1. IEEE P802.22 Wireless RANs Date: 2006-10-09
October 2006
doc.: IEEE /015r2
October 2006
ADAPTIVE TDD for WRANs
IEEE P Wireless RANs Date:
Authors:
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Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

2. October 2006
doc.: IEEE /015r2
October 2006
Why Adaptive TDD?
Adaptive TDD overcomes the problem of long TDD turn-around time in WRANs.
Adaptive TDD significantly increases the system capacity of WRANs.
Adaptive TDD can be implemented in a simple, flexible, efficient, and evolvable manner.
All the WG need to do is to add one extra subsection (less than 1 page) to the current Draft.
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

3. FRD Compliance Table – Req. 31:
October 2006
doc.: IEEE /015r2
October 2006
FRD Compliance Table – Req. 31:
31 (3)
If TDD is proposed, the proposal MUST include diagrams illustrating the proposed framing/timing and analysis of the impact of TDD turn-around time on system capacity, latency, and other factors affecting performance and overall system complexity. M Y
(analysis has been made in a separate document*)
* _ADAPTIVE_TDD.doc
Section of Draft 0.1: “TDD is the mandatory duplexing mode.”
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

4. Impact of TDD Turn-around Time
October 2006
doc.: IEEE /015r2
October 2006
Impact of TDD Turn-around Time
Assume 6Mhz channel, 2K FFT, ¼ CP, 50us max delay spread, 10ms frame.
Nearby CPEs need to be idle for:
More than 1 OFDMA symbol for cell radius of 30km.
More than 2 OFDMA symbols for cell radius of 100km.
=> For WRANs with large cell radius, TDD turn-around time leads to long DS-US gap and significantly reduces system capacity.
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

5. Concepts of Adaptive TDD
October 2006
doc.: IEEE /015r2
October 2006
Concepts of Adaptive TDD
TDD turn-around time BS
DS Subframe
US1-1
US 1,2
CPE1
DS Subframe
US1-1
US1-2
TPD1
TDS1
TSSRTG
CPE2
DS Subframe
US2-1
TPD2
TDS2
TSSRTG
TPD2
Allow nearby CPEs to start US transmission earlier than faraway CPEs.
Keep OFDMA symbol boundaries synchronized at BS.
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

6. Gain in UL Throughput October 2006
doc.: IEEE /015r2
October 2006
Gain in UL Throughput
Cell radius = 30km, nearby CPEs within 5km from BS can transmit 1 OFDMA symbol earlier. Nearby CPEs use 64QAM, ¾ code rate; faraway CPEs use QPSK, ½ code rate. DS and US traffics are symmetric.
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

7. Upstream Subframe Structure
October 2006
doc.: IEEE /015r2
October 2006
Upstream Subframe Structure
OFDMA Symbols
Divide US subframe into “Early zone” and “Normal zone”.
The size of each zone can be made flexible by adjusting the “Allocation Start Time”.
Slot-allocation rules are the same for the two zones.
Logical Subchannels
Early Start Time
Allocation Start Time
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

8. US MAP IE for AdaptiveTDD Bursts
October 2006
doc.: IEEE /015r2
US MAP IE for AdaptiveTDD Bursts
October 2006
Syntax
Size
Notes
US-MAP_IE() {
CID
16 bits
UIUC
4 bits
= 15
AdpativeTDD_IE()
24 bits
Padding Nibble
= 0x04 }
Using an US Extended UIUC, called AdaptiveTDD IE, to specify Adaptive TDD bursts.
To specify each AdaptiveTDD burst we need 48 bits, the same as that for normal uplink TDD bursts.
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

9. AdaptiveTDD_IE (Extended UIUC)
October 2006
doc.: IEEE /015r2
October 2006
AdaptiveTDD_IE (Extended UIUC)
Syntax
Size
Notes
AdaptiveTDD_IE() {
Extended UIUC
4 bits
0x0F
Length
0x02 (in bytes)
Burst_Profile
3 bits
From 6 to 12
Early Start Time = n
n = 0 => not employing Adaptive TDD
n > 0 => Adaptive TDD is employed, starts n 0FDMA
symbols earlier than what specified by the Allocation Start Time in the US-MAP.
Preamble Present
1 bit
Determines whether CPE shall send preamble before any data transmission
0 = Preamble shall not be used (default)
1 = Preamble shall be used
Duration}
8 bits
In number of MAC slots
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

10. Proposed Text Addition to Draft 0.1
October 2006
doc.: IEEE /015r2
October 2006
Proposed Text Addition to Draft 0.1
We propose to add a new section: “Section AdaptiveTDD IE” to the current v.0.1 document. In particular, the new Section will be as follows.
AdaptiveTDD IE
Each AdaptiveTDD IE is 3 byte long and is specified as in Table 1. The important fields of this extended IE are:
Extended UIUC: this 4 bit field is set to 0x0F to indicate an AdaptiveTDD IE.
Length: length of data field = 2 bytes.
UIUC: this 4 bit field specifies the burst profile of the corresponding AdaptiveTDD IE.
Early Start Time: this is a 2-bit field that specifies how early this AdaptiveTDD IE should start, with respect to the global Allocation Start Time specified in the US MAP.
Duration: The duration of the AdaptiveTDD IE in OFDMA slots.
Syntax
Size
Notes
AdaptiveTDD_IE() {
Extended UIUC
4 bits
0x0F
Length
0x02 (in bytes)
Burst_Profile
3 bits
From 6 to 12
Early Start Time = n
n = 0 => not employing Adaptive TDD
n > 0 => Adaptive TDD is employed, starts n 0FDMA symbols earlier than what specified in the US-MAP.
Preamble Present
1 bit
Determines whether CPE shall send preamble before any data transmission
0 = Preamble shall not be used (default)
1 = Preamble shall be used
Duration}
8 bits
In number of MAC slots
Table 1: Format of an AdaptiveTDD IE
Each AdaptiveTDD IE allocation shall be restricted within its early US portion, i.e., between its Early Start Time and the Allocation Start Time. Each
AdaptiveTDD IE allocation shall start at the unused MAC slot that has the lowest subchannel index followed by the lowest symbol index (i.e., subchannel
index has higher priority than symbol index). Each AdaptiveTDD IE allocation shall advance in the time domain and shall not overlap with the previous
AdaptiveTDD allocations. If the end of the Early US Portion has been reach, the allocation shall continue at the next subchannel at the first symbol,
specified by its Early Start Time.
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

11. Other Mandatory Requirements
October 2006
doc.: IEEE /015r2
October 2006
Other Mandatory Requirements
[26] In the upstream direction, protocols MUST provide the means to efficiently multiplex traffic from multiple CPEs and allocate transmission channel capacity.
[47] The PHY and MAC protocols SHALL flexibly accommodate communication between a base station and CPEs at distances of up to 100 km where feasible.
[53] Ranging SHALL be done to synchronize CPEs with a common time reference relative to base station timing  in order to maximize the transmission medium capacity by minimizing guard time requirements.
[87] The PHY and MAC protocols SHALL provide the ability to adaptively deliver different bit rates/capacities to individual subscribers.
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R

12. October 2006
doc.: IEEE /015r2
October 2006
Conclusions
Adaptive TDD mitigates the problem of long turn-around time and significantly increases system capacity.
Adaptive TDD can be implemented in a flexible, efficient, and evolvable manner.
All the WG need to do is to add one extra subsection (less than 1 page) to the current Draft.
Ying-Chang Liang, I2R
Ying-Chang Liang, I2R