36.213 series (1) - Power Control in LTE

Power control in LTE is not as critical as in WCDMA. This is due to two factors:

- In LTE uplink are orthogonal, therefore no intracell interference (at least in theory)

- LTE has the freedom to do frequency selective scheduling, which can be used to further reduce the interference from other cells.

However, power control still plays an important part in LTE because a proper power control can maximize system capacity, minimize inter-cell interference, also look after the fairness between cell centre and cell edge UEs.

UL Power Control

Different formulas are specified for UL PUSCH, PUCCH, SRS. Fundamentally the formulas can be considered as a sum of two main parts:

1. a basic open-loop operating point derived from static or semi-static parameters signaled by eNB
   
2. a dynamic offset updated from sub-frame to sub-frame.
   

Furthermore, the 1st part, basic open loop operating point is composed of two parts: A) a semi-static P0, which is a cell specific power level and a UE specific offset (to compensate for example UE's PL estimation error), and B) a open loop path-loss compensation.

Basic operating point = P0 + α ● PL

α is a factor to trade off the fairness of uplink scheduling against total cell capacity. Full path-loss compensation (alpha = 1) maximize fairness for cell edge UE, but causing more interference to neighboring cells. For PUCCH, full PL compensation is always used.

The 2^nd part, the dynamic offset is also composed of two parts, one part is MCS dependent power offset ∆_TF (TF is transport format), the other part the TPC command related power. The TPC can command UE to use a relative power offset comparing to its previous Tx power, or command UE to use a absolute power, regardless its previous Tx power.

Downlink Power Allocation

In the downlink, cell specific RS EPRE (Energy per RE) is semi-static, it only change when the eNB signals UE so. PDSCH EPRE is proportional to RS EPRE's, depends on PDSCH RE's position. If the PDSCH RE is on the same symbol where there is a RS (index 0, 4), ρ_B is used, otherwise ρ_A is used. The value of ρ_{A /} ρ_B is determined by P_A and P_B which are specified by eNB via MAC/RRC signaling, they are UE specific. For details, see 36.213 section 5.2. Below diagram is an example when P_A=0.8 (ρ_A = 0.8) and P_B=2 (ρ_{B /} ρ_A = 0.6)