1. Distributed Prioritized on Demand Contention
March 2008
doc.: IEEE yy/xxxxr0
July 2008
Distributed Prioritized on Demand Contention
IEEE P Wireless RANs Date:
Authors:
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Steve Shellhammer, Qualcomm
Steve Shellhammer, Qualcomm

2. Abstract Propose a new contention algorithm for spectrum sharing
July 2008
Abstract
Propose a new contention algorithm for spectrum sharing
The new algorithm benefits from the broadcast nature of the wireless channel significantly reducing the communication overhead
Employs a new adaptive back-off algorithm to further reduce the communication overhead
Steve Shellhammer, Qualcomm

3. Adaptive on Demand Contention
July 2008
Adaptive on Demand Contention
One of the spectrum sharing techniques described in the IEEE draft [1] for self coexistence between different WRANs
Contention destination (CD) is defined as the WRAN currently occupying the channel
Contention sources (CS) are defined as any WRAN that wishes to share the channel
Each CS will randomly generate a contention number (CN)
The CS will try transmitting the CN during the SCW (self-coexistence-window) through contention with other CS’s
After all CN’s are received at CD, the CD decided on the winning CS that has the largest CN
Steve Shellhammer, Qualcomm

4. Adaptive On Demand Contention
July 2008
Adaptive On Demand Contention
Generate a contention number
Generate a random back-off
Sense the channel
busy
Freeze the back-off
free
Decrease the back-off no
Back-off = 0 ?
yes
Transmit packet
yes
Collision ?
Steve Shellhammer, Qualcomm

5. July 2008
Disadvantage
Have to complete transmission of every CS and then choose the one with the highest contention number.
Large probability of collision due to the same size of random back-off window.
Broadcast nature of the wireless channel is not utilized
Longer scheduling time. If there are M CS’s, then at least M SCW frames are required to complete the process. Hence the algorithm does not scale well
If the winning CS only needs to transmit x frames with x small, then at least M-x frames are wasted till a new CS acquires the channel (where M is the number of CS’s)
Steve Shellhammer, Qualcomm

6. Dynamic Prioritized Contention (DPC)
July 2008
Dynamic Prioritized Contention (DPC)
Objective: to decrease the scheduling time via providing the CS with higher CN (higher priority) a smaller random back-off.
Size of random back-off window is based on the contention number (inversely proportional)
Every CS can listen to other CS’s contention number.
If a CS finds that its contention number is less than another CS’s contention number, it will stop trying to send the packet
Steve Shellhammer, Qualcomm

7. Proposed Algorithm Diagram
July 2008
Proposed Algorithm Diagram
Generate a contention number
Generate a back-off based on contention number no
Sense the channel
busy
Sensed a bigger contention number?
free
yes
Decrease the back-off
Stop trying to transmit in this slot no
Back-off = 0 ?
yes
Transmit packet
yes
Collision ?
Steve Shellhammer, Qualcomm

8. Generating the Random Back-off in DPC
July 2008
Generating the Random Back-off in DPC
CNmax: the size of contention number window
RBmax: the size of random back-off window
For each CS i
CN(i)=Uniformly distributed ~{0,1,…,CNmax-1)
ModRBmax=RBmax-round[CN(i)*(RBmax-1)/(CNmax-1)]
CSBackoff(i)=Uniformly distributed ~{0,1,…,ModRBmax-1};
Steve Shellhammer, Qualcomm

9. July 2008
Advantage
The CS with higher contention number (higher priority) will transmit the packet probably earlier, because it has a smaller size of random back-off window.
The CS will stop trying to transmit after it hears a higher contention number, so that the probability of collision is sharply decreased.
This advantage is significant in the case of large number of CS’s
Steve Shellhammer, Qualcomm

10. Simulation Results Maximum contention number Maximum back-off Window
July 2008
Simulation Results
Maximum contention number
CNmax=220
Maximum back-off Window
RBmax=24
Steve Shellhammer, Qualcomm

11. July 2008
Steve Shellhammer, Qualcomm

12. July 2008
Conclusions
The previous algorithm requires at least a number of frames equal to the number of contention sources. Hence does not scale well
The proposed algorithm scales well with increasing the number of contention sources (requires 1 or 2 frames on average)
This reduces the delay significantly for a WRAN to acquire the channel
Reduces the throughput loss when the current CD only needs to transmit for a few frames
Steve Shellhammer, Qualcomm

13. July 2008
References
[1] Draft Standard for Wireless Regional Area Networks IEEE P802.22™/ DRAFTv1.0
Steve Shellhammer, Qualcomm