ECE 4371, Fall, 2013
Introduction to Telecommunication
Engineering/Telecommunication Laboratory
Zhu Han
Department of Electrical and Computer Engineering
Class 19
Nov. 5th, 2014
Outline

OFDM
– Motivation
– System Model
– Applications
Introduction

A method of encoding digital data on multiple carrier
frequencies.

OFDM has developed into a popular scheme for wideband
digital communication
– Wireless and wire
– Digital television and audio broadcasting, DSL Internet access,
wireless networks, powerline networks, and 4G mobile
communications

A large number of closely spaced orthogonal sub-carrier signals
are used to carry data on several parallel data streams or
channels.
Introduction

OFDM is a specialized FDM
– Additional constraint


All the carrier signals are orthogonal to each other
Sub-carriers are orthogonal to each other
– Cross-talk between the sub-channels is eliminated
– Inter-carrier guard bands are not required

Greatly simplifies the design of
– Transmitter and the receiver;
Basic Idea

Divide a high bit- rate stream into several low bit- rate streams (
serial to parallel)

Robust against frequency selective fading due to multipath
propagation
OFDM

Low symbol rate modulation schemes suffer less from intersymbol interference caused by multipath propagation

Utilizes all carriers to transmit its data as coded quantity at each
frequency carrier, which can be QAM, etc..
Orthogonal frequency-division multiplexing
Special form of Multi-Carrier Modulation.
– Divide a high bit-rate digital stream into several low bit-rate
schemes and transmit in parallel
0.8
Normalized Amplitude --->

0.6
0.4
0.2
0
-0.2
-6
-4
-2
0
2
Normalized Frequency (fT) --->
4
6
Modulation techniques:
Single-carrier vs. Multi-carrier

Drawbacks
– Selective Fading
– Very short pulses
– ISI is comparatively long
– Poor spectral efficiency
because of band guards

Advantages
– Flat Fading per carrier
– N long pulses
– ISI is comparatively short
Modulation techniques:
Single-carrier vs. Multi-carrier

Compare communication system by power efficiency and
bandwidth efficiency.
– Power efficiency describes the ability of communication system to
preserve BER of the transmitted signal at low power levels.
– Bandwidth efficiency reflects how efficiently the allocated
bandwidth is utilized and is defined as the throughput data rate per
Hertz in a given bandwidth.

The bandwidth efficiency of OFDM with using optical fiber
channel is defined as
– Factor 2 is because of two polarization states in the fiber.
– where R_{s} is the symbol rate in Gsps, and B_{OFDM} is the
bandwidth of OFDM signal.
Modulation techniques:
Single-carrier vs. Multi-carrier

There is only 1 dBm increase in receiver power, but we get
76.7% improvement in bandwidth efficiency with using
multicarrier transmission technique.

There is saving of bandwidth by using Multicarrier modulation
with OFDM.
– Bandwidth efficiency of multicarrier system is larger than single
carrier system.
Transmission
Type
MQAM
No. of
Subcarriers
1.
Singlecarrier
64
1
2.
Multicarrier
64
128
Fiber length
Power at the receiver
(at BER of 10−9)
Bandwidth
efficiency
10
Gbit/s
20 km
-37.3 dBm
6.0000
10
Gbit/s
20 km
-36.3 dBm
10.6022
Bit rate
Key features - Advantages

High spectral efficiency as compared to
other double sideband modulation
schemes, spread spectrum, etc.

– Adequate channel coding and
interleaving
– Using close-spaced overlapping sub-carriers

Can easily adapt to severe channel
conditions without complex time-domain
equalization. Simpler equalization to flat
fading channel.
– Overall channel divided into multiple
narrowband signals that are affected
individually as flat fading sub-channels.
– Single carrier systems has high complexity
of the channel equalization applied across
the whole channel.
– OFDM is that using multiple sub-channels,
the channel equalization becomes much
simpler.
Against narrow-band co-channel
interference.
– Possible to recover symbols lost due
to the frequency selectivity of the
channel and narrow band
interference.

Robust against inter-symbol
interference (ISI) and fading
caused by multipath propagation.
– Low data rate on each of the
sub-channels

Efficient implementation using
Fast Fourier Transform (FFT).

Low sensitivity to time
synchronization errors.
Key features - Disadvantages

Sensitive to Doppler shift.

Sensitive to frequency synchronization problems.

High peak-to-average-power ratio (PAPR)
– An OFDM signal has a noise like amplitude variation and has a
relatively high large dynamic range, or PAPR.
– Impacts the RF amplifier efficiency as the amplifiers need to be
linear and accommodate the large amplitude variations and these
factors.
– Amplifier cannot operate with a high efficiency level, suffers from
poor power efficiency.

Loss of efficiency caused by cyclic prefix/guard interval.
OFDM
Sensitive to frequency synchronization problems.
Outline

OFDM
– Motivation
– System Model
– Applications
Frequency Response
Data coded in
frequency domain
Transformation to time domain:
each frequency is a sine wave
in time, all added up
Channel frequency
response
OFDM Effective Diagram

z[n]=H[n]s_n+w[n]
OFDM Transmitter and Receiver
Data on OFDM

The data to be transmitted on an OFDM signal is spread across
the carriers of the signal, each carrier taking part of the payload.

This reduces the data rate taken by each carrier.
– The lower data rate has the advantage that interference from
reflections is much less critical.

This is achieved by adding a guard band time or guard interval
into the system.
 This ensures that the data is only
sampled when the signal is
stable and no new delayed
signals arrive that would alter
the timing and phase of the
signal.
Guard Time and Cyclic Prefix

Since the duration of each symbol is long, it is feasible to insert a guard
interval between the OFDM symbols.

A Guard time is introduced at the end of each OFDM symbol for protection
against multipath.

The Guard time is “cyclically prefix” 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
Cyclic Prefix

The prefixing of a symbol with a repetition of the end

Receiver is typically configured to discard the cyclic prefix
samples, the cyclic prefix serves two purposes.
– As a guard interval

Eliminates the inter-symbol interference from the previous
symbol.
– As a repetition of the end of the symbol


The receiver will integrate over an integer number of sinusoid
cycles for each of the multi-paths when it performs OFDM
demodulation with the FFT.
For the cyclic prefix to be effective
– The length of the cyclic prefix must be at least equal to the length
of the multipath channel.
Transmitted Symbol

To have ISI-free channel,

Guard interval between OFDM symbols
ensures no ISI between the symbols.
OFDM Bit & Power loading

Bit Loading
– Map the rate with the sub-channel condition
– Sub-carriers with low gain should be assigned less complex
constellations and should carry fewer bits per symbol.

Extreme case: no bit should be assigned when channel gain
close to zero
– Sub-carriers with large gain should be assigned more complex
constellations and should carry more bits per symbol.

Power Loading
– Transmitter apply additional power to subcarrier.
– Water-filling
OFDM Bit & Power loading

Bit loading and power loading are often complementary at the
transmitter.
Outline

OFDM
– Motivation
– System Model
– Applications
OFDM Applications

Wireless
– The wireless LAN (WLAN) radio interfaces IEEE 802.11a, g, n, ac and HIPERLAN/2.
– The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD
Radio, T-DMB and ISDB-TSB.
– The terrestrial digital TV systems DVB-T and ISDB-T.
– The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T and MediaFLO forward
link.
– The wireless personal area network (PAN) ultra-wideband (UWB) IEEE 802.15.3a
implementation suggested by WiMedia Alliance.
– The OFDM based multiple access technology OFDMA is also used in several 4G and
pre-4G cellular networks and mobile broadband standards:
– The mobility mode of the wireless MAN/broadband wireless access (BWA) standard
IEEE 802.16e (or Mobile-WiMAX).
– The mobile broadband wireless access (MBWA) standard IEEE 802.20.
– the downlink of the 3GPP Long Term Evolution (LTE) fourth generation mobile
broadband standard. The radio interface was formerly named High Speed OFDM Packet
Access (HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA).
OFDM Applications

Cable
– ADSL and VDSL broadband access via POTS copper wiring,
– DVB-C2, an enhanced version of the DVB-C digital cable TV
standard,
– Power line communication (PLC),
– ITU-T G.hn, a standard which provides high-speed local area
networking of existing home wiring (power lines, phone lines and
coaxial cables). [2]
– TrailBlazer telephone line modems,
– Multimedia over Coax Alliance (MoCA) home networking.
OFDMA

OFDMA
– A multi-user version of the OFDM
– Multiple access is achieved in OFDMA by assigning subsets of
subcarriers to individual users


Combining OFDM with time division multiple access (TDMA)
or time-domain statistical multiplexing communication.
SC-FDMA
– A linearly pre-coded OFDMA scheme, in the sense that it has an
additional DFT processing step preceding the conventional
OFDMA processing.
– Lower peak-to-average power ratio (PAPR)
– Greatly benefits the mobile terminal in terms of transmit power
efficiency and reduced cost of the power amplifier.
Diagram of SC-FDMA & OFDMA
OFDM Time and Frequency Grid

Put different users data to different time-frequency slots
SC-FDMA VS OFDMA
Comparison
ADSL

Conventional voice band modem
– Dial up modem converts bits into waveforms that must fit to the
voice band (3.4kHz)
– Large QAM modulation require. (960-QAM)
– High transmission power and large complexity equalization.
– No phone call when surfing the Internet.

ADSL completely bypass the voice telephone system by
specializing in data service.
– Voice traffic continue to use voice band below 3.4 kHz.
– Achieved by Low pass Filter
– Bandwidth much larger than 4kHz
– Long copper wires suffer from attenuation at high frequencies
– Connection distance is short (1-5 km)
ADSL

OFDM can cope with this frequency selective attenuation and
with narrow-band interference
– Achieves high-speed data connections on existing copper wires.

ADSL2 uses variable sub-carrier modulation, ranging from
BPSK to 32768QAM.
Upstream and Downstream Subcarrier Allocation

The asymmetric data service
– Higher downstream than upstream

More downstream subcarriers than upstream carriers.
ADSL

However, DSL cannot be used on every copper pair;
– Interference may become significant if more than 25% of phone
lines coming into a central office are used for DSL.

OFDM is used in ADSL connections that follow the ANSI
T1.413 and G.dmt (ITU G.992.1) standards, where it is called
discrete multitone modulation (DMT).

OFDM is also used in the successor standards ADSL2,
ADSL2+, VDSL, VDSL2, and G.fast.
Wireless local area networks (LAN) and
metropolitan area networks (MAN)

OFDM is extensively used in wireless LAN and MAN
applications, including IEEE 802.11a/g/n and WiMAX.

IEEE 802.11a/g/n operating in the 2.4 and 5 GHz bands,
specifies a per-stream airside data rates ranging from 6 to 54
Mbit/s.

Four different modulation schemes are used: BPSK, QPSK, 16QAM, and 64-QAM.
– The multitude of choices allows the system to adapt the optimum
data rate for the current signal conditions.
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.

Five different PHY layers.

802.11a/g belongs to the High Speed WLAN category with peak data rate of 54Mbps

PHY Layer very similar to ETSI’s HIPERLAN Type 2
Outline

OFDM
– Motivation
– System Model
– Applications
–Thanks
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