cellular systems design fundamentals-123

Mobile Communication (ET4 153) 01/05/00

Cellular Systems 1

mc_02 #Cellular Systems 1

Mobile Communications (ET4 153)

2. Cellular System Design FundamentalsPart 2

Jos NijhofDelft University of Technology


Cellular Systems – Overview Part 1

• Frequency reuse• Cluster size (N)• Frequency reuse factor (1/N)• Co-channel interference• Co-channel reuse ratio (Q)• Signal-to-Interference ratio (S/I)• Trunking• Grade of Service (GOS)• Erlang-B formula• Cell splitting• Sectoring


Cellular Systems 2


Cellular Systems – Overview Part 2

• Exercises on cellular planning

• Channel assignment strategies

• Handoff (handover) strategies

• Power control


Example 1.1 - Problem statement

(1)

30.8

(3)

48.6

(4)

33.2

(2)

66.7

(6)

37.8

(1)

(7)32.6

(5)

38.2

Given:Total available channels: 395Each subscriber generates 0.03 erlangAverage holding time: 120 sSystem area: 1200 miles2

Grade of service: 2%

Compute:

(a) The number of channelsrequired in each cell

(b) The number of subscribersserved by the system

(c) The average number ofsubscribers per channel

(d) The number of callssupported by the system

(e) The subscriber density persquare mile

(f) The cell radius in miles


Cellular Systems 3


CellNumber

Traffic(erlang)

(An)

No. ofChannelsRequired

(a)

No. ofSubscribers

Per Cell(b)

No. ofCalls

Per Cell(d)

ChannelUtilisation

1 30.8 40 1026.7 924 0.77

2 66.7 78 2223.3 2001 0.86

3 48.6 59 1620.0 1458 0.82

4 33.2 43 1106.7 996 0.77

5 38.2 48 1273.3 1146 0.80

6 37.8 48 1260.0 1134 0.79

7 32.6 42 1086.7 978 0.78

Total 287.9 358 9596.7 8637

Example 1.1 - Solution

Fromerlang

table/chart

An

0.03(b) x 0.9

An

(a)



[ ] [ ]

radiusmiles 171.4=1200/7=area/cell :miles in radius cell : (f)

8.0=9597/1200 : mileper density subscriber : (e)

924=0.9 1026.7 : supported calls ofNumber : (1) Cell

calls/hr 0.9=36000.00025=calls/s 00025.0120

03.0

12003.0 :systemby supported calls ofnumber : d)(

26.8=3589597 :channelper ssubscriber ofnumber avg. : (c)

tablesee : (b) (a),

2

2

⇒

×

×==

×=⇒=

λ

λλhA


Cellular Systems 4


Example 1.3 - Problem statement

Compare the spectral efficiency of the digital system with respect to theanalogue system using the following data:

(a) The total number of channels in the analogue cellular system = 416(b) The number of control channels = 21(c) The number of voice channels = 395(d) The channel bandwidth = 30 kHz. The digital systems has 3 channels

per 30 kHz(e) The reuse factor N = 7(f) The total available bandwidth in each direction = 12.5 MHz(g) The total coverage area = 10,000 km2

(h) The required S/I ratio for the analogue system = 18 dB (63.1)(I) The required S/I ratio for the digital system = 14 dB (25.1)(j) The call blocking (GOS) = 2%



( )( ) ( )

( ) ( ) [ ]

Spectral efficiency - Total traffic carried by the system

Bandwidth Total coverage area

ANALOGUE SYSTEM:

No. of voice channels / cell: 395 / 7 = 56.4 56

Offered traffic load: , (from erlang table)

Carried traffic load: erlang / cell

Number of cells:

Spectral efficiency

mη =×

⇒= = ⇒ =

= − × = − × =

=

⇒ =×

×=

N B A

C B A

A R

RR

cell

56 0 02 459

1 1 0 02 459 44 98

10 000 10 000

2 6

44 9810 000

2 612 5 10 000

1384

2

2

2

. .

. . .

, ,

.

.,

.. ,

.


Cellular Systems 5



( ) ( )

( ) ( ) [ ]

( )( ) 34.5

384.1386.7

analoguedigital

/kmerlang/MHz 386.7

6307.06.25.12

4.151efficiency Spectral

6307.01.63

1.2566

lerlang/cel 5.15402.011 : load trafficCarried

table)erlang (from 5.15402.0,168 : load trafficOffered

168=356 = cellper channels voiceofNumber

3=kHz 30per channels of No.

:SYSTEM DIGITAL

area coverage TotalBandwidthsystem by the carried trafficTotal

= efficiency Spectral

222

2analoge

2digital2

1

241

==⇒

=××

=⇒

==⇒

=⇒

=

×−=×−==⇒==

×

×=

m

m

m

RR

Q

Q

I

SQ

I

SQ

ABC

ABN

ηη

η


GSM system architecture (1)

OMC

BSC

BSCBTS

BTS

BTS

MSC

EIRAUC

HLRVLR

GMSC

ISC

PLMN& International

PSTNISDNPDN

MS

MS

MS


Cellular Systems 6


GSM system architecture (2)

BTS Base Transceiving StationBSC Base Station ControllerMSC Mobile Switching CenterGMSC Gateway MSCISC International Switching CenterMS Mobile StationHLR Home Location RegisterVLR Visitor Location RegisterEIR Equipment Identity RegisterAUC Authentication CenterOMC Operation and Maintenance Center


A mobile radio environment

Multipath fading

MediumRadio path

Propagationloss

Basestation Mobile

station


Cellular Systems 7


The mobile radio channel: fading

10

0

-10

-20

-30

-40

-50

-600 1 2 3 4 5 6 7

time

Rayleigh fading(multipath reception)

Shadowing

Sig

nal L

evel

(dB

)

2

λ


GSM: Carrier frequencies, duplexing, and TDMA frames

124123

1

⋅⋅⋅

2⋅⋅⋅

960 MHz959.8 MHz

200 kHz

935.2 MHz935 MHz

124123

1

⋅⋅⋅

2⋅⋅⋅

915 MHz914.8 MHz

200 kHz

890.2 MHz890 MHz

1 2 3 4 5 6 7 8

Downlink

1 2 3 4 5 6 7 8

Data burst, 156.25 bit periods = 15/26 ms ≈ 576.9 µs

Delay45 MHzseparation

Uplink

25 MHz


Cellular Systems 8


Channel Assignment Strategies (1)

• Fixed channel allocation (FCA)– fixed assignment of frequencies to cell clusters and

cells.

– not very efficient if traffic load varies

– simple to use, but requires careful traffic analysis before installation

– used in the GSM system

• Variation: Borrowing channel allocation (BCA)– heavy loaded cell can “borrow” channels from a light

loaded neighboring cell

– problem: interference


Channel Assignment Strategies (2)

• Dynamic channel allocation (DCA)– each time a call request is made, the base station

requests a channel from the MSC

– MSC takes into account:• probability of future blocking within the cell• frequency of use of the channel• frequency reuse distance

– Advantages:• lower probability of blocking, increases trunking capacity of

the system

– Disadvantages:• increased storage and computational load


Cellular Systems 9


The handover process

Handover: Changing physical channels, radio channels of fixednetwork connections involved in a call, whilemaintaining the call

Two phases:

1. MONITORING PHASE• measurement of the quality of the current and possible candidate radio links• initiation of a handover when necessary

2. HANOVER HANDLING PHASE• determination of a new point of attachment (PoA)• setting up of new links, release of old links• initiation of a possible re-routing procedure


Two basic reasons for a handover

• MS moves out of the range of a BTS– signal level becomes too low

– error rate becomes too high

• Load balancing– traffic in one cell is too high ⇒ shift some MSs to other

cells with a lower load

The GSM standard identifies about 40 reasons for a handover!


Cellular Systems 10


Handover types

• Intra-cell handover– narrow-band interference ⇒ change carrier frequency– controlled by BSC

• Inter-cell, intra-BSC handover– typical handover scenario– BSC performs the handover, assigns new radio channel in the

new cell, releases the old one

• Inter-BSC, intra-MSC handover– handover between cells controlled by different BSCs– controlled by the MSC

• Inter-MSC handover– handover between cells belonging to different MSCs– controlled by both MSCs


Handover types

BSC BSC BSCBSC

MSC

BSC

MSC

Intra-BSC handover Inter-BSC / intra-MSChandover

Inter-MSC handover

handover

handoveroldPoA

newPoA

handover

PLMN

MSC MSC


Cellular Systems 11


Intra-MSC handover (mobile assisted)MS BTSold BSCold MSC BSCnew BTSnew

HO decision

measurementreport

measurementresult HO required

HO request

resource allocation

ch. activation

ch. activation ackHO request ackHO commandHO commandHO command

HO access

Link establishment

HO completeHO completeclear commandclear command

clear complete clear complete


Handover scenario at cell boundary

A B

Level at point A

Handoff threshold

Minimum acceptable signal level

Level at point B

Rec

eive

d si

gnal

leve

lR

ecei

ved

sign

al le

vel

BS1 BS2

Level at point B

Level at which handover is made

Improperhandover situation

Properhandover situation


Cellular Systems 12


Handover decision depending on receive level

receive levelBTSold

receive levelBTSnew

HO_MARGIN

MS MS

BTSold BTSnew

average level


Handover – 1st generation systems

• 1st generation systems (analog cellular):– signal strength measurements made by the BSs and

supervised by the MSC

– the BS constantly monitors the signal strengths of all the voice channels

– locator receiver measures signal strength of MSs in neighboring cells

– MSC decides if a handover is necessary or not.


Cellular Systems 13


Handover – 2nd generation systems

• 2nd generation systems (digital TDMA):– handover decisions are mobile assisted

– every MS measures the received power from surrounding BSs and sends reports to its own BS

– handover is initiated when the power received from a neighbor BS begins to exceed the power from the current BS (by a certain level and/or for a certain period)


Avoiding handovers: Umbrella cells

Large “umbrella” cell forhigh speed traffic

Small microcells forlow speed traffic


Cellular Systems 14


Power control

• Power levels transmitted by every MS are under constant control by the BSC.

• Assures that each MS within a BTS coverage area provides the same signal level to the BTS receiver.

• Goals:– to reduce interference

– to prolong battery life

– to combat the near-far problem in CDMA systems

cellular systems design fundamentals-123

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