Summary Machine‐to‐machine (M2M) communications being pivotal for internet of things (IoT) networks are characterized by low‐cost, low complexity, and often energy constrained terminals with low traffic duty cycle. Satellite networks provide an attractive low‐cost solution for such application, in particular when the terminals (both fixed and mobile) are distributed over a wide geographical area not well served by terrestrial infrastructure. An ALOHA random access (RA) scheme is a natural candidate for M2M communications since it is well matched to sporadic traffic, and it requires little terminals coordination. However, the classical ALOHA scheme suffers from a low throughput when operated in a load region requiring low probability of packet loss. To overcome this intrinsic ALOHA limitation, in the last decade, a lot of effort has been devoted to the investigation of evolutions of the ALOHA scheme reducing the probability of destructive packet collisions thus making it more attractive for satellite IoT. In such context, the Contention Resolution Diversity ALOHA (CRDSA) scheme and the Asynchronous Contention Resolution Diversity ALOHA (ACRDA) have emerged as promising solutions thanks to their high spectral efficiency achievable with low packet loss probability. The results reported in literature are however assuming ideal demodulator performance. This paper investigates the design and optimization of RA burst demodulator algorithms for single‐frequency and multifrequency CRDSA and ACRDA. It evaluates the performance of such algorithms in a number of system scenarios of practical interest and studies the impact of relevant system parameters on several performance metrics.
SUMMARYThe performance of the widely applied time-domain channel estimation for SISO-and MIMO-OFDM systems strongly depends on the preciseness of information regarding maximum channel impulse response (CIR) length. In practice the required CIR length is usually approximated by the length of the cyclic prefix (CP) which is an upper bound for most of the actual instantaneous CIR length. In this article, we introduce an appropriate channel length estimation method-named as Frequency Domain Channel Length Indicator (FCLI) which estimates the CIR length minimising the estimated mean square error (MSE) of an instantaneous channel estimate. The simulation results present that the FCLI outperforms the approximation of CP length. In addition, we investigate two applications of FCLI on (1) CP adaptation OFDM systems in which we minimise the overhead of the CP by the estimated CIR length and (2) CP-free OFDM systems in which we directly re-construct the structure of cyclic repetition with reduced error. Compared to the traditional channel estimation with the approximation of CP length, the overall system throughput of either CP adaptation or CP-free OFDM transmission scheme is significantly enhanced regardless of perfect synchronisation or practical distribution of time offsets.
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