RF planning plays a critical role in the Cellular design process. Accurate planning is essential in order to ensure that the system will provide both the increased capacity and the improvement in network quality where required.
A properly planned system should allow capacity to be added economically when traffic demand increases.The goal is to achieve optimum use of resources and maximum revenue potential whilst maintaining a high level of system quality.By doing a proper RF Planning by keeping the future growth plan in mind we can reduce a lot of problems that we may encounter in the future and also reduce substantially the cost of optimization.On the other hand a poorly planned network not only leads to many Network problems, it also increases the optimization costs and still may not ensure the desired quality.
Phases of Wireless Network planning
- Initial Network Design
It is a process with following outcome Number of Bus’s
- Sites Locations
- Site Configurations :Omni/ Sectored, Antenna Height, Antenna Type, Orientations, RF Power and other parameters
- Managing Future Growth in Subscriber Base
RF PLANNING PROCEDURES
- Initial Design
- Nominal Plan
- Site Selection
- Site Survey
- Site Planning
RF PLANNING PROCEDURES
- Initial design:
TOOLS USED FOR RF PLANNING
Network Planning Tool
- Planning tool is used to assist engineers in designing and optimizing wireless networks by providing an accurate and reliable prediction of coverage, doing frequency planning automatically, creating neighbor lists etc.
- With a database that takes into account data such as terrain, clutter, and antenna radiation patterns, as well as an intuitive graphical interface, the Planning tool gives RF engineers a state-of-the-art tool to:
– Design wireless networks
– Plan network expansions
– Optimize network performance
– Diagnose system problems
Propagation Test Kit
- The propagation test kit consists of
- Test transmitter.
- Antenna (generally Omni).
- Receiver to scan the RSS (Received signal levels).
- A laptop to collect data.
- A GPS to get latitude and longitude
- Cables and accessories
- A single frequency is transmitted a predetermined power level from the candidate site.
- These transmitted power levels are then measured and collected by the Drive test kit. This data is then loaded on the Planning tool and used for tuning models.
- This drive test is done to prepare generic models for network design.
- The objective is to collect field data to optimize or adjust the prediction model for preliminary simulations.
Propagation Tool Adjustment
- For a given network design there may be more than one model like dense-urban, urban, suburban, rural, highway etc.
- The predicted and measured signal strengths are compared and the model adjusted to produce minimum error.
- These models are then used for initial design of the network.
Traffic Modeling Tool
- Traffic modelling tool is used by the planning engineer for Network modelling and dimensioning.
- It helps the planning engineer to calculate the number of network elements needed to fulfil coverage, capacity and quality needs.
Project Management Tool
- Though not directly linked to RF Design Planning, it helps in scheduling the RF Design process and also to know the status of the project
- Site database : This includes RF data, site acquisition,power, civil ,etc.
- Inventory Control
- Fault tracking
- Finance Management
CELL SIZE/COUNT ESTIMATION :
Once the Maximum allowable pathloss is known, the achievable cell size can be evaluated. Cell radius is the distance from base station where the path loss equals MAPL. Beyond this radius, the signal is too weak to be acceptable.Cell radius is calculated using MAPL and Hata’s empirical formula. The designer should notice that the coverage objectives are usually different for Urban, Suburban and Rural areas and therefore MAPL has to be calculated for each area and then cell size determined separately.Once the cell radius is calculated, cell count estimates can be made.
|2. Nominal Plan:
A nominal plan is initially a hypothetical wireless network
and is starting point for the cell rollout process and will evolve into initial network
design. As physical sites are are identified and acquired the nominal plan is amended.
Creating Nominal Plan stages:
Planning of cell sites sub-area depending on clutter type and traffic required.
Positioning a hexagonal grid on sites over the desired coverage area
Analyzing the capacity of network so detecting:
Hot spots that require cell splits
Under-used cells that could be removed from the plan
Running Propagation Analysis by using generic models prepared by drive testing & prop test, running predictions for each cell depending on morphology type to predict the coverage in the given sub-areas. using the planning tool to calculate the path loss and received signal strength by based on the Co-ordinates of the site location, Ground elevation above mean sea level, Antenna height above ground, Antenna radiation pattern (vertical & horizontal) & antenna orientation, Power radiated from the antenna.
Reseting Cell Placement( Ideal Sites):
According to the predictions change the cell placements to design the network for contagious coverage and appropriate traffic
After site survey there also should be a modification of nominal plan according to the information gathered by site survey.
The major parts that should be modified and revised are as below:
There should be a comparing the nominal coordinates to the coordinates read by surveyor to confirm the exactness of site coordinates.
This process will start with evaluating the surroundings on the site according to site survey reports and deciding on the degree of importance of providing coverage and capacity on each sector. The degree of importance is rated from 1 to 5 where 1 is least important and 5 as most important.These data will be used for more extensive planning to balance traffic requirements and coverage ranges. And then continuing with filling up corresponding data under coverage and capacity on the survey form.This will be used as guidance for the project manager in making priorities in site rollout.
Radio frequency planning:
After getting the existing frequency information from site survey reports if interference is very much expected, RFI measurements should be performed to confirm if it is “safe” to put up a site in the proposed location or not.
Comparing the proposed antenna heights with the nominal antenna height. It is specified that the maximum allowable deviation of antenna height from the nominal or predicted height is 15%. As an example, if the nominal antenna height is 35 meters, variation up to 40.25 meters for the proposed height is allowable. If the proposed antenna height is lower than the nominal height, a tower or pole is considered to reach nominal height. Otherwise, another site location should be chosen.Antenna height is measured above mean ground level.
Analyzing the immediate vicinity for coverage considerations. the planner should notice that he should maintain the nominal antenna directions as planned as much as possible. However, nominal directions might not necessarily be the best directions to fulfill coverage objectives. Also the RF engineer will have to decide on the proposed antenna directions and configurations considering the following items:Are there immediate or upcoming obstacles along the nominal antenna directions such as buildings or hills?Do the antennas need to be re-directed to cover roads or more populated areas instead of open areas or water?Is an omni or two-sectored site sufficient to provide coverage and traffic?
Antenna type (beamwidth, gain, diversity):
Normally, antenna types are already determined from the plan. However, it might be necessary to change the type of antennas to be used to fulfill the site objective. The decision is affected by considering antenna characteristics such as beamwidth, gain and diversity type For example, urban may use the typical 65° antennas, while long roads may employ 30° high-gain antennas to provide long range coverage.
The tilt of antenna should also be determined in order to have better coverage and less interference with the adjacent sites.