1

Introduction to Shortened TTI And Processing Time for LTE

Sam Meng

HTC

2

• Background

• Design Considerations

• Specification

• Concluding Remarks

Table of Contents

3

Background

4

• Short for Transmission Time Interval

• But it can mean different things, e.g., TTI for

PBCH is 40 ms

• Here it refers to the smallest scheduling unit in

the time domain

• One subframe <=> 1 ms

TTI in LTE

#0 #1 #2 #3 #19#18

One radio frame, Tf = 307200Ts = 10 ms

One slot, Tslot = 15360Ts = 0.5 ms

One subframe

[1]

5

• Like TTI, it does not have a precise definition

• Here it refers to the standardized time delay for

UE/eNB to report HARQ-A/N for DL/UL data

transmissions

• FDD: 4m

• TDD: defined in a table

Processing Time in LTE

[2]

6

• Reduce packet data latency

• Directly affect perceived responsiveness;

video conferencing/driving/real-time

gaming/VR

• Indirectly determine throughput

• Improve resource efficiency

• Improve error rate given delay requirement

• Lower buffer requirement

• Advancements in hardware and processing

capability

Motivation to Reduce Latency

[9]

7

• Directly shorten the time unit for data

scheduling

• Has various impacts on the physical control

channel (PDCCH) and physical data channel

(PDSCH)

Reduce Latency by Shortening TTI

D

ow

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k s

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idth

1 subframe

1st slot 2nd slot

Unicast PDSCH

PD

CC

H

1 subframe

1st slot 2nd slot

sP

DS

CH

PD

CC

H

sP

DS

CH

sP

DS

CH

sP

DS

CH

sP

DC

CH

sTTI sTTI sTTI sTTI

sP

DC

CH

sP

DC

CH

sP

DC

CH

Unicast PDSCH

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1 subframe

1st slot 2nd slot

PD

CC

H

sP

DS

CH

sP

DC

CH

sTTI

1 subframe

1st slot 2nd slot

PD

CC

H

sP

DS

CH

sP

DC

CH

sTTI

Unicast PDSCH

(a) Cases corresponding to Alt 1. (b) Cases corresponding to Alt 2 and Alt 3.[4]

8

• Reduce latency for normal TTI too

• Easier to multiplex with legacy UEs’ data

Reduce Latency by Shortening Processing Time

0 1 2 3 4 5 6 7 8 9

0 1 2 3 4 5 6 7 8 9

Legacy

UE

UE with reduced

processing time

PUCCH resource

collision

DL

UL

HARQ

A/N

9

• For the actual time spent on processing data, yes

• But the catch is that UE can’t just report

HARQ-A/N to or received from eNB earlier

even if the HARQ-A/N is ready

Don’t We Already Have A Shorter Processing Time?

10

• This allows both sides to agree on a schedule (where to expect data, where to

expect HARQ-A/N)

• Lower complexity, facilitate resource scheduling

• Also affects the soft buffer management at the UE side

HARQ Timeline in LTE

[2]

11

Design Considerations

12

• Might not be that straightforward because

• Boundary alignment

• Control channel (PDCCH)

• HARQ timing does not proportionally

scale

• Limited UE tx power (UL coverage)

• Increased overhead of control signals

So, Just Shorten The TTI?

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1 subframe

1st slot 2nd slot

Unicast PDSCH

PD

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1 subframe

1st slot 2nd slot

sP

DS

CH

PD

CC

H

sP

DS

CH

sP

DS

CH

sP

DS

CH

sP

DC

CH

sTTI sTTI sTTI sTTI

sP

DC

CH

sP

DC

CH

sP

DC

CH

Unicast PDSCH

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1 subframe

1st slot 2nd slot

PD

CC

H

sP

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CH

sP

DC

CH

sTTI

1 subframe

1st slot 2nd slot

PD

CC

H

sP

DS

CH

sP

DC

CH

sTTI

Unicast PDSCH

(a) Cases corresponding to Alt 1. (b) Cases corresponding to Alt 2 and Alt 3.[4]

13

• For ease of scheduling and multiplexing with

legacy UE data, aligning sTTI at subframe

boundary is desirable

• Can sTTI length change within a subframe?

• More flexibility

• Higher complexity/signaling overhead

Boundary Alignment

14

• Inform UE there is a DL data scheduled for it

and where/how to receive the data

• For efficiency, PDCCH is designed in a

somewhat convoluted way

• Variable length

• Blind decoding

• E-PDCCH

PDCCH in LTE

[2]

15

• Reuse PDCCH for sTTI?

• Contradicts the intention to have a lower

latency (more granular scheduling

decision)

• A new (shortened) PDCCH has to be

designed and placed within each sTTI

sPDCCH for sTTI

[6]

16

• Span how many symbols?

• Unified or separate design for different

sTTI lengths?

• Same number of blind decoding in every

sPDCCH?

• Can the first sTTI be scheduled by the legacy

PDCCH?

sPDCCH for sTTI

sPD

CC

H

PD

CC

H for

2-sy

mbo

l sT

TI

sPD

CC

H sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H sPD

CC

H

PD

CC

H for

2-sy

mbo

l sT

TI

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

[7]

17

• HARQ timing is 4 ms in legacy LTE systems

• Processing capability

• Its advancement is what motivated this

feature

• Timing advance

• Determined by the supported cell size

HARQ Timing

18

• TA is determined by distance (cell size), and

does not scale with the length of TTI.

HARQ Timing

[2]

19

• UL coverage is limited by UE tx power

• TTI bundling is supported in LTE

• Allow combinations of a longer UL sTTI than

DL sTTI?

• Complicates system design and HARQ

Asymmetric DL/UL sTTI

2-symbol sTTI

DL

UL

7-symbol sTTI

Minimum processing time of n+k, where k=8

2-symbol sTTI

DL

UL

7-symbol sTTI

Minimum processing time of n+k, where k=4

[8]

20

• 8-layer MIMO transmission is supported in

LTE-A

• Extra control information

• More reference signal (RS)

• The added control information and RS

overhead eats into the available time frequency

resource for data transmission

Multi-layer Transmission

1 P

RB

subframe

2-symbol TTI

1 P

RB

subframe

1 P

RB

subframe

Subframe TTI

1 P

RB

subframe

Slot TTI

1 P

RB

subframe

3/4-symbol TTI 1-symbol TTI

1 P

RB

subframe

2-symbol TTI

1 P

RB

subframe

1 P

RB

subframe

Subframe TTI

1 P

RB

subframe

Slot TTI

1 P

RB

subframe

3/4-symbol TTI 1-symbol TTI

[10]

21

Specification

22

• No sTTI spans over subframe boundary

• Support DL/UL sTTI combinations

{2,2} {7,7} {2,7}

sTTI Arrangement

[5]

23

• Configurable bandwidth

• 1~2 OFDM symbols (configurable) in each

sTTI

• sDCI2 is still under discussion

sPDCCH

sPD

CC

H

PD

CC

H for

2-sy

mbo

l sT

TI

sPD

CC

H sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H sPD

CC

H

PD

CC

H for

2-sy

mbo

l sT

TI

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

sPD

CC

H

[7]

24

• For DL/UL sTTI 7 OFDM symbols, n+4 assuming a max TA of 0.33 ms is

supported

• For DL/UL sTTI 2 OFDM symbols, support

• n+6 assuming a maximum TA of 0.33 ms

• n+4 assuming a maximum TA of 0.067 ms

• For 1 ms TTI, n+3 assuming a maximum TA of 0.33 ms is supported

HARQ Timing

25

• 4-layer MIMO transmission with single codeword is supported for sTTI 2 OFDM

symbols

• 4-layer MIMO transmission with single codeword is supported for sTTI 7 OFDM

symbols

Multi-layer Transmission

26

Concluding Remarks

• Packet data latency, along with data rate, are the most important performance

metrics

• Before 3GPP Rel-14, enhancements on LTE/LTE-A systems are mostly on

increasing data rates (300 Mbps -> 4 Gbps -> 25 Gbps)

• Support of sTTI can potentially reduce latency to around 1/7th

The specification of sTTI is still in progress.

• The support of different TTI values is also more aligned with the design principle of

5G NR systems.

28

References

[1] “Physical channels and modulation,” 3GPP TS 36.211, V14.0.0, 2016-09.

[2] 4G: LTE/LTE-Advanced for Mobile Broadband, 2nd ed., Erik Dahlman, Stefan Parkvall, Johan Skold

[3] Draft Report of 3GPP TSG RAN WG1 meeting #86b.

[4] R1-164795, “Discussion on sPDSCH,” Samsung

[5] R1-1613691, “Way Forward on 2-symbols DL sTTI layout structure”

[6] R1-164068, “The co-existence consideration of legacy TTI and sTTI in one carrier ,” Huawei

[7] R1-166661, “Discussion on sPDCCH for shortened TTI ,” Sony

[8] R1-1609871, “Association timing for sTTI with different UL and DL lengths,” Sharp

[9] ITU-R, IMT Vision e Framework and Overall Objectives of the Future Development of IMT for 2020 and beyond, Recommendation ITU-R

M.2083, September 2015.

[10] R1-162706, “Downlink physical channel design for TTI shortening,” Samsung