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Power control is one of the most complicated subject in wireless communication technologies. Today we will try to have an overall idea about LTE Uplink Power Control with understanding each part of the calculation mechanism, but prior to diving to the topic let’s check a bit other things related to the power.

In physics, definition of power is the amount of energy transferred or converted per unit time. The unit of the power is Watt, but it can also be converted to other units such as dBm (Watt is a linear unit, while dBm is a logarithmic unit). Why we mentioned dBm here, because basically in radio telecommunication measurements the dBm or in longer version decibel-milliwatt is also used.

1 watt is equal to 30dBm (1W = 1000mW = 30dBm)

2 watts is equal to 33dBm (2W = 2000mW = 33dBm)

1 milliwatt is equal to 0 dBm (1mW = 0dBm)

Ok, but in the field when we open our drive test tool and we can see the measured signal level with negative values, e.g. -95dBm RSRP. Is it convertible to Watt too? Yes, we can with different magnitude: milliWatt(mW), microWatt(µW), nanoWatt(nW), picoWatt(pW), femtoWatt(fW), attoWatt(aW). Let see some examples for measured RSRP in Watt:

If two devices need to be communicated each other either over wire or wirelessly, then they need to transmit the required information with some amount of power. Let’s give an example from a mobile network. Imagine there are 100 mobile phone users under nearby same network cells and all the mobile phones are pushing their maximum power for transmission to the network cells in uplink direction. What will happen in this scenario? Intra-cell and inter-cell interference, huge handset power consumption. And consequently, it will cause for poor network performance and bad experience for customers.

Is there any way to eliminate the interference and avoid battery drain of the handsets? Yes, the answer is to control the transmission power of the mobile phones. Not to use the maximum power instead of that to use needed power if possible.

LTE Uplink Power Control

3GPP organization has technical specifications for power control in LTE. As we are talking about uplink direction here, then we need to mention PRACH, PUSCH, PUCCH and SRS power control. Each of them has own defined mathematical calculation formula. Vendors might have a bit different calculation including some extra features, but generally they should follow the specified standards. At first glance it might look as scary or boring, but don’t worry we will try to break into pieces and give simplified explanation for each part of the related formula.

Power Control on PRACH

Before initiation of physical random access procedure, the UE should receive the PRACH related information in advance from the higher layers through SIB2. You can find those information (parameters) in this page.

This is a short description from 3GPP: “From the physical layer perspective, the L1 random access procedure encompasses the transmission of random access preamble and random access response.  The remaining messages are scheduled for transmission by the higher layer on the shared data channel and are not considered part of the L1 random access procedure. A random-access channel occupies 6 resource blocks in a subframe or set of consecutive subframes reserved for random access preamble transmissions. The eNodeB is not prohibited from scheduling data in the resource blocks reserved for random access channel preamble transmission”

Now, let’s go one by one over each part of the PRACH Uplink Power control calculation formula:

1. P_PRACH is a preamble transmission power on physical random access channel (PRACH).
2. P_CMAX is configured maxium UE allowed output power. The transmitter characteristics are specified at the antenna connector of the UE with a single or multiple transmit antennas. For UE with integral antenna only, a reference antenna with a gain of 0 dBi is assumed. Here an integral antenna means permanent fixed antenna, which may be built-in, designed as an indispensable part of the user equipment. On the following shortened table, we can find UE power class per frequency band.

1. Preamble Received Target Power indicates target UE transmit power for the PRACH expected by the eNodeB. Lower value of this parameter will negative impact random access success rate, but higher value will improve it with causing interference on neighboring cells. Proper parameter definition is important (e.g. -104dBm, -106dBm etc.).
2. PL is downlink path loss estimation calculated by the UE for the serving cell. This is the related formula:

 PLc = referenceSignalPower – higher layer filtered RSRP

where referenceSignalPower is delivered by the network to the UE through SIB2 (see details). Higher layer filtered RSRP means that the measured raw RSRP might not be reported to the network and Layer3 filtering can be applied to the measured result before using for evaluation of reporting criteria or for measurement reporting, by the following formula:

where

M_n is the latest received measurement result from the physical layer;

F_n is the updated filtered measurement result, that is used for evaluation of reporting criteria or for measurement reporting;

F_n-1 is the old filtered measurement result, where F₀ is set to M₁ when the first measurement result from the physical layer is received; and

a = 1/2^(k/4), where k is the filterCoefficient for the corresponding measurement quantity received by the quantityConfig;

Power Control on PUSCH

Physical Uplink Shared Channel is used to transfer user data from the user equipment to the radio network. Like other physical channels when the data is transferred then it needs to be on a certain power for reaching to the destination. High transmit power can cause interference and fast UE battery drain, in opposite way with extreme low power may cause for undelivered data to the network. So, transmit power needs to be controlled.

But how the UE will be aware of the power control mechanism? The answer is serving cell itself. It will instruct the UE about the transmit power through PDCCH. The serving cell sends scheduling information to the UE over PDCCH, then the UE starts to transmit user data on PUSCH. The UE already knows which subframe, MCS and how many resource blocks will be used with the instruction. Now the further step is to calculate transmit power on PUSCH for each subframe (so calculation is based on every 1ms) with using below formula:

1. P_PUSCH is transmit power of UE on physical uplink shared channel (PUSCH).
2. P_CMAX is configured maxium UE allowed output power. For details see above in Power Control on PRACH. Same description is valid for PUSCH too.
3. M_PUSCH is the bandwidth of the PUSCH resource assignment expressed in number of resource blocks.
4. P_{0_PUSCH} is a composition of cell specific and UE specific components configured by higher layers and delivered to the UE to be used in power calculation to transmit on PUSCH, it is also described like eNodeB expected power or eNodeB target received power in some vendor libraries:

P_{0_PUSCH} =P_{0_NOMINAL_PUSCH} + P_{0_UE_PUSCH}

5. α (pronunciation is Alpha [ˈælfə]) is used as pathloss compensation factor. It is provided by the serving cell to the UE to be used for PUSCH power calculation in open loop. Following values can be setup {0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1}.
6. PL is downlink path loss estimation calculated by the UE for the serving cell. For details see above in Power Control on PRACH. Same description is valid for PUSCH too.
7. Δ_TF,c (i) depends on deltaMCS-Enabled parameter whether a UE needs to adjust the transmit power based on the MCS changes or not. It also has own small formula, let’s have a look:

where,

• BPRE is number of information bits per resource element for the scheduled MCS.
• K_s depends on deltaMCS-Enabled parameter switch. If it enabled then K_s is 0, if the switch is disabled then K_s is 1.25. The parameter indication is provided by the serving cell to the UE through RRC Connection Setup message.

Amazing! We have come a long way. It doesn’t look so easy, but we are trying to simplify for you. Let’s take a breath and go to the last piece of the formula which is closed loop part. So far the formula was presenting open loop power control section.

1. f_c (i ) is closed loop power control component and the current PUSCH power control adjustment state for serving cell which is defined by following formulas (a means cumulative, and b means absolute based on Accumulation-enabled parameter provided by higher layers):

δ_PUSCH  referred to as a TPC command and is signaled on PDCCH with DCI Format0 or DCI Format3/3A on subframe i – K_PUSCH for serving cell. For FDD, K_PUSCH is 4 subframes (4ms). For TDD, it depends on UL/DL configuration as below:

Above we mentioned TPC command, what does it mean? TPC or with a complete name Transmit Power Control is used in Closed Loop Power Control mechanism to instruct the user equipment to adjust its transmit power (increase or decrease).

Power Control on PUCCH

Physical Uplink Control Channel (PUCCH) is used to carry uplink control information from the UE to the radio network. The conveyed message might be formatted per information type, number of bits. And this type of formations is called PUCCH Formats. Here are some examples such as PUCCH Format1, PUCCH Format1a, PUCCH Format2 and others. The carried information range is large, here we can see just few of them: scheduling request, acknowledgement responses or retransmission requests, channel quality index.

As other physical uplink channels, there should also be some specified standards on PUCCH to control transmission power. Here are we have it:

1. P_PUCCH is transmit power of UE on physical uplink control channel (PUCCH).
2. P_CMAX is configured maxium UE allowed output power. For details see above in Power Control on PRACH. Same description is valid for PUCCH too.
3. P_{0_PUCCH} is a composition of cell specific and UE specific components configured by higher layers and delivered to the UE to be used in power calculation to transmit on PUCCH.
4. PL is downlink path loss estimation calculated by the UE for the serving cell. For details see above in Power Control on PRACH. Same description is valid for PUCCH too.
5. h(nCQI, nHARQ) is a PUCCH format dependent value, in different PUCCH Format the result might be different.

• nCQI: corresponds to the number of information bits for the channel quality information
• nHARQ: is the number of HARQ-ACK bits sent in subframe

6. Δ_{F_PUCCH}(F ) each value of this parameter corresponds to a PUCCH Format and impacts on PUCCH transmit power. It is delta value and the unit is dB. For instance, Δ_{F_PUCCH} for PUCCH Format1 might be set as -2dB, 0dB or 2dB.
7. ΔTxD(F’): if the UE is configured by higher layers to transmit PUCCH on two antenna ports, the value of this parameter is provided by higher layers for each PUCCH format; otherwise the value is 0.
8. g(i) is closed loop power control component and the current PUCCH power control adjustment state. It also has small formula, let’s skim on it:

• For FDD, M=1 and k₀ = 4
• For TDD, values of M and k_m depends on UL/DL configuration.
• δ_PUCCH referred to as a TPC command and is signaled on PDCCH with DCI format 1A/1B/1D/1/2A/2/2B/2C/2D

Power Control on SRS

Sounding Reference Signal (SRS) is a kind of transmitted signal from UE to the serving cell on the last symbol of the subframe of SC-FDMA. If the entire uplink bandwidth could be used all the time, then we would not need such kind of reference signal. But it cannot be, so we would need to use it. Then what is the purpose of this this? The purpose is to help the eNodeB to estimate channel quality of uplink path at different frequencies.

If we are talking about transmission between the UE and eNodeB then it means, there is a power usage and we need to control it not make side effects.

1. P_SRS is transmit power of UE on sounding reference signal (SRS).
2. P_CMAX is configured maxium UE allowed output power. For details see above in Power Control on PRACH. Same description is valid for SRS too.
3. P_{SRS_OFFSET}(m), basically SRS follows similar PUSCH power control calculation method. PSRS_OFFSET indicates the power offset of the sounding reference signal to the PUSCH. The unit is dB, and configurable (e.g. -4.5,-3,-1.5,0,1.5,3 depends on vendor).
4. M_SRS is the bandwidth of the SRS transmission expressed in number of resource blocks
5. Following three parameters are same with PUSCH power control parameters, you can find details above in PUSCH clause: P_{0_PUSCH} ,α, PL and f_c (i )

References: 3GPP 36.101, 3GPP 36.213, 3GPP36.331

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  Here we will try to give general idea for DRX feature in Idle Mode and Connected Mode. 

  Once new type of mobile terminals with wide screen arrived to the market, or another case many mobile applications started to be used by the subscribers then fast battery drain become bothering.

  Of course, there was a most effective solution for this issue, and it was DRX or in other word Discontinuous Reception. Similar features exist in legacy technologies too, but in this article the topic is only LTE DRX.

  Some of the mobile operators could achieve 40%-45% power saving with DRX in their trials, sometimes less or even no effectiveness was observed in some terminals (it might be related with chipset producer). But generally, DRX is a powerful option for energy saving.

  When a new feature was invented, definitely we can see its advantages and also disadvantages. We already know about its positive part which is energy saving at mobile equipment, but what about side effect?

  Side effect or negative part or whatever we want to call are packet delay and packet loss (e.g. eNodeB can schedule data at specified time but the UE may be in sleeping mode at that time). Parameters of DRX are very sensitive, if any unreasonable value are defined on it then it can cause for serious QoS degradation.

  Typically a UE (user equipment) should always listen to the network to see if there is a new data for downlink or need to wait for UL Grant to transfer the data. This notification arrives through PDCCH channel. The PDCCH mapped to the first OFDM symbols in every downlink subframe, it can be 1, 2, and 3 symbols too (depends on configuration).

Let’s come back to DRX

• Question 1: What is the relation between UE monitoring the PDDCH on each subframe and DRX ?
• Answer 1: The relation is power consumption, where the UE spends huge energy if it continuously keeps monitoring and decodes each PDCCH message.
• Question 2: How does the DRX help to save energy at UE side ?
• Answer 2: DRX will let the UE not to listen every PDCCH. The UE will monitor the PDCCH in a specified period and then it will switch OFF its transceiver.

On the following images we can see generic illustrations of DRX Disabled and DRX Enabled scenarios.

• On DRX disabled option, the UE keeps monitoring the PDCCHs on each subframe.
• On DRX enabled option, the UE monitors the PDCCH on a predefined time durations then goes to sleep.

This is not an actual DRX working procedure, it is just for giving an idea how the DRX is influencing the UE power consumption. DRX can work in RRC Connected and RRC Idle modes, it is also called C-DRX and Idle Mode DRX respectively.

• C-DRX has two stages as Short DRX Cycle and Long DRX Cycle. The Short DRX Cycle is optional, it is mostly used to avoid immediate action for large sleeping mode of Long DRX Cycle. It is clear from the below illustrations or in this version (see All in One).
• The PDCCH monitoring in Idle Mode DRX happens on Paging Occasion per Paging DRX cycle

  Now let’s have a look one complete DRX procedure including all steps. The parameter values are not from actual network configurations, so it should not be considered as reference. The purpose is to draw an exemplification of DRX.

From left to right:

• Each rectangle is a subframe, duration is 1ms
• Data Info: there is scheduled new data for the UE, obtained the information from PDCCH
• On green parts there are data transmission or reception
• DRX Inactivity Timer: Specifies the number of consecutive PDCCH-subframe(s) after successfully decoding a PDCCH indicating a new UL or DL user data transmission for this UE.
• DRX Short Cycle Timer: This parameter specifies the number of consecutive subframe(s) the UE shall follow the short DRX cycle after the DRX Inactivity Timer has expired.
• On Duration Timer: Specifies the number of consecutive PDCCH-subframe(s) at the beginning of a DRX Cycle where UE transceiver is ON to listen the PDCCH.

• C-DRX parameters are delivered to UE through RRC ConnectionReconfiguration or RRC Connection Setup. Once no any new data was decoded while listening period (onDurationTimer) and the times passed through Short DRX Cycles and Long DRX Cycles the time reaches to be releasing of RRC Connection with RRC Connection Timer expiration. This release command comes from the network. When the RRC Connection release command comes then the UE moves to Idle Mode, next hop is DRX Mode in Idle Mode where the UE wakes up on each Paging DRX Cycles and keeps its ear on PDCCH to see if there is any new data for the UE (e.g. Downlink) through paging messages.
• Idle Mode DRX parameters are broadcasted with SIB2. The frames that paging messages are transmitted over the Uu interface are fixed. These fixed positions are indicated by the paging frames (PFs) and the paging occasion (PO) subframes. One PF is one radio frame, which may contain one or multiple POs. One PO is a subframe containing the paging radio network temporary identifier (P-RNTI), which is transmitted over the PDCCH. The UE reads paging messages over the physical downlink shared channel (PDSCH) according to the P-RNTI.

   Let’s describe the process one more time. The UE made call setup then requested UL Grant to send data or received DL Grant for downlink data. We can see on green part. After some time, there is no any new scheduled data for the UE and DRX Inactivity Timer started. When the DRX Inactivity Timer expires as a second step Short DRX Cycles starts to run (it is optional, some operator do not use), if no any data arrives or requested during that period then next step is Long DRX Cycles. If the UE keeps its passive status (no UL/DL data) then RRC Connection will be released by the serving cell once the RRC Inactivity Timer expires. After release of RRC Connection the UE moves to Idle Mode and starts to use Idle Mode DRX for monitoring PDCCH.

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Radio side of the network is more complicated in comparison with other domains, because of mobility and user behaviour. A user can be nearby the serving cell under perfect radio environment, and another user can be in the cell edge with very bad radio coverage. So consequently, one of them may be able to get very good experience but other not. It is just one of the simple examples. Both users are very important for the operators and need to deliver better service to them. But how?

In that reason radio equipment manufacturers always spend a huge of amount for their research and development departments to invent or enhance the existing features to overcome such problems.

One of the crucial features in LTE is MIMO which we will talk about, it is used to for radio link robustness, improve spectral efficiency and enhance user data throughput. As we can see here MIMO has several options, not just to robust radio link and it can also give another benefit.

MIMO is a complex technique in terms of working principle and is already used for legacy technologies: e.g WiFi, HSPA+, WiMax, LTE and LTE-Advanced.

Multiple-input and Multiple-output is the complete name of MIMO, below quote is from Wikipedia:

“At one time, in wireless the term “MIMO” referred to the use of multiple antennas at the transmitter and the receiver. In modern usage, “MIMO” specifically refers to a practical technique for sending and receiving more than one data signal simultaneously over the same radio channel by exploiting multipath propagation.”

What we can understand from the quote, in modern usage the MIMO is mostly used for different signal streams to be transmitted simultaneously with taking advantage of multipath propagation and it will result higher data throughput.

MIMO Overview

In MIMO terminology we can see multiple antenna configurations:

SISO

Single Input Single Output, it is very simple configuration in antenna system. Only one transmit and one receive antenna is used. No diversity, no extra option to improve SINR and data throughput. Due to multipath signal effect, this system is vulnerable.

SIMO

Single Input Multiple Output, it is also known as Receive Diversity. The receiver will receive same data by multiple receiver points and will make selection of best signal (Switched Diversity) or combining (Maximum Ratio Combining – MRC) to improve signal to noise ratio. Because of the different transmission paths, the receiver sees two differently faded signals.

MISO

Multiple Input Single Output, also known as Transmit Diversity: same versions of the data will be transmitted over different path to improve link reliability. It is like redundant versions on different path.

MIMO

Multiple Input Multiple Output,

• More than one antenna in both receiver and transmitter sides
• Mostly intended for two types of benefits:
• Spatial Multiplexing: an information which is going to be transmitted will be separated into multiple streams and transmitted independently through different paths. Target is to improve data throughput at receiver side, but not helps for radio link reliability.
• Diversity Gain: each channel towards to the receiver may experience different level of fading and interference. Here on different channels, multiple versions of the same information are transmitted and selection or combining happens on receivers’ side. From such kind of multipath propagation, the diversity technique may exploit a gain which is called Diversity Gain. Tradeoff is obvious between Spatial Multiplexing and Diversity Gain.

Transmission Modes

3GPP specified variety versions of antenna techniques for different usage scenarios and classified them under Transmission Mode numbers for DL and UL separately. Classification might be due to capacity, link reliability, high speed user or low speed user, beamforming etc.

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