Discuss the relationship between RS power and PDSCH power in LTE networks. Explain the different types of REs (Pa, Pb) and how they are affected by the RS power and the offsets (PaPcOff, PB)? Also discuss the impact of boosting or deboosting a cell on the coverage and throughput of the network.
Unlike UMTS, LTE design was made simple but smart and beautiful. Downlink power allocation in LTE was based on the lessons learnt from UMTS which had an unnecessarily complicated and ineffective power scheme.
I always enjoy reviewing power settings in LTE and admire its subtle simplicity.
Yet there is one side of it that I was not paying attention to for a long time.
In LTE, data/control channels power are as independent as possible from the coverage channel (ReferenceSignal). There are 3 main players that set the total cell power:
1. Reference Signal power
2. Power of REs in symbols without Reference Singals (Typa A: Pa )
3. Power of REs in symbols with Reference signals (Type B: Pb)
Different vendors have taken different approaches. For example while in Huawei RS power is set with a direct value (e.g. 15.2 dBm), in Ericsson RS power is set implicitly by the total cell power and CrsGain.
Now let’s focus on the huawei approach.
Given an RS value like 15.2, the Pa power can be calculated by adding an offset (PaPcOff). offset 0 would mean that each PDSCH RE in Pa will have the same power as RS (15.2 dBm) and for offset -3 that power will be half (12.2).
For improving coverage, you need to increase the RS. But how can we do that? by borrowing power from other REs. Now here’s the small tricky detail i talked about. Which REs?
You can only borrow power from the REs that are in the same time symbol as RS, meaning Pb. Unless you make an error, Pa Power will NEVER decrease.
Pa offset is not for decreasing the power on Pa to add to RS, it’s for keeping the Pa power as it was.
Borrowing is done through the Pb offset (PB in Huawei, PdschTypeBGain in Ericsson).
With PB you will set the PDSCH power of Pb RE in relation to Pa Power.
Since for each RS we have at least 4 Pb REs, the loss is shared and there is minimum effect on PDSCH.
So when you are boosting/deboosting a cell, you are only borrowing power or returning it back to Pb.
If you face throughput decrease after boosting, it’s not because of losing PDSCH power, it is mainly because of attracting new users (only ~ 6 watts power and only in 2 symbols of each slot is borrowed if you change 152/0/0 to 182/1/-3). And if you increase throughput after deboosting (or Deboosting with positive Pa) it’s not because of more power given to PDSCH. It is because you have reduced coverage and hence the number of users.
