Encoding of control information and data for downlink broadcast of short packets

It is almost an axiom that every cellular wireless system, including the upcoming 5G systems, should be based on data transmissions organized in frames. The frame design is based on heuristics, consisting of a frame header and data part. The frame header contains control information that specifies the sizes of the data packets and provides pointers to their location within the data part. In this paper we show that this design heuristics is suboptimal when the messages in the data part are short. We consider a downlink scenario represented by an AWGN broadcast channel with K users, while the sizes of the messages to the users are random variables. Each data packet encodes a message to one user. However, if the message sizes are small, there is a significant overhead caused by the header and the data packets can not be encoded efficiently. This calls for revision of the established heuristics for framing control information and data. We show that grouping messages of multiple users allows more efficient encoding from a transmitter perspective. On the other hand, it has the undesirable implication that it requires each user to decode the messages of a whole group of users. We assume that the power spend by each user is proportional to the number of channel uses it needs to decode. Using recent results in finite blocklength analysis, we investigate the trade-offs between total transmission time from the transmitter perspective and the average power spend at each user. Our approach shows that the space of feasible protocols is significantly enlarged and thereby allows the designer to trade-off between average total transmission time and the average power spend by each user.