s types of terrain. The Modulation by digital Output Fading signal converter processing stage simulator Frequency hopping Signalling synthesizer Power shaping Fading level control profile Hopping sequences Figure 61.25 Principle of a typical signal generator The radio path or Physical Layer (Layer 1) 61/27 I/Q generation Bitsequence Gaussian filter Integrator (FSK Signal) digital signal processing Figure 61.26 Use of digital signal processing Figure 61.28 Principle of a fading simulator Table 61.14 Fading simulator test requirements Designation (km/h) Speed (km/h) Rural area (RA) Hilly terrain (HT) Figure 61.27 Some considerations for signal generators: (a) I/Q imbalance due to unequal gains in the two Typical urban (TU) modulation paths; (b) skew since phase difference between Equalizer test profile (EQ) the carriers is not 90°; (c) residual carrier components i.e. residual unmodulated carrier in the output signal Typical fading simulator requirements for GSM receiver tests area effects of the speed of the vehicle must also be built into the is given in Table 61.14. simulation model. The block diagram of Figure 61.28 shows the principle behind a fading simulator: 61.10.1.3 Sensitivity measurements 1. The signal is split and fed along several paths. The sensitivity of the receiver is a crucial GSM parameter, as it is 2. Each path contains simulated loss, delay and spurious modula- for other types of radio system. As measurements directly after the tion (Doppler effect). demodulator are, in general, not possible and would anyway give 3. The signal is recombined. different results for different designs, GSM test methods which do 61/28 Telecommunication system measurements not involve opening the radio set have been defined. The loop back method, in effect, makes the radio part of the test set up. If the decoder in the receiver detects an errored telegram (wrong parity), this fact is reported to the test set by the radio's own transmitter. This is indicated by the bad frame indication bit (BFI) in the transmit r.f. 