Cellular Networks

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Cellular Networks
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
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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

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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

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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)
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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
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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.

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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
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Frequency Reuse
The concept of frequency reuse is based on
assigning to each cell a group of radio channels
used within a small geographic area Cells are
assigned a group of channels that is completely
different from neighbouring cells The coverage
area of cells is called the footprint and is
limited by a boundary so that the same group of
channels can be used in cells that are far enough
apart
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Frequency Reuse

• Cells with the same number have the same set of
  frequencies

Frequency Reuse
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Frequency Reuse using 7 frequencies allocations
f2
f3
f7
f1
f4
f6
f2
f5
f2
f3
f7
f3
f7
f1
f2
f1
f4
f6
f3
f7
f4
f6
f5
f1
f5
f4
f6
f5
Each cell is generally 4 to 8 miles in diameter
with a lower limit around 2 miles.
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Problem with Smaller Clustersize
Interfering cells are closer by when clustersize
is smaller.
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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

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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

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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)

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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

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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

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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

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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)

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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

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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

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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

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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.

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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

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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

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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

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GSM Location Services
2
6
9
Gateway MTSC
Terminating MSC
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
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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
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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

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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

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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

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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

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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

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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
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• Fig 8-13

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• Table 8-3

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2G Technologies
cdmaOne (IS-95) GSM, DCS-1900 IS-54/IS-136PDC
Uplink Frequencies (MHz) 824-849 (Cellular) 1850-1910 (US PCS) 890-915 MHz (Eurpe)1850-1910 (US PCS) 800 MHz, 1500 Mhz (Japan) 1850-1910 (US PCS)
Downlink Frequencies 869-894 MHz (US Cellular)1930-1990 MHz (US PCS) 935-960 (Europa) 1930-1990 (US PCS) 869-894 MHz (Cellular)1930-1990 (US PCS) 800 MHz, 1500 MHz (Japan)
Deplexing FDD FDD FDD
Multiple Access CDMA TDMA TDMA
Modulation BPSK with Quadrature Spreading GMSK with BT0.3 p/4 DQPSK
Carrier Seperation 1.25 MHz 200 KHz 30 KHz (IS-136)(25 KHz PDC)
Channel Data Rate 1.2288 Mchips/sec 270.833 Kbps 48.6 Kbps (IS-136)42 Kbps (PDC)
Voice Channels per carrier 64 8 3
Speech Coding CELP at 13KbpsEVRC at 8Kbps RPE-LTP at 13 Kbps VSELP at 7.95 Kbps
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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. Bankruptcy
• Flash-OFDM 1.5 Mbps (upto 3 Mbps)

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Major Mobile Radio Standards USA
Standard Type Year Intro Multiple Access Frequency Band (MHz) Modulation Channel BW (KHz)
AMPS Cellular 1983 FDMA 824-894 FM 30
USDC Cellular 1991 TDMA 824-894 DQPSK 30
CDPD Cellular 1993 FH/Packet 824-894 GMSK 30
IS-95 Cellular/PCS 1993 CDMA 824-894 1800-2000 QPSK/BPSK 1250
FLEX Paging 1993 Simplex Several 4-FSK 15
DCS-1900 (GSM) PCS 1994 TDMA 1850-1990 GMSK 200
PACS Cordless/PCS 1994 TDMA/FDMA 1850-1990 DQPSK 300
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Major Mobile Radio Standards - Europe
Standard Type Year Intro Multiple Access Frequency Band (MHz) Modulation Channel BW (KHz)
ETACS Cellular 1985 FDMA 900 FM 25
NMT-900 Cellular 1986 FDMA 890-960 FM 12.5
GSM Cellular/PCS 1990 TDMA 890-960 GMSK 200KHz
C-450 Cellular 1985 FDMA 450-465 FM 20-10
ERMES Paging 1993 FDMA4 Several 4-FSK 25
CT2 Cordless 1989 FDMA 864-868 GFSK 100
DECT Cordless 1993 TDMA 1880-1900 GFSK 1728
DCS-1800 Cordless/PCS 1993 TDMA 1710-1880 GMSK 200
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• IEEE 802.11 vs 3G Cellular

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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)

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Engineering Issues

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

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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

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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)

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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

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What is WiMax?

• Worldwide Interoperability for Microwave Access
• Last mile wireless broadband access
• Alternative to cable and DSL
• Deliver data, voice, video
• Support hundreds to thousands of homes/business

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• Defined by IEEE as 802.16
• Typical target environment
• Targets fixed, portable, and mobile stations
• Environments with and without line of sight
• Cell radius of 3-10 kilometers
• Capacities of up to 40 Mbps per channel
• Mobile network deployments of up to 15 Mbps, 3
  km radius

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Builds on andExtends WiFi Technology

• Advantages of WiFi are
• Easy to deploy, unlicensed spectrum, low cost
• Supports (limited) mobility
• But WiMax needs to address the following

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WiFi limitations

• Susceptible to interference
• 802.11 targets short-range indoor operation
  (mostly)
• Security is a concern
• Limited level of mobility
• WiMax is intended to complement WiFi
• WiMax Forum promotes WiMax and looks after
  interoperability

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WiMax Deployment