1. Month Year
doc.: IEEE /xxxxr0
September 2009
Distributed Transmission Timing Adjustment for Synchronous Frame Arrival
Date:
Authors:
Name
Affiliations
Address
Phone
Sunggeun Jin
ETRI
Kapseok Chang
Hyungjin Kwon
Woo Yong Lee
Hyun Kyu Chung
Sunggeun Jin (ETRI)
VInko Erceg, Broadcom

2. Outline Background Current Limitation in the 802.11 WLANs
The necessity for the uplink multiuser MIMO
The reason why we need to consider transmission timing adjustment
Current Limitation in the WLANs
How to Solve in the and the 3GPP-LTE
New approach for the WLANs
Summary

3. Uplink Multiuser-MIMO
Why do we need uplink multiuser MIMO
There emerge new services requiring very high bandwidth in uplink direction
to upload a file with very large volume to a locally shared disk or file server
to transfer multimedia data from a portable device such as camcorder.
to send real-time frames with a highly stringent QoS requirement simultaneously for multiple stations to guarantee the QoS requirement.
Aggregate uplink throughput enhancement is required along with the downlink throughput improvement relying on the downlink multiuser MIMO

4. Why ? New Technical Issues in TGac/TGad Uplink Multi-user MIMO
Uplink multiuser MIMO (TGac) [1]
Stations’ cooperation (TGad) [2]
Uplink Multi-user MIMO
Independently working stations should transmit their signals in order to operate in uplink multi-user MIMO mode.
Cooperative Operation
A relay capable of cooperative transmission should send coded signal together with source station and the transmitted signals should be guaranteed to arrive at receiver within meaningful time deviation for service coverage extension and throughput improvement.
The both technologies become feasible only when the signals of multiple stations arrive at the receiver within meaningful time deviation without mutual interference.

5. Current Limitation Synchronization Multi-polling
In the IEEE standards, there is no explicit method to adjust signal transmission time.
Alternatively, SIFS access is a promising candidate for the purpose.
Multi-polling
Multiple stations should be allowed to occupy a common channel simultaneously for frame transmission.
Power Save Multi-Poll (PSMP) can be used for a scheduling policy to inform multiple stations to access a common channel. However, it is not exactly what to need for the synchronized signal arrival.

6. Promising Candidate: SIFS Access
Short Interframe Space (SIFS)
The shortest time duration preventing other stations from intervening frame exchange procedure.
Many pairs of frame exchanges are allowed to use the SIFS
Data frame and ACK, RTS and CTS, …

7. Potential Risk regarding SIFS
Close Look at SIFS
SIFS = 16 us (for a)
aSlotTime = 9 us (for a)
The standard said that “An IEEE implementation shall not allow the space between frames that are defined to be separated by a SIFS time, as measured on the medium, to vary from the normal SIFS value by more than 10% of aSlotTime for the PHY in use.”
 It implies that SIFS varies from 15.1 us to 16.9 us in worst case for a
Unallowable Time Offset Deviation
Even if we have strong assumption that SIFS is strictly constant value, it is possible that there exists signal arrival time difference since stations are scattered apart in general.
(We need to calculate signal arrival time offset deviation with an appropriate example)

8. Frame Arrival Time Difference
SIFS Access
In worst case, the arrival time difference is 1.8 us in the a network. However, 1.8 us is far from the defined cyclic prefix (=800 ns) for the a OFDM.
It implies that arrived signals work as interferences to each other.

9. How to Solve (3GPP LTE/802.16e)
Transmission Time Adjustment Procedure
3GPP-LTE and e solve the problem with contention based channel access procedures, i.e., random access and ranging procedure, respectively.
Base Station obtains the information about signal arrival time deviation and frequency offset through the random access procedures, and requests mobile stations to adjust transmission time and frequency offset.
Basic Concept about the Random Access Procedure
Since mobile stations’ transmitted signals are not exactly synchronized with each other, guard time is required such as Cyclic Prefix. The transmission time adjustment procedure targets at making mobile stations’ signals arrive within the guard time
Mobile Station (MS) A
Mobile Station (MS) B
RTD (Round Trip Delay)
≥ 2 * Distance / C (light of speed)
Base Station (or AP)
ARRIVAL TIME DIFFERENCE
Base Station (or AP)
Arrival time difference
MS A
MS B

10. Conceptual Procedure for Random Access (or Ranging)
Step 1. Message Transmission via Random Access
In 3GPP-LTE and IEEE e, stations transmit initial message via random access scheme
Step 2. Reply Message
Base Station replies with the corresponding message containing the information about transmission time offset adjustment.
Resource allocation for step 3
Step 3. L2/L3 Message (or management message)
Step 4. Indication Message
A message indicating whether the ranging procedure should proceed or not

11. New Approach for the 802.11 Requirements Why not in the 802.11?
Receiving station should detect the frame arrival time difference between the independently arriving frames, which more than two stations, at least, would transmit.
Receiving station should have the capability to adjust frame transmission times of transmitting stations.
Why not in the ?
A new procedure, which is similar to the random access (3GPP-LTE) and ranging (802.16e), is required to support Multi-User MIMO and cooperative transmission technologies.
The ranging procedure overhead in the WLANs is estimated to be negligible compared with that for cellular networks since the mobile stations are reasonably assumed to be stationary rather than cellular terminals

12. Summary
Uplink MIMO is required of consideration for aggregate uplink throughput improvement.
For the uplink MIMO, it is necessary to adjust fine grained transmission time and frequency offset.
The already-deployed technologies such as random access and ranging show that the fine grained offset adjustment is feasible.

13. References July 2009 Sunggeun Jin (ETRI) VInko Erceg, Broadcom
doc.: IEEE /xxxxr0
Month Year
July 2009
References
[1] IEEE /0852r0 (11TGac).
[2] IEEE /0769r0 (11TGad).
Sunggeun Jin (ETRI)
VInko Erceg, Broadcom