0195136047.pdf

724 COMMUNICATION SYSTEMS

whereAc,ωc, andφcare the peak amplitude, angular frequency, and phase angle of the mod-

ulated carrier, respectively. Equation (15.3.17) can be viewed as a carrierAccos(ωct+φc)

modulated by a digital signald(t) that is 0 or 1 in a given bit interval. The digital signal and

product device shown in Figure 15.3.14 are then equivalent to the waveform formatter and

modulator. Assuming that the received signalsR(t) differs only in amplitude fromsASK(t), one

can write

sR(t)=

{

Accos(ωct+φc), 0 <t<Tb, for 1

0 , 0 <t<Tb, for 0

(15.3.18)

The product device in the receiver’s demodulator acts like a synchronous detector that

removes the input carrier. The major disadvantage of the coherent ASK is that the required local

carrier must be phase-connected with the input signal. The input to the integrator is a unipolar

PCM signal, and the remainder of Figure 15.3.14 is a PCM receiver [see Figure 15.3.8(a)].

The noncoherent ASK system eliminates the need for a coherent local oscillator. Figure

15.3.15 shows the demodulator and code generator for a noncoherent ASK system. A matched

filter (that has its impulse response matched to have the same form as the carrier pulse at its input)

and envelope detector take the place of the synchronous detector and integrator of Figure 15.3.14.

While ASK systems are not as widely used as other systems for various reasons, the ASK

concept remains significant, particularly as applied to modern optical communication systems

which use intensity modulation of a light source.

Sampler

Sample at time Tb for bit interval 0 < t ≤ Tb

A cos (ωct + φc)

(local carrier)

Ac cos (ωct + φc)

Broad-band system noise

Digital

signal d(t) sASK(t) sR(t)

R ≥ 0 choose 1

< 0 choose 0

VT =

A^2 Tb

4

Σ Σ

+

+

+

−

∫ (.) dt D

Waveform formatterChannel Demodulator and code generator of receiver

and modulator

Tb

0

Figure 15.3.14Functions of a coherent ASK communication system.

Sample at time Tb for

bit interval 0 < t ≤ Tb

≥ 0 choose 1

< 0 choose 0

VT

A^2 Tb

sR(t) plus noise

4

Σ

+

−

Envelope

detector

Matched

R filter D

Figure 15.3.15Demodulator and code generator for a noncoherent ASK system.