Microwave Journal
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LTE ARQ and Re-segmentation

August 18, 2008

Many forms of data transmission utilize various forms of Automatic Repeat reQuest (ARQ), where data which has not been received or correctly decoded is automatically requested. This guarantees delivery of the full packet of data, for instance to make sure that the whole web page is displayed. When data is known not to have been received, there normally follows several further attempts to send/receive the data in the same format.


Usually this will be sufficient to ensure the data gets through, however if the data is still not received and decoded correctly even after several repeat attempts, it could be that more protection needs to be added to the basic data or for it to be formatted in a different manner. Long Term Evolution (LTE) utilizes a process known as re-segmentation, which essentially allows blocks of previously formatted data to be segmented further. The data can then be sent in smaller packets with added protection, hopefully giving the data a better chance of being received and decoded correctly.

This is a departure from the existing W-CDMA ARQ process, which does not use re-segmentation. When a block of data is packaged at the Radio Link Control (RLC) layer in W-CDMA, it is never re-segmented. Every attempt will be made to get that data packet through to the receiving entity, but there is no means possible for that data packet to be sent in smaller segments in order that it may be given a better chance. 2G Enhanced General Packet Radio Service (EGPRS) on the other hand, utilized a form of re-segmentation called split blocks, in which data could first be sent - for instance in Modulation Coding Scheme (MCS) 9, and if that was unsuccessful, it could be segmented further and sent as MCS6 or MCS3 with much better protection, but resulting in a heavily reduced data rate.

The effective data rates of the various EGPRS MSC’s were structured so that the transmission of four MCS3 or two MCS6 blocks would fit into one MCS9 block. In comparison to EGPRS, LTE has the possibility for enormous block size variability. LTE has effectively 2 variable dimensions, resource blocks and Modulation Coding Schemes. With 8 coding schemes, 3 modulation schemes and 1 to 100 resource block allocation, this gives a maximum data rate of 75376 bits per ms for 100 resource blocks (the full 20MHZ cell) and 64QAM, down to only 16 bits per ms SISO for 1 resource block and QPSK. This resource flexibility demands similar flexibility in PDU packaging at RLC.


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