Unified Ranging Structure for Femtocell BSs
Document Number: C80216m-0811
Date Submitted: 2009-04-24
Source:
Yih-Shen Chen, Peikai Liao and Paul Cheng
MediaTek
Chie-Ming Chou, Yung-Han Chen and Frank Ren
ITRI
E-mail: yihshen.chen@mediatek.com
E-mail: chieming@itri.org.tw, frank_ren@itri.org.tw
Venue: IEEE Session #61, Cairo, Egypt
Base Contribution: This is base contribution
Re: IEEE 802.16m-09/0021, “Call for Comments on Project 802.16m System Description Document (SDD)”
- CR to SDD: Chapter 15 Support of Femtocell
Purpose: For TGm members’ discussion and approval
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Ranging Channels in 802.16m
• In current SDD, there are three types of ranging channels:
– Non-synchronized ranging channel
• Initial ranging
– MS adjusts its frequency/timing/power with its potential
serving cell during initial network entry
• HO (handover) ranging
– MS adjusts its frequency/timing/power with its target cell
during handover
– Synchronized ranging channel
• Periodic ranging
– MS adjusts its frequency/timing/power with its potential
serving cell
– BR (bandwidth request) ranging
• MS sends out its transmission intention and requirements
2
Discussion on Synchronization Issue in
Femtocell
•
After DL synchronization with femtocell SCH, the maximal offset of a MS is
within synchronization requirement, given the cell coverage is 30 meters at
most
–
•
RTD: (2*30)/(3*108)  0.2μs
Synchronization requirement:
– For any duplexing, all SSs shall acquire and adjust their timing so that
all UL OFDMA symbols arrive time coincident at the BS to a accuracy
of ± 25% of the minimum guard-interval or better (8.4.10.1.2, Rev2/D8)
– WiMAX profile requirement:
•
Question: Do we still need the differentiation, given short-range?
3
Proposed Ranging Structure for Femtocell:
Unified Ranging Structure
• Single unified ranging channel configuration for
all the ranging purposes
– Option-1: BR ranging PHY structure in femtocell
– Option-2: Synchronized ranging structure in femtocell
4
Option-1:
BR channel as Unified Ranging Channel
• The PHY of the ranging channel consists of
preamble and message part (preamble+MSG
PHY; i.e., BR PHY)
– Benefit:
• High spectral efficiency
– No guard channel is required
– Few subcarriers are required ( ex:18)
• High bandwidth efficiency
– BW request for RNG-REQ is sent along with preamble
– MS ID may be sent with preamble for UL ranging if a
specific femto-only ID is assigned (CSG scenario)
5
Example (Preamble+MSG PHY)
Time
0
1
34
35
Frequency
36
37
70
71
Contention-based
ranging code
72
73
106
107
Ranging message
6
Procedure of Initial Ranging in Femtocell
Initial ranging preamble +MSG
(ID, RNG-REQ size)
Initial ranging preamble +MSG
(ID, RNG-REQ size)
Acknowledgement of the initial ranging
Acknowledgement of the initial ranging
UL grant for sending RNG-REQ message
RNG-REQ
Figure out
the grant
size
UL grant for BW-REQ
BR header for RNG-REQ
UL grant for RNG-REQ message
RNG-RSP
RNG-REQ
...
RNG-RSP
...
(a) if the MSG part is correctly decoded
(b) if the MSG part is not correctly
decoded
7
HO Ranging to Femtocell
• Bandwidth request as well as HO preamble
sequence are simultaneously sent to femtocell
target
– Shorten handover interruption time
8
Performance Analysis for Option-1
• The timing offset of initial ranging can be absorbed by CP
– Short coverage (≦30m)
• Low collision probability
– Small population number (≦10)
– ex: for 10 VoIP users, the arrival rate is 0.02/frame
– For OUG scenario, femto-BS can allocate more ranging opportunities to
reduce the collision probability
• Better spectral efficiency
– Assume PRBS code is reused for 16m initial ranging
• The total occupancy resources will be 144x6
• The saving for unified ranging channel is 88.9% per sub-frame
– 1- [ (6*6*3)/(6*6*3+144*6)] = 88.9%
– Spectral saving would be critical in femtocell deployment
because there might be more than thousand femtoBS
installments
• FDM/TDM spectral utilization is required
9
Implementation Impact for Option-1
• No modification is required on ranging PHY
structure
– Reuse BR PHY for initial/HO ranging
• No two analog filters are required
– In macrocell deployment, RCP of initial ranging
channel is different from CP of BR ranging channel
• Re-define some parameters in the quick access
message
– Reserve one or few IDs for initial/HO access
• Current only 12 out of 19 are defined for flow IDs (FID)
– 2-bit BW field can be re-defined as BW requirement
for RNG-REQ
10
Examples
AMS
Femtocell BS
Initial ranging code
AMS
Femtocell BS
AMS
Femtocell BS
RP+ message
RP+ message
RNG-RSP
(indicate RP, success)
RNG-RSP
(indicate RP, success)
CDMA_allocate-IE
CDMA_allocate-IE
CDMA_allocate-IE
New header (RP)
RNG-REQ
RNG-REQ
RNG-RSP
(indicate RP, success)
CDMA_allocate-IE
RNG-REQ
(i) Macrocell
(ii) Femtocell:
3-step success case
(iii) Femtocell:
3-step fail case
11
Exceptional Handling for Option-1(1/2)
~ corruption of message part
• What if message part is corrupted?
– Observation: ABS only knows that someone is
sending a ranging code with unidentified purpose
• Solution:
– Femtocell ABS treats it as BR ranging code by
sending CDMA_ALLOC A-MAP IE
– AMS responses with proper 6-byte header message
• BW-REQ message for BR request
• INIT_RNG-REQ message for initial ranging request
– After ABS figures out AMS’ intention, nothing else is
required to be modified
12
Exceptional Handling for Option-1 (2/2)
~ corruption of message part
• Required efforts on specification modifications:
– A new 6-byte header message (INIT_RNG-REQ)
needs to be defined
– Procedure to handle INIT_RNG-REQ transmission
• Required efforts on implementation:
– In MAC aspect, there is no much difference between
initial ranging and BR ranging (i.e., kind of
anonymous ranging)
• No frequency/timing adjustment in femtocell scenarios
– Common function blocks with different message to
transmit
13
Legacy Support Issue for Option-1
• In LZone, BR channel consists of three 4-by-6 titles
– Only preamble code can be sent
• However, BR channel as unified ranging structure can still be
applied
• Option-1a: Differentiated ranging opportunity allocation
– Pros:
• No protocol change. Same as 16e
– Cons:
• Lower spectral efficiency if new call arrival rate is low
• Option-1b: Same ranging opportunity allocation with different 6-byte
header
– Pros:
• Higher spectral efficiency for no trunking effect
– Cons:
• A new header for initial ranging is required
• MAC protocol modification for initial ranging is required
– The ranging purpose is identified by header, not code
14
Option-2:
Synchronized Ranging Channel as Unified Ranging
Structure
• Reuse Synchronized ranging structure for all ranging
purposes in femtocell BS
– Send initial ranging/HO ranging onto Synchronized channel
• Required efforts on specification modifications:
– AMS sends “periodic ranging code” onto “synchronized ranging
opportunity” for initial access in femtocell BS
• Here, we assume that the code structure of synchronized ranging is
different from that of un-synchronized ranging
15
Conclusion
• Unified ranging channel for femtocell can
provide
– Better spectral efficiency
– Easier deployment
– Shorter access latency
16
Text Proposal
[Adopt the following text modification into section
15.4.3 of P802.16m SDD]
17
Appendix
18
Unsynchronized Ranging Channel for
Macrocells
19
UL Ranging Procedure for 802.16e
• RNG-REQ size mismatch problem [1]:
– Multiple TLVs are optional in RNG-REQ  BS has to figure out an
appropriate size by itself
– This is an IOT issue, identified by Samsung in WiMAX TWG
Initial ranging preamble
Figure out
the grant
size
UL grant for sending RNG-REQ message
RNG-REQ
RNG-RSP
...
[1]080717_TWG_Samsung_r4_allocation_of_RNG_REQ_size.doc
20