POWER CONTROL IN LTE


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POWER CONTROL IN LTE

When dealing with wireless communication of any kind, one of the most important things to keep in mind is the kind of effect/change that the air interface is going to make on the signal. Unlike wires, air is not a static, immutable path whose effect on the signal can be predicted with a fairly high accuracy. The signal, when travelling through the air interface goes through distortion, attenuation, reflection through multiple surfaces, and many other things. So, it’s important for both the UE and the eNodeB, to have some kind of arrangement to take care of this.

Power Control is the mechanism through which the UE and the eNodeB handle the attenuation of the signal. There are two types of Power Control mechanisms as per Control Theory and LTE uses both of them. They are:


  • Open Loop Power Control
  • Closed Loop Power Control



What is Open Loop Power Control?


In case of Open Loop, no feedback is sent from the receiver to the transmitter informing the latter to either ramp up or lowers the power of the signal. This mechanism is used during the Random Access process.


  1. When a UE is powered on, after frequency scanning, it decodes the MIB and SIBs that are broadcasted periodically by the eNodeB. In SIB2, there is an IE called preambleInitialReceivedTargetPower.
  2. It tells the UE the value of transmission power (in dB) of the first uplink message that the UE will send to the eNodeB, which is the Random Access Request (RAR) message.
  3. In SIB2, the eNodeB also sends an IE called referenceSignalPower which tells the UE the power with which the reference signal was transmitted from the eNodeB.
  4. The UE decodes the value of the referenceSignalPower and also measures the strength of the reference signal.
  5. Based on this, it calculated the path loss (PL = Reference Signal Power – RSRP measurements) of the signal. When sending the RAR message, the UE will consider the path loss and the preambleInitialReceivedTargetPower and set the transmission power accordingly.
  6. If the UE does not receive a Random Access Response message from the first attempt, it will ramp up the transmission power by the step size mentioned in powerRampingStep IE in SIB2.

What is Closed Loop Power Control?


In Closed Loop mechanism, the receiver, on receiving a signal, sends a feedback to the transmitter telling it to increase/decrease the transmission power. As you may have guessed, a Closed Loop mechanism can only work once the transmitter and received have already established a communication i.e. the first signal that was transmitted was received and decoded properly and hence the feedback was sent and also decoded perfectly. This mechanism is used in LTE in the form of Transmission Power Control.

The TPC command is delivered to the UE through the following DCI Formats:

  • DCI format 3
  • DCI format 3A
  • DCI format 4
DCI 0 may be used for TCP command.
On establishing RRC connection with the eNB, the UE starts using Closed Loop Power Control on PUSCH and PUCCH channels.

PUSCH

The power with which the UE transmits data on the PUSCH is given by the formula:
Power = min{Pmax, 10log10(RBs) + PO + alphaPL + deltatf + f(i)}

Where,
10log10(RBs) - the amount of additional power that is needed based on the size of the RB allocation,with power being directly proportional to the number of RBs,

PO - It is the assumed value of interference that the UE is expected to overcome.
It is composed of:
P0_Nominal_PUSCH – it is communicated over SIB2. It is valid for all UEs in the cell.
P0_UE_PUSCH – it is a UE-specific value. It is optional.

alphaPL – PL stands for Path Loss and alpha(Rance-0,0.4,0.5,0.6,0.7,0.8,0.9,1) is the impact of the path loss. The value of alpha is communicated to UE in SIB2. If alpha = 1, then all of the Path Loss needs to be taken into account to calculate Power Control. The value of alpha could be hardcoded by vendors and it’s used for fractional power control.

deltatf – It is the MSC-specific component of the formula. It tells the UE if the eNB wants it to change itd power based on the MCS that is assigned to it.

f(i) – it is the closed loop feedback given by the eNB based on which the UE will add additional power to the transmission.

Thus, the two main components controlling power in PUSCH are:
a)     PO_nominal_PUSCH
b)    Alpha


PUCCH

The formula for power control in PUCCH is:
Power = min{Pmax, PO + alphaPL + deltatf + f(i)}

Where everything else is the same except PO is composed of P0_Nominal_PUCCH and P0_UE_PUCCH.
Also, the f(i) value is different for each format type of the PUCCH, so a different value is given to the UE in SIB2 for formats 1, 1a, 1b, 2, 2a and 2b.

Hence, the parameters that control the transmit power in PUCCH are:
a)     PO_nominal_PUCCH
b)    Alpha

One might think that a high value of UL transmission power will result in high BLER, high Throughput, and thus high UL SINR. But, in high capacity areas like airports, events, convention centers, etc. this might have the opposite effect. Thus, it is recommended that the UL RSSI should be analyzed in these types of venues during high capacity scenarios.

Both of the Power Control mechanisms mentioned above are used for uplink data transmission. For downlink transmission, instead of power control, rate control is used i.e. through Adaptive Modulation and Coding scheme, the Modulation scheme (QPSK, 16 QAM, 64 QAM) and Coding Rate are varied according to the channel conditions.


Reference: TS 36.213

By- Ankit Dhyani



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