1. Spectral Mask Implications
March 2008
doc.: IEEE yy/xxxxr0
September 2008
Spectral Mask Implications
IEEE P Wireless RANs Date:
Authors:
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Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm

2. March 2008
doc.: IEEE yy/xxxxr0
September 2008
Abstract
This updated presentation gives the results of an updated simulation for estimating the power spectral density (PSD) of the transmission
The previous simulation was a “quick and dirty” simulation. In particular, the previous simulation incorrectly aligned the PSD estimation window with the OFDM symbol edge, which gave an overly optimistic view of the PSD
This presentation describes the new simulator and provides simulation results
Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm

3. Outline Description of the simulator
September 2008
Outline
Description of the simulator
Simulated PSD estimate before the PA
Simulated PSD estimates after the PA, for various PA back-off values
Discussion on Next Steps
Steve Shellhammer, Qualcomm

4. Description of the Simulator
September 2008
Description of the Simulator
Synthesize a sequence of OFDM symbols
Each OFDM symbol includes
Support for both QPSK and 16QAM (random data)
Cyclic prefix
null tones
Oversampling (Increased sampling rate is needed to observe spectrum beyond channel of operation)
Third-order Butterworth filter
Non-linear PA model
Power Spectral density estimation
Steve Shellhammer, Qualcomm

5. Simulator Block Diagram
September 2008
Simulator Block Diagram
OFDM Signal Generator
With Oversampling
(Baseband)
Third-order Low-pass Filter
fc = 4.5 MHz
PSD Estimation
FFT using Hanning
Window
Non-linear PA Model
Steve Shellhammer, Qualcomm

6. Third Order Butterworth LPF
September 2008
Third Order Butterworth LPF
3 MHz
12 MHz
Signal Pass-band
Oversampling by 4x
Steve Shellhammer, Qualcomm

7. Spectral Estimation Capture a window of samples
September 2008
Spectral Estimation
Capture a window of samples
Multiply by a Hanning window
Calculate PSD estimate using FFT
Average PSD over multiple captured windows
Steve Shellhammer, Qualcomm

8. March 2008
doc.: IEEE yy/xxxxr0
September 2008
Non-Linear PA Model
The nonlinearity of the power amplifier (PA) distorts the OFDM spectrum
In this document the nonlinear PA model is used
This model was used in the evaluation of n proposals [3] and was originally used in modeling b [4].
This same form of non-linearity is specified in the channel models document [5]
Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm

9. Non-Linear PA Model Asat September 2008 March 2008
doc.: IEEE yy/xxxxr0
September 2008
Non-Linear PA Model
Asat
Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm

10. March 2008
doc.: IEEE yy/xxxxr0
September 2008
1-dB Compression Point
The 1-dB compression point is the point at which the PA output is 1 dB less than it would be in a perfectly linear amplifier
To ensure linear operation one must operate well below the 1-dB compression point
The closer the signal is to the 1-dB compression point the more distortion is produced
Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm

11. Simulation Parameters
March 2008
doc.: IEEE yy/xxxxr0
September 2008
Simulation Parameters
Signal
2k OFDM symbol
QPSK (Random data)
Null tones (Guard bands and DC)
1/4 Cyclic Prefix
Oversampling by 4x
Signal consists of 200 OFDM symbols
Third-order Butterworth Filter with fc = 4.5 MHz
Initially no PA
PSD Estimation
Window Size = 10k
Hanning window
Averaged PSD over more than 100 windows
Plot PSD over positive frequencies (symmetric)
Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm

12. September 2008
PSD of PA Input Signal
Steve Shellhammer, Qualcomm

13. Simulations with PA Back-off
September 2008
Simulations with PA Back-off
Added non-linear PA model to simulation
Adjusted PA input signal power to find the 1-dB compression point
PA output is 1 dB lower than an ideal PA
Ran simulations for three values of PA back-off, from the 1-dB compression point
8 dB Back-off
10 dB Back-off
12 dB Back-off
Plotted PSD of PA Output signal
Steve Shellhammer, Qualcomm

14. PSD of PA Output Signal with 8 dB Back-off
September 2008
PSD of PA Output Signal with 8 dB Back-off
Steve Shellhammer, Qualcomm

15. PSD of PA Output Signal with 10 dB Back-off
September 2008
PSD of PA Output Signal with 10 dB Back-off
Steve Shellhammer, Qualcomm

16. PSD of PA Output Signal with 12 dB Back-off
September 2008
PSD of PA Output Signal with 12 dB Back-off
Steve Shellhammer, Qualcomm

17. PSD of PA Input and PA Output for several values of PA Back-off
September 2008
PSD of PA Input and PA Output for several values of PA Back-off
8 dB Back-off
10 dB Back-off
12 dB Back-off
PA Input PSD
Steve Shellhammer, Qualcomm

18. Spectral Mask The draft [1] does not have a spectral mask
March 2008
doc.: IEEE yy/xxxxr0
September 2008
Spectral Mask
The draft [1] does not have a spectral mask
The draft [1] requires that the field strength outside the occupied channel is 4.8 V/m, measured at 3 meters in a 120 kHz bandwidth
According to Gerald [2] this equates to a spectral mask of -100 dBr outside the occupied channel
Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm

19. Possible Spectral Mask – Proposed by Gerald
September 2008
Possible Spectral Mask – Proposed by Gerald
-33 dB
-67.5 dB
Proposed Mask
12 dB Back-off
Steve Shellhammer, Qualcomm

20. Next Steps – Spectral Mask
September 2008
Next Steps – Spectral Mask
There are several paths that the working group can take regarding the spectral mask
Add a spectral mask to the draft which is consistent with the current signal design
Modify the signal design to improve the PSD so that a improved spectral mask can be included in the draft
Which approach does the WG prefer?
Steve Shellhammer, Qualcomm

21. March 2008
doc.: IEEE yy/xxxxr0
September 2008
References
IEEE P802.22™/ DRAFTv1.0 Draft Standard for Wireless Regional Area Networks Part 22: Cognitive Wireless RAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Policies and procedures for operation in the TV Bands, April 2008
Gerald Chouinard, Wireless Microphone Sensing, IEEE /530r0, November 2007
Adrian P Stephens, IEEE TGn Comparison Criteria, IEEE /814r19, March 2004
Mark Webster, Suggested PA Model for HRb, IEEE /294r0, September 2000
Eli Sofer and Gerald Chouinard, WRAN Channel Modeling, IEEE /55r7, August 2005
Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm