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

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

SIR Representation

Shannon Theorem

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;

Soft Frequency Reuse

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

About SI(N)R: Downlink

About SI(N)R: uplink

Impact of Power Control

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

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)

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

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

Inner/Outer Loop Power Control

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.

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

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.