Basics of Orthogonal Frequency Division Multiplexing

1. Insight into Orthogonal Frequency Division
Multiplexing (OFDM)
By
DR. Pravinkumar Patil

2. Outline
• Introduction
- Wireless Challenges
- Channel Impairments to wireless communication
- Fading in wireless environment
- Doppler shift
- Attenuation
- Insight into OFDM
- Single carrier Vs Multi-carrier (OFDM)
- OFDM features
- OFDM Applications
- OFDM spectrum and Symbol
- OFDM timing parameters
- OFDM mathematics
- OFDM MODEM

3. Wireless Research Challenges
• Next generation wireless networks are expected to provide reliable
Multimedia services (such as web browsing, music and video streaming,
video conference etc.) at very high rate of data transmission.
• This leads to challenges in wireless networks such as
-High data rates
-Reliable transmission(QoS)
-Multimedia service support in multi user environment.
-Efficient spectral utilization.
- Power efficient solution.
• Metrics
-Throughput
-BER
-PER
-Power consumption.

4. Channel Impairments to wireless
communication
• Fading in Radio environment
• Doppler shift
• Attenuation

5. 5
Wireless Propagation and fading
• As the wireless mobile device moves through the environment, the field
strength varies due to :
– Free space path loss: In free space, received power attenuates like 1/d2.
– Large scale fading: With reflections, diffraction and scattering received
power attenuate even more rapidly with distance.
– Small scale fading: Rapid fluctuations in received power due to
multipath propagation (constructive and destructive interference of the
transmitted waves)
log(distance)
Signal
Level
(dB)
pathloss
long term fading
shortterm fading

6. Doppler shift
• Changes in the received carrier frequency due to the relative motion
of the mobile to the base station
• f= fd = (v/l) cos(q)
– for f=900 MHz, v = 70 MPH (112 km/h)
– fD-max = v/l = 93.3 Hz
q
D=v. t

7. 7
Types of small-scale Fading

8. Insight into OFDM
• OFDM stands for “Orthogonal Frequency Division Multiplexing”
– “Orthogonal”: Multiple frequency channels are orthogonal to each
other. This provides demodulation of each information-bearing
subcarrier without interference from adjacent subcarriers
– “Frequency Division”: subcarriers are generated as frequency-disjoint
coupled carriers with fixed spacing
– “Multiplexing”: subcarriers are synthesized into a single channel
• OFDM based system : Multi-carrier modulation system.
• Definitions:
• A method of encoding digital data on multiple carrier frequencies for
transmitting large amounts of digital data at very high transmission rate
over a radio wave with high QoS.

9. Single carrier Vs Multi-carrier (OFDM)
• Single carrier
- Spectrally inefficient.
- Requires higher BW for higher data rates.
- Higher data rates results in less symbol period resulting in frequency
selective channel effect . Hence more susceptible to fading effects.
• OFDM
- data transmission on multiple orthogonal carriers improves spectral
efficiency.
- lower data rate on individual carrier , ensuring more symbol period
than max delay spread , hence converting frequency selective
channel to flat fading channel , therefore less prone to fading effects

10. OFDM features
• Reduced ICI –Orthogonal carriers.
• Reduced ISI –Due to CP.
• Higher spectral efficiency
• Less complex hardware-
IFFT- Modulator
FFT – Demodulator
• Simple equalization technique at receiver.
• Best candidate for transmit parameters adaptation.
Limitations-
-Frequency offset error.
- High PAPR.
-Data overhead- CP-Adverse effect on throughput.

11. Applications
• Digital Audio Broadcasting (DAB)
• Digital television
• Wireless LAN Network
• Broadband Wireless Access System
• ADSL (Asymmetric digital subscriber line)
• The LTE and LTE Advanced 4G mobile phone standards.

12. OFDM-Spectrum and symbol

13. OFDM-Timing parameters
TS = sampling interval = 1/FS
where
FS = frequency of sampling.
TFFT = NTs = OFDM symbol period.
N = No. of sub carriers.
TSY M = TG + TFF T = NTTS = Length of extended OFDM
symbol.
L = TG/TS = No. of sub carriers in CP.
TG = Guard interval. Typical value of which is selected such
that
TG/TFFT = 1/4, 1/8, 1/16, 1/32.

14. OFDM-equations

15. 15
OFDM Modulation/Demodulation
QAM
Mapping
IFFT
Cyclic
Prefix
S/P P/S
D/A
and
RF
(a)
RF
and
A/D
Strip
cyclic
prefix
S/P FFT P/S
QAM
decoding
(b)
FEQ
A QAM symbol is modulated onto each subcarrier
Use of IFFT/FFT for OFDM MOD/DEMOD

16. THANK YOU!