thus supporting four conversations in a single transmit receive channel in gsm the remainder of the time not used by a particular mobile for transmission and reception is available for the mobile 39 s receiver to scan other channels in order to determine for example when it should request a handover to a stronger base station this process is known as mobile assisted handover the transmit pulse formats are defined by the gsm specifications and include a guard time between pulses an up down ramping time to turn the transmitter power smoothly on and off to avoid spectral splatter into unsynchronized adjacent channels some guard bits at each end of the burst and the main body of the tdma burst which transmits 114 data bits 26 sync bits and two control bits at a flat power level the up down ramping times are adequate to prevent spectral spreading into an adjacent channel 200 khz away but are not designed to control harmonics of the 220 hz pulse repetition rate in the audio band of 300 hz to 4 khz absent the special transmission mode described below these harmonics cause audible noises in a hearing impaired user 39 s hearing aid fig2 shows an exemplary modified gsm reception and transmission format designed to control harmonics of the frame frequency the receive slots are the same as those illustrated in fig1 but the transmit slot timing has been centered midway between receive slots this permits a relatively slow almost sinusoidal up.