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Cellular Networks

VIEWS: 192 PAGES: 51

									    Chpater 8:Cellular Networks
• Overview
• 1G Analog Cellular
•2G CDMA - IS-95
•4G and Beyond
•Cellular Engineering Issues

              Ahmed Sameh
Data Rates
    2 Mbps

    1 Mbps                                               3G
                                                    (144Kbps to 2Mbps)

   100 Kbps
    10 Kbps
    1 Kbps

              1980             1990             2000           2010
       Cellular networks: From 1G to 3G
• 1G: First generation wireless cellular: Early 1980s
   – Analog transmission, primarily speech: AMPS (Advanced Mobile
     Phone Systems) and others
• 2G: Second generation wireless cellular: Late 1980s
   –   Digital transmission
   –   Primarily speech and low bit-rate data (9.6 Kbps)
   –   High-tier: GSM, IS-95 (CDMA), etc
   –   Low-tier (PCS): Low-cost, low-power, low-mobility e.g. PACS
• 2.5G: 2G evolved to medium rate (< 100kbps) data
• 3G: future Broadband multimedia
   – 144 kbps - 384 kbps for high-mobility, high coverage
   – 2 Mbps for low-mobility and low coverage
• Beyond 3G: research in 4G
     Issues Vital to cellular
•Frequency allocation
   •Many providers
•Multiple Access
   •Many users
   •Wide area of coverage
   •Traffic management
•Location management
   •High mobility (in cars, trains)
   •Multiple suppliers
   •Handoff management, roaming
•General principles
• Handled differently by different generations
                               Frequency Allocation
                 Cellular networks: Mostly around 900 MHz – 2GHz
Wavelength                                         Frequency


  0.1 m                                                     3000 GHz                Infrared
  1 mm                  THF - terribly high frequency       300 GHz
  10 mm                  EHF - extra high frequency         30GHz
   100 mm                SHF - super high frequency         3GHz
  1m                     UHF - ultra high frequency         300 MHz
  10 m                   VHF - very high frequency          30 MHz
                                                            3MHz                         Radio
 100 m                  HF - high frequency
 1 Km                   MF - medium frequency              300KHz
  10 Km                  LF - low frequency                 30Khz
  100 Km                 VLF - very low frequency           3KHz

         Source: Bekkers, R. and Smits, J., “Mobile Telecommunications”, Artech, 2000.
            Multiple Access Techniques: How to allocate users



                 Time                                                   Time Division
    Frequency Division                                                  Multiple Access (TDMA)
    Multiple Access (FDMA)                                               2G TDMA
     1G Cellular (AMPS)                      All sessions                3G TDMA

                                             based on a

             2G CDMA (IS-95)               Code Division
             3G CDMA                       Multiple Access (CDMA)
Cell 1
         A Cellular Network

                             Mobile               Public
                             Telephone            Switched
                             Switching            Telephone
         Cell 2              Center               Network
                             (MTSC)               (PSTN)

                           HLR       VLR

                   Mobile User              Base Transceiver Station (BTS)
                  Cordless connection    HLR = Home Location Register

                  Wired connection       VLR = Visitor Location Register
Overview of Location Services
 Cell-id based location.
    assigned an id of the cell that you are in.
    cell-id is stored in a database.
    As you move from one cell to another, you are assigned a
       different cell-id and the location database is updated.
    most commonly used in cellular networks. (HLR, VLR)
 Neighborhood polling: Connected mobile units only move to
  adjacent cells
 Angle of arrival (AOA). the angle at which radio waves from your
  device "attack" an antenna is used to calculate the location of
  the device.
 Time taken. In this case, the time taken between the device and
  the antenna is used to calculate the location of the device.
 Network assisted Global Positioning System (GPS). a GPS chip
  is installed inside a phone and thus the location of the user is
  Mobile telecommunications switching
             office (MTSO)
• Essentially an end office to connect calls between
  mobile units
• Several base stations connected to an MTSO
• In a large system, many MTSOs may be
  connected to a second level MTSO and so on
• MTSO connected to BSs, PSTN and each other
  through packet switching (ATM)
• Two types of channels available between mobile
  unit and BS
   – Control channels – used to exchange information
     having to do with setting up and maintaining calls
   – Traffic channels – carry voice or data connection
     between users
                        Cellular System
             Cell 1

                                       Mobile           Public
                                       Telephone        Switched
                                       Switching        Telephone
                       Cell 2
                                       Center           Network
                                       (MTSC)           (PSTN)
                                      HLR VLR

Handoffs (typically 30 mseconds):
1. At any time, mobile station (MS) is in one cell and under the control of a BS
2. When a MS leaves a cell, BS notices weak signal
3. BS asks surrounding BSs if they are getting a stronger signal
4. BS transfers ownership to one with strongest signal
5. MTSO assigns new channel to the MS and notifies MS of new boss
                  0G Wireless
• Mobile radio telephones were used for military
  communications in early 20th century
• Car-based telephones first introduced in mid 1940s
   – Single large transmitter on top of a tall building
   – Single channel used for sending and receiving
   – To talk, user pushed a button, enabled transmission and
     disabled reception
   – Became known as “push-to-talk” in 1950s
   – CB-radio, taxis, police cars use this technology
• IMTS (Improved Mobile Telephone System)
  introduced in 1960s
   – Used two channels (one for sending, one for receiving)
   – No need for push-to-talk
   – Used 23 channels from 150 MHz to 450 MHz
         First-Generation Cellular
• Advanced Mobile Phone Service (AMPS) invented at
  Bell Labs and first installed in 1982
• Used in England (called TACS) and Japan (called
• Key ideas:
  – Exclusively analog
  – Geographical area divided into cells (typically 10-25km)
  – Cells are small: Frequency reuse exploited in nearby (not
    adjacent) cells
  – As compared to IMTS, could use 5 to 10 times more users in
    same area by using frequency re-use (divide area into cells)
  – Smaller cells also required less powerful, cheaper,smaller
              Cell Design
                                         F          D
                        F          D          A
                                         G          C
                        G          C          B
                              B          E
                                    F          D
                                    G          C

•Cells grouped into a cluster of seven
•Letters indicate frequency use
•For each frequency, a buffer of two cells is used before reuse
•To add more users, smaller cells (microcells) are used
•Frequencies may not need to be different in CDMA (soft handoff)
  Cellular Network Organization
• Cell design (around 10 mile radius)
   – Served by base station consisting of transmitter,
     receiver, and control unit
   – Base station (BS) antenna is placed in high
     places (churches, high rise buildings) -
      • Operators pay around $500 per month for BS
   – 10 to 50 frequencies assigned to each cell
   – Cells set up such that antennas of all neighbors are
     equidistant (hexagonal pattern)
• In North America, two 25-MHz bands allocated to
   – One for transmission from base to mobile unit
   – One for transmission from mobile unit to base
Approaches to Increase Capacity
• Adding/reassigning channels - some channels
  are not used
• Frequency borrowing – frequencies are taken
  from adjacent cells by congested cells
• Cell splitting – cells in areas of high usage
  can be split into smaller cells
• Microcells – antennas move to buildings,
  hills, and lamp posts
              AMPS Operation
• Each phone has a 32 bit serial no and 10 digit phone no in
  its PROM
• When a phone is turned on, it scans for control signals
  from BSs
• It sends this info to BS with strongest control signal -
  passed to MTSO
• Subscriber initiates call by keying in phone number and
  presses send key
• MTSO verifies number and authorizes user
• MTSO issues message to user’s cell phone indicating send
  and receive traffic channels
• MTSO sends ringing signal to called party
• Party answers; MTSO establishes circuit and initiates
  billing information
• Either party hangs up; MTSO releases circuit, frees
  channels, completes billing
        Security Issues with 1G
• Analog cellular phones are insecure
• Anyone with an all band radio receiver can listen in
  (many scandals)
• Theft of airtime:
   – all band radio receiver connected to a computer
   – can record 32 bit serial number and phone number
     of subscribers when calling
   – can collect a large database by driving around
   – Thieves go into business - reprogram stolen
     phones and resell them
      Cellular Digital Packet Data (CDPD)
•   Developed by IBM, mostly used in North America
•   Packet switching built on top of AMPS
•   Sends IP packets over cellular phones
•   CDPD base stations are connected to IP routers
•   Same spectrum and physical infrastructure as
    analog cellular
    – Use available cellular capacity
       • Sniffing to find idle channels
       • Hops among available channels
       • Voice always higher priority
    – Share cellular infrastructure
       • Frequencies, Towers and antennas
• Raw bit rate: 19.2 kbs (actually closer to
    – Forward error correction and encryption
            CDPD Operation
     Landline Modem


        CDPD Modem

= base interface station                        Provider
(special unit that connects all base stations   Network
in CDPD provider network to routers)
    Second Generation Cellular
• Based on digital transmission
• Different approaches in US and Europe
• US: divergence
  – Only one player (AMPS) in 1G
  – Became several players in 2G due to competition
  – Survivors
     • IS-54 and IS-135: backward compatible with AMPS frequency
       allocation (dual mode - analog and digital)
     • IS-95: uses spread spectrum
• Europe: Convergence
  – 5 incompatible 1G systems (no clear winner)
  – European PTT development of GSM (uses new
    frequency and completely digital communication)
       Advantages of Digital
    Communications for Wireless
• Voice, data and fax can be integrated into a
  single system
• Better compression can lead to better
  channel utilization
• Error correction codes can be used for better
• Sophisticated encryption can be used
  Differences Between First and
   Second Generation Systems
• Digital traffic channels – first-generation systems
  are almost purely analog; second-generation
  systems are digital
• Encryption – all second generation systems
  provide encryption to prevent eavesdropping
• Error detection and correction – second-generation
  digital traffic allows for detection and correction,
  giving clear voice reception
• Channel access – second-generation systems allow
  channels to be dynamically shared by a number of
Integrating Data Over Cellular
• Direct access to digital channel
• Voice and data using one handset
• PCS 1900 (GSM-1900)
   –   9.6 kbps circuit switched data
   –   14.4 kbps under definition
   –   Packet mode specified
   –   Short message service
• IS-95-based CDMA
   – 13 kbps circuit switched data
   – Packet mode specified
   – Short message service
GSM (Global System for Mobile Communications)
  • Completely designed from scratch (no backward
  • Uses 124 channels per cell, each channel can
    support 8 users through TDM (992 users max)
  • Some channels used for control signals, etc
  • Several flavors based on frequency:
     – GSM (900 MHz)
     – GSM 1800 (called DCS 1800)
     – GSM 1900 (called DCS 1900) - used in North America
  • GSM 1900 phone only works in North America.
  • In Europe, you can transfer your SIM (Subscriber
    Identity Module) card to a phone of the correct
    frequency. This is called SIM-roaming.
          GSM (2G-TDMA)
• Circuit mode data
  – Transparent mode
  – Non-transparent mode using radio link protocol
  – Data rate up to 9.6kb/s
• Short message service
  – Limited to 160 characters
• Packet mode data: Plans for GSM Phase 2+
• Architecture specification very detailed
  (500 pages)
• Defines several interfaces for multiple
Mobile Station and Base Station Subsystem (BSS)
Mobile station
• Mobile station communicates across Um interface (air
  interface) with base station transceiver in same cell as
  mobile unit
• Mobile equipment (ME) – physical terminal, such as a
  telephone or PCS
   – ME includes radio transceiver, digital signal processors and
     subscriber identity module (SIM)
• GSM subscriber units are generic until SIM is inserted
   – SIMs roam, not necessarily the subscriber devices
• BSS consists of base station controller and one or more
  base transceiver stations (BTS)
• BSC reserves radio frequencies, manages handoff of
  mobile unit from one cell to another within BSS, and
  controls paging
  Network Subsystem Center
Mobile Switching Center (MSC) is at core; consists
  of several databases
• Home location register (HLR) database – stores
  information about each subscriber that belongs to
• Visitor location register (VLR) database –
  maintains information about subscribers currently
  physically in the region
• Authentication center database (AuC) – used for
  authentication activities, holds encryption keys
• Equipment identity register database (EIR) –
  keeps track of the type of equipment that exists at
  the mobile station
                               GSM Location Services
                                        6                     2
BTS                   Terminating               Gateway
             10       MTSC                      MTSC               Public         1
                                     10                       10   Switched
  9     10        7           8                                    Telephone
                                                 5        3
                                        4                          Network
                                                                   (PSTN)        10
                        VLR                       HLR
                                                          6. Call routed to terminating MSC
 1. Call made to mobile unit (cellular phone)
 2. Telephone network recognizes number                   7. MSC asks VLR to correlate call to
                                                          the subscriber
 and gives to gateway MSC
                                                          8. VLR complies
 3. MSC can’t route further, interrogates
                                                          9. Mobile unit is paged
 user’s HLR
                                                          10. Mobile unit responds, MSCs convey
 4. Interrogates VLR currently serving user
 (roaming number request)                                 information back to telephone
 5. Routing number returned to HLR and
 then to gateway MSC
      Legend: MTSC= Mobile Telephone Service Center, BTS = Base Transceiver Station
      HLR=Home Location Register, VLR=Visiting Location Register
                             GSM Protocol Architecture

  CM                                                                   CM
                                                   BSSMAP             BSSMAP

  RRM              RRM         BTSM      BTSM       SCCP               SCCP

  LAPDm            LAPDm LAPD            LAPD      MTP                 MTP
  Radio              Radio    64 Kbps    64 Kbps    64 Kbps
Mobile                                      Base Station
                     Base Transceiver                           Mobile Service
Station                                     Controller
                     Station                                    Switching Center

BSSMAP = BSS Mobile Application part     MM = Mobility Management
BTSM = BTS management                    MTP = Message Transfer Part
CM = Connection Management               RRM = Radio Resources Management
LAPD = Link Access Protocol, D Channel   SCCP = Signal Connection Control Point
Functions Provided by Protocols
• Protocols above the link layer of the GSM
  signaling protocol architecture provide
  specific functions:
  – Radio resource management: controls setup,
    termination and handoffs of radio channels
  – Mobility management: location and security
  – Connection management: connects end users
  – Mobile application part (MAP): between
  – BTS management: management base system
          2G CDMA Cellular
IS-95 is the best known example of 2G with
Advantages of CDMA for Cellular
• Frequency diversity – frequency-dependent
  transmission impairments have less effect on
• Multipath resistance – chipping codes used for
  CDMA exhibit low cross correlation and low
• Privacy – privacy is inherent since spread
  spectrum is obtained by use of noise-like signals
• Graceful degradation – system only gradually
  degrades as more users access the system
  Drawbacks of CDMA Cellular
• Self-jamming – arriving transmissions from
  multiple users not aligned on chip boundaries
  unless users are perfectly synchronized
• Near-far problem – signals closer to the receiver
  are received with less attenuation than signals
  farther away
• Soft handoff – requires that the mobile acquires
  the new cell before it relinquishes the old; this is
  more complex than hard handoff used in FDMA
  and TDMA schemes
Types of Channels Supported by
         Forward Link
• Pilot (channel 0) - allows the mobile unit to
  acquire timing information, provides phase
  reference and provides means for signal strength
• Synchronization (channel 32) - used by mobile
  station to obtain identification information about
  cellular system
• Paging (channels 1 to 7) - contain messages for
  one or more mobile stations
• Traffic (channels 8 to 31 and 33 to 63) – the
  forward channel supports 55 traffic channels
  Forward Traffic Channel Processing Steps
• Speech is encoded at a rate of 8550 bps
• Additional bits added for error detection
• Data transmitted in 2-ms blocks with forward error
  correction provided by a convolutional encoder
• Data interleaved in blocks to reduce effects of errors
• Data bits are scrambled, serving as a privacy mask
• Power control information inserted into traffic channel
• DS-SS function spreads the 19.2 kbps to a rate of 1.2288
  Mbps using one row of 64 x 64 Walsh matrix
• Digital bit stream modulated onto the carrier using QPSK
  modulation scheme
                Wireless Network Evolution to 3rd Generation
Enabling Technologies

    2 Mbps
                        CDMA Migration                      CDMA2000
                                                            3XRTT        W-CDMA
                        1G-2G Migration                     (UMTS)       (UMTS)
    500 kbps
                        TDMA Migration

  150 Kbps                                                        EDGE
  100 Kbps

   50 Kbps

   10 Kbps     1G
    1 Kbps      AMPS

             1980           1999      2000     2001      2002    2003
     Alternatives to 3G Cellular
• Major technical undertaking with                  many
  organizational and marketing overtones.
• Questions about the need for the additional
  investment for 3G (happy with 2.5G)
• Wireless LAN in public places such as shopping
  malls and airports offer options
• Other high-speed wireless-data solutions compete
  with 3G
   – Mobitex low data rates (nominally 8 Kbps), it uses
     a narrowband (2.5KHz) as compared to 30 KHz
     (GSM) and 5 MHz (3G).
   – Ricochet: 40 -128 kbps data rates. Bankrupcy
   – Flash-OFDM: 1.5 Mbps (upto 3 Mbps)
                 4G Systems
• Wireless networks with cellular data rates of 20
  Mbits/second and beyond.
• AT&T has began a two-phase upgrade of its wireless
  network on the way to 4G Access.
• Nortel developing developing features for Internet
  protocol-based 4G networks
• Alcatel, Ericsson, Nokia and Siemens found a new
  Wireless World Research Forum (WWRF) for research on
  wireless communications beyond 3G.
• Many new technologies and techniques (multiplexing,
  intelligent antennas, digital signal processing)
• Industry response is mixed (some very critical)
           Engineering Issues
•   Steps in MTSO controlled call
•   TDMA design
•   CDMA design
•   Handoff
•   Power control
•   Traffic engineering
    Steps in an MTSO Controlled
     Call between Mobile Users
•   Mobile unit initialization
•   Mobile-originated call
•   Paging
•   Call accepted
•   Ongoing call
•   Handoff
•   Call blocking
•   Call termination
•   Call drop
•   Calls to/from fixed and remote mobile subscriber
 Mobile Wireless TDMA Design
• Number of logical channels (number of time slots
  in TDMA frame): 8
• Maximum cell radius (R): 35 km
• Frequency: region around 900 MHz
• Maximum vehicle speed (Vm):250 km/hr
• Maximum coding delay: approx. 20 ms
• Maximum delay spread (m): 10 s
• Bandwidth: Not to exceed 200 kHz (25 kHz per
    3G-CDMA Design Considerations

• Bandwidth – limit channel usage to 5 MHz
• Chip rate – depends on desired data rate, need for error
  control, and bandwidth limitations; 3 Mcps or more is
• Multirate – advantage is that the system can flexibly
  support multiple simultaneous applications from a
  given user and can efficiently use available capacity by
  only providing the capacity required for each service
 Mobile Wireless CDMA Design
• Soft Handoff – mobile station temporarily
  connected to more than one base station
• RAKE receiver – when multiple versions of a
  signal arrive more than one chip interval apart,
  RAKE receiver attempts to recover signals from
  multiple paths and combine them
   – This method achieves better performance than simply
     recovering dominant signal and treating remaining
     signals as noise
      Mobile Radio Propagation
• Signal strength
   – Must be strong enough between base station and mobile
     unit to maintain signal quality at the receiver
   – Must not be so strong as to create too much cochannel
     interference with channels in another cell using the
     same frequency band
• Fading
   – Signal propagation effects may disrupt the signal and
     cause errors
   Handoff Performance Metrics
• Cell blocking probability – probability of a new
  call being blocked
• Call dropping probability – probability that a call
  is terminated due to a handoff
• Call completion probability – probability that an
  admitted call is not dropped before it terminates
• Probability of unsuccessful handoff – probability
  that a handoff is executed while the reception
  conditions are inadequate
Handoff Performance Metrics
• Handoff blocking probability – probability that a
  handoff cannot be successfully completed
• Handoff probability – probability that a handoff
  occurs before call termination
• Rate of handoff – number of handoffs per unit
• Interruption duration – duration of time during a
  handoff in which a mobile is not connected to
  either base station
• Handoff delay – distance the mobile moves from
  the point at which the handoff should occur to the
  point at which it does occur
                  Power Control
• Design issues making it desirable to include
  dynamic power control in a cellular system
   – Received power must be sufficiently above the
     background noise for effective communication
   – Desirable to minimize power in the transmitted signal
     from the mobile
      • Reduce cochannel interference, alleviate health concerns, save
        battery power
   – In SS systems using CDMA, it’s desirable to equalize
     the received power level from all mobile units at the BS
          Traffic Engineering
• Ideally, available channels would equal
  number of subscribers active at one time
• In practice, not feasible to have capacity
  handle all possible load
• For N simultaneous user capacity and L
  – L < N – nonblocking system
  – L > N – blocking system
  Blocking System Performance
• Probability that call request is blocked?
• What capacity is needed to achieve a certain
  upper bound on probability of blocking?
• What is the average delay?
• What capacity is needed to achieve a certain
  average delay?
                Traffic Intensity
• Load presented to a system:

                            A  h
      •  = mean rate of calls attempted per unit time
      • h = mean holding time per successful call
      • A = average number of calls arriving during average holding
        period, for normalized 

      • U = .h = system utilization
      • Q = no of users in Q ahead of you= U/1-U
      • U must be less than 0.5 for no blocking
    Factors that Determine the
    Nature of the Traffic Model
• Manner in which blocked calls are handled
   – Lost calls delayed (LCD) – blocked calls put in a queue
     awaiting a free channel
   – Blocked calls rejected and dropped
      • Lost calls cleared (LCC) – user waits before another attempt
      • Lost calls held (LCH) – user repeatedly attempts calling
• Number of traffic sources
   – Whether number of users is assumed to be finite or
                          Web sites
      • Bekkers, R. and Smits, J., “Mobile Telecommunications”, Artech, 2000.

      • PCS web site

      • GSM web site

      • wireless LAN Association

      • portable computers and communications association
• Online Magazines
      • Mobile Computing & Communications (
      •   Wireless Design Online (
      • Wireless Design & Development (
      • Wireless & Mobility (www.wireless
      • Wireless Review (
      • Wireless Systems Design (
      • Wireless Week (

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