Why tx frequency is kept higher than rx in repeaters in radio communication?
Answers
Well, what you mean by uplink and downlink depends on the type of communication you are referring to. Here, I am going to discuss about two major domains- satellite communication (satcomm) and mobile communication (mobcomm).
satcomm: downlink- signal from earth base station to satellite
uplink- signal from satellite back to earth
mobcomm: downlink: signal from base station to mobile station (cellphone)
uplink: signal from mobile station(cellphone) to base station
Now, as would have thought, separate frequency bands are always allocated for uplink and downlink signals, often separated by a gap (maybe for future allocation, since the span of 'guard bands' are relatively very less compared to the actual information-carrying bands, e.g., 100 KHz guard bands in case of GSM-900, where the uplink and downlink bands span 25 MHz each).
The main question that this article will be answering is pretty simple: If you observe the uplink and downlink channels carefully, you would notice that the uplink frequencies are higher than the corresponding downlink frequencies in the case of satcomm, whether in the case of mobcomm, it's just the reverse.
satcomm: C-band : U/L-6 GHz, D/L-4 GHz
Ku band: U/L-14 GHz, D/L-12 GHz
mobcomm: GSM-900: U/L-890-915 MHz, D/L- 935-960 MHz
GSM-1800: U/L-1710-1785 MHz, D/L- 1805-1880 MHz
WHY IS THAT SO?
The answer is simple too. It's all about power considerations.
In satcomm, the signals have to cross the atmosphere which presents a great deal of attenuation. The higher the frequency, the more is the signal loss and more power is needed for reliable transmission.
So now you would say why use higher frequencies if signal loss is more and you need more power? It's because lower frequencies get reflected by atmospheric bands and cannot penetrate to get through to the satellite.
Now, a satellite is a light-weight device which cannot support high-power transmitters on it. So, it transmits at a lower frequency (higher the frequency, higher is the transmitter power to accommodate losses) as compared to the stationary earth station which can afford to use very high-power transmitters. This is compensated by using highly sensitive receiver circuits on the earth station which is in the line-of-sight (LOS) of the satellite.