14. June 2017 at 11:52 #8220
Please could you answer these questions;
1) What happens when using (downlink_delay) larger than 1, Does that means the source of this delay comes from:
a) The delay that the feedback values will face during the transmission media in the downlink or the delay which has been generated by the uplink transmission?
b) or the delay means that the scheduler will be late in assigning the resource blocks?
c) or the delay from the process of estimation?
d) or all the above points ?
2) What is the action of the UE in case of receiving delayed feedback and why the throughput decreases?
Sara23. June 2017 at 8:25 #8251
1) Downlink delay is a number of TTIs (subframes) by which the calculated feedback information from the base station (CQI,RI, PMI) is delayed before being employed at the UE side. It means the feedback information is outdated by this number of TTIs.
2) On a time-variant channel, the throughput decreases with increasing downlink delay because the channel changes between the feedback calculation from an estimated channel between the transmission for which the obtained feedback parameters are employed. So when a delay is introduced in the downlink, the link adaptation does not work perfectly anymore, because the channel changes with time.
The UE cannot compensate for the feedback delay (in our simulator). What can be done as compensation is channel extrapolation (prediction) before feedback calculation at the base station before calculating and signalling the feedback information to the UE.
Stefan26. July 2017 at 1:37 #8337
Please could you answer this question
What is the difference between the link level and the system level simulator?
Sara2. August 2017 at 9:33 #83772. August 2017 at 16:30 #8379
Thank you very much,
Please could you explain the following case. I wonder why the (cell and UE ) throughput sometimes reaches to a (saturation value) with high SNRs , while the throughput should increase with the increase of SNR? for instance, I did a simulation test with 4×8 antennas and 1.4MHz with 2 users and up to 40dB SNR. The results show that the throughput of the UEs and the cell was saturated up to 8.5 MHz for UE and 17 MHz for the cell ! between (25 to 40 dB) SNRs.
Please, what is the reason for this saturation at the high SNRs?
Sara3. August 2017 at 9:52 #8380
with link adaptation and feedback, the modulation and coding scheme (MCS) or CQI is adapted to the channel conditions. With increasing SNR, higher CQIs are employed to reach higher spectral efficiency and throughput. Once the highest modulation and coding scheme (CQI15) is reached, no more increase in spectral efficiency is possible and the throughput saturates at its maximum.
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