A Combined Vertical Handover Decision Metric for QoS Enhancement in Next Generation Networks

Transcription

1 A Combined Vertical Handover Decision Metric for QoS Enhancement in Next Generation Networks Anna Maria Vegni 1, Gabriele Tamea 2,Tiziano Inzerilli 2 and Roberto Cusani 2 Abstract Vertical handover (VHO) techniques are applied when mobile users require service continuity and a seamless roaming between heterogeneous networks. A VHO decision can be taken on the basis of physical parameters, such as the received signal strength (RSS), data rate and signal-to-interference and noise ratio (SINR). In this paper, we compare three VHO decision criterions, each of them based on different physical metrics. A smart combination of several VHO criterions is the core of our proposed approach, which uses RSS and SINR parameters from different wireless access networks. To improve quality of service for mobile users, we also introduce the data rate gain parameter that lets an efficient VHO be executed. Simulation results are in terms of both end-users efficiency, i.e. cumulative received bits, and network performances, i.e. VHO frequency. Keywords- vertical handover; mobility management; data rate gain; VHO frequency. I. INTRODUCTION On next generation wireless networks heterogeneous broadband technologies coexist in order to guarantee a seamless connectivity to mobile users. Different network characteristics are basically expected to different multimedia applications [1]. So, as mobile applications require service and quality-of-service (QoS) continuity, cooperation of mobileaccess networks in heterogeneous environments is an important feature to assure. In this context, vertical handover (VHO) techniques can be applied when connectivity switching is needed [2] to preserve host connectivity and optimize QoS as perceived by the end user. A VHO is a process preserving user s connection on the move and following changes of network (i.e. from a base station (BS) UMTS to an access point (AP) WIFI) [2]. Many approaches are used to vertical hand over from a serving network (SN) to a candidate network (CN), with the goal of provide a maximization of throughput, and a limitation of unwanted and unnecessary vertical handovers [3]. This aspect is also called ping-pong effect [4] and leads to excessive network resource consumption and also affects mobile terminal s performance (i.e. battery life). The minimization of the number of VHOs is an important deal in handover management and many criterions have been proposed in the state of the art. In [3] the authors present an algorithm based on predicted distance information. However, no results 1 A.M. Vegni is with the Department of Applied Electronics, University of Roma Tre, Via della Vasca Navale 84, Rome, Italy. avegni@uniroma3.it. 2 G. Tamea, T. Inzerilli, and R. Cusani are with the Department of INFOCOM, University of Rome Sapienza, Via Eudossiana 84, Rome, Italy. {inzerilli, tamea, cusani}@ uniroma1.it. throughput or cumulative received bits by the mobile terminal (MT) have been evaluated. Limitation of the ping-pong effect is just obtained by MT s distance, but no physical network parameters are mentioned. Basically, several factors are considered as VHO decision criterion, as well as the traditional RSS parameter [5]. By the way, RSS based VHO does not give good performance for MT wide QoS requirements. The SINR parameter is particularly used as a metric in order to provide seamless handover with adaptive data rate (DR), as described in [8], while information about MT s location [6] or monetary cost [7] can be used to preventive VHOs. Normally, most effective VHO decisions are obtained by hybrid VHO approaches, based on two or more metrics, such as the RSS and the distance information between a BS and a MT [9]. In [10] a VHO decision function is obtained as a composition of the monetary cost, power requirements, security parameters, MT preference, network conditions, and MT s speed. No simulation results are reported for this approach. Then, in [8] the authors address on the SINR parameter as VHO metric, but do not consider it in the context of a hybrid vertical handover criterion. In this paper, the main physical parameters (i.e. RSS, SINR and data rate) are mixed in a hybrid VHO approach, in order to improve end-user QoS. Basically, a comparison between three different VHO decision functions (VHDFs), based on single and combined VHO decision metrics, has been evaluated. Single metrics are respectively RSS and DR, while the combined one is a contribution of RSS plus SINR and DR parameters. In Section II we introduce both the single-metric based VHDFs (i.e. called as RSS-VHDF, and DR-VHDF). Section III deals with the combined-metric based VHDF (C- VHDF). Effectiveness of C-VHDF is tested in terms of maximization of cumulative received bits and limitation of number of VHOs (Section IV). Finally, the main findings of our study are summarized in Section V. II. SINGLE-METRIC BASED VHDF In Subsection A, the RSS-VHDF method is illustrated, while the DR-VHDF is presented in Subsection B. A. RSS-VHDF The RSS-VHDF is based on RSS parameter, measured by different MT s Network Interface Cards (NICs), (i.e. UMTS, and WIFI NICs) [5]. Basically, smartphones and mobile devices are equipped with several NICs, such as UMTS, GSM, WIFI, Bluetooth, GPS, and so on. Let us assume that a MT is

2 moving in an outdoor heterogeneous network environment, composed by different network hotspots (i.e. UMTS macrocells and WIFI microcells). The MT moves at low speed (i.e. 0.5 m/s, depicting a pedestrian environment) and in each position it can receive the RSS parameter by the SN, and measures the RSS parameter by a CN, if available. Without loss of generality, we can assume that UMTS is the SN, and WIFI a possible CN. The RSS-VHDF performs a power monitoring of the SN at regular intervals of times, called as waiting times, in order to limit both the battery life of the MT and the number of vertical handovers [5, 6]. The main steps of RSS-VHDF are the following: 1. First VHO alarm: when the RSS level from the SN is decreasing under a fixed value (i.e. P SN < S SN + Th 1 ), a first VHO alarm is given by the MT. Basically, S SN is the MT s sensibility for a particular SN, and Th 1 a threshold to avoid unexpected connection interruptions; 2. Network discovery: after the first VHO alarm has occurred, the network discovery is initiated in order to find an available CN to handover to; 3. Power Testing: if a CN is available and its RSS parameter is higher than a VHO threshold (i.e. P CN > H CN-VHO ), the channel estimation phase starts; 4. Channel estimation: the channel estimation is evaluated when the MT requires a QoS enhancement. The goodput parameter is instantaneously estimated both for the SN and the CN (i.e GP SN, and GP CN, respectively). If GP CN > GP SN, a VHO is performed; otherwise the MT will repeat this phase later, after waiting T wait seconds [5, 6]; 5. Idle mode: just after a VHO occurrence, the MT is in idle mode for at least the waiting time parameter T wait. The values of T wait are independent on network technology. The RSS-VHDF approach is described in Fig. 1. The power testing phase is the core of RSS-VHDF, as a VHO occurs only if the CN s RSS parameter exceeds a VHO threshold, i.e. as follows: Equation (1) represents a necessary condition for VHO execution, after channel estimation is performed [5]. If (1) is not verified, no VHO will be executed. The waiting time parameter is introduced to limit the ping-pong effect, (i.e. when a MT moves on the boundary of a wireless cell and makes frequent handovers). The waiting time parameter is defined as the minimum time that must be assured between two consecutive handovers [5, 6]. The greater is the waiting time the smaller will be the number of performed vertical handovers, and for all the waiting time duration the MT is in idle mode. We considered the same values for T wait, both if the MT is in the SN, and in the CN, as described in [5] and given by the following formula: (1)

3 where is the total transmitting power of the k-th UMTS BS, is the transmitting power of the j-th UMTS BS to the i-th MT, and is the channel gain between the j-th UMTS BS and the i-th MT. From (3) and (4), it derives the maximum achievable data rate for the i-th MT connected with the j-th UMTS BS, and WIFI AP, respectively. Then, by considering the downlink transmission from the AP/BS to the MT, respectively for WIFI and UMTS networks, for a given carrier bandwidth W, and by Shannon capacity formula, the data rate expression can be written as [11, 12] where represents the channel coding loss factor [db]. Equation (5) for WIFI and UMTS network becomes, respectively: (5) (6) that from WIFI to UMTS, (i.e. ), for different values, in order to be VHO type independent. The overall DR-VHDF process is illustrated in Fig. 2. This approach has main analogies with RSS-VHDF, but the VHO decision that is based on a data rate gain. Just after the first VHO alarm and the network discovery, like in RSS-VHDF, the DR-VHDF introduces the Data Rate gain phase, as a VHO decision criterion. No power testing phase is considered because the metric is different. The core step of DR-VHDF is the Data Rate gain, whose condition is expressed by (8) and (9), when UMTS is the SN, and WIFI the CN, and vice versa. Briefly, the following steps compose the DR-VHDF algorithm: 1. First VHO alarm: as in RSS-VHDF; 2. Network discovery: as in RSS-VHDF; 3. Data rate gain: if a CN is available, a data rate comparison is performed between SN and CN. If condition (8) or (9) occurs (i.e. depending if WIFI is the CN and UMTS the SN, and vice versa), a VHO is initiated to the CN; 4. Idle mode: as in RSS-VHDF. (7) where W WIFI = 22MHz [11] and W UMTS = 5MHz [12] are carrier bandwidth. The parameters WIFI and UMTS are the channel coding loss factors, corresponding to 3dB [11] and 12dB [12], respectively. In order to assure service continuity, the data rates from two different networks have to be the same or differ of a bandwidth gain value R gain [Mbps]. As an example, if we consider a handover from UMTS to WIFI, data rate comparison is given by where represents the data rate gain when moving from the UMTS to the WIFI network. Again, in case of a handover from WIFI to UMTS, the data rate condition will be where represents the data rate gain when moving from the WIFI to the UMTS network. Hence, in both types of transitions, a vertical handover occurs only if the network switching can offer a gain of data rate. In Section IV, the data rate gain for VHO from UMTS to WIFI has been set equal to (8) (9) Figure 2. Main steps of DR-VHDF approach. III. COMBINED-METRIC BASED VHDF In the C-VHDF, the combined metric is based on the power control and the data rate gain; the first is typical from RSS- VHDF, the latter from the DR-VHDF. In this way, a VHO is performed if the power testing is verified according to (1), and then a data rate gain is assured by the CN, as expressed in (8) or (9). The C-VHDF process is depicted in Fig. 3. The main steps are: 1. First VHO alarm: common to RSS and DR-VHDF; 2. Network discovery: common to RSS and DR-VHDF; 3. Power testing: common to RSS-VHDF; 4. Data Rate gain: common to DR-VHDF; 5. Idle mode: common to RSS and DR-VHDF.

4 The combined RSS and DR metrics avoid reducing the VHO frequency thought maximize network performance (i.e. cumulative received bits). The comparison of the three VHDF approaches is illustrated in the following Section IV. CRBs) shows the effectiveness of C-VHDF algorithm. As a matter of fact, the average of CRBs sampled at the 2500-th step for C-VHDF has low different values, for each value of waiting times, while RSS and DR-VHDF have high different values, decreasing for increasing values of waiting time parameter. The C-VHDF gives maximum performances, independently of the waiting time parameter. TABLE I. BS/AP Max. Tx Power Cell Radius MAIN SIMULATION PARAMETERS Channel Gain Sensibility Background noise UMTS 43 dbm 600 m 20% 100 dbm 100 dbm WIFI 20 dbm 120 m 60% 100 dbm 96 dbm Figure 3. Main steps of C-VHDF approach. Power testing and data rate gain are the main aspects of the combined metric based approach. IV. SIMULATION RESULTS A Matlab simulation environment is set up in order to validate the effectiveness of the C-VHDF algorithm, with respect to RSS and DR-VHDF. The simulation results were averaged over several heterogeneous scenarios (i.e. 100 scenarios), each of them based on 3 UMTS cells and 30 WIFI APs, modeled in a region of 2Kmq by a map of zones of 5mq, as in [5, 6]. The MT moves for 2500 steps in a pedestrian environment, (i.e. 0.5 m/s speed). In order to approximate a real pedestrian and outdoor environment, each scenario is generated in a random way, such as the AP and BS positions are unknown a priori, and the MT moves in this area in random directions. The probability to perform a handover from UMTS to WIFI and vice versa is strictly dependent by the network setup. Unwanted and unnecessary VHOs can occur as the MT moves in an area with radio coverage (i.e. UMTS and WIFI) unknown a priori. Table I collects the main parameters for WIFI and UMTS cells [5, 11]. For WIFI and UMTS cell radius typical values were set to 120 m [13] and 600 m [14], respectively. According to (3) and (4), the interference power is 40% in WIFI channel, while the ratio of total allocated BS transmits power to UMTS channel is 80%, respectively. Then, the transmitted power in the middle of UMTS and WIFI cell is about 43 and 30dBm, respectively [13, 14]. For the VHO thresholds H CN-VHO for WIFI and UMTS, we respectively set 94dBm and 97dBm, values greater than the sensibility parameter. Finally, the RSS-VHDF performances are obtained by exponential smoothing average channel bandwidth estimation [5], while for DR and C-VHDF we initially considered R WIFI gain = R UMTS gain = 1Mbps. Fig. 4 shows the average of CRBs [Bit], at the end of the MT s walk, versus different values of waiting time parameters (2). The maximization of system performance (i.e. in term of Figure 4. Average total received bits at the end of the MT s path, for different values of waiting time. As the tradeoff between maximization of CRBs and restriction of number of VHOs is an issue for VHO management, the single-metric based VHDFs have lower performance than combined-metric based VHDF. This is shown in Fig. 5, by the average of the number of vertical handovers occurred for the RSS, DR, and C-VHDF vs. the waiting time parameter, respectively. The VHO frequency represents the amount of VHOs initiated and then concluded to a CN, during MT s path. It can be expressed as: N x VHDF, j = n VHO i ( T wait,k ), (10) i=1 where x-vhdf, j is VHO frequency for the j-th waiting time parameter T wait, k (i.e. j= k+1, with j J, J =8), evaluated for a particular VHDF (i.e. x=rss, DR, and C). N is the total number of steps of the MT s path (i.e. N=2500). The value of is 1 if a VHO has occurred in the i-th step, 0 otherwise. As it is noticeable in Fig. 5, the higher the VHO frequency the lower the waiting time parameter (i.e. j=1), and vice versa (i.e. j=6).

5 In Table II the maximum and minimum values of x-vhdf, j are collected for RSS, DR, and C-VHDF, respectively. The value of C-VHDF,1 is lower than all the other maximum VHO frequency values for RSS and DR-VHDF. It shows that the C- VHDF strongly reduces the ping-pong effect, but maintains high CRBs values. VHDF is around [1, 5], while for DR-VHDF is [13, 44]. This proves the effectiveness of a combined VHO approach, respect to a single criterion-based algorithm, while keeping high network performance. Figure 5. Average VHO frequency over 100 simulated scenarios. The DR-VHDF performance are stricltly dependent on the waiting time parameter. Figure 6. Average number of VHOs for DR-VHDF. TABLE II. MAXIMUM AND MINIMUM VALUES OF VHO FREQUENCY RSS-VHDF, j DR-VHDF, j C-VHDF, j j = j = Let us consider the following parameter as an indicator of VHO frequency: (11) It represents the VHO frequency band, that is the range of all possible values of VHO frequency for a particular VHDF. It is obtained as the difference between the maximum and minimum value of averaged VHO frequency. From Table II, we get that RSS-VHDF = 25, while for DR and C-VHDF the VHO frequency bands are the same values, such as DR- VHDF= C-VHDF = 2. Again, the C-VHDF is independent of the waiting time parameter, unlike the single-metric based VHO approaches. Finally, we have compared the DR and C-VHDF performances for different values of data rate gain, i.e. R WIFI gain = R UMTS gain = 0.3, 0.5, 0.7, 0.9, and 1 Mbps. Figs. 6 and 7 depict the VHO frequency for DR and C-VHDFs, respectively. For the same value of data rate gain (i.e. 0.5 Mbps), C-VHDF has lower number of VHOs than that for DR-VHDF. The C- VHDF algorithm never joins the minimum number of VHOs reached by DR-VHDF, independently of the waiting time parameter. As a matter, the range of VHO frequencies for C- Figure 7. Average number of VHOs for C-VHDF. CONCLUSIONS We have presented a comparison between three different VHO decision approaches, i.e. the RSS, the DR, and the C- VHDFs, based on single and combined metrics, respectively. Simulation results show that C-VHDF can assure better network performance in terms of limitation of VHO frequency, respect to single metric-based VHDF algorithm. The main aspect of the proposed C-VHDF algorithm is that a VHO is performed if an available CN can assure a data rate gain. The C-VHDF provides the same performances of RSS and DR- VHDF, in terms of maximization of CRB, but with a reduction of the ping-pong effect.

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