PRACH preamble power considerations in LTE

Unlike UMTS, the PRACH in LTE is used only for the transmission of random access preambles. These are used when the UE wants to access the system from RRC idle, as part of the RRC re-establishment procedure following a radio link failure, during handover or when it finds itself out of sync.

As part of the PRACH procedure the UE needs to determine the power to use for the transmission of the preamble and for this it looks at SIB2 for the preambleInitialReceivedTargetPower IE. As shown from the extract above (taken from a live network) this is expressed in the dBm and in this specific case it is set to -104dBm. So this is the expected power level of the PRACH preamble when it reaches the eNodeB.

What is also broadcasted is the reference signal power, which in our case is set to 18dBm. Based on this and a current measurement of the RSRP, the UE can determine the pathloss. Once it knows the pathloss it can then determine how much power it needs to allocate the PRACH preamble to reach the enodeB at -104dBm.

So lets say that the UE measures an RSRP of -80dBm. Based on the broadcasted reference signal power it can calculate the pathloss, PL = 18 - (-80) = 98dB. This means that for a preamble to reach the eNodeB at -104dBm it needs to be transmitted at PPRACH = -104 + 98 = -6dBm. That is fine.

But what happens if we consider other values of RSRP? For example cell edge? Cell edge can be determined by the value of the qRxLevMin. Looking at SIB1 from the same network we can see that this is set to -128dBm (IE x 2). 

So at an RSRP of -128dBm the pathloss is PL = 18 - (-126) = 144dB. So the UE needs to transmit the preamble at PPRACH = -104 + 144 = 40dBm. Is this ok? Actually no, as LTE UEs are only capable of transmitting at a maximum power of 23dBm. Does this mean the UE does not even go through the PRACH procedure? No, but it will be limited to transmitting at 23dBm meaning that the preamble will reach the eNodeB at - 121dBm, which means that the probability of a successful detection is very low.

In actual fact based on this network we can say that anywhere in the cell where the RSRP is below -109dBm will lead to a power limited PRACH attempt and a lower probability of detection. This is something to think about next time your LTE signal strength is low and your phone seems unresponsive..