Optimal spectrum refarming for LTE

When looking to refarm some spectrum for LTE (e.g. 1800MHz spectrum from GSM) the following simple approach will lead to optimal results.

Start by thinking of how much spectrum you would ideally refarm. This will typically be 20MHz. Assuming this was possible pick the centre frequency for this allocation. This will be your EARFCN. Then look at how much spectrum you can actually refarm. This will typically be less, as the traffic on the legacy RAT might not have reduced enough or frequency re-planning your whole legacy network will take time. Most operators go for 10MHz, but in some cases 5MHz is also used.

Deploy your network.

After some time has passed and more spectrum is available, keep the centre frequency the same and just expand the bandwidth. Some cells might be using 10MHz, some 15MHz or 20MHz but because the centre frequency has not changed, all mobility can be intra-frequency. No need for inter-frequency handovers, no need for additional neighbour planning, no need for measurement gaps, no need for additional SIBs being broadcasted. UEs will seamlesly reselect & handover taking into account the used bandwidth every time as this is broadcasted in the MIB which is read in idle mode and after every handover.

Although the above might sound like the obvious way of doing things, both EE in the UK (see here) and other LTE deployments (see here) don't follow this but rather offset their two bandwidth allocations leading to needless inter-frequency mobility.

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..