Cell Planning Overview

Cells

A cell may be defined as an area of radio coverage from one BTS antenna system.  Its is smallest builtding block in a mobile network and is the reason why mobile network are often reffered to as cellular networks.  Typically, cell are represented graphically by hexagons.

There are two main types of cell:

a.     Omni directional cell

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

b.   Sector cell

A sector cell is the area coverage from an antenna, which transmits, in a given direction only.  For example, this may be equal to 120° or 180° 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 term such as two-sectored sites and more commonly, three-sectored sites.

                       

Typically, omi-directional cell are used to gain coverage, whereas sector cells are used to gain capacity.  The border between the coverage area of two cells is the set of points at which the signal strength from both antennas is the same.  In reality, the environtment will determine this line, but for simplicity, it is represented as a straight line.

If six BTS’s are placed around an original BTS, the coverage area-that is, the cell-takes on hexagonal shape.

Step 1: Traffic and Coverage Analysis

Cell Planning begins with traffic and coverage analysis.  The analysis should produce information about the geographical area and the expected capacity (traffic load).  The types of data collected are:

�     Cost

�     Capacity

�     Coverage

�     Garde of Service (GOS)

�     Available Frequencies

�     Speech Quality

�     System Growth Capability

The basis for a cell planning is he traffic demand, i.e. how many subscribers use the network and how much traffic they generate.  The Erlang (E) is unit of measurement of traffic intensity.  It can be calculate with the following formula :

Where : A          = offered traffic from one or more users in system n          = number of call per hour T          = average call time in seconds

The geographical distribution of traffic demand can be calculated by the use ofdemographic data such as :

�     Population distribution

�     Car usage distribution

�     Income level distribution

�     Land usage data

�     Telephone usage statistics

�     Other factors, like subscription/call charge and price of MSs

Calculation of Required Number of BTS

To determine the number and layout BTS’s the number of subscribers and the Grade of Service (GOS) have to be known.  The GOS is the presentage of allowed congested calls and defined the quality of the service.

Step 2: Nominal Cell Plan

A nominal cell plan can be produced from the data compiled from traffic and coverge analysis.  The nominal sell plan is a graphical representation of the network and looks like a cell pattern on a map.  Nominal cell plans are the first cell plans and form the basis for further planning.

Successive planning must be taken into account the radio propagation properties of the actual environtment.  Such planning needs measurement technique and computer-aided analysis tools for radio propagation studies.  Ericsson planning tools, Test Mobile System (TEMS) Cell Planner, includes a prediction package which provides:

�     Coverage prediction

�     Composite coverage synthesis

�     Co-channel interference predictions

TEMS call planner is software package designed to simplify the process of planning and optimizing a cellular network.  It is based on ASSET by Airtouch.

With TEMS CellPlanner, traffic can be spread around on map to determine capacity planning.  The traffic can be displayed using different color for different amount for Erlang/km² or the user can highlight the cell that do not meet the specified GOS.

Its possible to import data from the test MS and display it on map.  TEMS CellPlanner can olso import radio survey files, which can be used to tune the prediction model for the area where the network is to be planned.  Data can also be imported from and exported to OSS.

For example, if there are doubts about the risk of time dispersion at a particular site the following steps could be taken :

�     The site location can be changed

�     The site can be measured with recpect to time dispersion

�     The site could be analyzed with the carrier-to-reflection ratio (C/R) predictional tool.

Radio Propagation

In reality, hexagon are extremely simplified models of radio coferage pattern because radio propagation is highly dependent on terrain and other factors.  The problem of path loss, shadowing and multipath fading all effect the coverage of an area.  For example, time dispersion is a problem caused by reception of radio signals, which are reflected off far away objects.  The carrier-to-reflection (C/R) is defined as the ratio between the direct signal (C) and the reflected signals.

Also, due the problem of the time alignment the maximum distance a MS can be form a BTS is 35 km.  This is the maximum radius of a GSM cell.  In areas where large coverage with small capacity is required, it is possible to allocate two consecutive TDMA time slots to one subscriber on a call.  This is enable maximum distance from the BTS of 70 km.

Frequency Re-use

Modern cellular networks are planned using the technique of frequency re-use.  Within a cellular networks, the number of calls that the network can support is limited by the amount of radio frequencies allocated  to that network.  However, a cellular network can overcome this constraint and maximize the number of subscriber that it can service by using frequency re-use.

Frequncy re-use means that two radio channel within the same network can use exactly the same frequencies, provided that there is a sufficient geographical distance (the frequency re-use distance) between them so they will not interference with each other.  The tighter frequency re-use plan, the greather the capacity potential of the network.

Based on traffic calculation, the cell pattern and frequency re-use plan are worked out not only for the initial network, but so that future demand can be meet.

Co – Channel Interferences (C/I)

Cellular networks are more often limited by problems caused by interference rather than by signal stregth problems.  Co-channal interference is caused by the used of a frequency close to the exact same frequency.  The former will interference with letter, leading to terms interfering frequency.  The former will interfering frequency (I) and carrier frequency (C).

The GSM specification recommend that the carrier-to-interference (C/I) ratio is greater that 9 decibles (dB). This (C/I) ratio is influenced by the following factors:

�     The location of the MS

�     Local gography and type of local scatters

�     BTS antenna type, site elevation and position

Adjacent Channel Interferences (C/A)

Adjacent Interferences (A), that is frequencies shifted 200kHz from the carrier frequency (C), must be avoided in the same cell and preferably in neighboring cell also.  Although adjacent frequencies to the carrier frequency they can still cause interference and quality problems.

The GSM specification states that the carrier-to-adjacent ratio (C/A) must be larger than -9dB.  Ericsson recommends that higher than 3 dB be used as planning criterion.

By planning frequency re-use in accordance with well established cell pattern, neither co-channel interference nor adjacent channel interference will cause problems, provided the cell have homogenous propagation properties oe the radio waves.  However, in reality cell vary in size depending on the amount of traffic they are expected to carry.  Therefore, real cell palns must be verified by means of prediction or radio measurements to ensure that the interference does not become a problem.  Nevertheless, the firs cell plann based on hexagons, the nominal cell plan, provides a good picture of system planning.

Cluster

Groups of frequencies can b e placed together into pattern of cells called clusters.  A cluster is a groups of cells in which all available frequencies have been used once and only once.

Since the same frequencies can be used in neigbouring clusters, interference may become a problem.  Therefore, the frequency reuse distance must be kept as large as possible.  However, to maximize capacity the fequency re-use distance should be kept as low as possible. 

The re-use pattern recomded for GSM are the 4/12 and the 3/9 pattern.  4/12 means that there are four three-sector sites supporting twelve calls using twelve frequency groups.  4/12 cell pattern is in common use by GSM network operators.

 

Below is an example of how a network operator could drive 24 vailable frequencies (1-24) into a 3/9 cell pattern):

Frequency Group	A1	B1	C1	A2	B2	C2	A3	B3	C3
No.RFC	1	2	3	4	5	6	7	8	9
	10	11	12	13	14	15	16	17	18
	19	20	21	22	23	24

In the  3/9 cell pattern there are always 9 channel separating each frequency in a cell.  However when compared with the 4/12 pattern, cell A1 and C3 are neighbours and use adjacent frequencies (10 and 9 respectively).  Therefore, the C/A interference will increase.  In this case, an operator, may use frequency hopping which, if plannd correctly, could reduce the possibility of such adjacent channel interference.

In real network the allocation of channels, to cell will not be as uniform as in possible as in table above, as some cell will require more channels and some will require less.  In this case, a channel may be taken from a cell will low traffic load and moved to one will higher traffic load.  However, in doing so, it is important to ensure that interference is still minimized.

Step 3: Surveys

Once nominal cell plan has been completed and basic coverage and interference predictions are available, the site surveys and radio measurements can be performed.

Site surveys are performed for all proposed site location. The following must be checked for each site.

�     Exact location

�     Space for equipment, including antennas

�     Cable runs

�     Power facilities

�     Contract with site owner

Radio Measurement

Radio measurement are performed to ajust the parameter used in planning tools in reality.  That is, adjustment are made to meet the spesific sites climate and terrain requerements.  For eaxample, parameter used in a cold climate will differ from those used in a tropical climate.

A test transmitter is mounted on vehicle, and signal strength is measured while driving around the site area.  Afterword the results from these measurement can be compared to the values to planning tools produce when the simulating the same type of transmitter.  The planning parameter can then be adjusted to match the actual measurements.

Step 4: System Design

Once the planning parameters have been adjusted to match the actual measurements, dimension of the BSC, TRC and MSC/VLR can be adjusted and the final cell plan produced.  As name implies, this plan can than be used for system installation.

New coverage and interference prediction are run at this stage, resulting in Cell Design Data (CDD) documenys containing cell parameters for each cell.

Step 5 and 6 : System Implementation and Tuning

Once the system has been installed, it is continuously monitored to determine how well it meet demand.  This called system tuning.  It involves:

�     Checking that the final cell plan was implemented successfully

�     Evaluating customer complains

�     Checking parameters and taking other measurement, if necessary

TEMS (TEst Mobile System)

TEst Mobile Systems (TEMS) is a testing tool used read and control the information sent over the air interface between the BTS and the MS.  It can be used for radio coverage measurement.  In addition, TEMS can be used both for field measurements and post processing.

TEMS consist of an MS with special software, a portable Personal Computer (PC) and optionally a Global Positioning System (GPS) receiver.

The MS can be useed in active and idle mode.  The PC is used for presentation, control and measurements storage.

The GPS receiver provides the exact position of the measurements by utilizing satellites.  When satellite signals are shadowed by obstacle, the GPS system swithches to dead rockoning.  Dead reckoning consist of a speed sensor and a gyro.  This provides the position if the satellite signals are lost.

TEMS measurements can be imported to TEMS CellPlanner.  This means that measurements can be displayed on a map.  For example, this enables measurement can also be downloaded to spreadsheet and word processing packages.

Step 7: System Growth/Change

A Cell planning is an on going process.  If the network needs to be expanded because of an increase in traffic or because of an environtment (e.g. a new building), then the operator must perform the cell planning process again, starting with new traffict and coverage analysis.

Hierarchical Cell Structures (HCS)

The feature Hierarcihcal Cell Structures (HCS) divides the cell network into up 8 layer.  The higher layers are used for large cells and the lower layers for small cells.  For example, large cells are added to a cellular network to provide coverage at coverage gaps.  The large cells then acts as umbrella cell for medium sized cells.  Additionally, micro cells can be added to cellular network in order to provide hot spot capacity.  The medium sized cells the act as umbrella cells for the micro cells.

The different cell layers can be seen as priority designation with the lower layer as the higest priority.  Thus, when selecting a BCCH carrier, an MS will choose an acceptable signal in as low a layer as possible.  HCS make it possible to pass between cell layers in a controlled way, facilitating dimensioning and cell planning in cell structures where large and small cell are mixed

Overlaid/Underlaid Subcells

Overlaid/Underlaid Subcells feature provides a way to increase the traffic capacity in a cellular network withouth building new sites.

A set of channel in BTS is assigned to transmit at a certain poer level.  These are the underlaid subcell channels.  Another set channels in the same BTS are assigned to transmit at a power level.  These are overlaid subcell channels.

The features make it possible to use two different frequency re-use pattern; one pattern for overlaid subcell serve a smaller area than the corresponding underlaid subscell can therefore be made shorter.  Consequently, the number of frequencies per cell can be increased providing an increased traffic capacity traffic capacity in the cellular network.