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PHR Power Head Room LTE

Power headroom is a measure of how much additional transmission power a UE has available beyond its current usage. The UE reports its power headroom value using a MAC control element (PHR) to help the eNodeB estimate the UE's available uplink bandwidth. There are two triggers for the UE to transmit a PHR - a significant change in path loss or a periodic timer specified in RRC messages. The power headroom value maps to an actual power level that can be used to determine the UE's remaining transmission capacity.

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710 views2 pages

PHR Power Head Room LTE

Power headroom is a measure of how much additional transmission power a UE has available beyond its current usage. The UE reports its power headroom value using a MAC control element (PHR) to help the eNodeB estimate the UE's available uplink bandwidth. There are two triggers for the UE to transmit a PHR - a significant change in path loss or a periodic timer specified in RRC messages. The power headroom value maps to an actual power level that can be used to determine the UE's remaining transmission capacity.

Uploaded by

Muhammad Rizki
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LTE Quick Reference Go Back To Index Home : www.sharetechnote.

com

PHR(Power Headroom Report)

What is Power Headroom ?

Power headroom indicates how much transmission power left for a UE to use in addition to the power being used by
current transmission. Simply put, it can be described by a simple formula as below.

Power Headroom = UE Max Transmission Power - PUSCH Power = Pmax - P_pusch

If the Power Headroom value is (+), it indicates "I still have some space under the maximum power" implying "I can
transmit more data if you allow".
If the power Headroom value is (-), it indicate "I am already transmitting the power greater than what I am allowed
to transmit".

How does UE report Power Headroom Value ?

PHR is a type of MAC CE(MAC Control Element) that report the headroom between the current UE Tx power
(estimated power) and the nominal power. eNodeB (Network) use this report value to estimate how much uplink
bandwidth a UE can use for a specific subframe. Since the more resource block the UE is using, the higher UE Tx
power gets, but the UE Tx power should not exceed the max power defined in the specification. So UE cannot use
much resource block (bandwidth) if it does not have enough power headroom.

You will find the following fig and table from 36.321.

How can I figure out real power value from this report value ? You can find the mapping table from 36.133 as shown
below.

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When does UE transmit the Power Headroom Report ?

There are two triggers for PHR (Power Headroom Report).

i) Path Loss Change greater than a certain threshold : UE can calculate the path loss based on RS(Reference
Signal) power notified by network and the measured RS power at UE antenna port. If this value changes over a
certain threshold UE transmit PHR.
ii) By some peridic Timer.

These triggers are specified in RRC messages (e.g, RRC Connection Setup, RRC Connection Reconfiguration) as
shown below.

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