5G NR Transmission Time Interval

Different numerologies in 5G NR affect slot duration and TTI. Source: Qualcomm 2018, slide 13.
Different numerologies in 5G NR affect slot duration and TTI. Source: Qualcomm 2018, slide 13.

Transmission Time Interval (TTI) is composed of consecutive OFDM symbols in the time domain in a particular transmit direction. By combining different number of symbols, different TTI durations are possible.

In the frequency domain, different numerologies are permitted, corresponding to different Sub-Carrier Spacing (SCS). A higher SCS shortens symbol duration and hence shortens TTI. A combination of numerology and TTI determines how many bits and in what manner they're transmitted on the air interface.

Compared to LTE, 5G NR improves the design of TTI towards lower latency and more efficient use of radio resources. For 5G's Ultra-Reliable and Low-Latency Communications (URLLC) use case covering driverless cars, disaster response, factory automation and more, low latency is essential. It's been noted that,

Latency is in general a function of the TTI.

Discussion

  • How does TTI design in 5G compare against 4G/LTE?

    In 4G/LTE, TTI is fixed to 1ms and it's composed of 14 OFDM symbols. This is only the transmission time on the air interface. Other delay components include processing delay (at base station and UE) and HARQ retransmissions. In uplink, making a scheduling request and waiting for the grant adds to the delay. Estimates of one-way delay assuming 10% block error rate are 4.8ms (DL) and 11.3ms (UL). This is significant delay for URLLC use case.

    LTE Release 15 introduced shortened TTI of 2 symbols and 7 symbols. In FDD-LTE, both are allowed. In TDD-LTE, only 7-symbol shortened TTI is allowed. The release also specifies how processing times can be shortened. Making these changes to LTE was a challenge due to requirements of backward compatibility, that is, UEs that don't support shortened TTI should coexist with those that do.

    5G design includes scalable TTI, corresponding to slot durations 62.5µs to 1ms. One or more consecutive slots allocated to either DL or UL make up a TTI. 5G also incorporates mini-slot transmissions, which is similar to what LTE calls shortened TTI.

  • Could you describe 5G NR's scalable TTI design?
    5G NR allows for scalable TTI. Source: Qualcomm 2016, fig. 15.
    5G NR allows for scalable TTI. Source: Qualcomm 2016, fig. 15.

    By using different SCS in 5G NR, different slot durations and TTIs are configurable. For example, 15kHz SCS with 14 symbols spanning the entire subframe corresponds to LTE's configuration. At 240kHz SCS (for control only), 14 symbols are squeezed into a 62.5µs slot. In Release 16, 120kHz is the maximum SCS allowed for data. Hence, the lowest achievable slot duration is 125µs. At 60kHz, there's an option to use extended cyclic prefix (CP), thus limiting the slot duration to 12 symbols.

    Unfortunately, as SCS increases, CP decreases. This means that in deployments where there's a long delay spread, CP will not offer sufficient protection against inter-symbol interference (ISI). Thus, reducing TTI by increasing SCS is not always possible. It's for this reason 5G NR also allows for mini-slot transmissions.

    With mini-slots, a TTI can be as small as 2, 4 or 7 symbols. For example, at 30kHz SCS, the corresponding durations are about 70µs, 140µs and 250µs. Thus, we can cater to URLLC use case even at a lower SCS.

  • Could you explain pipeline processing in relation to 5G NR TTI and TB?
    Reducing processing time via pipeline processing. Source: Takeda et al. 2017, fig. 5.
    Reducing processing time via pipeline processing. Source: Takeda et al. 2017, fig. 5.

    MAC sublayer sends a transport block (TB) to PHY for transmission every TTI. At the receiving side, MAC layer can't process the TB until it's fully received. In extreme cases, a TB can have more than a million bits.

    A method to reduce the processing delay is to break up the TB into code blocks. Each code block independently goes through channel coding and rate matching. This feature, which also exists in LTE, is called code block segmentation.

    To distribute the processing across time, code blocks are mapped to different OFDM symbols. Processing can be decoupled from TTI duration. Processing can start as soon as a code block is received and happen in parallel with next code block reception. This is called pipeline processing. Ultimately, this reduces the processing time at the receiver.

    Pipeline processing is possible because time-domain interleaving is not done. Mapping of bits to resource blocks is also done on a frequency-first manner.

  • What's TTI bundling in 5G NR?
    Slot aggregation or TTI bundling in 5G NR. Source: Swamy 2019.
    Slot aggregation or TTI bundling in 5G NR. Source: Swamy 2019.

    TTI bundling is when the same TB is transmitted across multiple TTIs. This is done for the sake of redundancy and higher chance of successful transmission. For some time-critical services such as voice, delayed HARQ retransmissions are not useful. By pre-emptively transmitting multiple copies of the same data, quality is improved.

    TTI bundling reduces retransmissions and round trip time. It may be useful towards UEs that are the cell edge.

    TTI bundling exists in LTE and is not new to 5G NR.

    5G NR standard doesn't often use the term TTI bundling. Instead, slot aggregation is a more common term. The feature is enabled via RRC signalling. RRC Information Elements (IEs) pdsch-AggregationFactor and pusch-AggregationFactor signal the number of repetitions, which can be 2, 4 or 8.

  • Besides TTI, what are other techniques in 5G NR design that reduce latency?
    Latency components in DL transmission and ways to control them. Source: 5G Americas, fig. 4.1.[(5G Americas, fig. 4.1)]
    Latency components in DL transmission and ways to control them. Source: 5G Americas, fig. 4.1.[(5G Americas, fig. 4.1)]

    Besides shorter TTI via numerology and use of mini-slot TTI, other ways to reduce latency include frequency transmission opportunities to minimize wait time, shorter processing time via pipeline processing, grant-free UL transmission and flexible TDD frame structure.

    DMRS is front loaded, that is, it comes at the start of frame. A UE can therefore start channel estimation and decoding at the earliest.

    To enable quick HARQ feedback, 5G NR uses a self-contained subframe structure. A subframe contains DL control, DL data, guard period and UL control. Thus, ACK/NACK for DL data can be sent in the same subframe. A similar self-contained uplink-centric subframe exists.

    Multiple code blocks are grouped into a Code Block Group (CBG). If there's an error, only that CBG is retransmitted, not the entire TB. With fewer bits, errors at CBG are less likely than at TB. Moreover, slot aggregation tries to minimize retransmissions.

    For channel coding, 5G NR uses Low-Density Parity Check (LDPC). LDPC has a highly parallelizable decoder, thus reducing processing time.

Milestones

Mar
2016

Durisi et al. note that some use cases such as Machine-to-Machine (M2M) communications send short packets. Likewise, there are use cases that require low latency. Given these requirements, they propose performance metrics and protocols more suited for short packets.

Jun
2016
Shortened TTI and shorter processing in LTE Release 15. Source: Qualcomm, via Mavrakis 2018.
Shortened TTI and shorter processing in LTE Release 15. Source: Qualcomm, via Mavrakis 2018.

A proposal by Ericsson identifies shortened TTI and processing time for LTE. From its beginning in Release 8 to the most recent Release 13, LTE has focused on increasing data rates and no enhancements have come to lower latency. Lower latency will improve TCP throughput and reduce L2 buffer space requirements. Eventually, this proposal is standardized in LTE Release 15.

Apr
2017

Qualcomm files a patent titled TTI Bundling for URLLC UL/DL Transmissions. For selected UEs that may benefit from this feature, the base station signals the bundle length. Subsequently, data or control packets are repeated to those UEs. Number of repetitions correspond to signalled bundle length.

Aug
2020

As part of Release 17 work, modifications to TTI bundling are proposed to improve PUSCH coverage. One proposal is to allow repetition across non-consecutive slots. Another is to consider symbol-level repetition.

References

  1. 3GPP. 2016. "RP-160872: New Work Item on shortened TTI and processing time for LTE." 3GPP TSG RAN Meeting #72, Busan, Korea, June 13-16. Accessed 2021-03-31.
  2. 3GPP. 2017. "TR 38.804: Study on new radio access technology; Radio interface protocol aspects." V14.0.0, March. Accessed 2021-02-27.
  3. 3GPP. 2020. "R1-162386: Email discussion/approval on PUSCH coverage enhancement." 3GPP TSG RAN WG1 Meeting #102-e E-meeting, August 17-28. Accessed 2021-03-31.
  4. 5G Americas. 2018. "New Services & Applications with 5G Ultra-Reliable Low Latency Communications." White paper, 5G Americas, November. Accessed 2021-03-29.
  5. Dahlman, Erik, Stefan Parkvall, and Johan Skold. 2018. "5G NR: The Next Generation Wireless Access Technology." Academic Press. Accessed 2021-02-23.
  6. Durisi, Giuseppe, Tobias Koch, and Petar Popovski. 2016. "Towards Massive, Ultra-Reliable, and Low-Latency Wireless Communication with Short Packets." arXiv, v4, March 1. Accessed 2021-03-30.
  7. ETSI. 2021a. "TS 138 300: 5G; NR; NR and NG-RAN Overall description; Stage-2." V16.4.0, January. Accessed 2021-02-23.
  8. ETSI. 2021b. "TS 138 212: 5G; NR; Multiplexing and channel coding." V16.4.0, January. Accessed 2021-03-31.
  9. ETSI. 2021c. "TS 138 331: 5G; NR; Radio Resource Control (RRC); Protocol specification." V16.3.1, January. Accessed 2021-03-31.
  10. Kihero, Abuu B., Muhammad Sohaib J. Solaija, and Hüseyin Arslan. 2019. "Inter-Numerology Interference for Beyond 5G." IEEE Access, vol. 7, October 21. Accessed 2021-03-30.
  11. Li, Chih Ping, Chong Li, Jing Jiang, Wanshi Chen, Hao Xu, and Haitong Sun. 2018. "TTI bundling for URLLC UL/DL transmissions." US Patent App. US20180103468A1, April 12. Filed 2017-04-05. Accessed 2021-03-30.
  12. Matthe, Maximilian. 2018. "The Cyclic Prefix for OFDM." Blog, DSP Illustrations. Accessed 2021-03-31.
  13. Mavrakis, Dimitris. 2018. "Accelerating the Path to 5Gwith LTE Advanced Pro." White paper, ABI Research. Accessed 2021-03-31.
  14. Pokhrel, Shiva Raj, Jie Ding, Jihong Park, Ok-Sun Park, and Jinho Choi. 2020. "Towards Enabling Critical mMTC: A Review of URLLC Within mMTC." IEEE Access, vol. 8, July 20. Accessed 2021-03-30.
  15. Qualcomm. 2016. "Making 5G NR a reality." White paper, Qualcomm, December. Accessed 2021-03-29.
  16. Qualcomm. 2018. "Designing 5G NR." Presentation, Qualcomm, April. Accessed 2021-03-30.
  17. ShareTechnote. 2021. "5G/NR - Channel Coding." ShareTechnote. Accessed 2021-03-31.
  18. Swamy, Kumara. 2019. "5G NR: PDSCH Resource Allocation in Time-Domain." How LTE Stuff Works?, December. Accessed 2021-03-30.
  19. Takeda, Kazuki, Li Hui Wang, and Satoshi Nagata. 2017. "Latency Reduction toward 5G." IEEE Wireless Comms, June. Accessed 2021-03-29.
  20. Techplayon. 2019. "TTI Bundling for better HARQ and Latency." Techplayon, May 15. Accessed 2021-03-30.
  21. Tweet4technology. 2016. " TTI (Transmission Time interval) Bundling FDD." Blog, Tweet4technology, November 29. Accessed 2021-03-30.
  22. Wu, Hao. 2020. "A Brief Overview of CRC Implementation for 5G NR." IntechOpen, March 17. Accessed 2021-03-31.

Further Reading

  1. Takeda, Kazuki, Li Hui Wang, and Satoshi Nagata. 2017. "Latency Reduction toward 5G." IEEE Wireless Comms, June. Accessed 2021-03-29.
  2. Parvez, Imtiaz, Ali Rahmati, Ismail Guvenc, Arif I. Sarwat, and Huaiyu Dai. 2018. "A Survey on Low Latency Towards 5G: RAN, Core Network and Caching Solutions." arXiv, v2, May 29. Accessed 2021-03-30.
  3. Bennis, Mehdi, Mérouane Debbah, and H. Vincent Poor. 2018. "Ultra-Reliable and Low-Latency Wireless Communication: Tail, Risk and Scale." arXiv, v2, August 23. Accessed 2021-03-30.
  4. Durisi, Giuseppe, Tobias Koch, and Petar Popovski. 2016. "Towards Massive, Ultra-Reliable, and Low-Latency Wireless Communication with Short Packets." arXiv, v4, March 1. Accessed 2021-03-30.
  5. Malhotra, Dheeraj. 2020. "Introduction to Numerology." Blog, NR LTE related tech oriented blog, April 25. Accessed 2021-03-30.

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Devopedia. 2021. "5G NR Transmission Time Interval." Version 4, March 31. Accessed 2021-09-09. https://devopedia.org/5g-nr-transmission-time-interval
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Last updated on
2021-03-31 13:04:25