In this work, we derive the maximum a posteriori (MAP) symbol detector for a multiple-input multiple-output system in the presence of Wiener phase noise due to noisy local oscillators. As in single-antenna systems, the computation of the optimum receiver is an analytically intractable problem and is unimplementable in practice. In this purview, we propose three suboptimal, low-complexity algorithms for approximately implementing the MAP symbol detector, which involve joint phase noise estimation and data detection. Our first algorithm is obtained by means of the sum-product algorithm, where we use the multivariate Tikhonov canonical distribution approach. In our next algorithm, we derive an approximate MAP symbol detector based on the smoother-detector framework, wherein the detector is properly designed by incorporating the phase noise statistics from the smoother. The third algorithm is derived based on the variational Bayesian framework. By simulations, we evaluate the performance of the proposed algorithms for both uncoded and coded data transmissions, and we observe that the proposed techniques significantly outperform the other important algorithms from prior works, which are considered in this work. Index Terms -Maximum a posteriori (MAP) detection, phase noise, sum-product algorithm (SPA), variational Bayesian (VB) framework, extended Kalman smoother (EKS), MIMO. , IEEE Transactions on Signal Processing 2 I. INTRODUCTIONEmploying multiple-input multiple-output (MIMO) systems has been shown to significantly enhance performance in terms of data rate and link reliability in wireless fading environments [1]. In general, the analysis and design of MIMO system is based on the assumption that the carrier phase is perfectly known at the receiver, and that there is no phase noise in the system. The phase noise manifests in a MIMO system as the random, time-varying phase differences between the oscillators connected to the antennas at the transmitter and the receiver. Practical designs of MIMO systems based on this assumption can result in significant performance losses and have to be addressed appropriately [2]. The detrimental effects of phase noise can be even more pronounced in scenarios where independent oscillators are connected to each transmit and receive antenna (or a subset of them). This scenario is particularly relevant for line-of-sight MIMO systems that operate at carrier frequencies of around 10 GHz or lesser. Here, separate oscillators are needed for each antenna [3], since the antennas are placed far from each other [4]. The scenario under consideration also corresponds to a massive MIMO system [5], [6], where a large number of antennas are placed at the base station and each user terminal is equipped with a single antenna.The problem of designing receiver algorithms in the presence of random, time-varying phase noise due to noisy local oscillators has been studied extensively for single-antenna systems. We refer the readers to [7]-[10] and the references therein. To address the problem of designin...