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stds-802-16-tg3: Re: OFDMA Ranging



Dear Itzik

Thank you  so much for highly esteemed your answer on my question
I have a question on the backoff value described in 6.2.2.3.3. Uplink Channel Descriptor(UCD) message of 802.16/D5-2001.
There are 2 kinds of backoff value in table 16, but I did not fully understand differences between Ranging backoff start/end and Request backoff start/end. As I mentioned and you agreed in following attached e-mail, 3 types Ranging code such as initial, periodic and BW request are refering the same single Ranging backoff start/end. But there is no exact references and description for Request backoff start/end parameters.
Would you clarify the difference between two parameters?
Thanks
Changhoi Koo

Ranging backoff value

Itzik Kitroser wrote:
Dear Chkoo,
I'm sorry for the delay in my response.
I still believe that one backoff value which will be tuned according to the BW request domain is enough.
The proportion between the number of codes and the Backoff window size is inverse but not linearly, but rather depends also on
the number of users and expected usage scenario                        .
I understand your concerns and motivation, but I think that 3 different Backoff values are not required due to the redundant added
value that we will receive.
I do encourage you to examine this subject further and provide analysis and simulations if it interests you.
 
Regards,
Itzik.
-----Original Message-----
From: chkoo [mailto:chkoo@metro.telecom.samsung.co.kr]
Sent: Thursday, May 02, 2002 7:44 AM
To: chkoo
Cc: itzikk@runcom.co.il; 'chkoo'; 'Jerry Krinock'; stds-802-16-tg3@ieee.org; 'Mike Paff'; 'Manoneet Singh'; 'VanWaes Nico (NET/MtView)'; 'Yossi Segal'; 'Yigal Leiba'
Subject: Re: OFDMA Ranging

Dear Mr. Itzikk and Jerry Krinock and all
I have sent the following mail abot 2 weeks ago, regarding Ranging code and its back off time allocation.
Your response must make me happy and enlight me on BWA systems.
Thanks
Changhoi Koo


chkoo wrote:
Dear All.
Thanks for your responses

Regarding 5), I have more detail concern and question.
As we understood, Ranging codes for 3 groups has only one Backoff value with some specific ranges to calculate the backoff window. However,  3 catagories, N, M and L will be assigned  dynamically and look up the same value.
I think collision probability strongly depends on the number of Ranging code allocated to each group and backoff scheme is conventionally used to provide the time distribution which can a little bit resolve the collision due to retransmission instance. Thus, I believe that we have 2 level contention  resolution scheme, i.e., multiple codes alocation and backoff time for SSs.
In my understanding, the number of multiple code and backoff time range are in inverse proportion.
For example, as you mentioned, BW-request is dominant precedures rather than other ranging procedures so that the number of Ranging code for BW-request can be greater than other ranging code. This assignment can have more less collisions. In contrast, in case of initial ranging procedures. Ranging Code will be allocated relatively less than other cases. Therefore, collision probability can be relatively higher.

What I have question or concern is that 3 kinds of backoff value assignment for each N, M and L, , respectively, are better than the assignment of a single backoff value for all described in the current specification. 

How do you think about?
Would you enlight me on this stuff?

Thanks
Changhoi Koo

Itzik Kitroser wrote:
Dear changhoi,
Please find my answers incorporated in your mail.
 
Regards,
Itzik.
-----Original Message-----
From: chkoo [ mailto:chkoo@metro.telecom.samsung.co.kr ]
Sent: Tuesday, April 23, 2002 6:07 AM
To: Jerry Krinock
Cc: Itzik Kitroser; chkoo; stds-802-16-tg3@ieee.org ; Mike Paff; Manoneet Singh; VanWaes Nico (NET/MtView); Yossi Segal; Yigal Leiba
Subject: Re: OFDMA Ranging

Dear Jerry Krinonk, Itzik Kitoser and all
Thanks for your kind response enlightend me.
And I'd like to say  " I'm so sorry for too late response"

So far, we has discussed about Ranging code selection and its utilization based on Qualsi-random selection proposed by Samsung in the previous meetings.
Here are my clarifications and some of questions to more understnad IEEE 802.16 BWA system.

1.The ranging code-selection by the BS
Given that your corrections and clarification, I fully agreed with them, I had carefully review the current specification and discussion with our physical layer delegates several tmes. As concluding, we, Samsung,agree with your comments about the "Ranging subchannel selection". Basically, subchannel is uniques to each cell, any interference problem due to the same ranging code selection, will be an obsolete.
Thank you so much...

2. The ranging code-selection by the SS
Original Samsung's proposal was to prohibit the selection of the same ranging code from the multiple SSs in the same cell comparing that the current specification is only describing the Random selection". To do that,our proposal is to give Pseudo dedicated Ranging code allocation and selection based on SS's ID and total allocated Ranging codes.And I believe that it may have an enough performance hopping. However, I did find out s ome
un
clea
re description for allocationg the Ranging code and associated procedures through the current specification as followings.

In page 211, IEEE P802.16a/D3-2002, Ranging code generation and allocation schemes are described as follows. "The number of available codes for Ranging procedures is 48, These codes are divided in three usage groups. e.g., Initial Ranging, Periodic ranging and BW-request ranging. And these code are allocated to the groups by the BS. The default number of codes for each group is two." And an unknown number for each group, N, M and L, are produced and allocated by the BS. AT this stage, I have several preliminary questions on the Ranging code allocation.
1) Which message would includ this information? (In my view, UL_MAP message including UIUC, but I could not really find out the exact information field describing which codes are allocated to which ranging procedures). So would you enlighte me?
 Itzik> Please look at page 224 of the P80216a_D3.pdf draft, there you will find a table describing the TLVs relevant to the UCD message, types 13,14 and 15
defines the number of codes allocated to each of the Ranging domains.

2) According to the current specification, th
e BS can dynamically allocate the Ranging codes(N for Initial, M for periodic and L for BW-request). I believe that the BS can distribute the ranging code and it strongly depends on the currecnt cell status. For, example, If the number of SSs in the traffic state are relatively greater than those in the idle state, the BS will re-allocate the Ranging code, M or L, for periodic and BW-request, which is greater than initial Rangng code. This means that the Ranging code for each group can be assigned in an imbalance status and it can issue the more colliding from the small ranging code assigned to SS. Would you correct what I am thinking is correct or not?
Itzik> The BS can allocate the codes according to the cell status, implementation perspective or other reasons. The important issue that the N,M and L values are dynamic and are learned by the SS to react according to the strategy selected by the BS. 

3) Regarding understanding(2), from the imbalance Ranging code allocation, if the lack of Ranging code allocation occures due to cell status, the Quasi-random selection may be one of effective control scheme which selects Ranging code based on SS's connection ID I believe. Would you correct me?
 
Itzik> I don't understand why Quasi-random selection can improve any situation, you just change to collision probability domain from code distribution to CID distribution.   I think that you can see from the answers of Jerry,Nico and myself that Quasi-random does not improve the collision probability in a major factor, this was also what I saw in my simulations.

4) From point
of implementation view, you mentioned that the Ranging code selection by the SS are jointed with implementation and it will be a manufacture propritery techniques. However, in traffic state, because the BS is recognizing the connection ID of the SS and it can estimate the Ranging code sequence corresponding to the ranging procedures, to be transmitted to the BS, the Quasi-random selection can give more benefit and estimated demodulation or something like opertion such as quick acquisition, at the BS. However, it can not have any benefits if that kind of selection shceme are not mentioned in the specification. Would you correct me also?
 
Itzik> I don't agree with you that the BS can recognize a SS using the code that the SS selected in the Quazi-random scheme, taking a BS with 500 SS's with uniform CID distribution, and having 16 codes for each domain, you will have a code reuse factor of ~30. Therefore the SS is still anonymous, from the BS's point of view.

5) In current specification, I think that there is only one backoff valus in the UCD message for the Ranging procedures. What I have question is that this single backoff valuse should be applied to all Ranging procedures such as initial, periodic and BW-request.
This is also not unclear to me. Would you lead me from the cloud?  
 
Itzik> The most dominant usage of the "Ranging procedures" or CDMA contention access is for BW requests, so the backoff values should be configured according to the status is this domain, the other domains (i.e. Initial and Maintenance ranging) can use the same values. Do you have other ideas?  
 
 Thanks
Changhoi Koo




Jerry Krinock wrote:
[This is a continuation of the "Re: How Are You" thread which has been going
on privately.]

Itzik,

You emailed something a few days ago which got my attention:
Itzik Kitroser wrote:

Some of the sub-channels can be used for Ranging sub-channels, in this case,
each Ranging sub-channels is build out of two regular sub-channels and
will contain 106 carriers. The MAC messages that contain reference to
Ranging sub-channels.

This is not my impression and not what it says in the current draft IEEE
P802.16a/D3-2002. On pg. 209 line 53 it says (in text which I wrote!):

The MAC shall define a single ranging channel. This single ranging channel
is composed of one or more subchannels.

and on page 211, line 19:

The length of each ranging code is a multiple of 53 bits. The default
is two subchannels allocated as the ranging channel; in this case the ranging
code length is 106.

I hope you see the difference between your remarks and the text in the
current draft. Your remarks say that there may be MORE THAN ONE "ranging
channel", and that each ranging channel is comprised of two subcarriers;
thus the ranging code length is always 2x53=106. In the current draft, it
says that there is ALWAYS ONE ranging channel, and that the code length
varies with the number of subchannels allocated to ranging. If two
subchannels are allocated to ranging, then the code length is 106 bits. If
four subchannels are allocated to ranging, then it is 212 bits. However,
the BS may also allocate one, three, five, six, etc. subchannels which would
result in code lengths of 53, 159, 265, 318, etc. respectively.

When I ran my simulations of the ranging process last year, I simulated it
both ways: with multiple ranging channels as you described, and with a
single ranging channel as in t
he
current draft
. I found that, for a given
ranging subchannel allocation (of, say, 4 subchannels), performance was
slightly better using the single ranging channel as in the current draft
(say, a single ranging channel of code length 212) rather than as you
described (say, two ranging channels of 106 bits each). It seemed that the
increased coding gain was slightly more beneficial than the decreased
collisions in the frequency domain, which you get from having two channels
available.

In KiHo's contribution IEEE C802.16a-02/33r1, he describes selection of a
"Ranging Subchannel Index". According to the current draft, since there is
only a single ranging channel and therefore no need to index one channel.
Thus, it appears that KiHo he believes the same as you.

The way I wrote the current draft is the way I thought it was intended by
Runcom's original contributions. If you want it changed to the way you
underst and, you
must
submit a comment s
omeday to change it. Although my
simulations show slightly better performance using the method in the current
draft, I personally do not care that much which way we do it, because in my
opinion the CDMA scheme performs poorly in either case. (But that is
another discussion!)

Regarding the current discussion by Changhoi, I have re-read the
contributions from Samsung and agree with Itzik that inter-cell interference
is sufficiently randomized by the our use of different subchannel
permutations in adjacent cells, assuming adjacent cells have different
values of the parameter ID_cell. I do not believe that the proposed
quasi-random selection will have any advantage.

Changhoi, thank you very much for restarting this discussion which brought
this misunderstanding to light. I hope you can contribute some simulations,
and prove us wrong if you may. May I suggest you please post a top-level
descript
ion or blo
ck diagram of your alg
orithms before you code the routines
in detail, so that others can review the basic assumptions. Otherwise,
we'll just have another set of simulation results that no one understands
and no one agrees with.

One final note: I have copied this message to the TGa email reflector. I
believe that this email reflector is underutilized, and I discourage the use
of private email lists for technical discussions. There may be new members
of the working group who can contribute, or at least need to begin
understanding the issues!


-- 
Changhoi Koo Ph.D
Senior Engineer
Global Stand and Strategy
Telecommunication R&D Center


Samsung Electronics CO.LTD
Suwon P.O BOX 105
21st FL., IT center.
416, Maetan-3dong, Paldal-gu
Suwon-si, Gyunggi-Do, Korea 442-742
TEL : +82-31-279-5091
FAX : +82-31-279-5130
Mobile : +82-16-9530-5091
E-mail :
chkoo@samsung.com
chkoo@telecom.samsung.co.kr


-- 
Changhoi Koo Ph.D
Senior Engineer
Global Stand and Strategy
Telecommunication R&D Center


Samsung Electronics CO.LTD
Suwon P.O BOX 105
21st FL., IT center.
416, Maetan-3dong, Paldal-gu
Suwon-si, Gyunggi-Do, Korea 442-742
TEL : +82-31-279-5091
FAX : +82-31-279-5130
Mobile : +82-16-9530-5091
E-mail : chkoo@samsung.com
chkoo@telecom.samsung.co.kr


-- 
Changhoi Koo Ph.D
Senior Engineer
Global Stand and Strategy
Telecommunication R&D Center


Samsung Electronics CO.LTD
Suwon P.O BOX 105
21st FL., IT center.
416, Maetan-3dong, Paldal-gu
Suwon-si, Gyunggi-Do, Korea 442-742
TEL : +82-31-279-5091
FAX : +82-31-279-5130
Mobile : +82-16-9530-5091
E-mail : chkoo@samsung.com
chkoo@telecom.samsung.co.kr


-- 
Changhoi Koo    Ph.D
Senior Engineer
Global Stand and Strategy
Telecommunication R&D Center


Samsung Electronics CO.LTD
Suwon P.O BOX 105
21st FL., IT center.
416, Maetan-3dong, Paldal-gu
Suwon-si, Gyunggi-Do, Korea 442-742
TEL : +82-31-279-5091
FAX : +82-31-279-5130
Mobile : +82-16-9530-5091
E-mail : chkoo@samsung.com
             chkoo@telecom.samsung.co.kr