Cellular Interference

8/10/2019 Cellular Interference

1/20

ECE 4730: Lecture #5 1

Cellular Interference

Two major types of system-generated interference :

1) Co-Channel Interference (CCI)

2) Adjacent Channel Interference (ACI)

Co-Channel Interference caused by frequency reuse

Many cells in given coverage area use same set ofchannel frequencies to increase system capacity (C)

Co-channel cells cells that share same set offrequencies

VC & CC traffic in co-channel cells are interfering sourcesto neighboring (not adjacent!) co-channel cells


2/20


Co-Channel Interference

Possible solution :

A) Increase base station Tx power to improve radio signal

reception?

NO!!Why ??

increases interference from co-

channel cells by the same amount!no net improvement


3/20



Possible solution :

B) Separate co-channel cells by some minimum distance to

provide sufficient isolation from propagation of radio

signals? YES!!Why ??

if all cell sizes same then co-

channel interference is independentofTx power


4/20



CCI depends on : R: cell radius

D: distance from BS to centerof nearest co-channel

cellD/Rthen spatial separation relative to cell

coverage area

Improved isolation from co-channel RF energy

Q= D/R: co-channel reuse ratio For hexagonal cells Q= D/R= N3


5/20


Fundamental Tradeoff

Tradeoff in cellular system design:

Small Q small cluster size more frequency reuse larger system capacity great!!

But also small co-channel cell separation increasedCCI reduced voice quality not so great!

Tradeoff: Capacity vs. Voice Quality


6/20



Signal to Interference ratio S/I(not S/Nor SNR!!)

Equation (1) where

AverageRx power at distance d

Rx signal decays as power law relationship with distance between Txand Rx

oi

i

iI

S

I

S

1

n

rd

dPP

0

0

S: Rx power from desired signal

Ii: Interference power from ith

co-channel cell

io: # of co-channel interfering cells

P0: Rx power at close-in reference point

d0: close-in reference distancen: path loss exponent


7/20ECE 4730: Lecture #5 7


If base stations have equal Tx powerand propagationconstant (n) is the samethroughout coverage area (not

always true!) then

Equation (2) where

Di: Distance from ithinterferer to mobile Rx power

@ mobile (Di)n

n: Path loss exponent orpropagation constant

Free space or LOS (no obstruction) n= 2 (1 / 4 R2)

Urban cellular n= 3 to 5

oi

i

ni

n

D

R

I

S

1

)(


8/20ECE 4730: Lecture #5 8


If all interfering base stations are equidistant (D)from mobile unit and considering only first layer (or

tier) of co-channel cells then

Equation (3)

W

W

i

N

i

Q

iR

D

I

S

o

n

o

n

o

n 2/)3(1


9/20ECE 4730: Lecture #5 9


What determines acceptable S/I? Voice quality Subjectivetesting

1G AMPS S/I18 dB (assumes n= 4) Solving Eq. (3) forNusing S/I=18 dB = 101.8= 63.1, n= 4, and

io= 6 interfering co-channel cells

N= 1/3 [ (S/I) io]2/n= 1/3 [ (63.1)6 ] 2/n= 6.5 7

N= 7 was verycommon choice for 1G AMPS

2G GSM S/I10 dB 2G IS-95 (CDMA) S/I7 dB (7 1 dB)


10/20ECE 4730: Lecture #5 10


Many assumptions involved in Eq. (3) Same Tx power for all cell BSs

Hexagonal geometry Propagation constant, n, same throughout area

DiD(not true forN= 4 non-hexagonal)

Optimistic result in many cases Computer propagation tools used to calculate S/Iwhen

assumptions are not valid S/Iis usually the worstwhen mobile is at cell edge Fig. 3.5, pg. 71 N= 7 and S/I17 dB


11/20ECE 4730: Lecture #5 11


Worst-case S/Ion forward

channel

mobile is at cell edge

low signal power

high interference power

1stTier of Co-

Channel Cells


12/20ECE 4730: Lecture #5 12


Equations (1)-(3) are (S/I) for forwardlink only Co-channel base Tx interfering with desired base station

transmission to mobileunit Interference occurs @ mobile unit

What about reverselink co-channel interference? Less important b/c signals from mobile antennas (near

ground!) dont propagate as well as those from tallbasestation antennas

Obstructions near ground level significantly attenuatemobile energy in direction of base station Rx

Also weaker b/c mobile Tx power is variable powercontrol (needed to mitigate ACI!)


13/20ECE 4730: Lecture #5 13

Adjacent Channel Interference

Adjacent Channel Interference (ACI) Caused by imperfect Rx filters that allow energy from

adjacent channels to leak into passband of desired signal

f1 f2

Signal BWDesired Ideal

Filter Response

ActualFilterResponse

Signal

Energy Leaks

into Rx from

Adjacent

Channel


14/20ECE 4730: Lecture #5 14


ACI can affect both forward & reverse channel links Reverse Link mobile-to-base

Interference @ base station Rx from nearbymobile Tx when desiredmobile Tx is faraway from base station

Forward Link base-to-mobile Interference @ desired mobile Rx from nearbybase Tx whensecondary mobile Rx is faraway from base station

Near/Far Effect

Interfering source (Tx) is nearsome Rx when other source is faraway

ACI is primarilyfrom mobiles in same cell

Some cell-to-cell ACI does occur as well secondary source


15/20



Reverse Link ACI

X

BS

X

X

MS 1

Undesired

Nearby

StrongSignal

MS 2

DesiredSignal

Far Away

& Weak

Interference @ BS Rx


16/20



Forward Link ACI

X

BS

X

X

MS 2: Secondary

Mobile, Far Away

with from BS

MS 1: DesiredMobile,

Nearby with Strong Tx

Signal and ACI from BS

Interference @ MS Rx


17/20


Minimizing ACI

Dont allocate channels within a given cell from contiguousbandof frequencies

Maximize channel separation

Typical separation of 6 passband bandwidths

Many channel allocation schemes separate byNbandwidths Some schemes seek to minimize ACI from neighboring cells

Use high Q filters (sharp rolloff) in base stations

Better filters possible since not constrained by physical size as muchas in mobile Rx

Makes reverse-link ACI lessof a concern than forward-link ACI Also true b/c of power control (discussed next)


18/20


Minimizing ACI

1G AMPS Channel Allocation

Example 3.3 Page 75

395 VC and 21 CC per service provider (A & B)

21 VC sector groups with 19 channels/group

21 channel separation for each sector group

ForN= 7 3 VC groups/cell (antenna sectorization!)

57 channels/cell 7 channel separation for each cell group


19/20


Minimizing ACI

Example 3.3 Page 75

ControlChannels

A, B, C =

Sectors

1, 2 ..7 =

Cell # for

N =7 Reuse


20/20


Minimizing ACI

Mobile Unit Power Control

Effective technique to minimize ACI Base station & MSC constantly monitor mobile RSS

Mobile Tx power varied (controlled) so that only thesmallestTx power is used to produced quality reverse

link signal

Dramatically improves adjacent channel S/Iratio

Most beneficial for ACI on reverselink

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Cellular Interference

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