Mobility Management in Mobile Wireless Systems

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Mobility Managementin Mobile Wireless Systems

• Lecture 9

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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.

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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.

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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.

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Figure 1
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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.

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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.

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Figure 2 Location of Cell Site
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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

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Figure 3
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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.

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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.

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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.

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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.

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• 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.

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• 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.

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Figure 4
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• 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.

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Figure 5
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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.

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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.

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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.