Analog and Digital Communication Notes

Analog and Digital Communication Notes

Analog and Digital Communication

Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator. A device that can do both operations is a modem (a contraction of the two terms).

In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a constellation diagram.

In analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.

Analog modulation techniques

Angular modulation

Phase modulation (PM)

Frequency modulation (FM)

Amplitude modulation (AM)

Single-sideband modulation (SSB, or SSB-AM), very similar to single-sideband suppressed carrier modulation (SSB-SC)

Vestigial-sideband modulation (VSB, or VSB-AM)

Sigma-delta modulation (∑Δ)

Digital modulation techniques

Any form of digital modulation necessarily uses a finite number of distinct signals to represent digital data.

In the case of PSK, a finite number of phases are used.

In the case of FSK, a finite number of frequencies are used.

In the case of ASK, a finite number of amplitudes are used. This is very similar to pulse code modulation

Pulse modulation

These are hybrid digital and analogue techniques.

Pulse-code modulation (PCM)

Pulse-width modulation (PWM)

Pulse-amplitude modulation (PAM)

Pulse-position modulation (PPM)

Pulse-density modulation (PDM)

Miscellaneous techniques

The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.

Adaptive modulation

Trellis coded modulation (TCM) also known as trellis modulation

 

Generation and Detection of Am

Introduction to Modulation

Basic concept is to vary a carrier signal c(t) = Ac cos (2πfct relative to an analog (information) wave form m(t) orbits {bi}.

Analog modulation varies the amplitude (AM), frequency (FM), or phase (PM) of the carrier c(t).

Digital modulation varies the amplitude (M-AM), phase (PSK), pulse characteristics (PAM), or amplitude and phase (MQAM) of the carrier.

Amplitude Modulation

Amplitude modulation varies the carrier amplitude according to an analog information signal m(t)

•In standard AM modulation, a constant term is added to the information signal to yield the transmitted signals(t)=Ac[1+kam(t)] cos(2πfct).

•The constant term greatly simplifies demodulation but is wasteful of power.

•The envelope of the transmitted signal is a(t)=Ac|1+kam(t)|.

•If |kam(t)|<=1t then a(t) is always nonnegative, which simplifies demodulation (can demodulate envelope only), but hurts SNR.

 

•The spectrum of modulated signal is S(f) = .5 kA[M (f−fc) + M(f+fc)] + .5 A[ (f−fc) + δ(f+fc)]

•The percentage modulation of the signal is defined as max t[100|kam(t)|].

•The band width of the modulated signal is twice that of the information signal.

Generation of AM Waves

• Multipliers difficult to build in hardware (at least circa 1920)

• AM waves typically generated using a nonlinear device to obtain the desired multiplication

• Square law modulator sums carrier c (t) and information m (t) signals, and then squares the m using a nonlinear device. Unwanted terms are filtered out with a band pass filter.

• Switched modulation sums c (t) and m (t) then passes sum through a switch, which approximately multiplies it by a periodic square wave. This generates the desired signal plus extra terms that are filtered out.

Detection of AM Waves

• AM detection typically entails tradeoffs between performance and complexity (cost).

• Square law detector squares the received signal followed by a low pass filter. This detection is simple but introduces an unwanted distortion term proportional to m2(t).

• Envelope detector is a simple circuit for AM detection consisting of resistors, a capacitor, and a diode. It only works when |kam(t)| <= 1t (Can’t detect sign change). The RLC circuit must track envelope but not the carrier (f−1c<<

Main Points:

• Modulation is the process of encoding a message signal orbits into a carrier signal.

• AM modulation modulates the amplitude of the carrier wave form with a message signal.

• A constant term is added to the message signal to simplify demodulation: this is wasteful of power and hurts SNR.

• AM waves are typically generated using nonlinear devices.

• AM waves can be demodulated using nonlinear devices with some distortion.

• Envelope detectors are simple cheap devices to detect AM waves with no distortion, but they have poor SNR.

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