On Models for Interference Calculations between Radio Communication Systems

Sammanfattning: When installing a new radio system, it is of great importance to know that it will not interfere with the already existing systems, and that they can coexist side by side. The problems that will follow if interference occurs, can be of huge magnitude, e.g. radars not working, trains stopping etc. The idea of this thesis is to find an analysing method that can take several parameters into account when evaluating the scenario and its possible conflicts, and since many of the radio systems today are digital, give the bit error rate as a result. The problem is to find a way to model both the victim signal and the interfering signal(s), that also takes as many variables as possible into account when doing so. The model should be as general as possible, making it applicable on many different scenarios. Since many of the older radio systems are narrowband systems (e.g. GSM), and many of the newly deployed radio systems are wideband systems (e.g. UMTS, LTE), it is also necessary be able to represent such systems in the same environment. Also, is it possible to replace the modulated interferer with some random process? When a good model to describe the signal and interference was found, simulations were made in Matlab to test it. Variables such as modulation, power, frequencies, losses and filters were all included in the implementation. The results of the simulations show bit error rates, i.e. how much the interfering system affects the victim system, as a function of the geographical distance between them, and also their difference in frequency. In the case with a GSM-R system with a cell radius of 8 km being interfered by a LTE system, the minimum distance from the LTE transmitter to the GSM-R receiver to keep a BER < 10-3 was found to be 1.5 km. If a frequency guard band were used, those minimum distances were lowered to 900 m and 800 m, respectively. Replacing the LTE transmission (OFDM) with a sequence generated from a Middleton Class A process, the same results could not be obtained. The method described in this thesis to evaluate the possible conflict between different radio systems is general enough to be able to handle any kind of systems, together with their specific parameters and design choices.