Mobile Communications Systems 1

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Mobile CommunicationsSystems 1

• Rolando A Carrasco
• Professor in Mobile Communications
• BSc(Hons), PhD, CEng, FIEE
• R.Carrasco_at_ncl.ac.uk
• School of Electrical, Electronic and Computing
  Engineering
• University of Newcastle upon tyne

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Introduction (1)

• Mobile Computing Systems
• Types of wireless communication systems
• Cellular System
• Intra-cell/Inter-cell operation
• Frequency Re-use
• Channel Assignment Strategies
• Handoffs
• Interference and System Capacity
• Types of Interference
• Capacity/Interference Relation
• Improving Capacity in Cellular Systems

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Introduction (2)

• Multiple Access in the Mobile Environment
• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Capacity and Interference for FDMA and TDMA
• Commercial Applications for FDMA TDMA
• Spread Spectrum Multiple Access Techniques, Code
  Division Multiple Access (CDMA)
• General Concepts and Characteristics
• Capacity and Interference in CDMA
• Other Multiple Access Techniques
• Universal Mobile Telecommunication Systems
  (UMTS)-ATM Integration and Wireless ATM (WATM)
• UMTS-ATM Integration
• UMTS-ATM Network Architecture

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Mobile Computing Systems

• Rely on radio transmission as the final link
  between terminals
• Finite resource, spectrum available is strictly
  limited
• Multipath propagation, fading interference
• Terminals ability to move, complicates the system
• The term mobile
• Any radio terminal, that can be moved during
  operation
• Radio terminal that is attached to a high speed
  platform (cellular telephone inside a vehicle)
• The term portable
• A radio terminal that can be hand-held used at
  walking speed

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Radio Transmission Impairments
PSTN, ISDN BISDN,...
Fixed Networks
MSC
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Mobile Computing Systems

• Mobiles users communicate through fixed base
  stations (BS)
• BSs are controlled by the radio network
  controller (RNC in 3G)
• RNC allows the system to contact the fixed
  backbone network

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Types of wireless communication systems

• type of transmission used is evaluated
• Analogue transmitting unbroken electromagnetic
  waves, closely correspond to the waveforms
  produced by the original sounds.
• Digitalconvert information (e. g. voice or data)
  into a series of coded pulses, transmitted at a
  fast rate.
• Depending on the direction of the transmission
  and the simultaneity of the communication
• Simplex communication in one direction only.
• Half-duplex two-way communication over the same
  radio channel. A user can only transmit or
  receive (no simultaneity).
• Full-duplex two simultaneous but separate
  channels to achieve a two-way communication

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Types of wireless communication systems

• A final classification can be made depending on
  the type of service provided
• Paging systems
• Cordless
• Adhoc
• Cellular

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Paging Systems

• Systems that send brief messages (numeric,
  alphanumeric or voice) to a subscriber.
• message is called a page and is sent in one
  direction only (simplex transmission)
• messages are broadcasted to inform the subscriber
  about the attempts made by other users to contact
  them or to receive news headlines, faxes or other
  types of information.

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An example Paging System
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Cordless

• Fully duplex, using a radio channel to connect a
  portable handset to a dedicated Base Station.
• Connected to fixed network via a specific
  telephone number
• Connection over short distances
• 1G cordless few metres
• 2G cordless, DECT few hundred metres

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Mobile Adhoc Network (MANET)

• Mobiles communicate bouncing off each other.
• They are not fixed to using a BS to connect to
  fixed network
• Useful in disaster situations

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Cellular Mobile Communication system

• The cellular concept is a system level idea where
  many low power transmitters replace a single high
  power transmitter, covering a large geographic
  area, each covering a portion of the service area
  called a cell

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Cellular Concept
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Cellular Mobile Communication system

• Mobile Units (MS)
• mounted on vehicles or carried as portables
• contain transceivers, antennas and control
  circuitry,
• communicate with an assigned Base Station through
  duplex radio channels
• Base stations (BS)
• several transmitters and receivers that through
  antennas communicate simultaneously with all
  mobiles within the area of coverage (cell) and
  are connected to radio network controller (RNC)
  via telephone lines or microwave links.

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Cellular Mobile Communication system

• Each BS is allocated a portion of the total
  number of channels available to the entire
  system, which are used within a small geographic
  area (cell).
• Neighbouring BSs are assigned different groups of
  channels so that all the available channels are
  assigned to a relatively small number of Base
  Stations.
• Radio Network Controller (RNC)
• controls a number of cells
• arranges Base Stations and channels for the
  mobiles
• handles connections with the fixed Public
  Switched Telephone Network (PSTN) and other fixed
  networks

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Intra-cell/Inter-cell operation

• Base Station and mobiles communication is defined
  by a common air interface (CAI)
• four different types of channels
• two for voice or data transmission
• The voice channel used to transmit from the Base
  Station to the Mobile Station is called the
  forward voice channel (FVC)
• the channel used in the opposite direction is the
  reverse voice channel (RVC)
• two for control and signalling
• forward control channel (FCC)
• reverse control channel (RCC)
• in charge of the call set-up, channel quality
  measurements, handoff procedure and other
  management functions.

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Intra-cell/Inter-cell operation

• Each cell has allocated a number of channels
• used for voice or signalling traffic
• assigned according to a frequency re-use pattern
• An active mobile registers with an appropriate BS
• The mobile information and its cellular location
  are stored in the RNC
• When a call is set up either from or to the
  mobile
• control signalling system assigns a channel
  (from those available to the base station with
  which the mobile is registered) and instructs the
  mobile to use the corresponding channel. This is
  the channel assignment operation.
• A connection is established via the BS to the
  fixed network.

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Intra-cell/Inter-cell operation

• The quality of the channel (radio link) is
  monitored by the BS for the duration of the call
  and reported to the RNC
• RNC makes decisions concerning the quality and
  instructs the MS and BS accordingly.
• When a mobile enters a different cell while a
  connection is in progress
• the system, controlled by the RNC, assigns the
  mobile to a new BS which has to provide new
  channels to the connection, if these are
  available.
• This process is called a handoff.

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Frequency Re-use

• Each BS is allocated a different set of carrier
  frequencies
• Each cell has a usable bandwidth associated with
  these carriers
• No. of carrier frequencies available is limited
• It is therefore necessary to re-use the available
  frequencies many times in order to provide
  sufficient channels for the required demand
• This process is called frequency re-use
• All the cells with a different set of frequencies
  form a cluster

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Frequency Reuse Channel Assignment
Co-Channel Interference Fixed Channel Allocation

Dynamic Channel Allocation Centralised or
Distributed
Frequency reuse concept. Cells with the same
letter use the same set of frequencies.
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Types of Interference

• Co-channel Interference
• -is independent of the transmitted power
• -is in function of the radius of the cell (R)
• -and the distance to the centre of the nearest
  co-channel cell (D)
• Adjacent channel Interference
• Interference resulting from signals, which are
  adjacent in frequency to the desired signal is
  called adjacent channel interference

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Capacity/Interference Relation

• R is radius of the cell
• D is the distance to the center of the nearest
  co-channel cell D
• Function
• The parameter Q called the co-channel reuse ratio
  is related to the cluster size N for hexagonal
  geometry
• A small value of Q provides larger capacity since
  the cluster is small but increases the
  interference
• A large value of Q improves the transmission
  quality, due to a smaller level of co-channel
  interference.

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Co-channel reuse

• Cluster size co-channel
  reuse ratio
• N
  Q
• 7
  4.58
• 12
  6
• 13
  6.24
• Let i0 be the number of co-channel interfering
  cells. Then the carrier to interference ratio C/I
  for a mobile receiver
• Where C is the desired carrier power from the
  desired Base station
• Ii is the interference power caused by the ith
  interference co-channel cell base station

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Propagation measurements in a Mobile radio base
station

• The average power Pr at distance d from the
  transmitting antenna is approximated by
• - where P0 is the power at a close in reference
  point in the far field region of the antenna at a
  small distance do from the transmitting antenna
• -n is the path loss exponent. Consider the
  forward link where the mobile is at distance R
  from the serving base station
• - If Di is the distance of the ith interference
  from the mobile

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Carrier to interference

• C/I for a mobile can be approximated as
• Considering only the first tier of interfering
  cell. If all the interfering base stations are
  equidistant from the desired base station and if
  this distance is equal to the distance D between
  cell centres
• For AMPS Systems C/I17db, n4
• For cell N7 C/I18db

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Carrier to Interfering

• In figure 1.2 it can be seen that for a N7 cell
  cluster with the mobile until at cell boundary,
  the mobile is a distance D-R from the two nearest
  co-channel interfering cells and approximately
  DR/2,D,D-R/2,and DR from the other interfering
  cells in the first tier can be approximated as

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Co-Channel Cells
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Channel Assignment Strategies

• The way the channels are assigned inside a cell
  affects the performance of the system
• especially when a change of BSs occurs
• Fixed Channel Allocation Schemes (FCA)
• Dynamic Channel Allocation (DCA)
• Hybrid Channel Allocation (HCA)

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Fixed Channel Allocation (FCA)

• channels are divided in sets
• allocated to a group of cells reassigned to
  other groups, according to some reuse
  pattern
• Different considerations are taken before the
  assignment of the channels (i.e. signal
  quality, distance between BSs, traffic
  per BS)
• they are fixed (i.e. a cell can not use channels
  that are not assigned to it)
• assignment of frequency sets to cells when the
  system is designed does not
  change unless restructured
• Any call attempt within the cell can only be
  served by the unused channels in that
  cell
• If all the channels in that cell are busy, the
  service is blocked
• simple method but does not adapt to changing
  traffic conditions
• introduction of new BSs supposes frequency
  reassignment for the complete system

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Dynamic Channel Allocation (DCA)

• Channels are placed in a pool
• assigned to new calls depending on the carrier
  to interference ratio (CIR) and other
  criteria.
• Each time a call is made the serving base station
  requests a channel from the RNC
• The switch then allocates a channel to the
  requested cell following an algorithm that
  takes into account the likelihood of future
  blocking within the cell
• the frequency of use of the candidate channel
• the reuse distance of the channel, and other cost
  functions.
• The RNC only allocates a given frequency if that
  frequency is not presently in use
  in the cell or any other cell which falls within
  the minimum restricted distance of frequency
  reuse to avoid interference
• reduces the likelihood of blocking, which
  increases the trunking capacity of the system,
  since all the available channels in a market are
  accessible to all of the cells

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Dynamic Channel Allocation (DCA)

• Require the RNC to collect real-time data on
• channel occupancy
• traffic distribution
• radio signal strength indications (RSSI) of all
  channels on a continuous basis
• This increases the storage and computational load
  on the system but provides the advantage of
  increased channel utilisation and decreased
  probability of a blocked call
• Allocation of channels is more complex since
  additional information is needed, but is also
  more flexible to traffic changes (i.e.
  non-uniform traffic).

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Hybrid Channel Allocation (HCA)

• a combination of both FCA and DCA
• some channels are pre-assigned
• others are shared dynamically
• One of these approaches is based on the principal
  of borrowing channels from a neighbouring cell
  when its own channels are occupied
• Known as the borrowing strategy
• RNC supervises such borrowing procedures
  ensures that the borrowing of a channel does not
  disrupt or interfere with any of the calls in
  progress in the donor cell

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Allocation Comparison

• FCA better for high uniform traffic loads
• Max reusability of channels is always achieved
• DCA performs better for non-uniform traffic loads
• allocation of channels is flexible
• FCA schemes behave like a no. of small groups of
  servers
• DCA provides a way of making these small groups
  of servers behave like a larger server, which is
  more efficient.
• FCA call must always be handed off into another
  channel
• same channel is not available in adjacent cells.
• DCA the same channel can be used if interference
  does not occur.

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Allocation Comparison

• variations in traffic that are typical of
  microcells are not well handled in FCA.
• DCA techniques perform better in microcells
• Implementation complexity of DCA is higher than
  FCA.
• FCAeach cell has a number of channels and the
  channel selection is made independently
• DCA the knowledge of occupied channels in other
  cells is necessary (i.e. heavy signalling load).
• A great deal of processing power to determine
  optimal allocations is also required.

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Allocation Control

• Centralised fashion
• channels are assigned by a central controller,
  usually the RNC
• Distributed fashion
• Channels are selected either by the local BS or
  by the mobile
• BS control BSs keep info about current available
  channels in its vicinity.
• Updated by exchanging data between BSs. In a
  mobile control system the mobile chooses the
  channel based in its local CIR measurements (i.e.
  lower complexity but less efficiency).
• FCA is suitable for a centralised control system.
  
• DCA is applicable to a centralised or
  decentralised control system