Wireless communication

Lecture 13

Amplitude Modulation

 

Overview:

 

Typical AM Communication System


Transmitter:

Data:  (voice, analog, digital) is assumed to be bandlimited to fdata (for voice, for instance, this is 300 - 3300 Hz).  This signal is BASEBAND. 

Baseband Filter: limits data to fdata (removing noise, and limiting the band of the data)

Local Oscillator (at the transmission frequency, 1900 MHz for digital phones, for instance)

"Up-converter":  The mixer in this case is called an "up-converter" because it increases the frequency of the signal to something that can be transmitted.  The mixer is a frequency multiplier that produces the sum and difference frequencies (and also all of their harmonics)

Power Amplifier:  High frequency amplifier to increase power output to the antenna.  This will produce the "transmitted power".  It does not have to be "low noise", as the channel will add so much noise that this extra is not critical. 

Antenna:  Transmission antenna. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Receiver:

 


Receive Antenna:

Low Noise amplifier:  The received signal is very low (-100 dBm, for instance).  The amplifier cannot add too much noise at this point, because it will not be dispersed by the environment and will go directly into the system.  This is also a high frequency amplifier, and generally one of the most expensive components of the system.

Receiver local Oscillator:  The local oscillator could take the data straight to the base band at this point, but usually it is taken to an intermediate frequency to be filtered and amplified

IF Filter and AMP:  Filter and amplify intermediate frequency.

Detector: regain original data

 

Typical FM Communication System

 

Transmitter:

 

(Frequency of signal varies according to the magnitude of the message signal (m(t))

VCO = voltage controlled oscillator… change in input voltage changes output frequency.

 

Slope Detector Receiver

 

 

Slope filter has a gain that is proportional to frequency (magnitude varies linearly with frequency). The output of the slope filter is the differentiation of the received signal.  The output of the envelope detector includes the original signal and a DC offset that is proportional to the carrier frequency. 

 

Comparison of AM and FM modulations:

                                                AM                                                      FM

Information stored in:    amplitude of the signal              frequency of the signal

 

Noise immunity

            Atmosperic       worse                                                   better

            Burst                worse                                                   better

            Fading              worse                                                   better

            Phase               better                                                   worse

 

Bandwidth use fixed                                                     adaptable

            (modulation index)

 

Power TX/RX              depends on signal amplitude                  fixed

Amplifiers                     Class A or AB                                      Class C

                                    (30-40% efficiency)                              (70% efficient)

                                    half the battery life

 

co-channel interference worse                                       better

                                                                                                (capture effect… picks up the

strongest signal but not weaker)

 

RX weak signals                       better                                       worse

 

Frequency Band                       Less                                         more

 

Equipment complexity   simple                                       complex