  Low-latency communication plays an increasingly important role in
delay-sensitive applications by ensuring the real-time information exchange.
However, due to the constraint on the maximum instantaneous power, guaranteeing
bounded latency is challenging. In this paper, we investigate the
reliability-latency-rate tradeoff in low-latency communication systems with
finite-blocklength coding (FBC). Specifically, we are interested in the
fundamental tradeoff between error probability, delay-violation probability
(DVP), and service rate. Based on the effective capacity (EC), we present the
gain-conservation equations to characterize the reliability-latency-rate
tradeoffs in low-latency communication systems. In particular, we investigate
the low-latency transmissions over an additive white Gaussian noise (AWGN)
channel and a Nakagami-$m$ fading channel. By defining the service rate gain,
reliability gain, and real-time gain, we conduct an asymptotic analysis to
reveal the fundamental reliability-latency-rate tradeoff of ultra-reliable and
low-latency communications in the high signal-to-noise-ratio (SNR) regime. To
analytically evaluate and optimize the quality-of-service-constrained
throughput of low-latency communication systems adopting FBC, an
EC-approximation method is conceived to derive the closed-form expression of
that throughput. Our results may offer some insights into the efficient
scheduling of low-latency wireless communications, in which statistical latency
and reliability metrics are crucial.
