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At a uplink site, there would be a transmitter transmitting hundreds of watts and a receiver trying to receive micro volts. If this was done on a common frequency it would be impossible to stop the receiver "receiving" the output of the local transmitter, so if they are on different frequencies, filters can be used to do this.
Frank
I get the boxing analogy, but the frequencies could be the same...after all, this is how radar works...the send frequency is the same as the received frequency.....you simply time domain multiplex so you're either sending or receiving and not both at the same time. -This has the advantage of only needing one antenna.
2 frequencies allow duplex (sending and receiving at the same time), 1 frequency allows simplex (you're either sending or receiving but not both). If GSM was like radar, one person would talk (like a radar ping) and the other would listen. Once the talker stopped, the listener could then talk (the echo) and the original talker must listen. 2 way communication is possible but not in a manner that people have grown accustomed to. In fact, the early mobile phones were simplex. Technology evolved to allow mobile communications to mimic land-line operation.
Actually, it's a bit more complicated than that, GSM is also time multiplexed. It works on the principle that several users can be on the same frequency simultaneously but each is allocated a time slot to transmit in. A central 'time slot assigner' carrier issues the slots to users in range so they can individually identify themselves and then communicate as necessary.
The main reason for the two frequencies has been explained, a diplexer in the phone and at the cell base station keeps the two frequencies apart from each other to allow full duplex operation, no matter which station is using the time slots. I like the boxing analogy!
The uplink dish is pointed toward a specific satellite and the uplinked signals are transmitted within a specific frequency range, so as to be received by one of the transponders tuned to that frequency range aboard that satellite.
The transponder 'retransmits' the signals back to Earth but at a different frequency band (to avoid interference with the uplink signal), typically in theC band and/orKu band
C-band transmission is susceptible to terrestrial interference while Ku-band transmission is affected byRain fade (as water is an excellent absorber of microwaves).C band is defined as frequency band between 0.5 and 1 GHz (0.3 and 0.6 m).
The Ku band is a portion of theElectromagnetic spectrum in the Microwave range of frequencies ranging from 10 to 18 Hertz.
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