0195136047.pdf

15.3 DIGITAL COMMUNICATION SYSTEMS 727

t

0

Digital signal d(t)

Phase of si(t)

(a)

(b)

0

0

π

−

+

π

Initial level

11100001 1 110 0

Example digital bit sequence

1

− 1

t

t

t

t

a(t − Tb)

Phase of si(t − Tb)

Sign of sd(t)

1

− 1

t

a(t)

1

− 1

Figure 15.3.18(a) Example sequence

of message binary digits and modulator

waveforms in DPSK.(b)Phase and polar-

ity relationships as applied to DPSK mes-

sage recovery.

Sampler

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

A cos (ω 1 t + φ 1 )

A cos (ω 2 t + φ 2 ) ≥ 0 choose 1

< 0 choose 0

Σ

+

−

Σ

+

+

D

∫ (.) dt

∫ (.) dt

Digitalsignal d(t) VCO

Broad-band system noise

Modulator Channel Demodulator and code generator of receiver

sFSK(t) sR(t)

R

Tb

0

Tb

0

Figure 15.3.19Functions of a coherent FSK system.