Re: [STDS-802-16] turbo-codes and subchannelization
Ambroise
My understanding had been your second interpretation (it
sort of matches the CC coding of a "long and thin" allocation), but I agree that
the interpretation is ambiguous (and of course more complex than the shift
register of a CC, as it requires buffering of the coded
data).
If your first interpretation were correct, then the BS
scheduler would have another (complex) constraint to deal with (e.g. only do BTC
on 1/4 or larger subchannels), and the reach of the system would be compromised:
the CTC/BTC could not be used in the cases where the SS needs to be
concentrating power on few tones and where the additional SNR gains from using
BTC/CTC would be most beneficial.
The language describing the number of subchannels itself is
(in places) a bit vague. I try to think of 31 possible subchannels, of
which 1 is full bandwidth and the others are 16*(1/16), 8*(1/8), 4*(1/4),
2*(1/2) (indexed by the sub-channel index of Table 211), but there are places in
the document (e.g. the CTC) where the alternative ("n subchannels") is, I think,
used to describe an aggregation of n*(1/16) (n = 2^m, m in
{0,1,2,3,4}). Otherwise the value of Nsub in Table 218 (page 438,
439) would be stuck at 1, because, in an allocation, you cannot allocate multiple
(1/16) subchannels: you allocate using a single index..
I doubt this was what was intended. A similar lax style was used in BTC
section, and also in the UCD channel encodings table (page
658).
Regards
David
Dr David Castelow
Airspan Communications Ltd
Principal Systems
Engineer Cambridge House, Oxford Road
DSP & Systems Design
Uxbridge UB8 1UN, UK
http://www.airspan.com
+44 1895 467281
In the 802.16-2004
OFDM PHY, turbo-codes have been extended to support uplink subchannelization.
A minimum
block size is defined both in BTC (96 bits) and CTC (48
bits).
How do you
accomodate this with the use of a small number of
subchannels?
A first
interpretation would be that you cannot use turbo-codes with a small number of
subchannels (for instance in BTC QPSK 1/2, you can only support 4 or
8 subchannels to reach the minimum block size of 96
bits).
A second
interpretation is that you must code over several OFDM symbols to reach desired
block size (for instance in BTC QPSK 1/2 with one used subchannel, a coded block
is necessarily a multiple of 4 symbols).
Best
regards,
Ambroise
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