Wireless & Personal Communication Systems – CSE5807 Lecture: 04 1 Wireless Personal Communications...

1Wireless & Personal Communication Systems – CSE5807Lecture: 04

Wireless Personal Communications Systems – CSE5807

Topic – Cellular Wireless Networks

Lecture: 04

Stephen Giles and Satha K. Sathananthan

Faculty of Information Technology

Monash University

Modified by Peter Granville August 2006

These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak .


Cellular technology

is the foundation

of mobile wireless communications

and supports users

in locations

that are not easily served by

wired networks.


Wireless Networks

Limited bandwidth.

Noisy channel and Multipath propagation.>> Interference.

Limited coverage => Roaming

Security.

Power consumption.


Wireless Networks

1001010.1 Mbps

Vehicle

Walk

Fixed

Walk

FixedIndo

or

O

utdo

or

Wideband CellularG

SM

, CD

MA

WLAN

WPAN

LAN

WAN3G

2G

WPAN – Wireless Personal Area Network


Cellular Systems• First Generation Systems: Analog

– Advanced Mobile Phone Service (AMPS): US, Australia, Southeast Asia.

– Total Access Communication System (TACS): EU

– Nippon Telephone and Telegraph (NTT): Japan

– Being phased out

• Second Generation Systems: Digital– Global System of Mobile communications (GSM): Europe, Asia

– Code Division Multiple Access (CDMA) systems (IS-95): US, Asia

• Third Generation Systems: Digital & Packet switching– High Speed Transmission

– Wideband CDMA

– CDMA2000


Wireless Channel

• Compared to wire/fiber, mobile radio channels have major problems with noise and interference.

- Environmental effects. - Large amounts of noise. - Leakage from adjacent channels and distant transmitters on the same channel. - Multi-path fading (Rayleigh) and Doppler effect.

• Signal coverage:- Essential for deployment of wireless networks.- Influenced by the radio frequency of operation, transmitted power and the terrain.


Wireless Channel

TxRx

Diffraction

Reflection

Scattering


Multipath Propagation - Effects System Performance

• Reflection– A signal can be reflected by obstacles (obstacle > signal wave) so that

multiple copies of the signal with varying delays can be received.

– Depending on the differences in the path lengths of the direct and reflected waves, the composite signal can be either larger or smaller than the direct signal.

• Scattering– Occurs when Size of obstacle <= size of signal wave

– An incoming signal is scattered into several weaker outgoing signals

• Diffraction– Occurs at the edge of an impenetrable body (large cf signal).

– When a radio wave encounters such an edge, waves propagate in different directions with the edge as the source.

– Thus signals can be received even when there is no unobstructed LOS from transmitter.


Cellular Concepts• The concept of cellular radio emerged in the late 1940s:

• high power transmitter• 80km radius cell• 25 channels

•To get around the limitations on available frequencies, the following approach was implemented:

• A large number of low-power transmitters (<=100W) with shorter radius• Because the range of the transmitter is small, an area can be divided into cells• one base station per cell - transmitter, receiver, control unit• A range of frequencies allocated to each cell.


Cellular Concepts

• Frequency allocation such that co-channel interference is limited. Adjacent cells assigned different frequencies.

• Cells sufficiently distant from each other to allow frequency reuse

• “Hand-over“ (handoff) techniques for mobile units moving from cell to cell.


Cell Shape• Square Cell Pattern

– If cell width is d, then a cell has four neighbours at a distance d and 4 neighbours at a distance SQRT(2d) from cell centre

– As a mobile user moves towards cell’s boundaries, it is best if all adjacent antennas are equidistant. This simplifies the task of determining when to switch the user to an adjacent antenna and which antenna to use.

• Hexagonal Cell Pattern– Provides for equidistant antennas

– Radius R, is radius of circle that circumscribes it

– Distance between all adjacent cell centres is d = SQRT(3R)

• Refer Stallings fig 10.1


Cell Shape• Variations from ideal hexagon are due to:

– Topographical limitations

– Local signal propagation conditions

– Practical limitations on siting antennas


Cellular Concepts

• Areas divided into cells:– Each served by its own antenna => multiple low-power

transmitters.

– Served by base station consisting of transmitter, receiver, and control unit.

– Band of frequencies allocated.

– Adjacent cells assigned different frequencies to avoid interference or crosstalk.

=>> Frequency reuse.


Cellular Concepts: Frequency Reuse

coverage

coverage

Base station (BS)

coverage

Backbone Network

• To transmit signals on a particular frequency band with “limited power” so that the same frequency band can be reused in other location.• Fig 10.2 Stallings



4-cell frequency reuse (N=4)

7-cell frequency reuse (N=7)



• N = Frequency reuse factor• R = Radius of a cell• dadj = Distance between centers of adjacent cells • Dmin = Minimum distance between centers of cells that use the

same band of frequencies (cochannel)• K = Total number of channels (frequency bands) allocated to

the systems.

R d

D



,...21,19,16,13,12,9,7,4,3,1

,.....2,1,0, )(22

N

JIJIJIN

N

KNcpc

Rd 3

• In a hexagonal cell pattern:

NR

D3

• If each cell is assigned equal number of channels, then the number of channels per cell:


Calculation

NR

D3

Dmin = 1.6 x SQRT(3 x 7) = 1.6 x 4.58 = 7.3 km

dadj = SQRT(3 x 1.6) = 2.19 km

Given a cell radius of R = 1.6 km, Nreuse = 7

calculate:

1) Minimum distance Dmin, between centers of cells that use the same band of frequencies (co-channels)

2) Distance dadj, between centers of adjacent cells

Rd 3


CalculationGiven a system with Ncells = 32, R =1.6 km, total number of traffic channels K = 336, Nreuse = 7, calculate: Ncpc number of traffic channels per cell Tcalls total number of concurrent calls that can be handled

Tcalls = Ncpc x Ncells Ncpc = K/Nreuse

Ncpc = 336/7 = 48 traffic channels per cell

Tcalls = 48 x 32 = 1536 concurrent calls (ie 1536 channels available)


Cellular Concepts: Interference and Capacity

• Interference affects reuse plan.• Major interference:

- Cochannel (or same frequency interference): => most important.- Adjacent Channel: => less important.

• The smaller value of Nreuse the: Wider available bandwidth in each cell. Higher interference.

• For AMPS, K=395 and Nreuse = 7 is the smallest pattern that can provide sufficient isolation between two uses of the same frequency, about 57 frequency channels per cell


Cellular Concepts: Cochannel Interference

7-cell frequency reuse (weaker)

4-cell frequency reuse (stronger)

Calculate Dmin with R=1.6km


Cellular Concepts: Adjacent Channel Interference

wanted

Interference

…

pow

er

Adjacent channelinterference

Frequency band of the mobile phone


Cellular Concepts: Increasing Capacity

• In time as more customers use the system, traffic may build up so that there are not enough frequencies assigned to a cell to handle its calls.


Cellular Concepts: Increasing Capacity

• 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.

– Power level used must be reduced to keep the signal within the cell

– 1.5 km close to practical limit

– As cells get smaller, handoffs become more frequent

– Fig 10.3 Stallings

• Cell sectoring: – Cells are divided into a number of wedge-shaped sectors, each with their

own set of channels.


Cellular Concepts: Cells• Different sizes and types of cells are used in a cellular network. The choices of a cell depend on the bandwidth usage in a region.

- Macrocells: - Used to serve low density traffic area.- Tens of kilometers, served by base stations.

- Microcells: - Used to serve high density traffic area.- 100m to 1km, base station antenna moved to rooftops of small buildings and finally to lamp posts.

- Sectored cells:- Used to reduce cochannel interference.

- Umbrella cells: - Used to reduce the need for handover in microcells.


Cellular Concepts: CellsLarge cells are used to serve low traffic areas.

Microcells are used for high traffic demand regions.

Umbrella cells are used in areas where users are moving fast from one cell to another (eg. freeways)


Cellular Concepts: Sectored Cells• 3 or 6 sectors per cell:

• The output power of an antenna in a sectored cell:

-3dB

coverag

e

antenna

Rhombic Hexagonal Triangular


Cellular Concepts: Interference in Sectored Cells

interference

7-cell frequency reuse

• Each sector is operated at a different frequency band.

• The number of main interference is reduced from 6 to 2


Spatial Diversity Multipath in wireless transmissions results in

“Rayleigh Fading” (or fast fading).

Multiple antennas are used to receive signals from a mobile phone to reduce Fading effects.

Base stationtower

Two receivers


Operation of Cellular System


Operation of Cellular Systemfig 10.6 Stallings

• Mobile equipment (ME):– Physical terminal, includes radio transceiver, digital signal processors and

subscriber identity module (SIM).

• Base Station (BS):– Includes antenna, controller, and a number of receivers.

– Use multiple low-power transmitters.

– Areas divided into cells, and each served by its own antenna.

– Band of frequencies allocated.

• Mobile telecommunications switching office (MTSO):– Connects calls between mobile units.

• Two types of channels available between mobile unit and BS.– Control channels: used to exchange information for setting up and

maintaining calls.

– Traffic channels: carry voice or data connection between users.


Operation of Cellular System• Mobile unit initialization:

– Scanning => Select the strongest setup channel.

– Handshake => Identify and register location.

• Mobile-originated call:– Request for connection on the pre-selected setup channel.

• Paging:– MTSO sends message to certain BSs to identify the called number.

• Call accepted:– Mobile recognizes the call respond to BS -> MTSO.

– MTSO assigns traffic channels.

• Ongoing call:– Monitoring stage.

• Handoff:


Handoff or Handover

Handoff occurs when a mobile phone moves from one cell to another.Power levels are constantly measured by base stations and/or mobile phones to decide whether a handoff is needed.

cell boundary

Base statio

n

Base statio

n f1 f2


Handoff or Handover

• Handoff types– Network initiated, based on received signals from the mobile unit.

– Mobile assisted, via providing info to network concerning signals received at mobile unit

• Performance metrics for handoff decision.– Cell blocking probability: probability of a new call being blocked, due

to heavy load on Base Station traffic capacity. What happens <1>?

– 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.


Handoff or Handover• Principal parameter used to make the Handoff Decision is

measured signal strength from the mobile at BS.

• BS averages the signal strength over a moving window of time to remove the rapid fluctuations due to multi-path effects

• Handoff strategies


Handoff Strategies• Refer fig 10.7 Stallings:

• Relative signal strength.

– The mobile unit is handed off from BS A to BS B when signal strength at B first exceeds that at A - L1

– Can result in ping-pong effect


Handoff Strategies• Relative signal strength with threshold.

– Handoff only occurs if :

(1) the signal at current BS is sufficiently weak (less than a pre-defined threshold) and

(2) the other signal is the stronger of the two

– The intention is that so long as the signal at current BS is adequate, handoff is unnecessary

– High Threshold Th1 - handoff at L1 as above

– Th2 - handoff at L2

– Th3 - handoff at at L4, reduces quality of the link and may result in


Operation of Cellular System• Call blocking:

– No free traffic channels, busy tone returned to user.

• Call termination:

– MTSO is informed, traffic channels released

• Call drop:

– Resulting from weak signal, MSTO is informed

• Calls to/from fixed and remote mobile subscriber:

– MTSO sets up the connection.


Power Control• 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 spread spectrum systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS.


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:

• Received signal power level.• Received signal to noise ratio.• Received bit error rate.

– BS makes power adjustment decision and communicates to mobile on control channel.

– Also used to adjust power in forward channel.• Mobile provides power adjustment information to BS.

• Refer table 10.2 Stallings


Traffic Engineering• Ideally, available channels would equal number of subscribers

active at one time• In practice, not feasible to have capacity to handle all possible

users.

• For N simultaneous user capacity and L subscribers• L < N => non-blocking system• L > N => blocking system

• Blocking system:– Blocking probability (B):

• Probability that call request is blocked.

– What capacity is needed to achieve a certain upper bound on probability of blocking, B?


Traffic Engineering• Traffic intensity (A):

– Load presented to a system:

= mean rate of calls attempted per unit time.

• h = mean holding time per successful call.

• A = traffic intensity = * h Erlang

hA

• 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.


Traffic Engineering - Example• h = mean holding time per successful call = 3mins

= mean rate of calls attempted per unit time = 20 calls/min

• Hence A = traffic intensity = 3 * 20 = 60 Erlang

• Erlang a measure of traffic intensity

• Number of channels needed ?– 120 expect 50% utilization

– 50 clearly inadequate

– 60 would meet average demand, but would not cater for fluctuations around the mean rate A


Traffic Engineering - Example• Refer fig 10.8 Stallings

• Cell capacity 10 channels

= Mean Rate of calls per min = 97/60 = 1.62

• h = Mean Call holding time = 294/97 = 3 mins

• Hence A = traffic intensity = 3 * 1.62 = 4.86 Erlang, so an average of 4.86 channels are engaged


Traffic Engineering• Performance is measured by the call blocking probability.

• Blockage rate depends on the number of lines available, the number of initiated calls, and the length of the call.

• Erlang B formula:- Calls are lost if a line is not available.

N

i

i

N

iA

NA

ANB

0!

!),(


Call Blocking - ExampleInstalling a Base Station

• You are required to determine how many lines the Base Station is required to support

• Project a 50% growth in cell traffic over the next five years

• What probability of call blocking are you prepared to accept ?

• Currently mobile users are generating an average of 2 calls/min (the call arrival rate) and the average connection time per call is 3 mins (the call holding time)


Call Blocking - Example

• (Call Arrival Rate) = 2 * 1.5 = 3 calls/min

• (Call Holding Time) = 3 mins

• You need to calculate the traffic load that the Base Station will be required to handle A – the Traffic Intensity

– A is a measure of incoming traffic = * = 9 Erlang

• The Probability of a Call Blocking is given by:

where N is the number of outgoing lines

/1

/1

N

n

nNB nANAP

0)!/(/)!/(


Call Blocking - Example

Results of Calculations

• Let N = 3 outgoing lines

• n! = n x n-1 x n-2 x ......x 1, 0! = 1, eg 3! = 3 x 2 x 1 = 6

• Result = 1 + 9 + 40.5 + 121.5 = 172

• PB = 121.5/172 = 0.71

• A/N the utilisation per link = 9/3 = 3 as A/N exceeds 1 the probability of blocking rises rapidly

!3/9!2/9!1/9!0/9)!/9( 323

0

10 n

n n


Traffic Engineering - Example• Refer table 10.3 Stallings

• A larger capacity system is more efficient than a smaller capacity system for a given grade of service:

– Consider two cells:

• each with a capacity of 10 channels

• they have a joint capacity of 20 channels

• can handle a combined offered traffic intensity of (2 * 3.43) 6.86 Erlangs for a grade of service of 0.002

– However, a single cell:

• capacity 20 channels

• can handle 10.07 Erlangs at a grade of 0.002


Traffic Engineering - Example• Refer table 10.3 Stallings

• A larger capacity system is more susceptible to reduction of the grade of service as traffic increases:

– Consider a cell of 10 channels:

• giving a grade of service of 0.002 for load of 3.43 Erlangs

• A 30% increase in traffic to 4.46 Erlangs reduces the grade to 0.01

– However, a cell of capacity 70 channels:

• giving a grade of service of 0.002 for a load of 51 Erlangs

• only a 10% increase in traffic to 56.1 reduces the grade to 0.01


Analog Cellular Networks (1 G) An analog cellular network is operated at 900 MHz.

FDMA is used to allow multiple mobile phones to share a single base station in a cell.

Voice signals are transmitted with no coding scheme.

The major analogue cellular systems are based on the original AMPS design:

- System bandwidth: 25MHz - 25 or 30 kHz channels - AMPS (USA, EIA-553) 800MHz- TACS (UK) 900MHz - NMT (Nordic countries) 450 and 900 MHz


AMPS Parameters• Downlink: 869 to 894 MHz

• Uplink: 824 to 849 MHz

• Channel bandwidth: 30 kHz

• Spacing between forward and reverse channel: 45 MHz

• Number of full-duplex voice channels: 790

• Number of full-duplex control channels: 42

• Mobile unit maximum power: 3 watts

• Cell radius: 2 to 20 km

• Data transmission rate: 10 kbps

• Modulation schemes: FM and FSK


AMPS 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


1 G and 2 G Comparison1G 2G

System Analog Digital

Multiple Access FDMA TDMA, CDMA

Scheme: FDMA

Voice quality: Low Good

Bandwidth efficiency: Low High

Power consumption High Low

on mobile phones:

Security: Low High

Value added service: Difficult Easy

System complexity: Low High


GSM• Use of several carrier frequencies.• Cell sizes vary : 100 m up to 35 km (user density, geography, transceiver power etc).

• Multiple Access = FDMA/TDMA FDMA 200kHz TDMA 8 slots in a frame

ie. each channel = 200kHz/8 = 25kHz (Bandwidth)

A B

0 1 2 3 4 5 6 7 0 … time

freq

f3

f2

f1

cell A


GSM Parameters• Downlink: 935 to 960 MHz

• Uplink: 890 to 915 MHz

• Channel bandwidth: 200 kHz

• Users per channel: 8

• Spacing between forward and reverse channel: 45 MHz

• Number of duplex channels: 125

• Mobile unit maximum power: 20 watts

• Speech coding bit rate: 13 kbps

• Modulation schemes: GMSK


GSM Speech Signal Processing


GSM Network Architecture


GSM: 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.


GSM: Base Station Subsystem (BSS)• BSS consists of base station controller and one or more

base transceiver stations (BTS).

• Each BTS defines a single cell.– Includes radio antenna, radio transceiver and a link to a base

station controller (BSC).

• BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging.


GSM: Network Subsystem (NS)• NS provides link between cellular network and public switched

telecommunications networks:– Controls handoffs between cells in different BSSs.

– Authenticates users and validates accounts.

– Enables worldwide roaming of mobile users.

• Central element of NS is the mobile switching center (MSC).


GSM: Mobile Switching Center (MSC) 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



fixed network

BSC

BSC

MSC MSC

GMSC

VLR

HLR

NSSwith OSS

RSS

VLR

BTS

MS

MS



NSS

MS MS

BTS

BSC

GMSCIWF

OMC

BTS

BSC

MSC MSC

Abis

Um

EIR

HLRVLR VLR

A

BSS

PDNISDN, PSTN

RSS

radio cell

radio cell

MS

AUCOSS

signaling

O


GSM: Channels Physical: TCH (Traffic Channel)

Logical channels, they are used for controls and signaling. Some examples:Synchronization channel (SCH):

- to supply mobile phones with training sequence to achieve synchronization.

Random access channel (RACH):- to allow a mobile phone to request

for a channel.Paging channel (PCH):

- for a base station to page individual mobile phones and many others.


Required Reading

• W. Stallings, “Wireless Communications and Networks” Prentice-Hall, 2000.

>> Chapter 10

Optional Reference

• K. Pahlavan and K. Krishnamurthy “Principles of Wireless Networks”, Prentice-Hall, 2002.

Wireless & Personal Communication Systems – CSE5807 Lecture: 04 1 Wireless Personal Communications...

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