4g LTE: LTE MAC UL CE: Power Headroom Report

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

* If periodicPHR-Timer is set, UE will send periodically PHR to network.

* When down link pathloss threshold is reached (dl-PathlossChange).

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}

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

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