Mobility Management in Mobile Wireless Systems Lecture 9

Cellular Communication No matter what frequencies are used. There is no radio frequency which can carry data or voice to long distances without serious attenuation. The entire communication is, therefore, achieved in multiple “communication slots” to overcome this limitation. A communication slot is a geographical area within which the RF can be used to set up communication. In cellular terminology this communication slot is referred to as a cell, and the entire communication infrastructure is known as cellular communication. The size of a cell is defined, keeping in mind the energy level of RF to be used and the power of the transceiver.

Cellular Communication To provide communication over a geographical area of any shape- for example, a city- it is logically divided into a set of cells. This logical division helps to identify the best position to install the transceiver for achieving the “best coverage,” which means that a mobile device is able to receive communication from any point of a cell. A mathematical model is used to identify the “best coverage” pattern.

Cellular Communication To understand the meaning of the best coverage, let us use a circle to develop a coverage pattern. Figure (1a) (in slide 5) illustrates an example of covering an area using circles. –It is obvious that the entire area cannot be fully covered by using circle of any size. –The space at the junction of three circles will always remain uncovered. Figure (1b) illustrates the coverage using hexagons which has no uncovered space anywhere.

Figure 1

Cellular Communication A cell must have a wireless component for managing the communication. One of the important coverage factors is the location of its “cell site,” which is the point in the cell where the transceiver is installed. The main objective here is to cover maximum number of calls efficiently.

Cellular Communication Figure (2a) (in slide 8) shows the arrangement used in practice. –The cell site is the smaller circle at the junction of three hexagons which represent cells. –The transceiver covers a portion, referred to as “sector” of each cell, and provides each sector with its own set of channels. This arrangement can also be visualized as shown in Figure (2b). –It appears here that a cell site is located at the center of a cell.

Figure 2: Location of Cell Site

Cellular Communication In a cellular architecture a number of wireless and wired components are required to establish the desired point-to-point or point-to-multipoint communication. A Mobile Database System (MDS) is interested in the components that are directly connected through wireless channel with mobile devices. One such component is the transceiver, which is usually referred to as a base station (BS). A BS functions under the supervision of a telecommunication switch called Mobile Switching Center (MSC) and connected to it through wired line. It is the MSC which connects the entire mobile system with PSTN (Public Switched Telephone Network). Figure 3 illustrates a communication path

Figure 3

Frequency Reuse The continuous connectivity may also be affected by interference, which can be defined as interaction between two signals. –Two communication sessions may interfere with each other if the frequencies they use are quite close to each other or even identical. –It can also occur if the base stations of two closely or if there is another active mobile unit communicating mobile units are located in the vicinity or if there is an active call in a nearby cell.

Frequency Reuse To avoid co-channel interference, two frequencies are kept apart at least by 45 Hz, but in the case of higher frequencies the separation may be less. Each cell is usually assigned 25 to 30 channels for communication, which depends on the expected traffic volume in the cell. This will support only 25 to 30 communication sessions. The number of channels for a cell is determined by a number of factors such as: the density of callers, which indicates the number of callers per square meter; the average activity factor, which relates to the average use of phone in an hour; the probability of blocked calls; and so on.

Frequency Reuse In cellular communication the system has to deal with co- channel interference and adjacent channel interference. Co-channel interference occurs when the same frequency is used for communication in two nearby cells. –Co-channel interference could be tolerable in a voice communication because humans can guess reasonably accurately the words in the presence of noise. –This is not acceptable in data communication because it can corrupt the data to an extent which cannot be recovered by the receiver.

Frequency Reuse Co-channel interference is solved by keeping the cells, which plan to use the same set of frequencies, apart by a distance called frequency reuse distance. A frequency reuse distance is the minimum safe distance between two cells which can reuse the same frequencies without interference. This distance is expressed in terms of intervening cells between the two cells where same frequencies are reused.

Frequency reuse is implemented through cell clustering. A cell cluster is made up of a number of same size cells and the cluster size depends on the entire coverage area, the frequency plan, acceptable co- channel interference, so on. A cluster may be made up of 3,4,7,9, 12, 13, 16, etc., cells; but out of these, 7-cell and 4-cell clusters are most commonly used to cover an area.

Figures 4a through 4d illustrate the composition of clusters of sizes. In reality, an area is covered by a number of clusters, each one composed of different size cells as illustrated by Figure 4e. This cluster organization was in fact used by the Vodafone company to provide communication service around the area covered by M25 ringroad in England. The inner area was populated by smaller-size cells to manage high communication traffic from highly populated user community.

Figure 4

To reuse the same set of frequencies in another cell, it must be separated by frequency reuse distance, which is usually denoted by D. Figure 5 illustrates the separation of cells by frequency reuse distance in a cluster of seven cells.

Figure 5

Location and Handoff Management Mobile unit movement is random and therefore its geographical location is unpredictable. This situation makes it necessary to locate the mobile unit and record its location to HLR and VLR when a call has to be delivered to it.HLR VLR

Location and Handoff Management The entire process of the mobility management component of the cellular system is responsible for two tasks: –(a) location management- that is, identification of the current geographical location or current point of attachment of a mobile unit which is required by the MSC (Mobile Switching Center) to route the call- and –(b) handoff- that is, transferring (handing off) the current (active) communication session to the next base station, which seamlessly resumes the session using its own set of channels.

Location and Handoff Management The entire process of location management is a kind of directory management problem where locations are current locations are maintained continuously. One of the main objectives of efficient location management schemes is to minimize the communication overhead due to database updates (mainly HLR). The other related issue is the distribution of HLR to shorten the access path.