemes are calculated and explained next.
10 "Distance-based scheme for vertical handoff in heterogeneous wireless networks" by REF .
Distance-Based Scheme for Vertical Handoff in Heterogeneous Wireless Networks
Journal of Emerging Technologies in Web Intelligence
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