1. Network Kernel Architectures and Implementation (01204423) Physical Layer
Chaiporn Jaikaeo
Department of Computer Engineering Kasetsart University
Materials taken from lecture slides by Karl and Willig

2. Overview Frequency bands Modulation
Signal distortion – wireless channels
From waves to bits

3. © Jochen Schiller, FU Berlin
RF Spectrum
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency
1 Mm
300 Hz
10 km
30 kHz
100 m
3 MHz
1 m
300 MHz
10 mm
30 GHz
100 m
3 THz
1 m
300 THz
visible light
VLF LF MF HF
VHF
UHF
SHF
EHF
infrared UV
optical transmission
coax cable
twisted pair
© Jochen Schiller, FU Berlin

4. Frequency Allocation Some frequencies are allocated to specific uses
E.g., Cellular phones, analog radio broadcasting
ISM bands
Industrial, Scientific, Medical
License-free operation
Some typical ISM bands
Frequency
Comment
MHz
– MHz
40.66 – MHz
433 – 464 MHz
Europe
900 – 928 MHz
Americas
2.4 – 2.5 GHz
WLAN/WPAN
5.725 – GHz
WLAN
24 – GHz

5. Example: US Freq Allocation

6. Overview Frequency bands Modulation
Signal distortion – wireless channels
From waves to bits

7. Signals General form of signal Parameters
A(t) – amplitude
f(t) – frequency
(t) – phase
When the above parameters are all constant, the signal becomes a basic sine wave

8. Modulations Process of encoding data into signal
By changing the three parameters
Different ways of setting parameters
Set the parameter to an arbitrary value
 analog modulation
Choose values from a finite set of legal values
 digital modulation (keying)
Resulting signal requires a certain bandwidth to be transmitted
Centered around frequency of the basic sine wave

9. Modulation/Keying Examples
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
© Tanenbaum, Computer Networks

10. © Forouzan, Data communications and Networking
QAM
QAM – Quadrature Amplitude Modulation
A combination of ASK and PSK
© Forouzan, Data communications and Networking

11. Receiver: Demodulation
Receiver matches signal with corresponding data bits
Problems
Carrier synchronization: frequency may vary
Bit (symbol) boundary
Frame boundary
Received signal is not the transmitted signal!

12. Dealing with Synchronization
E.g., Mica motes

13. Overview Frequency bands Modulation
Signal distortion – wireless channels
From waves to bits

14. Signal Distortion Wireless transmission distorts transmitted signal
Wireless channel  abstract model describes these distortion effects
Sources of distortion
Attenuation
Reflection/refraction
Diffraction
Scattering
Doppler fading

15. Suitable Frequency Attenuation depends on the used frequency
Can result in a frequency-selective channel
with large enough bandwidth span © ©

16. Path Loss Friis free-space equation: Other than free-space
Ptx – Transmission Power
Gt – Tx antenna gain
Gr – Rx antenna gain
L  1 – Loss in circuitry
2    6 – Path-loss exponent

17. Non-Line-Of-Sight (NLOS) Paths
Different paths have different propagation times
Results in delay spread of the wireless channel
Closely related to frequency-selective fading properties of the channel
With movement: fast fading
LOS path
NLOS path

18. Signal Strength in Multi-Path Environment
Brighter color = stronger signal
Simple free-space attenuation formula is not sufficient
© Jochen Schiller, FU Berlin

19. General Path-Loss Formula
 > 2 is the path-loss exponent
Rewrite in logarithmic form (in dB):
Take obstacles into account by a random variation
Add a Gaussian random variable with 0 mean, variance 2 to dB representation

20. Overview Frequency bands Modulation
Signal distortion – wireless channels
From waves to bits

21. Noise and interference
Due to effects in receiver electronics
Depends on temperature
Interference from third parties
Co-channel interference
Adjacent-channel interference

22. Symbols and Bit Errors
Error ratio depends on signal strength compared to noise
Captured by signal to noise and interference ratio (SINR)
SINR allows to compute bit error rate (BER)
Depends on modulation and symbol rate
E.g., formula for BPSK

23. SINR vs. BER Example

24. SINR vs. BER Example

25. Summary
Wireless radio communication introduces many uncertainties and vagaries into a communication system
Handling the unavoidable errors will be a major challenge for the communication protocols