Class Report

林宏穎 :

OFDM Introduction

OFDM History

1957: Kineplex multi-carrier HF modem1966: Chang, Bell Labs: OFDM paper & patent1971: Weinstein & Ebert propose use of FFT and guard interval1985: Cimini describes use of OFDM for mobile communications1987 Alard & Lasalle: OFDM for digital broadcasting1995: ETSI DAB standard: first OFDM-based standard1997: DVB-T standard1998: Magic WAND project demonstrates OFDM modems for wir

eless LAN1999: IEEE 802.11a and HIPERLAND/2 standards for wireless L

AN2000: V-OFDM for fixed wireless access2001: OFDM considered for new IEEE 802.11 and 802.16 standar

ds

Introduction to OFDM• Basic idea

– 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, 802.11a, DAB, DVB, etc.

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

frequency

FDM

frequency

OFDM

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

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)

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

Modulation System

P/SQAM

decoder

channelestima-tion &

equalizer

S/P

quadrature amplitude

modulation (QAM) enco

der

N-IFFTadd

cyclic prefix

P/SD/A +

transmit filter

N-FFT S/Premove

cyclic prefix

TRANSMITTER

RECEIVER

N subchannels N complex samples

N complex samplesN subchannels

Receive filter

+A/D

multipath channel

Single carrier modulation

Multi carrier modulation

MulticarrierMapping

Mapping

Mapping

Filter

Filter

Filter

f0

f1

fN-1

f2f1f0 fN-1

Bandlimitedsignals

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

Rate R

Rate R

Rate RRate NR

OFDM Source of Impairment

PilotInsertion

QAMMapping

FECCoding

ChannelCorrection

QAM De-Mapping

FECDecoding

IFFT(TX)

FFT(RX)

InsertGuard

Interval

RemoveGuard

Interval

DAC

ADC

IQModulator

Multi-pathFading

Channel

TimingFrequency

Synchronization

Fixed-PointComputation

Error

Power AmplifierNon-Linear

Phase noiseFrequency offset

HPA

LNA

Phase noise

AGC Amp

AGC Response Time

ADCnoise

FrequencyCorrected

Signal

Symbol timing

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

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)

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

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

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