Salient features

1

Salient features

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

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

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.

2

OFDM Transceiver

Coding

Binary Input Data

Interleaving QAM mapping

PilotInsertion S - P

IFFTFFT

DecodingDe-Interleaving QAM demapping

Channel Correction P - SBinary

Output Data

S - P

P - SAdd Cyclic extension

& Windowing

DACRF Tx

Remove Cyclic

extension

Timing &Freq.Sync.

ADCRF Rx

3

Input Vector IFFT Mapped to Output Time Series, Up-Sampled, Converted Via DAC to Waveform, and I-Q Up-Converted

4

The IFFT as Signal Generator and Interpolator

5

Adjacent Symbol Interference (ASI) Symbol Smearing Due to Channel

6

Guard Interval Inserted Between Adjacent Symbols to Suppress ASI

7

Cyclic Prefix Inserted in Guard Interval to Suppress Adjacent Channel Interference (ACI) and retain orthogonality

8

Data Length Defines Sinc Width:Spectral Spacing Matches Width

9

Extended Data Length Reduces Sinc Width: Spectral Spacing Preserved

10

11

Selection of OFDM parameters

Bandwidth, bit rate, delay spread Guard time Tg

2 to 4 times delay spread 2 to 4 depends on the order of modulation employed

Symbol duration > Guard time to maximize SNR More subcarriers, smaller spacing, implementation

complexity, more sensitivity to phase noise & frequency offset, high PAPR

Symbol duration 5 x Guard time ( 1-dB SNR loss ) Ts = 5 x Tg Tofdm = Ts + Tg

Subcarrier spacing f = 1 / Ts Number of subcarriers = 3-dB BW / f

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Example : Bit rate = 20 Mbps Tolerable delay spread = 200 ns Bandwidth < 15 MHz

Tg = 800 ns Tofdm = 5 x Tg + Tg = 4.8 sec

f = 1 / 4 sec = 250 KHz Number of bits in one OFDM symbol = 20 Mbps x 4.8 sec = 96

16-QAM with rate ½ Conv. Coding 2 bits / symbol / subcarrier 48 subcarriers 48 x 250 KHz = 12 MHz < 15 MHz

QPSK with rate ¾ coding 1.5 bits / symbol / subcarrier 64 subcarriers 64 x 250 KHz 16 MHz > 15 MHz

64 point IFFT / FFT 16 zero subcarriers oversampling

Given requirements

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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

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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.

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OFDM DRAWBACKS High sensitivity inter-channel/carrier interference, ICI

OFDM is sensitive to frequency, clock and phase offset

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

amplifier non-linear amplification destroys the

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

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OFDM Symbol: Time and Spectra Channel Input and Output

20 40 60 80 100 120 140 160 180

-0.4

-0.2

0

0.2

0.4

0.6Real Part of Time Series, Input to Channel

20 40 60 80 100 120 140 160 180

-0.4

-0.2

0

0.2

0.4

0.6Real Part of Time Series, Output of Channel

-0.5 0 0.5-30

-25

-20

-15

-10

-5

0

5

10Spectrum

-0.5 0 0.5-30

-25

-20

-15

-10

-5

0

5

10Spectrum

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Test Bench: Demonstration of Receiver I-Q Imbalances, Carrier Offset, and Timing Offset

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Time and Spectra of Sparse OFDM Symbol

0 10 20 30 40 50 60 70 80 90 100-1

-0.5

0

0.5

1Real Part OFDM Time Series

Normalized Time

Am

plitu

de

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-60

-50

-40

-30

-20

-10

0

10Spectrum

Normalized Frequency

Log

Mag

nitu

de (d

B)

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Carrier Offset: 4% of FFT Bin Width

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Time and Spectra With Frequency Offset = 0.1 Bin

0 10 20 30 40 50 60 70 80 90 100-1

-0.5

0

0.5

1Real Part OFDM Time Series with Offset Frequency = 0.1 Bin Width

Normalized Time

Am

plitu

de

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-60

-50

-40

-30

-20

-10

0

10Spectrum With Frequency Offset = 0.1 Bin Width

Normalized Frequency

Log

Mag

nitu

de (d

B)

21

Timing Offset: 10% of Sampling Time Period

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Timing Clock Offset: 5% of Sampling Time Period per Frame

23

Time and Spectra With Sample Clock Offset = 1.02 fs

0 10 20 30 40 50 60 70 80 90 100-1

-0.5

0

0.5

1

Real Part OFDM Time Series with Sampling Clock = 1.02 fs

Normalized Time

Am

plitu

de

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-60

-50

-40

-30

-20

-10

0

10

Spectrum With Sampling Clock = 1.02 fs

Normalized Frequency

Log

Mag

nitu

de (d

B)

24

Time and Spectra With Sample Clock Offset = 0.98 fs

0 10 20 30 40 50 60 70 80 90 100-1

-0.5

0

0.5

1

Real Part OFDM Time Series with Sampling Clock = 0.98 fs

Normalized Time

Am

plitu

de

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-60

-50

-40

-30

-20

-10

0

10

Spectrum With Sampling Clock = 0.98 fs

Normalized Frequency

Log

Mag

nitu

de (d

B)

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Gain Imbalance: 10% Error

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Phase Imbalance: 0.1 Radian Error

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I-Q Mixer Imbalance; 20% Gain, 0.2 Radians

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Differential Delay to I/Q Mixers, 10% of Sample Interval

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Power Amplifier Non-Linearity

0 1 2 3 40

0.5

1

1.5

2

2.5

3

3.5

4Nonlinear Transfer Function of Amplifier

1-dB Compression Point

0 2 4 6 8 10-2

-1.5

-1

-0.5

0

0.5

1

1.5

2Input and Output of Non-Linear Amplifier

-0.5 0 0.5-60

-50

-40

-30

-20

-10

0

10Spectrum of Two Input Sinusoids

Normalized Frequency-0.5 0 0.5

-60

-50

-40

-30

-20

-10

0

10Spectrum of Two Output Sinusoids

Normalized Frequency

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OFDM based Applications

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

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IEEE 802.11a Overview Carrier frequency= 5 GHz Total allotted bandwidth= 20 MHz x 10 =

200MHz Size of the FFT= 64 Number of data subcarriers= 48 Number of Pilot subcarriers= 4 FFT period= 3.2 µs Channel bandwidth used= 64/3.2 µs => 20

MHz

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Typical Configuration 52 subcarriers, 64 point FT/IFFT Symbol time 4 µs Guard time 800 ns BPSK, QPSK, 16-QAM, 64-QAM Coding rates 1/2,3/4,2/3 Bit rates 6,12,18,24,36,48,54 Mbps Channel spacing 20 MHz Tolerable delay spread about 250 ns at 24

Mbps

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DFT (FFT) as Signal Generatorfor Complex Sinusoids

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DFT (FFT) As Signal Analyzer for Complex Sinusoids

1,...,2,1,0:)()(1

0

2

NkenhkH

N

n

nkNj

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Radix-2 FFT Flow Diagrams

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OFDM Modulation With IFFTand Interpolator

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OFDM Demodulation With FFT

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OFDM Transceiver

39

Linear Versus Circular Convolution

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Fast Circular Convolution with the FFT