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Modulation & Demodulation
Modulation
Modulation - process (or result of the process) of translation the baseband message signal to bandpass (modulated carrier) signal at frequencies that are very high compared to the baseband frequencies.
Demodulation is the process of extracting the baseband message back the modulated carrier.
An information-bearing signal is non-deterministic, i.e. it changes in an unpredictable manner.
Why Carrier?
Effective radiation of EM waves requires antenna dimensions comparable with the wavelength:
Antenna for 3 kHz would be ~100 km long
Antenna for 3 GHz carrier is 10 cm long
Sharing the access to the telecommunication channel resources
Modulation Process
Modulation implies varying one or more characteristics (modulation parameters a1, a2, … an) of a carrier f in accordance with the information-bearing (modulating) baseband signal.
Sinusoidal waves, pulse train, square wave, etc. can be used as carriers
Continuous Carrier
Carrier: A sin[t +]
A = const
 = const
 = const
Amplitude modulation (AM)
A = A(t) – carries information
 = const
 = const
Frequency modulation (FM)
A = const
 = (t)– carries information
 = const
Phase modulation (PM)
A = const
 = const
 = (t) – carries information
Amplitude Shift Keying (ASK)
Pulse shaping can be employed to remove spectral spreading
ASK demonstrates poor performance, as it is heavily affected by noise, fading, and interference
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Differential Modulation
In the transmitter, each symbol is modulated relative to the previous symbol and modulating signal, for instance in BPSK 0 = no change, 1 = +1800
In the receiver, the current symbol is demodulated using the previous symbol as a reference. The previous symbol serves as an estimate of the channel. A no-change condition causes the modulated signal to remain at the same 0 or 1 state of the previous symbol.
DPSK
Differential modulation is theoretically 3dB poorer than coherent. This is because the differential system has 2 sources of error: a corrupted symbol, and a corrupted reference (the previous symbol)
DPSK = Differential phase-shift keying: In the transmitter, each symbol is modulated relative to (a) the phase of the immediately preceding signal element and (b) the data being transmitted.
Pulse Carrier
Carrier: A train of identical pulses regularly spaced in time
Pulse-Amplitude Modulation (PAM)
Modulation in which the amplitude of pulses is varied in accordance with the modulating signal.
Used e.g. in telephone switching equipment such as a private branch exchange (PBX)
Pulse-Duration Modulation (PDM)
Modulation in which the duration of pulses is varied in accordance with the modulating signal.
Deprecated synonyms: pulse-length modulation, pulse-width modulation.
Pulse-Position Modulation (PPM)
Modulation in which the temporal positions of the pulses are varied in accordance with some characteristic of the modulating signal.
Ultra-Wideband (UWB) Systems
Radio or wireless devices where the occupied bandwidth is greater than 25% of the center frequency or greater than 1.5 GHz.
Radio or wireless systems that use narrow pulses (on the order of 1 to 10 nanoseconds), also called carrierless or impulse systems, for communications and sensing (short-range radar).
Radio or wireless systems that use time-domain modulation methods (e.g., pulse-position modulation) for communications applications, or time-domain processing for sensing applications.
Demodulation & Detection
Demodulation
Is process of removing the carrier signal to obtain the original signal waveform
Detection – extracts the symbols from the waveform
Coherent detection
Non-coherent detection
Coherent Detection
An estimate of the channel phase and attenuation is recovered. It is then possible to reproduce the transmitted signal and demodulate.
Requires a replica carrier wave of the same frequency and phase at the receiver.
The received signal and replica carrier are cross-correlated using information contained in their amplitudes and phases.
Also known as synchronous detection