The hexagon cell shape
MY REVEAL - CELLULAR
If we have two BTSs with omniantennas and we require that the
border between the coverage area of each BTS is the set of points
where the signal strength from both BTSs is the same, we obtain a
straight line. If we repeat the procedure placing 5 more BTSs around
the original one, the obtained coverage area, i.e. the cell, has a
hexagonal shape.
CELL PLANNING
Introduction
Every cellular network needs cell planning, in order to ensure
coverage and avoid interference. The cell planning process consists
of many different tasks, all together making it possible to achieve a
well working network.
Definitions
The hexagons have become a symbol for cells in a radio network.
Real–world planning must, however, consider the fact that radio
Some definitions are important to understand, before going deeper
propagation is very much dependent on terrain and other factors, and
into the cell planning process:
that hexagons are extremely simplified models of radio coverage
• Radio coverage
Received signal strength in the MS (from the BTS) above a chosen
value.
• Cell
The area that is covered from a BTS.
• Omni cell
A cell with an omnidirectional BTS antenna system.
patterns. Still, the first geometrical plan based on hexagons (the
nominal cell plan) gives a good view when planning a system.
CELL PLANNING PROCESS
Cell planning can briefly be described as all the activities involved in
determining which sites should be used for the radio equipment,
which equipment should be used, and how the equipment should be
configured. To ensure coverage and to avoid interference, each
cellular network needs planning. The major activities involved in the
cell planning process are represented in Figure 1-1.
• Sector cell
A cell with a (uni-) directional BTS antenna system.
• Site
The geographical location where the RBS equipment is stored, and
the BTS antennas are mounted.
• 3–sector–site
A site with equipment for three sector cells.
So what is the maximum size of a cell? Well, there are limiting
factors for how big an area a base station can cover. A crucial factor
is the ability for the sent burst from the MS to arrive in the intended
time slot at the base station. This depends on the relation between
how far away the MS is, and the timing advance parameter. With 8
time slots per carrier a maximum distance between the base station
and the cell border is 35 km. 4 time slots per carrier extends the
allowed distance to 72 km.
[1] TRAFFIC AND COVERAGE ANALYSIS
(SYSTEM REQUIREMENTS)
The cell planning process is started by a traffic and coverage
analysis.
The
analysis
should
produce
information
about the
geographical area and the expected capacity need. The different
types of data collected are:
Cost
Capacity
Coverage
Grade of Service (GoS)
Available frequencies
Speech Quality Index
System growth capability
The traffic demand (that is, how many subscribers access the system
and how much traffic is generated) provides the basis of cellular
network engineering. The geographical distribution of the traffic
Traffic per subscriber is calculated with the Erlang formula, as
demand can be calculated using demographic data, such as:
below:
Population distribution
Car usage distribution
Income level distribution
Land usage data
Telephone usage statistics
Other factors, such as subscription charges, call charges, and
costs of mobile stations
Traffic calculations
The input for the traffic calculations is mentioned above. The output
should be information about how many sites and cells are needed. In
order to be able to decide this, the available number of frequencies
per cell, as well as the Grade Of Service (GOS), have to be known.
Available number of frequencies per cell can only be decided
when knowing which cell pattern should be used; (see Figure 104
Example of traffic calculation
Input data:
Traffic per subscriber: 25 mE; Number of subscribers: 10 000;
Number of available frequencies: 24; Cell pattern: 4/12 (12 frequency
groups); GOS: 2%. How many 3–sector-sites are needed?
and Figure 105). Then, the total number of available frequencies are
• frequencies per cell = 24/12 = 2 frequencies
evenly divided into frequency groups.
• traffic channels per cell = 2 x 8 - 2 (control channels) = 14 TCH
Which cell pattern to choose depends on the type of system, as it is
• traffic per cell = 14 TCH, 2% GOS Æ 8.2 E/cell (use the Erlang
based upon frequency re–use distance. This will be explained below
table)
(see Frequency re–use).
GOS is defined as allowed percentage of unsuccessful call set–ups
• subscribers per cell = 8.2 E / 0.025 E = 328 subscribers per cell
due to congestion. Normally, a value between 2% and 5% is
• needed number of cells = 10 000 / 328 = 30 cells
applicable in mobile telephone systems.
• needed number of 3–sector–sites = 30 / 3 = 10 ! The Answer
The Erlang table is used when wanting to find out the third factor,
Frequency re–use
when knowing two of the three factors: number of traffic channels,
traffic (in Erlang) and GOS.
A fundamental principle in the design of cellular systems is the
frequency re–use patterns. Frequency re–use is defined as the use of
radio
channels
on
the
same
carrier
frequency, covering
geographically different areas. These areas must be separated from
one another by a sufficient distance, in order to avoid co–channel
interference. Based on the traffic calculations, the cell pattern and
frequency plan are worked out. Not only for the initial network but
with the possibility to adapt smoothly to the demands of traffic
growth.
Interference
• C/I
Therefore, a reduction in the number of frequency groups would allow
each site to carry more traffic, reducing the total number of sites
needed for a given traffic load. However, decreasing the number of
The carrier–to–interference ratio (C/I) is defined as the ratio of the
frequency groups and reducing the frequency re–use distance will
level of the received desired signal to the level of the received
result in a lower average C/I distribution in the system.
undesired signal.
There are three types of frequency re–use patterns: 7/21, 4/12 and
3/9. Only 4/12 and 3/9 are interesting for CME 20. In all three cases
the site geometry has the following features:
• Three cells (sectors) at each site. The antenna pointing azimuths of
the cells are separated by 120 degrees and the cells are arranged
with antennas pointing at one of the nearest site locations thus
forming cells in a cloverleaf fashion. Each cell uses one 60–degree
transmitting antenna and two 60–degree diversity receiving antennas
with the same pointing azimuths.
• Each cell approximates the shape of a hexagon.
This C/I ratio is dependent on the instantaneous position of the
mobile and is due to irregular terrain and various shapes, types and
We assume that the traffic is homogeneously distributed within the
cells.
numbers of local scatterers. Other factors such as antenna type,
The cell size is normally given in terms of the distance between two
directionality and height, site elevations and positions, and the
neighboring sites. The cell radius R (= the side of the hexagon) is
number of local sources of interference also affect the distribution of
always one–third of the site–to–site distance when 3–sector sites are
the C/I ratio in a system.
used.
GSM states C/I > 9dB, with frequency hopping implemented, and
A group of neighboring cells using all the channels in the system, but
recommends C/I > 12dB when frequency hopping is not employed.
not re–using them, according to the patterns described below is
• C/A
The carrier–to–adjacent ratio (C/A) is defined as the relation in dB in
signal strength between the serving and an adjacent frequency. In
called a cluster.
The 4/12 cell pattern uses 12 frequency groups in a 4 site re–use
pattern.
GSM, a multiple of 200 kHz away. GSM specifies C/A > -9dB.
Cell patterns
The distribution of the C/I ratio desired in a system determines the
number of frequency groups, F, which may be used. If the total
allocation of N channels is partitioned into F groups, then each group
will contain N/F channels. Since the total number of channels (N) is
fixed, a smaller number of frequency groups (F) would result in more
channels per set and per cell.
The 3/9 cell pattern uses 9 frequency groups in a 3 site re–use
pattern.
pattern.
Example of how to divide the available frequencies into
frequency groups:
24 frequencies in a 3/9 cell pattern
[3] SURVEYS (AND RADIO MEASUREMENTS)
The nominal cell plan has been produced and the coverage and
interference predictions have been roughly verified. Now, it is time to
visit the sites where the radio equipment is to be placed and perform
radio measurements. The former is important because it is necessary
to assess the real environment to determine whether it is a suitable
site location for a cellular network. The latter is even more important
because better predictions can be obtained using field measurements
of the signal strengths in the actual terrain where the mobile station
It should be noted, that when using 3/9, there will be adjacent
channels in neighboring cells, which gives lower C/A values.
To see this, the example can be compared with Figure 3/9 Cell
Pattern above. Cells with frequency groups A1 and C3 are neighbors,
as well as A2–C1, and A3–C2.
[2] NOMINAL CELL PLAN
Upon compilation of the data received from the traffic and coverage
analysis, a nominal cell plan is produced. The nominal cell plan is a
graphical representation of the network and it simply looks like a cell
pattern on a map. However, there is a lot of work behind it (as
is to be located.
Site surveys
Site surveys are performed for all proposed site locations. Many
issues have to be checked and verified, such as:
• Exact location
• Space for equipment, including antennas
• Cable runs
• Power facilities
previously described).
• Contract with owner
Nominal cell plans are the first cell plans produced and these form
Also, the radio environment has to be checked, so that there is no
the basis of further planning. Quite often, a nominal cell plan,
other radio equipment on the site that will cause intermodulation
together with one or two examples of coverage predictions, is
problems, or too high buildings surrounding the possible site.
included in tenders.
Radio measurements
Coverage and interference predictions are usually initiated at this
Radio measurements are performed to be able to adjust the
stage. Such planning needs computer-aided analysis tools for radio
parameters used in the planning tool to reality, to the specific climate
propagation studies.
and terrain in the area of interest. Parameters used in Sweden,
would be different to the ones to be used in a tropical country, for
example.
A test transmitter is mounted, and then the signal strength is
nodes are tested for full functionality on their own ! this is called
installation test.
measured while driving around in the area. Back in the office, the
Secondly, the interworking function is tested ! this is called
results from the measurements can be compared with the values the
integration test. The two tests together is called the network element
planning tool produces when simulating the same type of transmitter,
test, which is further explained below.
and the parameters for the planning are adjusted to match reality.
Network Element Tests
[4] SYSTEM DESIGN
The picture below shows the main process steps of the Network
After optimization and when the predictions generated by the
element test of the BSC and RBS.
planning tool can be considered reliable, a dimensioning of the RBS
equipment, BSC, and MSC is performed. The final cell plan is
produced. As the name implies, this plan is later used at system
installation. In addition, a document called Cell Design Data (CDD)
containing all cell parameters for each cell is completed.
[5] IMPLEMENTATION
System installation, commissioning, and testing are performed
following the final cell planning and system design.
Installation Engineering
[6] SYSTEM TUNING
Once the system has been installed, it is continually evaluated to
determine how well it meets the demands. This is called system
tuning and it involves:
· A check that the final cell plan has been implemented
successfully
· An evaluation of customer complaints
Figure 1-2 illustrates the main steps of the implementation of a new
radio site.
· A check that the network performance is acceptable
Changing parameters and undertaking other measures (if
needed)
The output from the system design step in the cell planning process
results in a hardware order (for example, BSC, RBS) to the factory.
Installation engineering personnel perform site investigations, which
The system needs constant re-tuning, due to the fact that the traffic
and number of subscribers continuously increase.
means taking a closer look at the actual location where the site is to
Eventually, the system reaches a point where it must be expanded
be built. This results in an installation documentation, which is put
so that it can manage the increasing load and new traffic. At this
into a binder for each site. The installation documentation contains all
point, a coverage analysis is performed and the cell planning process
information needed to build the site, for example, floor plan, cable
cycle starts all over again.
drawings, antenna arrangement drawing, grounding plan, site material
list, etc. The material needed to build the site is then ordered
according to the installation documents.
When all equipment has arrived the installation can begin. After
installing the equipment, it is time to check its functionality. Firstly, the
nodes are tested for full functionality on their own ! this is called