Achieving maximum capacity while maintaining an acceptable grade of service and good speech quality is the main issue for the network planning. Planning an immature network with a limited number of subscribers is not the real problem. The difficulty is to plan a network that allows future growth and expansion. Wise re-use of site location in the future network structure will save money for the operator.
Various steps involved in planning procedure
Planning means building a network able to provide service to the customers wherever they are. This work can be simplified and structured in certain steps. The steps are
-System requirements
-Define radio planning
-Initial network plan
-Surveys
-Individual site design
-Implementation
-Launch of service
-On-going testing
This process should not be considered just as it is depicted, in a single flow of events. For instance, the radio planning and surveying actions are interlinked in an ongoing iterative process that should ultimately lead to the individual site design
Planning models
Propagation in land mobile service at frequencies from 300 to 1800MHz is affected in varying degrees by topography, morphography, ground constants and atmospheric conditions. A very common way of propagation loss presentation is the usage of so called propagation curves, normally derived from some measurement formulae are :
- Okumara Y. and others, for field strength and its variability in VHF and UHF land Mobile Radio Service.
- Hata. M, Empirical formula for Propagation Loss in Land Mobile Radio Services.
- Cost –207, Digital Land Mobile Radio Communication.
- Cost-231, Urban Transmission Loss for Mobile Radio in the 900 and 1800MHz bands.
Planning tools
Tools are the software packages that help for planning the network.
Cellular planning tool is based on utilization of digitized map and measurement results. The design database includes the parameters of the base stations, antennas, propagation models and system parameters.
The basic package includes:
• Coverage area calculation
• Composite coverage area dominance
• Point to point calculation
• Interference area calculation etc.,
Planning for cellular network
For a well-planned cell network planner should meet the following requirements
- Coverage as required and predicted.
- Co channel and adjacent channel interference levels as predicted for maintaining good quality of service.
- Minimum antenna adjustments during the optimization process.
- Maximum the network capacity (Erl/km2) with limited frequency band (MHz) by reusing the same frequencies.
- Minimum changes to the BSS parameters/database during the optimization phase.
- Facilitate easy expansion of the network with minimal changes in the system.
In general the planning process starts with the inputs from the customer. The customer inputs include customer requirements business plans system characteristics and any other constraints. After the planned system is implemented the assumptions made during the planning process nee to be validated and corrected wherever necessary through an optimization process.
Total planning process can be divided in to four parts
- Capacity Planning
- Coverage Planning
- Parameter Planning
- Optimization
CAPACITY PLANNING
Network dimensioning
Network Dimensioning (ND) is usually the first task to start the planning of a given cellular network. The main result is an estimation of the equipment necessary to meet the following requirements.
- Capacity
- Coverage
- Quality
ND gives an overall picture of the network and is used as a base for all further planning activities.
Network dimensioning input
The inputs are
- Capacity related
Spectrum available.
Subscriber growth forecast
Traffic density map (Traffic per subs)
- Coverage related
Coverage regions
Area types information
- Quality related
MS classes
Blocking probability
Location probability
Redundancy
Indoor coverage.
The operator normally supplies the input data, but use of defaults is also possible. The technical parameter and characteristics of the equipment to be used are another very important part of the input. This includes the basic network modules (MSC, BSC, BTS) as well as some additional elements (antennas, cables…)
Capacity calculation
The capacity of a given network is measured in terms of the subscribers or the traffic load that it can handle. The former requires knowledge of subscriber calling habits (average traffic per subscriber) while the latter is more general. The steps for calculating the network capacity are
- Find the maximum no of carriers per cell that can be reached for the different regions based on the frequency reuse patterns and the available spectrum.
- Calculate the capacity of the given cell using blocking probability and the number of carriers.
- Finally the sum of all cell capacities gives the network capacity.
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Spectrum efficiency
= S / (n X A X B)
S - total spectrum available
n - reuse factor
A - cell area
B - channel bandwidth
Erlang B table
To calculate the capacity of the given cell using blocking probability and the number of carriers we need the well-known Erlang B table or formulas and the no of traffic channels for different number of carriers. The result we get is the traffic capacity in Erlangs, which can easily be transferred into the number of subscribers.
Erlangs = n X t / 3600
- n = no of calls attempted
- t = total duration in seconds
Frequency reuse schemes
A cellular network can easily be drawn as a combination of hexagons or circles by the help of regular grids. One of the advantages is the possibility to try different frequency reuse patterns (clusters) and calculate the expected co-channel interference. This is required to assign a frequency reuse no (cluster size) to any of the network regions area types. It is clear that the high-density regions (big cities) are the most problematic parts of the network.
Power budget calculations
To guarantee a good quality in both directions (uplink and downlink) the power of BTS and MS should be in balance at the edge of the cell. The main idea behind the power budget calculations is to receive the maximum output power level of BTS transmitter as a function of BTS and MS sensitivity levels, MS output power, antenna gain (Rx & TX), diversity reception, cable loss, combiner loss, etc
The power budget calculations provides following useful results:
BTS transmitted power:
BTS transmitted power is adjusted to provide a balanced radio link (i.e. Uplink Downlink radio link performance is the same) for given BTS and MS receiver performance, MS transmitter performance, antenna and feeder cable characteristics.
Isotropic path loss:
this is the maximum path loss between BTS and MS according to given radio system performance requirements.
Coverage threshold:
downlink signal strength at coverage area border for given location probability.
Cell range for indoor and outdoor coverage:
this is a rough indication about cell range in different area types and can be used for network dimensioning. It can also be used for comparing the effect of different equipment specification and antenna heights for the cell range
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Downlink
UPlink
Power Budget calculation tables
Transmitting
end
|
MS
|
BTS
|
Tx
RF output power
|
33
dBm
|
43
dBm
|
Combiner
loss
|
0
dB
|
4
dB
|
Feeder
loss
|
0
dB
|
3
dB
|
Tx
antenna gain
|
0
dB
|
16
dB
|
EIRP
|
33
dBm
|
52
dBm
|
Receiving
end
|
BTS
|
MS
|
Rx
sensitivity
|
-104
dBm
|
-102
dBm
|
Rx
antenna gain
|
16
dB
|
0
dB
|
Diversity
gain
|
4
dB
|
0
dB
|
Feeder
loss
|
3
dB
|
0
dB
|
Rx
power
|
-104
dBm
|
-102
dBm
|
Required
isotropic Rx power
|
-121
dBm
|
-102
dBm
|
Maximum
permissible path loss
|
154
dB
|
154
dB
|
Path loss calculation
The general Path loss equation is given by (Okumara-Hata urban propagation model)
Lp =Q1+Q2 log (f)-13.82 log (Hbts)-a (hm)+{44.9-6.55log(hbts)} log (d)+Qo
Lp =path loss in dB
f =Frequency in MHz
d =distance between BTS and the mobile (1-20 Kms)
Hbts=base station height in meters (30 to 100m)
a (hm)= Correction required if mobile height is more than 1.5 meters and is
given by:
a (hm)={1.1 log (f)-0.7}hm –{1.56 log (f)-0.8} for urban areas and
=3.2{log (11.75hm)2-4.97for Dense urban areas
hm = mobile antenna hei8ght (1-10m)
Q1 =69.55 for frequencies from 150 to 1000MHz
=46.3 for frequencies from 1500 to 2000MHz
Q2 = 26.16 for 150 to1000 MHz
= 33.9 for 1500 to 2000 MHz
Qo = 0 dB for urban
= 3 dB for Dense urban