A New Technique for Sidelobe Suppression in OFDM

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

Overview w Problem Definition and Techniques for Sidelobe Suppression w Principle of Cancellation Carriers w Simulation Results w Comparison With Existing Methods w Application: OFDM Overlay Systems w Summary and Outlook Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 2

Spectrum of an OFDM Signal significant out-of-band radiation Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 3

Techniques for Sidelobe Suppression w Pulse shaping e. g. raised-cosine pulse instead of rectangular pulse intersymbol interference (ISI) (I)FFT can’t be applied for modulation/demodulation w Windowing of transmission signal in frequency domain expansion of signal in time domain intersymbol interference (ISI) w Guard bands at the borders of the OFDM spectrum, e. g. DVB-T waste of scarce spectral ressources, DVB-T: ca. 16% Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 4

New Idea: Cancellation Carriers cancellation carrier used for data transmission data carriers sidelobes that should be suppressed Ø cancellation carriers are not sidelobes that should be suppressed Ø cancellation carriers carry complex weighting factors Ø weighting factors are determined such that the sidelobes of the transmission signal are minimized Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 5

Optimization: Constrained Least Squares s: Vector of C: Matrix with g: Vector of sampled signal in optimization range non-weighted sampled cancellation carriers in the columns 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 Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 6

Spectrum With Weighted Cancellation Carriers amplitude data carriers OFDM signal with cancellation carriers Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 7

Spectrum With and Without Cancellation Carriers Parameters: BPSK, 12 data carriers 2 x 1 cancellation carrier optimization range: 32 sidelobes ø - 19 d. B Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 8

Spectrum With and Without Cancellation Carriers symbol vector: 1 1 1 Parameters: BPSK, 12 data carriers 2 x 2 cancellation carriers optimization range: 32 sidelobes ø - 40 d. B Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 9

Suppression of Sidelobes x-axis: all possible symbol vectors y-axis: mean power spectral density of sidelobes for each symbol vector - 16 d. B - 34 d. B BPSK 12 data carriers unconstrained optimization • 1 cancellation carrier mean power for CCs: 19% of total power • 2 cancellation carriers mean power for CCs: 45% of total power Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 10

Power Ratio of Cancellation Carriers to Total Power for All Possible Symbol Vectors unconstrained mean: 0. 45 max: 0. 92 power of cancellation carriers: 30% mean: 0. 26 max: 0. 30 87 % of symbol vectors use maximum amount of power for cancellation carriers Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 11

Mean Sidelobe Suppression for Different Constraints - 20 d. B - 23 d. B - 26 d. B Parameters: BPSK 12 data carriers 2 x 2 cancellation carriers optimization range: 32 sidelobes - 28 d. B - 34 d. B 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 Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 12

Bit Error Rate Performance Parameters: 2 x 2 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 Real SNR loss unconstrained (0%) 3. 5 d. B 50% 3 d. B (50%) 2. 4 d. B 40% 2. 22 d. B (67%) 1. 9 d. B 30% 1. 55 d. B (83%) 1. 4 d. B 20% 0. 96 d. B (91%) 0. 89 d. B 10% 0. 45 d. B (98%) 0. 44 d. B 5% 0. 22 d. B (100%) 0. 22 d. B (symbol vectors with max. power) Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation . 13

Application: OFDM Overlay Systems subcarriers used by licenced systems free subcarriers used by OFDM overlay system challenges: Ø co-existence of both systems Ø avoid interference towards licenced system task: suppress sidelobes Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 14

Application: OFDM Overlay Systems subcarriers used by licenced systems free subcarriers used by OFDM overlay system Parameters: BPSK 13 (=5+8) data carriers 4 x 2 cancellation carriers optimization range: all displayed sidelobes joint optimization of all cancellation carriers Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 15

Comparison With Existing Methods Advantages: w (I)FFT can still be applied for modulation/demodulation w no additional ISI w smaller guard bands (some guard carriers can be used for data transmission) Possible drawbacks: w slight loss in BER performance w computational complexity of least squares optimization Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 16

Summary and Outlook w 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 w Further investigations: – optimization of minimization algorithm and parameters – implementation in overlay systems – influence on PAPR Sinja Brandes, German Aerospace Center (DLR), Institute of Communications and Navigation 17