  To ensure the low end-to-end (E2E) delay for tactile internet, short frame
structures will be used in 5G systems. As such, transmission errors with finite
blocklength channel codes should be considered to guarantee the high
reliability requirement. In this paper, we study cross-layer transmission
optimization for tactile internet, where both queueing delay and transmission
delay are accounted for in the E2E delay, and different packet loss/error
probabilities are considered to characterize the reliability. We show that the
required transmit power becomes unbounded when the allowed maximal queueing
delay is shorter than the channel coherence time. To satisfy quality-of-service
requirement with finite transmit power, we introduce a proactive packet
dropping mechanism, and optimize a queue state information and channel state
information dependent transmission policy. Since the resource and policy for
transmission and the packet dropping policy are related to the packet error
probability, queueing delay violation probability, and packet dropping
probability, we optimize the three probabilities and obtain the policies
related to these probabilities. We start from single-user scenario and then
extend our framework to the multi-user scenario. Simulation results show that
the optimized three probabilities are in the same order of magnitude.
Therefore, we have to take into account all these factors when we design
systems for tactile internet applications.
