LTE MOBILE COMMUNICATIONS
This section presents some specific aspects related to the LTE cellular networks planning in Xirio. The LTE and LTE-A characteristics and the complexity of its radio interface require planning this systems differently than other mobile systems.
Proper LTE planning involves the analysis of both the network coverage and the system's capacity against the users traffic demand. The presence of a greater traffic load limits the system behavior since the radio resources to meet the demand are limited. Likewise, a higher level of interference is generated between cells as the charge increases.
When starting from known traffic loads, the LTE network simulation can be performed statically, obtaining coverage and interference levels throughout the service area. If there are not real data available on the traffic load, it is necessary to make a capacity study to determine them.
The capacity studies can be based on random traffic generators (Monte Carlo type), however, this type of simulations waste computing time and do not always faithfully recreate the system behavior. The capacity calculation introduced by Xirio is an evolution within the current simulations of mobile communications focusing on obtaining an overall average network behavior in short calculation time.
In Xirio the following LTE calculations can be performed according to the user requirements:
•Static coverage of the network. As with 3G mobile communications planning, if you do not wish to undertake the study of interference and capacity, it is possible to obtain estimates of network coverage through a simplified study based on classical techniques, ie link balances and propagation attenuation calculations. This analysis has some limitations, and its results are less accurate.
•Static analysis of SINR interference (Signal-to-Interference-plus-Noise Ratio). Initially the traffic load supported by each base station is not known. In this way a static analysis of SINR interference is carried out from the traffic load configured in the sectors LTE parameters.
This calculation provides a rough idea of the network coverage under user-defined load conditions through a coverage spot and without the need for capacity analysis.
•Capacity analysis. From a user distribution in environments and the static analysis of previous SINR interferences, Xirio obtains an approximate result of the network resources demand.
Then, steps of access network planning process are listed, highlighting in each of them, and when necessary, the points to which the user must pay special attention in these networks planning.
NEEDS ANALYSIS - CONFIGURATION
•Cartography. Depending on the service area for the network on project, this type of studies may require the use of rural, urban or mixed cartography.
When the stations are located in rural areas, terrain models of 100 and 25 meter resolution are often used. For simulating the effect of multipath losses in the coverage of these stations over urban areas, it is usually used a morphography layer in which the user can include additional losses associated with each urban area.
If the stations are located within an urban area, is advisable to use urban mapping for radio planning. It is adequate a terrain model of between 1 and 2 meter resolution. In this case it would not be added any additional losses to the propagation.
Finally, it is possible to use multiple layers of altimetry to perform mixed calculations, where the station is located in proximity to an urban center and the user wants to analyze the propagation toward its interior.
•Calculation methods. Different mobile communication technologies work in very different frequency bands and require many customisations when defining a calculation method applicable to their planning. Here are some proposals for specific environments and technologies:
oRural environment. Planning in such environments is usually performed using deterministic methods such as UIT-R Rec.P.526 or Deygout method, provided that cartography of the rural area where the network is deployed is available. In these cases it is necessary to consider that the simulation results assure the levels exceeded in 50% of time and locations, so that, in order to ensure higher percentages it will be necessary to use the "fading margin", which is configurable in parameters of the calculation method.
Together with these methods, and if morphographic layers are available, it is customary to introduce additional losses associated with the terrain, especially to see the effects of multipath in urban environments.
In cases where no precise morphographic layers from the service area are available, it is possible to use empirical methods such as Okumura-Hata, particularized for rural settings.
oUrban environment. Planning for urban or mixed areas (rural-urban) can be done by empirical methods (the Okumura-Hata method is an adequate approximation of the range of the signal into a radius from the base station) or deterministic if precise enough urban cartography is available (1 or 2 meters resolution). As a deterministic method, can be used as ITU-R Rec. 1411, Xia-Bertoni or modulated Okumura-Hata method to obtain the signal propagation in city taking into account the diffractions on the buildings roofs, and reflection effects in buildings.
Whether using empirical methods, or detailed urban cartography, the use of corrections due to terrain use (clutter) is discouraged, since it would be introducing redundant correction on estimates of propagation.
Quality objectives for these networks require to ensure a minimum coverage for a certain percentage of time and locations. These minimum percentages are translated into signal margin over the estimated average value in the simulations, which is obtained from a statistical distribution that simulates the effects of fading of the received signal by the mobile terminal in a given propagation environment. Therefore, in order to ensure higher percentages it will be necessary to use the "fading margin" configurable in the parameters of the calculation method.
•Link budget. Since the simplified study based on link budget and calculation of attenuation must be study separately for each transmission path (uplink and downlink), Xirio incorporates into coverage and link studies the ability to select the path under study as well as specific parameters (load factor base station, number of users, etc) in sectors and receivers of LTE communications services for the dynamic calculation of the reception sensitivity from them.
•Stations and sectors. LTE mobile communications networks are typically composed of a stations distribution in a cellular mesh. Each station can consist of one or more sectors. The way to define these entities in Xirio is to define a site as the transmitting station location and as many transmitter as sectors. Particularly for third generation mobile communications is necessary to specify parameters like bit rate, load factor, etc. to establish a dynamic sensitivity that characterizes calculation.
•Best server by signal. The user has to give the criteria that Xirio will use to determine the desired sector (best server) at each location. This will be the sector to which the mobile would be linked at every location in normal conditions. The usual approach is to work with a better server by signal, that is, the mobile would be linked to the transmitter which provides the highest signal level.
The result obtained is a static signal level spot. In scenarios with low traffic demand, for example in rural environments, the result obtained may suffice. Otherwise a study of the network interference and capacity will be necessary.
NETWORK INTERFERENCE, SINR
•LTE network specific parameters. To obtain a static signal spot based on link balances, it is enough to define some sectors parameters. However, in order to perform an interference calculation, it is necessary to establish how the network stations interrelate with each other.
These specific parameters are configured in the LTE parameters of the multi-coverage study. It is also necessary to configure some parameters of the sectors and the mobile terminal that are not necessary in the basic signal calculation.
If static ICIC is used when setting up the stations it will be necessary to configure each sector with a different ICIC static label. This will mitigate the intercellular interference effect.
This calculation is especially optimized for base stations formed by three sectors. If it is a station composed of a single sector it is advisable to try as far as possible that adjacent cells have different labels.
The LTE interference result consists of global coverage spots along with individual reports and charts for each sector.
•Cartography. In the case of calculations in urban environments it is advisable to use an additional digital elevation model in capacity calculations. Normally terminals distribution in the study scenario varies from indoor to outdoor and it will be necessary to have adequate cartography to take into account this effect. From multi-coverage study can be accessed to configure this cartography.
•LTE capacity parameters. The distribution of users/terminals in the study scenario is done through the environments definition. In each environment it is possible to configure the different user/terminal types with their respective indoor and outdoor densities. The configuration of these LTE capacity parámeters is done from the parameter window of the multi-coverage study.
The capacity result consists of a detailed report and graph of demand for network resources.
Once the capacity calculation is done, it is advisable to obtain the definitive SINR coverage spot. To do this, a second interference calculation must be made, adjusting the sectors traffic load according to the capacity statistics obtained.