MOBILE TELEPHONY CONCEPTS
In spite of the overall complexity of wireless technology, many of the concepts and principals that it is based on are disarmingly simple to understand. This chapter describes some of the major terms and concepts in mobile telephony.
Let’s begin with a fundamental concept that underlies the operation of most forms of radio frequency communication – namely the carrier frequency.
Carrier frequency
Consider a simple radio communication scenario involving two people, Alice and Bob. Let’s assume that Alice is communicating with her friend Bob using a very basic radio frequency transmitter and receiver set. Let’s also assume that Alice is the only one transmitting in the room. The microphone used by Alice converts the air pressure waves of sound coming out of Alice’s mouth into an electrical signal. Alice’s radio frequency transmitter will transmit this electrical signal as a radio frequency waveform through the air. Now assume that Bob is the only one listening in another room. If Bob has a radio frequency receiver, his receiving unit will catch the radio signals coming out of Alice’s transmitter. Bob’s receiver will amplify (i.e. increase the power of) the incoming radio signal using an electronic amplification unit and feed it into the speaker, which will convert the electrical signal into an acoustic pressure wave. Bob will thereby hear Alice’s speech coming out his speaker.
This arrangement basically works fine as long as Alice is the only person talking in that room. If another person say, Charlie begins transmitting his voice by using his radio transmitter, Bob’s receiver might catch both Alice and Charlie’s radio signals and convert them into an essentially indecipherable audio signal.
The concept of a carrier frequency solves this problem.
Figure 69: The concept of Carrier Frequency Modulation (FM)
Let’s assume that both Alice and Bob have purchased a better radio transmitter & receiver that uses the concept of carrier frequency. What this means is that when Alice speaks into the microphone, her new transmitter transmits at a certain “center-line” frequency, known as the carrier frequency. Alice’s FM radio transmitter contains a special electronic device known as a “modulator” which has two inputs and one output. One of its two inputs is the carrier frequency signal used by Alice. The other input is the electrical signal generated by Alice’s microphone. The modulator will combine these two inputs into an output signal in which the carrier frequency signal is “modulated” or nudged up or down slightly around the center-line value, exactly in proportion to variations in the electrical signal produced by Alice’s microphone. Recollect that the variations in the electrical signal produced by Alice’s microphone, are in turn proportional to the changes in the audio frequency and pitch of the words that Alice speaks into her microphone. Therefore, the modulated output signal generated by the modulator now effectively encodes Alice’s speech. This modulated signal is always in the radio frequency band. This “Frequency Modulated” or FM radio signal is transmitted into the air by Alice’s transmitter. Bob’s FM radio receiver has the ability to ‘tune’ into the carrier frequency that Alice’s transmitter is transmitting on. Therefore Bob’s receiver will “catch” this signal transmitted by Alice. Bob’s FM radio receiver also contains a special kind of electronic device known as a “demodulator” which works exactly in the opposite fashion as the “modulator” device installed inside Alice’s transmitter set. The demodulator has two inputs and produces one output. One of its inputs is the carrier frequency signal shared by Alice and Bob’s transmitter-receiver units. The other input into the demodulator is the modulated radio frequency signal received by Bob’s receiver. The demodulator simply subtracts out the carrier frequency component from the modulated signal and produces an output signal which is a proportionate replica of the electrical signal produced by Alice’s microphone. This signal, which encodes Alice’s speech is amplified and fed into Bob’s speaker which in turn reproduces Alice’s speech.