Downlink Transmission of Short Packets: Framing and Control Information Revisited

Abstract

Abstract-Cellular wireless systems rely on frame-based transmissions. The frame design is conventionally based on heuristics, consisting of a frame header and a data part. The frame header contains control information that provides pointers to the messages within the data part. In this paper, we revisit the principles of frame design and show the impact of the new design in scenarios that feature short data packets which are central to various 5G and Internet of Things applications. We treat framing for downlink transmission in an AWGN broadcast channel with K users, where the sizes of the messages to the users are random variables. Using approximations from finite blocklength information theory, we establish a framework in which a message to a given user is not necessarily encoded as a single packet, but may be grouped with the messages to other users and benefit from the improved efficiency of longer codes. This requires changes in the way control information is sent, and it requires that the users need to spend power decoding other messages, thereby increasing the average power consumption. We show that the common heuristic design is only one point on a curve that represents the trade-off between latency and power consumption.

Wireless Access in Ultra-Reliable Low-Latency Communication (URLLC)

“…This results in a lower required bandwidth, as the figure shows. 4 The trick with increasing the blocklength in order to attain a more efficient transmission, given the reliability and the latency constraints, can also be used in a scenario in which a Base Station (BS) broadcasts to multiple terminals, as in [18]. Namely, given the total number of channel uses for broadcast, the BS can concatenate all packets intended for different users and use the resulting large packet as a input to the encoder.…”

“…Intuitively, this offers the highest reliability, but the price is that each node needs to spend energy to decode data that it does not need, as it decodes the whole packet before seeing if there is any data intended for that node and, if yes, extract it. The tradeoff between the reliability and energy expenditure is analyzed in [18]. This technique has been termed concatenateand-code in [19], which has extended the work towards crosslayer scheduling.…”

Energy-Efficient Non-Orthogonal Transmission under Reliability and Finite Blocklength Constraints

“…This new capacity formula explicitly characterizes the relationship between transmission rate, code blocklength and decoding reliability, thus particularly suitable for evaluating the performance of URLLC systems. The capacity of FBC has been successfully applied to the study of wireless communications with strict latency constraints, as in [6][7][8][9][10][11]. For example, reference [6] considered the energy-efficient packet scheduling problem and showed that the classical Shannon capacity may significantly underestimate the energy under FBC.…”

Wireless Access for Ultra-Reliable Low-Latency Communication: Principles and Building Blocks

“…Finally, the two approaches can be considered the extremes of a trade-off between frame duration and power consumption at the devices. In this context, finite blocklength information theory can help in finding the optimal operating point on this trade-off curve, a problem addressed in greater detail in [7].…”