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/XFHQW7HFKQRORJLHVBell Labs Innovations

EG5: Frequency PlanningLM: 4

Engineering Guideline

401 - 380 - 335Version 0.9February 1998

Lucent Technologies ProprietaryThis document contains proprietary information of Lucent Technologies and is not

to be disclosed or used except in accordance with applicable agreements.Copyright 1998 by Lucent TechnologiesUnpublished and Not for Publication

All Rights Reserved


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Engineering Guideline EG5: Frequency Planning

Lucent TechnologiesPROPRIETARY

See notice on first page

2 Version: 0.9 LM: 4


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Version: 0.9 LM: 4 3

This material is protected by the copyright and trade secret laws ofthe United States and other countries. It may not be reproduced,distributed or altered in any fashion by any entity, including otherLucent Technologies Business Units or Divisions, without theexpressed written consent of the Customer Technical Support andInformation organisation.

Notice

Every effort was made to ensure that the information in this

document was complete and accurate at the time of printing.However, information is subject to change.


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Table of Contents

1.1 About this Guideline 7

1.2 Re-using Carrier Frequencies 7

1.3 Frequency Clusters 7

1.4 Interference 7

1.4.1 Shadow Margin 8

1.5 Frequency Re-Use 9

1.5.1 Omni Cell layout 9

1.5.2 Sector Cell Layout 10

1.5.3 Cloverleaf Model 10

1.6 Network Growth 11


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1.1 About this GuidelineThis document explains how the limited number of available channel frequencies

can be allocated between cells, to provide the most effective radio coverage

throughout the network. The concept and practical application of frequency reuseis discussed, together with the problems the network frequency plan must

overcome.

1.2 Re-using Carrier FrequenciesFrequency reuse is the core concept of cellular mobile radio systems, since the

number of permitted carrier frequencies is fixed. A frequency can be reused

simultaneously in different cells, provided cells using the same frequency are

sufficiently distant to keep co-channel interference at an acceptable level most of

the time, in most of the covered area. This distance is dependent upon a number of

factors such as transmitter power levels, antenna design, terrain etc.

1.3 Frequency ClustersFor the purpose of devising a frequency plan, the cells are arranged into clusters.

The available frequency spectrum is shared by the cells in this cluster , which is

repeated over the entire network. In this way an optimum frequency reuse pattern

is achieved,

Theoretically, a large cluster (C) is desired. In practice, the total number of

allocated frequencies is fixed. When C is too large the number of frequencies

assigned to each of the cluster cells becomes too small. Trunking inefficiency and

limited traffic capacity will be the result.

The challenge is to find the smallest value of C which can still meet the system

performance requirements. This involves :

Estimation of Co-channel interference

Calculation of the minimum frequency reuse distance D to meet the Co-

channel interference criterion

Practical values of C are in the range from 3 to 21.

1.4 InterferenceGSM Rec 05.05 specifies the necessary interference protection ratios for co-

channel and adjacent channels. The values are given in Table 1.

Table 1: Interference protection ratios specified by GSM Rec 05.05

Relation Frequency

Spacing (kHz)

Minimum

C/I (dB)

Co-channel 0 9

1st

adjacent channel 200 -9

2nd


3rd



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The minimum frequency spacings according to GSM Rec 05.05 are: shown in

Table 2.

Table 2: Minimum frequency spacings specified by GSM Rec 05.05 .

Where ... Minimum Spacing

kHz

within a cell 600between two co-cell sites 400

between two neighbouring cells 200

The specified protection ratios already take into account multipath propagation

effects (Raleigh fading). That is, given these values the receivers have to meet a

certain performance in terms of Bit Error Rate and Frame Erasure Rate. Other

effects like signal degradation due to shadowing are not included in these values.

1.4.1 Shadow Margin

Table 3 shows the additional margins which need to be considered on top of the

above specified interference protection ratios. This is required in order to achievesufficient performance in a predefined percentage of locations on the cell fringe or

the entire cell area.

Table 3: Additional margins for predefined percentage of locations.

Edge freedom of

interference

Area freedom of

interference

Margin (=7dB,

n=3.5)

0.70 0.86 5 dB

0.75 0.90 7 dB

0.80 0.91 8 dB

0.85 0.94 10 dB

0.90 0.96 13 dB0.95 0.99 16 dB

These values are valid under the assumption that the shadowing effects of serving

and interferer signal can be statistically described by a lognormal distribution with

a standard deviation of 7 dB and a decrease of the field strength level by 35

dB/decade (n=3.5) versus the distance.

In order to achieve 94 percent freedom of interference in the entire cell area we

have to add at least 10 dB margin to the reference interference protection ratio. For

example from the GSM 05.05 recommendation, co-channel interference requires a

minimum C/I of 9dB. To take into account shadow margin the frequency reuse in

the network has to be designed in such a way that a C/I value of at least 19 dB co-channel is guaranteed.

It is possible to improve the C/I ratio by good frequency management e.g. grouping

the channels into sub-sets. Alternatively if the quality of the received signal is poor

an intracell handover to another frequency or timeslot should occur. It is also

possible to reduce the signal strength of the interfering signal. This can be done by

lowering the antenna height, changing the radiation pattern of the antenna used or

applying electrical downtilt.

For other methods of improving the carrier to interference ratio see the

Engineering Guideline on Capacity Enhancement Techniques.


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1.5 Frequency Re-UseThe C/I performance in radio networks with different cell layouts is also different.

This leads to different frequency reuse distances in networks of different cell

configurations

1.5.1 Omni Cell layout

In an Omni cell network all base stations are equipped with omnidirectional

antennas i.e. the signal is radiated uniformly in any direction. This leads to the

well known hexagonal cell structure.

Table 4 shows the C/I which can be achieved for various frequency reuse distances

in a homogeneous omni cell network. In the tables the reuse distance D is

normalised to the cell radius R. The calculated C/I values are given for various

antenna heights assuming a mobile at the cell boundary and considering the two

strongest interferers.

Associated to the normalised reuse distance D/R in a regular cellular network isthe so-called cluster (C). If the frequency allocation applied to such a cluster is

periodically repeated according to certain rules a regular frequency reuse pattern

can be achieved throughout the entire network. Only certain cluster sizes are

possible, due principally to the geometry of a hexagon. These can be derived using

the following formula

C = i*i + i*j + j*j i,j = 0,1,2,3,4...

Using these clusters the overall interference situation is exactly the same for each

cell in the entire network. The relation between the cluster size C and the

normalised reuse distance D/R is given by the following formula:

D/R = SQRT(3*C)

Table 4: C/I values for various antenna heights, mobile at cell fringe.

Cluster

C

D/R C/I (dB)

Hb = 35m

C/I (dB)

Hb = 45 m

C/I (dB)

Hb = 100 m

C/I (dB)

Hb = 200m

1 1.73 -3.0 -3.0 -3.0 -3.0

3 3.00 9.1 8.8 8.1 7.4

4 3.46 11.7 11.4 10.4 9.6

7 4.58 17.1 16.7 15.4 14.2

9 5.20 19.2 18.8 17.3 16.1

12 6.00 22.0 21.5 19.9 18.5

13 6.24 22.5 22.0 20.3 18.9

16 6.93 24.3 23.7 21.9 20.419 7.55 25.9 25.3 23.4 21.8

21 7.94 26.6 26.0 24.1 22.4

25 8.66 28.0 27.4 25.4 23.6

27 9.00 28.9 28.2 26.1 24.3

28 9.17 29.1 28.4 26.3 24.5

31 9.64 29.8 29.2 27.0 25.2

At a base station height of 35m and minimum required C/I of 19dB the size of the

cluster required is 9 with a D/R of 5.2.

For example if the cell radius is 6km the frequencies can be re-used at a transmitter

approximately 31 km away .


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1.5.2 Sector Cell Layout

In a Sector cell layout, typically 3 cells (120 degree sectors) are installed at each

site i.e. each sector illuminates a 120 degree area. The main advantages of a sector

cell network are

Increased coverage area per site as higher gain antennas can be

used

Better C/I performance compared to Omni cell Layout if

frequencies are reused in such a way that all sectors with the

same frequency are pointing in the same direction.

Table 5 shows the C/I performance in 3-sector cell network assuming that all

sectors using the same frequency are pointing in the same direction. The C/I values

have been calculated considering the two worst interferers.

Table 5: C/I performance in 3-sector cell network (refer to text).

Cluster C D/R C/I (dB)

Hb

= 35m

C/I (dB)

Hb

= 45 m

C/I (dB)

Hb

= 100 m

C/I (dB)

Hb

= 200m

1 1.73 -0.4 -0.4 -0.5 -0.5

3 3.00 13.4 13.1 12.1 11.2

4 3.46 17.6 17.1 15.8 14.7

7 4.58 19.3 18.8 17.4 16.2

9 5.20 23.1 22.6 20.9 19.4

12 6.00 24.0 23.4 21.7 20.2

13 6.24 25.2 24.6 22.8 21.2

16 6.93 27.1 26.5 24.6 22.8

19 7.55 27.3 26.7 24.7 23.0

21 7.94 28.8 28.1 26.1 24.3

25 8.66 30.3 29.6 27.5 25.627 9.00 30.2 29.5 27.3 25.4

28 9.17 30.7 30.0 27.8 25.9

31 9.64 31.7 31.0 28.7 26.8

1.5.3 Cloverleaf Model

The Cloverleaf model is generally used for high capacity macrocellular networks

which feature uniform traffic distribution.

The site geometry of the Cloverleaf model can be characterised as follows:

3-cells (sectors) per site.

The pointing azimuth for the three collocated cells are 120 degrees

separated and are oriented with the neighbouring sites to produce a

cloverleaf pattern.

Each cell has one directional transmit antenna and two directional

diversity receive antennae with the same azimuth.

High-gain antennas with a horizontal beamwidth of 65 are used.

The 4/12 Cloverleaf scheme is shown in Figure 1.


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Network growth can also be required for other reasons, for example if the existing

outdoor coverage needs to be upgraded to indoor coverage. When new BTSs or

TRXs are required then the integration of these into the existing system needs to

be carefully planned.

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