1. Digital Communications 0909.402.03/0909.504.01 Fall 2002
Lecture 6 October 14, 2002
Shreekanth Mandayam
ECE Department
Rowan University

2. Plan Baseband Data Transmission
Recall: Detection in the presence of AWGN
Receiver Model
Matched Filter Detector
BER for Unipolar and Bipolar Signaling
BER for multilevel signaling
Correlation receiver
Bandlimited Channels and ISI
Pulse Shaping: Raised Cosine Response
Eye Diagrams

3. Digital Communications Transceiver
Anti-
aliasing
Filter
Data
Encryption
Encoder
Error
Control
Encoder
Source
Encoder
Channel/
Line
Encoder
Sampling
Quantization
MUX
Modulator
ADC
Analog i/p
CODEC
MODEM
Multiple access channel
Analog o/p
Data
Encryption
Decoder
Error
Control
Decoder
Audio
Amp
Reconstruction/
DAC
Source
Decoder
Equalization /
Decision
Circuits
DEMUX
Demod-ulator

4. Recall: Receiver ModelTX S
s(t)
n(t)
LPF
Comparator
r(t)
t=t0 VT m ~
r0(t)
r0(t0) m

5. Recall: Multilevel Signaling
Binary
Source
l-bit
DAC
Binary signal
D symbols/second
= R bits/second
L=2l Multilevel Signal
D symbols/second = R/l
R bits/second 4 2 -2 -4 Tb Ts T0
t (ms)

6. Average Symbol Error
demos/BER_multilevel_baseband.m

7. Matched Filter/Correlation Receiver
s(t)
Matched
Filter
h(t)
r0(t) = s0(t) + n0(t)
r(t) = s(t) + n(t) A T
s(-t) A -T
h(t)=s(T-t) X
s(t)
Integrator
t=t0
r0(t)
r0(t0)
s(t) + n(t) A T
h(t)*s(t) A2
T 2T
t=t0=T

8. Bandlimited Channels: Inter-Symbol Interference (ISI)
Band-unlimited Channel f
H(f)
Input pulse
Output pulse t t
Transmission
Channel
Band-limited Channel
Output pulse
Input pulse
H(f)
-B B f t t

9. Pulse Shaping: Nyquist’s First Method for Zero ISI
HE(f)
Transmit
Filter
HT(f)
Channel
Characteristics
HC(f)
Receiver
Filter
HR(f)
vi(t)
v0(t) t t

10. Pulse Shaping: The Raised Cosine Response
He(f) fD fD
1.0
0.5
-B f f f f B f

11. Pulse Shaping: The Raised Cosine Response
he(t)
2f0 t
-3/2f0 -2/2f0 -1/2f /2f0 2/2f0 3/2f0

12. Summary