Ibrahim DEMIRDOGEN

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Exploring the Opportunity In OFDM-Based System
for Cognitive Radios

• Ibrahim DEMIRDOGEN
• U29388800

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Outline

• OFDM
• Cognitive Radio
• Opportunity in OFDM System
• Simulation Results
• Conclusions

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OFDM

• Why OFDM?
• Robust to time dispersive channel
• Efficient Spectrum Usage (Orthogonal subcarr.
  positioning)
• Less equalizer complexity at the receiver
• Cons
• High PAPR (Input Back Off)
• High Sidelobes (Spectral Re-growth- ACI)
• Sensitive to loss of Orthogonality

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OFDM Cont.

• Two fundamental elements of OFDM structure are
  IDFT and DFT
• IDFT
• DFT
• IFFT and FFT functions of MATLAB are
  implementation of these transforms in order to
  have less computational complexity.

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Cognitive Radio (CR)

• Goal
• Increase the bandwidth efficiency or spectrum
  usage
• How ?
• By taking advantage of vacant part of the spectrum

One day spectrum utilization of Lichtenau,
Germany, in 2001 (50 MHz- 1GHz)
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Cognitive Radio Cont.

• Primary users(PUs) have no obligation to be aware
  of secondary user
• Secondary users(CRs) have to leave when PUs need
  to use spectrum
• Secondary users(CRs) also should not interfere
  with each other.
• In order to have these capability, CR should have
  
• Intelligence to adapt itself to environment
• Awareness of user requirements
• Awareness of resources (spectrum and power supply)

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OFDM and CR

• OFDM meets the requirements of CR
• FFT operation eases the spectrum sensing in
  frequency domain.
• OFDM has adaptable parameters FFT size,
  sub-carr. spacing/power, CP size, modulation
• OFDM has an immunity to narrow-band
  interference(turn-off)
• It is interoperable with current technologies
  such as (WLAN-WMAN-WPAN)
• It has a support for multiuser access inherently.
  (OFDMA)

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Motivation

• How current methods affect the detection of
  opportunity?
• What current methods are using to find out the
  opportunity?
• Are current methods sufficient enough to exploit
  the opportunity?
• Future work
• If not, What can be done further to find out the
  opportunity?

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Exploiting the Opportunity for CR in OFDM-Based
System

• For OFDM based system, opportunity consists of
  taking advantage of unused subcarriers.
• How to find out the opportunity?
• Well-Known Method Searching the spectrum for
  empty carriers by observing the power level of
  the OFDM signal
• Searching for Opportunity in OFDM
• High sidelobes mislead the opportunity detection
  process
• Suppress sidelobes
• Change the subcarrier allocation method

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Sidelobe Suppression Techniques in OFDM

• Inserting Cancellation Carriers
• Increases PAPR
• Subcarrier weighting
• Increase BER and reduction is not as much as CC
• Using guard bands on both sides of the OFDM
  spectrum
• Using Windowing techniques before transmission

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Subcarrier Allocation Methods

• There are two types of subcarrier allocation
  methods
• Block-wise subcarrier allocation
• Preferable to exploit the opportunity
• Vulnerable to channel effects
• Scattered subcarrier allocation
• More robust to channel effects
• Less number of subcarriers are detected as
  opportunity

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Simulation

• Opportunity is searched for under these
  conditions and compared
• Windowed vs. without Windowed OFDM signal
• Block-wise and Scatter Subcarrier Allocations

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Simulation Parameters

• Total Subcarrier Number (N) 256
• Cyclic Prefix Size (Cp) N/4 (64)
• Window RC Window with 32 sample (16 LRC 16
  RRC )
• Channel Medium Delay Spread-Vehicular B Ch.
  (Most dispersive) with AWG Noise (6-Tap)
• SNR Values from 10 dB to 40 dB
• Subcarr. Block Sizes L 1-2-4-8-16-32-64-128
• Numbers of employed and unemployed subcarriers
  are equal which is 128

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Some of the Simulation PlotsUsed Window- Raised
Cosine (RC)
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RC Filtered OFDM Signal
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Window Effect on OFDM Signal (10 dB)
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Window Effect on OFDM Signal (20 dB)
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Window Effect on OFDM Signal (30 dB)
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Effect of Block Allocation Size
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Simulation Results

• Effect of window is observed above a certain SNR
  value ( 20 dB )
• Subcarrier allocation method affects the finding
  out the opportunity by only looking at the power
  spectrum of the signal
• Windowing has less effect in scattered subcarrier
  allocation
• Also, definition of the threshold value has
  critical importance
• PFA ( detected as unused)/( used subcarriers)
  should be kept minimum.
• For 20 dB SNR value with a quite low PFA
• Percentage of Detected subcarriers
• 8 for L2 and 50 for L16 with windowing
• 6 for L2 and 30 for L16 without
  windowing

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Conclusions

• Sidelobes of OFDM signal effect the opportunity
  detection for CR
• Detection of the opportunity can be increased to
  some extent by using sidelobe suppression
  techniques or using block based subcarrier
  assignment
• Even though these techniques are used, we still
  cannot fully exploit the opportunity
• So, we may need to look different dimensions !!!

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Thank You

• Questions??