Mobility Management in Mobile Wireless Systems - PowerPoint PPT Presentation

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Mobility Management in Mobile Wireless Systems

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Mobility Management in Mobile Wireless Systems Lecture 9 – PowerPoint PPT presentation

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Title: Mobility Management in Mobile Wireless Systems


1
Mobility Managementin Mobile Wireless Systems
  • Lecture 9

2
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.

3
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.

4
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.

5
Figure 1
6
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.

7
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.

8
Figure 2 Location of Cell Site
9
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

10
Figure 3
11
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.

12
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.

13
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.

14
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.

15
  • 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.

16
  • 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.

17
Figure 4
18
  • 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.

19
Figure 5
20
Location and HandoffManagement
  • 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.

21
Location and HandoffManagement
  • 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.

22
Location and HandoffManagement
  • 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.
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