Downlink Transmission of Short Packets: Framing and Control Information Revisited

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 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 tradeoff between latency and power consumption.

I. Introduction

Wireless networks heavily depend on reliable and efficient transmission of large data packets through the use of coding and information theory. The advent of machine-to-machine (M2M), vehicular-to-vehicular (V2V), and various streaming systems have spawned a renewed interest in developing information theoretical bounds and codes for communication of short packets [1]–[3]. Additionally, these applications often have tight reliability and latency constraints compared to typical wireless systems today. Communication at shorter blocklengths introduces several new challenges which are not present when considering communication of larger data packets. For example, the overhead caused by control signals and header data is insignificant if large data packets are sent, and hence, this overhead is often neglected in the analysis of protocols. However, more stringent latency requirements lead to shortened blocklengths for transmission such that the size of control information may approach the size of the data part in packets. This is especially true for multiuser systems such as broadcast channels, two-way channels, or multiple access channels, where the control information must include information about the packet structure, security, and user address information for identification purposes.