What is 5G ultra reliable low latency communications (URLLC)?


Introduced in 3GPP release 15 to address the requirements of ITU-R M.2083, ultra-reliable low latency communications (URLLC) is one of the key pillars of 5G New Radio (NR). As the underlying feature required to support dense sensor grids of IoT endpoints, it is a primary enabler for a number of unique use cases in the areas of manufacturing, energy transmission, transportation and healthcare. With the need to support end-to-end latencies as low as 5ms, the delay budget for individual interfaces can be as low as 1ms. This means that optimizations must be made at every step of the uplink and down link transmission process. While outside the scope of 3GPP specifications, the need to reduce data processing response times is also leading to the emergence of highly distributed edge computing strategies.

3GPP technical specification (TS) 38.912 performed a study on new radio (NR) access technology that set the stage for standards-track specifications, such as TS 38.201-202 and TS 38.2011-215. These documents detail the physical channels and modulation techniques employed within 5G frames. Many of those technologies either directly address low latency communications or define approaches that support the co-existence of low latency data and other traffic characteristics, commonly referred to as multi-numerology. There are five fixed OFDM (subcarrier spacing) numerologies in 5G, the first (0) enables coexistence with current 4G/LTE radio and all of which can be mixed at the transmitter for further flexibility. Using the classic definition of numerology, the subcarrier spacing parameter can be directly correlated to the deployment event. A 700MHz macro cell using 15 kHz spacing for broad coverage, for example, or a 28 GHz mmWave micro cell using 120 kHz spacing for delivering enhanced mobile broadband (eMBB).


Multi-numerology in 5G new radio

Specifically addressing the issue of low latency communications, new radio allows for a variable transmission time interval (TTI) which can scale from 1ms, the (compatible) setting fixed in LTE, down to ~140 microseconds, depending on whether spectral efficiency (eMBB) or low latency (URLLC) is the goal. The maximum number of retransmissions can also be set according to traffic type (e.g. 2 for URLLC and 4 for eMBB). Plus, as a bonus, NR permits the multiplexing of different TTI’s on the same frequency, so spectrum can be shared without delay sensitive traffic being stuck waiting for slower transmissions to conclude. The techniques employed to enable MIMO antenna arrays also support advancements in lowing latencies, specifically the self-contained integrated subframe where transmissions from the RF antenna and acknowledgements from the user equipment occur on the same subframe.

There is one other modification to the NR frame structure, described within TS 38.912, that contributes significantly to delivering on URLLC. Every transmission slot comprises 14 OFDM symbols and each OFDM symbol represents an individual bitstream employing quadrature phase-shift keying (QPSK / 4-QAM) or 16, 64 or 128 quadrature amplitude modulation (QAM) constellations. With numerologies employing wider subcarriers and higher QAM modulation orders, the 5G specifications allow us to create a mini-slot from a sub-set (e.g. 2 or 4) of the individual OFDM Symbols. This provides the ability to service critical traffic flows with fine scheduling granularity to reduce transmission latency. The lower the transmission speed, the smaller subcarrier spacing becomes and the fewer mini-slots per slot we can create. While that dependency on mmWaves might have 5G naysayers questioning the value of mini-slots, the majority of early URLLC applications will originate within controlled enterprise manufacturing environments where utilization of the higher bands will be easier and therefore more prevalent.

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