1. 1 11 Frequency Reuse Techniques for Attaining both Coverage and High Spectral Efficiency in OFDMA Cellular Systems Zheng Xie and Bernhard Walke RWTH Aachen University Aachen, Germany WCNC 2010

2. 2 22 Outline Introduction Previous Schemes Soft Frequency Reuse (SFR) Incremental Frequency Reuse (IFR) Enhanced Fractional Frequency Reuse (EFFR) Evaluation Conclusion

3. 3 Frequency Reuse Factor Effective reuse of resources can highly enhance the system capacity Frequency reuse factor (FRF) K defines frequency reuse pattern With a smaller frequency reuse factor (FRF), more available bandwidth can be obtained by each cell

4. 4 Previous Frequency Reuse Schemes With the usage of FRF-1, the most user terminals (UTs) are afflicted with heavy Inter-cell interference (ICI) Especially near the cell edge The conventional method to figure out this problem is by increasing the FRF mitigate the ICI efficiently but decrease on available bandwidth The most representative approaches improving cell-edge performance while retaining spectrum efficiency Soft Frequency Reuse (SFR) scheme Incremental Frequency Reuse (IFR) scheme

5. 5 Soft Frequency Reuse (SFR) Scheme CCU: cell-centre users CEU: cell-edge users

6. 6 Limitations of SFR How to define the borderline to divide cell area for CCUs and CEUs is a key issue Generally, there are more CEUs than CCUs in a cell since the outer surface area is much larger than the inner part CEUs have maximum one third of the entire bandwidth to utilize, which results in lower spectrum efficiency More ICI could happen even in a low traffic-load situation, while there are still subchannels in idle and underutilized in the system The resource allocation via the SFR scheme starts always from the first subchannel up

7. 7 Incremental Frequency Reuse (IFR) Scheme (1) The only difference between the IFR design and the classical reuse-1 Classical reuse-1: allocate resources always from the first subchannel IFR: start dispensing resources from different points

8. 8 Incremental Frequency Reuse (IFR) Scheme (2) IFR scheme can overcome the low spectrum reuse efficiency problem and the more ICI at low loading traffic problem IFR scheme only performs better when just fewer traffic exists in a system When the loading factor is greater than 0.3, it is inferior to the SFR scheme

9. 9 Enhanced Fractional Frequency Reuse (EFFR) (1) Enhanced Fractional Frequency Reuse (EFFR) scheme intends to retain the advantages of the both approaches Concept Define 3 cell types for directly contiguous cells in a cellular system Reserves for each cell-type a part of the whole frequency band named Primary Segment The Primary Segments among different type cells should be orthogonal The Primary Segment of each cell will be further divided into reuse-3 part: cannot be reused by directly neighboring cells reuse-1 part: is at the same time a part of the Secondary Segments belonging to the other two cell-types

10. 10 Enhanced Fractional Frequency Reuse (EFFR) (2)

11. 11 Power Allocation and SINR Estimation Transmission Power Allocation Any cell-type is not allowed to use the reuse-3 subchannels dedicated to the other two cell types The power allotted to the reuse-3 subchannels can be tripled Signal-to-Interference-Ratio (SINR) Estimation A cell acts on the Secondary Segment as a guest, and occupying secondary subchannels is actually reuse the primary subchannels belonging to the directly adjacent cells Reuse on the Secondary Segment should conform to two rules: monitor before use resource reuse based on SINR estimation  Each cell listens on every secondary subchannel all the time  Before occupation, a cell makes SINR evaluation and chooses resources with best estimation values for reuse  If all available secondary resources are either occupied or not good enough to a link, this cell will give up scheduling resources

12. 12 Resource Allocation The reuse-3 subchannels will be assigned to CEUs If there are still resources remained after all CEUs are served, they will be continuing allotted to such CCUs with relatively poor SINR values When the reuse-3 subchannels are exhausted, the remaining reuse-1 subchannels in the Primary Segment are allocated to residual unsatisfied users If still resources are requested, available reuse-1 subchannels in the Secondary Segment will be scheduled to adequate users by applying interference-aware- operation

13. 13 Distinctions between the EFFR Scheme and the Two Aforementioned Schemes CEUs are very susceptible against ICI, the reuse-3 subchannels in the Primary Segment are exclusively available for the users To reduce excessive ICI to the neighboring cells, packets will be sent on a reuse-1 subchannel in lower strength Allocation of reuse-1 subchannels in the Secondary Segment is not blindly carried out, but in an interference-aware way according to SINR estimation In the Primary Segment unsatisfied users, whether they are CCUs or CEUs, have the same chance to get resources in the Secondary Segment IFR SFR EFFR

14. 14 Relevant Factors The following relevant factors play paramount roles and could influence the system performance The ratio of the number of reuse-3 subchannels M to reuse-1 subchannels N in the Primary Segment The power ratio of high power level to low power level Range definition for partition of CCUs and CEUs This is the first work to present simulation results of the SFR scheme with varying range definitions SINR threshold for reuse

15. 15 Simulation Environment (1) Simulation Tool The Open Wireless Network Simulator (OpenWNS)

16. 16 Simulation Environment (2) For all simulations We consider an OFDMA uplink cellular system in an omni-cell case UTs are uniformly distributed within each hexagonal cell Each UT has a maximal transmission power of 200mW Different cell specific power

17. 17 Scenario with 15 UTs in each cell Mean Overall Uplink Cell Capacity

18. 18 Scenario with 15 UTs in each cell Mean Weakest User Uplink Throughput

19. 19 Scenario with 25 UTs in each cell Mean Overall Uplink Cell Capacity

20. 20 Scenario with 25 UTs in each cell Mean Weakest User Uplink Throughput

21. 21 Conclusion A novel frequency reuse technique named the EFFR scheme for ICI mitigation in OFDMA networks is proposed The EFFR scheme uses dedicated FRF-3 and higher transmission power for CEU allows CCU to occupy resources with FRF-1 and interference awareness With respect to the range definition for division CCU- zone and CEU-zone, the EFFR scheme can provide more flexibility and robustness than the SFR scheme