1. Wireless communication channel

2. Effects on Radio Communication
Signal degradation can be classified by type :
Path Loss
happen during distance covered by the radio signal, it is called “Free space
path loss “, it can be calculated by
LFS = log F (MHz) +20 log d (Km)
Signal attenuation
Resulting from shadowing effects introduced by the obstacles between transmitter and receiver
Fading of the signal
Caused by numerous effects all of which are related to the Radio
propagation phenomenon

3. Wireless Multipath Channel
One of the most problem in communication channel is
fading

4. Fading Problems
Shadowing (Normal fading):
The reason for shadowing is the presence of obstacles like large hills or buildings in the path between the site and the mobile.
The signal strength received fluctuates around a mean value while changing the mobile position resulting in undesirable beats in the speech signal.

5. Fading Problems
Raleigh Fading (Multi-path Fading):
The received signal is coming from different paths due to a series of reflection on many obstacles. The difference in paths leads to a difference in paths of the received components.

6. Parameters of multi-path channel
Time Domain Frequency Domain
1- Max delay spread: Coherence BW:
3- Coherence Time : Doppler Shift:

7. Doppler Shift Phase change due to path length difference
Doppler shift (apparent change in freq.)

8. Types of fading

9. At High Data Rate 1- 2- High data rate transmission Problems
short symbol time compared to the delay spread.
= Delay spread
= Symbol period
Problems 1-
ISI
= signal BW
= coherence BW 2-

10. Orthogonal frequency division multiplexing
(OFDM)

11. History
OFDM was introduced in 1950 but was only completed in 1960’s Originally grew from Multi-Carrier Modulation used in High Frequency military radio.
Patent was granted in 1970’s
Earlier OFDM wasn’t popular Large arrays of sinusoidal generators and coherence demodulator Too expensive and complex.
Later when DFT and IDFT became a known solution to the arrays of generators and demodulators.
It was still not popular as there is no efficient method to perform the IFFT and FFT operation.
Advances in VLSI technology allows implementation of fast and cheap FFT and IFFT operation drive OFDM popularity.

12. Applications

13. OFDM Orthogonal Frequency Division Multiplexing
-Divide the information over several carriers
Instead of using one big truck
When The truck is lost…
All is lost!
Use several small trucks
When one truck is lost…
Only a portion of the shipment is lost!

14. Concept of an OFDM signal
Ch.1
Ch.2
Ch.3
Ch.4
Ch.5
Ch.6
Ch.7
Ch.8
Ch.9
Ch.10
Conventional multicarrier techniques
frequency
Ch.2
Ch.4
Ch.6
Ch.8
Ch.10
Ch.1
Ch.3
Ch.5
Ch.7
Ch.9
Saving of bandwidth
50% bandwidth saving
Orthogonal multicarrier techniques
frequency

15. . . OFDM changes Frequency Selective Fading to Flat Fading Channel
N number of subcarrier . .

16. Solution to Frequency Selective Fading
When the data rate is lower
= Delay spread
= Symbol period
= signal BW
= coherence BW
Frequency Selective => Flat Fading
In flat fading, the amplitude varies but there is no ISI

17. Multicarrier Modulation
Divide broadband channel into narrowband subchannels
No ISI in subchannels if constant gain in every subchannel and if ideal sampling
Orthogonal Frequency Division Multiplexing
Based on the fast Fourier transform
Standardized for DAB, DVB-T, IEEE a, a, HyperLAN II
Considered for fourth-generation mobile communication systems
channel
subcarrier
magnitude
subchannel
frequency
Subchannels are 312 kHz wide in a and HyperLAN II

18. OFDM Frequency Spectrum
Use many carriers that are equally spaced:
k = 0, 1, … , N-1
Ts = Symbol Time
Carrier 1 has a maximum
where
all other carriers are 0 1 2 3 4 5
frequency

19. OFDM Many carriers with small spacing => Long symbol time
But many carriers carry a lot of information!
Long symbol time is an advantage!
Delay Spread (Multipath)
Direct Path
Symbol n-1
Symbol n
Symbol n+1
Delayed Path
Symbol n-1
Symbol n
Symbol n+1
ISI
ISI
ISI = Inter Symbol Interference

20. OFDM Avoid ISI and preserve Orthogonality => Guard Interval
Total Symbol length
Useful Symbol length
Guard
Symbol n
Guard
Direct Path
Guard
Symbol n-1
Guard
Symbol n
Guard
Symbol n+1
Delayed Path
Guard
Symbol n-1
Guard
Symbol n
Guard
Symbol n+1
Symbol n is added constructively or destructively according to phase
Integration
Period

21. Avoid ICI and preserve Orthogonality => cyclic prefix
N samples
v samples
CP: Cyclic Prefix CP
s y m b o l i
s y m b o l ( i+1)
copy

22. Discrete versus Fast Fourier Transform
Discrete (DFT):
For each frequency sample ‘k’ (0 to N-1) loop ‘n’ (over 0 to N-1) => N2 complex multiplications
Fast (FFT, Cooley-Tukey algorithm):
“An efficient algorithm to calculate a DFT”
N.log(N) complex multiplications

23. OFDM Block Diagram

24. Main advantages • High spectral efficiency. And high data rate.
• Efficient in multipath environments.
• Simple digital realization by using the FFT operation.
• Low complex receivers due to avoidance of ISI.
•Different modulation schemes can be used on individual sub-carriers.

25. x Drawbacks Large Peak to Average Ratio (PAR).
Added sinusoid cause large PAR and issue of amplifier non-linearity arises.
Accurate frequency and time synchronization is required.
More sensitive to Doppler spreads than single-carrier schemes.
Sensitive to frequency offset and phase noise caused by imperfections in the transmitter and the receiver oscillators.
Guard interval causes loss in spectral efficiency