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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
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From: owner-stds-802-16@listserv.ieee.org [mailto:owner-stds-802-16@listserv.ieee.org] On Behalf Of Ambroise Popper
Sent: 13 October 2004 15:51
To: STDS-802-16@listserv.ieee.org
Subject: [STDS-802-16] turbo-codes and subchannelization

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 Popper
tel   : +33 1 44 89 48 11
cell : +33 6 60 63 57 20
 
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