University of Hertfordshire

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Original languageEnglish
JournalIEEE Transactions on Vehicular Technology
Publication statusPublished - 20 Jan 2020


Cell-free (CF) massive multiple-input-multiple-output (MIMO) has emerged as an alternative deployment for conventional cellular massive MIMO networks. As revealed by its name, this topology considers no cells, while a large number of multi-antenna access points (APs) simultaneously serves a smaller number of users over the same time/frequency resources through time-division duplex (TDD) operation. Prior works relied on the strong assumption (quite idealized) that the APs are uniformly distributed, and actually, this randomness was considered during the simulation and not in the analysis. However, in practice, ongoing and future networks become denser and increasingly irregular. Having this in mind, we consider that the APs locations are modeled by means of a Poisson Point Process (PPP) which is a more reaslistic model for the spatial randomness. In particular, by virtue of stochastic geometry tools, we derive both the downlink coverage probability and achievable rate. Notably, this is the only work providing the coverage probability and shedding light into this aspect of CF massive MIMO systems. Focusing on the extraction of interesting insights, we consider small-cells (SCs) as a benchmark for comparison. Among the findings, CF massive MIMO systems achieve both higher coverage and rate with comparison to SCs due to properties of favorable propagation and channel hardening. Especially, we showed that increasing the APs density results in lower coverage and increasing the number of users decreases the achievable rate in both architectures but CF massive MIMO systems take advantage of the aforementioned properties and outperform SCs. In general, the performance gap between CF massive MIMO systems and SCs is enhanced with increasing the APs density. Another interesting observation concerns that a higher path-loss exponent decreases the rate while the users closer to the APs affect more the performance in terms of the rate.


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