1. 1 A Distributed Approach to Interference Mitigation between OFDM-based 802.16 Systems Operating in License-exempt Spectrum Omar Ashagi, Sean Murphy and Liam Murphy School of Computer Science and Informatics College of Engineering, Mathematics and Physical Science UCD Dublin, Ireland IEEE ICC 2007

2. 2 Outlines  Introduction  System Architecture  Distributed Approach  Simulation  Conclusion

3. 3 Introduction_ 802.16(1)  Wireless systems operating in license-exempt spectrum some advantages: No cost No need for interaction with a regulator Minimal constraints on how the spectrum can be used  License-exempt spectrum is 5.25-5.35GHz and 7.725-7.825GHz

4. 4 Introduction_ 802.16(2) and goal channel Subchannel Subcarrier SS BS SS BS SS BS SS Distributed Approach

5. 5 Introduction_ 802.16(3) Lower guard subcarriersUpper guard subcarriers SS BS SS BS SS guard subcarriers, assumed equal to upper guard subcarrier

6. 6 System Architecture_ Assumptions  m is minimum number of subcarriers which a BS can operate on  The channel has 256 OFDM subcarriers

7. 7 System Architecture_ equations(1) SS BS SS BS SS α is the number of active subcarriers K is the number of OFDM subcarriers on the channel β is the number of null or guard subcarriers η is the number of BSs operating on the channel SS BS SS Lower guard subcarrier Upper guard subcarrier

8. 8 System Architecture_ equations(2) the first data subcarrier index The BS subchannel ID BS 3 BS 1 BS 2 (3-1)*4+(3-1)*1=10 β =1+3*1=4 α=(16-4)/3=4

9. 9 SS BS SS BS SS BS SS BS SS (3-1)*4+(3-1)*1=10 β =1+3*1=4 α=(16-4)/3=4 Subchannel 1 Subchannel 2 Subchannel 1 Subchannel 2 Subchannel 3 Distributed Approach _ BS mode

10. 10 SS BS SS BS SS BS SS idle Random backoff Distributed Approach _ SS mode

11. 11 Simulation_ Parameters

12. 12 Simulation_ 6, 7 and 8 BSs (100 for each)

13. 13 Simulation_ 5 BSs and 79 SSs

14. 14 Simulation_ (3) Total number of subcarriers used in the system

15. 15 Simulation_ OFDM subcarrier distribution when BS 2 started first

16. 16 Simulation_ OFDM subcarrier distribution when BS 2 started last

17. 17 Conclusion  A distributed approach can enable the channel to be shared by intelligently activating and deactivating OFDM subcarriers  Future work will involve addressing some of these convergence and stability problems