principles of ultra-reliable low latency communications (URLLC) Petar Popovski Aalborg University Denmark petarp@es.aau.dk 5G V2X Communications @ KCL, London, June 11, 2018
outline ▪ future connectivity landscape ▪ URLLC performance and statistics ▪ URLLC building blocks ▪ wireless network slicing in 5G 5G V2X Communications @ KCL, London, June 11, 2018
the future wireless connectivity landscape ▪ can be seen as eigenvalues for composing services, e.g. in Virtual Reality, rather than three isolated services. 5G V2X Communications @ KCL, London, June 11, 2018
future wireless connectivity landscape 5G but a lot of (great!) other wireless systems ▪ connectivity type not necessarily provided by the 5G radio interface ▪ LPWA, 802.11ah, etc. 5G V2X Communications @ KCL, London, June 11, 2018
distilled service requirements eMBB ▪ acceleration of 4G, large payloads, active over longer periods maximize rate , moderate reliability (e.g. 10E-3) ▪ mMTC ▪ fix low rate, unknown active subset from a massive device set maximize arrival rate , low reliability (e.g. 10E-1) ▪ URLLC ▪ intermittent transmissions, but from a much smaller device set offer high reliability (e.g. 10E-5) while localized in time ▪ 5G V2X Communications @ KCL, London, June 11, 2018
the IoT modes: massive and ultra-reliable access 100 Mbps 95% of the time 1 Mbps from 100 devices or or 100 kbps 99.999% of the 10 kbps from 10000 time devices data rate access reliability limit data rate protocol for control limit information # devices error probability 5G V2X Communications @ KCL, London, June 11, 2018
adoption of ultra-reliable communication we need to divide the applications into two groups ▪ cable replacement how would we design a system if we could trust to the wireless as much as to the wired? ▪ ” native ” wireless applications which new systems can we think of once we are empowered with wireless connectivity? 5G V2X Communications @ KCL, London, June 11, 2018
MTC use cases URLLC mMTC ▪ commercial and public safety ▪ environmental monitoring of large areas ▪ industrial control and automation ▪ large infrastructures ▪ smart energy and smart grid roads, ports, industrial plants ▪ V2X and UAV control ▪ available parking places ▪ Augmented Reality (AR) ▪ management of object fleets and digital interaction with vehicles, bicycles physical objects 5G V2X Communications @ KCL, London, June 11, 2018
a simple communication-theoretic model a communication engineer models known unknowns user activity 𝑧 = ℎ ∙ 𝛽 ∙ 𝑦 + 𝑨 + 𝑗 noise channel interference state objective: find 𝛽 and, if 𝛽 = 1 , find also 𝑦 5G V2X Communications @ KCL, London, June 11, 2018
a simple communication-theoretic model 𝑧 = ℎ ∙ 𝛽 ∙ 𝑦 + 𝑨 + 𝑗 ultra-reliability requires to ▪ model accurately the known unknowns ▪ bound the impact of the unknown unknowns ▪ the standard culprit 𝑨 seems easy ▪ interference can be arbitrarily varying 5G V2X Communications @ KCL, London, June 11, 2018
a simple communication-theoretic model 𝑧 = ℎ ∙ 𝛽 ∙ 𝑦 + 𝑨 + 𝑗 sources of uncertainty ▪ activity 𝛽 is the problem of a MAC protocol ▪ ℎ is the problem of channel estimation and channel knowledge ▪ 𝑗 is a matter of interference management and spectrum regulation – spectrum license is paid to acquire the right to control interference. 5G V2X Communications @ KCL, London, June 11, 2018
the user activity 𝛽 the worst case is when there is no prior information about the user activity ▪ random access grant-free access means that the packet reception in the uplink is not conditioned on a correct downlink reception ▪ can improve latency, even reliability its uncertainty is removed by ▪ scheduling ▪ the receiver predicts the activity variable 5G V2X Communications @ KCL, London, June 11, 2018
URLLC performance and statistics 5G V2X Communications @ KCL, London, June 11, 2018
latency-reliability characterization t R : time of data reception reliability Pr(𝑢 𝑆 ≤ 𝑢) diversity: 1 1- Pe time? frequency antennas interfaces latency t 5G V2X Communications @ KCL, London, June 11, 2018
design targets broadband rate- ultra-reliable low latency oriented systems communication URLLC reliability reliability 1 1 latency latency 5G V2X Communications @ KCL, London, June 11, 2018
a simple error model 𝛿 𝑡 Pr 𝐹 = Pr < 𝛿 𝑢ℎ 1 + 𝛿 𝐽 SINR ▪ in absence of interference, we need to characterize the lower tail of 𝛿 𝑇 ▪ if 𝛿 𝑇 is known, we need to characterize the upper tail of 𝛿 𝐽 5G V2X Communications @ KCL, London, June 11, 2018
channel uncertainty in URLLC ▪ assume that the interference is absent. ▪ we (somehow) know that the channel is Rayleigh. ▪ the target error rate is 𝜁 𝑉 , average SNR is ҧ 𝛿 𝑇 how do we choose the rate R? Pr 𝐹 = Pr log 2 1 + 𝛿 𝑡 < 𝑆 1 𝑆 = log 2 1 + ҧ 𝛿 𝑇 ln 1 − 𝜁 𝑉 where is the problem? 5G V2X Communications @ KCL, London, June 11, 2018
channel uncertainty in URLLC ▪ the knowledge of average SNR is based on n collected samples ▪ when n is low, the rate should R be chosen very conservatively ▪ online update of the estimate and rate (or power) adaptation P. Popovski et al., ”Ultra -Reliable Low Latency Communication is Difficult: A Statistical Assessment”, in preparation 5G V2X Communications @ KCL, London, June 11, 2018
building blocks for URLLC 5G V2X Communications @ KCL, London, June 11, 2018
▪ channel models ▪ transmission of short packets ▪ high diversity ▪ lean protocol design with respect to latency – focus on control information ▪ network architecture ▪ wireless slicing and coexistence with other services 5G V2X Communications @ KCL, London, June 11, 2018
▪ channel models ▪ transmission of short packets ▪ high diversity ▪ lean protocol design with respect to latency – focus on control information ▪ network architecture ▪ wireless slicing and coexistence with other services 5G V2X Communications @ KCL, London, June 11, 2018
wireless channel model behavior in ultra-reliable regime currently there is lack of experimental evidence for URC-relevant statistics of wireless channels initial analysis of common wireless channel models in URC regime ▪ block fading ▪ 𝑄 𝑆 is the minimal SNR to decode data rate 𝑆 ▪ the analysis reveals the URC-behavior: 𝛾 Pr 𝑄 ≈ 𝜁 ≈ 𝛽 𝑄 𝑆 𝑆 𝑄 < 𝑀 ത ത 𝑄 5G V2X Communications @ KCL, London, June 11, 2018
outdoor physical setup outdoor physical setup H TX H RX λ λ ⇡ ⇡ − − Γ λ λ ⇡ ⇡ fl two-wave model with equal amplitudes φ ⇢ ⇢ represents one of the worst cases 5G V2X Communications @ KCL, London, June 11, 2018
indoor physical setup indoor case dominated by diffuse components, good for high reliability P. Eggers, M. Angjelichinoski, and P. Popovski, ”Wireless Channel Modeling Perspectives for Ultra- Reliable Low Latency Communications”, available on Arxiv, 2018. 5G V2X Communications @ KCL, London, June 11, 2018
an example of a short packet format UNB (ultra narrowband) system reliability of the packet reception is a product of the reliabilities of different parts Pr 𝑡𝑣𝑑𝑑𝑓𝑡𝑡 = Pr 𝑄𝐵 Pr 𝑡𝑧𝑜𝑑 Pr 𝐽𝐸 Pr 𝑒𝑏𝑢𝑏 … 5G V2X Communications @ KCL, London, June 11, 2018
▪ repetition coding for control information inefficient ▪ the proverbial 1-bit feedback becomes questionable Philippe Petit, 5G V2X Communications @ KCL, London, June 11, 2018 http://www.msnbc.com/msnbc/philippe-petit-twin-towers-balancing-act-remembered#slide1
communication theory and protocol information 5G V2X Communications @ KCL, London, June 11, 2018
fundamental theory of finite blocklength transmission at short blocklengths there is a penalty that keeps the rate away from capacity. AWGN SNR 0 dB 5G V2X Communications @ KCL, London, June 11, 2018
gain in reliability ▪ low SNR ▪ 10 bytes control information ▪ 10 bytes data ▪ same amount of channel uses probability of error probability of error 10e-3 10e-6 5G V2X Communications @ KCL, London, June 11, 2018
mixing data and control information has energy cost frequency frequency M M D D time time the notion of frame in cellular systems should be revisited 5G V2X Communications @ KCL, London, June 11, 2018
connection between packetization and energy efficiency Alice separated data and metadata useful for energy efficiency Bob Carol data for Bob, Carol turns off her receiver after the metadata Alice joint data and metadata better coding of the metadata Bob Carol however, everybody decodes everything 5G V2X Communications @ KCL, London, June 11, 2018
some observations ▪ basic tradeoff between energy efficiency and ultra-reliability ▪ departure from the common causal relationship metadata -> data ▪ low latency usually means sending with few channel uses (DoF) – DoF can be increased in e.g. frequency or space 5G V2X Communications @ KCL, London, June 11, 2018
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