Introduction to Mobile Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006.
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Transcript of Introduction to Mobile Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006.
Introduction to Mobile Introduction to Mobile CommunicationsCommunications
TCOM 552, Lecture #6Hung Nguyen, Ph.D.
10 October, 2006
10/10/2006Hung Nguyen, TCOM 552, Fall 20062
OutlineOutline
Cellular Communications Principles Control and Traffic Channels Frequency Reuse Hand-Off 1st Generation Cellular Wireless Networks
and AMPS
10/10/2006Hung Nguyen, TCOM 552, Fall 20063
Cellular/PCS EvolutionCellular/PCS Evolution
Bell Labs proposes cellular in 1968 First cellular system begins operation in early
1980’s using analog (AMPS) system, by AT&T -- First Generation systems (1G)– These are called cellular systems, in the 800 and 900 MHz
range mostly. Large, bulky phones Digital system using TDMA starts early 1990’s (2G)
– Both in 800 and 900 MHz, and around 1800 to 1900 MHz– These, along with GSM and CDMA below, sometimes
labeled PCS, Personal Communications Systems (usually when the frequency range is in the 1800 and 1900 MHz range)
10/10/2006Hung Nguyen, TCOM 552, Fall 20064
Cellular/PCS Evolution (cont’d)Cellular/PCS Evolution (cont’d)
European GSM starts operation early 1990’s (2G)– Similar frequency range as TDMA but not exactly--- US
and Europe usually differ– GSM used nearly worldwide, more users than US
standards First CDMA operation 1996-1997 -- Qualcomm
holds patents (2G)– Used by half of 2G systems in US, S. Korea, parts of Latin
America First limited 3G operation in Japan in May 2001--
wideband CDMA mostly– Worldwide (semi) standardization agreed to in 1999, for
high data rate services to 2 Mbps – New frequency allocation around 1800 MHz in most of
world, with some variations– US allocations not made yet, probably 2002, best ones
already being used
10/10/2006Hung Nguyen, TCOM 552, Fall 20065
Cellular System OverviewCellular System Overview
10/10/2006Hung Nguyen, TCOM 552, Fall 20066
Cellular Systems ElementsCellular Systems Elements
Base Station (BS) – includes an antenna, a controller, and a number of receivers– Made up of Base Transceiver Station (BTS) and Base
Station Controller (BSC) --- often one BSC controls multiple BTS’s
Mobile telecommunications switching office (MTSO) – connects calls between mobile units --Also called Mobile Switching Center (MSC)
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
10/10/2006Hung Nguyen, TCOM 552, Fall 20067
Principles of Cellular CommunicationsPrinciples of Cellular Communications
Base Stations cover an area called a cell Cell phones communicate with Base Station
over wireless links Base Stations interconnected and handle
users as they move from cell to cell Base Stations connected to PSTN to switch
calls to/from cell phone users Wireless links involve radio communications
between users and Base Stations. Cell phone is a radio.
Used initially for voice but increasingly for data, email, Internet and later video
10/10/2006Hung Nguyen, TCOM 552, Fall 20068
Cellular Network OrganizationCellular Network Organization
Use multiple low-power transmitters (100 W or less)
Areas divided into cells– Each served by its own antenna– Served by base station consisting of transmitter,
receiver, and control unit– Band of frequencies allocated– Cells set up such that antennas of all neighbors
are equidistant (hexagonal pattern) See Tutorial at
– http://www.iec.org/tutorials/cell_comm/index.html– http://www.iec.org/online/tutorials/cell_comm/
10/10/2006Hung Nguyen, TCOM 552, Fall 20069
From IEC online
10/10/2006Hung Nguyen, TCOM 552, Fall 200610
Cells usually adjacent to cover someselected area, often more along hightraffic highways, downtowns, popular suburban shopping areas, business areas
From IEC online
10/10/2006Hung Nguyen, TCOM 552, Fall 200611
How Cellular WorksHow Cellular Works
A cellular telephone system provides a wireless connection to the PSTN for any user in the radio range of the system. It consists of:
– mobile stations (cell phones)– base stations– the Mobile Switching Center
(MSC) The base station provides a
connection to the cell phone via wireless links or the ‘air interface’
The base station is the bridge between the mobile users and the MSC via phone lines or microwave links.
The MSC connects the entire cellular system to the PSTN.The MSC, its interconnections and databases is called the ‘core network’
MSC PSTN
From iec.org
10/10/2006Hung Nguyen, TCOM 552, Fall 200612
Cellular Architecture and MobilityCellular Architecture and Mobility
Cellular Architectures Include– Mobiles
– Base Stations In Adjacent Cells
– Switching Centers: MSC’s
– Interfaces to the PSTN
– With Networked MSC’s A Key Factor is Mobility
– Requires Handoff as User Moves from Cell to Cell
– Requires Databases to Track User Mobility -- Used by MSC’s for Mobility Management
Figure adapted from IEC cellular and wireless tutorials
OtherMSC’s
10/10/2006Hung Nguyen, TCOM 552, Fall 200613
Cellular ChannelsCellular Channels
Two-Way Cellular Communication relies on a variety of channels to ensure call connection and delivery.
– Control Channels: Channels used as beacons for idle mobiles and access to traffic channels. Signaling channels sometimes grouped here also -- in AMPS these control channels are called forward control channel and reverse control channel
– Traffic Channels: Channels used to support customer voice calls and messages. Separate forward and reverse channels
– Signaling Channels: A type of control channel used to provide network and hand-off signals during customer communications -- in AMPS these are called ‘voice channels’
Base StationFo
rwar
d C
hann
el
(dow
nlin
k)
Reverse C
hannel
(uplin
k)
Modified from iec.org
10/10/2006Hung Nguyen, TCOM 552, Fall 200614
Cellular Control ChannelsCellular Control Channels
The base station/mobile link
• Defined by the Air Interface which specifies several different channels. Example: (not AMPS)
Forward Access Channel (FAC) Forward Traffic Channel (FTC) Forward Pilot Channel (FPC) Forward Synch Channel (FSC) Reverse Access Channel (RAC) Reverse Traffic Channel (RTC)
Mobile User
RAC
RTC
FAC
FTC
FPC
FSC
MSC From iec.org
10/10/2006Hung Nguyen, TCOM 552, Fall 200615
Steps in an MSC Controlled Call Steps in an MSC Controlled Call between Mobile Usersbetween Mobile Users
Mobile unit initialization (power on) Mobile-originated call (push send button) Base Station and MSC accept route call Paging -- to called user via his Base Station Call accepted by called user Ongoing call (traffic channels with signaling) Handoff as mobile unit moves to adjacent
cell
10/10/2006Hung Nguyen, TCOM 552, Fall 200616
Additional Functions in an MSC Additional Functions in an MSC Controlled CallControlled Call
Call blocking (when no traffic channels available)
Call termination (at end, re-assign the traffic channels to other calls)
Call drop (when handoff does not work) Calls to/from fixed wireline subscribers
– MSC acts like a PSTN switch, I/F to PSTN Calls to/from remote mobile subscribers
– MSC has to find out where remote subscriber is and route call to Base Station there
10/10/2006Hung Nguyen, TCOM 552, Fall 200617
Frequency ReuseFrequency Reuse
Adjacent cells assigned different frequencies to avoid interference or crosstalk
Objective is to reuse frequency in nearby cells but not adjacent ones– Multiple frequencies assigned to each cell– Transmission power controlled to limit power at
that frequency escaping to adjacent cells– The issue is to determine how many cells must
intervene between two cells using the same frequency
10/10/2006Hung Nguyen, TCOM 552, Fall 200618
Sectoring permits more freq. reuse -- e.g.AMPS from 12 to7, due to lower interference
Fig. from iec.org
SEVEN CELL REUSE
Each cell number has a frequency allocation, so e.g., cell 1 only interfers with other cell 1’s, and so on.
This is an example of 7 cell reuse, cluster sizes is 7 cells.
Also used 3, 4 cell clusters, 12, 13, 19
CDMA does one cell reuse
10/10/2006Hung Nguyen, TCOM 552, Fall 200619
From iec online
Sectoring used to limit interference to sectors, thus increasing capacity
SectoringSectoring
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Mobile Radio Propagation EffectsMobile Radio Propagation Effects
Okumura-Hata Propgation Model– Based on empirical data– Include base station and mobile heights– Equation 10.1
Example 10.2
10/10/2006Hung Nguyen, TCOM 552, Fall 200621
Interference, Capacity, and Frequency Interference, Capacity, and Frequency ReuseReuse
Maximum number of users in cellular determined by interference levels that can be tolerated
Min. SNR (=S/N) needed to receive a signal in interference become SINR (=S/(I+N)), where I is interference power
But for maximum capacity I>>N, so (S/I)min is the minimum required S/I to make a receiver work– e.g., AMPS, (S/I)min for FM to work is 18 dB
But in cellular I is proportional to S– I is due to signals (S) transmitted in nearby cells at same
frequencies ---- but attenuated by being further away– So S/I determined largely by distance separations, D/R– D/R in turn determined by frequency reuse factor --- how
often in cellular topology is the same frequency reused
10/10/2006Hung Nguyen, TCOM 552, Fall 200622
FREQUENCY REUSEFREQUENCY REUSE
D/R= 4.6for S/I=17-18 dB
D
RL= 7(see next page why)
10/10/2006Hung Nguyen, TCOM 552, Fall 200623
Frequency ReuseFrequency Reuse
L (Rappaport, others use N, N not noise) is number of cells in cluster– Frequencies not reused in cluster, but divided up, reused from
cluster to cluster– L=7 in example
If D=distance to next cluster and R is radius of cell– Hexagonal geometry yields L1/3(D/R)^2– For example, D/R=4.6, L=7– AMPS used L=12 and L=7– For D/R=3, L=3 --- D/R=6, L=12
S/I is related to D/R by D/R=>(6(S/I))^(1/n) – Because D/R determines relative distance to interferers
compared to distance to wanted signal, n from 1/(R^n) propagation loss
– For L=7, S/I=17.8 dB – Since AMPS requires 18 dB min. (for clear voice) it is close for
L=7 and OK except worse cases near edge of cell--- sometimes L=12 was used --- it is better for TDMA and CDMA
10/10/2006Hung Nguyen, TCOM 552, Fall 200624
Increasing Capacity, Frequency Reuse Increasing Capacity, Frequency Reuse and Sectoringand Sectoring
Frequency reuse determines capacity for a given technology– e.g., AMPS has 416 channels for a service provider-21 control
channels so has 395 traffic channels --- With L=7 reuse each cell has 395/7= 56 and 3/7 channels max. capacity
Capacity can be increased by– More spectrally efficient technology (e.g., AMPS to TDMA)– Better frequency reuse through various means to control
interference Typically, Sectoring --- each cell divided into (e.g., 3) sectors
– For L=7, this reduces the interference from adjacent cells to 1/3 of them
• Makes S/I=24.2 dB, so AMPS for sure well now• Still AMPS not able to go to L=4 or 5
– GSM uses sectoring to reduce interference so reuse is 4 or 5– CDMA uses sectoring to make reuse 1 (ie, L=1)
Cell splitting (more, smaller cells, microcells -- more cells more capacity as 56 and 3/7 above is calls/cell), smart antennas
10/10/2006Hung Nguyen, TCOM 552, Fall 200625
Base Station --- Diversity Receivers Base Station --- Diversity Receivers Each SectorEach Sector
RECEIVER A
RECEIVER B
Transceiver
MobileAUDIOout
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Handoff Performance MetricsHandoff 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
10/10/2006Hung Nguyen, TCOM 552, Fall 200627
Handoff Performance MetricsHandoff 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
10/10/2006Hung Nguyen, TCOM 552, Fall 200628
Handoff Strategies Used to Determine Handoff Strategies Used to Determine Instance of HandoffInstance of Handoff
Relative signal strength Relative signal strength with threshold Relative signal strength with hysteresis Relative signal strength with hysteresis and
threshold Prediction techniques
10/10/2006Hung Nguyen, TCOM 552, Fall 200629
Handoff Approaches UsedHandoff Approaches Used
Network Controlled Handoff -- NCHO– Network makes measurements and controls when handoff
ocurs– In AMPS, when signal level from mobile falls below -100
dBm (or so, proprietary schemes) at serving basestation (BS), it looks for another BS and requests it to make measurements, when it’s -90 dBm there it hands off
– Hysteresis, no immediate handoff, stabilizes for no ping pong
– Done by MTSO Mobile Assisted Handoff
– Mobile is making measurements and reporting them to BS, from its cell and adjacent cells, MTSO still controls it
– Used in TDMA, GSM and CDMA
10/10/2006Hung Nguyen, TCOM 552, Fall 200630
Power ControlPower 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
IS-95 CDMA adjusts the power of each handset 800 times per second, thus minimizing interference
10/10/2006Hung Nguyen, TCOM 552, Fall 200631
Types of Power ControlTypes of Power Control
Open-loop power control– Depends solely on mobile unit– No feedback from BS– Not as accurate as closed-loop, but can react
quicker to fluctuations in signal strength Closed-loop power control
– Adjusts signal strength in reverse channel based on metric of performance, usually power measured at BS
– BS makes power adjustment decision and communicates to mobile on control channel
– Used in AMPS and others
10/10/2006Hung Nguyen, TCOM 552, Fall 200632
IS-41 -- Part 1IS-41 -- Part 1
Established to provide automatic roaming services in US, preceded by similar system in GSM
Open interface --- used to be no standards Roaming is being served by a System other than
your Home System --- a license to operate in one area by one company defines a System -- for AMPS many Systems are controlled by one MTSO, some more
Can use the PSTN using SS7 or ISDN, or dedicated trunks, it is at application layer (L7) in ISO, as it defines messages for the network management application related to roaming
MSC’s is term used instead of MTSO for IS-41
10/10/2006Hung Nguyen, TCOM 552, Fall 200633
10/10/2006Hung Nguyen, TCOM 552, Fall 200634
IS-41 -- Part 2IS-41 -- Part 2
IS-41 allows roaming by providing 3 functions and 2 key databases
HLR and VLR -- mobile location registers-- normally at MSC’s
Exchange of messages between Visiting and Home MSC’s
Mobility management– Allows users to be registered in visiting Systems, while
having the Home System be updated so it can keep track of location
– Handoff across MSC’s, forward, back, to third (as it proceeds)
Authentication, through MSC messages Call Management, using HLR/VLR to find mobile
– Serving MSC usually uses SS7 to set up path through PSTN
10/10/2006Hung Nguyen, TCOM 552, Fall 200635
First-Generation AnalogFirst-Generation Analog
Advanced Mobile Phone Service (AMPS)– 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
– Each band has two service providers (A and B) to encourage competition
– Frequency reuse exploited
10/10/2006Hung Nguyen, TCOM 552, Fall 200636
Cellular HistoryCellular History
1983 FCC allocation 306 Metropolitan Statistical Areas (MSAs),
424 Rural Statistical Areas A Carrier B Carrier
10/10/2006Hung Nguyen, TCOM 552, Fall 200637
AMPS OperationAMPS Operation
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
10/10/2006Hung Nguyen, TCOM 552, Fall 200638
AMPS StandardAMPS Standard
– 30 kHz channels in 50 MHz AMPS spectrum – Analog voice, FDMA, FM modulation– Control Channels are FSK – Reuse factor 7 usually with sectoring – 416 channel pairs/ carrier (395 traffic)– Mobile to base 824-849 MHz (A and B providers)– Base to mobile 869-894 MHz (A and B
providers)– 25 MHz per service provider: 56 and 3/7
calls/cell – Spectral Efficiency -- 2.26 calls/MHz/cell
10/10/2006Hung Nguyen, TCOM 552, Fall 200639
Cellular PowerCellular Power
BASE Stations– BSAs: 100 - 500 watts– MSAs: 20 - 100 watts
Mobiles:– 0.006 to 4 watts
10/10/2006Hung Nguyen, TCOM 552, Fall 200640
AMPS Spectral EfficiencyAMPS Spectral Efficiency
In 25 MHz there are 416 dual-30KHz channels, one uplink, one downlink
21 are used for control, leaving 395 traffic channels Since 7 cell reuse is needed, 395/7=56 and 3/7 are
available in each cell (56 in 4 cells, 57 in 3). This means each cell can support 56 and 3/7 two-way conversations, or calls.
Spectrum efficiency – is thus 56 and 3/7 calls per cell, with a total allocated
bandwidth of 25 MHz, or 56 and 3/7 divided by 25, – equals 2.26 calls/cell/MHz
Sometimes spectral efficiency done in calls/MHz/km^2 -- just figure how many km^2/cell, and divide into 2.26
10/10/2006Hung Nguyen, TCOM 552, Fall 200641
AMPS Control and Signaling Channels -- AMPS Control and Signaling Channels -- Common and DedicatedCommon and Dedicated
Both done digitally, as Manchester coded FSK, as tones near the carrier, + and - 8 KHz deviation, at 10 kbps– Protected by block codes, BCH, repeated 5 and 11 times
to make sure no errors Common channels, called control channels, used
for control and paging information --- including the information to set up calls– Forward are broadcast channels (FOCC)– Reverse (RECC) are random access, controlled by status
on FOCC Signaling channels while call is active called ‘voice
channels’, forward and reverse, FVC, RVC, in band, inserted maybe 1 or 2 times/minute in blank and burst lasting 100 msec, used for handoff, power control, end
10/10/2006Hung Nguyen, TCOM 552, Fall 200642
AMPS Terminal States/FunctionsAMPS Terminal States/Functions
Initialization– Terminal scan 21 control channels -- transmitted
continuously by BS’s --- it locks on to strongest one, and usually uses it as a paging channel as well (or told where)
– Control channels are reused on 21 factor basis, 21 BS’s, reuse freq*3 (# SAT’s -- SAT’s are supervisory tones, 3, inserted at 6 KHz), so terminal could access cell further away if stronger
Idle– Monitors paging channel from cell, looks for its number
Access– Requests traffic channel from BS, gets it and ‘voice
channels’ Conversation
– Still some signaling, in FVC/RVC, to control call
10/10/2006Hung Nguyen, TCOM 552, Fall 200643
ID Codes and Channel TypesID Codes and Channel Types
ID Codes– MIN --- mobile ID #, its phone number, 10 digits– ESN -- electronic serial number on terminal, factory assigned– System ID -- SID, 15 bit, ID of System, used by mobile to check
vs its SID, if different it’s roaming, otherwise at Home– SAT -- supervisory audio tone, in FVC/RVC, so mobile can tell
which is his BS, BS can tell which are his terminals, analog, 3 of them
– DCC -- like SAT, for digital control signals, dig. color code Channels are described at 3 levels: physical, logical, network
– Physical is analog (FM), digital (FSK)– Logical is functions and format -- traffic, control, ‘voice’, and how
bits are arranged– Network -messages -- e.g., call setup, handoff, etc
10/10/2006Hung Nguyen, TCOM 552, Fall 200644
Network OperationsNetwork Operations
– Call management (control sequences)– Mobility management
Registration messages into databases, pages on most likely cells, updates of mobile location into HLR, VLR registrations, handoff
– Radio Resource Management (BS channels mgt.) call admission, channel assignment, power control of
terminals (8 levels), MTSO controlled handoff --threshold with hysteresis (-100 dBm to -90 dBm), also intracell
– Authentication (but no privacy) phone number+serial number(on terminal), can be
intercepted
– Information Transport– OA&M
10/10/2006Hung Nguyen, TCOM 552, Fall 200645
10/10/2006Hung Nguyen, TCOM 552, Fall 200646
System GSM IS-136 (IS-54)
IS-95
Access FDMA/ TDMA
FDMA/ TDMA
CDMA
Channel BW
200 kHz
30 kHz
1.25 MHz
Vocoder speech
13 kb/s 8 kb/s 0.8 to 8 kb/s (variable)
Max User Power 125 mw 100 mw 200 mw (variable)
Users/ Chan.
8 3 64 Max.
Some Cellular System ParametersSome Cellular System Parameters
10/10/2006Hung Nguyen, TCOM 552, Fall 200647
Single Cell Capacity Improvement
Capacity Improvement
Analog FDMA 1.0
TDMA IS 136 3
GSM 2
CDMA IS-95
14 (varies)
Capacity ImprovementCapacity Improvement
10/10/2006Hung Nguyen, TCOM 552, Fall 200648
Approaches to Cope with Increasing Approaches to Cope with Increasing Capacity NeedsCapacity Needs
More spectrum Adding new channels 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 Cell sectoring – cells are divided into a number of
wedge-shaped sectors, each with their own set of channels --- permits greater frequency reuse
Microcells and picocells – antennas move to buildings, hills, and lamp posts -- but not AMPS
Higher spectral efficiency technology -- e.g., AMPS to TDMA or CDMA