1. 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

2. 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

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

4. 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%

5. 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

6. 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

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

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

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

10. 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

11. 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

12. 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

13. 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 .

14. 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

15. 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

16. 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

17. 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