Cellular Wireless Networks

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Cellular Wireless Networks
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Introduction

• Cellular technology is the underlying technology
  for most mobile wireless communications and data
  transfers
• Main concept Use of multiple low-power
  transceivers
• An area is divided into cells, each of which are
  served by a base station, consisting of
  transmitter, receiver, and a control unit
• Adjacent cells are assigned different frequencies
  to avoid interferences and crosstalk. Cells at
  sufficient distance from each other can use the
  same frequency bands
• The cells are hexagonal in shape
• Each cell has a base transceiver the
  transmission power is carefully controlled to
  avoid interferences with neighboring cells
• About 10-50 frequencies are assigned to each cell

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Cellular Geometries

• Handout

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Frequency Reuse

• How many cells must intervene between two cells
  using the same frequency?
• D Minimum distance between centers of cells that
  use the same band (co-channels)
• R Radius of a cell
• d distance between adjacent cells (d?3 R)
• N Number of cells in a repetitious pattern
  (reuse factor)
• D/R v(3N)
• or D/d vN

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Frequency Reuse

• Handout

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Increasing Capacity

• Adding new channels
• add unused channels
• Frequency borrowing
• borrowed from adjacent cells
• Frequencies can be also assigned dynamically
• Cell Splitting
• Original cells (about 6.5-13km in size) can be
  split to smaller cells (1.5 km minimum)
• Powers of transmitters are lowered
• Handoff becomes more frequent
• Cell Sectoring A cell is divided into number of
  wedge-shaped sectors
• Each sector is assigned a separate subset of
  cells channel
• Directional antennas at the base station are used
  to focus on each sector

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Operations
Mobile Telecom Switching Office (MTSO)
Base Station
Base Station
Base Station
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Operations cont.

• Base stations (BS) includes an antenna, a
  controller, and a number of transceivers, for
  communicating on the channels assigned to that
  cell
• Each BS is connected to an MTSO.
• One MTSO serves a single or multiple BSs
• MTSO assigns the voice channel to each call,
  performs handoff, and monitors the call for
  billing information
• Channels between the mobile units and the base
  stations
• Control channels used to exchange information
  having to do with setting up and maintaining
  calls
• Traffic channels carry a voice or data
  connections between users

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Typical Call - 1

• Mobile Unit Initialization
• The unit scans and selects the strongest set-up
  control channel
• A handshake takes place between the mobile unit
  and the MTSO controlling this cell, through the
  BS in this cell
• The scanning process is repeated periodically
• Mobile-originated call
• The number of the called unit is sent on the
  preselected set-up channel to the BS
• The BS sends the request to the MTSO
• Paging
• The MTSO sends a paging message to certain BSs
• Each BS transmit the paging signal on its own
  assigned set-up channel

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Typical Call - 2

• Call accepted
• The called mobile unit recognizes its number on
  the set-up channel being monitored and responds
  to that BS, which send the response to the MTSO
• MTSO sets up a circuit between calling and called
  BSs
• MTSO selects an available traffic channel within
  each BSs cell and notifies each BS, which in
  turn notifies its mobile unit.
• Ongoing call
• While the connection is maintained, the two
  mobile units exchange voice or data signals,
  going through their respective BSs and the MTSO
• Handoff
• If a mobile unit moves out of range of one cell
  and into the range of another during a
  connection, the traffic channel has to change to
  one assigned to the BS in the new cell

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Handoff

• Handoff Initiation
• Network-initiated or mobile assisted
• Performance Metrics
• Cell blocking probability
• Call dropping probability
• Call completion probability
• Probability of unsuccessful handoff
• Handoff blocking probability
• Handoff probability
• Rate of handoff
• Interruption duration
• Handoff delay

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Handoff When to?

• Handout

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First Generation (Analog Systems)

• Advanced Mobile Phone System (AMPS) developed
  by ATT (early 1980s)
• Uses FDD scheme
• AMPS Parameters
• Base station transmission band 869-894 MHz
• Mobile unit transmission band 824-849 MHz
• Spacing between forward and reverse channel 45
  MHz
• Channel bandwidth 30 kHz
• Number of full-duplex voice channels 790
• Number of full duplex control channels 42
• Data transmission rate 10 kbps
• Cell size 2-20km radius

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AMPS - Operation

• Each AMPS-capable cellular telephone includes a
  numeric assignment module (NAM) in read-only
  memory
• NAM contains telephone number (provided by
  service provider) and the serial number of the
  phone
• When turned on, the phone transmits its serial
  number to MTSO
• MTSO maintains a database to take care of
  authentication and billing issues.

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Sequence of Events

• The subscriber dials a number
• The MTSO authorizes and issues a message to the
  users cell phone indicating which traffic
  channel to use for sending and receiving
• The MTSO sends a ringing signal to the called
  party. When the party answers, the MTSO
  establishes a circuit between the two parties and
  initiates billing information
• When one party hangs up, the MTSO releases the
  circuit, frees the radio channels, and completes
  the billing information

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Second Generation (TDMA/CDMA)

• Goal higher quality signals, higher data rates,
  and greater capacity
• Advances
• Digital traffic channels second generation
  systems provide digital traffic channels,
  supports digital data voice traffic is encoded
  in digital form before transmitting
• Encryption
• Error detection and correction
• Channel access allows multiple users per channel
  using TDMA and CDMA
• Second Generation Cellular Telephone System
• GSM
• Interim Standard 136 (IS-136)
• IS-95

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Standards

• GSM (Global System for Mobile Communications)
• Introduced in 1990, TDMA-based
• Transmission range BS935-960 MHz, MS 890-915
  MHz
• Channel bandwidth 200 kHz, 8 users/channel
• Deployed widely in Europe, Asia, Australia, South
  America, PCS spectrum in US
• IS-136 (Also known as North American Digital
  Cellular)
• Introduced in 1991, TDMA-based
• Transmission range BS 869-894 MHz, MS
  824-849 MHz
• Channel BW 30 kHz, 3 users/channel
• Deployed in North South America, Australia
• IS-95 (Also known as cdmaOne)
• Introduced in 1993, CDMA-based
• BS and MS transmission bands same as IS-136
• Channel bandwidth 1250 kHz, 64 users/channel
• Deployed in North South America, Korea, Japan,
  China, Australia

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Global System for Mobile (GSM) Communications

• Services
• Teleservices mobile telephony
• Data services packet-switched traffic
• User services
• Telephone services
• Data services
• packet switched protocols and data rates from 300
  bps to 9.6 kbps
• Supplementary ISDN services
• Calling features, SMS, and cell broadcast
• On-the-air privacy
• The digital bit stream sent by a GSM transmitter
  is encrypted according to a specific secret
  cryptographic key that is known only to the
  carrier. The key changes with time for each user.

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GSM System Architecture

• Three subsytems
• Base System Subsystem (BSS)
• Network and Switching Subsystem (NSS)
• Operation Support Subsystem (OSS)
• Mobile station (MS) is considered a part of BSS
• Refer to the GSM Architecture (handout)

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Mobile Station

• Mobile Equipment (ME)
• Device including keypad, speaker, microphone,
  radio transceiver, Digital signal processor
• Subscriber Identity Module (SIM)
• A pluggable memory device that stores information
  such as subscriber identification number, the
  network and countries where the subscriber is
  entitled to service, privacy keys, and other
  user-specific information
• Calls in GSM are directed to SIM

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GSM Subsystems

• BSS (also known as radio subsystem)
• Provides and manages radio transmissions between
  MSs and Mobile Switching Center (MSC)
• Manages the radio interface between all
  subsystem, reserves radio frequencies, manages
  handoff, controls paging
• Each BSS consists of a Base Station Controllers
  (BSC) and multiple Base Transceiver Stations
  (BTS)
• Connects the MS to the NSS via the MSCs
• NSS
• Manages the switching functions of the system and
  allows the MSCs to communicate with other
  networks such as PSTN and ISDN
• OSS
• Supports the operations and maintenance of GSM
  monitor, diagnose, troubleshoot all aspects of
  GSM system

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Base Station Subsystem (BSS)

• The MSs communicate with the BSS over the radio
  interface
• BSS consists of one or more BSCs which connect to
  a single MSC
• Each BSC typically controls about several hundred
  Base Transceiver Stations (BTS)
• BTS may be colocated with the BSC or remotely
  distributed
• Mobile handovers between two BTS under the
  control of the same BSC are handled by the BSC
  reduces switching burden on MSC

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GSM Interfaces

• Um radio interface
• Abis interface connecting a BTS to a BSC
• Carries traffic and maintenance data
• Standardized for all manufacturers
• BSCs are physically connected to MSCs (A
  interface)
• Uses an SS7 protocol called the signaling
  correction control part (SCCP) to support
  communications between the MSC and BSSs

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NSS

• NSS
• Handles the switching of GSM calls between
  external networks and the BSCs
• Responsible for managing and providing external
  access to several customer databases
• MSC is the central unit in the NSS and controls
  traffic among all BSCs

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Databases in NSS

• Home Location Register (HLR)
• Contains subscriber information and location
  information for each user who resides in the same
  city as MSC
• Each subscriber in a particular GSM market is
  assigned a unique International Mobile Subscriber
  Identity (IMSI)
• Visitor Location Register (VLR)
• Temporarily stores the IMSI and customer
  information for each roaming subscriber who is
  visiting the coverage area of a particular MSC
• Once a roaming mobile is logged in the VLR, the
  MSC sends the necessary information to its HLR so
  that the calls can be appropriately routed
• Authentication Center (AUC)
• Handles authentication and encryption keys for
  each of the subscriber in the HLR and VLR
• Equipment Identity Register (EIR)
• Keeps the international mobile equipment identity
  (IMEI) that reveals manufacturer, country of
  production, terminal type
• Helps in managing faults and thefts
• Implementation is optional to the service provider

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OSS

• OSS supports one or several operation maintenance
  centers (OMC), which are used to monitor and
  maintain performance of each MS, BS, BSC, and MSC
• Functions
• Maintain all telecom hardware and network
  operations
• Maintain all charging and billing procedures
• Manage all mobile equipments in the system

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Second Generation CDMA (IS-95)

• Forward link Up to 64 logical CDMA channel
  each occupying 1228 kHz bandwidth
• Types of channels
• Pilot (channel 0) allows MS to acquire timing,
  phase reference, and signal strength reference
• Synchronization (channel 32) 1200 bps channel
  used by MS to obtain identification information
  (system time, code state, protocol version, etc.)
• Paging (Channels 1-7)
• Traffic channels (8-31, 33-63) up to 14.4 kbps

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IS-95 (continued)

• Reverse Link Up to 94 logical CDMA channels,
  each occupying 1228 kHz bandwidth
• Supports up to 32 access channels and up to 64
  traffic channels
• The access channel is used by an MS to initiate a
  call, respond to paging, and for location update
• Traffic channels is reverse links are mobile
  unique

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Third Generation (3G) Wireless Networks

• Motivations
• Multi-megabit Internet access
• Very high network capacity
• VoIP
• Integration of multiple technologies
• ITU formulated a plan to implement a global
  frequency band in the 2000 MHz range that would
  support a single, ubiquitous wireless
  communication standard for all countries called
  International Mobile Telephone (IMT-2000)
• 3G evolution of CDMA cdma2000
• 3G evolution of GSM wideband CDMA (W-CDMA),
  also called Universal Mobile Telecommunications
  Service (UMTS)