Introduction to Shortened TTI And Processing Time for LTE. Sam Meng HTC

Transcription

1 Introduction to Shortened TTI And Processing Time for LTE Sam Meng HTC 1

2 Table of Contents Background Design Considerations Specification Concluding Remarks 2

3 3 Background

4 TTI in LTE Short for Transmission Time Interval One slot, T slot = 15360T s = 0.5 ms One radio frame, T f = T s = 10 ms But it can mean different things, e.g., TTI for PBCH is 40 ms #0 #1 #2 #3 #18 #19 One subframe Here it refers to the smallest scheduling unit in [1] the time domain One subframe <=> 1 ms 4

5 Processing Time in LTE 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 [2] 5

6 Motivation to Reduce Latency 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 [9] capability 6

7 Reduce Latency by Shortening TTI 1 subframe 1 subframe Directly 1 subframe shorten the 1 subframe time unit for data 1st slot 2nd slot 1st slot 2nd slot 1st slot 2nd slot scheduling 1st slot 2nd slot Unicast PDSCH Unicast PDSCH Downlink system bandwidth PDCCH Unicast PDSCH PDCCH spdsch spdsch spdsch spdsch stti stti stti stti Has various impacts on the physical control Downlink system bandwidth channel (PDCCH) and physical data channel PDCCH (PDSCH) spdsch PDCCH spdsch stti stti [4] (a) Cases corresponding to Alt 1. (b) Cases corresponding to Alt 2 and Alt 3. 7

8 Reduce Latency by Shortening Processing Time Reduce latency for normal TTI too DL UL Legacy UE UE with reduced processing time Easier to multiplex with legacy UEs data HARQ A/N PUCCH resource collision 8

9 Don t We Already Have A Shorter Processing Time? 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 9

10 HARQ Timeline in LTE 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 10 [2]

11 11 Design Considerations

12 So, Just Shorten The TTI? 1 subframe 1 subframe Might 1 subframe not be that straightforward 1 subframe because 1st slot 2nd slot 1st slot 2nd slot 1st slot 2nd slot 1st slot 2nd slot Boundary alignment Control channel (PDCCH) Unicast PDSCH Unicast PDSCH Downlink system bandwidth PDCCH Unicast PDSCH PDCCH spdsch spdsch spdsch spdsch stti stti stti stti Downlink system bandwidth PDCCH HARQ timing does not proportionally scale Limited UE tx power (UL coverage) spdsch PDCCH spdsch Increased overhead of control signals stti stti [4] (a) Cases corresponding to Alt 1. (b) Cases corresponding to Alt 2 and Alt 3. 12

13 Boundary Alignment 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 13

14 PDCCH in LTE 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 [2] E-PDCCH 14

15 for stti 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 [6] 15

16 for stti Span how many symbols? Unified or separate design for different PDCCH for 2-symbol stti stti lengths? Same number of blind decoding in every? PDCCH for 2-symbol stti Can the first stti be scheduled by the legacy PDCCH? [7] 16

17 HARQ Timing 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 17

18 HARQ Timing TA is determined by distance (cell size), and does not scale with the length of TTI. [2] 18

19 Asymmetric DL/UL stti UL coverage is limited by UE tx power TTI bundling is supported in LTE DL UL 2-symbol stti 7-symbol stti Minimum processing time of n+k, where k=4 Allow combinations of a longer UL stti than DL stti? Complicates system design and HARQ [8] 2-symbol stti DL 7-symbol stti Minimum processing time of n+k, where k=8 UL 19

20 Multi-layer Transmission 8-layer MIMO transmission is supported in LTE-A subframe subframe subframe subframe subframe Extra control information More reference signal (RS) 1 PRB PRB 1 PRB PRB 1 PRB 1 PRB 1 PRB 1 PRB The added control information and RS overhead eats into the available time frequency [10] Subframe 3/4-symbol TTI TTI 2-symbol Slot TTI TTI 3/4-symbol 1-symbol TTI TTI 2-symbol TTI resource for data transmission 1-symbo 20

21 Specification 21

22 stti Arrangement No stti spans over subframe boundary Support DL/UL stti combinations {2,2} {7,7} {2,7} [5] 22

23 Configurable bandwidth PDCCH for 2-symbol stti 1~2 OFDM symbols (configurable) in each stti PDCCH for 2-symbol stti sdci2 is still under discussion [7] 23

24 HARQ Timing 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 ms For 1 ms TTI, n+3 assuming a maximum TA of 0.33 ms is supported 24

25 Multi-layer Transmission 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 25

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 1/7 th The specification of stti is still in progress. 26 The support of different TTI values is also more aligned with the design principle of 5G NR systems.

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28 References [1] Physical channels and modulation, 3GPP TS , V14.0.0, [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] R , Discussion on spdsch, Samsung [5] R , Way Forward on 2-symbols DL stti layout structure [6] R , The co-existence consideration of legacy TTI and stti in one carrier, Huawei [7] R , Discussion on for shortened TTI, Sony [8] R , 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 [10] R , Downlink physical channel design for TTI shortening, Samsung 28