Advanced Wireless Networks - 4G Technologies

298 CROSSLAYER OPTIMIZATION

where

D( p)is the total expected distortion from mapping N scalable frames to K different priority classes with an allocation policy p, D0 is the expected distortion if no video data are received, D j is the distortion reduction if video layer j is correctly received. The term j ≤ j [1 − β j (θ p j , φ p j )] in Equation (10.2) is the probability that video layer j and all video layers j , on which video layer depends, are correctly received while video layer j is transmitted over the priority class p j . The priority class p j has QoS exponents θ p j and φ p j , which correspond to its guaranteed buffer overflow and delay bound probability, respectively (for details see Chapter 8). On the other hand, β j (θ p j , φ p j ) is the probability that video layer j is lost due to either buffer overflow or playback deadline violation when transmitted

over priority class p j (see again Chapter 8 for details).

The optimal mapping problem can be formally stated as follows. Given the set of rate constraints under the priority transmission system in described above, and the expected channel service rate r, which can be considered stationary in a time period t, corresponding to one GOP, what is the optimal mapping policy p from one GOP with N scalable frames (coded in M video layers) to K priority classes such that D( p) is minimized? This can be

j, p j =i

K

μi (ki ) < r

i=1

where μi (ki )is the rate constraint of priority class i, and bpj j is the size of video layer j, which will be conveyed by priority class pj. The solution to the optimization problem follows a constrained-based search that exploits the dependency among the layers [57]. This is a source coding problem and will not be discussed within this chapter.

Video adaptation is based on using a set of QoS bounds to characterize the range of video quality requirements and transmission service capabilities. Within this set of bounds, QoS parameters of video and transmission service can be adjusted to cope with the timevarying and nonstationary wireless link quality. Owing to the time-varying characteristics of video content and time-varying wireless channel, the set of bounds is also time-varying. Specifically, the QoS bound for video application at time t can be defined as the video

where εi,L (θi,L ) and εi,U (θi,U ) are the respective lower and upper bounds of the guaranteed buffer overflow probability by priority corresponding to QoS exponent θi,L and θi,U . Similarly, the rate constraint corresponding to the statistical QoS guarantee can be expressed as

300CROSSLAYER OPTIMIZATION

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306 MOBILITY MANAGEMENT

LOCATION

MANAGEMENT

LOCATION

CALL

REGISTRATION

DELIVERY

(UPDATE)

Figure 11.1 Components of location management process.

Thus, mobility management supports mobile terminals, allowing users to roam while simultaneously offering them incoming calls and supporting calls in progress. Mobility management consists of location management and handoff management.

Location management is a process that enables the network to discover the current attachment point of the mobile user for call delivery. The main components of the process are shown in Figure 11.1. The first segment is location registration (or location update). In this stage, the mobile terminal periodically notifies the network of its new access point, allowing the network to authenticate the user and revise the user’s location profile. The second segment is call delivery. Here the network is queried for the user location profile and the current position of the mobile host is found. The main issues in location management involve database architecture design, design of messaging procedures and the transmission of signaling messages between various components of a signaling network. Other issues include: security, dynamic database updates, querying delays, terminal paging methods and paging delays.

Handoff (or handover) management enables the network to maintain a user’s connection as the mobile terminal continues to move and change its access point to the network. The three-stage process for handoff first involves initiation, where the user, a network agent or changing network conditions identify the need for handoff. The second stage is new connection generation, where the network must find new resources for the handoff connection and perform any additional routing operations. Under network-controlled handoff (NCHO) or mobile-assisted handoff (MAHO), the network generates a new connection, by finding new resources for the handoff and performing any additional routing operations. For mobile-controlled handoff (MCHO), the mobile terminal finds the new resources and the network approves. The final stage is data-flow control, where the delivery of the data from the old connection path to the new connection path is maintained according to agreed-upon QoS. The segments of handoff management are presented in Figure 11.2.

Handoff management includes two conditions: intracell handoff and intercell handoff. Intracell handoff occurs when the user moves within a service area (or cell) and experiences signal strength deterioration below a certain threshold that results in the transfer of the user’s calls to new radio channels of appropriate strength at the same (BS). Intercell handoff occurs when the user moves into an adjacent cell and all of the terminal’s connections must be transferred to a new BS. While performing handoff, the terminal may connect to multiple BSs simultaneously and use some form of signaling diversity to combine the multiple

Figure 11.2 Components of handoff management.

signals. This is called soft handoff. On the other hand, if the terminal stays connected to only one BS at a time, clearing the connection with the former BS immediately before or after establishing a connection with the target BS, then the process is referred to as hard handoff. Handoff management issues are: efficient and expedient packet processing, minimizing the signaling load on the network, optimizing the route for each connection, efficient bandwidth reassignment and refining quality of service for wireless connections. Below we will discuss the handoff management in some of the component networks of 4G integrated wireless network concept as suggested by Figure 1.1.

11.1.1 Mobility management in cellular networks

Mobile terminals (MTs) are free to travel and thus the network access point of an MT changes as it moves around the network coverage area. As a result, the ID of an MT does not implicitly provide the location information of the MT and the call delivery process becomes more complex. The current systems for PLMN location management strategies require each MT to register its location with the network periodically. In order to perform the registration, update and call delivery operations described above, the network stores the location information of each MT in the location databases. Then the information can be retrieved for call delivery.

Current schemes for PLMN location management are based on a two-level data hierarchy such that two types of network location database, the home location register (HLR) and the visitor location register (VLR), are involved in tracking an MT. In general, there is an HLR for each network and a user is permanently associated with an HLR in his/her subscribed network. Information about each user, such as the types of services subscribed and location information, are stored in a user profile located at the HLR. The number of VLRs and their placements vary among networks. Each VLR stores the information of the MTs (downloaded from the HLR) visiting its associated area.

Network management functions, such as call processing and location registration, are achieved by the exchange of signaling messages through a signaling network. Signaling system 7 (SS7), described in Chapter 1 [34, 38, 63], is the protocol used for signaling exchange, and the signaling network is referred to as the SS7 network.

The type of cell site switch (CSS) currently implemented for the PLMN is known as a mobile switching center (MSC). Figure 11.3 shows the SS7 signaling network which