1 Salient features 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. 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.
Salient features. 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. Time-domain spreading: - PowerPoint PPT Presentation
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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)
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Timing Offset: 10% of Sampling Time Period
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Timing Clock Offset: 5% of Sampling Time Period per Frame
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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)
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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