Download Cellular Networks

Ch 8. Cellular Networks
Myungchul Kim
[email protected]
Cellular Networks
Overview
• 1G Analog Cellular
• 2G TDMA - GSM
• 2G CDMA - IS-95
• 2.5G
• 3G
• 4G and Beyond
• Cellular Engineering Issues
•
Overview
Data Rates
2 Mbps
3G
(144Kbps to 2Mbps)
1 Mbps
100 Kbps
2.5G
(10-150Kbps)
10 Kbps
1 Kbps
2G
(9.6Kbps)
1G
(<1Kbps)
1980
1990
2000
Years
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
•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
Frequency Allocation
Wavelength
Cellular networks: Mostly around 900 MHz – 2GHz
Frequency
Gamma-rays
X-rays
3000 GHz
0.1 m
300 GHz
1 mm
THF - terribly high frequency
10 mm
EHF - extra high frequency
30GHz
SHF - super high frequency
3GHz
100 mm
1m
UHF - ultra high frequency
300 MHz
10 m
VHF - very high frequency
30 MHz
3MHz
100 m
HF - high frequency
1 Km
MF - medium frequency
Infrared
Micro
Waves
300KHz
10 Km
LF - low frequency
30Khz
100 Km
VLF - very low frequency
3KHz
Source: Bekkers, R. and Smits, J., “Mobile Telecommunications”, Artech, 2000.
Radio
Waves
Multiple Access Techniques: How to allocate users
Session1
Time
Frequency
Frequency Division
Multiple Access (FDMA)
1G Cellular (AMPS)
All sessions
based on a
code
Time
Time Division
Multiple Access (TDMA)
2G TDMA
3G TDMA
Time
2G CDMA (IS-95)
3G CDMA
Session4
Session3
Session2
Session2
Frequency
Frequency
Session3
Session1
Session4
Code Division
Multiple Access (CDMA)
Cell 1
A Cellular Network
Cell 2
Public
Switched
Telephone
Network
(PSTN)
Mobile
Telephone
Switching
Center
(MTSC)
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
tracked.
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
Cell 2
Mobile
Telephone
Switching
Center
(MTSC)
HLR VLR
Public
Switched
Telephone
Network
(PSTN)
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
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
Cell Design
E
E
A
D
F
B
C
B
C
G
A
G
D
F
E
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)
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
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
9.6kbps)
– Forward error correction and encryption
CDPD Operation
Interne
t
Landline Modem
Router
Router
CDPD Modem
= base interface station
(special unit that connects all base stations
in CDPD provider network to routers)
CDPD
Provider
Network
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
quality
• 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
users
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
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.
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
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
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
GSM Location Services
6
9
BTS
9
Terminating
MSC
10
10
10
8
7
4
VLR
2
Gateway
MTSC
5
10
3
Public
Switched
Telephone
Network
(PSTN)
1
10
HLR
5
1. Call made to mobile unit (cellular phone)
2. Telephone network recognizes number
and gives to gateway MSC
3. MSC can’t route further, interrogates
user’s HLR
4. Interrogates VLR currently serving user
(roaming number request)
5. Routing number returned to HLR and
then to gateway MSC
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
Legend: MTSC= Mobile Telephone Service Center, BTS = Base Transceiver Station
HLR=Home Location Register, VLR=Visiting Location Register
GSM Protocol Architecture
CM
CM
MM
MM
RRM
RRM
LAPDm
LAPDm LAPD
Radio
Mobile
Station
Radio
BTSM
64 Kbps
Base Transceiver
Station
BSSMAP = BSS Mobile Application part
BTSM = BTS management
CM = Connection Management
LAPD = Link Access Protocol, D Channel
BSSMAP
BSSMAP
BTSM
SCCP
SCCP
LAPD
MTP
MTP
64 Kbps
64 Kbps
Base Station
Controller
64Kbps
Mobile Service
Switching Center
MM = Mobility Management
MTP = Message Transfer Part
RRM = Radio Resources Management
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
(MTSO)
– Connection management: connects end users
– Mobile application part (MAP): between
HLR,VLR
– BTS management: management base system
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
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
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
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
3G
2 Mbps
CDMA2000
3XRTT
(UMTS)
CDMA Migration
1G-2G Migration
500 kbps
TDMA Migration
2.5G
150 Kbps
EDGE
CDMA-2000
1XRTT
100 Kbps
GPRS
2G
50 Kbps
10 Kbps
1G
IS-95
GSM
1 Kbps
AMPS
1980
1999
2000
2001
2002
2003
W-CDMA
(UMTS)
• Fig 8-13
• Table 8-3
Alternatives to 3G Cellular
• Major technical undertaking with
organizational and marketing overtones.
many
• 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)
• IEEE 802.11 vs 3G Cellular
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
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)
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
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.wirelessdesignmag.com)
• Wireless & Mobility (www.wireless mag.com)
• Wireless Review (www.wirelessreview.com)
• Wireless Systems Design (www.wsdmag.com)
• Wireless Week (www.wirelessweek.com)