Transcript IEEE C802.16m-10/0500

Transmission Timing Offset Broadcasting for Ranging in
Femtocells
Document Number: IEEE C802.16m-10/0500
Date Submitted: 2010-04-30
Source:
Pei-Kai Liao, Yih-Shen Chen and Paul Cheng
MediaTek Inc.
E-mail: [email protected], [email protected]
Kuhn-Chang Lin, Yu T. Su
NCTU
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


Uplink network timing synchronization


Can be achieved by backbone network signaling
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 macroBS according to the deployment location of the femto-BS

The reserved TTG is smaller if the femto-BS is farther away from the macroBS
 The reserved RTG is larger if the femto-BS is farther away from the macroBS
2
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
3
Introduction (3/4)
Distance C
Distance A
Femto-BS1
Macro-BS
Distance B
MS1
MS2
Served by
Macro-BS
Served by
Femto-BS2
Femto-BS2



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
4
Introduction (4/4)
TTG
Macro-BS1
RTG
DL (Tx)
UL (Rx)
TTG
Femto-BS1
DL (Tx)
UL (Rx)
DL (Tx)
UL (Rx)
RTG
DL (Tx)
UL (Rx)
DL Propagation Delay
UL Timing Advance for MS2
MS2
synchronized to
Macro-BS1
DL (Rx)
TTG
Femto-BS2
with no TTG/RTG
adjustment
UL (Tx)
DL (Tx)
DL (Rx)
UL (Tx)
RTG
UL (Rx)
DL (Tx)
UL (Rx)
Timing Difference
Interference Happens!!
5
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
6
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
7
Problem Statement (3/4)
TTG
Macro-BS
DL (Tx)
UL (Rx)
TTG with
Adjustment
Femto-BS with
TTG/RTG
adjustment
DL (Tx)
Femto-BS with
TTG/RTG
adjustment (Rx)
DL (Tx)
UL (Rx)
RTG with
Adjustment
UL (Rx)
DL (Tx)
UL (Rx)
Time Difference
MS not knowing
TTG/RTG
adjustment
RTG
DL (Rx)
UL (Tx)
TTG
Expected UL
Start Time
DL (Rx)
UL (Tx)
RTG
Incomplete Receiving!!
Rx Observation Window
Expected Duration
of RCP
Expected Duration of
Ranging Preamble
MS not knowing
TTG/RTG
adjustment (Tx)
RCP
Ranging Preamble
Problem in initial ranging!
Time Difference
8
Problem Statement (4/4)
TTG
Macro-BS
DL (Tx)
UL (Rx)
TTG
Femto-BS with
no TTG/RTG
adjustment
MS with
TTG/RTG
adjustment
Femto-BS with
RTG
DL (Tx)
DL (Tx)
UL (Rx)
RTG
UL (Rx)
DL (Tx)
UL (Rx)
Time Difference
RTG with Adjustment
DL (Rx)
UL (Tx)
DL (Rx)
UL (Tx)
TTG with Adjustment
Expected UL
Start Time
Incomplete Receiving!!
no TTG/RTG
adjustment (Rx)
Rx Observation Window
Expected Duration
for RCP
Expected Duration for
Ranging Preamble
MS with
TTG/RTG
adjustment (Tx)
RCP
Ranging Preamble
Problem in handover ranging!
Time Difference
9
Proposed Solution

S-RCH transmission timing offset broadcast in femtocells

S-RCH Transmission timing offset is broadcasted by femtoABSs 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
10
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
11
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

12
Why S-RCH in Femtocells,
Instead of NS-RCH?


Since the timedomain structure of
NS-RCH is different
from other channels,
the orthogonality can
be destroyed
When marco-AMS
transmits data
channel and femtoAMS transmits
ranging channel,
ranging performance
in femtocell
degrades
significantly
13
Text Proposal
14