**1. Introduction**

[25] NS-2 The Network Simulator, "http://www.isi.edu/nsnam/ns/", 2012.

[27] Evalvid Tool-Set. "http://www.tkn.tu-berlin.de/research/evalvid/", 2012.

[28] Z. Wang, L. Lu, Bovik AC (2004) Video quality assessment based on Structural dis‐ tortion measurement, Signal Processing: Image Communication, vol. 19, no. 2 2004.

[26] NIST WiMAX, "http://ndsl.ie.cuhk.edu.hk/", 2012.

84 Selected Topics in WiMAX

In the past, the earlier cellular systems concentrated on voice calls as the main application that has tobe consideredtofulfil end-userrequirements.However,nowadayswiththevarietyofthe user application and their requirements, the new 3G/4G systems have to consider many applications such as voice calls video streaming/conference, online gaming, peer-to-peer applicationandmanyotherapplicationandtheirdifferentrequirementsasshowninTable1[1-2].


**Table 1.** WiMAX Application Types

© 2013 Ben-Mubarak et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Although Mobile WiMAX promises to deliver the triple play services, handover mechanism still has some unsolved issues, which may affect the end user QoS requirements [3]. Cell selection is one of the main phases which may affect the user requirements after the handover process. After the Serving Base Station (SBS) advertisement message, MOB\_NBR-ADV and the MS handover scanning, the MS will examine the collected PHY information on the neighbour BSs (nBSs) channel conditions to select the Target Base Station (TBS) for the coming handover [4]. The conventional cell selection scheme in Mobile WiMAX is based on signal quality, i.e. the nBS which has the best signal quality in terms of Received Signal Strength, will be considered as the TBS for the coming handover [5]. However, a single criterion like signal quality is not sufficient as a basis to choose the best BS for different end user application's requirements. As an illustration in Figure 1, suppose the MS is in an overlapping area of two or more BSs that have similar signal quality, there will be an ambiguity on which one will the MS choose for different user application requirements. Putting a cell selection criterion on signal quality entirely may make an MS choose a TBS with a good signal quality but one which may incur higher delay or smaller bandwidth, this may affect some real-time applications such as VoIP, video conferencing, streaming media, multiplayer interactive gaming.

another idle cell to get the best channel. In order to select a TBS that best meets the end-user application requirements this paper proposes a smart way for selecting the right TBS based on a set of multiple decision criteria. The method uses Analytic Hierarchy Process (AHP) method for criteria weighting and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) for the TBS ranking based on some criteria such as CINR, queue length, and

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

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

87

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,

**Carrier-to-Interference-plus-Noise-Ratio (CINR):** The signal quality is the main criteria for

bandwidth.

**Figure 1.** Cell selection

BW, and congestion delay.

choosing the TBS, but this is not a sufficient criterion.

**2. Handover cell selection criteria**

Handover operation in Mobile WiMAX can be divided into two main phases; the pre-handover phase, and the actual handover phase. In the pre-handover phase the cell-reselection algorithm is one of the most important steps and this may affect the end-user QoS after the handover. On the other hand, the actual handover phase mainly focuses on handover execution and network re-entry [4]. Most of the published works are on the actual handover phase, about how to execute the scheme with a small number of messages [6-10], while there are only a few researches that focus on cell selection scheme.

The 802.16 standard [6] defines the receive signal strength indicator (RSSI) or carrier-tointerference plus noise ratio (CINR) as the handover trigger factor and cell selection scheme whereas WiMAX supports many multimedia data services therefore, it is not sufficient to let the signal strength be the only reference criteria.

In [11], the authors introduced an enhanced handover target cell selection algorithm for WiMAX network based on the effective capacity estimation and neighbour advertisement. The algorithm let each BS to estimate its idle capacity and broadcasts its effective idle capacity via the media access control (MAC) management message, MOB NBRADV, to help each MS to select the handover target cell. Therefore, the cell selection algorithm uses two criteria, idle capacity and signal strength as a weighted cost function to select the target cell. The result shows that this scheme enhances performance in terms of packet loss ratio and system throughput. Nevertheless, other criteria can be taken into account to provide better services for different user applications. In vertical handover schemes, i.e. handover between different network technologies, some researchers have introduced multi-criteria based network selection as described in [12-13].

Because of the increasing number of mobile stations (MS) and the support of high speed Internet and multimedia data services, the base station (BS) of WiMAX often works at a data rate close to maximum capacity. Therefore, it is not proper to let the signal strength to be the only reference criteria to choose the next cell. The MSs should decide intelligently to switch to another idle cell to get the best channel. In order to select a TBS that best meets the end-user application requirements this paper proposes a smart way for selecting the right TBS based on a set of multiple decision criteria. The method uses Analytic Hierarchy Process (AHP) method for criteria weighting and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) for the TBS ranking based on some criteria such as CINR, queue length, and bandwidth.

**Figure 1.** Cell selection

Although Mobile WiMAX promises to deliver the triple play services, handover mechanism still has some unsolved issues, which may affect the end user QoS requirements [3]. Cell selection is one of the main phases which may affect the user requirements after the handover process. After the Serving Base Station (SBS) advertisement message, MOB\_NBR-ADV and the MS handover scanning, the MS will examine the collected PHY information on the neighbour BSs (nBSs) channel conditions to select the Target Base Station (TBS) for the coming handover [4]. The conventional cell selection scheme in Mobile WiMAX is based on signal quality, i.e. the nBS which has the best signal quality in terms of Received Signal Strength, will be considered as the TBS for the coming handover [5]. However, a single criterion like signal quality is not sufficient as a basis to choose the best BS for different end user application's requirements. As an illustration in Figure 1, suppose the MS is in an overlapping area of two or more BSs that have similar signal quality, there will be an ambiguity on which one will the MS choose for different user application requirements. Putting a cell selection criterion on signal quality entirely may make an MS choose a TBS with a good signal quality but one which may incur higher delay or smaller bandwidth, this may affect some real-time applications such

as VoIP, video conferencing, streaming media, multiplayer interactive gaming.

researches that focus on cell selection scheme.

86 Selected Topics in WiMAX

the signal strength be the only reference criteria.

selection as described in [12-13].

Handover operation in Mobile WiMAX can be divided into two main phases; the pre-handover phase, and the actual handover phase. In the pre-handover phase the cell-reselection algorithm is one of the most important steps and this may affect the end-user QoS after the handover. On the other hand, the actual handover phase mainly focuses on handover execution and network re-entry [4]. Most of the published works are on the actual handover phase, about how to execute the scheme with a small number of messages [6-10], while there are only a few

The 802.16 standard [6] defines the receive signal strength indicator (RSSI) or carrier-tointerference plus noise ratio (CINR) as the handover trigger factor and cell selection scheme whereas WiMAX supports many multimedia data services therefore, it is not sufficient to let

In [11], the authors introduced an enhanced handover target cell selection algorithm for WiMAX network based on the effective capacity estimation and neighbour advertisement. The algorithm let each BS to estimate its idle capacity and broadcasts its effective idle capacity via the media access control (MAC) management message, MOB NBRADV, to help each MS to select the handover target cell. Therefore, the cell selection algorithm uses two criteria, idle capacity and signal strength as a weighted cost function to select the target cell. The result shows that this scheme enhances performance in terms of packet loss ratio and system throughput. Nevertheless, other criteria can be taken into account to provide better services for different user applications. In vertical handover schemes, i.e. handover between different network technologies, some researchers have introduced multi-criteria based network

Because of the increasing number of mobile stations (MS) and the support of high speed Internet and multimedia data services, the base station (BS) of WiMAX often works at a data rate close to maximum capacity. Therefore, it is not proper to let the signal strength to be the only reference criteria to choose the next cell. The MSs should decide intelligently to switch to
