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Cell-Free Massive MIMO with Nonorthogonal Pilots for Internet of Things

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Document pages: 26 pages

Abstract: We consider Internet of Things (IoT) organized on the principles of cell-freemassive MIMO. Since the number of things is very large, orthogonal pilotscannot be assigned to all of them even if the things are stationary. Thisresults in an unavoidable pilot contamination problem, worsened by the factthat, for IoT, since the things are operating at very low transmit power. Tomitigate this problem and achieve a high throughput, we use cell-free systemswith optimal linear minimum mean squared error (LMMSE) channel estimation,while traditionally simple suboptimal estimators have been used in suchsystems. We further derive the analytical uplink and downlinksignal-to-interference-plus-noise ratio (SINR) expressions for this scenario,which depends only on large scale fading coefficients. This allows us to designnew power control algorithms that require only infrequent transmit poweradaptation. Simulation results show a 40 improvement in uplink and downlinkthroughputs and 95 in energy efficiency over existing cell-free wirelesssystems and at least a three-fold uplink improvement over known IoT systemsbased on small-cell systems.

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