**2. Handover cell selection criteria**

The conventional scheme of cell selection in Mobile WiMAX is based on a single criterion which is signal quality. The nBS which has the best signal quality (ex. RSSI or CINR) will be considered as the TBS for the coming handover [5-6]. As highlighted earlier, this is inefficient because besides the signal quality each user's application has their own respective QoS requirements.

In this paper, two types of application are studied; VoIP as an example of real-time applica‐ tions, and Media Content Downloads as an example of non real-time application [2]. As shown in Table 1, VoIP is a latency or delay sensitive application with low data rate demand. On the other hand, Media Content Download is a delay-tolerant application and generally demand high bandwidth. Thus, there are some other criteria that need to be considered when the cell selection decision is to be taken. In this paper, three criteria will be considered; they are CINR, BW, and congestion delay.

**Carrier-to-Interference-plus-Noise-Ratio (CINR):** The signal quality is the main criteria for choosing the TBS, but this is not a sufficient criterion.

**Bandwidth (BW):** This metric refers to the available bandwidth in WiMAX cell. It is simply the difference between the total capacities and the aggregated used BW in Kbps.

preference on handover criteria is modelled as weights assigned by the user on the criteria; for

The CINR and congestion delay are considered as important for voice application because as studied by [15-16], to get a minimum jitter and delay the CINR has to be good enough, while the bandwidth and CINR are considered important for the non real-time application such as

WSM is the most popular multi criteria decision making (MCDM) method. It is the simplest way of evaluating the number of alternatives (*m*) in terms of a number of decision criteria (*n*) [17]. The overall score of an alternative is calculated as the weighted sum of all the attribute

( ) , 1,2,3,....... .

Where *Ai* is the evaluated score of an alternative, *Wj* is the weight value for criteria *j*, *n* number of criteria. Because the decision matrix value could be in different scales such as BW could be 10 Mbps and the cell load could be 50% or 0.50 the decision matrix has to have a comparable scale (normalized) by using (5) for the benefit criteria (i.e. stronger CINR, larger BW) and (6)

is the normalization value of *xi <sup>j</sup>*

**3.2. Technique for Order Preference by Similarity to Ideal Solution (TOPSIS)**

TOPSIS is one of MCDM methods based on the concept that the chosen alternative should have the shortest distance from the positive ideal solution (PIS) and the farthest from the negative ideal solution (NIS) for solving a multiple criteria decision making problem. Briefly,

*<sup>n</sup> WSM i j ij j*

*Wr w w w rrr* <sup>123</sup> = é ù ë û (2)

http://dx.doi.org/10.5772/55894

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Hybrid AHP and TOPSIS Methods Based Cell Selection (HATCS) Scheme for Mobile WiMAX

*Wnr w w w nr nr nr* <sup>123</sup> = é ù ë û (3)

*A wx i m* = = å (4)

.

*ij ij* / , 1,... , 1,.... *MAX <sup>j</sup> r x x i mj n* = == (5)

*MIN* / , 1,... , 1,.... *ij ij <sup>j</sup> r x x i mj n* = == (6)

is the performance score of alternative *Ai* with

VoIP and Media Content Download which are shown in (2) and (3).

Media Content Download.

**3.1. WSM (Weighted Sum Model)**

values as shown in equation (4).

respect to criterion *xj*

for cost criteria (i.e. more delay). In (5) and (6) *xij*

and *rij*

**Congestion delay:** This is the delay of packets due to queuing until they can be processed.
