Machine Learning Methods for User Positioning With Uplink RSS in Distributed Massive MIMO

We consider a machine learning approach based on Gaussian process regression (GP) to position users in a distributed massive multiple-input multiple-output (MIMO) system with the uplink received signal strength (RSS) data. We focus on the scenario where noise-free RSS is available for training, but only noisy RSS is available for testing purposes. To estimate the test user locations and their 2σ error-bars, we adopt two state-of-the-art GP methods, namely, the conventional GP (CGP) and the numerical approximation GP (NaGP) methods. We find that the CGP method, which treats the noisy test RSS vectors as noise-free, provides unrealistically small 2σ error-bars on the estimated locations. To alleviate this concern, we derive the true predictive distribution for the test user locations and then employ the NaGP method to numerically approximate it as a Gaussian with the same first and second order moments. We also derive a Bayesian Cramer-Rao lower bound (BCRLB) on the achievable rootmean-squared-error (RMSE) performance of the two GP methods. Simulation studies reveal that: (i) the NaGP method indeed provides realistic 2σ error-bars on the estimated locations, (ii) operation in massive MIMO regime improves the RMSE performance, and (iii) the achieved RMSE performances are very close to the derived BCRLB.