RE: stds-802-16-tg3: support for AAS (C802.16a-02/49)
Hi everybody,
I hope everybody would find C802.16a-02/49 a constructive effort towards
improvement of the AAS support in the standard.
Responding to Nico's comment about still having two incompatible OFDMA
modes, I wanted to draw everybody's attention to the fact that even without
OFDMA2 we have 32 sub-carrier permutations in OFDMA (see equation 24 in page
203 of IEEE P802.16a/D3-2002, and note the parameter IDcell). The reason for
having these permutations is to combat inter-sector and inter-cell
interference.
It could be argued that having these permutations (which exist in the uplink
as well as the downlink) makes the OFDMA mode non-interoperable with itself,
but this is not the case, since the OFDMA signal can be identified and
synchronized to without knowing the permutation in use. After this
synchronization is achieved, a simple scanning procedure can be used to
identify the permutations (there are only 32 possibilities). This scanning
procedure is only used on initial network entry.
Given this, the addition of yet another permutation, adds a negligible
complexity to OFDMA implementation, and certainly does not render the modes
non-interoperable.
Regards,
Yigal
> -----Original Message-----
> From: owner-stds-802-16-tg3@majordomo.ieee.org
> [mailto:owner-stds-802-16-tg3@majordomo.ieee.org]On Behalf Of
> nico.Vanwaes@nokia.com
> Sent: Monday, May 13, 2002 2:38 AM
> To: stds-802-16-tg3@ieee.org
> Subject: stds-802-16-tg3: support for AAS (C802.16a-02/49)
>
>
> Colleagues,
>
>
> Reading through C802.16a-02/49, I find that an excellent
> effort has been made to rewrite and improve the AAS support
> in a concise manner. It's an effort I hope will find
> widespread support.
>
> There is however in my opinion one major exception to which
> I'd like to draw some early attention, so that the interested
> people have some remaining time to reconsider the matter.
>
> This one exception is the text discussed as:
>
> The substantial difference between OFDMA modes is how logical
> carriers are mapped to physical carriers.
> OFDMA2 uses sub-channels that are composed of adjacent
> sub-carriers. A separate permutation mapping can represent
> this. Using this permutation in conjunction with adaptive
> array makes the array implementation simpler, and is
> therefore justified.
>
> which finds its suggested implementation on page 212, line 55
> in the 23rd suggested modification.
>
> Imho, the main problem with this language is that it once
> again introduces two entirely incompatible OFDMA modes. Yet
> instead of clearly indicating the failure to compromise on
> one harmonized carrier allocation by having an OFDMA and
> OFDMA2 PHY, the proposal is yet again to stuff this principle
> incompatibility into an "optional" subclause; a situation
> similar to what we started out with way too many meetings ago.
>
> There is currently, to my understanding, a single major
> technical argument for either carrier allocation. The fully
> distributed carrier allocation is claimed to be preferable
> for optimum frequency diversity, the contiguous carrier
> allocation is claimed to be preferable for minimum array
> processing overhead, as only one weight per contiguous
> allocation is needed.
> In my mind, as somebody who isn't hung up on either
> implementation, it seems to be, entirely from a technical
> perspective, worth it to ponder compromising alternatives to
> either, such as for example a scheme in which 4 clusters of
> 13 contiguous carriers are distributed with a modified
> version of the current mandatory permutation algorithm, in
> order to retain significant frequency diversity and
> drastically reduce the AAS implementation complexity.
>
> This may be painful for the proponents of both current
> carrier allocation schemes, but imho, this is the only
> alternative to voting one scheme off the table.
>
> Regards,
>
> /Nico
>
>
>
>