GSM network design for Stavanger and Sandnes

Transcription

1 GSM network design for Stavanger and Sandnes Jan Magne Tjensvold November 6, 007 Contents 1 Introduction 1 Background.1 Geography and population Technology and equipment Calculations Omni directional antenna Sectored antenna Voice channels Traffic density Cells Equipment Cost estimate 8 References 9 1 Introduction This report investigates the design of a GSM network for the cities of Stavanger and Sandnes in Norway. It provides a cost estimate of the required infrastructure in these two cities and for the highway connecting them. The estimate only includes equipment costs and not maintenance costs or other operational costs which would be needed to keep the network fully functional during its lifetime. It only includes the cost for the base transceiver stations (BTS), base station controllers (BSC) and the mobile switching center (MSC). 1

2 Figure 1: Map of Norway with the location of Stavanger and Sandnes. Figure : Rogaland county with the municipality of Stavanger in red and Sandnes in blue. Background.1 Geography and population Stavanger and Sandnes is located in the county of Rogaland, Norway as seen by Figure 1 and. Statistics Norway (SSB) [1] provides maps showing the population density of an area. One such map of Stavanger and Sandnes has been used as the basis for drawing an outline of the urban areas in Figure 3 on the next page. SSB provided some vital statistics for the two municipalities which is shown in Table 1 on the following page. The table also contains some other statistics in addition to SSB s population and area statistics. Since we are designing a GSM network for the city areas we only use the urban area and urban population statistics. We also make the assumption that the population growth within each municipality is uniformly distributed over the area it covers. Using the distance calculation tool from Temakart Rogaland [] the length of highway E39, as shown in Figure 3, was found to be about 10.7 km. We also assume that we will be able to get a market share of 35%.

3 Figure 3: Outline of the urban areas of Stavanger and Sandnes along with highway E39. Statistics Stavanger Sandnes Highway Area 71.4 km km 10.7 km Urban area 43.4 km 1.0 km Population Urban population Annual growth rate 1.% 1.36% Market penetration 40% 45% Path loss slope, γ Bandwidth, BW 8.33 MHz 8.33 MHz MHz Traffic/subscriber, E Erlangs Erlangs Table 1: Area and population statistics for Stavanger and Sandnes. 3

4 . Technology and equipment LGC Wireless, Inc. [3] has been chosen as the supplier for the GSM network equipment. In Table you can see the specifications of their UltraWAVE range of BTSs, BSCs, MSCs and HLRs (Home Location Register is sold together with MSC). The equipment costs are estimated since none of the vendors was willing to provide their prices to this project. Component Capacity Connections Cost (NOK) BTS Erlangs 84 channels BSC 466 Erlangs 31 BTSs MSC 500 Erlangs 56 BSCs HLR subscribers Table : GSM network equipment specifications from LGC Wireless. GSM requires a signal-to-interference ratio (SIR) of at least 14 db. GSM channel bandwidth is 00 khz and by employing Time Division Multiple Access (TDMA) each channel can carry 8 full-rate voice channels. We will use a frequency reuse factor of N = 4. Roll out time (ROT ) is estimated to 4 years, in which no additional equipment is expected to be installed. This means that the capacity will remain constant during this time. We also aim for a grade of service (GoS) of 1%. This means that in average 1 call is blocked for every 100 call attempts. 3 Calculations 3.1 Omni directional antenna First we must find out if we should use omni directional or sectored antennas. For omni directional antennas the following formula for SIR applies ( 3N ) γ 1 SIR = 10 log j (1) where N is the frequency reuse factor, γ is the path loss slope and j is the number of surrounding cells. For a hexagonal cell layout, as seen in Figure 4 on the following page, j is 6. To simplify (1) we replace 3N with q using the following equation q = d r = 3N () where d is the distance between the center of two cells using the same frequency and r is the radius of a cell. When inserting () into (1) we get the 4

5 Figure 4: Hexagonal cell layout. following formula [ (q 1) γ ] SIR = 10 log j By applying our frequency reuse factor N = 4 to () we can calculate the value of q q = 3N = 3 4 = We can now find the SIR in the two cities by using the specified path loss value of γ = 3.5 like this [ (q 1) γ ] SIR city = 10 log j [ ] ( ) 3.6 = 10 log 6 = db SIR city is less than the 14 db we require so we cannot use omni directional antennas in the cities. We use a similar formula to calculate the SIR for the highway. Since the highway does not use a hexagonal cell layout, but rather a linear cell layout, we use the following formula SIR highway = 10 log [(N + 1) γ + (N 1) γ ] = 10 log [ ( 4 + 1).7 + ( 4 1).7] = 7.55 db We find that SIR highway is well above the required 14 db. This means that we can safely use omni directional antennas for the highway. (3) 5

6 3. Sectored antenna Since we are unable to use omni directional antennas with N = 4 in the cities we must examine the possibility of using sectored antennas instead. In GSM a sectored antenna usually covers 10 and in a group of 3 sectored antennas they cover a total of 360. To find the SIR for GMS type of sectored antennas we use [ SIR = 10 log 1 q γ + (q + 0.7) γ Using the previously discovered value of q and γ together with (4) we find that the SIR for the cities is [ ] 1 SIR city = 10 log q γ + (q + 0.7) [ γ ] 1 = 10 log ( ) 3.6 = 17.6 db which is above the requirement of 14 db. This allows us to use sectored antennas in the city areas. 3.3 Voice channels To find the number of voice channels m we use the following formula BW m = Channels 1 SpeechP errf N Sectors The 1 channel is the control channel which we have to make room for. We fill (5) with our numbers for the two cities and the highway and get 8.33 MHz m city = 00 khz 1 8 = 6 channels MHz m highway = 1 8 = 38 channels 00 khz Traffic density By assuming uniform distribution of the subscribers we can use this formula to determine the traffic density T D of each city T D = E P op P ene (1 + Growth)ROT Share Area where E is the traffic per subscriber (call intensity) in Erlangs, P op is the population, P ene is the market penetration, Growth is the annual population growth, ROT is the roll out time in years and Share is the market 6 ] (4) (5) (6)

7 share. Using the values we have found together with (6) we find the traffic density of each city the following way T D Stavanger = ( ) km = Erlangs/km T D Sandnes = ( ) km = Erlangs/km Next we need to determine how much traffic we can serve in each sector with the number of voice channels and our target GoS. The Erlang-B function gives us a starting point GoS = B(E, m) = E m m! m k=1 Ek k! where m is the number of voice channels and E is the call intensity in Erlangs. Using (7) we determine experimentally that, to achieve a GoS of 1% for the given number of voice channels, we will be able to serve the following traffic per sector (T S) 3.5 Cells T S city = Erlangs T S highway = 7.53 Erlangs To find out how many cells our network needs, and thus how many BTSs we need to deploy, we first have to find out how much area a single cell covers. If we assume that the subscribers are distributed uniformly the following formula will give us the answers we need (7) CellArea = T S Sectors T D Applying (8) to the T D and T S values we have found gives (8) CellArea Stavanger = CellArea Sandnes = Erlangs 3 = 3.64 km Erlangs/km Erlangs 3 = km Erlangs/km To find the minimum number of cells we need to cover the given areas we divide the total area on the cell area like this Area Cells = (9) CellArea 7

8 We use this formula to fine the number of cells required for Stavanger and Sandnes 43.4 km Cells Stavanger = 3.64 km = 14 cells 1.0 km Cells Sandnes = km = 6 cells For the highway the cell radius can be much larger since it will not be completely surrounded by other cells. Seeing as the longest distance the GSM specification supports in practical use is 35 km and our highway is 10.7 km we can just use one single cell for the entire stretch of road. 3.6 Equipment Cells highway = 1 cell The minimum number of BTSs needed is T otalcells = = 1 cells/btss Each BTS can only handle Erlangs and the total cell traffic is CellT raffic = 3 T S city = = Erlangs so we do not need to increase the number of BTSs. The UltraWAVE BSC can connect 31 BTSs, which should be enough, but it has a traffic limitation of only 466 Erlangs. The total traffic intensity is T otalt raffic = T otalcells CellT raffic = = 1068 Erlangs which is more than a single BSC can handle. We find the number of BSCs we need by T otalt raffic 1068 = = 3 BSCs T rafficlimit 466 One UltraWAVE X100 MSC can handle 56 BSCs and 500 Erlangs which should be more than enough for our purpose. Using the population numbers, annual growth, market penetration and market share we find that our total number of subscribers will be about at the end of year 4. This is well within the limits of the UltraWAVE X100 HLR. 4 Cost estimate Given the cost of each piece of equipment from Table on page 4 we calculate the total cost T otalcost = = NOK 8

9 18.3 million NOK is the cost of the all the essential GMS network equipment for the cities of Stavanger and Sandnes pluss the highway. This number relies on some variables like expected market share and market penetration which might be hard to predict. The estimate is also very sensitive to the path loss slope values and the estimated traffic per subscriber. References [1] Statistics Norway (SSB). URL [] Temakart Rogaland. Detailed web map of Rogaland with distance and area tools., URL [3] LGC Wireless, Inc. URL 9