SFN INTERFERENCE PARAMETERS
The operation of SFN (Single Frequency Networks) requires special treatment since the different stations are broadcasting the same program in unison and signals presence from several sources at a point could be considered to be constructive or destructive interference depending on delays in different signals with respect to the dominant one. .
The interference that elements of a SFN network create for other types of networks is evaluated using normal method. In the same way, interference that elements from other networks cause on the SFN network is evaluated using normal method. However, the contribution of different network elements on a SFN network receiver is considered in accordance with following criteria:
•The delay of the interference different sources regarding to the dominant signal is evaluated. This delay is a function of difference in distances from the transmitters to reception point and also of transmitters relative delays ("delay" parameter in the transmitter element).
•The transmitter which signal imposes the reference symbol, from which the guard interval starts. This transmitter can be:
o"Beginning best server". The desired transmitter itself.
o"Beginning first pre-echo strong enough". The transmitter that, reaching the receiver before best server, puts a signal level sufficiently high with regard to the best server. It is considered that the pre-echo signal level is sufficiently high when it is inside a margin with regard to highest signal level imposed by the user. To set the window beginning all pre-echoes calculated both with the main propagation model and complementary one will be assessed. However, calculation of the desired signal (C) only will take into account signals calculated with the main propagation model.
o"Beginnig first pre-echo stable strong enough". The transmitter that, reaching the receiver before best server, puts a signal level sufficiently high with regard to the best server. It is considered that the pre-echo signal level is sufficiently high when it is inside a margin with regard to highest signal level imposed by the user. To set the window beginning only pre-echoes calculated with the main propagation model will be assessed.
The choice of the guard interval beginning depends on the receiver type to simulate. The "first pre-echo strong enough" assure higher constructive interference possibilities in most cases. The next comparative graphic clarifies this aspect:
Depending on delays, the application evaluates whether the interference produced by each transmitter of the SFN is constructive or destructive. The method used to determine interference produced by a transmitter, as well as protection ratio that should be used in each case depends on the service (DAB, Fixed DVB or Portable DVB) and the signal delays respect the beginning of the guard interval window.
•C (constructive part) = Wi * Ni (Interference level)
•I (destructive part) = (1-Wi) * Ni (Interference level)
•If the study service is DAB, Wi will be calculated as:
•Ts is the symbol interval.
•Tg is the guard interval.
•Tf is the equalization interval.
•Tu is the useful time that is obtained as Ts - Tg.
If the service is fixed DVB the calculation Wi is based on the following formula:
The protection ratio will be calculated as the effective protection ratio (EPT) using the following formula:
In the mobile DVB service, the calculation of Wi is the same as in fixed DVB. The EPT calculation will use the following equation:
The last section allows to define DVB-T2 parameters:
•MISO Group (Multiple Input - Single Output): Two groups of transmitters simultaneously broadcast two different versions in time and frequency of desired signal. This improves the system SNR and, consequently, the coverage and speed of network data.
•Consider TFS gain (Time-Frequency Slicing): Several statistically multiplexed services are transmitted over more than one RF channel, providing additional gain.