IEEE 802.16m Identifying Femtocells Subscriber Groups with Less Overhead

Document Number: IEEE C80216m-09_0451r2Date Submitted: 2009-04-24

Source: Ying Li, Zhouyue Pi, Baowei Ji, Email: yli2@sta.samsung.com Phone: +1-972-761-7903Farooq Khan, Jaehee Cho, Jung Je Son, Anshuman Nigam

Samsung Electronics

Venue: IEEE Session #60Base Contributions:

Re: Change Request for IEEE 802.16m SDD, Section 17

Purpose: Discussion and ApprovalNotice:

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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.

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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.

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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.

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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.

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Subscriber Group ID Differentiated by SFH CRC Scrambling (1/2)

• Illustration of femto BS operation (an example) – OSG Femtocell BS and specific CSG Femtocell BS are differentiated by

scrambling the CRC of a SFH burst with different masks (called s-code herein).

– Send: M + (CRC Xor s-code), where M is the SFH message, CRC is produced from M, s-code is a scrambling code.

Each CSG Femtocell BS uses the s-code corresponding to its assigned CSG ID

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• Illustration of MS operation (an example) MS decodes SFH by

Step 1. decode the message (M^)Step 2. calculate the CRC^Step 3. calculate s-code^ = CRC^ Xor (CRC Xor s-code)– If s-code^ = any CSG-ID in MS’ list, it is a CSG Femto BS with CSG-ID s-

code^– Otherwise, CRC fails and MS keeps searching/measurement.

Subscriber Group ID Differentiated by SFH CRC Scrambling (2/2)

Advantages(1/2)• The operation is for femtocell only. No change for

macrocell.• Advantage via an example: Scramble 24bit CSG ID with

24bit CRC. • Compare with: 24bit CSG ID as payload in SFH, plus 16bit

CRC• Advantages

– Less overhead, 24 vs 40, save 40%– Mostly enhanced CRC capability

• If an MS subscribes 2^a (a<=8) CSGs, the CRC capability is roughly enhanced by (8-a) bits.

– E.g., MS subscribes 2^8=256 CSGs, CRC same capability– E.g., MS subscribes 2^7=128 CSGs, CRC enhanced by 1 bit.– E.g., MS subscribes 2 CSGs, CRC enhanced by 7 bits.

• Typically, MS may subscribe to its femto at home, in office, and tens of memberships such as café, airport, etc.

• Most likely, MS only subscribes less than 2^8=256 CSGs [note that multiple femtocells can share one CSG ID).

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Advantages (2/2)• Advantage via more examples• Given CSG ID x bits (x<=24) Scramble x-bit CSG ID with 24-bit CRC.• Compare with: x-bit CSG ID as payload in SFH, plus 16bit CRC• The example is only meant to show the advantage of the approach.

Specific approach can be FFS for stage-3 text.

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16bit CSG ID 20bit CSG ID 24bit CSG ID

CSG ID + 16bit CRC

CSG ID scrambling with 24bit CRC

CSG ID + 16bit CRC

CSG ID scrambling with 24 bit CRC

CSG ID + 16bit CRC

CSG ID scrambling with 24 bit CRC

Overhead 16+16=32bits

24bits.24:32, save 25%

20+16=36bits

24bits.24:36, save 33%

24+16=40bits

24bits24:40, save 40%

CRC capability

[Assume an MS subscribes 2^a (a<=8) CSGs]

16bit (24-a) bitsEnhanced by (8-a) bits

16bit (24-a) bitsEnhanced by (8-a) bits

16bit (24-a) bitsEnhanced by (8-a) bits

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 mostly enhances CRC capability.

• If not in full length, a part of the CSG ID can be carried by the approach, to reduce the overhead.

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Proposed text

Insert the following text in 15.2.

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.