n. Measured figure = Nn Overload 20/15 System band limiting Channel stop filter filter frequency f(c) Channel pass filter frequency f(c) SYSTEM SHAPED WHITE NOISE NOISE RECEIVER GENERATOR Figure 20.14 System loaded noise test The Noise Power Ratio (NPR) is given by 10log(Ns/Nn) dB. The total noise power falling into the channel can be determined Thermal noise Intermodulation noise from Equation 20.31, where N is the total weighted noise contribu- tion over 4kHz in the specified channel in dBmOp, k = B/4N where B is the bandwidth of the system bandwidth limiting filter in kHz, and N is the number of channels. N C = - NPR - 18.6 10 log dBmOp (20.31) The term 10log k is to correct for the fact that the actual traffic load has gaps between the channels and is not continuous. The total noise power is also given in pWOp by Equation 20.32. (20.32) Antilog Traffic loading (relative to nominal) Several channels are selected for measurement over the band by varying the frequency f(c) of both the channel stop filter in the Figure 20.15 Noise power ratio variation for change in transmitter and the channel pass filter in the receiver. transmission loading Also of interest is to vary the channel loading factor L (nominally -15dBmO) and determine the NPR for various conditions of traffic load. This is a measure of the system's robustness to peak loads and channel loading factor (see Table 20.6); and n is the number of transmission level changes outside the normal design guides. The active channels. normal expected NPR curve is shown in Figure 20.15. As the transmission load is lowered then the basic or Gaussian noise predominates. The NPR increases linearly with decreasing traffic power. As the transmission load is increased then intermodu- lation noise predominates and the NPR rapidly rises as more higher order intermodulation products contribute. (20.33) + 9.9 + The optimum working point of the system is easily determined from Figure 20.15 The value of n is derived from assuming that the probability n channels are active in a