This document discusses various techniques for encoding signals for wireless transmission, including: - Digital data must be converted to analog signals using techniques like amplitude-shift keying, frequency-shift keying, and phase-shift keying. Analog signals can also be modulated to higher frequencies for transmission. - Analog signals can be converted to digital for transmission using pulse code modulation (PCM) or delta modulation (DM). PCM assigns a binary code to analog samples while DM approximates the analog signal as a staircase function. - Key factors in signal encoding are the signal-to-noise ratio, data rate, bandwidth, clocking, interference immunity, and cost/complexity of the scheme. Higher data rates

1. Signal Encoding
NETW4004 LECTURE 3
SOURCE: STALLINGS CHAPTER 6

2. Encoding Techniques in Wireless
2
Digital-to-analog
 Digital data and digital signals must be converted to
analog signals for wireless transmission
Analog-to-analog
 Baseband signals must be modulated onto a higher-frequency
carrier for transmission.
Analog-to-digital
 Digitising analog signals for digital transmission so as to
improve quality and take advantage of TDM schemes.
Digital-to-digital

3. Signal Encoding Criteria
3
What determines how successful a receiver will be in
interpreting an incoming signal?
 Signal-to-noise ratio
 Data rate
 Bandwidth
An increase in data rate increases bit error rate
An increase in SNR decreases bit error rate
An increase in bandwidth allows an increase in data
rate

4. Comparison of Encoding Schemes
4
Signal spectrum
 With lack of high-frequency components, less bandwidth
required (discuss)
 No DC component: AC coupling via transformer possible
Clocking
 Ease of determining beginning and end of each bit
position
Signal interference and noise immunity
 Performance in the presence of noise
Cost and complexity
 The higher the signal rate to achieve a given data rate, the
greater the cost

5. Basic Encoding Techniques I
5
Digital data to analog signal
 Amplitude-shift keying (ASK)
 Amplitude difference of carrier
frequency
 Frequency-shift keying (FSK)
 Frequency difference near carrier
frequency
 Phase-shift keying (PSK)
 Phase of carrier signal shifted
Fig. 6.2 Modulation of Analog Signals for Digital
Data

6. Amplitude-Shift Keying (ASK)
6
One binary digit represented by presence of
carrier, at constant amplitude (1)
Other binary digit represented by absence of
carrier (0)
( )
A ( f t) c cos 2p
0
ïî
ïí ì
s t =
binary 1
binary 0
 where the carrier signal is Acos(2πfct)
Used to transmit digital data over optical fiber
Susceptible to sudden gain changes
Inefficient modulation technique

7. Binary Frequency-Shift Keying (BFSK)
7
Two binary digits represented by two different
frequencies near the carrier frequency
( )
ïî
ïí ì
s t =
A ( f t) 1 cos 2p
A ( f t) 2 cos 2p
binary 1
binary 0
 where f1 and f2 are offset from carrier frequency fc by equal but
opposite amounts
Less susceptible to error than ASK
Used for high-frequency (3 to 30 MHz) radio
transmission

8. Using Multiple Frequencies (MFSK)
8
More than two frequencies are used in FSK
More bandwidth efficient
Used for frequency hopping in spread spectrum
si (t ) =Acos2pfit
1£i£M
f = fc + (2 i - 1 -
M )
fd
i L = =
M number of different signal elements 2
=
L number of bits per signal element

9. Using Multiple Frequencies (MFSK)
9
Example (6.1-P143):
With fc=250 kHz, fd=25 kHz and M=8 (L=3 bits), we
have the following frequency assignments for each of
the 8 possible 3-bits data combinations:
f fc i M fd i = +(2 -1- )
f1= 75 kHz 000 f2=125 kHz 001 f3=175 kHz 010
f4=225 kHz 011 f5=275 kHz 100 f6=325kHz 101
f7=375 kHz 110 f8=425 kHz 111

10. Phase-Shift Keying (PSK)
10
Two-level PSK (BPSK):Uses two phases to represent binary digits
( )
A ( f t) c cos 2p
A ( f t) c - cos 2p
ïî
ïí ì
s t =
binary 1
binary 0
Differential PSK (DPSK): Phase shift with reference to previous bit
 Binary 0 – signal burst of same phase as previous signal burst
 Binary 1 – signal burst of opposite phase to previous signal burst

11. Phase-Shift Keying (PSK)
11
Four-level PSK (QPSK)
 Each element represents two bits
 Phase shift in multiples of p/4
( )
Acos 2pf t p c 11
ì
Acos 2pf t 3p c
ï ïî
ïïí
s t =
÷øö çè
æ +
ö çè
Acos 2pf t 3p c
ö çèæ -
Acos 2pf t p c
ö çè
OQPSK: Introducing a time-delay
 Phase change less than p/2
 Therefore less interference
4
÷ø
æ +
4
÷ø
4
÷ø
æ -
4
01
00
10

12. QPSK & OQPSK Diagram
Fig. 6.6
12

13. Quadrature Amplitude Modulation
13
QAM is a combination of ASK and PSK
 Two different signals sent simultaneously on the same
carrier frequency
s(t) d (t) f t d (t) f t c c cos 2p sin 2p 1 2 = +

14. Demodulation of QAM
14

15. Reasons for Analog Modulation
15
Modulation of digital signals
When only analog transmission facilities are
available, digital to analog conversion required
Modulation of analog signals
A higher frequency may be needed for effective
transmission
Modulation permits frequency division
multiplexing

16. Analog Modulation
16

17. Basic Encoding Techniques II
17
Analog data to digital signal
Pulse code modulation (PCM)
Delta modulation (DM)
Once analog data have been converted
to digital signals, the digital data
can be transmitted using NRZ-L
can be encoded as a digital signal using a
code other than NRZ-L
can be converted to an analog signal

18. Pulse Code Modulation
18
Based on the sampling
theorem
Each analog sample is
assigned a binary code
 Analog samples are referred to
as pulse amplitude modulation
(PAM) samples
The digital signal consists of
block of n bits, where each n-bit
number is the amplitude
of a PCM pulse

19. Pulse Code Modulation
19
By quantizing the PAM pulse, original signal is
only approximated
 Leads to quantizing noise
Signal-to-noise ratio for quantizing noise
 Each additional bit typically increases SNR by 6 dB, or a
factor of 4.
SNR ratio can be improved by nonlinear encoding
such as non-uniform quantization.

20. Delta Modulation (DM)
20
In DM, analog input is approximated by staircase
function
 Moves up or down by one quantization level (d) at each
sampling interval
The bit stream approximates derivative of analog
signal (rather than amplitude)
 1 is generated if function goes up
 0 otherwise
Two important parameters
 Size of step assigned to each binary digit (d)
 Sampling rate
Accuracy improved by increasing sampling rate
 However, this increases the data rate

21. DM
21
 Advantage of DM over PCM is the simplicity of its implementation.
 Used for audio signal encoding in Bluetooth.
 PCM exhibits better SNR at the same data rate.

22. Recap
22
Signal encoding
Basic encoding techniques
 Digital to analog
 Analog to analog
 Analog to digital
Problems 6.1, 6.10, 6.16