ofdm modulation

7/30/2019 ofdm MODULATION

1/36

9. Multi Carrier Modulation and OFDM


2/36

Transmission of Data Through Frequency Selective Time Varying Channels

We have seen a wireless channel is characterized by time spreadand frequencyspread.

Time Spread

Frequency Spread


3/36

ifsymbol duration >> time spreadthen there is almost no Inter Symbol Interference

(ISI).

1 0 time

channel

1 0

phase still recognizable

Problem with this: Low Data Rate!!!

Single Carrier Modulation in Flat Fading Channels


4/36

this corresponds to Flat Fading

Frequency Frequency

channel

Flat Freq. Response

Frequency

in the Frequency Domain


5/36

ifsymbol duration ~ time spreadthen there is considerable Inter Symbol Interference

(ISI).

1 0

time

channel

? ?

phase not recognizable

Single Carrier Modulation in Frequency Selective Channels


6/36

One Solution: we need equalization

channel equalizer

1 0

time

1 0

time

Channel andEqualizer

Problems with equalization: it might require training data (thus loss of bandwidth)

if blind, it can be expensive in terms computational effort

always a problem when the channel is time varying


7/36

let symbol duration >> time spreadso there is almost no Inter Symbol Interference(ISI);

send a block of data using a number of carriers (Multi Carrier)

1 0

time channel

time

time

0

0

1

1

symbol symbol

The Multi Carrier Approach


8/36

Compare Single Carrier and Multi Carrier Modulation

Frequency Frequency

channel

0 1 0 1 1 1

Block ofsymbols

subcarriers

Each subcarrier seesa Flat Fading

Channel: EasyDemod

MC

Frequency

1

One symbol

Frequency

Flat Fading Channel:Easy Demod

SC

101 1

0 1 0 1 1 1


9/36

In MC modulation each MC symbol is defined on a time interval and it contains ablock of data

data intervalguard interval

OFDM Symbol

data datadatadata

data

MAX channel time spreadwith

Structure of Multi Carrier Modulation


10/36

the guard time is long enough, so themultipath in one block does not affect

the next block

Data BlockData Block

TX RX

We leave a guard time betweenblocks to allow multipath

Guard Time

data+guard

Guard Time

TX

RX

NO Inter Block Interference!


11/36

Baseband Complex Signal:

MC Signal

Transmitted Signal:


12/36

Orthogonal Subcarriers and OFDM

data intervalguard interval

Choose:

Orthogonality:


13/36

still orthogonal at the receiver!!!

Orthogonality at the Receiver

transientresponse

Transmittedsubcarrier Channel

(LTI)

Receivedsubcarrier

steady stateresponse

OFDM b l i di t ti


14/36

Let

be the sampling frequency;

be the number of data samples in each symbol; the subcarriers spacing

Then:

with the guard time.

OFDM symbols in discrete time

S OFDM S b l


15/36

Summary OFDM Symbol

Sampling Interval

guard data

TIME:

Freq spacing

FREQUENCY:

# samples# subcarriers


16/36

OFDM Symbol and FFT

Where:

positive subcarriers

negative subcarriers

unused subcarriers


17/36

Guard Time with Cyclic Prefix (CP)

CP from the periodicity

IFFT{X}CP

OFDM Demod lator


18/36

OFDM Demodulator

with

See each block:

No Inter Block Interference

O ll St t f OFDM C S t


19/36

Overall Structure of OFDM Comms System

IFFT +CP P/S

FFT -CP S/P

Simple One Gain Equalization


20/36

To recover the transmitted signal you need a very simple one gain equalization:

received transm. noise

channel

Use simple Wiener Filter:

Simple One Gain Equalization

OFDM as Parallel Flat Fading Channels


21/36

OFDM as Parallel Flat Fading Channels

Significance: a Freq. Selective Channel becomes NFlat Fading Channels

OFDMMod

OFDMDemod

FrequencySelectivechannel

NFlatFading

Channels

OFDM Parameters


22/36

OFDM Parameters

Summarize basic OFDM Parameters:

sampling rate in Hz

N length of Data Field in number of samples

L length of Cyclic Prefix in number of samples

total number of Data Subcarriers

data

time

data

frequency

guard guard guard


23/36

IEEE 802.11a:

Frequency Bands: 5.150-5.350 GHz and 5.725-5.825 GHz (12 channels)

Modulation OFDM

Range: 100m

IEEE 802.11g

Frequency Bands: 2.412-2.472GHz

Modulation: OFDM

Range: 300m

Channel Parameters: FCC


24/36

Channel Parameters: FCC

Example: the Unlicensed Band 5GHz U-NII (Unlicensed National Information Infrastructure)

4 channels in the range 5.725-5.825GHz

8 channels in the range 5.15-5.35GHz

Channel Parameters: Example IEEE802 11


25/36

Channel Parameters: Example IEEE802.11

In terms of a Transmitter Spectrum Mask (Sec. 17.3.9.2 in IEEE Std 802.11a-1999)

Typical SignalSpectrum

Typical BW~16 MHz

In either case:


26/36

In either case:

Sampling frequency

FFT size

Cyclic Prefix

DATACP


27/36

Sub-carriers: (48 data + 4 pilots) + (12 nulls) = 64

Pilots at: -21, -7, 7, 21

Frequency TimeIFFT


28/36

DATA

Frequencies:

Subcarriers index


29/36

Time Block:

Overall Implementation (IEEE 802.11a with 16QAM).


30/36


1. Map encoded data into blocks of192 bits and 48 symbols:

data

EncodeInterleave

010011010101

Buffer(192 bits)

111001111000

1101

4x48=192 bits

Map to16QAM

+1+j3

-1+j

+3-j3

+1-j



31/36


2. Map each block of48 symbols into 64 samples

+1+j3

-3-j

+3-j3

+1-j

IFFT

time domainfrequency domain

null

null

24 data2 pilots

24 data2 pilots

Channel Parameters: Physical


32/36

Constraints on OFDM Symbol Duration:

to minimize CP overhead

roughly!!!

Frequency Spread

Time Spread

for channel Time Invariant

C y

S f OFDM d Ch l P t


33/36

Summary of OFDM and Channel Parameters

Channel:

1. Max Time Spread sec

2. Doppler Spread Hz

3. Bandwidth Hz

4. Channel Spacing Hz

OFDM (design parameters):

1. Sampling Frequency

2. Cyclic Prefix

3. FFT size (power of 2)

4. Number of Carriers

Example: IEEE802.11a


34/36

Channel:

1. Max Time Spread

2. Doppler Spread

3. Bandwidth

4. Channel Spacing

OFDM (design parameters):

1. Sampling Frequency

2. Cyclic Prefix

3. FFT size (power of 2)4. Number of Carriers

p

Applications: various Area Networks


35/36

According to the applications, we define three Area Networks:

Personal Area Network (PAN), for communications within a few meters. This is the typical

Bluetooth or Zigbee application between between personal devices such as your cell phone,desktop, earpiece and so on;Local Area Network (LAN), for communications up 300 meters. Access points at theairport, coffee shops, wireless networking at home. Typical standard is IEEE802.11 (WiFi) orHyperLan in Europe. It is implemented by access points, but it does not support mobility;Wide Area Network (WAN), for cellular communications, implemented by towers. Mobilityis fully supported, so you can move from one cell to the next without interruption. Currently itis implemented by Spread Spectrum Technology via CDMA, CDMA-2000, TD-SCDMA,EDGE and so on. The current technology, 3G, supports voice and data on separate networks.For (not so) future developments, 4G technology will be supporting both data and voice on thesame network and the standard IEEE802.16 (WiMax) seems to be very likely

pp

More Applications


36/36

pp

1. WLAN (Wireless Local Area Network) standards and WiFi. In particular:

IEEE 802.11a in Europe and North America

HiperLAN /2 (High Performance LAN type 2) in Europe and North America MMAC (Mobile Multimedia Access Communication) in Japan

2. WMAN (Wireless Metropolitan Network) and WiMax

IEEE 802.16

3. Digital Broadcasting Digital Audio and Video Broadcasting (DAB, DVB) in Europe

4. Ultra Wide Band (UWB) Modulation

a very large bandwidth for a very short time.

5. Proposed for IEEE 802.20 (to come) for high mobility communications (cars,trains )

ofdm modulation

Documents