Power Control for Reducing Interference
To ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel. Power Control for Reducing Interference becomes very important.
- In practical cellular radio and personal communication systems the power levels transmitted by every subscriber unit are under constant control by the serving base stations.
- This is done to ensure that each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel.
- Power control not only helps prolong battery life for the subscriber unit, but also dramatically reduces the reverse channel S/I in the system.
- Power control on the downlink has less impact on intercell interference than on the uplink, since the downlink transmissions all originate from the cell center, whereas uplink transmissions can come from the cell boundaries, which exacerbates interference to neighboring cells.
- Thus we will focus on the effect of power control on the uplink. Consider the two cells shown in Figure 5.7. Suppose that both mobiles B1 and B2 in cell B transmit at the same power.
- Then the interference caused by the mobile B1 to the base station in cell A will be relatively large, since it is close to the boundary of cell A, while the interference from B2 will generally be much weaker due to the longer propagation distance.
- If water-filling power adaptation is employed, then B1 will generally transmit at a lower power than B2, since it will typically have a worse channel gain than B2 to the base station in cell B as it is farther away.
- This has the positive effect of reducing the intercell interference to cell A. In other words, water filling power adaptation reduces intercell interference from mobiles near cell boundaries, the primary source of this interference.
- A similar phenomenon happens with multiuser diversity, since users transmit only when they have a high channel gain to their base station, which is generally true when they are close to their cell center.
- Conversely, under channel inversion the boundary mobiles will transmit at a higher power to maintain the same received power at the base station as mobiles near the cell center.
- This has the effect of increasing intercell interference from boundary mobiles.
Figure 5.7 Effect of Power Control on Intercell Interference