Coverage planning is normally performed with prediction modules on digital map database. The basic input information for coverage planning includes:
-Coverage regions
-Coverage threshold values on per regions (outdoor, in-car, indoor)
-Antenna (tower height limitations)
- Preferred antenna line system specifications
- Preferred BTS specification
Activities such as propagation modeling, field strength predictions and measurements are usually referred to as coverage planning.
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Site selection:
Coverage planning and site selection are performed on parallel with the site acquisition in interactive mode. Both network planning team and site acquisition should team have well defined responsibilities and means to communicate.
Propagation models
Propagation models are essentially curve fitting exercises. Propagation tests are conducted at different frequencies, antenna heights, and locations over different periods and distances. The receive signal data is analyzed using mathematical tools and are fitted to an appropriate curve. Formula to match these curve are then generated and used as models.
Some of the major propagation models are:
- Long-distance propagation model
- Longley-Rice model (irregular terrain model)
- Okumara
- Hata
- Cost 231-Hata (similar to Hata: for 1500-2000 MHz band)
- Wolfish-Ikegami Cost 231
- Wolfish-Xia JTC
- XLOS (Motorola proprietary model)
- Bullington
- Du Path loss model
- Diffracting Screens model
Coverage predictions
The possibilities for rough coverage calculations based on propagation curves formulas. These average values are not enough for the detailed network planning; therefore many computer-aided tools based on digital maps usage have been developed to improve the quality of the predictions.
Digital maps
There are different types of information that can be digitized and used for coverage predictions. The most important from the network planning point of view are topography (terrain heights), morphography (area types), rods traffic density.
For the micro cell modeling, which is required in a dense urban environment, more information and heighten resolution maps should be used. Information about the buildings and streets is essential, so the pixel size from 5m to 25m is reasonable. The streets can be stored and used in vector format.
Point to point and cell coverage
Using a given digital map it is not difficult to obtain the path profile between any two points, say BS and MS. Furthermore the profile can be related to the corresponding area types, thus making possible the calculation of specific propagation loss. Normally different corrections, such as the diffraction loss or mixed land-sea path correction are added to the basic propagation loss. the result of such point to point calculations can be used for cell coverage prediction. There are two basic approaches:
- Radial calculations
- Pixel by pixel calculations
The latter one gives better possibilities for the interference predictions, so the results should be transferred to the raster format even if the radial approach is used.
Field strength measurements
The field strength measurements are needed for determination of coverage areas as well as for tuning the propagation model of network planning system. In case of measurement before base station installation the site should equipped with the test transmitter. Possible test transmitter configurations are mobile station base station channel unit signal generator with power amplifier. The selection of routs to be measured depends on the purpose of the measurements. The most critical routs are typical located in urban or hilly areas. Where it is difficult to predict the field strength values with high accuracy.
During the field strength measurement, the measuring system normally takes the samples from the signal received by the antenna. The field strength samples recorded by a control computer with time, location marks. Using the samples it is possible to calculate the average values.
Propagation Model Tuning
The propagation models are not universal. The predictions must be verified by measurements and the models tuned accordingly. The model testing and tuning is a very sophisticated and challenging task, which requires detailed knowledge of the propagation nature. It should be done for every area type in a given country or region before the detailed network planning is started.
Extension of cell coverage area
The cellular area extension can be done with cellular repeaters and preamplifiers.
The cellular repeater amplifies the RF signal in both uplink and downlink directions, i.e. it is a device which compensates the propagation loss between the base station antenna and mobile station antenna. The cellular repeater antenna is connected between two antennas: the first antenna is pointed to the base station site and the second one (reradiating elements) is pointed to the area to be covered.
Radiating cable (leaky cable) can be used in tunnels as a reradiating element to provide homogeneous field strength inside the tunnel.
Mast Head Preamplifiers (MHPx) are---- installed at the base station antennas mast after the Rx antenna to amplify the uplink signal. The preamplifier has a low noise figure and adjustable gain to compensate the Rx antenna feeder attenuation. It can be very helpful when low-power hand portables are used in the network.
FREQUENCY PLANNING
The main goal of the frequency-planning task is to increase the efficiency of the spectrum usage, keeping the interference in the network below some predefined level. Therefore it is always related to interference predictions. There are two basic approaches to solve the frequency assignment problem.
- Frequency reuse patterns
- Automatic frequency allocation
Some software’s are used with automatic frequency allocation algorithms for finding the optimum solutions. The frequency allocation is generally guided by the following information:
- Channel requirement on cell basis according to the capacity planning
-Channel spacing limitations according to BTS specification
- Quality of service requirement which is conserved to acceptable interference probability
- Traffic density distribution over the service area
- Performance of advanced system features (frequency hopping, IUO, etc….)
The frequency allocation is based on cell-to-cell interference probability estimation according to the network topology, field strength distribution and traffic load. This results in customized frequency performance of the selected radio network elements.
The starting point of automatic frequency allocation is much better, since the exact site coordinates and BTS characteristics are available. Usage of propagation model based on digital maps, we are able to obtain interference predictions very near to reality.
Frequency Reuse
A frequency used in one cell can be reused in another cell at a certain distance. This distance is called reuse distance. The advantage of digital system is that they can reuse frequencies more efficiently than the analogue ones, i.e. the reuse distance can be shorter, and the capacity increased. A cellular system is based in reuse of frequencies. All the available frequencies are divided into different frequency groups so that a certain frequency always belongs to a certain frequency group. The frequency groups together form a cluster.
“A cluster is an area in which all frequency groups are used once, but not reused.”
The frequencies can be divided into different frequency groups. This introduces the terms reuse patterns and reuse grids. The most common reuse patterns in GSM is “4/12” and “3/9”.
4/12 means that the available frequencies are divided into 12 frequency groups, which in turn are located at 4 base stations sites. This assumes that the base station has three cells connected to it. The frequency groups are often assigned a number or name such as A1, B1, C1, D1, A2,…….. D3.
3/9 means that the available frequencies are divided into 9 frequency groups located at 3 sites. Problem with C/A might appear in certain parts of a cell, arising from adjacent frequencies in neighboring cells.
Example: channel assignment of 24 frequencies in a 3/9-cell plan.
Interference calculations
The reference interference ratio is defined in GSM as the interference ratio for which the required performance in terms of frame erasure, bit error rate or residual bit error rate is met. The reference interference ratios for BS and all types MSs are the following:
- Co channel interference: C/Ic <= 9 dB
- First adjacent channel interference: C/Ia1 <= -9 dB
- Second adjacent channel interference: C/Ia2 <= -41 dB
Co channel interference
The carrier to interference (C/I) ratio at a given mobile receiver can be calculated as follows:
C/I = C / (I1 + I2 + …….. +Ik)
Where k is the number of co channel interfering cells. For regular grid case it is possible to simplify the calculations by using the popular path loss expressions
Time dispersion
Some interference effects may be caused from the reflected signals if received outside the equalizer window. This happens only when the difference between direct path and the reflection path is larger than the equalizer window (about 4.5 km) and the reflected signal is strong enough. The reflection outside the equalizer window should be regarded as an independent co channel interferer, therefore the same reference C/I <= 9 dB should be used.
Digital maps based co channel interface
From the coverage areas calculated by the help of digital maps it is quite easy to obtain the expected interference areas. Since the frequency plan is still to be done, the multiple interferences cannot be calculated. Thus the process works for every pair of BS checking the ratio between the two-signal pixels. The probability of future multiple interference can be reduced by adding some margin, say 6 dB to the reference interference ratio. If the percent of the interfered area is larger than a given predefined level (depending on the required service quality), the pair cannot operate in the same channel. The results are presented as a matrix with elements giving the minimum slowed channel difference (in this case only 0 and 1) for every pair of BSs.