M Y R E V E A L - C E L L U L A R

Transcription

1 M Y R E V E A L - C E L L U L A R The hexagon cell shape If we have two BTSs with omniantennas and we require that the border between the coverage area of each BTS is the set of points where the signal strength from both BTSs is the same, we obtain a straight line. If we repeat the procedure placing 5 more BTSs around the original one, the obtained coverage area, i.e. the cell, has a hexagonal shape. CELL PLANNING Introduction Every cellular network needs cell planning, in order to ensure coverage and avoid interference. The cell planning process consists of many different tasks, all together making it possible to achieve a well working network. Definitions Some definitions are important to understand, before going deeper into the cell planning process: Radio coverage Received signal strength in the MS (from the BTS) above a chosen value. Cell The area that is covered from a BTS. Omni cell A cell with an omnidirectional BTS antenna system. Sector cell The hexagons have become a symbol for cells in a radio network. Real world planning must, however, consider the fact that radio propagation is very much dependent on terrain and other factors, and that hexagons are extremely simplified models of radio coverage patterns. Still, the first geometrical plan based on hexagons (the nominal cell plan) gives a good view when planning a system. CELL PLANNING PROCESS Cell planning can briefly be described as all the activities involved in determining which sites should be used for the radio equipment, which equipment should be used, and how the equipment should be configured. To ensure coverage and to avoid interference, each cellular network needs planning. The major activities involved in the cell planning process are represented in Figure 1-1. A cell with a (uni-) directional BTS antenna system. Site The geographical location where the RBS equipment is stored, and the BTS antennas are mounted. 3 sector site A site with equipment for three sector cells. So what is the maximum size of a cell? Well, there are limiting factors for how big an area a base station can cover. A crucial factor is the ability for the sent burst from the MS to arrive in the intended time slot at the base station. This depends on the relation between how far away the MS is, and the timing advance parameter. With 8 time slots per carrier a maximum distance between the base station and the cell border is 35 km. 4 time slots per carrier extends the allowed distance to 72 km. [1] TRAFFIC AND COVERAGE ANALYSIS

2 (SYSTEM REQUIREMENTS) The cell planning process is started by a traffic and coverage analysis. The analysis should produce information about the geographical area and the expected capacity need. The different types of data collected are: Cost Capacity Coverage Grade of Service (GoS) Available frequencies Speech Quality Index System growth capability The traffic demand (that is, how many subscribers access the system and how much traffic is generated) provides the basis of cellular network engineering. The geographical distribution of the traffic demand can be calculated using demographic data, such as: Traffic per subscriber is calculated with the Erlang formula, as below: Population distribution Car usage distribution Income level distribution Land usage data Telephone usage statistics Other factors, such as subscription charges, call charges, and costs of mobile stations Traffic calculations The input for the traffic calculations is mentioned above. The output should be information about how many sites and cells are needed. In order to be able to decide this, the available number of frequencies per cell, as well as the Grade Of Service (GOS), have to be known. Available number of frequencies per cell can only be decided when knowing which cell pattern should be used; (see Figure 104 and Figure 105). Then, the total number of available frequencies are evenly divided into frequency groups. Which cell pattern to choose depends on the type of system, as it is based upon frequency re use distance. This will be explained below (see Frequency re use). GOS is defined as allowed percentage of unsuccessful call set ups due to congestion. Normally, a value between 2% and 5% is applicable in mobile telephone systems. The Erlang table is used when wanting to find out the third factor, when knowing two of the three factors: number of traffic channels, traffic (in Erlang) and GOS. Example of traffic calculation Input data: Traffic per subscriber: 25 me; Number of subscribers: ; Number of available frequencies: 24; Cell pattern: 4/12 (12 frequency groups); GOS: 2%. How many 3 sector-sites are needed? frequencies per cell = 24/12 = 2 frequencies traffic channels per cell = 2 x 8-2 (control channels) = 14 TCH traffic per cell = 14 TCH, 2% GOS Æ 8.2 E/cell (use the Erlang table) subscribers per cell = 8.2 E / E = 328 subscribers per cell needed number of cells = / 328 = 30 cells needed number of 3 sector sites = 30 / 3 = 10! The Answer Frequency re use A fundamental principle in the design of cellular systems is the frequency re use patterns. Frequency re use is defined as the use of radio channels on the same carrier frequency, covering geographically different areas. These areas must be separated from one another by a sufficient distance, in order to avoid co channel interference. Based on the traffic calculations, the cell pattern and frequency plan are worked out. Not only for the initial network but with the possibility to adapt smoothly to the demands of traffic growth.

3 Interference C/I The carrier to interference ratio (C/I) is defined as the ratio of the level of the received desired signal to the level of the received undesired signal. Therefore, a reduction in the number of frequency groups would allow each site to carry more traffic, reducing the total number of sites needed for a given traffic load. However, decreasing the number of frequency groups and reducing the frequency re use distance will result in a lower average C/I distribution in the system. There are three types of frequency re use patterns: 7/21, 4/12 and 3/9. Only 4/12 and 3/9 are interesting for CME 20. In all three cases the site geometry has the following features: Three cells (sectors) at each site. The antenna pointing azimuths of the cells are separated by 120 degrees and the cells are arranged with antennas pointing at one of the nearest site locations thus forming cells in a cloverleaf fashion. Each cell uses one 60 degree transmitting antenna and two 60 degree diversity receiving antennas with the same pointing azimuths. Each cell approximates the shape of a hexagon. This C/I ratio is dependent on the instantaneous position of the mobile and is due to irregular terrain and various shapes, types and numbers of local scatterers. Other factors such as antenna type, directionality and height, site elevations and positions, and the number of local sources of interference also affect the distribution of the C/I ratio in a system. GSM states C/I > 9dB, with frequency hopping implemented, and recommends C/I > 12dB when frequency hopping is not employed. C/A The carrier to adjacent ratio (C/A) is defined as the relation in db in signal strength between the serving and an adjacent frequency. In GSM, a multiple of 200 khz away. GSM specifies C/A > -9dB. We assume that the traffic is homogeneously distributed within the cells. The cell size is normally given in terms of the distance between two neighboring sites. The cell radius R (= the side of the hexagon) is always one third of the site to site distance when 3 sector sites are used. A group of neighboring cells using all the channels in the system, but not re using them, according to the patterns described below is called a cluster. The 4/12 cell pattern uses 12 frequency groups in a 4 site re use pattern. Cell patterns The distribution of the C/I ratio desired in a system determines the number of frequency groups, F, which may be used. If the total allocation of N channels is partitioned into F groups, then each group will contain N/F channels. Since the total number of channels (N) is fixed, a smaller number of frequency groups (F) would result in more channels per set and per cell. The 3/9 cell pattern uses 9 frequency groups in a 3 site re use

4 pattern. Example of how to divide the available frequencies into frequency groups: 24 frequencies in a 3/9 cell pattern It should be noted, that when using 3/9, there will be adjacent channels in neighboring cells, which gives lower C/A values. To see this, the example can be compared with Figure 3/9 Cell Pattern above. Cells with frequency groups A1 and C3 are neighbors, as well as A2 C1, and A3 C2. [2] NOMINAL CELL PLAN Upon compilation of the data received from the traffic and coverage analysis, a nominal cell plan is produced. The nominal cell plan is a graphical representation of the network and it simply looks like a cell pattern on a map. However, there is a lot of work behind it (as previously described). Nominal cell plans are the first cell plans produced and these form the basis of further planning. Quite often, a nominal cell plan, together with one or two examples of coverage predictions, is included in tenders. Coverage and interference predictions are usually initiated at this stage. Such planning needs computer-aided analysis tools for radio propagation studies. [3] SURVEYS (AND RADIO MEASUREMENTS) The nominal cell plan has been produced and the coverage and interference predictions have been roughly verified. Now, it is time to visit the sites where the radio equipment is to be placed and perform radio measurements. The former is important because it is necessary to assess the real environment to determine whether it is a suitable site location for a cellular network. The latter is even more important because better predictions can be obtained using field measurements of the signal strengths in the actual terrain where the mobile station is to be located. Site surveys Site surveys are performed for all proposed site locations. Many issues have to be checked and verified, such as: Exact location Space for equipment, including antennas Cable runs Power facilities Contract with owner Also, the radio environment has to be checked, so that there is no other radio equipment on the site that will cause intermodulation problems, or too high buildings surrounding the possible site. Radio measurements Radio measurements are performed to be able to adjust the parameters used in the planning tool to reality, to the specific climate and terrain in the area of interest. Parameters used in Sweden, would be different to the ones to be used in a tropical country, for

5 example. A test transmitter is mounted, and then the signal strength is measured while driving around in the area. Back in the office, the results from the measurements can be compared with the values the planning tool produces when simulating the same type of transmitter, and the parameters for the planning are adjusted to match reality. [4] SYSTEM DESIGN After optimization and when the predictions generated by the planning tool can be considered reliable, a dimensioning of the RBS equipment, BSC, and MSC is performed. The final cell plan is produced. As the name implies, this plan is later used at system installation. In addition, a document called Cell Design Data (CDD) containing all cell parameters for each cell is completed. nodes are tested for full functionality on their own! this is called installation test. Secondly, the interworking function is tested! this is called integration test. The two tests together is called the network element test, which is further explained below. Network Element Tests The picture below shows the main process steps of the Network element test of the BSC and RBS. [5] IMPLEMENTATION System installation, commissioning, and testing are performed following the final cell planning and system design. Installation Engineering [6] SYSTEM TUNING Once the system has been installed, it is continually evaluated to determine how well it meets the demands. This is called system tuning and it involves: A check that the final cell plan has been implemented successfully An evaluation of customer complaints Figure 1-2 illustrates the main steps of the implementation of a new radio site. The output from the system design step in the cell planning process results in a hardware order (for example, BSC, RBS) to the factory. Installation engineering personnel perform site investigations, which means taking a closer look at the actual location where the site is to be built. This results in an installation documentation, which is put into a binder for each site. The installation documentation contains all information needed to build the site, for example, floor plan, cable drawings, antenna arrangement drawing, grounding plan, site material list, etc. The material needed to build the site is then ordered according to the installation documents. A check that the network performance is acceptable Changing parameters and undertaking other measures (if needed) The system needs constant re-tuning, due to the fact that the traffic and number of subscribers continuously increase. Eventually, the system reaches a point where it must be expanded so that it can manage the increasing load and new traffic. At this point, a coverage analysis is performed and the cell planning process cycle starts all over again. When all equipment has arrived the installation can begin. After installing the equipment, it is time to check its functionality. Firstly, the