ECE 6332, Spring, 2014
Wireless Communications
Zhu Han
Department of Electrical and Computer Engineering
Class 23
April 16th, 2014
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
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)
Normalized Amplitude --->
0.8
0.6
0.4
0.2
0
-0.2
-6
-4
-2
0
2
Normalized Frequency (fT) --->
4
6
OFDM
Transmitted Symbol

To have ISI-free channel,

Guard interval between OFDM symbols
ensures no ISI between the symbols.
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.
guard
guard
guard
Symbol
guard
Symbol
Multipath component that does not cause ISI
guard
Symbol
Multipath component that causes ISI
guard
Mathematical description
Mathematical description
OFDM Timing Challenge
OFDM bit loading

Map the rate with the sub-channel condition

Water-filling
OFDM Time and Frequency Grid

Put different users data to different time-frequency slots
OFDM Transmitter and Receiver
OFDM
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
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.
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.
Applications

WiMax

Digital Audio Broadcast (DAB)

Wireless LAN
Applications

High Definition TV (HDTV)

4G Cellular Communication systems

Flash -OFDM
Proprietary OFDM Flavours
Wireless Access (Macro-cellular)
Wideband-OFDM
(W-OFDM) of Wi-LAN
www.wi-lan.com
-- 2.4 GHz band
-- 30-45Mbps in 40MHz
-- large tone-width
(for mobility, overlay)
Flash OFDM
from Flarion
www.flarion.com
-- Freq. Hopping for
CCI reduction, reuse
-- 1.25 to 5.0MHz BW
-- mobility support
Vector OFDM
(V-OFDM) of Cisco, Iospan,etc.
www.iospan.com
-- 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)
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
Calculations for 802.16a -- Example: 5MHz
Carrier frequency
Channel Bandwidth
Number of inputs to IFFT/FFT
Number of data subcarriers
Number of pilots
Subcarrier frequency spacing f
Period of IFFT/FFT Tb
Length of guard interval
Length of the preamble for Downlink
Length of the preamble for Uplink
Guard interval for Uplink preamble
OFDM symbol duration
2-11 GHz
5 MHz
256
192
8
19.53125 KHz (5 MHz/256)
51.2 s (1 / f)
12.8 s (Tb / 4)
128 s (640 sub-carriers)
76.8s (384/5 MHz)
25.6 s (128/5 MHz)
64 s (320/5 MHZ)
Broadband Access Standards -- contd.

IEEE LAN and MAN standards
IEEE 802.16
(10 to 66 GHz)
IEEE 802.16a,b
(2 to 11 GHz)
1-3 miles, non-LoS
2-5 miles, LoS(> 11GHz)
IEEE 802.11a or
.11b, or .11g
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
The IEEE 802.11 Standard
Evolution of Radio Access Technologies
In Nov. 2004, 3GPP began a project to define the long-term
evolution (LTE) of Universal Mobile Telecommunications System
(UMTS) cellular technology
802.16m
802.16d/e

LTE (3.9G) :
3GPP release 8~9

LTE-Advanced :
3GPP release 10+
LTE vs. LTE-Advanced
27
DS-CDMA versus OFDM
a0
Impulse
Response h(t)
Input
(Tx signal)
Frequency
Response H(f)
a3
time
DS-CDMA can
exploit
time-diversity
Output
(Rx signal)
channel
OFDM can exploit
freq. diversity
freq.
Comparing Complexity of TDMA, DSCDMA, & OFDM Transceivers
TDMA
Timing Sync.
Freq. Sync.
Timing Tracking
Freq. Tracking
Channel
Equalisation
Analog Front-end
(AGC, PA, VCO, etc)
Easy, but requires
overhead (sync.) bits
Easy, but requires
overhead (sync.) bits
CDMA
OFDM
Difficult, and requires
sync. channel (code)
Very elegant, requiring
no extra overhead
More difficult than TDMA
Gross Sync. Easy
Fine Sync. is Difficult
Modest Complexity
Complexity is high in
Asynchronous W-CDMA
Usually not required
within a burst/packet
Easy, decision-directed
techniques can be used
Modest Complexity
(using dedicated correlator)
Requires CPE Tones
(additional overhead)
Modest to High Complexity
(depending on bit-rate and
extent of delay-spread)
Very simple
(especially for CPM signals)
RAKE Combining in CDMA
usually more complex than
equalisation in TDMA
Fairly Complex
(power control loop)
Frequency Domain
Equalisation is very easy
Complexity or cost is
very high (PA back-off
is necessary)
Comparing Performance of TDMA, DSCDMA, & OFDM Transceivers
TDMA
Fade Margin
(for mobile apps.)
Range
Re-use & Capacity
FEC Requirements
Variable Bit-rate
Support
Spectral Efficiency
CDMA
OFDM
Modest requirement
(RAKE gain vs powercontrol problems)
Required for mobile
applications
Range increase by reducing
allowed noise rise (capacity)
Difficult to support large
cells (PA , AGC limitations)
Modest (in TDMA) and
High in MC-TDMA
Modest
Re-use planning is
crucial here
FEC optional for voice
FEC is usually inherent (to
increase code decorrelation)
FEC is vital even for
fixed wireless access
Required for mobile
applications
Very easy to increase
cell sizes
Low to modest support
Very elegant methods
to support VBR & VAD
Modest
Poor to Low
Powerful methods
to support VBR
(for fixed access)
Very High
(& Higher Peak Bit-rates)
LTE vs. LTE-Advanced
31
LTE vs. LTE-Advanced
32