Transcript IEEE C802.16m-09/1232r1

IEEE 802.16m Fast and Efficiently Identifying Femtocells Subscriber Groups
Document Number: IEEE C80216m-09_1232r1
Date Submitted: 2009-07-10
Source:
Ying Li, Zhouyue Pi, Baowei Ji,
Farooq Khan, Jaehee Cho, Jung Je Son,
Anshuman Nigam, Kaushik Josiam,
Sudhir Ramakrishna
Email: [email protected]
Phone: +1-972-761-7903
Samsung Electronics
Venue:
IEEE Session #62
Base Contributions:
Re:
Change Request for IEEE 802.16m SDD, Section 15
Purpose:
Notice:
Discussion and Approval
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Use cases to support
• Use cases will direct and shape our specific design
• The possibilities of use cases could be, for example,
– 1. Home
• E.g., a home owner can buy a femto and open it up only to family
members and friends
– 2. Enterprise
• E.g., a company can buy tens/hundreds femtocells to enhance the
connectivity, then such femtos can be open only to the company
employees
– 3. Membership
• E.g., a use can buy a membership of Starbucks, which means all the
femtocells owned by Starbucks should be open to the user
– 4. Operator
• E.g., the operator uses femtocells to fix coverage holes, then such
femtocells should be open to subscribers of this operator
– And so on.
2
Motivation (1/3)
• IEEE 802.16m System Requirements
– Shall support CSG (Closed Subscriber Group)
– Shall allow dense deployment of large number of Femtocells
– Shall support preferred access and handover of MS’s to their designated
Femtocell BSs
• Some operations (such as Cell Selection, Network RE-/Entry)
could be different for Femtocells and macro cells, and for CSG
and OSG (Open Subscriber Group) Femtocells.
• Therefore, an MS may need to differentiate Femtocells from
macro cells, CSG from OSG, or one CSG from the other CSG.
3
Motivation (2/3)
• Accessibility of a BS
– Macro BS: accessible to all MSs
– OSG Femtocell BS: accessible to all MSs
– CSG Femtocell BS: only accessible to its authorized MSs, though open to all
MSs for emergency access.
• An MS needs to determine whether it is allowed to access a CSG
Femtocell
– CSG BS may not be accessible to many MSs, the chance that an MS cannot
access to a CSG BS is large, so the accessibility should be checked early.
– The number of SCH symbols for cell IDs may not be large enough to
differentiate a potential large number of CSG Femtocells.
– PBCH can be a good place to offer information for MS to check accessibility.
– Consider methods avoiding using expensive PBCH payload.
4
Motivation (3/3)
• CSG ID – to ease the management
– Several Femtocell BSs could belong to a same CSG entity, which have the
same group of authorized MSs.
– Benefits of using a common CSG ID for this set of BSs
 Shorten the list of allowable femtocells stored in MS
 To ease the management at the paging controller, and avoid location update
when the MS is moving inside the CSG.
 To ease the management of subscriber groups. E.g., an MS subscribes a
membership of Starbucks, which has stores throughout country. Consider
Starbucks installs a new femto BS,
 Without CSG ID, Starbucks has to ask the operator to update it by adding this new femtocell to the
white list of all its membership subscribers
 With CSG ID, such update is not needed.
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Challenges
• Given potential large number of subscriber groups, CSG ID
may need quite a few bits.
• CSG ID needs to be broadcast by femtocells.
• The additional bits in SFH to identify femtocells may consume
the expensive SFH payload.
– Consider approaches to avoid/reduce the payload consumption, such
as by using the CRC scrambling with different masks on SFH.
6
Subscriber Group ID
Differentiated by SFH CRC Scrambling (1/2)
• Illustration of femto BS operation
– CSG ID is carried by scrambling CRC.
Femto BS
broadcasts
Information excluding CSGID
(Info)
CRC XOR CSGID
• Illustration of MS operation
– MS recovers CSGID.
MS
Received Information
excluding CSGID (Info_R)
Generate
CRC
(CRC_G)
Received
(CRC XOR CSGID)_R
XOR
CSGID_R
CSGID_R
in Set S
Set S: the set of CSGs that an MS
subscribes, with N CSGIDs in set S.
N
Y
CSGID_R
Discard.
Continue search
7
Subscriber Group ID
Differentiated by SFH CRC Scrambling (3/3)
• Rationale:
– An MS may only subscribe a few CSGs among the entire CSGID
space. In information theory prospective, this gives the chance of
compressing, e.g., reducing the overhead by scrambling CRC.
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Analysis and Comparison (1/4)
Option 1
Option 2
Information (include 24bit CSGID)
Information (exclude
24bit CSGID)
24bit
• Preparation
16bit CRC
24bit CRC scrambling
24bit CSGID
– Denote p1a, p1b as Pr{ received not equal to sent} for information
part (not including CRC16) and CRC part, respectively, in option 1
– Denote p2a, p2b as Pr{ received not equal to sent} for information
part (not including last 24bits) and last 24bits, respectively, in option2
– Assume that any corruption of data affects data in a completely
random way, i.e., such that the corrupted data is totally uncorrelated
with the original data.
• For Option 1, there are two error events
– Mis-detection: information part (not including CRC16) received is
correct, but it does not pass CRC check.
• Not a big issue: discard the message and wait for next chance
• Probability: (1-p1a)*p1b
– False-alarm: information part received is in error, but it passes CRC
check.
• This type of error is not desired. Repeated transmission may be used to
overcome this.
• Probability approx.: p1a*2^{-16}=p1a*1.5e-5
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Analysis and Comparison (2/4)
• For option 2
Femto BS
broadcasts
MS
Information excluding CSGID
(Info)
Received Information
excluding CSGID (Info_R)
CRC XOR CSGID
Received
(CRC XOR CSGID)_R
Generate
CSGID_R
CRC
XOR
CSGID_R
in set S
(CRC_G)
N
Set S: the set of CSGs that an MS
subscribes, with N CSGIDs in set S.
Y
CSGID_R
Discard.
Continue search
Scenarios
Issues
1
Info_R ≠ Info, CSGID_R not in Set S
/
2
Info_R ≠ Info, CSGID_R in Set S
False alarm , not desired
3
Info_R =Info, CSGID_R = CSGID
/
4
Info_R = Info, CSGID_R not in Set S
Mis-detection, discarded, no big issue
5
Info_R = Info, CSGID_R in Set S, CSGID_R ≠ CSGID
CRC not affected, accessibility not affected, but
may affect operations (if any) which use CSGID
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Analysis and Comparison (3/4)
Option 1
Option 2
Information (include 24bit CSGID)
Information (exclude
24bit CSGID)
24bit
16bit CRC
24bit CRC scrambling
24bit CSGID
• Consider that an MS has subscribed a set S of N CSGs, with N CSGIDs in set S.
• For Option 2, there are three error events
– Mis-detection: information part (not including last 24bit as shown in the figure)
received is correct, but the recovered CSGID is not in set S hence CRC check is not
passed.
• Not a big issue: discard the message and wait for next chance
• Probability approx. (1-p2a)*p2b*(1-(N-1)/(2^{24}-1)) ≈ (1-p2a)*p2b*(1-N*2^{-24})
– False-alarm: information part received is in error, but it gives CSGID in set S.
• This type of error is not desired. It may be corrected in next repeated SP1s in SFH.
• Probability approx. p2a*N*2^{-24}=p2a*N*5.96e-8
– CRC not affected but CSGID wrong (if N>=2): information received is correct, and
recovered CSGID lies in the set S, but the recovered CSGID is different from the
CSGID sent.
• This type of error will NOT affect the accessibility of CSG femto check, since the recovered
CSGID is still in the allowable femto set S. It does NOT affect CRC either since CRC check is
passed correctly.
• It may only impact the operations (if any) which uses CSGID as input parameters.
• It may be corrected in next repeated SP1s in SFH.
• Probability approx. (1-p2a)*p2b* ((N-1)/(2^{24}-1)) ≈ (1-p2a)*p2b*N*2^{-24}
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Analysis and Comparison (4/4)
Option 1
Option 2
(our design)
Information (include 24bit CSGID)
Information (exclude
24bit CSGID)
16bit CRC
24bit
24bit CRC scrambling
24bit CSGID
• Save 16bits in transmission and mostly enhanced CRC
Mis-detection
[information received is
correct, but it does not
pass CRC check]
False alarm
[information received
is in error, but it passes
CRC check]
CRC not affected but CSGID wrong
[information received is correct,
recovered CSGID lies in set S, but
different from the CSGID sent
Option
1
(1-p1a)*p1b
p1a*2^{-16}
=p1a*1.5e-5
/
Option
2
(1-p2a)*p2b*(1-N*2^{24})
p2a*N*2^{-24}
=p2a*N*5.96e-8
(1-p2a)*p2b*N*2^{-24}
=(1-p2a)*p2b*N*5.96e-8
p1a, p1b: Pr{received not equal to sent} for info and CRC, respectively, in option 1
p2a: p2b: Pr{received not equal to sent} for info and last 24bits, respectively, in option 2.
An MS has subscribed a set S of CSGs, with N CSGIDs in set S.
If N<=256, the false alarm p2a*N*5.96e-8 is less than p1a*1.5e-5. Enhanced CRC!
In option 2, CSG wrong probability is less than False alarm, since usually p2b<p2a
N<=128 : (False alarm + CSG wrong) in option 2 less than False alarm in option 1.
The error events may be corrected in next repeated SP1s in SFH.
12
Proposed Detailed Solution Based on SFH
Design in AWD 80216m-09_0010r2
• In current AWD 80216m-09_0010r2,
– S-SFH SPs have fixed size.
– 16-bit CRC is used for S-SFH.
– In S-SFH SP1, there is a 16bit field for BS_EIRP
• What we propose is
– Femtocell uses 8bit for BS_EIRP, instead of 16bit, in SP1
– Femtocell uses 24bit CRC, instead of 16bit
– Femtocell 24bit CSGID scrambles 24bit CRC
• Rationale:
– SP1 for femtocells different from macrocells, which is logically possible since
femto and macro are differentiated by preamble partitions.
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Advantages of Our Proposal
• Advantages
– Fast: Both femto BSID and CSGID (separately) can
be carried in SP1.
– Reliable: If an MS subscribes less than 2^8=256
groups (which may be in most cases), the CRC is
enhanced compared to 16bit CRC.
– Efficient: 24bit femto CSGID is carried within a
fixed size SFH SP. No need to enlarge the size of
the current SP1 for macro/femto.
14
Summary
• CSG ID of femtocells is carried by using CSG ID
as the mask to scramble CRC
• The operation is for femtocell only. No change
for macrocell.
• The approach reduces overhead, and
enhances CRC capability.
15
Proposed text
Insert the following text to the end of 15.3.2.
Subscriber groups of femtocells may be differentiated by
scrambling the CRC of an SFH burst with different masks. Each
CSG femtocell BS uses the mask corresponding to the CSG ID.
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