Introduction to Mobile Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006.

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Introduction to Mobile Introduction to Mobile Communications Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006

Transcript of Introduction to Mobile Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006.

Page 1: 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

Page 2: Introduction to Mobile Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006.

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OutlineOutline

Cellular Communications Principles Control and Traffic Channels Frequency Reuse Hand-Off 1st Generation Cellular Wireless Networks

and AMPS

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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)

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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

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Cellular System OverviewCellular System Overview

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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

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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

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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/

Page 9: Introduction to Mobile Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006.

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From IEC online

Page 10: Introduction to Mobile Communications TCOM 552, Lecture #6 Hung Nguyen, Ph.D. 10 October, 2006.

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Cells usually adjacent to cover someselected area, often more along hightraffic highways, downtowns, popular suburban shopping areas, business areas

From IEC online

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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FREQUENCY REUSEFREQUENCY REUSE

D/R= 4.6for S/I=17-18 dB

D

RL= 7(see next page why)

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Cellular HistoryCellular History

1983 FCC allocation 306 Metropolitan Statistical Areas (MSAs),

424 Rural Statistical Areas A Carrier B Carrier

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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

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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

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Cellular PowerCellular Power

BASE Stations– BSAs: 100 - 500 watts– MSAs: 20 - 100 watts

Mobiles:– 0.006 to 4 watts

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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

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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

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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

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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

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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

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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

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Single Cell Capacity Improvement

Capacity Improvement

Analog FDMA 1.0

TDMA IS 136 3

GSM 2

CDMA IS-95

14 (varies)

Capacity ImprovementCapacity Improvement

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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