or 20 watts output; frequency in GSM 900/1800 MHz bands.

Portable mast Adjustable up to 5m. With 1m antenna on top, effective height above ground is 6m.

Transmit antenna High gain Omni or directional antenna as required.

Receiver (TEMS

mobile), Hand held mobile phone with RS232 connection to a lap top.Or, an accurate portable RF

sensitivity meter/ CW receiver if model calibration is required. Positioning System GPS system,with PCMCIA card

Computer Lap top PC with TEMS software

and GPS software.

Cables & accessories: Calibrated cable lengths (10m) of low loss feeder with known attenuation values; 12 Volts battery with appropriate cable to connect to transmitter.

Power meter VSWR meter

Planning Tool Preparation and Model Calibration

The concepts of RF planning were covered in Section 3. It is possible to do this planning for a number of sites by manually handling the propagation test results.

However, to do iterative operations and get a coverage map it is essential to use planning tools to automate the process.

There are many planning tools available to day: NetPlan ( Motorola)

PlaNet (MSI) CellCad (LCC) Odyssey (Aethos) Asset (Aircon)

Planning tool Requirements:

The planning tool must be easy to use, should be compatible with tools like the TEMs, should be economical and require minimum hardware.

Obtain Maps:

It is important to have the map of the city/area under consideration. Such map information could be obtained as paper copies from authorized sources or from a satellite image. The latter option is very expensive. The maps could be obtained from the local authorities. Where contour maps are required they could be obtained from the Survey of India department. The maps should be preferably on 1:50000 or 1:25000 scale. Normally the data would be in 50m resolution. Less than this is not suitable for macro cell designs as it calls for large memory requirements for the planning tool.For micro cells, less than 30 m resolutions can be considered, where a “ Ray Tracing Tool” is used.The geographical data of the area is digitized under 3 different categories:

_ Land Use

_ Digital Terrain Map (DTM)

_ Vectors ( Roads, Railway tracks etc).

Planning tool preparation and Model Calibration: There are many planning tools available to day:

NetPlan (Motorola) PlaNet (MSI) CellCad (LCC) Odyssey (Aethos) Asset (Aircon)

A planning tool should be:

Easy to Use

Compatible with tools like TEMs Minimum Hardware requirements Economical.

Maps collected from authorized sources.

1:50000 or 1:25000 scale 50 m resolution for macro cells.

Less than 30 m resolution possible for Micro cell planning using “ Ray Tracing Tool” .

Maps are digitized under 3 categories:

Land Use

Digital Terrain Map ( DTM ) Vectors ( Roads, Railways etc).

Most Planning tools use corrections for the land use or clutter. The propagation model can be tuned by assigning values to:

Clutter factor (gain or loss due to clutter) Clutter heights (for diffraction modelling)

Different types of clutter are defined in these models/tools: Dense Urban

Urban Suburban

Suburban with Dense Vegetation. Rural

Industrial Area

Utilities (marshalling yards, docks, container depots etc ) Open area

Quasi Open area Forest

Water

Too many clutter type definitions complicate the planning process. 10 to 15 is typical DTM. Provided by the map vendor

Provides Contour information as a digital map. Vectors Highways Main Roads Railways Canals/Water ways Coast line Rivers

Each category is digitized as separate layers Displayed separately if required. Map information is set up in the planning tool. Model Calibration carried out.

Model Calibration

Typically RF propagation tests are conducted for 5-8 proposed BTS sites covering as much of the area as possible. Efforts should be made to include all types of clutters during the propagation test. Almost all the planning tools have provision for changing the clutter values to match the propagation test results. They all have different directory structures and means of handling the geographical data. As an example, the PlaNet has a procedure for tuning the Macro Cell models. NetPlan has a Custom Path Loss Model which enables the planer to play with the values of various types of clutter and fine tune the model. For the sake of illustration, an excerpt from the “ User Reference Guide” of PlaNet is placed as an Annexure.

All tools have provision for manipulating clutter values.

Different tools have different Directory structures and means of handling geographical data. Macro Cell Tuning procedure described in “ User Reference Guide” of PlaNet kept in Annexure for illustration purposes.

The procedure mainly talks about ensuring correct data header files to include: BTS location

EIRP of BTS Antenna Type BTS antenna height

Description of surrounding area.

Procedure uses a general core model equation:

The equation has constants k1 to k6 and a constant for clutter, kclutter.

Initial values for the constants are set as per the model chosen (say Okumara-Hata).

PlaNet programme is run repeatedly to make RMS error values for all data files ZERO or a minimum. For each run of the programme, the values of k1 to k6 are manipulated.

This completes Model calibration.

Link Budget and Other Steps

Once the geographical data is entered in to the planning tool, a coverage map is required to be generated for each site.

The most important step is then to prepare a Link Budget for the site under consideration. Some of the key points to be considered are:

What is the desired probability of the receive signal strength at the mobile, within the entire coverage area? ( usually 90 or 95% and is decided in consultation with the customer).

What is the expected in-building coverage?

What is the probability on the “ EDGE” of the coverage area that the receive signal strength is sufficient for the mobile to make and hold a call?

What is the fade margin available?

What is the maximum permissible path loss ( from the Link Budget) What is the radius of the cell?

What are the areas of different types of coverage planned for? For example Main business area could be 100 sq. Km and the suburban area could be 200 sq. Km and so on.

How many sites are required for each type of area? ( from coverage point of view) Is the number of sites calculated as above adequate for capacity?

Decide on more sites for capacity.

Link Budgets and Other Steps

Key Points to be considered: Coverage Probability

Expected in-building coverage? Edge Probability

Fade Margin required

Maximum permissible path loss (from the Link Budget) What is the radius of the cell?

Is the number of sites calculated as above adequate for capacity? Decide on more sites for capacity.

Coverage Estimation

Having calculated the Maximum permissible path loss, the cell radius can be determined as explained in Section 2.

For a given cell radius, we can calculate the estimated coverage area based on the formula:

The site separation distance D = (3N) R. ( R is the radius of the cell, same as d) For a given area, we can then calculate the number of sites required.

Number of Sites = Total Area / Cell area.

Capacity Considerations

Having determined the number of sites, it is then required to check if the capacity requirements are also met. This depends on the spectrum available, which will decide the site configurations. The availability of other features like frequency hopping etc is also to be taken into account. If capacity requirements are not met, then more sites may have to be added. If the number of sites as calculated above is not acceptable to the customer, then a second round of calculations may be required, assuming 50% in building coverage in place of 75%.

Having prepared the preliminary cell plan, the planning tool can be used for returning coverage predictions. These will be more accurate than the rough models used for estimating the site density.Typically this activity will go through a number of iterations in consultation with the customer.

Search Areas:

Based on the final plan, “ search areas” are issued for each site location. In addition to the rough location, information like latitude and longitude of the place, approximate antenna heights, specific areas if any and the size of the search areas also needs to be given. The search area size would depend upon the criticality of the site. As per the information given by the RF planner, site acquisition teams scout for suitable buildings.

Site Selection

The choice of a site location depends on factors like the antenna heights above ground as well as above clutter. A generic rule of thumb could be used depending upon the type of area under consideration.

Central business area:

Here the search area size is very critical, usually within 100m

the antenna should be at or slightly above the average surrounding clutter height All near field obstructions should be avoided

Antenna orientation may be along major roads

Try and mount antennas close to solid structures like a lift motor room, this minimizes back lobe radiation effects

As far as possible avoid towers on building tops as this might cause interference to neighboring cells.

Residential/Suburban areas:

Here the site separations may be larger than the first case and to that extent the location of the site is also less critical. Search areas are typically 200 m

Antenna height should be 3 to 5 metres above the average clutter height Try and locate sites close to major routes or at large junctions

Back lobe radiation is less critical, though it is desirable to avoid it.

Industrial areas:

A suitable location as much at the centre of the area as possible should be selected.

Quasi Open areas:

Search areas can be bigger, say, about 500 m

Propagation is more dependent on the terrain than on the clutter. Hence more care is needed in hilly areas Use of towers is common

Antenna orientation is not a critical factor. Availability of infrastructure like land, power etc is important.

High ways:

Search areas are along the road and are close High gain antennas required

Chapter 7 Extending Cell Range

Contents:

Extending Cell Range Objectives

Extending Cell Range

Increasing BTS Transmit EIRP Air Combining Techniques Improving BTS Sensitivity

Objectives :

Explain the reason for extending the range of cells Explain the various methods used for range extension Discuss the results of the various methods

Extending Cell Range

Most operators are nowadays looking for achieving maximum coverage with a minimum or optimum number of sites. This requirement becomes significant especially in the 1800 MHz band. For in these frequencies, the path loss in more and cell sizes are much smaller than GSM 900 frequencies. Hence to get more coverage, the number of sites increases.

Extending the range of each cell helps in reducing the number of sites required. The range of a cell depends upon the BTS transmit power, BTS receive sensitivity and the mobile’ s receive sensitivity. If we can improve these parameters, the cell can be ‘extended’ .

Increasing BTS Transmit EIRP

To maximize BTS o/p power, single carrier cells can be used, this will avoid the combination losses of multiple carrier cells.

The output power at the top of the cabinet could be set to 40Watts, giving an increase in signal strength of 3 dB.

Another way to maximize Tx and Rx signals is to implement low loss feeder cables. A typical 7/8” Andrews coaxial cable has an attenuation of 3.92 dB/100mt. If a 1-5/8” Andrews cable with an attenuation of 2.16 dB/100mt is used, then an increase of 1.6 dB can be obtained per 100m.