HARQ

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HARQ

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Attached is a graphical representation of a Data Download performed on the AT&T LTE N/W using an LTE device.

This view was created using Qualcomm's QXDM tool.  The Throughput shows an avg of 20 Mbps.  The BLER

graph shows the HARQ for all 8 HARQ processes.  I did read a little about HARQ but I am not 100% clear on

exactly how this works and how it improves the overall performance on both UL and DL.  Could someone please

provide me with a detailed explanation of the HARQ process and what the BLER (%) tells us about the performance

of the device under test.

Verified Answer
  • At the Physical Layer for LTE, the target error rate is 10%; in UMTS, the target could be configured, and was often set to 1% or 2%. You can read more about HARQ in 36.321, sections 5.3 and 5.4.

    Uplink adaptation is really up to the eNB vendor. Using the available feeback from uplink transmissions (such as DMRS, SRS and HARQ stats), the eNB can adjust its uplink allocations (and the UE's power level) in order to achieve the target error rate.

All Replies
  • The Block Error Rate (BLER) is a measure of how successful a data transmission is over the air at the Physical/MAC layer level. If a transport block is successful decoded (the CRC calculated by the receiver matches the CRC sent in the block, then the transmission is successful. The target BLER for the radio link is typically set to 10% (meaning that the transmission must be sent in such a way as to achieve a 90% success rate on average).

    If the transmission fails, then the receiver indicates this with a HARQ NAK (negative acknowledgement); the sender can then send additional information (e.g. error correction bits) to help the receiver decode the original transmission (a so-called HARQ retransmission). The receiver combines the new bits with the original bits, and tries to decode the block again; if successful, the transmission is complete, if not, another HARQ retransmission can provide more help. Generally speaking, HARQ will try 3 times to get the block through (original + 2 retransmissions), after which the transmission is considered to have failed.

    36.321 Sections 5.2 and 5.3 provide the details on HARQ.

  • And what is the role of channel decoder? If a frame has some errors it can be correct by channel decoder and is not  necessary retransmissions. Only in the case when the channel decoder cant correct the errors the receiver send a HARQ NAK.

  • Correct. However, not all of the error correction bits are sent in the original transmission. If the decoder is able to successfully decode the transmission, then no further action is needed; if it can't, HARQ can supply additional error correction information to improve the decoding results.

  • Don - Isnt the target BLER 1% ? Also can you throw some light on Uplink Adaptation using Channel Quality and BLER . Also what spec to refer?

  • At the Physical Layer for LTE, the target error rate is 10%; in UMTS, the target could be configured, and was often set to 1% or 2%. You can read more about HARQ in 36.321, sections 5.3 and 5.4.

    Uplink adaptation is really up to the eNB vendor. Using the available feeback from uplink transmissions (such as DMRS, SRS and HARQ stats), the eNB can adjust its uplink allocations (and the UE's power level) in order to achieve the target error rate.

  • Hello  Djamschid (DJ) Mehrpuyan,

    Which version of QXDM are you using ? I am using 3.09.19 , but after installation could not see any support for LTE in this tool. Can you share some more information/documents regarding setting up QXDM for the first time .

    -Ankit

  • Hi Don,

    I just want to understand it right,. What you are  saying is  that BLER can be configured from the network, right? if so, that means it is not necessarily be always 10% ?

    Thanks,