1. Range in 802.11y and Ramifications on OFDM Symbol Format
January 2006
doc.: IEEE /xxxxr0
January 2006
Range in y and Ramifications on OFDM Symbol Format
Date:
Authors:
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Steve Shellhammer, Qulacomm
Steve Shellhammer, Qualcomm

2. January 2006
doc.: IEEE /xxxxr0
January 2006
Abstract
The transmit power in the 3560 MHz band is higher than that allowed in the 2.4 GHz and 5 GHz band.
This results in longer range than in those two bands.
Estimates of range are given in this presentation
The longer range will lead to larger multipath delay spread which will require larger guard interval (GI) than is used in a/g
Suggestions on methods of modifying the OFDM symbol format with larger GI are given
Steve Shellhammer, Qulacomm
Steve Shellhammer, Qualcomm

3. Power Allowed in 3650 MHz Band
January 2006
Power Allowed in 3650 MHz Band
Fixed Devices
25 watts (44 dBm) Maximum
1 watt per 1 MHz (PSD limit)
Mobile Devices
1 watt (30 dBm) Maximum
40 mwatts per 1 MHz (PSD limit)
Steve Shellhammer, Qulacomm

4. Estimates of 802.11y Range Relative to 802.11a
January 2006
Estimates of y Range Relative to a
This section estimates the range of y relative to a
Approach
Estimate difference in Link Budget between y and a
For a typical path loss formula translate increase in path loss to the increase in range
Steve Shellhammer, Qulacomm

5. Estimates of 802.11y Range Relative to 802.11a
January 2006
Estimates of y Range Relative to a
Difference in path loss due to lower frequency
Difference in TX power for Fixed Devices as a function of bandwidth (BW in MHz)
Difference in link budget for Fixed Devices
Steve Shellhammer, Qulacomm

6. Estimates of 802.11y Range Relative to 802.11a
January 2006
Estimates of y Range Relative to a
Difference in TX power for Mobile Devices as a function of bandwidth (BW in MHz)
Difference in link budget for Fixed Devices
Steve Shellhammer, Qulacomm

7. Estimates of 802.11y Range Relative to 802.11a
January 2006
Estimates of y Range Relative to a
Assume the BW is a power-of-two fraction of 20 MHz
ΔLB (Fixed Device)
ΔLB (Mobile Device)
5 MHz
20 dB
6 dB
10 MHz
23 dB
9 dB
20 MHz
26 dB
12 dB
The actual bandwidth is not yet specified
Estimate range assuming full 20 MHz bandwidth based on increased in link budget
Steve Shellhammer, Qulacomm

8. Estimates of 802.11y Range Relative to 802.11a
January 2006
Estimates of y Range Relative to a
Use a simple exponential path loss formula with an exponent of α=3
Ratio of range of 11y and 11a
Steve Shellhammer, Qulacomm

9. Estimates of 802.11y Range Relative to 802.11a
January 2006
Estimates of y Range Relative to a
Ratio in Range of 11y and 11a
d11y/d11a
20 MHz
7.3
2.5
10 MHz
5.8
2.0
5 MHz
4.6
1.6
2.5 MHz
3.7
1.3
Fixed devices have a range of up to approximately 7 times that of a
Mobile devices have a range of up to approximately 2.5 times that of a
Steve Shellhammer, Qulacomm

10. Estimates of 802.11y Range Relative to 802.11a
January 2006
Estimates of y Range Relative to a
Two class of applications
Long-range WLAN
Both Fixed and Mobile devices
Estimated range approximately 2.5 times the range of a
Mesh y AP
All fixed devices
Estimated range approximately 7 times the range of a
Steve Shellhammer, Qulacomm

11. Ramifications on OFDM Symbol Format
January 2006
Ramifications on OFDM Symbol Format
Longer range leads to larger multipath delay spread
The OFDM Guard Interval (GI) will need to be increased
Currently there are no specific delay spread requirements
Once specific delay spread requirements we can be more precise in our statements regarding GI requirements
Will consider several methods of obtaining the OFDM symbol format
Scaled version of a
Scaled version of 40 MHz mode of y
New OFDM format
Steve Shellhammer, Qulacomm

12. January 2006
Scaled Version of a
Scale the a OFDM symbol by scaling the clock BW GI
10 MHz
1.6 s
5 MHz
3.2 s
2.5 MHz
6.4 s
Scaling the GI to 6.4 s the bandwidth becomes only 2.5 MHz, resulting in lower permitted TX power
The practical limit on this approach is scaling by a factor of 4, resulting in a 5 MHz bandwidth
Steve Shellhammer, Qulacomm

13. Scaled Version of 40 MHz Mode of 802.11n
January 2006
Scaled Version of 40 MHz Mode of n
Scale the 40 MHz n OFDM symbol by scaling the clock BW GI
20 MHz
1.6 s
10 MHz
3.2 s
5 MHz
6.4 s
Scaling the clock by 4 is a reasonable choice and results 10 MHz bandwidth
This results in 3 dB more power permitted TX power than scaling a
Scaling clock by 8 is probably impractical in this case, since the resulting 5 MHz bandwidth does not permit enough TX power to justify such a large GI
Steve Shellhammer, Qulacomm

14. January 2006
New OFDM Symbol Format
To obtain full range increase we would need larger BW
Use a larger FFT
Say that we target a 6.4 s GI
FFT duration 25.6 s
FFT BW
256
10 MHz
512
20 MHz
256 size FFT gives approximately 5.8 times the range of 11a
512 size FFT gives approximately 7.3 times the range of 11a
Steve Shellhammer, Qulacomm

15. Trade-off between GI size and TX Power
January 2006
Trade-off between GI size and TX Power
For a fixed FFT size there is an inverse relationship between GI size and BW
This assumes a fixed GI overhead
Based on PSD regulatory limit maximum permitted TX power scales with BW
There are two conflicting factors that limit range
TX power GI
The following figure illustrated this trade-off
Steve Shellhammer, Qulacomm

16. Trade-off between GI size and TX Power
January 2006
Trade-off between GI size and TX Power
GI Limits on Range
512 Point FFT
256 Point FFT
Range
128 Point FFT
64 Point FFT
TX Power Limits on Range
Guard Interval (GI) Size (s)
Steve Shellhammer, Qulacomm

17. January 2006
Summary of Options
To get a range increase of approximately a factor of four
Scale either a or 40 MHz mode of n, by scaling the clock down by a factor of four
To get a range increase of approximately a factor of eight
New OFDM symbol format with larger FFT size
Larger GI
Larger permitted TX power
All these calculations are based on link budget with an assumed path loss model
More detailed requirements are needed to decide between the approaches
Clearly, reusing available silicon is a Very Useful approach
Steve Shellhammer, Qulacomm