Transcript IEEE C802.16m-09/1564r1

Dedicated BR preamble and tile assignment in Femtocell (AWD-Femto)
Document Number: IEEE C802.16m-09_1564r1
Date Submitted: 2009-07-09
Source:
Kanchei(Ken) Loa, Tsung-Yu Tsai, Yi-Ting Lin, Jiun-Je Jian, Chun-Yen Hsu,
Chiu-Wen Chen, Youn-Tai Lee, Yung-Lan Tseng
E-mail: [email protected]
Institute for Information Industry (III)
Venue:
Base Contribution: N/A
Re : Call for Contribution on 80216m-09_0028r1
Purpose: Discussion and approval of the proposal into the IEEE 802.16m AWD
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Introduction
• Bandwidth request channel (BR channel) is
used in Femtocell
– A BR channel occupies an LRU which is consist of 3
6X6 tiles separated in the physical band.
– During BR, AMSs contend for bandwidth by
transmitting <preamble | quick access message>
in one of the tile in the BR channel.
Introduction
• In the Femtocell’s BR procedure in the quick access channel, an AMS
transmits <preamble | quick access message>
– If the quick access message is decoded successfully, the Femtocell BS grants the
bandwidth for the following data according to the content in the quick access message.
Thus the BW-REQ message can be omitted.
AMS
Femtocell BS
AMS
Femtocell BS
<preamble | quick access message>
<preamble | quick access message>
UL granted for BW-REQ
UL grant
BW-REQ
Data
UL grant
Data
If the quick access message is not decoded
successfully (5-step BR procedure)
If the quick access message is decoded
successfully (3-step BR procedure)
Dedicated preambles
• Since the Femtocell BS serves only limited
number of AMSs, it can assign a dedicated
<preamble, tile> to each registered AMS for
BR
– Guarantee no preamble collision
• If two or more AMS send the same preamble in the
same tile, it is difficult to early detect the potential
error in either 3-step or 5-step procedure
– Reduce the contention delay
– Omit 12-bit STID in the quick access message
• The quick access message only contains about 12~16
bit information
Mechanism for dedicated assignment
• During each request, a predefined pseudo random
assigning rule is used for the Femtocell BS and AMSs
to obtain dedicated <preamble, tile>.
– The predefined pseudo random assigning rule is
synchronized among the Femtocell BS and AMSs.
– Pseudo random assigning prevents some pairs of AMSs
always suffer from message collision when they send the
request in the same BR channel.
– Pseudo random assigning could further minimize the
average of message collision probabilities among all pairs
of AMSs.
Pseudo random assignment
• Each <preamble, tile> is associated to a unique index
between 1 and D
• D = (number of dedicated preambles) * (number of tiles in
a quick access channel)
• During network entry, each AMSi is assigned to an
unique integer, Vi, which is between 1 and D.
• When an AMS transmits a request, the preamble and
tile is determined as follows:
1. Generate a random integer R that is between 1 to D. R
may be generated by a pseudo random function of
frame and subframe number.
2. AMSi uses the <preamble, tile> associated with the
element aVi,R in the scramble matrix A to perform BR.
Definition of Scramble Matrix
• Scramble matrix A is a D*D matrix with
following properties
1.Each entity of A are integers between 1 and D.
2.The elements of each column of A are different.
3.The elements of each row of A are different
Example of Scramble Matrix
• A simple and scalable D*D scramble matrix is
generating a circular matrix with which the
first row is [1 2 … D].
For example, D  72
1
72

A 

3
 2
2
1
72
3
...
2


71


72
72
71


2
1 
Performance Evaluation
Simulation Setup
•
•
•
•
•
Number of preambles : 24
Number of tiles in an LRU : 3
D = 24 * 3 = 72
Number of requesting AMSs in an BR channel: 1~10
Evaluate the preamble collision probability
– Preamble collision is the event that more than one AMS send the
same preamble in the same tile
Simulation Result
Collision Probability of Preamble
0.5
Random Access
Dedicated Assignment
0.45
0.4
Collisoin Probability
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
1
2
3
4
5
6
Number of Request Users
7
8
9
10
Summary
• When the number of requested AMS in a BR channel is larger
than four, the probability of preamble collision exceeds 8%
for random access
– Preamble collision is difficult for immediate detection and significantly
degrade the efficiency and robustness of BR procedure in either 3step or 5-step scenarios
• Dedicated preamble and tile guarantees no preamble collision
– The efficiency of BR procedure is improved
– Besides, dedicated preamble and tile could save the space of STID in
the quick access message
• Allow the quick access message to carry more information and CRC which ensures
the robustness of the quick access channel
Proposed Text
17.x Bandwidth request in Femtocell
For bandwidth request in the BR channel, each AMS
is assigned with a pair of dedicated preamble and tile.
A pseudo-random assigning rule is used to assign
each AMS’s dedicated preamble and tile in each
request. The pseudo random assigning rule is
synchronized among Femtocell BS and AMSs. A
D*D circular matrix A, which has the first row vector
[1 2 … D], could be used to facilitate the pseudorandom assignment.
where D is defined as NP * Nt . Where NP is the number
of dedicated preambles, and Nt is the number of tiles in a
BR channel. The value of NP is TBD.
By the definition, the circular matrix A is
1
D

A

3
 2
2 ...
1
4
3 ...
D 1
D 
D  2 D  1


1
2 
D
1 
The mechanism of pseudo-random assignment is as
follows:
Each pair of preamble and tile is associated to a
unique index between 1 and D.
During network entry, each AMSi is assigned to
an unique integer, Vi, which is between 1 and D.
When an AMS transmits a request, the steps of
determining assigned dedicated preamble and tile is
as follows:
Step 1: Generate a random integer R by a pseudo
random function of frame and subframe number
taking value from 1~D
Step 2: AMSi uses the <preamble, tile> associated
with the element aVi,R in the circular matrix A to
perform bandwidth request.