1 Short Block-length Codes for Ultra-Reliable Low Latency Communications Mahyar Shirvanimoghaddam, Mohamad Sadegh Mohamadi, Rana Abbas, Aleksandar Minja, Chentao Yue, Balazs Matuz, Guojun Han, Zihuai Lin, Yonghui Li, Sarah Johnson, and Branka Vucetic Abstract—This paper reviews the state of the art channel rigorous latency and reliability levels. For example, power coding techniques for ultra-reliable low latency communication electronics based industrial control needs the overall network (URLLC). The stringent requirements of URLLC services, such latency to be less than 0.1msec and reliability of (1 − 10−9) as ultra-high reliability and low latency, have made it the most challenging feature of the fifth generation (5G) of mobile [2]. networks. The problem is even more challenging for the ser- vices beyond the 5G promise, such as tele-surgery and factory Physical layer design of URLLC is very challenging be- automation, which require latencies less than 1ms and packet cause URLLC should satisfy two conflicting requirements: error rates as low as 10−9. This paper provides an overview ultra-low latency and ultra-high reliability. One could use short on channel coding techniques for URLLC and compares them in packets to reduce latency which in turn causes a severe loss terms of performance and complexity. Several important research in coding gain. Alternatively, the system bandwidth should be directions are identified and discussed in more detail. widened, which is not always possible especially for some Index Terms—Mission critical communication, short block- URLLC applications in industrial control that might operate length channel codes, URLLC. over unlicensed spectrum [2]. On the other hand, for enhancing reliability, we need to use strong channel codes eventually I. INTRODUCTION paired with retransmission techniques which indeed increase HE third generation partnership project (3GPP) has de- the latency. fined three main service categories in 5G. Enhanced T For high reliability transmissions of URLLC data, a channel mobile broadband (eMBB) is the service category designed code with low code rates is generally used [3]. Several for services which have high requirements for bandwidth candidate channel codes such as low density parity check such as virtual reality, augmented reality, and high-resolution (LDPC), Polar, tail-biting convolutional code (TB-CC), and video streaming. The second category is massive machine- Turbo codes, were considered for both eMBB and URLLC type communication (mMTC) which promises to support the data channels. While it was recognized that LDPC , Turbo massive number of machine-type devices and ultra-low power and Polar codes have similar performance at large block sizes, consumption to increase the device lifetime. they could have big performance differences at small block The third service category is ultra-reliable and low latency sizes. LDPC has been already selected for eMBB data channel communication (URLLC) which focuses on delay sensitive and Polar codes were selected for the eMBB control channel. applications and services (see Fig. 1). Factory automation However, recent investigations showed that there exists some and tele-surgery have the strictest reliability requirement of error floors for LDPC codes constructed using base graph (BG) − −9 with an end-to-end latency of less than 1ms. (1 10 ) 2, which has been considered for short block low rate scenarios Other services such as smart grids, tactile internet, intelligent [3]. Moreover, as shown in [3], Polar codes outperform LDPC transportation systems, and process automation have more arXiv:1802.09166v4 [cs.IT] 5 Sep 2018 codes without any sign of error floor. This means that it is not relaxed reliability requirements of − −3 ∼ − −6 at (1 10 ) (1 10 ) straight-forward to extend/modify eMBB channel coding for latencies between 1ms to 100ms [1]. As shown in Fig. 1, 5G URLLC with very diverse latency and data rate requirements. may not be able to achieve the requirements for some industrial It has been clearly specified that channel coding for URLLC and medical applications with a very strict latency requirement should be further studied, especially for information blocks of of less than 1ms and block error rate (BLER) of −9. These 10 less than 1000 bits [3]. systems might need to have their own standards with more In this paper, we compare the main contenders channel M. Shirvanimoghaddam, R. Abbas, C. Yue, Z. Lin, Y. Li, and B. Vucetic are with the School of Electrical and Information Engineering, The University codes for URLLC, with the aim to achieve a favorable trade- of Sydney, NSW, Australia. off between the latency and reliability. We mainly focus on M. S. Mohammadi is with Silicon Laboratories. short blocks, i.e., in the order of a few hundreds bits for A. Minja is with the Faculty of Engineering (aka Faculty of Technical Sciences), University of Novi Sad, 21000 Serbia. URLLC. We review existing short channel codes and compare B. Matuz is with the the Institute of Communications and Navigation of them in terms of rate efficiency at the reliability of interest for the German Aerospace Center (DLR), Munchner Strasse 20, 82234 Wessling, some URLLC applications. We show that existing candidate Germany. G. Han is with the School of Information Engineering, Guangdong Univer- channel codes for URLLC still show a considerable gap to sity of Technology, Guangzhou 510006, China. the normal approximation [4] benchmark, therefore there are S. Johnson is with the School of Electrical Engineering and Computing, still rooms for further improvements. We highlight several The University of Newcastle, NSW, Australia. Corresponding Author: M. Shirvanimoghaddam (email: mah- important research directions to improve the performance of [email protected]) URLLC channel codes. 2 5G eMBB Target 10−2 5G URLLC Target 5G Promise 10−3 ITS −4 traffic efficiency 10 Tactile road safety Internet automated guided vehicles 10−5 urban intersection remote Remote Control −6 consulta- 10 tion Smart Grids Block Error rate Process 10−7 haptic feedback Automation −8 Tele-surgery Factory Automation 10 remote surgery Packaging machines 10−9 Machine tools Printing machines Manufactuing cell 1msec 4msec 10msec 100msec End-to-End Latency Fig. 1. Latency and reliability requirements for different URLLC services. II. KEY METRICS, REQUIREMENTS, AND PERFORMANCE 4) Performance Benchmark: There are two effects which BENCHMARK should be distinguished here to better understand the code design problem for short blocks. The first one is the gap to 1) Latency: In the physical layer, we mainly focus on user the Shannon’s limit, that is if we decrease the block length, the plane latency, which is defined as the time to successfully coding gain will be reduced and the gap to Shannon’s limit will deliver a data block from the transmitter to the receiver via the increase. This is not a problem of code design but is mainly radio interface in both uplink and downlink directions. User due to the reduction in channel observations that comes with plane latency consists of four major components: the time-to- finite block lengths. We will use the normal approximation transmit latency, the propagation delay, the processing latency, (NA) [4], that incorporates the reduction in channel observa- e.g., for channel estimation and encoding/decoding, and finally tions, as the performance benchmark for comparison. For a the re-transmission time. Propagation delay is typically defined coding block of length N, the normal approximation is given as the delay of propagation through the transmission medium, by [4]: and it depends on the distance between the transmitter and receiver. The time-to-transmit latency is required to be in the V −1 1 order of a hundred microseconds, which is much less than the R = C − Q (ǫ)+ log2(N), 1ms currently considered in 4G [1]. rN 2N 2) Reliability: Reliability is defined as the success probabil- where is the code rate, is the channel capacity, is the ity of transmitting K information bits within the desired user R C V plane latency at a certain channel quality. Sources of failure channel dispersion, ǫ is the average block error rate (BLER), from a higher layer perspective are when the packet is lost, and Q(.) is the cumulative distribution function of the standard or it is received late, or it has residual errors. It is essential to normal distribution. Fig. 2 shows the normal approximation for maximize the reliability of every packet in order to minimize different code rates and information block lengths. As can be seen, when the block length increases, the gap to the Shannon’s the error rate, so as to minimize the number of retransmissions. 1 In this paper, we use block error rate (BLER) as a metric to limit [4] decreases . compare different channel codes in terms of reliability. The second effect is the gap to the finite length bounds, 3) Flexibility: The flexibility of the channel coding scheme that is if we decrease the block length, modern codes, such as is an important aspect along with the evaluation of the coding LDPC or Turbo codes, show a gap to finite length bounds. This performance. Bit-level granularity of the codeword size and is often due to the suboptimal decoding algorithms. As can be code operating rate is desired for URLLC [5]. The actual seen in Fig. 2, long term evolution (LTE) Turbo and TB-CC coding rate used in transmission could not be restricted and codes show a considerable gap to the normal approximation optimized for specified ranges [5]. The channel codes therefore at short blocks. However, when the block length of the Turbo need to be flexible to enable hybrid automatic repeat request code increases, the gap to the normal approximation and the (HARQ). The number of retransmissions however needs to be Shannon’s limit decreases. kept as low as possible to minimize the latency.