ofdm and multicarrier communications – an overview, issues and knowledge gaps

OFDM and Multicarrier Communications – an overview, issues and knowledge gaps

POZNAN UNIVERSITY OF TECHNOLOGYPOZNAN UNIVERSITY OF TECHNOLOGY

Hanna BoguckaHanna Bogucka

WPR1 Kick-off meeting, Paris,WPR1 Kick-off meeting, Paris, June June 2525, 2004, 2004

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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

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OFDM Transceiver

Encoder Interleaver QAM mapper

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Decoder De-interleaver

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A/DRF RXTiming and frequency

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OFDM spectrum, cyclic prefix and postfix

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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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Partial Transmit Sequences

PAPR reduction - Multiple signal representation, cont.

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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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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 non-

reliable blocks exceeding a certain threshold Rejection of R-S symbols containing bits transmitted on

non-reliable subcarriers

Interesting directions of research, cont.

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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, pre-equalization, joint detection for multiuser OFDM, and so on.

Interesting directions of research, cont.

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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 ?