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Understanding Power Control

Meghana Manjunath

In wireless communication, one of the main challenges for the transmitted signal is to overcome the degradation caused by the channel. The energy drops by the time the signal reaches the receiver. In order to successfully transmit a signal, it must overcome the variations of the wireless channel. There are various methods and techniques that can be used.

  • Power Control
  • Modulation Scheme
  • Coding Rate

These predefined procedures exist at Layer 2 and Layer 1 of network architecture and help in preparing the signals from getting corrupted by the variations in the environment.

Power control is a technique where the transmitted power is varied in accordance with the channel quality to provide a fixed data rate. It is used in both uplink and downlink directions in wireless communication standards like LTE, 5G. Let us focus on uplink as some of the main concerns for UE is to reduce power consumption and limit intra-cell and inter-cell interference which can be achieved by controlling power.

The power control scheme can be either conventional or fractional.

Conventional Power Control 

Conventional Power Control is used to maintain a constant Signal to Interference plus Noise Ratio (SINR) at the receiver. In this method the UE increases the transmit power to compensate any increase in path loss. 

Fractional Power Control

Fractional Power Control allows the received SINR to decrease as the path loss increases. The UE transmits power at a reduced rate as the path loss increases.

The power control mechanism has adopted two methods in order to compensate for losses that occur during transmission i.e open loop and closed loop.

Open Loop Power Control

Open Loop Power Control is the capability of the UE transmitter to set its uplink transmit power to a specified value suitable for the receiver. It is a unidirectional control process with no feedback from the network. The UE determines the transmission power by Power Setting Algorithm which has inputs given by the UE itself. The transmission power at which PRACH is sent is determined by open-loop power control.

Uplink Power can be determined by

POL = min {Pmax, 10·log10M + P0+α·PL}

POL is the uplink power, set by open-loop power control. The choice of α depends on whether a conventional or fractional power control scheme is used. Using α = 1 leads to conventional open loop power control while 0 < α < 1 leads to fractional open loop power control.

  • Pmax is the maximum allowed transmit power. It depends on the UE power class. 
  • M is the number of physical resource blocks (PRB). 
  • P0 is a cell/UE specific parameter signaled by radio resource control (RRC). 
  • α is the path loss compensation factor. 
  • PL is the downlink path loss estimate. It is calculated in the UE based on the reference symbol received power (RSRP). 

However, Open Loop Power control cannot compensate for issues like slow fading which is a disadvantage. For this reason, Closed Loop Power Control is introduced.  

Closed Loop Power Control

Closed-Loop Power Control is the capability of the UE to adjust the uplink transmit power in accordance with the feedback given by the network (eNodeB or gNodeB) to the UE. Based on the Transmit Power Control command sent by the eNodeB, UE will either increase or decrease its power to compensate for the losses. It is mainly adopted to compensate for issues such as slow fading. The PUCCH or PUSCH channel power is determined by the closed-loop power control mechanism.

In a closed-loop power control system, the SINR of the received signal is estimated by the uplink receiver at the eNB. Later it is compared with the desired SINR target value. If the received SINR is below the target, a TPC command is transmitted to the UE in order to increase the transmitter power. Otherwise, the TPC command will request to decrease the transmitter power.

The UE power for PUSCH can be determined by

PPUSCH = min{Pmax, 10 · log10 · M + P0 + α · PL + δmcs + f(Δi)} [dBm]

     where,

  • Pmax is the maximum allowed transmit power. It depends on the UE power class. 
  • M is the number of physical resource blocks (PRB). 
  • P0 is a cell/UE specific parameter signaled by radio resource control (RRC). 
  • α is the path loss compensation factor. 
  • PL is the downlink path loss estimate. It is calculated in the UE based on the reference symbol received power (RSRP). 
  • δmcs is a cell/UE specific modulation and coding scheme defined in the 3GPP specifications for LTE.  
  • f (Δi) is UE specific. Δi is a closed-loop correction value and f is a function that permits to use accumulate or absolute correction value.

P0=α·(SNR0 +Pn)+(1-α)(Pmax-10·log10 M0) [dBm]

    where,

  • SNR0 is the open loop SNR target.
  • Pn is the noise power per PRB. 
  • M0 defines the number of PRBs for which the SNR target is reached with full power.

So in this way the UE is able to control the Uplink power. It can be concluded that having a Power Control mechanism is important in a network to overcome losses achieved due to environmental changes and achieve good performance.

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