CALCULATIONS: TRAFFIC CAPACITY INTRODUCTION

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14 CALCULATIONS: TRAFFIC CAPACITY

14.1 INTRODUCTION

Traffic capacity calculations provide support when designing networks for desired levels of capacity and blocking. The calculations are performed using three tools; a Traffic Map tool, a Traffic Capacity tool and a subscriber tool. The first step is to define a traf-fic map. The second step is to calculate how many subscribers each server has to handle. Finally, when editing the server result, capacity calculations are performed.

Three subscriber types can be defined in the traffic map expressed as the number of sub-scribers in a point, along a line, within a polygon or in a circular area. Every terrain code can also be assigned a subscriber density. This is done as a geographical parameter in the function that handles settings for the geographical databases.

Figure 14.1: Traffic Map tool.

An object is a point, a line, a polygon or a circle specified by its co-ordinates. Each ob-ject is described with one or several of the defined subscriber types. An object thereby defines a raster in which every point is described by three values. Each value is the number of subscribers for the type. In the circle object the subscribers can be distributed

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uniformly or Gaussian. When selecting the Gaussian distribution the relation of densities between the centre and the periphery is specified also. A new object can be created by using an area defined in the Map Viewer. The traffic map can be saved to a file and retrieved later. While creating the traffic map it can be shown in the Map Viewer. It is possible to show one of the three subscriber types for one or several objects.

The second step is to calculate how many subscribers each server has to handle based on the traffic map and a best server calculation. This is essentially done in the same way as in the coverage tool. The calculation of the best server can be done with noise or inter-ference limitation.

Figure 14.2: Traffic Capacity tool, main window.

When calculating the best server it is possible to include one traffic map and terrain code values for the subscriber types. When calculating with a traffic map the objects have different priorities. When objects overlap in a raster point in the map only one of the objects determine which number of subscribers to use. The order of priority is; point, line, polygon, circle and terrain code. Thus, on raster points where there is an object of the type point, it will always determine the number of subscribers used in the calcula-tion. The best server is displayed for each raster point in the map. For each server the number of served subscribers is presented. If no object is defined the terrain code values

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of subscriber density are used as defined in the window for setting of land cover para-meters as part of the definitions for the geographical data.

Traffic capacity results appear under their own headline in the usual result folder. A result is edited to perform the third step, where the number of subscribers that a station serves is presented and further calculations may be carried out.

The number of traffic channels for each station can be edited locally in the result table. This is done by double-clicking on the desired station. Subsequent calculations are per-formed with the new number of channels. These changed values are not stored to the project file as a station parameter, just locally in the result.

Figure 14.3: Traffic Capacity tool, information window.

For each station in the calculation it is possible to see the number of subscribers the sta-tion serves within the covered area. The subscribers are presented by type and as the total number. The result may be shown in the Map Viewer.

Under the parameters tab the user may select Erlang B or Erlang C to calculate the offered traffic. Erlang B is used when the server has no queue; hence an arriving sub-scriber is either served or rejected. Erlang C is used when subscribers may be queued for later processing. Both formulas assume that there are more subscribers than channels available.

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Figure 14.4: Traffic Capacity tool, parameters window.

Input parameters are requested grade of service, number of calls per hour and holding time. For Erlang C the delay time is specified also. The result is presented in the map. It is possible to change the parameters and repeat the calculations as long as the edit win-dow is open.

The grade of service specifies the probability that a subscriber will be rejected (Erlang B) or that the queuing time > delay time (Erlang C) when requesting service. Low values of the grade of service are the most acceptable to the user. The holding time is the aver-age duration of calls. The delay time is the average time a subscriber spends in the queue.

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Figure 14.5: Traffic Capacity tool, Grade of Service window.

The grade of service is calculated from the traffic map and the number of channels the station has as specified in WRAPdB. The column A+B+C shows the GoS obtained with the total number of channels specified in WRAPdB (No. of channels A + No. of chan-nels B + No. of channels C) and the total number of subscribers. By double-clicking on the station name extended information is displayed.

Figure 14.6: Traffic Capacity tool, extended information for Grade of Service (Station Base 1 double-clicked).

Results are calculated for several numbers of channels. The number that is specified in WRAPdB is pointed out. Erlang C calculations give in addition the following results: WA(s) for the average waiting time for all calls; WD(s) for the average waiting time for delayed calls and PD for the probability that a call will be delayed (queued).

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Figure 14.7: Traffic Capacity tool, subscribers window.

The number of subscribers that can be served by the station is calculated for each sub-scriber type. The input parameters used are GoS as specified under the Parameters tab and the number of channels. The column A+B+C shows the number of subscribers that can be served for all subscriber types when the minimum GoS as specified under the Parameters tab is used.

Figure 14.8: Traffic Capacity tool, Traffic window.

The traffic in Erlang that can be served by the station is calculated for each subscriber type. The input parameters used are GoS as specified under the Parameters tab and the

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number of channels. The column A+B+C shows the traffic that can be served for all subscriber types when the minimum GoS as specified under the Parameters tab is used.

Figure 14.9: Traffic Capacity tool, Channels window.

The number of channels required to achieve the specified grade of service is calculated for each subscriber type. The column A+B+C shows the number of channels required when all subscribers are treated together and the minimum GoS as specified under the Parameters tab is used.

Figure 14.10: Traffic Capacity tool, Presentation window.

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It is possible display one of the results at time. It is also possible to display the traffic map.

14.1.1 Example 1: Working with the traffic map

As a preparation for calculations with the Traffic Capacity tool there is a need to create a traffic map. This map defines the distribution and density of subscribers over the area of interest and thus constitutes a very important design input to the subsequent calculations. The aim is to finally arrive at a balanced network, with the proper design in terms of individual and composite base station coverage, where each base station provides the required grade of service.

A traffic map is comprised of the following traffic objects:

Points Lines Polygons Circles Each object can be assigned a number of subscribers.

The following procedure will create a traffic map that is suitable to use together with the BaseStation.WPR project for traffic calculations. The complete map is also available as a file Ostergotland.WTM traffic map file on the CD-ROM.

Open the project BaseStation.WPR and show all the 16 base stations on the map.

Select Traffic Map - New from the Menu bar, or click on the icon New traffic map. This opens the traffic map main window. Name the map to be generated OstergotlandTrafficMap.

Choose the Polygon tab. Draw a polygon enclosing all of the base stations in the map. This is done by right-clicking with the cursor in the map area and selecting Area - Polygon. The first point in the poly-gon is defined with left-clicking in the map. Subse-quent points are defined by renewed left-clicks. The polygon is finally closed by right-clicking.

Click button [Get from Map Viewer] in the traffic map main window. This imports the polygon coordinates to the traffic map.

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Click on Objects: [Edit...]. This opens the Edit Traf-fic Polygon window, where the total number of sub-scribers of each of the three types A, B and C can be defined. Enter A:2000, B:50 and C:40. Clicking [OK] closes the window. Again clicking on Objects: [Edit...] opens the edit window and the display looks like Figure 14.11. Close the Edit Traffic window.

Figure 14.11: Traffic map generation with the Polygon type of object.

Select the Circle icon in the Map Viewer. Draw a circle just enclosing one of the three largest urban areas shown in the map Viewer. Click [Get from Map Viewer]. Repeat this for all three main areas. Circle objects C1, C2 and C3 have now been created.

Mark object C1 and select Object: [Edit...]. This opens the Edit Traffic Circle window. Check the box Subscribers are Gaussian distributed over the area. This means that the defined number of subscribers will be dispersed over the circle according to a truncated Gaussian distribution, with highest density at the centre of the circle. The shape of distribution is defined by the Gauss distribution parameters. Use the default parameters and enter number of subscribers A:1000, B:20 and C:10. Click [OK]. Repeat this for the other two circles. See Figure 14.12 which shows the appearance of this window after closing and opening

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again by clicking on Objects: [Edit...]. Close the Edit Traffic window.

Figure 14.12: Edit Traffic Circle window with Gaussian distribution of subscribers.

Select the tab Points to define a few local hot spots of traffic. Select the Pointsicon in the Map Viewer and place 8 points at remaining urban areas. Using the same Objects: [Edit...] procedure, assign traffic of subscriber type A:300, B:10 and C:5 to each of the eight points.

It is also possible to define traffic along a line drawn in the map. The line can be segmented to follow for instance a road where significant traffic may be expected. We will not do that for this example, though.

The contents of the traffic map may be displayed in the Map Viewer. The object that is selected in the Objects list is highlighted in bold contour if the box Update in Map Viewer is checked. The traffic map can also be shown using colour coding of the levels (num-ber of subscribers) by suitable selections in the win-dow opened from Levels: [Edit...].

When the traffic map is ready it can be exported to a file by clicking on Export to be used later or sent to a colleague for further work. Export this map to a suitable file name and location, for instance naming it OstergotlandTrafficMap.WTM. Click Save also. This

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will save the map under the name defined in the Name field in the traffic map window. It can thereafter be opened again by selecting - from the menu bar. The list of available traffic maps will then be shown.

This process has generated a traffic map comprised of the following:

A number of subscribers distributed over all of the anticipated coverage area of the base station network (the polygon)

In addition, a number of subscribers in the main urban areas (the three circles). These are distributed with higher density in the city centres through the choice of the Gaussian distribution of subscribers.

In addition, a number of subscribers at an additional eight smaller urban areas (the eight points).

Each terrain code can be assigned a certain subscriber density expressed as the number of subscribers per square km (subscribers/km2). This is a setting in the geographical data (Settings in the main window Menu bar, then Geographical - Edit - Land Cover). The terrain code subscriber density has a lower priority and is overridden by the polygon and circle values. So in this example any subscriber density defined for the terrain codes will not be used. The polygon and circle values will be used as they have higher priority.

14.1.2 Example 2: Traffic calculations in a GSM network

The following example shows how to use the Traffic Capacity tool to analyse and design a GSM base station network for good performance fulfilling requirements for Grade of Service. The required number of channels to cover the intended area and serving the defined number of subscribers will be calculated.

The example is based on the BaseStation.WPR project, which has 16 GSM base sta-tions. The previously created traffic map can be used, or the Ostergotland map which is on the CD-ROM.

Open the project BaseStation.WPR and show all the 16 base stations on the map.

Double-click on one of the base stations to study its parameters. The original demonstration project has a single frequency pair assigned to each station and the number of traffic channels is for subscriber type A:20, B:1 and C:2. Since each GSM frequency can carry a maximum of 8 channels this is obviously not correct. However, as a starting point for the calculation it works fine.

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The base stations have an EIRP of 19 dBW and omni-directional antennas with the transmitter and receiver antenna at the same height. The base receiver sensitivity is set at -105 dBm. The characteristics of the associated mobile station can be checked by open-ing the folder WRAP Database - Equipment - Mobiles and double-clicking on the GSM Mobile in the station list. The EIRP is 3 dBW and the receiver sensitivity is set to -100 dBm. The downlink power budget thus allows 149 dB transmission loss and the uplink power budget allows 147 dB, so the link is very nearly balanced.

The BaseStation.WPR project includes calculated results for the uplink and downlink coverage, using the Detvag-90/FOI terrain dependent propagation model. You can display the results in the Map Viewer and see the slight difference in coverage.

Open the traffic map that will be used for this exam-ple by selecting in the map viewer or main window menu bar - OstergotlandTrafficMap. This is the map that was described in the previous example. The objects of the traffic map are displayed in the Map Viewer. The purpose of this is to define a suitable calculation area for the Traffic Capacity tool. This can be done for instance by selecting area type Polygon in the Map Viewer and drawing a new polygon along the contours of the traffic map polygon. Another way is just to draw a circle enclosing the complete traffic map.

Mark all of the 16 base stations and select the Traffic Capacity tool on the Menu bar. Note that only those base stations that are marked will be used in the following calculation, and in this case we want to calculate for all of them. The main window of the Traffic Capacity tool opens.

Make the following selections in the Traffic Capacity main window. - Select Calculation: Best server, noise limited - Select Mobile: GSM Mobile - Downlink - Margin: Default 0 dB - Calculation area: Defined in Map Viewer - Traffic Map: OstergotlandTrafficMap. - Un-check Use terrain code values - Resolution: Average - Propagation Model: Longley-Rice, default parameters - And set a suitable name in the Description field. The appearance of the Traffic Map window after these selections is shown in Figure 14.13. Click on [OK],

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which starts the calculation. Most of the time for the calculation process deals with determining the areas where each base station provides the best service, i e strongest signal or (for the selection of Best Server, interference limited) best S/I ratio.

Figure 14.13: Traffic Capacity main window.

Open the folder This Project - Results - Traffic Capa-city Results and double-click on the result that was just calculated. This opens the Edit Result window as shown in Figure 14.14. The first tab gives the identi-fication of the result and presents the number of sub-scribers, as defined by the traffic map, that are within each base stations best server area. Already in this result it is indicated if certain base sta-tions will need to serve an unproportionally large number of subscribers.

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Figure 14.14: Traffic Capacity result window: Number of offered subscribers within each best server area.

Selecting the next tab Parameters allows the entry of traffic characteristics of each subscriber type and the selection of traffic formulas. Erlang B is used for systems where calls to a base station that has no available channels are rejected immediately. Erlang C is used for systems where such calls are queued (put on hold) for a specified maximum time, the Delay time. If a channel becomes available within this time the call is served. Otherwise it is rejected. Select Erlang B for the following calculations and set the subscriber traffic profiles to the following:

Subscriber A Subscriber B Subscriber C

Grade of Service 0.05 0.05 0.05

Calls/hour 0.5 10 1

Holding time (s) 180 2 100

Selecting the tabs Grade of Service, Subscribers, Traffic and Channels will now present the calculated results. These are based on the traffic map, the net-work layout and technical characteristics, the se-

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lected traffic formula and the traffic characteristics of the subscribers. See Figure 14.15 for the Grade of Service result. This is equivalent to the probability of blocking that a subscriber will experience.

Figure 14.15: Grade of Service for the offered traffic and current number of channels.

The columns for the individual subscriber types show the result when each subscriber type uses the dedicated set of channels, respectively. The column Subscr A+B+C shows the result when all channels are pooled together and are available to all types of subscribers. The minimum GoS as specified under the Parameters tab is used for the pooled case.

In each of the tabs Grade of Service, Subscribers, Traffic and Channels there is a detailed result avail-able for each base station. This is presented by double-clicking on a selected base station. See Figure 14.16, where the Erlang C formula was first selected under tab Parameters. The detailed information shows results for a range of channels around the current number of channels, which is indicated by the arrow.

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Figure 14.16: Grade of Service for the offered traffic and current number of channels. This shows extended information for base station 1. Erlang C formula with delay time 3 s was selected under Parameters.

Click on tab Channels. This will display the result according to Figure 14.17. The result shows that the network will work with fewer than the current number of channels, which assumes that each base station has a total of 24 channels thus needing three frequencies each (Eight channels per frequency for GSM).

Figure 14.17: Number of channels needed to achieve the required Grade of Service.

Finally, click on the Presentation tab. Here you can select from a list of calculated results to display in the Map Viewer. Select Best Server and press [OK]. This will show the best server map as in Figure 14.18.

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Figure 14.18: Best Server map for GSM network. Propagation model Longley-Rice.

Go back to the Information tab. Double-click on one of the base stations to open the Edit Channels window, see Figure 14.19. Change the number of channels and open the other tabs to see the change in results. Note that changing the number of channels here does not automatically change these values for the stations as stored in the project or in the WRAPdB database. The change is only performed locally in the result.

Figure 14.19: Editing the number of traffic channels for Base Station 01.

Printout of the calculated results in text form is available by right-clicking on the result in the folder Traffic Capacity Results. Information for all base stations will then be printed.

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For a more detailed analysis and design of the network you should use a terrain depend-ent propagation model, in this case Detvag-90/FOI is well suited. This will make calculations slower, but initially you can use a low calculation resolution to reduce the time and anyway get good quality results. The traffic capacity calculations may indicate a need to adjust network parameters for instance to balance the traffic between base sta-tions. This is done with the standard methods such as power adjustment, antenna pattern and gain adjustment and raising or lowering the base station antenna. The locations of base stations may also be changed. To accommodate the traffic load it may be necessary to change the number of channels available in selected base stations, which in turn may reflect to a need to assign more or less frequencies to these base stations. This is per-formed with the Frequency Assignment tool as is described in the section about that tool.