无线系统的微波与射频设计9

1. Signal-Sideband Modulation
m(t): bandlimited modulating waveform fM: the maximum frequency of m(t)
m(t) cos 2 fmt 0 fm fM
n0/2: two-sided power spectral density of Gaussian white noise. Contributed by the transmitter channel and noise generated
codes.
§9.1 Analog Modulation §9.2 Binary Digital Modulation §9.3 Error Probabilities for Binary Modulation §9.4 Effect of Rayleigh Fading on Bit Error Rats §9.5 M-ary Digital Modulation
Conclusion: SSB demodulator does not degrade the input SNR.
Ni
2 n0 2
fM
n0 fM
S0 Si N0 n0 fM
2. DSB-SC Modulation
Double-sideband suppressed carrier (DSB-SC)
m
t
1 2
xt
Input narrowband noise power:
Ni
E{n2 (t)}
1 2
E{x2 (t)}
1 2
E{y2 (t)}
E{x2 (t)} E{y2 (t)}
Output noise power:
N0
E{[ 1 2
x(t)]2}
1 4
E{x2 (t)}
Ni 4
The output SNR: S0 Si N0 Ni
carrier wave has the effect on:
➢ ①controlling the radiated frequency spectrum. ➢ ②more efficient use of the allocated RF bandwidth. ➢ ③flexibility in accommodating different baseband
But
NiБайду номын сангаас
2g2
fm
gn0 2
2n0 fM
S0 Si N0 n0 fM
Conclusion: The coherent SSB and DSB-SC demodulators have the same SNR performance.
Microwave and RF Design of Wireless Systems
Chapter 9
Modulation Techniques
Dr. Zhang Yonghong
Comparing with transmitting baseband signal directly, to transmit data by modulating a higher frequency
by the input stages of the receiver.
The demodulator LO is identical in frequency and phase with the modulator LO-synchronous or coherent demodulator.
mt
A 2
cos(2IF
m )t
1 2
x(1
cos
2IF t)
1 2
y(t)
sin
2IF t
v0 (t)
A 2
cos
m
t
1 2
xt
Input signal power: Si A2 / 2
Output signal power:
S0
1 ( A)2 22
A2 8
Si 4
v0 (t)
A 2
cos
vi (t) Acos(IF m )t n(t)
n(t) x(t) cosIF t y(t) sin IF t
v1(t) vi (t) cosIFt
cos cos cos cos / 2 cos2 1 cos 2 / 2 sin 2 2sin cos
A 2
cos
➢ ①more efficient use of the radio spectrum. ➢ ②usually requires less power. CDMA ➢ ③over a fading communications channel. ➢ ④more compatible with the use of error correcting
signal formats.
V (t) Acos(t )
Amplitude Frequency Phase
AM
FM
PM
➢Analog modulation (vary continuously)
➢Digital modulation (change in discrete steps)
In contrast to analog modulation, digital modulation has:
vi (t)
A 2
cos(IF
m )t
A 2
cos(IF
m )t
n(t)
S0 2 Si
N0
Ni
Conclusion: the DSB-SC demodulator improves the input signal-to-noise ratio by a factor of two.
FSSB=2FDSB
§9.1 Analog Modulation
Basic analog modulation: AM (SSB, DSB), FM, PM PPM (Pulse Position Modulation) is used in UWB (ultra wideband) system.
1. Signal-Sideband Modulation 2. DSB-SC Modulation 3. DSB-LC Modulation 4. Envelope Detection of DSB-LC Modulation 5. Frequency Modulation (FM)