Thus sub-carriers are able to be overlapped without interfering and to Thus, sub carriers are able to be overlapped without interfering and to maximize spectral efficiency without causing adjacent channel interference (in ideal system condition)
In addition, these sub-carriers are able to be detected correctly, since the maximum power of each sub-carrier corresponds directly with the minimum
f h dj h l
power of each adjacent channel.
April, 2011 21
Basic concept of OFDM: Rectangle pulse-shaping on the time-domainBasic concept of OFDM: Rectangle pulse shaping on the time domain
(a) DC centered spectrum with equally spaced zeros
(b) Shift spectrum with linear phase on DC pulse: move spectrum to first spectral zero
OFDM System
Basic concept of OFDM: Rectangle pulse-shaping on the time-domainBasic concept of OFDM: Rectangle pulse shaping on the time domain
Real and imaginary parts of complex exponential time series: Integer number of cycles per interval
April, 2011 23
Basic concept of OFDM: Rectangle pulse-shaping on the time-domainBasic concept of OFDM: Rectangle pulse shaping on the time domain
Spectra of complex exponential time series: Integer number of cycles per interval
OFDM System
Basic concept of OFDM: orthogonal transmissionBasic concept of OFDM: orthogonal transmission
Serial-to-parallel (S/P): once the bit-stream composing of N data symbols has been divided among the individual sub-carriers, each sub-carrier is modulated as if it was an individual
April, 2011 25
Basic concept of OFDM: orthogonal receptionBasic concept of OFDM: orthogonal reception
Efficient OFDM transmission using inverse discrete Fourier transform (IDFT)
OFDM System
Efficient OFDM transmission using inverse discrete Fourier transform (IDFT)
April, 2011 27
OFDM transceiver structure using inverse fast Fourier transform (IFFT) and OFDM transceiver structure using inverse fast Fourier transform (IFFT) and FFT at transmitter and receiver, respectively
OFDM System
OFDM converts the frequency-selective channel to frequency-flat channel in OFDM converts the frequency selective channel to frequency flat channel in terms of each frequency-bin.
Which means that OFDM system has more robust transmission
Property than SC system in such a channel.
April, 2011 29
OFDM system still suffers from ISI effect OFDM system still suffers from ISI effect
the ISI affects orthogonality between subcarriers, which leads to severe inter-(sub)carrier interference (ICI) problem
The advantages of robustness to frequency-selective channel and spectral efficiency do not be guaranteed anymore.
OFDM System
Insertion of guard-interval (GI) between OFDM symbols to prevent the ISI Insertion of guard interval (GI) between OFDM symbols to prevent the ISI effect
The GI length is larger or equal to the maximum delay spread of a channel
GI insertion with zero-padded (ZF) symbol
No ISI, but ZF still affects the orthogonality after FFT operation, since it broken the continuity of sub-carriers.
April, 2011 31
Insertion of cyclic-prefixed (CP) symbol between OFDM symbols instead of ZP Insertion of cyclic prefixed (CP) symbol between OFDM symbols instead of ZP symbol
It completely eliminates ISI and ICI.
It maintains subcarrier orthogonality.
OFDM System
Key blocks of OFDM TransceiverKey blocks of OFDM Transceiver
Pilot insertion to estimate channel information and the amount of syn. offset
Non-linear amplifier effect to IFFT output
Relation between symbol timing offset and ISI effect
Relation between frequency offset and orthogonality
One-tap channel equalization on the frequency-domain
April, 2011 33
Pilot insertionPilot insertion
Channel information estimation on the time-/-frequency domain
Compensates symbol timing and frequency offsets
Representative pilot patterns
Block-type pilot pattern arrangement
Comb-type pilot pattern arrangement
Scattered-pilot pattern arrangement
Cf) Known-symbol insertion
Known-symbol is sometimes appended in the front of frame block composing
Known symbol is sometimes appended in the front of frame block composing
OFDM System
Block-type pilot pattern arrangement Block type pilot pattern arrangement
The channel estimation can be performed by either periodically inserting pilot tones into all sub-carriers (frequency axis)
It is usually used in a severe frequency-selective channel
Channel varies slowly enough so that the channel estimation will have a good accuracy
April, 2011 35
Comb-type pilot pattern arrangement Comb type pilot pattern arrangement
The number of pilots used for channel estimation is usually much smaller than the number of sub-carriers
This method is usually used in systems having significant channel variation over a short period of time
OFDM System
Scattered-pilot pattern arrangement Scattered pilot pattern arrangement
Block-type pilot pattern + comb-type pilot pattern
This method is commonly be used in systems having significant doubly selective channels
April, 2011 37
High PAPR problemHigh PAPR problem
IFFT output shows Gaussian distribution, approximately.
High-peak random signals (a high PAPR problem) are often observed, which are distorted on the non-linear region of amplifier.
OFDM System
OFDM Input and output of non-linear amplifierOFDM Input and output of non linear amplifier
April, 2011 39
ISI effect according to symbol timing offset ISI effect according to symbol timing offset
Symbol timing offset corresponds to the starting position of FFT window.
Cases of timing offset within CP symbol
OFDM System
April, 2011 41
ICI effect according to frequency offset ICI effect according to frequency offset
Due to oscillator mismatch or Doppler Shift
Breaking orthogonality -> ICI problem
-> performance degradation
OFDM System
Efficient receiver processing according to the use of CP symbolEfficient receiver processing according to the use of CP symbol
Convert linear channel matrix to circular matrix after removing CP symbol
April, 2011 43
Simple one-tap frequency-domain equalization (FDE) realizationSimple one tap frequency domain equalization (FDE) realization
Each subcarrier can be processed independently, which means that only one-tap multiplier is sufficient in term of implementation.
Consequently, it is obvious that the FDE used for OFDM system has lower computational complexity than TED for SC system
OFDM System
An estimation of channel frequency response (CFR) using comb-type pilot An estimation of channel frequency response (CFR) using comb type pilot arrangement
Calculate the channel estimates at the pilot subcarriers
Interpolate the estimates for the other subcarriers
1
Channel, B andwidth, and S am ples
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
E x tended Reflec tion
1 Interpolated
M agnitude of Interpolation E rror For In-B and Frequenc ies
April, 2011 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 45
0 0.005
Norm aliz ed Frequenc y
Constellations before and after performing FDE Constellations before and after performing FDE
Robustness to frequency selectivity one-tap equalizer