APPENDIX A 



PER-CHANNEL SIGNAL ENERGY AS A FUNCTION OF THE NUMBER OF 



ACTIVE CHANNELS 



The Equivalent Isotropic Radiated Power (EIRP) on a per-channel 

 basis is given by 



W 

 EIRP/„. ^^ , = watts (in watts per channel) (1) 



X + N 



where W = the total EIRP available over the 



400-kHz DCS channel in watts (not 

 in dBm). This is normally 2.5 watts, 



X = the number of equal power users 



transmitting sinmltaneously . The non- 

 equal power users case will be discussed 

 later, and 



N = the ndise-to-signal ratio at the input to 

 the spacecraft receiver. 



This ratio must be converted into a numerical form and not expressed 

 in dB. The noise power is calculated over the entire 400-kHz DCS 

 bandwidth and not over just the 100-Hz DCS data bandwidth. Typical 

 numbers are: noise power over a 400-kHz bandwidth at the input to 

 the spacecraft receiver = -117dBm and the signal energy from a 

 fixed platform is approximately -127dBm. Hence, the noise-to- 

 signal ratio is nominally lOdB which also happens to be a numerical 

 ratio of 10. 



The EIRP as given by equation (1) will be given in watts which must 

 be converted into an equivalent dBm number for most link 

 calculations. 



Applying the above to equation (1) gives,' for typical cases, 



2 5 

 EIRP = xTio watts/channel. (2) 



As can be seen from equation (2), when the number of channels 

 transmitting simultaneously is small, say less than about five, 

 the EIRP per channel is approximately constant at about 0.2 watts 

 (= 23dBm) per channel. When the number of channels transmitting 

 simultaneously is large, greater than say about 20, the EIRP will 

 decrease linearly in watts as the number of channels transmitting 

 increases. 



67 



