OFDM, Quick Intro

K7DEN

OUTLINE

OVERVIEW ABOUT OFDMDEFINITION AND PRINCIPLESOFDM ADVANTAGES & DRAWBACKSAPPLICATIONSCONCLUSIONS AND PERSPECTIVES

OVERVIEW ABOUT OFDM

OFDM was invented more than 40 years ago. OFDM has been adopted for several technologies:

Asymmetric Digital Subscriber Line (ADSL) services.

IEEE 802.11a/g, IEEE 802.16a. Digital Audio Broadcast (DAB). Digital Terrestrial Television Broadcast: DVD in Europe, ISDB in Japan 4G, IEEE 802.11n, IEEE 802.16, and IEEE 802.20.

WHY OFDM ?

High bit rate needs are clumped by the nature of communication channels.

Multi-path Propagation effects forbid increasing of transmission rates.

OUTLINE

OVERVIEW ABOUT OFDMDEFINITION AND PRINCIPLESOFDM ADVANTAGES & DRAWBACKS

OFDM DEFINITION

OFDM = Orthogonal FDM Carrier centers are put on orthogonal

frequencies ORTHOGONALITY - The peak of each signal

coincides with trough of other signals Subcarriers are spaced by 1/Ts

Modulation

PRINCIPLES

BASIC IDEA : Channel bandwidth is divided into multiple subchannels to reduce ISI and frequency-selective fading.

Multicarrier transmission : Subcarriers are orthogonal each other in frequency domain.

PRINCIPLES

Time-domain spreading: Spreading is achieved in the time-domain by

repeating the same information in an OFDM symbol on two different sub-bands => Frequency Diversity.

Frequency-domain spreading: Spreading is achieved by choosing conjugate

symmetric inputs for the input to the IFFT (real output)

Exploits frequency diversity and helps reduce the transmitter complexity/power consumption.

FDM OFDM

Frequency Division Multiplexing

OFDM frequency dividing

EARN IN SPECTRAL EFFICIENCY

The baseband OFDM signals can be written as

Where is the central frequency of the mth sub-channel and is the corresponding transmitted symbol.

The signals are orthogonal over [0, T ] as illustrated below:

OFDM THEORY

1

0

0 ,2exp)(N

mm Ttt

T

mjXtx

T

mfm

mX

tT

mj 2exp

ml

T

dttT

ljt

T

mjT )2exp().2exp(1

0

FEC IFFT

DAC

LinearPA

add cyclic extension

bits

fc

OFDM symbol

Pulse shaper &

view this as a time tofrequency mapper

Generic OFDM Transmitter

Complexity (cost) is transferred back from the digital to the analog domain!

Serial toParallel

AGC

fc

VCO

Sampler FFTError

gross offset

Slot &

fine offset

Freq. Offset

Estimation

TimingSync.

(of all tones sent in one OFDM symbol)

Generic OFDM Receiver

RecoveryP/S and

Detection

OUTLINE

OVERVIEW ABOUT OFDMDEFINITION AND PRINCIPLESOFDM ADVANTAGES &

DRAWBACKSAPPLICATIONS

OFDM ADVANTAGES OFDM is spectrally efficient

IFFT/FFT operation ensures that sub-carriers do not interfere with each other.

OFDM has an inherent robustness against narrowband interference. Narrowband interference will affect at most a couple of subchannels. Information from the affected subchannels can be erased and recovered via the forward error correction (FEC) codes.

Equalization is very simple compared to Single-Carrier systems

OFDM ADVANTAGES OFDM has excellent robustness in multi-path environments.

Cyclic prefix preserves orthogonality between sub- carriers. Cyclic prefix allows the receiver to capture multi- path energy more efficiently.

Ability to comply with world-wide regulations: Bands and tones can be dynamically turned on/off to comply with changing regulations.

Coexistence with current and future systems: Bands and tones can be dynamically turned on/off for enhanced coexistence with the other devices.

OFDM DRAWBACKS

High sensitivity inter-channel interference, ICI

OFDM is sensitive to frequency, clock and phase offset

The OFDM time-domain signal has a relatively large peak-to-average ratio tends to reduce the power efficiency of the RF

amplifier non-linear amplification destroys the

orthogonality of the OFDM signal and introduced out-of-band radiation

OUTLINE

OVERVIEW ABOUT OFDMDEFINITION AND PRINCIPLESOFDM ADVANTAGES & DRAWBACKSAPPLICATIONS

Standards

Digital Audio Broadcasting (DAB)Digital Video Broadcasting (DVB)Asymmetric Digital Subscriber Line (ADSL)Wireless LAN IEEE 802.11a

Wireless networking, device connectivityProposed for 802.16 standard

Connection between subscriber's transceiver station and a base transceiver station

Digital Radio

Coded OFDM Transmitter

COFDM Transmitter  

DRM

For HF Frequencies 4.5 – 5 kHz for half channel

Data Rates of 4 – 14.5 kbit/s

9 – 10 kHz for full channelData Rates of 8 – 35kbit/s

18-20 kHz for double channelData Rates of 31 – 72kbit/s

Products

AOR ARD9800 http://www.hamradio-dv.org/aor.htm

Icom D-StarWinDRM

Proprietary OFDM Flavours

Wideband-OFDM(W-OFDM) of Wi-LAN

www.wi-lan.com

Flash OFDMfrom Flarion

www.flarion.com

Vector OFDM(V-OFDM) of Cisco, Iospan,etc.

www.iospan.com

-- 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 MANCisco leads the Broadand Wireless Internet Forum (BWIF)