1. A novel hidden station detection mechanism
January 2006
doc.: IEEE /0098r0
January 2006
A novel hidden station detection mechanism
Date:
Authors: Youngsoo Kim, Sunghyun Choi, Kyunghun Jang, Dongjun Lee
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Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

2. Contents Background Impact of hidden station
January 2006
doc.: IEEE /0098r0
January 2006
Contents
Background
Impact of hidden station
Definition of hidden station
Definition in IEEE k D2.2
Problem of the definition using Data transmission range
Common definition using Carrier sense range
Hidden area
Proposed detection mechanism
Discussion
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

3. January 2006
Background
Hidden station and hidden station report mechanism were defined in k/D2.2
Text of hidden station was removed during the comment resolution, and hence there is no text for hidden station in k/D3.0 and later versions
Hidden stations, however, could severely degrade the system performance when they exist
We propose to re-consider the hidden station issue with an alternative and technically correct definition
Youngsoo Kim, Samsung

4. Impact of hidden station – (1/2)
January 2006
doc.: IEEE /0098r0
January 2006
Impact of hidden station – (1/2)
Assumption
Ideal rate adaptation
8 stations with star topology
Cell radius: 11m – 45m
Frame size: 1500 bytes
The transmitter has infinite number of packets
Channel error is not considered
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

5. Impact of hidden station – (2/2)
January 2006
doc.: IEEE /0098r0
January 2006
Impact of hidden station – (2/2)
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

6. Definition of hidden station
January 2006
doc.: IEEE /0098r0
January 2006
Definition of hidden station
Definition in IEEE k D2.2
3.56 hidden station: A hidden station is a station that is known to exist, since MPDUs with RA set to the hidden station’s MAC address are received, however frames with TA set to the MAC address of the hidden station are not received
Using Data transmission range
Need to decode both PLCP header and data
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

7. Problem of the definition using Data transmission range
January 2006
doc.: IEEE /0098r0
January 2006
Problem of the definition using Data transmission range
STA3 can hear the packet transmission from both AP and STA2, however it can’t decode the MAC address of STA2  it’s not hidden, however it’s considered hidden as the previous definition
Assumption
TX rate adaptation
ACK rate = lowest rate
STA3
STA2
BSA
DATA transmission range
Carrier sense range
STA1 AP
STA1 can only hear the packet transmission from AP  Hidden from STA2
STAs located in this circle may decode the MAC address in the frame transmitted from STA2 correctly
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

8. Common definition using Carrier sense range
January 2006
doc.: IEEE /0098r0
January 2006
Common definition using Carrier sense range
Hidden station is the station which can’t sense the packet transmission and may transmit during the packet transmission
The STAs which can decode PLCP header are not considered hidden stations
RX STA
TX STA
Carrier sense range
DATA transmission range
STAs in this area can decode the PLCP header,
but can’t decode the Data  Data could be broken
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

9. January 2006
Proposed definition
Add a definition of hidden stations as "A hidden station to a particular station is a station that is not capable of making CCA busy at the particular station, but, at the same time, the hidden station is capable of making CCA busy at a third party station which is capable of making CCA busy at the particular station."
Youngsoo Kim, Samsung

10. Hidden area January 2006 5 Data ACK 4 Not HIDDEN 1 3 2 Not HIDDEN 2 3
doc.: IEEE /0098r0
January 2006
Hidden area 5
Data
ACK
MAC part 4 1
Not HIDDEN
RX STA 3 2
Not HIDDEN
TX STA 2 3
Not HIDDEN 1 4
HIDDEN 5
HIDDEN
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

11. Proposed detection method
January 2006
doc.: IEEE /0098r0
January 2006
Proposed detection method
Cases 1, 2, and 3
STAs in these area are not hidden from the TX STA
Cases 4 & 5
Detect the existence of the hidden station
An ACK reception when the medium has been idle over SIFS interval
The MAC address of hidden station can be determined
A reception of PLCP header including Length = 14 (i.e., either ACK or CTS) octets while the FCS failure occurs from the frame reception and the medium has been idle over SIFS.
The MAC address of hidden station cannot be determined
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

12. Proposed detection method
January 2006
doc.: IEEE /0098r0
January 2006
Proposed detection method
Start No
Idle > SIFS
11b?
Yes
Yes No
Check
Length (msec) and rate No
Frame detection
Check length (bytes)
Yes
CRC check?
Calculate length (bytes)
Error OK
If length (bytes) = 14
Frame type No
Not ACK
Yes
ACK
Detect hidden station
Detect the existence
of hidden station
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

13. Performance Evaluation
January 2006
doc.: IEEE /0098r0
January 2006
Performance Evaluation
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology

14. Summary Hidden station detection Effectiveness of proposed scheme
January 2006
doc.: IEEE /0098r0
January 2006
Summary
Hidden station detection
Can be used to decide if the STA use the RTS/CTS for protection
Should be considered with RTS/CTS threshold
E.g. throughput performance
Effectiveness of proposed scheme
Fast hidden station detection
No change of current specification
Youngsoo Kim, Samsung
Youngsoo Kim, Samsung Advanced Institute of Technology