A New Technique for Sidelobe Suppression in OFDM Systems Sinja Brandes German Aerospace Center (DLR) Institute of Communications and Navigation Oberpfaffenhofen, Germany COST 289, 7th MCM, Oberpfaffenhofen, Germany 7 March, 2005

Overview Problem Definition and Techniques for Sidelobe Suppression Principle of Cancellation Carriers Simulation Results Comparison With Existing Methods Application: OFDM Overlay Systems Summary and Outlook

Spectrum of an OFDM Signal significant out-of-band radiation

Techniques for Sidelobe Suppression Pulse shaping e.g. raised-cosine pulse instead of rectangular pulse  intersymbol interference (ISI)  (I)FFT can’t be applied for modulation/demodulation Windowing of transmission signal in frequency domain  expansion of signal in time domain Guard bands at the borders of the OFDM spectrum, e.g. DVB-T  waste of scarce spectral ressources, DVB-T: ca. 16%

New Idea: Cancellation Carriers cancellation carriers are not used for data transmission cancellation carriers carry complex weighting factors weighting factors are determined such that the sidelobes of the transmission signal are minimized data carriers sidelobes that should be suppressed sidelobes that should be suppressed

Optimization: Constrained Least Squares s: Vector of sampled signal in optimization range C: Matrix with non-weighted sampled cancellation carriers in the columns g: Vector of weighting factors Constraint: Limit power of cancellation carriers Degrees of freedom: Position of cancellation carriers in the spectrum Number of cancellation carriers Different constraints Optimization range

Spectrum With Weighted Cancellation Carriers OFDM signal with cancellation carriers cancellation data carriers data carriers amplitude

Spectrum With and Without Cancellation Carriers Parameters: BPSK, 12 data carriers 2x1 cancellation carrier optimization range: 32 sidelobes ø - 19 dB

Spectrum With and Without Cancellation Carriers symbol vector: 1 1 1 1 1 1 1 1 1 1 1 1 Parameters: BPSK, 12 data carriers 2x2 cancellation carriers optimization range: 32 sidelobes ø - 40 dB

Suppression of Sidelobes x-axis: all possible symbol vectors y-axis: mean power spectral density of sidelobes for each symbol vector BPSK 12 data carriers unconstrained optimization 1 cancellation carrier mean power for CCs: 19% of total power 2 cancellation carriers 45% of total power - 16 dB - 34 dB

power of cancellation carriers: 30% Power Ratio of Cancellation Carriers to Total Power for All Possible Symbol Vectors unconstrained power of cancellation carriers: 30% mean: 0.45 max: 0.92 mean: 0.26 max: 0.30 87 % of symbol vectors use maximum amount of power for cancellation carriers

Mean Sidelobe Suppression for Different Constraints Parameters: BPSK 12 data carriers 2x2 cancellation carriers optimization range: 32 sidelobes - 28 dB - 26 dB - 23 dB power of cancellation carriers … …unconstrained …limited to 50% of total power …limited to 40% of total power …limited to 30% of total power …limited to 20% of total power - 20 dB - 34 dB

Bit Error Rate Performance Parameters: 2x2 cancellation carriers, 12 data carriers, BPSK no coding, AWGN+Rayleigh-fading channel, Zero Forcing+Hard Decision Amount of power spent for CCs Maximal SNR loss (symbol vectors with max. power) Real unconstrained  (0%) 3.5 dB 50% 3 dB (50%) 2.4 dB 40% 2.22 dB (67%) 1.9 dB 30% 1.55 dB (83%) 1.4 dB 20% 0.96 dB (91%) 0.89 dB 10% 0.45 dB (98%) 0.44 dB 5% 0.22 dB (100%) 0.22 dB .

Application: OFDM Overlay Systems task: suppress sidelobes subcarriers used by licenced systems free subcarriers used by OFDM overlay system challenges: co-existence of both systems avoid interference towards licenced system task: suppress sidelobes

Application: OFDM Overlay Systems subcarriers used by licenced systems free subcarriers used by OFDM overlay system Parameters: BPSK 13 (=5+8) data carriers 4x2 cancellation carriers optimization range: all displayed sidelobes joint optimization of all cancellation carriers

Comparison With Existing Methods Advantages: (I)FFT can still be applied for modulation/demodulation no additional ISI smaller guard bands (some guard carriers can be used for data transmission) Possible drawbacks: slight loss in BER performance computational complexity of least squares optimization

Summary and Outlook Principle of cancellation carriers: cancellation carriers are not used for data transmission, but carry complex weighting factors weighting factors are determined such as to minimize sidelobes of the transmission signal significant reduction of sidelobes promising approach for sidelobe suppression in overlay systems Further investigations: optimization of minimization algorithm and parameters implementation in overlay systems influence on PAPR