The Received Signal Strength Indicator (RSSI) in the uplink is affected by the parameter settings that govern open loop power control in LTE. Open loop power control is used during Random Access.

The random access is often the first transmission from the UE, and it is a short transmission (less than 3 ms at most). Consequently, the network does not have an opportunity to power control the PRACH transmitted by the UE. Instead, the UE must estimate the minimum amount of power it needs to send the access request without causing excessive interference.

The UE receives a number of key parameters for PRACH power control in SIB 2, including:

  • Preamble Initial Received Target Power: The power level the eNB would like to receive for a random access. The default value is -104 dBm.
  • Power Ramping Step: The amount of additional power to be used every time the random access is attempted again. This can be 0, 2, 4 or 6 dB.
  • Preamble Trans Max: The maximum number of times a random access can be attempted before the UE gives up, to a maximum of 10 tries.
  • RA Response Window Size: The number of subframes the UE will wait for a response after a random access, between two and 10 subframes.
  • T300: the time the UE has to receive the RRC connection Setup message from the eNodeB.

The UE will determine the initial power level based on the Preamble Initial Received Target Power value and an estimate of the uplink Path Loss (PL) as follows:

Pinitial = min (Pmax, Preamble Initial Received Target Power + PL); where Pmax is the maximum transmit power of the UE, based on its category.

If the eNB fails to respond to the random access in the designated time window (RA Response Window Size), then it can repeat the random access (after waiting at least four more subframes), increasing its power level by the Power Ramping Step value. If no response is received after Preamble Trans Max attempts, then the UE will return an access failed indication.

The values of Preamble Initial Target Power and Power Ramping Step directly affect RSSI. High values of both parameters may result in high RSSI, particularly in indoor environments (i.e.: Airports, convention centers, etc.) and events (i.e.: foot ball games at stadiums, concerts, etc.). In this type of environments, a high concentration of UEs exists and often times, many of them try accessing the network at the same time. Even when a different UE has selected a different preamble and has calculated the right amount of power, the eNodeB often times will only answer to ONE of them (this is vendor implementation dependent) per sub-frame, leading to the rest of the UE to increase their transmit power. If this condition prevails, quickly, all UEs will be transmitting at is maximum transmit power in less than a second, producing a high RSSI at the eNodeB.

Values of -110 to -112 dBm are recommended for Preamble Initial Target Power and 2dB for Power Ramping step for events and indoor environments. For outdoor environments, depending on the load and RSSI, values of -104 dBm and 4dB could be used, respectively.