# 5G NR Transmission Time Interval

## Summary


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?

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?

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?

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?

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

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.