[e2e] ARQ/HARQ

[Detlef Bosau]

In some private communication, my peer pointed out that in many cases a 
packet sent via a wireless link would reach the receiver either in only 
some few trials, say one or two, or would fail even in quite a lot of 
trials.

Hence, the number of retransmission attempts could be set rather low, 
because either only one or few attempts would finally suffice - or even 
20 or 30 attempts would not achieve the transmission.

Unfortunately, my peer did not had hard facts to argue for this. 
However, one reason for this behaviour could be the burstiness of 
wireless errors. Another observation from actual BLER/SNR measurements 
is, refer to
http://www.detlef-bosau.de/wrapper.php?file=from-symbols-to-packets/R4-020612_html_14c9b9cd.gif
which is taken from 
http://www.3gpp.org/ftp/tsg_ran/WG4_Radio/TSGR4_22/Docs/R4-020612.zip
that the transition from a nearly error free SNR range to a unusable SNR 
range is rather steep. Actually, in this HSDPA related example the 
difference from an SNR where an appropriately coded packet is conveyed 
nearly error free and an SNR where the transmission will fail is 1 dB or 
even less.

Actually, I think this problem is addressed (at least in part) by HARQ 
approaches, which avoid the dumb repetition of inappropriately coded 
packets in case of transmission failure.

If so, this would support a "binary" Model (like e.g. a Gilbert Markov 
Model, although a real wireless channel will hardly be Markovian) which 
only discriminates "link on" and "link off". For HSDPA, one would surely 
extend this in order to reflect several codings, were finaly a coding 
could be appropriate for an actual SNR, hence the transmission would be 
successful, or inappropriate, hence the transmission would fail.


When I think of standards like the GPRS standards, this would leave QoS 
profiles which provide for a BLER of 10^(-9) rather questionable, 
because these are achieved, particularly in GPRS, by local retransmission.

Are these thoughts reasonable?

Detlef

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