1. ECE 7930 Advanced Topics in Wireless Networks
Topic 5: Soft Frequency Reuse and Power Control
Prof. Rose Qingyang Hu
Electrical and Computer Engineering Department
Utah State University

2. Motivation for SFR What is the proper reuse factor for OFDM system?
Frequency reuse increase the SIR while decreasing the available bandwidth for each cell
There must exist an optimal trade off to achieve the maximum channel capacity

3. SIR Representation

4. Shannon Theorem

5. Basic Principle for Soft Frequency Reuse
Total available bandwidth are divided in to 3 groups;
Every cell/sector selects one group as its major bands, and others as its minor bands.
Higher upper limit of transmit power for major bands than minor bands,
Major bands can be used in the whole cell area;
Minor bands are used only in the inner zone of the cell with limited transmit power;

6. Soft Frequency Reuse

7. Power Control so as to…
Mitigate interference ↔Increase Network Capacity
Conserve energy ↔Prolong battery life
Adapt to channel variations ↔Support QoS
Greening the Internet
All the above are dependent on…SI(N)R

8. About SI(N)R: Downlink

9. About SI(N)R: uplink

10. Impact of Power Control

11. Uplink Power Control
If MS is near the BS or in LOS situation, power to/from the MS can be reduced
Helps Reduce CCI
Save battery power
Alleviate health concerns
In CDMA systems all MS use same frequency
Fine power control is crucial to mitigate the near-far effect

12. Power control types Three types of power control:
Open loop power control (Power allocation)
Close loop power control
Inner loop power control (QoS/BLER target adjustment)
Outer loop power control (SIR-target adjustment)

13. Open-Loop Power Control
MS measures power on downlink
its uplink power and vice versa
Power measurements averages the Rayleigh fading
Depends on distance and shadowing only
No feedback from BS
Not as accurate as closed-loop power control

14. Closed-Loop Power Control
Performed on top of the Open Loop Power Control
BS measures the uplink power from MS
BS transmits power control commands on the forward link
MS steps its power up or down accordingly

15. Inner/Outer Loop Power Control

16. Mobile association
In the conventional wireless networks, mobiles associate with a base station that has the best downlink SINR.
In a heterogeneous network, base stations, such as pico nodes, with smaller transmission powers will have a very small footprint. Thus pico cell coverage is significantly limited in the presence of macro coverage.
In a heterogeneous network, mobiles with larger Macro SINR may have smaller path loss to micro/pico nodes thus causing significant uplink interference at the lower power base stations.

17. Mobile association in HetNet
Default (best-power): UEs are assigned with the micro or macro BSs to be associated with according to their received signal strength
Range expansion: biased cell selection expands the micro-cell area and hence balance the loads among macro and micro BSs

18. Range expansion based cell selection
Range extension can be used to expand coverage area for low-power base station
UE association is determined by minimal path loss
Range expansion can achieve better spectrum efficiency and network capacity
Achieve better spatial reuse efficiency similar to cell splitting
Multiple relay nodes can simultaneously use the resource vacated by the macro base station
Lower traffic load on macro base station
Resource coordination schemes are essential to manage interference and optimize throughput performance in a relay network.