In this paper a method combining different types of simulation is used to evaluate transient interferences over a digital communication system at its frequency band. The method combines different simulation stages; the first one is an electromagnetic simulation, with FDTD numerical method, which is carried out to establish the level of the interference coupled to the digital communication system; at the second stage, the simulation of the transmitted data at the communication system is performed introducing the EM interference calculated at the first stage. The result of this co-simulation will be a BER and other useful indicators to quantify the degradation produced by the transient interference. To show the effectiveness of the co-simulation method first of all the method is validated comparing the results with other evaluation techniques, measuring a real interference over an RFID system. The RFID system is according to the ISO 14443 type B standard, and the interference generated is a burst according to 61000-4-4. Once the co-simulation method is validated, the co-simulation system is used to quantify and decide if an EMC redesign technique of the previous scenario is efficient enough to avoid degradation at the RFID system produced by the burst transient interference.