Broadcast of Transmission Timing Offset for Ranging in Femtocells

Document Number: IEEE C802.16m-10/0500r1

Date Submitted: 2010-05-12

Source:Pei-Kai Liao, Yih-Shen Chen and Paul Cheng E-mail: pk.liao@mediatek.com, yihshen.chen@mediatek.comMediaTek Inc.

Kuhn-Chang Lin, Yu T. SuNCTU

Venue: IEEE 802.16 Session #67, Bangalore, India

Base Contribution: This is base contribution

Re: Letter Ballot #31a on the Draft Amendment (IEEE P802.16m/D5)

Purpose: For TGm members’ discussion and approval

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Introduction (1/4) Network timing synchronization for a femto-BS can be categorized as the

following two aspects Downlink network timing synchronization

Can be achieved by backbone network signaling Uplink network timing synchronization

Can be achieved by femto-BS ranging

Due to downlink and uplink network timing synchronization requirement for a femto-BS, the TTG/RTG reserved to avoid the collision of downlink and uplink transmission may be different from that of macro-BS according to the deployment location of the femto-BS The reserved TTG is smaller if the femto-BS is farther away from the macro-

BS The reserved RTG is larger if the femto-BS is farther away from the macro-

BS

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Introduction (2/4) In D4, the value of TTG/RTG is fixed, regardless of cell type

Without TTG/RTG adjustment in femtocell, the uplink receiving of the femto-BS will be interfered by the uplink transmission of macro-MSs due to non-aligned uplink subframes if the time difference is not an integer number of OFDM symbols It is illustrated by two figures in the following slides

Therefore, femto-BS may need to adjust its TTG/RTG so as to synchronize with the network signals over the air

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Introduction (3/4)

Femto-BS2

Femto-BS1Macro-BS MS1

MS2Served by Macro-BS

Served by Femto-BS2

Distance A

Distance B

Distance C

Distance A and Distance B are much larger than Distance C Distance A is approximately equal to Distance B Femto-BS1 is very close to Macro-BS

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Introduction (4/4)

DL (Tx)Macro-BS1

Femto-BS1

MS2 synchronized to

Macro-BS1

DL (Tx)

UL (Rx)

UL (Rx)

DL (Tx)

DL (Tx)

UL (Rx)

UL (Rx)

DL (Tx) UL (Rx) DL (Tx) UL (Rx)

DL (Rx) UL (Tx) DL (Rx) UL (Tx)

TTG RTG

TTG RTG

DL Propagation Delay

Femto-BS2 with no TTG/RTG

adjustment

RTG

Interference Happens!!

Timing Difference

TTG

UL Timing Advance for MS2

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Problem Statement (1/4) According to D5 Section 16.4.6 , “A Femto ABS should be synchronized with the

overlay ABS network, where the synchronization means the aligned frame boundary, and the aligned DL/UL split in TDD systems.” Femto-ABS is not mandatory to synchronize with overlay ABS network It is mandatory only when Femto-ABS share the same frequency with Macro-ABS

For AMSs which are turned on in the femtocell, there will be a large time difference between the ranging signal transmission by AMSs and the femto-ABS observation window if femto-ABS synchronizes with the overlay macro-ABS The time difference is the same as the uplink transmission timing adjustment of the

femto-ABS The adjustment value is equal to the round-trip time from the femto-ABS to the

overlapping macro-ABS

For AMSs which handover from macrocell to femtocell, there is still problem if the femto-ABS does not synchronize with the overlay macro-ABS The time difference is the same as the uplink transmission timing adjustment of the AMS The adjustment value is equal to the round-trip time from the AMS to the overlapping

macro-ABS

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Problem Statement (2/4)

Problem 1: The time difference results in incomplete receiving of ranging signal at the femto-ABS It is illustrated in the figure in the following slides

Problem 2: The non-aligned transmission of ranging signal also introduces serious interference to the receiving of uplink data from other AMSs at the femto-ABS

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Problem Statement (3/4)

Problem in initial ranging!Ranging PreambleRCP

Femto-BS with TTG/RTG adjustment

DL (Tx) DL (Tx)

UL (Tx)

DL (Tx)

UL (Rx)

MS not knowing TTG/RTG adjustment

Macro-BS DL (Tx)

UL (Tx)

UL (Rx)

UL (Rx)

DL (Rx) DL (Rx)

UL (Rx)

Femto-BS with TTG/RTG

adjustment (Rx)

MS not knowing TTG/RTG

adjustment (Tx)

Time Difference

Time Difference

Rx Observation Window

Incomplete Receiving!!Expected UL Start Time

TTG

TTG with Adjustment

RTG with Adjustment

RTG

TTG RTG

Expected Duration of RCP

Expected Duration of Ranging Preamble

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Problem Statement (4/4)

Problem in handover ranging!

Ranging PreambleRCP

Femto-BS with no TTG/RTG

adjustment

DL (Rx) DL (Rx)

UL (Rx)

DL (Tx)

UL (Tx)MS with TTG/RTG adjustment

Macro-BS DL (Tx)

UL (Rx)

UL (Tx)

UL (Rx)

DL (Tx) DL (Tx)

UL (Rx)

Femto-BS with no TTG/RTG

adjustment (Rx)

MS with TTG/RTG

adjustment (Tx)

Time Difference

Time Difference

Rx Observation Window

Incomplete Receiving!!Expected UL Start Time

TTG

TTG with Adjustment

RTG with Adjustment

RTG

TTG RTG

Expected Duration for Ranging Preamble

Expected Duration for RCP

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Proposed Solution S-RCH transmission timing offset broadcast in femtocells

S-RCH Transmission timing offset is broadcasted by femto-ABSs only

Transmission timing offset is applied to S-RCH transmission to fix timing alignment problem among different cell types

No matter whether femto-ABS is synchronized to the overlay macro-ABS, it works well

Specification modifications S-SFH SP1 content modification for femtocell

Use 3 bits for the broadcasting in femto-ABS only It does not increase any bit to S-SFH SP1

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Number of Bits Required for S-RCH Transmission Timing Offset Broadcast According to SRD, system performance has to be

optimized for the cell coverage up to 5 km Maximal round trip delay considered is 33.33 us

The maximal tolerable uplink timing error for synchronous ranging channel is < ±5.73 us (1/8 CP)

If 3 bits are used, the granularity is around 4.17 us, which introduces at most ±2.085 us timing error < ±5.73 us

Therefore, 3 bits are quite enough for the signaling

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Why S-RCH Transmission Timing Offset is Round Trip Delay? Assume TTG is fixed, the expected UL start time for a macro-AMS with no

adjustment is Start_Time_ULmacro-AMS = End_Time_DLmacro-ABS + Propagation_Delay + TTG = Start_Time_ULmacro-ABS + Propagation_Delay

To align with the UL subframes of macro-ABS, the uplink timing advance should be applied Start_Time_ULmacro-ABS = Start_Time_ULmacro-AMS – UL_Timing_Advance + Propagation_Delay UL_Timing_Advance = Start_Time_ULmacro-AMS + Propagation_Delay – Start_Time_ULmacro-ABS

= (Start_Time_ULmacro-ABS + Propagation_Delay) + Propagation_Delay – Start_Time_ULmacro-ABS

= 2 × Propagation_Delay RCH_Tx_Timing_Offset = 2 × Propagation_Delay

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Why S-RCH in Femtocells, Instead of NS-RCH? Since the time-

domain structure of NS-RCH is different from other channels, the orthogonality can be destroyed

When marco-AMS transmits data channel and femto-AMS transmits ranging channel, ranging performance in femtocell degrades significantly

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Text Proposal

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