Basic concepts of Orthogonal Frequency Division Multiplexing

1. OFDM Basic Idea
 Orthogonal frequency-division multiplexing
 Divide a high bit- rate stream into several low bit- rate streams (
serial to parallel)
 Robust against frequency selective fading due to multipath
propagation

2. Orthogonal frequency-division multiplexing
 Special form of Multi-Carrier Transmission.
 Multi-Carrier Modulation.
– Divide a high bit-rate digital stream into several low bit-rate
schemes and transmit in parallel (using Sub-Carriers)
-6 -4 -2 0 2 4 6
-0.2
0
0.2
0.4
0.6
0.8
Normalized Frequency (fT) --->
Normalized
Amplitude
--->

3. OFDM

4. Transmitted Symbol
 To have ISI-free channel,
 Guard interval between OFDM symbols
ensures no ISI between the symbols.

5. Guard Time and Cyclic Extension...
 A Guard time is introduced at the end of each OFDM symbol for protection
against multipath.
 The Guard time is “cyclically extended” to avoid Inter-Carrier Interference
(ICI) - integer number of cycles in the symbol interval.
 Guard Time > Multipath Delay Spread, to guarantee zero ISI & ICI.
Multipath component that does not cause ISI
guard Symbol guard
guard Symbol guard
guard Symbol guard
Multipath component that causes ISI

6. Mathematical description

7. Mathematical description

8. OFDM Timing Challenge

9. OFDM bit loading
 Map the rate with the sub-channel condition
 Water-filling

10. OFDM Time and Frequency Grid
 Put different users data to different time-frequency slots

11. OFDM Transmitter and Receiver

12. OFDM

13. Multiband OFDM
- Simple to implement
- Captures 95% of the multipath channel energy in the Cyclic Prefix
- Complexity of OFDM system varies Logarithmically with FFT size i.e.
- N point FFT  (N/2) Log2 (N) complex multiplies for every OFDM
symbol

14. Pro and Con
 Advantages
– Can easily be adopted to severe channel conditions without complex
equalization
– Robust to narrow-band co-channel interference
– Robust to inter-symbol interference and fading caused by multipath propagation
– High spectral efficiency
– Efficient implementation by FFTs
– Low sensitivity to time synchronization errors
– Tuned sub-channel receiver filters are not required (unlike in conventional
FDM)
– Facilitates Single Frequency Networks, i.e. transmitter macro-diversity.
 Disadvantages
– Sensitive to Doppler shift.
– Sensitive to frequency synchronization problems
– Inefficient transmitter power consumption, since linear power amplifier is
required.

15. OFDM Applications
 ADSL and VDSL broadband access via telephone network copper wires.
 IEEE 802.11a and 802.11g Wireless LANs.
 The Digital audio broadcasting systems EUREKA 147, Digital Radio
Mondiale, HD Radio, T-DMB and ISDB-TSB.
 The terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T.
 The IEEE 802.16 or WiMax Wireless MAN standard.
 The IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) standard.
 The Flash-OFDM cellular system.
 Some Ultra wideband (UWB) systems.
 Power line communication (PLC).
 Point-to-point (PtP) and point-to-multipoint (PtMP) wireless applications.

16. Applications
 WiMax
 Digital Audio Broadcast (DAB)
 Wireless LAN

17. Applications
 High Definition TV (HDTV)
 4G Cellular Communication systems
 Flash -OFDM

18. Proprietary OFDM Flavours
Wideband-OFDM
(W-OFDM) of Wi-LAN
www.wi-lan.com
Flash OFDM
from Flarion
www.flarion.com
Vector OFDM
(V-OFDM) of Cisco, Iospan,etc.
www.iospan.com
Wireless Access (Macro-cellular)
-- 2.4 GHz band
-- 30-45Mbps in 40MHz
-- large tone-width
(for mobility, overlay)
-- Freq. Hopping for
CCI reduction, reuse
-- 1.25 to 5.0MHz BW
-- mobility support
-- MIMO Technology
-- non-LoS coverage,
mainly for fixed access
-- upto 20 Mbps in MMDS
Wi-LAN leads the OFDM Forum -- many proposals submitted to
IEEE 802.16 Wireless MAN
Cisco leads the Broadand Wireless Internet Forum (BWIF)

19. 19
OFDM based Standards
 Wireless LAN standards using OFDM are
– HiperLAN-2 in Europe
– IEEE 802.11a, .11g
 OFDM based Broadband Access Standards are getting
defined for MAN and WAN applications
 802.16 Working Group of IEEE
– 802.16 -- single carrier, 10-66GHz band
– 802.16a, b -- 2-11GHz, MAN standard

20. Key Parameters of 802.16a Wireless MAN
• Operates in 2-11 GHz
• SC-mode, OFDM, OFDMA, and Mesh support
• Bandwidth can be either 1.25/ 2.5/ 5/ 10/ 20 MHz
• FFT size is 256 = (192 data carriers+ 8 pilots +56 Nulls)
• RS+Convolutional coding
• Block Turbo coding (optional)
• Convolutional Turbo coding(optional)
• QPSK, 16QAM, 64QAM
• Two different preambles for UL and DL

21. Calculations for 802.16a -- Example: 5MHz
Carrier frequency 2-11 GHz
Channel Bandwidth 5 MHz
Number of inputs to IFFT/FFT 256
Number of data subcarriers 192
Number of pilots 8
Subcarrier frequency spacing f 19.53125 KHz (5 MHz/256)
Period of IFFT/FFT Tb 51.2 s (1 / f)
Length of guard interval 12.8 s (Tb / 4)
Length of the preamble for Downlink 128 s (640 sub-carriers)
Length of the preamble for Uplink 76.8s (384/5 MHz)
Guard interval for Uplink preamble 25.6 s (128/5 MHz)
OFDM symbol duration 64 s (320/5 MHZ)

22. IEEE 802.16
(10 to 66 GHz)
Broadband Access Standards -- contd.
 IEEE LAN and MAN standards
IEEE 802.11a or
.11b, or .11g
IEEE 802.16a,b
(2 to 11 GHz)
2-5 miles, LoS(> 11GHz)
1-3 miles, non-LoS

23. The IEEE 802.11a/g Standard
 Belongs to the IEEE 802.11 system of specifications for wireless LANs.
 802.11 covers both MAC and PHY layers.
 802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps
 FFT 64, Carrier 2.4G or 5G. Total bandwidth 20 MHz x 10 =200MHz

24. The IEEE 802.11 Standard

25. Evolution of Radio Access Technologies
 LTE (3.9G) :
3GPP release 8~9
 LTE-Advanced :
3GPP release 10+
802.16d/e
802.16m
In Nov. 2004, 3GPP began a project to define the long-term
evolution (LTE) of Universal Mobile Telecommunications System
(UMTS) cellular technology

26. 26
LTE vs. LTE-Advanced

27. DS-CDMA versus OFDM
channel
Input
(Tx signal)
Output
(Rx signal)
Impulse
Response h(t)
time
a3
a0
freq.
Frequency
Response H(f)
DS-CDMA can
exploit
time-diversity
OFDM can exploit
freq. diversity

28. Comparing Complexity of TDMA, DS-
CDMA, & OFDM Transceivers
Timing Sync.
Freq. Sync.
Timing Tracking
Freq. Tracking
Channel
Equalisation
Analog Front-end
(AGC, PA, VCO, etc)
TDMA OFDM
Very elegant, requiring
no extra overhead
CDMA
Easy, but requires
overhead (sync.) bits
Difficult, and requires
sync. channel (code)
Easy, but requires
overhead (sync.) bits
More difficult than TDMA
Gross Sync. Easy
Fine Sync. is Difficult
Modest Complexity
Usually not required
within a burst/packet
Requires CPE Tones
(additional overhead)
RAKE Combining in CDMA
usually more complex than
equalisation in TDMA
Modest Complexity
(using dedicated correlator)
Easy, decision-directed
techniques can be used
Frequency Domain
Equalisation is very easy
Complexity or cost is
very high (PA back-off
is necessary)
Very simple
(especially for CPM signals)
Complexity is high in
Asynchronous W-CDMA
Modest to High Complexity
(depending on bit-rate and
extent of delay-spread)
Fairly Complex
(power control loop)

29. Comparing Performance of TDMA, DS-
CDMA, & OFDM Transceivers
Fade Margin
(for mobile apps.)
Range
Re-use & Capacity
FEC Requirements
Variable Bit-rate
Support
Spectral Efficiency
TDMA OFDM
Required for mobile
applications
CDMA
Required for mobile
applications
Modest requirement
(RAKE gain vs power-
control problems)
Range increase by reducing
allowed noise rise (capacity)
Difficult to support large
cells (PA , AGC limitations)
Modest (in TDMA) and
High in MC-TDMA
Re-use planning is
crucial here
FEC is vital even for
fixed wireless access
FEC is usually inherent (to
increase code decorrelation)
FEC optional for voice
Powerful methods
to support VBR
(for fixed access)
Very High
(& Higher Peak Bit-rates)
Modest
Modest
Low to modest support
Poor to Low
Very elegant methods
to support VBR & VAD
Very easy to increase
cell sizes

30. 30
LTE vs. LTE-Advanced

31. 31
LTE vs. LTE-Advanced