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
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
OFDM Transceiver RF TX D/A Binary input QAM mapper Encoder Interleaver Pilot insertion S/P P/S CP and windowing IFFT FFT QAM demapper Decoder De- interleaver Equalizer P/S S/P CP removal Binary output Timing and frequency synchronization RF RX A/D
OFDM spectrum, cyclic prefix and postfix Normalized frequency
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).
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
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
Interesting directions of research • PAPR reduction • Clipping and windowing, recursive clipping • Reference signal subtraction
PAPR reduction, cont. • Coding • Golay complementary codes • generalized Reed-Muller codes • Parallel combinatory OFDM signaling • Multiple signal representation • Pre-distortion
PAPR reduction - Multiple signal representation • Selected Mapping
PAPR reduction - Multiple signal representation, cont. • Partial Transmit Sequences
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
Interesting directions of research, cont. • Impulse noiserejection 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. • 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.
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 • ?