Vertical Handoff Decision Algorithms

Published Date: 02 Nov 2017

CHAPTER 3

This chapter describes two main existing vertical handoff techniques: classical approach and fuzzy based VHDA in detail. These two algorithms are studied which represent the VHD algorithms based on threshold comparison of metrics and fuzzy logic based dynamic programming techniques, respectively. In case of classical approach, the vertical handoff decision is based on Received Signal Strength (RSS) of the Access Point (AP) of WLAN, whereas in case of the Fuzzy based VHDA the vertical handoff decision is based on fuzzy control theory. The work involves the implementation of these two techniques and their comparison and analysis with respect to the proposed vertical handoff technique.

3.1 Classical approach to VHDA

This is one of the traditional methods of handoff. When the mobile user approaches the cell boundary, horizontal handoff takes place in case of homogeneous networks and vertical handoffs in case of heterogeneous networks. Received signal strength is used as the main handoff decision criterion in this approach. Various strategies have been developed to compare the RSS of the current point of attachment with that of the candidate point of attachment.

In the classical approach [76], the handoff decision is based on received signal strength (RSS) of the access point (AP) of WLAN. The RSS in a mobile radio channel shows that the average RSS at any point decreases logarithmically as a power law of the distance between a transmitter and receiver, whether indoor or outdoor [76]. The average received power (Pr) at a distance (d) from the transmitting antenna is approximated by

Pr = P0 (d/d0) - n (3.1)

Pr = P0 – [10 * n * log(d/d0)] (3.2)

where, n is path loss exponent

Pr is the average received power at a distance d from the transmitting antenna

P0 is the transmitted power or power received at a close-in-reference point in the far field region of antenna at a small distance d0.

Value of n depends upon specific propagation environment as given in Table 3.1 [76]. The value of n lies in the range 1.6 to 6. It is 1.6 in the building line of sight, 2 in free space and still higher in case of obstructions in buildings or factories.

Table 3.1: Path loss exponent values for different propagation environments

Propagation environment

Value of path loss exponent (n)

Free space

2

Urban area cellular radio

2.7 to 3.5

Shadowed urban cellular radio

3 to 5

In building line of sight

1.6 to 1.8

Obstructed in building

4 to 6

Obstructed in factories

2 to 3

RSS is continuously monitored by mobile device. When a mobile terminal moves in a WLAN coverage area overlaid by cellular network coverage shown in Fig.1.3, it may continue to be connected to WLAN if RSS of AP is above threshold. While moving, if the RSS falls below the threshold, vertical handoff takes place between WLAN to cellular in order to avoid disconnection. However, if the mobile terminal is in cellular network then it does not handoff to WLAN till RSS of AP exceeds threshold.

To maintain an acceptable voice quality, the handoff threshold shall be kept slightly above minimum usable signal, (Δ) which is given as [76]

Δ = Pr handoff - Pr minimum usable (3.3)

where, Pr handoff is the average received power at a distance d when handoff is initiated,

Pr minimum usable is the minimum usable received power for acceptable voice quality

The value of delta (Δ) should neither be too large nor too small. If value of Δ is too large unnecessary handoffs may occur and if it is too small there may be insufficient time to complete handoff before a call is lost due to weak signal conditions.

For handoff decision, the received power (Pr) at a distance d from the transmitting antenna is represented by RSSAP and its value is compared with Pr handoff represented as threshold [77]. Accordingly, for classical RSS based handoff algorithm the pseudo code is given in Fig. 3.1.

The mobile terminal is working in an overlay network and is initially connected to access point in WLAN. It keeps on measuring RSS of the access point of WLAN at fixed intervals.

Initially user working in overlay network

Case 1: User connected to AP in WLAN

if RSSAP ≤ Threshold

{

WC ++;

handoff from WLAN to cellular network;

}

else continue working in WLAN

Case 2: User connected to BS in cellular network

if (location of MT== near WLAN) and (RSSAP > Threshold)

{

CW ++;

handoff from cellular network to WLAN ;

}

else continue working in cellular network

where

RSS AP is equal to Pr (defined in equation 3.2)

WC maintains a counter for handoffs from WLAN to cellular network

CW maintains a counter for handoffs from cellular network to WLAN

Threshold is equal to Pr handoff (defined in equation 3.3)

Figure 3.1. Pseudo code for classical RSS based approach to vertical handoff

If the strength of RSSAP is below the threshold, vertical handoff to cellular network is initiated; otherwise MT stays in the current access network. If the MT is initially connected to base station in cellular network and is near WLAN than strength of RSSAP is measured. If the strength of RSSAP is above the threshold, vertical handoff to WLAN is initiated; otherwise MT stays in cellular network. The flow is shown in

Fig. 3.2.

Approaching boundary of WLAN ?

If RSSAP > Threshold

MT connected to AP in WLAN

Handoff to cellular network and increment the counter

If RSSAP

≤ Threshold

N

Y

Y

N

Y

MT connected to BS in cellular

Network

Measure RSSAP

Measure RSSAP

Handoff to WLAN and increment the counter

B

B

A

A

N

.

Figure 3.2. Flow chart for classical RSS based VHDA

3.2 Fuzzy approach to VHDA

In Fuzzy approach to VHDA, the handoff decision is based on fuzzy control theory. The fuzzy based VHDA [45] takes into account three factors for vertical handoff decision i.e. power level (PL) received from candidate base station which is same as RSS, cost of network operation ( C ) to which the candidates base station attaches to i.e. tariff paid by subscriber and the amount of unused bandwidth of candidate base station (BW).

In non-fuzzy algorithm the network parameter contains only 0 and 1values after normalization, whereas, fuzzy system based algorithm considers discrete values within a particular range along with the crisp values, 0 and 1. The fuzzy vertical handoff decision algorithm performs the vertical handoff by applying membership function and weight vector to the input parameters such as RSS, cost and bandwidth.

3.2.1 Membership function for input parameters

Membership function of Received Signal Strength (μ1) is calculated by using normalization factor as proposed in paper [29].

μ1= 0 if 0 ≤ RSS(x) ≤ RSSTH (3.4a)

μ1= (RSS(x) - RSSTH) / (RSSmax - RSSTH ) if RSS(x) > RSSTH (3.4b)

where, RSS(x) is the actual power level of received signal, RSSTH is threshold power level of received signal and RSSmax is maximum power level of received signal that can be received from base station.

Membership function of cost (μ2) is calculated as proposed in paper [29] as

μ2= (1- (C(x)/ CTH) if 0 ≤ C(x) ≤ CTH (3.5a)

μ2= 0 if C(x) > CTH (3.5b)

where, C(x) is the actual cost of operation network which a candidate base station belongs to, and CTH is threshold cost. When C(x) > CTH the user will consider that the operation network is too expensive to accept.

Membership function of bandwidth (μ3) is calculated as proposed in paper [29] as

μ3 = (B(x)/ Bmax ) if 0 ≤ B(x) ≤ Bmax (3.6a)

μ3= 0 if B(x) > Bmax (3.6b)

where, B(x) is amount of unused bandwidth of a candidate base station and Bmax is the maximum amount of bandwidth the candidate base station can provide.

After establishing the membership functions, the membership degree (µk) is calculated as function of input parameters given as

µk = f (RSSk, Ck, BWk) (3.7)

3.2.2 Weight Vector

In order to achieve the optimized vertical handoff decision, dynamic weight vector should be chosen so as to magnify the dominant difference of membership degrees among candidate base stations. The weight Vector is defined as

W = (w1,w2,w3) = [σ 1/ i=1σ i , σ 2 / i=1 σ i , σ 3/ i=1 σ i ] (3.8)

σ i = [1/(n-1) *( nk=1 µi,k (x)2– (1/n*( nk=1 µi,k (x) * nk=1 µi,k (x) )) )]½ (3.9)

where σi is the standard deviation of µi,1(x), µi,2 (x) …. µi,n (x) respectively, for i =1,2,3 where i =1 represents RSS, i = 2 represents Cost and i = 3 represents Bandwidth.

3.2.3 Fuzzy Vertical Handoff Decision

The fuzzy vertical handoff decision value (F) is evaluated for each of the base station (k) by using µk= {µ1,k µ2,k µ3,k} and W={w1,w2,w3}

Fk = W µk (3.10)

where, W is the weight vector

µk is the membership degree for the kth base station.

Fk is fuzzy vertical handoff decision value for kth base station

The base station with maximum value of Fk function is chosen for vertical handoff and to avoid unnecessary vertical handoffs the difference between the functional value Fk of next chosen base station and the current base station should be greater than value of Handoff-Threshold. The pseudo code for the implementation of the vertical handoff decision algorithm based on fuzzy control theory is given in Fig. 3.3.

The power level of the signal measured is taken as RSSAP from the transmitting antenna and its value is compared with threshold which is taken as Pr handoff (defined by equation 3.3). If power level (which is same as RSS of AP in WLAN) falls below threshold, vertical handoff to cellular network may take place depending upon the value of Fk i.e. if Fk is greater than Handoff-Threshold. The value of Fk in turn depends upon two other input parameters, cost and unused bandwidth. The Handoff-Threshold is the minimum value at least by which the next station to be selected is good than the current one so as to avoid unnecessary handoffs. If the mobile terminal is in cellular network then it does not handoff to WLAN till the power level or RSS is above threshold (which is taken as Pr handoff defined by equation 3.3) and it is near the WLAN. The handoff takes place depending upon the value of Fk, which is evaluated depending upon the value of other two input parameters: cost and unused bandwidth. If Fk is above variable Handoff-Threshold (which is the minimum value at least by which the next station to be selected is good than the current) the handoff takes place.

Initially user working in overlay network

Case 1: User connected to AP in WLAN

if RSSAP ≤ Threshold

{

if (max. of Fk > Handoff-Threshold)

{ WC ++;

handoff from WLAN to cellular network;

}

}

else continue working in WLAN

Case 2: User connected to BS in cellular network

if (location of MT = = near WLAN) and (RSSAP > Threshold)

{

if (max. of Fk > Handoff-Threshold)

{ CW ++;

handoff from cellular network to WLAN;

}

}

else continue working in cellular network

where

WC maintains a counter for handoffs from WLAN to cellular network

CW maintains a counter for handoffs from cellular network to WLAN

Threshold is equal to Pr handoff. (given by equation 3.3)

Handoff-Threshold is a minimum value at least by which the next station to be selected is good than the current one so as to avoid unnecessary handoffs.

Fk is defined as fuzzy vertical handoff decision value for kth base station (given by equation 3.9)

Fig. 3.3. Pseudo code for fuzzy approach to vertical handoff

The flow chart for the same pseudo code is shown in Fig. 3.4.

Input RSS

Evaluate

F

k

F

k

>

Handoff_Thr

eshold

Stay in current

access network

Handoff to C

ellular

Network and

increment

the

counter

Input RSS

RSS

AP

>

Threshold

&

location = near

WLAN

Input

B

W, C

Evaluate F

k

F

k

>

Handoff_Thr

eshold

Stay in current

access network

Handoff to WLAN

and increment the

counter

MT connected to AP

in WLAN

Y

Y

N

N

MT connected to BS

in cellular network

RSS

AP

≤

Threshold

Input

B

W, C

Y

Y

N

B

A

B

A

N

Fig. 3.4. Flow chart for the fuzzy based VHDA

3.3 Summary

This chapter presents two main existing vertical handoff techniques: classical RSS based and fuzzy based VHDA in detail. In case of classical RSS based, the vertical handoff decision is based on Received Signal Strength (RSS) of the Access Point (AP) of WLAN, whereas in case of the Fuzzy VHDA the vertical handoff decision is based on fuzzy control theory. These two algorithms are taken as the representative VHD algorithms and are discussed in detail with the help of pseudo code and flowcharts.