An Introduction to URLLC for 5G-NR: Ultra-Low Latency

[This article is part 2 of two-part series of articles on URLLC. This part covers ultra-low latency technologies in 5G , top IP players and important patents in  URLLC. Please read part 1 which covers ultra-reliability here]

Recap from the previous article:

• URLLC is one of the different types of technologies that are supported by the 5G New Radio (NR) standard, as stipulated by 3GPP (3rd Generation Partnership Project) Release 15. It will allow the expansion of communication service to reach the areas with stringent service requirements especially in terms of high reliability and low latency which is needed in mission-critical applications such as autonomous driving, industrial automation, robotic surgeries, etc. The technical empowerment is thus desirable and hence 3GPP in its Rel-15 and Rel-16 have laid out some of the key technologies using which will turn it into a real-time service. Some of the key enablers of URLLC include integrated frame structure, incredibly fast turnaround, efficient control and data resource sharing, grant-free based uplink transmission, and advanced channel coding schemes, etc. Time-sensitive networking is another component of the 5G URLLC capabilities.

Now let’s dive into low latency part of the story!!!

Ultra-low latency:  Latency is defined as the round-trip time taken by the data to reach the target receiver (UE) from source equipment (NR-gNB). In previous technologies like in 3G, 4G the round-trip delay was of the order of hundreds of milliseconds. To further reduce this to around 1 millisecond we use below mentioned techniques.

Numerology: Numerology in URLLC is about the frame configuration that includes frame component elements such as slots, sub-frame elements, subcarrier spacing, etc. Ideal frame configuration will help in achieving low latency.  Different key concepts related to frame configuration are mentioned below-

• Subcarrier spacing is no longer fixed to 15 kHz i.e. it can be 30, 60,120 or 240 kHz. The number of slots increases as numerology (µ) increases. The number of slots in one frame can be 1,2,4 or 8 similarly corresponding slot length will be 1ms, 500us, 250 us and 125 µs. Ideally, a standard slot has 12 symbols but in mini slots, the number of symbols can be 7,4 or 2.
• Low latency is achieved using different dynamic slots numbers in a frame depending upon requirements and applications. Slots can be DL, UL, or flexible.NR slot structure allows dynamic assignment of the link direction in each OFDM symbol within the slot. With this, the network can dynamically balance UL and DL traffic.
• Below figure explains sub-frame arrangement

Grant free transmission: It is a technique that reduces the signaling overhead caused by conventional grant-based schemes. It reduces latency considerably because channel resources are accessed without undergoing assignments through a handshake process. This is proposed in 5G New Radio as an important latency reducing solution. However, it comes at an increased likelihood of collisions resulting from uncontrolled channel access, when the same resources are pre-allocated to a group of users.

Following figures (a) and (b) show “with” and “without grant” transmission scenarios. In with grant free transmission fig (a), two additional steps (Scheduling Request (SR) and Scheduling Grant (SG)) are involved in comparison to “without grant free” transmission fig. (b). so, the latency is reduced in grant free transmission as the time taken in SR and SG is saved.

Transmission Time Interval: TTI refers to the duration of a transmission on the radio link. The TTI is related to the size of the data blocks passed from the higher network layers to the radio link layer. To combat errors due to fading and interference on the radio link, data is divided at the transmitter into blocks and then the bits within a block are encoded and interleaved. The length of time required to transmit one such block determines the TTI. The below figure shows that how a subframe is divided into TTIs. TTIs can be of varying symbol sizes depending upon the requirement of latency and spectral efficiency as shown in figure. In URLLC, to achieve low latency and high reliability, shorter TTIs are used because shorter TTIs take less time in retransmission and error detection. The figure below shows variation in size of TTIs as per application requirements.  One resource block is equal to one TTI.

• Figure: TTI representation in a sub-frame

3GPP TDocs (URLLC):

The following table gives information about the documents and various releases for URLLC, 5G-NR

IP Players in URLLC: Major IP filing companies in URLLC are Qualcomm, Huawei, LG, Samsung, and Ericsson.

The following graph (source: pcs.dolcera.com) gives an idea about top IP players in URLLC, 5G-NR domain.

Important patents of URLLC of the top players: