Power Headroom Report (PHR) is a MAC Uplink Control Element.
LCID value is “11010” in binary and 26 in decimal
* Every UE will have a maximum transmission power defined by their power class in UP Link.
* PHR is used to inform eNB about the relative transmission power left in UE.
* Formula to calculate PHR is
Power Headroom = UE Max Transmission Power – PUSCH Power
PUSCH Power = Pmax – P_pusch
* So in the above Power Headroom formula, if the value is +ve then UE has some transmission power left and thus network can assign more resource blocks to UE.
* If it is -ve, then UE doesn’t have additional power left and thus network will not assign more resource blocks to UE.
* PHR ranges from -23db to +40db
Reasons for UE to trigger PHR
* If periodicPHR-Timer is set, UE will send periodically PHR to network.
* When down link pathloss threshold is reached (dl-PathlossChange).
IE’s to be concentrated on
We need to check RRC Connection Setup, RRC Connection Reconfiguration message to get the PHR configuration.
“phr-Config” is the IE to be concentrated.
radioResourceConfigDedicated
mac-MainConfig
phr-config
setup
periodicPHR-Timer
prohibitPHR-Timer
dl-PathlossChange
periodicPHR-Timer: Timer for PHR reporting. Possible values can be {sf10, sf20, sf50, sf100, sf200, sf500, sf1000, infinity}
sf10 means 10 subframes, sf20 means 20 subframes.
prohibitPHR-Timer: Timer for PHR reporting. It means, how long UE should not send PHR transmission. Possible values can be {sf10, sf20, sf0, sf10, sf20, sf50, sf100, sf200, sf500, sf1000}
sf10 means 10 subframes, sf20 means 20 subframes.
dl-PathlossChange: Path loss is a reduction in power density when an electromagnetic wave is traveled through space. The value will be in db. Possible values can be {dB1, dB3, dB6, infinity}
Decoding PHR value
The size of PHR is 1 octate
From the above image, we can see that,
The first 2 bits are “R” it means reserved.
Next 6 bits represents that value of Power Headroom Level. It ranges from 000000 to 111111 => 0 to 63
Below are the 2 tables that we refer to when we are decoding PHR value
Example 1:
Suppose we have a MAC PDU “1A 2C”
First we need to decode “1A” and check if there are any additional sub headers and check if there is a MAC CE
1A => 0001 1010
Here “E = 0”. So there are no additional sub headers.
Next “11010” represents PHR MAC CE.
So next octate is a MAC PHR CE “2C”
2C => 0010 1100
Here first 2 bits are reserved.
Hence we need to check next 6 bits “101100”
Value is 44. According to the table, the value will be 21db.
Reference: TS 36.321