Network Assisted Femtocell BS Detection

Document Number: C80216m-09/0838

Date Submitted: 2009-04-24

Source:Fang-Ching (Frank) Ren, Yan-Xiu Zheng , Chie Ming Chou, Yung-Han Chen E-mail: frank_ren@itri.org.tw ITRI

Venue: IEEE Session #61, Cairo, Egypt

Base Contribution: N/A

Re: Comments on 802.16m SDD (802.16m-08/003r8)

Purpose: Discussion and Approval

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Motivation

• To support handover between macro cell and femtocell, fast cell search on femtocell BSs is necessary for MS.

• However, the number of femtocells overlaid within a macro cell may be large.– The method to announce femtocells in a broadcating

neighborhood list is not efficient.– Also, femtocell may not be deployed everywhere. The

process of femtocell search should be performed in right time (i.e., the MS is really in coverage of femtocells) for a limited number of candidates.

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SDD description in 802.16m-08/003r8

• 15.2.1 PHY level cell identifier

Femtocell BSs and macro BSs are differentiated using Cell IDs, which shall be obtained from the A-PREAMBLE. It enables MSs to quickly identify cells types, avoid too frequent handover attempts into and out of a Femtocell BS, and avoid performing unnecessary network entry/re-entry.

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Proposal I: Femtocell Existence Detection

• IEEE 802.16m Synchronization structure– Hierarchical structure composed of PA-Preamble and

SA-Preamble– PA-Preamble:

• Less sequences: less than 20• Less complexity for sequence matching• Cell type identification in PA-Preamble for IEEE 802.16m

except for OSG and CSG femtocell BS– SA-Preamble

• Physical Cell identification• Large number of sequences: at least 512• Huge effort and complexity for cell scan

• OSG/CSG Femtocell existence can be detected based on PA-Preamble detection.

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Macro

Camp on suitable macro

cell

Cell Type Identification

CSG

Detect PA-Preamble

OSG

Proposal I: Femtocell Existence Detection

• Concept of Cell Identification

Identify a SA-Preamble from

macro cell

OSG Femtocell

Identification

Camp on suitable OSG

Femtocell

CSG Femtocell

Identification

Camp on suitable CSG

Femtocell

Cell Identification on SA-Preamble may also be differentiated to prevent unnecessary identification attempts.

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Proposed Text15.2.1 PHY level cell identifier

CSG/OSG Femtocell BSs and macro BSs are differentiated using Cell IDs, which shall

be obtained from the A-PREAMBLE. It enables MSs to quickly identify cells types,

avoid too frequent handover attempts into and out of a Femtocell BS, and avoid

performing unnecessary network entry/re-entry.

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Proposal II: Femtocell Identification

• Based on PA-Preamble can be used to distinguish signal from macro BSs, OSG Femtocell BSs, and CSG Femto BSs, – a MS may start Femtocell identification after

OSG/CSG Femtocell PA-Preamble detected.– The candidate set of SA-Premable can be

further reduced for cell identification (ID).

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Proposal II: Femtocell Identification• Ranging-based femtocell BS

identification procedure – Step1:

• MS sends an preserved ranging code in a preserved raging slot.

– Step2: • Femtocell BS responses an

localized Identification Code.

– Step3: • MS sends a selected ranging

code to reply the one of BS

– Step4: • The selected Femtocell BS

send Cell ID message, e.g. Cell ID, SA-Preambe sequence, etc

(1) Preserved ranging code

(2) Localized BS identification codes

(3) Selected initial ranign code

(4) Cell ID message

Cell search based on Cell ID message

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Proposal II: Femtocell Identification

• Example for supporting Macro to Femtocell HO – In subclause 15.5.1, SDD said “The network provides

certain system information to MSs for supporting handover between a macro BS and a Femtocell BS”

• Reserved ranging code (said C0) and reserved ranging slot in Femtocell.

• The channel allocation for transmission of “Femtocell Localized ID codes”.

• Femtocell Localized ID codes (D1~DN) and corresponding ranging codes (C1~CN).

• The channel allocation for the selected Femtocell BS to send Cell ID message.

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Proposal II: Femtocell Identification• Example (Cont’d)

– Step 0: MS detects the PA-Preamble from Femtocells.

– Step 1: MS send ranging code C0.

– Step 2: Femtocells response localized ID code D1,D2, D3, and D5 to the MS.

– Step 3: The MS sends ranging code C1 due to localized ID code D1 was detected and selected.

– Step 4: Femtocell D1 sends Cell ID message.

D1D2

D3 D4

D5

D5D4 Macro BS

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Proposed Text15.4.2 Femtocell BS detection An MS needs to know the existence of nearby Femtocell BSs operating in the same or different frequency of the overlay BS. Femtocell BS or macro BS may transmit control signal or message on the frequency of the overlay BS for an MS to identify the existence of the Femtocell BSs, and resource used by the femtocell BS or macro BS to transmit the control signal or message may be indicated to the MS by the overlay BS. The control signal or message contains the information necessary to help AMS to access the neighbour Femtocell BSs.. The transmission of control signals by the Femtocell BS does not cause severe intereference or denial of service An MS can initiate this femtocell BS detection procedure when the MS identifies the existence of femtocell BSs around the MS, e.g., detecting the specific P-SCH from femtocell BSs. After identifying the existence of femtocell BSs, the MS can transmit a signal in a preserved radio block among neighboring femtocell BSs for acquiring a candidate femtocell list. The radio block reservation may be pre-configured by the overlaid macro BS. Then the MS can identify a femtocell BS as the candidate target