1. 林宏穎: OFDM Introduction
Class Report
林宏穎:
OFDM Introduction

2. OFDM History 1957: Kineplex multi-carrier HF modem
1966: Chang, Bell Labs: OFDM paper & patent
1971: Weinstein & Ebert propose use of FFT and guard interval
1985: Cimini describes use of OFDM for mobile communications
1987 Alard & Lasalle: OFDM for digital broadcasting
1995: ETSI DAB standard: first OFDM-based standard
1997: DVB-T standard
1998: Magic WAND project demonstrates OFDM modems for wireless LAN
1999: IEEE a and HIPERLAND/2 standards for wireless LAN
2000: V-OFDM for fixed wireless access
2001: OFDM considered for new IEEE and standards

3. Introduction to OFDM Basic idea Advantages Disadvantages
Using a large number of parallel narrow-band sub-carrier instead of a single wide-band carrier to transport information
Advantages
Very easy and efficient in dealing with multi-path
Robust against narrow-band interference
Disadvantages
Sensitive to frequency offset and phase noise
Peak-to-average problem reduces the power efficiency of RF amplifier at the transmitter
Adopted by various standards
DSL, a, DAB, DVB, etc.

4. OFDM Definition
The technique of OFDM is based on the well-known technique of FDM
FDM technique:
Different streams of information
are mapped onto separate parallel
frequency channels
Guard bands are inserted to reduce interference between adjacent channels
OFDM technique
Multiple carriers carry the
information stream
Carrier spectrum are are overlapped
and orthogonal to each other
A guard time is added to each symbol
to combat the channel delay spread
FDM
frequency
OFDM
frequency

5. Concept of OFDM A type of multi-carrier modulation
Single high-rate bit stream is converted to low-rate N parallel bit stream
Each parallel bit stream is modulated on one of N sub-carriers
Each sub-carrier can be modulated by QFSK or QAM
Add a guard time to each OFDM symbol to avoid inter-symbol interference of fading channel
To achieve high bandwidth efficiency, the sub-carriers are closely spaced and overlapped
Sub-carriers are orthogonal over the symbol time
Use coding to correct errors for sub-carriers in deep fading environment

6. Advantages of OFDM Robust in multi-path propagation environment
Successful Examples:
DAB, DVB-T, Wireless LAN
More tolerant of delay spread
Due to the use of many sub-carriers, the symbol duration is increased, relative to delay spread
Inter-symbol interference is avoided through the use of guard interval
Simplified or eliminate equalization needs, as compared to single carrier modulation
More resistant to fading
Low symbol rate per carrier provides the robustness against frequency selective fading or narrowband interference
FEC is used to correct for sub-carriers that suffer from deep fade
Multi-carrier with single frequency network (SFN)

7. OFDM Good for Broadband Systems
Most broadband systems are subjects to multipath transmission
Conventional solution to multipath is an equalizer in the receiver
Equalizers are too complicated at high data rates
With OFDM there is a simple way of dealing with multipath
Relatively simple DSP algorithms

8. quadrature amplitude modulation (QAM) encoder
Modulation System
Single carrier modulation 
Multi carrier modulation
N subchannels
N complex samples
S/P
quadrature amplitude modulation (QAM) encoder
N-IFFT
add cyclic prefix
P/S
D/A +
transmit filter
TRANSMITTER
multipath channel
RECEIVER
N subchannels
N complex samples
P/S
QAM decoder
channel
estima-
tion &
equalizer
N-FFT
S/P
remove cyclic prefix
Receive filter +
A/D

9. Multicarrier
Rate R
Mapping
Filter f0
Rate R
Mapping
Filter f1
Rate NR
Rate R
Mapping
Filter
fN-1
Bandlimited
signals f0 f1 f2
fN-1
 The transmission bandwidth is divided into sub-bands which
are transmitted in parallel
 Ideally, each sub-band is narrow enough so that the fading
it experiences is flat (no ISI)
 Disadvantages
-- Requires filter bank at receiver
-- Spectrally inefficiency

10. OFDM Source of Impairment
Power Amplifier
Non-Linear
Insert
Guard
Interval
FEC
Coding
QAM
Mapping
Pilot
Insertion IQ
Modulator
DAC
IFFT
(TX)
FFT
(RX)
HPA
Fixed-Point
Computation
Error
Multi-path
Fading
Channel
Frequency
Corrected
Signal
Phase noise
ADC
noise
FEC
Decoding
Remove
Guard
Interval
AGC Response Time
QAM De-
Mapping
Channel
Correction
ADC
Symbol timing
AGC Amp
LNA
Timing
Frequency
Synchronization
Phase noise
Frequency offset

11. Performance Loss Detection Loss of synchronized Detection
SNR (dB) required to achieve the performance of perfect channel knowledge . (Infinite Precision arithmetic assumed)
Algorithms for channel model description
Implementation Loss
SNR (dB) resulting from finite precision arithmetic
Computation complexity, architecture selection, cost

12. Problems of OFDM Modulation
ICI (Inter-channel interference): interference between symbol in adjacent frequencies
ISI (inter-symbol interference): interference of successive OFDM frames
Highly vulnerable to synchronization errors and frequency offsets
Highly vulnerable to the non-linearity of the Pas (in the RF analog front end)

13. Challenges for OFDM Synchronization challenges
Transmitter frequency  Receiver frequency
Mesochronous: same frequency, different phase
Pleisochrnous: slightly different frequencies
Asynchronous: totally different frequencies
Transmitter sampling time  Receiver sampling time
Symbol timing is unknown to receiver
Peak-to Average Power Ratio (PAPR)
Dynamic range at output of IFFT is much larger than at input
it is about 2 dB higher than that of the ATSC 8-VSB system. A larger Tx (more dynamic range) might be required or using pre-distortion and better filtering to reduce the first adjacent channel interference
Channel estimation for time varying environment

14. Impact of Symbol Duration
The symbol duration of OFDM is much larger than that of single carrier system under the similar overall transmission bandwidth
A larger symbol duration will enhance the effective bit rate and power utilization if the delay spread is about fixed
The larger OFDM duration when compared with the channel coherence time can reduce the ability to combat the fast temporal fading
The channel coherence time is inversely proportional to the maximum Doppler shift

15. Impact of Sub-Carrier Spacing
Because of the time-frequency duality, some of the time-domain arguments can be translated to the frequency domain
The large number of OFDM sub-carriers makes the bandwidth of the individual sub-carriers small relative to the overall signal bandwidth and the channel coherence bandwidth
The fading on each sub-carrier is frequency flat and can be better modeled as a constant complex channel gain.
The narrower sub-carrier spacing will be easier to cause inter-carrier interference