CELL PLANNING AND FREQUENCY REUSE

Overview

Conventional Cell and Frequency Planning

Radio Network Features and their impact

Real” Cell and Frequency Planning

Site selection

Parameters to cell planning

Spectrum availableTraffic

Cost

GoS

QoS

Quality

Coverage

Speech Quality

C/I

Algorithms

Lee’s model and other empirical models

Ploss = PR1 + g10log(d / d1) + n10 log( f / f0) - a0

PR1 is the reference loss at d1(normally 1 mile) (e.g. -84dBm in a city like Tokyo and -

49dBm for open areas) g depends on the type of terrain (value between 2 and 4) n is between 2 and 3

Co-channel interferenceInfobits

Codingbits

CodeRate

Max data rate(kbs) /TS

Required C/I (dB)

modulation

GSM 260 196 0.5 13.3 9 GMSK

CS-1 181 275 0.45 9.05 9 GMSK

CS-2 268 188 0.65 13.4 13 GMSK

CS-3 312 144 0.75 15.6 15 GMSK

CS-4 428 28 21.4 23 GMSK

MCS-1 176 0.53 8.4 9 GMSK

MCS-2 224 0.69 11.2 13 GMSK

MCS-3

296 0.89 14.8 15 GMSK

MCS-4 352 1 16.8 23 GMSK

MCS-5 448 0.38 22.4 14.5 8PSK

MCS-6 592 0.5 29.6 17 8PSK

MCS-7 896 0.78 44.8 23.5 8PSK

MCS-8 1088 0.92 54.4 29 8PSK

MCS-9 1184 1 59.2 32 8PSK

Adjacent Channel interference

for co-channel interference C/Ic=9 Db

for adjacent (200 kHz) interference C/Ia1=-9 dB

for adjacent (400 kHz) interference C/Ia2=-41 dB

for adjacent (600 kHz) interference C/Ia3=-49 dB

GSM Cell

GSM cell are basically geographical areaCovered by radio frequencies of BTS antennasThe hypothetical shape of a GSM cell is a Hexagon.

Two types of GSM cell

Omni directional

Sector cell

The Hexagon

dc

a e

f

b

R

d

Area A=3(3)½R²/2

Distance between centersof two adjacent cells:d = (3)½R

Omni directional Cell

An omni-directional cell (or omnicell) is served by a BTS with an antenna which transmits equally in all directions (360 degrees).

BTS

Sector cell

A sector cell is the area of coverage from an antenna, which transmits, in a given direction only. For example, this may be equal to 120 degrees or 180 degrees of an equivalent omni- directional cell. One BTS can serve one of these sector cells with a collection of BTS’s at a site serving more than one, leading to terms such as two-sectored sites and more commonly, three-sectored sites.Typically, omni-directional cells are used to gain coverage, whereas sector cells are used to gain capacity.

Hexagonal or sectored cell

BTS

GSM Frequency reuse

• The frequency reuse concept is to use same frequency channel with another cell within the same GSM cell cluster.

• Since cells are more or less close to each other there is not perfect solution.

• Cells will always interfere with each other even if they are not immediate neighbors.

• How many cells must we consider when we do our planning? how many closest neighbors do we have?

• The frequency re-use patterns recommended for GSM are the 4/12 and the 3/9 pattern. 

The only (almost) noise we have to consider comes from interfering base stations.If we want to reduce interference from neighbors we need to increase the D/R ratio.If D = R sqr(3k) then sqr(3k) should be big, that is increase k!If we need a C/I ratio of 18dB then we need k > 6.

In GSM networks a frequency reuse pattern with k = 3, 7 or 12.In the 900-band, which is 2x25MHz wide, we can have 124 carriers. If these are divided into groups of 12 frequencies we can have 10 groups.One cell can thus be covered by 10 carriers.Each carrier can have 8 connections thus amaximum of 80 calls in an area covered by a cell.

Re-use distance

reuse distance D

k = 6number of cells inpattern

U

V

j

i30º

Re-use distance

D = (i2 + ij + j2)½2Rcos 30°D = (i2 + ij + j2)½ (3) ½ RNumber of cells in there-use patternN = i2 + ij + j2i in (1,2,3,4 …..)j in (0,1,2,3,4 …..)D/R = (3N)½

Hierarchical Cells

Umbrella Cell:

Macro Cell: Antenna above average rooftop height

Micro Cell: Antenna below average rooftop height

Pico Cell: Indoors

DTX- DiscontinuousTransmission

Average Voice activity is around 50%

DTX is a feature that allows to be transmitted only

when there is something to be transmitted

Uses VAD (Voice Activity Detector)

It safes on battery power

Improves the overall network quality by reducing

unnecessary interference

Dynamic Power Control

This enable the BTS and the Mobile to transmit

only the power necessary for effective

communications

Power Control Commands are via the SACCH

This improves the battery live of Mobile Phones

And it improve the overall network quality by

reducing unnecessary interference

Base Band Frequency Hopping Number of frequencies equal to number of transceiivers

ControllerCall 1

ControllerCall 2

ControllerCall 3

ControllerCall 4

Tx and Rx on f0

Tx and Rx on f1

Tx and Rx on f2

Tx and Rx on f3

COMBINER

f2 f3 f0 f1

f3 f0 f1 f2

f1 f2 f3 f0

f0 f1 f2 f3

Synthesised Hopping Number of frequencies more or equal to number of transceiivers

ControllerCall 1

ControllerCall 2

ControllerCall 3

ControllerCall 4

Tx and Rx hopping

Tx and Rx hopping

Tx and Rx hopping

Tx and Rx hopping

f2 f3 f0 f1

f3 f0 f1 f2

f1 f2 f3 f0

f0 f1 f2 f3

Frequency Diversity

Raleigh fading is frequency dependent

Diversity: combining two or more uncorrelated versions

of the same signal

For “conventional” frequency diversity the info is sent on

two different frequencies at the same time.

To be uncorrelated the two frequencies should be more

than 1/(multi-path spread), where the multi-path spread is

dependent on the environment.

For urban areas the frequencies should be more than

600kHz apart

Why does hopping work?

Review interleaving

If one timeslot gets completely lost during

transmission 1/8 of two speech frames are lost.

At the receiver the speech frames are de-interleaved

The channel coding can recover from the 12.5% BER.

Interleaving and Channel Coding is part and parcel of

the GSM standard - it works even without hopping.

Interference Diversity

Extent of Interference diversity depends on: Interference load (DTX and Power Control) Frequency reuse: low re-use -> low gain; dependent on area type. Number of Frequencies (less -> less gain) Cyclic or Random

Interference diversity gain reached with 25% load, 12 frequencies in Urban area with random hopping is 2.5dB - mostly it is less.

Planning for FH network

Use separate frequency blocks for TCH and BCCH

BCCH frequency channel must be Always On No

hopping over BCCH.

Plan TCH layer:

MAL : Mobile radio frequency channel Allocation List

HSN: Hopping sequence number

MAIO: Mobile Allocation Index Offset

MAI: Mobile Allocation Index

Selecting a BCCH block

Why a BCCH block? Identifying the source of interference Re-evaluation of the neighbor list For collecting data for a measurement based plan Optimum size? Where a change in a BCCH carrier will on

average make the same difference as a change in a TCH carrier in the optimized plan

Selecting a BCCH block

Block Size BCCH =

Total Number of Carriers Available/(Average Traffic on TCH layer per cell/8)*Scaling (DTX.PC) + 1

TCH layer

MAIO MA

MAI 0 2 1A 2A 3A 1B 2B 3B 1C 2C 3C

1 1 3 1 2 3 4 5 6 7 8 9

2 2 4 10 11 12 13 14 15 16 17 18

3 3 1 19 20 21 22 23 24 25 26 27

4 4 2 28 29 30 31 32 33 34 35 36

HSN = X 4 1 2 3 2 4 3 1TRX1 ON 1A has MAIO = 0 28 1 10 19 10 28 19 1TRX1 ON 1A has MAIO = 2 10 19 28 1 28 10 1 19

THANK YOU

SHASHANK ASTHANA