Bumblebee Killer

Abstract: A common problem in GSM terminals is an interfering signal nicknamed the “Bumblebee”. This interference is generated by the switching nature of TDMA cellular telephony, the radio circuits are switched on and off with the radio access rate. In GSM, this frequency is approximately 217 Hz, This frequency and its harmonics get into the analog microphone signal, and produce a very annoying periodic noise in the uplink speech. This thesis contains a study of four differerent software solutions, to suppress this interference. These four different methods are denoted, Notch filter, Matched filter, Correlators and Interpolation in frequency. It is shown that the best result is achieved with the Correlatars. Since the frequency comlponents of the Bumblebee are well known, it is possible to estimate the phase and the amplitude of these with Correlators. This is done by correlating the microphone signal with sinusoids having the same frequencies as the Bumblebee, hence the name Correlators. By generating sinusoids with these phase and amplitude estimates, and then subtracting them from the microphone signal, the Bumblebee is suppressed. Since it is subtracted in the time domain it is also possible to consider a recurring pause in the interference caused by the so called idle frame in the transmission.