Frame Length Reduction for Massive-Machine Communications

Machine type communications (MTC) require short- length frames to improve the latency and to achieve high reliability when combined to advanced automatic request (ARQ) mechanisms. The use of short frames has a direct impact on the physical layer, especially on the forward-error correction code (FEC) performance. In this article, lattice based codes are used to achieve an efficient joint decoding of the code and the modulation. The linear group structure of lattices makes it possible to design a decoder based on a sphere decoder. To improve the performance for lower spectral efficiencies, the decoder design takes into account the inevitable lattice shaping processing of the transmitter. With a small number of dimensions, the frame lengths are very short and well suited to MTC. A comparison in frame error rate is performed between the proposed lattice code and an LTE inspired baseline, designed upon the LTE turbo code. Simulation results show that for equivalent spectral efficiencies, a gain in frame length is obtained. This frame length reduction can be employed for increasing the number of users in a machine type communication system.