Chpater 8:Cellular Networks

1
Chpater 8Cellular Networks

• Overview
• 1G Analog Cellular
• 2G TDMA - GSM
• 2G CDMA - IS-95
• 2.5G
• 3G
• 4G and Beyond
• Cellular Engineering Issues

Ahmed Sameh
2
Overview
Data Rates
2 Mbps
3G (144Kbps to 2Mbps)
1 Mbps
100 Kbps
2.5G (10-150Kbps)
10 Kbps
2G (9.6Kbps)
1 Kbps
1G (lt1Kbps)
1980
2010
1990
2000
Years
3
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 (lt 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

4
Issues Vital to cellular

• Frequency allocation
• Licensed
• 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

5
Frequency AllocationCellular networks Mostly
around 900 MHz 2GHz
Wavelength
Frequency
Gamma-rays
X-rays
3000 GHz
Infrared
0.1 m
300 GHz
1 mm
THF - terribly high frequency
30GHz
10 mm
EHF - extra high frequency
Micro Waves
3GHz
100 mm
SHF - super high frequency
300 MHz
1m
UHF - ultra high frequency
30 MHz
10 m
VHF - very high frequency
Radio Waves
3MHz
100 m
HF - high frequency
300KHz
1 Km
MF - medium frequency
30Khz
10 Km
LF - low frequency
3KHz
100 Km
VLF - very low frequency
Source Bekkers, R. and Smits, J., Mobile
Telecommunications, Artech, 2000.
6
Multiple Access Techniques How to allocate users
Session4
Session2
Session3
Session3
Session1
Session4
Frequency
Session2
Frequency
Session1
Time
Time
Time Division Multiple Access (TDMA) 2G TDMA
3G TDMA
Frequency Division Multiple Access (FDMA) 1G
Cellular (AMPS)
All sessions based on a code
Frequency
Time
2G CDMA (IS-95) 3G CDMA
Code Division Multiple Access (CDMA)
7
A Cellular Network
Cell 1
Public Switched Telephone Network (PSTN)
Mobile Telephone Switching Center (MTSC)
Cell 2
HLR
VLR
Base Transceiver Station (BTS)
Mobile User
HLR Home Location Register VLR Visitor
Location Register
Cordless connection
Wired connection
8
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 tracked.

9
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

10
Cellular System
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
11
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

12
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
  MCS-L1)
• 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 devices

13
Cell Design
E
D
F
E
A
D
F
C
G
A
B
C
G
E
B
D
F
A
C
G
B

• 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)

14
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
  AMPS
• One for transmission from base to mobile unit
• One for transmission from mobile unit to base

15
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

16
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 users 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

17
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

18
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
  9.6kbps)
• Forward error correction and encryption

19
CDPD Operation
Internet
Landline Modem
Router
Router
CDPD Modem
CDPD Provider Network
base interface station (special unit that
connects all base stations in CDPD provider
network to routers)
20
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)

21
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
  quality
• Sophisticated encryption can be used

22
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
  users

23
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

24
GSM (Global System for Mobile Communications)

• Completely designed from scratch (no backward
  compatability)
• 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.

25
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 suppliers

26
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
• 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

27
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
  it
• 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

28
GSM Location Services
2
6
9
Gateway MTSC
Terminating MTSC
BTS
1
Public Switched Telephone Network (PSTN)
10
10
10
8
7
9
10
3
5
4
VLR
HLR
10
5
6. Call routed to terminating MSC 7. MSC asks VLR
to correlate call to the subscriber 8. VLR
complies 9. Mobile unit is paged 10. Mobile unit
responds, MSCs convey information back to
telephone
1. Call made to mobile unit (cellular phone) 2.
Telephone network recognizes number and gives to
gateway MSC 3. MSC cant route further,
interrogates users HLR 4. Interrogates VLR
currently serving user (roaming number request)
5. Routing number returned to HLR and then to
gateway MSC
Legend MTSC Mobile Telephone Service Center,
BTS Base Transceiver Station HLRHome Location
Register, VLRVisiting Location Register
29
GSM Protocol Architecture
CM
CM
MM
MM
BSSMAP
BSSMAP
RRM
BTSM
BTSM
RRM
SCCP
SCCP
LAPDm
LAPD
LAPD
LAPDm
MTP
MTP
Radio
64 Kbps
64 Kbps
Radio
64 Kbps
64Kbps
Base Station Controller
Mobile Station
Base Transceiver Station
Mobile Service Switching Center
MM Mobility Management MTP Message Transfer
Part RRM Radio Resources Management SCCP
Signal Connection Control Point
BSSMAP BSS Mobile Application part BTSM BTS
management CM Connection Management LAPD
Link Access Protocol, D Channel
30
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 (MTSO)
  
• Connection management connects end users
• Mobile application part (MAP) between HLR,VLR
• BTS management management base system

31
2G CDMA Cellular

• IS-95 is the best known example of 2G with CDMA
• Advantages of CDMA for Cellular
• Frequency diversity frequency-dependent
  transmission impairments have less effect on
  signal
• Multipath resistance chipping codes used for
  CDMA exhibit low cross correlation and low
  autocorrelation
• 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

32
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

33
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
  comparison
• 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

34
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

35
Enabling Technologies

• Wireless Network Evolution to 3rd Generation

3G
2 Mbps
CDMA2000 3XRTT (UMTS)
CDMA Migration
W-CDMA (UMTS)
1G-2G Migration
500 kbps
TDMA Migration
EDGE
2.5G
150 Kbps
CDMA-2000 1XRTT
100 Kbps
GPRS
2G
50 Kbps
IS-95
10 Kbps
1G
GSM
AMPS
1 Kbps
1999
2000
2001
2002
2003
1980
36
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)

37
4G Systems

• Wireless networks with cellular data rates of 20
  Mbits/second and beyond.
• ATT 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)

38
Engineering Issues

• Steps in MTSO controlled call
• TDMA design
• CDMA design
• Handoff
• Power control
• Traffic engineering

39
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

40
Mobile Wireless TDMA Design Considerations

• 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
  channel)

41
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 reasonable
• 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

42
Mobile Wireless CDMA Design Considerations

• Soft Handoff mobile station temporarily
  connected to more than one base station
  simultaneously
• 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

43
Mobile Radio Propagation Effects

• 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

44
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

45
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
  time
• 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

46
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, its desirable to
  equalize the received power level from all mobile
  units at the BS

47
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
  subscribers
• L lt N nonblocking system
• L gt N blocking system

48
Blocking System Performance Questions

• 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?

49
Traffic Intensity

• Load presented to a system
• ? 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

50
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 infinite

51
Web sites

• Bekkers, R. and Smits, J., Mobile
  Telecommunications, Artech, 2000.
• www.pcsdata.com PCS web site
• www.gsmdata.com GSM web site
• www.wlana.com wireless LAN Association
• www.pcca.org) portable computers and
  communications association
• Online Magazines
• Mobile Computing Communications
  (www.mobilecomputing.com)
• Wireless Design Online (www.wirelessdesignonline
  .com)
• Wireless Design Development (www.wirelessdesign
  mag.com)
• Wireless Mobility (www.wireless mag.com)
• Wireless Review (www.wirelessreview.com)
• Wireless Systems Design (www.wsdmag.com)
• Wireless Week (www.wirelessweek.com)