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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. Overview.

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Presentation Transcript
slide1

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
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
slide3

Spectrum of an OFDM Signal

significant out-of-band radiation

slide4

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

 intersymbol interference (ISI)

  • Guard bands at the borders of the OFDM spectrum, e.g. DVB-T

 waste of scarce spectral ressources, DVB-T: ca. 16%

slide5

New Idea: Cancellation Carriers

cancellation

carrier

cancellation

carrier

  • 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

slide6

Optimization: Constrained Least Squares

s:Vector of

sampled signal in optimization range

g: Vector of

weighting factors

C: Matrix with

non-weighted sampled

cancellation carriers

in the columns

Constraint:

Limit power of cancellation carriers

  • Degrees of freedom:
  • Position of cancellation carriers in the spectrum
  • Number of cancellation carriers
  • Different constraints
  • Optimization range
slide7

data carriers

data carriers

OFDM signal

with cancellation

carriers

cancellation

carriers

cancellation

carriers

Spectrum With Weighted Cancellation Carriers

amplitude

slide8

Spectrum With and Without Cancellation Carriers

Parameters:

BPSK,

12 data carriers

2x1 cancellation carrier

optimization range:

32 sidelobes

ø - 19 dB

slide9

Spectrum With and Without Cancellation Carriers

Parameters:

BPSK,

12 data carriers

2x2 cancellation carriers

optimization range:

32 sidelobes

symbol vector: 1 1 1 1 1 1 1 1 1 1 1 1

ø - 40 dB

slide10

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
  • mean power for CCs:
  • 45% of total power

- 16 dB

- 34 dB

slide11

Power Ratio of Cancellation Carriers to Total Power for All Possible Symbol Vectors

power of cancellation carriers: 30%

unconstrained

mean: 0.26

max: 0.30

mean: 0.45

max: 0.92

87 % of symbol vectors use maximum

amount of power for cancellation carriers

slide12

Mean Sidelobe Suppression for Different Constraints

Parameters:

BPSK

12 data carriers

2x2 cancellation carriers

optimization range: 32 sidelobes

- 28 dB

- 26 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

- 23 dB

- 20 dB

- 34 dB

slide13

Bit Error Rate Performance

Parameters: 2x2 cancellation carriers, 12 data carriers, BPSK

no coding, AWGN+Rayleigh-fading channel, Zero Forcing+Hard Decision

.

slide14

Application: OFDM Overlay Systems

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

slide15

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

slide16

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
slide17

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