Uplink Power Control in LTE
Malaka Dahanayake
Telecommunication Engineering Professional | Expertise in 4G/5G Radio Access Network
LTE Power Control:
Downlink power control determines the energy per resource element (EPRE). The term resource element energy denotes the energy prior to CP insertion. The term resource element energy also denotes the average energy taken over all constellation points for the modulation scheme applied. Uplink power control determines the average power over a SC-FDMA symbol in which the physical channel is transmitted.
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This article will address LTE uplink power control.
Uplink Power Control (ULPC) in Long-Term Evolution (LTE) is a mechanism used to adjust the transmission power of mobile devices in the uplink direction. It aims to optimize the quality and efficiency of the wireless communication by ensuring that the transmitted signals are received at the base station with sufficient strength while minimizing interference to other users and conserving battery power in the mobile devices. ULPC continuously adjusts the transmit power levels based on feedback from the base station, environmental conditions, and network requirements to maintain an optimal balance between signal quality and power consumption.
There are roughly two different ways of power control mechanism.
One is called Open Loop Power control and the other one is called Closed Loop Power Control.
Overall Flow for Open Loop Power Control
One of the most common example of Open Loop Power Control is the initial PRACH power. This PRACH power is determined as illustrated below.
Once initial PRACH is detected, the UE power is controlled dynamically by TPC (Transmission Power Control) command (MAC CE and TPC field in DCI 0). It means UE Transmission Power is controlled by some feedback input from eNB. In this way, overall power control process form a loop (closed loop). That's why it is called Closed Loop Control.
Overall Flow for Closed?Loop Power Control
If explained in simple terms,
i) Transmit send a signal to receiver
ii) Receiver measure the power of the signal from the transmitter
iii) if the measured power is too low, the receiver send a special command saying "increase the power". And if the measured power is too strong, it would send another command saying "decrease the power"
Basic uplink power control schemes
Power Control - PUSCH (Physical Uplink Shared Channel)
If the UE transmits PUSCH without a simultaneous PUCCH for the serving cell, then uplink PUSCH power will be given by ,
As per the above from the 3GPP specification TS136 213, the User Equipment (UE) will utilize either PCMAX or the value derived from the formula above, selecting the option with the lowest value.
Let's delve deeper into each component to gain a better understanding of PUSCH UL power control.
Effective PUSCH UL power control helps maintain a balance between signal quality, interference management, and power efficiency, thereby improving overall network performance. By dynamically adjusting the transmit power levels based on channel conditions and network requirements, PUSCH UL power control contributes to maximizing spectral efficiency, minimizing interference, and extending battery life for mobile devices. This ultimately leads to enhanced user experience, increased network capacity, and improved system reliability.
P_CMAX
The P_CMAX in various case is derived from various formula and tables, recommendation is? to refer to "6.2.5 Configured transmitted Power" of 36.101.
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What is PpowerClass? It is the maximum UE transmitter power specified in 3GPP 36.101. UE manufacturer must make it sure that UE does not transmit any power greater than this power.?
UE maximum output power
The following UE Power Classes define the maximum output power for any transmission bandwidth within the channel bandwidth for non CA configuration and UL-MIMO unless otherwise stated. The period of measurement shall be at least one sub frame (1ms).
M_PUSCH
MPUSCH is the PUSCH bandwidth during subframe ‘i’ expressed in terms of Resource Blocks. This variable is used to increase the UE transmit power for larger resource block allocations.
The UE transmit power is increased in direct proportion to the number of allocated Resource Blocks.
P_O_PUSCH(j)
This is target received power at eNodeB. In other words , This is a power that NodeB require for the safe decoding for the received signal. Configuring this at the eNodeB can be different from vendor to vendor. Typically, we only encounter cell-specific nominal P0 values. In the scenarios I have worked on, I have not come across a value for P0_UE other than 0.
PL(Path Loss)
This is calculated by referenceSignalPower
“Reference Signal Power" is defined by the following Information Element (SIB2, Refer to 36.331).
α - Alpha (Factor to enable or disable Fractional Power Control?? )
alpha(j) : For j = 0, 1, alpah(j) can be any one of {0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1}. The specified value come from higher layer (e.g, SIB2).
– The path loss compensation factor alpha can be used to have higher received PUSCH power for UEs close to the site than for UEs experiencing higher path loss.
– Care needs to be taken when setting alpha. Too low values may cause degraded UE performance, since SINR gets too low at cell border.
In systems that use orthogonal transmission scheme it is beneficial to use fractional compensation of path loss (0 < α < 0). It is apparent that the received power is decreasing with growing path loss. This diminution depends on the path loss compensation factor as it is shown on the Fig. 3. The knee point is where the UE reaches the maximum transmission power allowed. Position of this point also depends on α.
It has been proved, that fractional power control brings better spectral efficiency and is especially effective in small cells up to 1 km. Fractional power control scheme generally increases aggregate data rate of the cell up to 40% [2] and particularly improves the situation for users on the cell edges. These reach the maximum transmission power but do not experience such high interferences like they would, if users in adjacent cells, finding themselves before the knee, would be using the full compensation scheme.
As an example and comparison among the two schemes. UE transmit power and received power spectral density as a function of path loss are shown below.
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It increases the UE transmit power when transferring a large number of bits per Resource Element. This links the UE transmit power to the Modulation and Coding Scheme (MCS). The number of bits per Resource Element is high when using 64 QAM and a large transport block size. The number of bits per Resource Element is low when using QPSK and a small transport block size. Increasing the UE transmit power helps to achieve the SINR requirements associated with higher order modulation schemes and high coding rates.
f(i)
Following table defines how you should interpret the TPC value in DCI into the real power changes. As you see here, the same TPC value will convert to different physical power changes depending on whether the power control mode is 'Accumulated mode' or 'Absolute mode'.
Power Control PUCCH(Physical Uplink Control Channel)
UE TX power for PUCCH in subframe i is set by
Same as PUSCH, selecting the option with the lowest value from below formula.
Let's examine each component in detail.
P_CMAX
This is same as in PUSCH.
P_O_PUSCH(j)
P_O_PUCCH:? P_O_NOMINAL_PUCCH + P_O_UE_PUCCH,
where P_O_NOMINAL_PUCCH and P_O_UE_PUCCH came from higher layer(SIB2 or RRC Connection Setup or RRC Connection Reconfiguration).
PL(Path Loss)
Path Loss calculation is same as explained in the PUSCH part.
h(nCQI , nHARQ, nSR )
You may refer TS 136 213 for more details.
Delta_F PUCCH(F)
The parameter ΔF_PUCCH (F) is provided by higher layers. Each ΔF_PUCCH (F) value corresponds to a
PUCCH format (F) relative to PUCCH format 1a, where each PUCCH format (F ) is defined in Table 5.4-1 of TS 36.213.
Delta_TxD(F')
g(i)
Below is a extract from TS 136.213
g(i) = g(i-1) + delta_PUCCH(i-4), This shows that the current g(i) is determined by the previous subframe g(i) and g(i) of 4 subframe earlier.
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delta_PUCCH is determined by the value carried by DCI format 1A/1B/1D/1/2A/2/3 and the following table of 36.213.
In a summery :
In open loop power control, the UE utilizes downlink Reference Signal (RS) measurements to roughly estimate the RF path loss and, in combination with P0, determines the optimal power for transmission over PUSCH (this only occurs briefly if closed loop is supported).
In closed loop power control (which is active most of the time), the eNB utilizes received power measurements per UE and per RB to issue TPC (transmit power control) commands over the downlink control channel to the UE (PDCCH). This closed loop power control instructs the UE to increase or decrease its power in 1dB or 3dB increments at a slow rate, and is designed to mitigate slow fading in the RF channel. The eNB can detect when the UE is near or at full power through the UE transmit power headroom reports sent in PUCCH (uplink control channel).
Uplink in LTE, Power control is used. As the battery of the phone(UE) is power limited compared to base station power in the DL.
Uplink power control is used mainly for the following two reasons.
Lets see a real world example of power control happened at the eNodeB
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In this example ,
P0 Nominal configured for Pusch is -98 dbm and accumulation is enabled
Trace output from L1 traces, here you can see the last TPC command and the receive power at eNodeB for a particular UE.
References :
1) TS 136.213
2) TS 136.211
3) TS 136.101
4) TS 136.331