Journal of Computer Networking, Wireless and Mobile Communications (JCNWMC) Vol.2, Issue 1 Sep 2012 16-38 © TJPRC Pvt. Ltd.,
INTEROPERABILITY MECHANISMS AND CRITERIA FOR LTE AND MOBILE-WIMAX
ANWAR MOUSA
University of Palestine, P.O. Box: 1219, Gaza- Palestine
ABSTRACT
In this paper, basic mechanisms of interoperability between Long-Term Evolution (LTE) as 4G cellular system and mobile WIMAX networks as 4G wireless are introduced. Two cost-based mechanisms are investigated to represent two interoperability functions: Initial Network Selection (INS) and Inter-Network Handover (INH). Simplified approaches that ease the evaluation of related Cost Functions (CF) are proposed . The necessary assumptions for the implementation of a joint LTE-WIMAX system level simulator platform, through a real coexistence scenario, are proposed. Numerical results show a considerable enhancement in terms of selected performance metrics such as blocking and dropping probabilities. Weight values readjustments are also tested to highlights the critical key factors affecting interoperability mechanisms.
KEYWORDS: LTE, Mobile WIMAX, interoperability, Initial Network Selection, Inter-Network Handover, 4G .
INTRODUCTION
It is an established fact that seamless interoperability among heterogeneous networks represents the major challenge for the success of 4G systems with different evolving access technologies [1]. LTE advanced [2] and mobile WIMAX, IEEE 802.16m [3] are considered as the strongest two candidates of 4G systems. LTE and mobile WiMAX have many similarities but also some significant differences, e.g. in the air interface used in the uplink (SC-FDMA and OFDMA respectively) and in the length of their subframes (1msec. and 5 msecs. respectively). WIMAX possesses independent RAN architecture to enable seamless interoperability with 3GPP (LTE) networks and existing IP operator core network. WIMAX and LTE technologies are available today; WIMAX can offer 70 Mbps peak data rate depending on spectrum allocation and other issues; LTE is able to offer more than 300 Mbps peak data rate downstream.
Based on Mobile IP (MIP) [4], many researchers discussed cellular and wireless interoperability scenarios and mechanisms. Interoperability scenarios among heterogeneous networks manage general tasks such as network selection, authentication, charging, security, service continuity… etc [5]. Depending on service selection and continuity, the 3GPP TR 22.934 [6] defined six common scenarios. 17 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
But regarding the level of coupling between cellular and Wireless networks, two main integration architectures are broadly considered; loose and tight/very tight coupling. In [7], the advantages and limitations of each kind of coupling were thoroughly discussed. In this paper, however, decision key factors based on all affecting parameters such as terminal type, user speed, Prediction, connection history, traffic load and Carrier-to-Interference ratio (C/I), are analyzed. They are evaluated based on simplified cost functions for Initial Network Selection (INS) and Inter-Network Handover (INH) between cellular and Wireless networks. Few researches have illustrated the theoretical analysis for specific individual parameters such as traffic load balance strategies, as clearly illustrated in [8]. But no research in literature, to the extent of my knowledge, has addressed all the aforementioned decision key factors as a whole, as done in this paper.
Initial Network Selection (INS) is one of the basic functions of interoperability process between heterogeneous networks. A clever selection of a suitable network by users would result in lower blocking probability, higher capacity and enhanced QoS. These benefits can be achieved only if INS enables an efficient use of network resources. As a result, users can select the most appropriate network with an enhanced QoS with respect to the desired service requirements. The achievement of these enhancements depends on the integration architecture of the two technologies and on developing efficient INS mechanisms and criteria. An enhanced algorithm for INS between the LTE and WIMAX networks is proposed. When a network has been selected, the user is subject to change the initially selected network according to various conditions; hence the importance of an efficient Inter-Network Handover (INH) arises. INH is based on a simplified cost function evaluation. The proposed cost function covers all possible inter-network handover’s key factors for a specific coexisting deployment scenario of the two networks.
The rest of the paper is organized as follows. Section 2 introduces interoperability architecture and process. In section 3, the two interoperability functions (INS and INH) will be analyzes. Section 4 presents basic assumptions for the implementation of a joint LTE-WIMAX system level simulator and section 5 illustrates the achieved numerical results. Finally, section 6 concludes the paper. INTEROPERABILITY ARCHITECTURE AND PROCESS
LTE-WIMAX Interoperability Architecture
The chosen solution, in this paper, for interoperability between WIMAX and LTE is loose coupling integration architecture. In loose-coupling approach, WIMAX is connected to LTE core network (MME/GW) indirectly through an external IP network such as the Internet. This type of architecture allows for the flexibility and independence of implementing individually different mechanisms within each network. Besides, it allows the gradual deployment of WIMAX network with no or little modification on the LTE networks. On the contrary, in tight coupling architecture, the WIMAX network is connected to the LTE core network as one LTE radio access network. Various drawbacks arise by tight coupling: First, a large volume of WIMAX traffic will go through the LTE core network, possibly making congestion to the LTE. Second, the WIMAX needs to have a protocol stack Anwar Mousa 18 compatible with that of the LTE networks and finally an interface in LTE core networks exposed to WIMAX is required! Figure 1 illustrates a simplified architecture for interoperating LTE networks with mobile WIMAXs where the latter is integrated to external IP networks as loose-coupling.
In LTE, all the network interfaces are based on IP protocols. The eNBs are interconnected by means of an X2 interface and to the Mobility Management Entity/Gateway (MME/GW) entity by means of an S1 interface as shown in Figure 1. The Gateway (GW) is split into two logical gateway entities namely the serving gateway (S-GW) and the packet data network gateway (P-GW) [9]. The S-GW acts as a local mobility anchor forwarding and receiving packets to and from the eNB serving the UE. The P- GW interfaces with external packet data networks (PDNs) such as the Internet. The P-GW also performs several IP functions such as address allocation, policy enforcement, packet filtering, routing…etc [10].
Internet
IP Backbone
L S1 Router W T S1 AA I E M S1 A A N S1 X e
t
w B o Distribution System a r s k e d
I P
N WBS e WBS t w
eNB: enhanced Node B MME/GW: Mobility Management Entity/Gateway WBS: WIMAX Base Station MMS: Multi-Media Messenging Service
Figure 1: Interoperating Architecture for LTE/4G and Mobile WIMAX. WIMAX Is Integrated to External IP Networks.
LTE-WIMAX Interoperability Process 19 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
Figure 2 shows a general LTE-WIMAX Interoperability Process where each entity is illustrated as follows:
INS/Switch on Terminal Trigger INH/ coverage shortage, radio difficulties, Interoperability location change …etc
No Coexisting Area
Yes
CF-Evaluation Weight values re-adjustment
Network Performanc Selection /Access e Control Procedure
End
Figure 2: LTE-WIMAX Interoperability Process
Trigger Interoperability
Interoperability process is triggered in two situations
1. When a user switch on his terminal. According to his profile, QoS requirements and the capacity of the covering networks, the user has to select the most appropriate network in terms of call/session quality and network capacity,
2. When a user who is already connected to a network observes his call/session quality to drop under a lower limit or when another network is detected to be more appropriate for the specific service requirements. For example, a user may experience bad conditions in his current network caused by population overloading, network coverage shortage, radio propagation difficulties or location change…etc. This user tries to handover to another network and thus increases the possibility to terminate normally his call/session.
Coexisting Area
Interoperability process is performed solely in the coexisting area of the two networks, otherwise it is terminated.
CF Evaluation
In general, a CF can be expressed as follows [11]: Anwar Mousa 20
= β β = CF ∑ PfX (X ) with ∑ X 1 X X
Equation 1: General CF equation
Where βX are the weights corresponding to the preference function Pf(X) for a given factor X.
The purpose of the weights denoted β is to perform a balance between the different criteria. For some reasons, a criterion can be more important than the other ones. These weights are pre-determined in order to meet the system optimization but can be re-adjusted if a particular situation claims it. However, they conform to the following basic rules: ∃ = ∧ ∀ ≠ ≠ β = ∧ β = 1. X / Pf (X ) 0 Y X , Pf (X ) 1⇒ X 1 Y 0
∃ = ∧ ∀ ≠ ≠ β = ∧ β = 2. X / Pf (X ) 1 Y X ,Pf (X ) 0 ⇒ X 1 Y 0
∃ = ∧ ∃ = ∀ β = 3. X1 / Pf (X1) 0 X 2 / Pf (X 2 ) 1⇒ X , X /1 N
∀ ∈ ∀ β = λ 4. X ,Pf (X [1,0]) ⇒ X , X
Equation 2: Rules for the Weights of the Cost Function
Where N is defined as the number of monitored criteria and λ is a scalar.
Network Selection and Access Procedure
Network Selection
The output of the cost function ranges between 0 and 1 and provides also the traffic preference i.e. the preference to connect to one network. The output of the cost function determines the traffic preference according to the following rules: ≤ ≤ ρ 0 CF 1 ⇒ Traffic preference 1
ρ ≤ ≤ ρ 1 CF i ⇒ Traffic preference 2
ρ ≤ ≤ ρ i CF j ⇒ Traffic preference i
ρ ≤ ≤ j CF 1 ⇒ Traffic preference j
Equation 3: Rules for the Traffic Preference
Letting ρ increase in 0.25 step size, the output of the cost function determines the traffic preference according to:
0 ≤ CF ≤ 25.0 ⇒ Traffic preference 1
25.0 ≤ CF ≤ 5.0 ⇒ Traffic preference 2
21 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
5.0 ≤ CF ≤ 75.0 ⇒ Traffic preference 3
75.0 ≤ CF ≤ 1 ⇒ Traffic preference 4
The traffic preference determines the preferred/target network. This preference is scaling from 1 to 4 and is defined as follows:
• Traffic Preference 1: Requests a WIMAX connection and cannot access the LTE network,
• Traffic Preference 2: Requests a WIMAX connection (but can be allocated to the LTE network as well),
• Traffic Preference 3: Requests an LTE connection (but can be allocated to a WIMAX network as well),
• Traffic Preference 4: Requests an LTE connection and cannot access the WIMAX network.
The preference functions express the preference for either WIMAX or LTE network regarding a criterion (or a key factor) and its output varies, in general, within the range [0, 1]. The preference function for a given factor depends on the probability of use of this factor regarding each of the two networks. A probability of use can be seen as the probability to decide that a network is efficient for different values of a factor X. It is calculated on the basis of real information i.e. on measurement or on the basis of assumptions or predicted data that translate in certain cases a specific strategy in order to enhance the overall system performance. Moreover, when the probability of use is based on objective data, it represents the probability to connect successfully to one network. In this respect, the probability of use should be pre-computed by running stand-alone simulations for the given factor X.
A simplified approach that eases the evaluation of the CF is proposed in this paper. This approach focuses on the weight values assigned to the key factors and eliminates the need for pre- determined data and statistics. The CF will no longer depend on preference functions and probabilities of use but only the edge values i.e., 0 or 1, will be assigned for each preference function. In this scope, the value 0 defines the preference for WIMAX and 1 for LTE system, for example. However, values could be reversed. The CF becomes a summation of weight values, each assigned to one of the key factors of the interoperability (INS or INH) process. The purpose of the weights is to perform a balance between the different factors where for some reasons, a factor can be more important than the other ones. Each weight takes a value from the range [0, 1] and the summation of all weight values should not exceed 1. Hence the value of the CF ranges from 0 to 1. These weights can be re-adjusted in order to meet performance optimization. The re-adjustment is carried out by controlling the overall performance by some predefined metrics.
The remaining entities of the LTE-WIMAX Interoperability Process: The Network Access, Performance Control and Weight values re-adjustment will be discussed in details in the next section for each of the interoperability functions: INS and INH.
Interoperability Mechanisms Anwar Mousa 22
In this the two interoperability functions will be analyzes: INS resulting in the choice of a network between WIMAX and LTE networks as the user switches his terminal on, INH leading to the decision for switching from one network to the other. Initial Network Selection (INS)
In this section, an approach is developed for INS of one of the following two interoperating networks: the LTE network as cellular wide area coverage and the WIMAX network as Wireless Medium Area coverage Network (WMAN). This approach, a basic element of interoperability process between these two heterogeneous networks, is based on cost function criteria and covers all possible selection key factors.
INS Key Factors
The basic INS key factors are defined as follows:
• Terminal Type Factor: 4G-Mobile Phones and Laptops are the two major terminal types considered. Laptop is better served by the WIMAX network than the LTE since laptops have a high probability of use in accessing Wireless systems as they replace workstations in office environments and are equipped with wireless access points that enable wireless access in general.
• Speed Factor: The two networks are differently adapted to user speed; the WIMAX network is adapted for speeds from 0 up to 100 Km/h where the LTE network is adapted for speeds from 0 up to around 350 Km/h [12].
• Network Connection History Factor: If one network’s history shows significant number of connection failure for the current user or previous users due to some shortcoming or unstable circumstances then the user is preferred to join the other network.
• Traffic Load Factor: The user should be assigned to the network which has free resources. This is achieved if the selected network load is less than a predefined threshold. The goal is to achieve traffic balance between the two networks. For instance situations where one of the systems is overloaded, thus interference limited, alternative in load balancing is to be investigated. The load factor plays its role in the co-existing coverage area only.
INS Decision Criteria
The decision for INS of one of the two networks is taken in two steps: First, the CF is computed and a preliminary decision is taken depending on the output of the CF to assign the user to one of the two networks. Second, the effect of some mandatory aspects is taken into account to finalize the decision.
CF Computation
The simplified CF covers the aforementioned four INS key factors. It is computed as follows: 23 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
CF = W-Speed*Pref-Speed + W-Load*Pref-Load + W-Terminal*Pref-Terminal + W-History *Pref-History
Equation 4: Simplified INS CF Equation
Where W-Speed, W-Load, W-Terminal and W-History represent the weight assigned to the speed factor, the Load factor, the terminal type factor and the connection history factor respectively while Pref-Speed, Pref-Load, Pref-Terminal and Pref-History represent the preference to either of the two networks.
The preference functions are assigned 0 (preference to WIMAX) or 1 (preference to LTE) according to the following criteria:
• Pref_Terminal: Laptops are assigned 0 while 4G-Mobile Phones are assigned 1.
• Pref_Load: If the actual LTE load exceeds a predefined threshold the Pref-Load is assigned (privilege to WIMAX selection ) while If the actual WIMAX load exceeds a predefined threshold the Pref-Load is assigned 1 (privilege to LTE selection ). If the load in both networks does (or does not) exceed the threshold, then no weight is assigned to this factor.
• Pref_Speed: If the user speed is between 0 and 60 km/h then the Pref_Speed is assigned 0. For speeds higher than 60 km/h, it is assigned 1.
• Pref_History: If the WIMAX network history shows abnormal connection failure statistics then the Pref_History is assigned 1. On the other hand, if the LTE network history shows abnormal connection failure statistics it is assigned 0. If the two networks have similar connection history (whether success or failure) then it is assigned 0 to privilege initial connection to WIMAX. The output of the CF determines a preliminary decision for network selection according to the rules in Equation 3 as preliminary selection:
Reflection of Mandatory Aspects
• If the preliminary selected network was WIMAX then this selection will be final if the speed of the user does not exceed 100 Km/h1 otherwise the user is assigned to the LTE network (preliminary selection is reversed).
• The user may be blocked if the finally selected network’s enhanced-Node-B or Base Station does not fulfill the required link quality in terms of SNR and QoS. Again blocking is executed if a predefined time delay is exceeded.
1 WIMAX is well adapted for speeds from 0 to 60 Km/h, however speeds up to 100 Km/h could be handled by the network while speeds over this range is considered prohibited. Anwar Mousa 24
INS Procedures
INS is normally triggered when a user, located on the coexisting coverage, switches on his mobile terminal. The network is selected after CF computation, taking into account the mandatory aspects. Connection is performed to the enhanced Node B (e-NB) of LTE network or to the WIMAX Base Station (WBS) providing the best link quality. If the available link does not fulfill the minimum QoS, connection is tried again until the user exceeds a given delay. Once this delay is reached the user is dropped. In Figure 3, T is the time executed by the user searching for an acceptable connection and the Delay is the maximum allowed time before blocking. To evaluate the performance of the INS process, the Blocking probability metric is defined as the percentage of blocked users over the number of connection trials.
Connection Trial
No No T>Delay Successful Connection
Yes Yes Blocking
End
Figure 3: Selection Procedure for Initial Network Selection (INS) Algorithm
Weight Values Re-Adjustment
The value of weight assigned to each CF factor reflects the importance of this parameter in the initial assignment process. This is generally measured by a predefined performance metric that is used to re-adjust the weights in order to optimize the INS performance. Instead of equal weights assignment policy, each factor could be assigned different weights from the possible [0, 1] values starting from 0 with a suitable step size. The re-adjustment of the weight values is carried out at a predefined time period.
Inter-Network Handover (INH)
Following is a proposed approach for INH between LTE and WIMAX networks based on a simplified cost function evaluation. The cost function covers all possible handover key factors. The basic internetwork handover key factors are presented for a specific coexisting deployment scenario of the two 25 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX networks. The cost function for INH decision and the metrics needed for performance evaluation are also produced.
INH Decision Key Factors and Cost Function Computation
The basic key factors needed to make the decision for triggering the internetwork handover (INH) between the WIMAX and LTE networks are introduced. The CF is the summation of weight values, each assigned to one of the key factors. The purpose of the weights is to perform a balance between the different factors. For some reasons, a factor can be more important than the other ones. These weights are pre-determined in order to meet the system optimization but can be re-adjusted if a particular situation claims it.
Each weight takes a value from the range [0, 1] and the summation of all weight values should equal to 1. INH is triggered if the CF exceeds a pre-defined threshold (INH-threshold). INH requests are rank ordered in a priority queue according to their CFs’ values. The effects of each factor and weight assignment are discussed in details as following:
1. Carrier-to-Interference ratio (C/I) factor: The C/I is the factor considered to assess the signal quality of the user. The interference considered consists of thermal noise, intra- system (intra and inter-cell) interference. A weight is assigned to this factor if the C/I ratio results in a Packet Error Rate (PER) that exceeds the upper bound defined by the minimum QoS requirements.
2. Speed factor. The two networks are differently adapted to user speed as follows:
• WIMAX users: The speed is a critical factor for the WIMAX users since this network is adapted for speeds from 0 up to 60 Km/h. If the user speed exceeds this range then a strong weight should be assigned to this factor for seek of INH to the LTE network otherwise there is a big risk to be dropped.
• LTE users: The LTE network is adapted for speeds from 0 up to around 350 Km/h. A weight is assigned to a user if his speed becomes adequate for the WIMAX network, i.e., below 60 Km/h.
3. Traffic load factor: A weight is assigned to this factor if the actual load in one system exceeds a predefined threshold, or if more capacity is required than provided by the system, i.e., not all users can be served, while the other network has still resources available. The goal is to achieve traffic balance between the two networks. The load factor plays its role in the co-existing coverage area only.
4. Prediction factor: INH statistics are recorded for each user. The goal is to prevent users from triggering INH if their statistics indicate previous failures. Hence, a weight is assigned to a user with low record of handover failure. Anwar Mousa 26
The CF is computed as follows:
CF=weight_C/I+weight_Speed+weight_trafficload+weight_prediction
Equation 5: Simplified INH CF Equation
It has to reflect some mandatory aspects like:
° A WIMAX user exits from the coexisting coverage or increases his speed to a level that could not be handled by the WIMAX network. In this case the user is granted maximum value for the CF, (i.e., 1) in order to have the highest priority for INH, otherwise the user will be dropped!
° The CF is valid for users who are allowed to perform INH. Some users are NOT allowed to join the other network due to various reasons (user preference, license …etc.) hence the CF value should always be 0 for these users.
INH Procedure
If the CF value exceeds the (INH-threshold) then the INH procedure initiates. Hard handover mechanism is considered between the two networks, i.e., connection of the current network is cut before the establishment of the new link to the other network. The user requests a connection to the WBS/e-NB from which he receives the best C/I ratio. If that WBS/e-NB cannot satisfy the minimum QoS requirements, the user will scan again all the neighbouring WBS/e-NB until the user exceeds a given delay – once this delay is reached the user is dropped. In
4, T is the time executed by 27 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX the user searching for an acceptable connection and the Delay is the maximum allowed time before dropping.
No CF-value>INH- threshold
Yes
INH Trial
Yes No No T>Delay Successful INH
Yes Yes Dropping
End
Figure 4: Network Access Procedure for (INH) Algorithm
Performance Control
To evaluate the performance of the INH process, the following metrics are defined:
° Handover rate: defined as the number of successful handover to a given network in one second.
° Handover dropping probability: defined as the percentage of dropped handovers over the number of handover trials.
° Average throughput per WBS/e-NB: defined as the average number of bits successfully transmitted per second.
Weight Values Re-Adjustment
The value of weight assigned to each CF factor is re-adjusted, in the same manner as done for INS case, in order to optimize the INH performance.
Joint LTE-WIMAX Considered Scenario and Simulation Environment Anwar Mousa 28
A set of basic assumptions for the implementation of a joint LTE-WIMAX system level simulator is presented, in the following section, including the coexistence scenario.
SYSTEM LEVEL ASSUMPTIONS
LTE Cell Deployment
The LTE cellular layout considered for macro-cell system simulations consists of a hexagonal grid assuming 19 cell sites and three sectors per site with a total of 57 sectors as shown in Figure 5. The sectored architecture is selected for interference reduction. For the sector of interest all dominant interferers (dark shaded sectors) and sub-dominant interferers (light shaded sectors) are detected.
For the 3-sector sites the antenna bore sight points toward the flat side of the cell. The cell radius is calculated as ISD/ √3, where ISD is the inter-site distance with hexagonal cell layout (1732m). The users are dropped uniformly in the entire cell. The minimum distance between the UE and the Node- B is assumed as 35 m [12]. Therefore, if a user is dropped within 35 m from the Node-B, the drop is cancelled and a new drop is attempted.
29 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
8
9 19
10 2 18
3 7 * *** 1 11 ******** 17 ********** ******** *** 4 * 6
12 5 16
13 15
14
** * **** Sector of Interest * **** * Sec.2 Dominant Interfering Sec.1 Sector Numbers Sector Sec.3
Subdominant Interfering Sector
Figure 5: The LTE Cellular Layout
The Antenna Pattern
The antenna pattern (horizontal) for 3-sector cell sites using fixed pattern is given by Eq. 4:
θ 2 ()θ = − − ≤θ ≤ A min 12 , Am where 180 180 θ 3dB
Equation 6: Antenna Pattern (Horizontal) for 3-Sector Cell Sites
Anwar Mousa 30
where θ3 dB is the 3 dB beam-width and Am is the maximum attenuation. The actual antenna pattern for multi-antenna transmission depends upon the MIMO precoding and beam-forming scheme used. In the case of the 3-sector antenna, we assumed θ3 dB = 70 ◦ and Am = 20 dB.
LTE Propagation Issues
Suitable propagation models should be adopted for the interoperating networks according to the specified deployments and scenarios. In an urban and suburban environment, the propagation model introduced in [13] will be used for the LTE deployment as shown in Eq. 5:
L(dB)=128.1+37.6log10(R)
Equation 7: Urban and Suburban Environment, Propagation Model
where R is the distance between the UE and Node-B in kilometers , Slow Fading is added, with Standard deviation of 8dB at carrier frequency=2 GHz. Minimum distance between UE and Node-B ≥ 35 meters and Total e-NB Tx power at 10MHz BW = 46 dBm. UE power class 21dBm (125mW) and 24 dBm (250mW) [14].
LTE User Mobility
In the case of LTE, the speed is initialized according to a speed density function illustrated in Table 1. Speed is kept constant during a simulation run while the user direction may be updated randomly after the mobile travels a predefined distance in the deployment.
Table 1: Speed Probability Density Function for LTE
Speed (Km/h) 0 3 10 20 30 60 90 120 150 2000 250 350
Probability 3 4 5 7 9 10 11 13 17 10 9 2
WIMAX Cell Deployment
As far as the WIMAX case is concerned, the cell deployment consists of 16 omni-directional Base Stations placed in circular cells (square cells are drawn in figure 6 for convenience).
14 15 16 13
9 10 11 12
5 6 7 8
1 2 3 4
Figure 6: The WIMAX Cell Numbers 31 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
WIMAX Propagation Issues
For WIMAX network, COST-Hata-Model is adapted. The COST-Hata-Model is the most often cited of the COST231 models [15], also called the Hata Model PCS Extension. It is a radio propagation model that extends the Hata Model (which in turn is based on the Okumura Model) to cover a more elaborated range of frequencies. It is formulated in Eq. 6 as:
L = 46.3 + 33.9log10(f) − 13.82log10(hB)− a(hR) + (44.9 − 6.55log10(hB))log10(D) + C
Equation 8: COST-Hata- Propagation Model
Where L is the total path loss, f is the transmission frequency, hB is the base station antenna effective height 2, d is the length of the path, hR is the subscriber station antenna effective height, C is the correction factor equal to 0dB for medium cities and suburban areas, and 3dB for metropolitan areas. a(hR) is the subscriber station antenna height correction factor as described in the Hata Model for Urban Areas. It is computed as a(hR) = (11log10f − 0.7)hR −1.56log10f − 0.8
This model is mainly applied in the 1.5-2GHz carrier frequency, 30-300m Base Station height, 1-10m subscriber station height and 1-20km distance. The WiMAX Forum recommends using COST231 model for system simulations and network planning of macro-cellular systems in both urban and suburban areas for mobility applications. The WiMAX Forum also recommends adding a 10dB fade margin to the path loss to account for shadowing.
WIMAX User Mobility
In case of WIMAX, the speed is initialized according to a speed density function illustrated in Table 2. Speed is kept constant during a simulation run while the user direction may be updated randomly after the mobile travels a predefined distance in the deployment.
Table 2: Speed Probability Density Function for WIMAX
Speed (Km/h) 0 1 3 8 10 20 30 50 70 80 90 100
Probability 6 8 5 7 9 8 11 13 12 10 7 4
Joint LTE-WIMAX Considered Scenario
The considered joint scenario represents the coexistence of the LTE macro deployment with an outdoor WIMAX as illustrated in Figure7. In the coexisting area of the two networks, some users although located on the WIMAX coverage could be connected to the LTE e-NB’s. Anwar Mousa 32
8
9 19
10 2 18
3 7 * *** 1 11 ******** 17 ********** ******** *** 4 * 6
12 5 16
13 15
14
Sec.2 LTE Cells & Sec.1 WIMAX Network Sec.3 Sector numbers
Figure 7: Combined LTE-WIMAX Deployment
The developed software simulation platform- using C++ language and based on IST- FITNESS Simulation Platform [16] - incorporates the appropriate functionalities to examine seamless interoperability between the two networks. The platform takes into consideration: networks configurations, propagation conditions, fast fading, service requirements and the aforementioned interoperability algorithms (INS and INH).
Numerical Result Following are the numerical results for each of the interoperability algorithms (INS and INH).
INS Numerical Results
The achieved numerical results regarding INS are classified as follows: 33 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
Blocking Probability
Error! Reference source not found. 8 shows the blocking probability for users who have initially selected one of the interoperating networks for two test cases: The first represents the interoperating case between WIMAX and LTE networks where INS algorithm is activated with equal weights assigned for each factor of the CF. The second simulates the standalone case where INS algorithm is deactivated. It can be seen that the blocking probability drops nearly to the third when INS algorithm is activated; from 8.7% to 3.2% for LTE network and from13% to 4.3% for WIMAX. This indicates that INS gives an alternative chance for users to join the other network. Thus, the potential for the users being blocked will be minimized.
Blocking Probability in Interoperating Networks
0.14 0.12 0.1 0.08 WITH INS 0.06 WITHOUT INS 0.04 0.02 Blocking Probability 0 LTE WIMAX Netw ork Type
Figure 8: Blocking Probability for Interoperating Networks with and without INS Algorithm.
Weight Values Re-Adjustment
The value of weight assigned to each CF factor reflects the importance of this parameter in the initial selection process. This is generally measured by a predefined performance metric that is used to re-adjust the weights in order to optimize the INS performance. In Error! Reference source not found. 9 and 10, equal weights were assigned for the four factors constituting the CF; each factor was assigned 0.25. In the following scenario, the blocking probability was used as a performance metric to study the effect of assigning different combinations of weight values for each factor. Each factor is assigned four different weights from the possible [0, 1] values starting from 0 with 0.25 as a step size weight. The remaining factors equally share the rest weight. For instance, if a factor is assigned 0.75, each of the remaining three factors takes (1.0 – 0.75)/3 = 0.25/3 = 0.083 as assigned weight. Consequently, the four curves intersect only at the point where they have equal weights, i.e., 0.25. The re-adjustment of the weight values is carried out by the algorithm every a predefined time period (10 seconds in the simulation) where the blocking probability is calculated for different weight combinations.
For the LTE network, the curves for the four factors are plotted in Error! Reference source not found. 9. By examining the speed factor, it can be noticed that when high weight is assigned to it, the Anwar Mousa 34 blocking probability is increased. This indicates that the speed factor is not critical (not important and should be assigned low weight) for INS, which is expected since the LTE network is adapted for low and high speeds as well. The critical factor is shown to be the load, which yields the minimum blocking probability at a weight equal 0.75. Note that the connection history can be seen as the second important factor, being more critical than terminal type.
In
0.1 0.09 0.08 0.07 Speed 0.06 Terminal 0.05 History 0.04 Load 0.03 0.02 Blocking Probability 0.01 0 0 0.25 0.5 0.75 1 Weight
Figure 10, the curves of the four factor of the CF are plotted for the WIMAX network. The speed seems to be the most critical factor for INS. Assigning high weight to the speed factor (0.5) yields the minimum blocking probability. This is not surprising since the WIMAX network is not adapted for high speeds where the risk of blocking is significant. Note that the terminal type is not critical for the selection process since the WIMAX network is adapted for both types of terminals (Laptop and 4G mobile). 35 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
0.3 0.25 0.2 Speed Terminal 0.15 History 0.1 Load
Blocking Probability 0.05 0 0 0.25 0.5 0.75 1 Weight
Figure 9: The Blocking Probability Versus Different Weight Combinations of the CF Factors for the LTE Network
0.1 0.09 0.08 0.07 Speed 0.06 Terminal 0.05 History 0.04 Load 0.03 0.02 Blocking Probability 0.01 0 0 0.25 0.5 0.75 1 Weight
Figure 10: The Blocking Probability versus Different Weight Combinations of the CF Factors for WIMAX Network
INH Numerical Results Anwar Mousa 36
The achieved numerical results regarding INH are classified as follows:
Dropping Probability
Figure 11 shows the dropping probability in the two interoperating networks for two test cases: The first represents the case when INH is activated (in both directions) in addition to the usual intra- Network handover. The second represents the case when INH is deactivated and the network supports intra-Network handover only. It can be seen that that the dropping probability decreases nearly to the fifth in LTE network and to the fourth in WIMAX when INH is activated. This indicates that INH gives an alternative chance for users whose channel conditions are so bad that cannot continue with their original network hence prevent them from being dropped. We notice that the dropping probability for the LTE network is less than that for the WIMAX one. This is thanks to the sectorization implemented in the LTE network, which decreases interference and enhances the overall channel characteristics for users.
Dropping Probability in Interoperating Networks
90 80 70 60 50 WITH INH 3 40 WITHOUT INH 30 20 10 0 Dropping Probability,x10- LTE WIMAX Network Type
Figure 11: Dropping Probability for Interoperating Networks with and without INH Algorithm.
Weight Values Adjustment
The value of weight assigned to each factor of the CF reflects the importance of this factor in the inter-network handover process. This is measured by the performance metrics and used to re-adjust the weights to optimize the INH performance. The dropping probability is used as performance metric to study the effect of assigning different combinations of weight values for each factor. For the LTE network, the curves for the four factors are plotted in Error! Reference source not found. 12. Examining the speed factor, for example, we notice that assigning high weight to it increases the dropping probability, which means that the speed is not critical for INH from LTE to WIMAX. This is expected since the LTE network is adapted for low and high speeds as well. The critical factor is shown to be the load, which yields the minimum dropping probability at a weight equal 0.5.
In Error! Reference source not found. 13, the curves of the four factor of the CF are plotted for the WIMAX network. The speed can be seen to be the most critical factor for INH from WIMAX to 37 Interoperability Mechanisms and Criteria for LTE and Mobile-WIMAX
LTE network. Assigning high weight to the speed factor (0.75) yields the minimum dropping probability. This is not surprising since the WIMAX network is not adapted for high speeds while the destination network, LTE, is well adapted.
In the two Figures 12 and 13, we notice that the C/I factor has nearly the same behavior and seems to be the second important factor for the two networks. Other performance metrics could be used to direct weight readjustment like handover rate and average throughput. This depends on the interests and priorities of the service provider.
35 30 25 Speed 20 C/I 15 Prediction 10 Load 5 Dropping Probability,x10-3 Dropping 0 0 0.25 0.5 0.75 1 Weight
Figure 12: The Dropping Probability versus Different Weight Combinations of the CF Factors for the LTE Network
35 30 25 Speed 20 C/I 15 Prediction 10 Load 5 Dropping Probability,x10-3 Dropping 0 0 0.25 0.5 0.75 1 Weight
Figure 13: The Dropping Probability versus Different Weight Combinations of the CF Factors for WIMAX Network
CONCLUSIONS Anwar Mousa 38
Two cost-based mechanisms were investigated to represent two interoperability functions: Initial Network Selection (INS) and Inter-Network Handover (INH) between LTE and mobile WIMAX networks. Simplified approaches that ease the evaluation of related cost functions were proposed. Besides, the necessary assumptions for the implementation of a joint LTE-WIMAX system level simulator platform, through a real coexistence scenario, were presented. Simulation results have shown a significant gain of implementing interoperability functions over standalone cases through selected performance metrics: The blocking probability drops nearly to the third when INS algorithm is activated and the dropping probability decreases nearly to the fifth in LTE network and to the fourth in WIMAX when INH is activated. Moreover, the value of weight assigned to each decision key factor was readjusted to reflect the critical parameters and optimize interoperating networks ‘performances: The critical factor for LTE network was shown to be the load, which yields the minimum blocking and dropping probabilities while the speed can be seen to be the most critical factor for WIMAX network.
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TABLE OF FIGURES
Figure 1: Interoperating architecture for LTE/4G and mobile WIMAX.
Figure 2: LTE-WIMAX Interoperability Process
Figure 3: Selection Procedure for Initial Network Selection (INS) algorithm
Figure 4: Network Access Procedure for (INH) algorithm
Figure 5 : The LTE cellular layout
Figure 6: The WIMAX Cell Numbers
Figure 7: Combined LTE-WIMAX deployment Figure 8: Blocking probability for interoperating networks with and without INS algorithm.
Figure 9: The blocking probability versus different weight combinations of the CF factors for the LTE network.
Figure 10: The blocking probability versus different weight combinations of the CF factors for WIMAX network
Figure 11: Dropping probability for interoperating networks with and without INH algorithm
Figure 12: The dropping probability versus different weight combinations of the CF factors for the LTE network.
Figure 13: The dropping probability versus different weight combinations of the CF factors for WIMAX network
Anwar Mousa 40