Transcript OFDM and Multicarrier Communications – an overview, issues and knowledge gaps POZNAN UNIVERSITY OF TECHNOLOGY Hanna Bogucka WPR1 Kick-off meeting, Paris, June 25, 2004

OFDM and Multicarrier Communications –
an overview, issues and knowledge gaps
POZNAN UNIVERSITY OF TECHNOLOGY
Hanna Bogucka
WPR1 Kick-off meeting, Paris, June 25, 2004
Outline
 OFDM and multicarrier modulation concept
 Applications
 OFDM advantages
 OFDM drawbacks
 Interesting directions of research
 Knowledge gaps
 Discussion
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OFDM and multicarrier modulation concept
Orthogonal Frequency Division Multiplexing (OFDM), is a form of
multicarrier transmission, in which a high-rate data stream is
transmitted in a parallel manner over a number of low-rate orthogonal
subcarriers


 N 1

y(t )  ex(t )  e  d n,k  exp( j 2f k t ) 
 k 0

f k  k  f
f  T1
U
N 1
x n, m   d n, k e
for nTU  t  (n  1)TU ,
t  m t
j 2 km
N
k 0
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OFDM Transceiver
RF TX
Binary
input
Encoder
Interleaver
QAM
mapper
Pilot
insertion
S/P
D/A
P/S
CP and
windowing
S/P
CP
removal
IFFT
FFT
Binary
output
Decoder
Deinterleaver
QAM
demapper
Equalizer
P/S
RF RX
A/D
Timing and
frequency
synchronization
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Power Spectral Density [dBr]
OFDM spectrum, cyclic prefix and postfix
TS
Tprefix
TU
Tpostfix
Normalized frequency
TS
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Applications
 High-speed digital subscriber lines (HDSL, ADSL, VDSL)
 Digital broadcasting systems (DAB, DVB-T)
 Wireless LANs (HIPERLAN/2, IEEE 802.11a, IEEE 802.16,
MMAC)
 Fixed broadband wireless access (IEEE 802.11g)
 Considered for wireless ATM network (Magic-WAND),
 Considered for future broadband radio area networks and
multimedia communications (European and American
projects).
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OFDM Advantages
 Robustness against multipath fading and intersymbol interference
and narrow-band interference (MC-CDMA)
 High spectral efficiency, no side bandwidth required for system
operation,
 Simple frequency domain equalization
 Single-frequency network configuration possibility
 Inherent flexibility, especially with the application of
– adaptive bit and power loading,
– adaptive modulation and coding,
– adaptive subcarrier allocation,
– space-time processing, MIMO, smart antennas,
– multicarrier CDMA, MC-DS-CDMA, and so on
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OFDM Drawbacks
 High Peak-to-Average Power Ratio (PAPR) of the transmitted
signal resulting in:
 clipping noise (limited quantization levels, rounding and truncation,
during IFFT and FFT computation)
 nonlinear distortions of power amplifiers
 BER performance degradation,
 energy spilling into adjacent channels,
 intermodulation effects on the subcarriers, warping of the signal
constellation in each subchannel.
 Sensitivity to frequency offset and phase noise, which are
sources of intercarrier interference,
 Whitening of impulse noise
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Interesting directions of research
 PAPR reduction
 Clipping and windowing, recursive clipping
 Reference signal subtraction
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PAPR reduction, cont.
 Coding
 Golay complementary codes
 generalized Reed-Muller codes
 Parallel combinatory OFDM signaling
 Multiple signal representation
 Pre-distortion
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PAPR reduction - Multiple signal representation
 Selected Mapping
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PAPR reduction - Multiple signal representation, cont.
 Partial Transmit Sequences
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Interesting directions of research, cont.
 Conflating adaptive modulation and coding for OFDM
 Adaptive bit and power loading
 Adaptive resource management (subcarrier and spreading code
allocation)
 Throughput-oriented, power-oriented and delay-oriented adaptation
strategies
 Frequency-domain processing for single-carrier modulation
 The use of IOTA (Isotropic Orthogonal Transfer Algorithm) function
 Fast OFDM (FOFDM) with intercarrier frequency spacing equal to
half of the inverse of the signalling interval
 Filtered multitone FDMA
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Interesting directions of research, cont.
 Impulse noise rejection for OFDM
– Non-linear clipping
– Median filtering and spectrum analysis
– Application of guard frequencies
– Application of pilots and DSP algorithms
– MSE monitoring and R-S coding
 Adaptation of decision areas for data symbols
 Rejection of OFDM block in case of a number of nonreliable blocks exceeding a certain threshold
 Rejection of R-S symbols containing bits transmitted on
non-reliable subcarriers
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Interesting directions of research, cont.
 MC-CDMA and MC-DS-CDMA
 Frequency hoping OFDMA
 MC-CDMA for the down-link in 4G cellular system
 MC-CDMA for the up-link with parallel interference cancellation or
joint detection
 MC-DS-CDMA for the up-link (power efficiency of a terminal)
 Adaptive MC-CDMA with capacity optimization
 Impulse noise rejection
 OFDM MIMO, space - time diversity, e.g. cyclic delay diversity
 Smart algorithms for phase-noise and frequency-offset
compensation, channel (turbo) estimation and equalization, preequalization, joint detection for multiuser OFDM, and so on.
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Knowledge gaps
 PAPR reduction methods without or minimized transmission
overhead
 For OFDM
 For MC-CDMA
 Fast link adaptation techniques for mobile users
 Fast channel prediction
 Minimization of feedback information delay
 Simple optimisation algorithm
 Cross-layer optimization (set up the basis in the physical layer)
 Multi-criteria optimization for high capacity energy-aware networks
and systems
 „Safe” impulse noise rejection
 ?
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