We study the problem of optimizing the frame structure of a massive MIMO system under channel aging. We argue that the conventional TDD frame structure with lumped training is suboptimal under ...rapidly aging channels. We, therefore discuss a generalized frame structure allowing for the training of a fraction of the total number of users, with the conventional lumped and interspersed training frames as its special cases. We then derive the achievable uplink and downlink rates for this system, incorporating the cost for switching from uplink to downlink and vice versa. The analysis, and the subsequent numerical results clearly bring out the dependence of the rates achievable in a massive MIMO system on the choice of the frame structure.
The time-varying nature of the wireless propagation channel under high user mobilities, termed as channel aging, is a major performance impediment in many communication systems. In this paper, we ...discuss deep learning models for semi-blind channel estimation in a single input single output wireless communication system under channel aging. In our proposed scheme, we first use pilot based training to obtain initial channel estimates. Following this, we treat the detected symbols as pilots and perform further channel estimation using an Encoder-Decoder LSTM network for constant and sliding window schemes. To show the effectiveness of our method, we show the training capabilities of our models and the BER vs SNR graphs for multiple simulations. We discuss integrating these Encoder-Decoder LSTM models with deep learning enabled symbol detection techniques like the DetNet to further improve spectral efficiency. The Encoder-Decoder LSTM network gives us a low BER, with the moving window scheme outperforming the constant window scheme.
In this paper, we investigate the coexistence of a single cell massive MIMO communication system with a MIMO radar. We consider the case where the massive MIMO BS is aware of the radar's existence ...and treats it as a non-serviced user, but the radar is unaware of the communication system's existence and treats the signals transmitted by both the BS and the communication users as noise. Using results from random matrix theory, we derive the rates achievable by the communication system and the radar. We then use these expressions to obtain the achievable rate regions for the proposed joint radar and communications system. We observe that due to the availability of a large number of degrees of freedom at the mMIMO BS, results in minimal interference even without co-design. Finally we corroborate our findings via detailed numerical simulations and verify the validity of the results derived previously under different settings.
In cellular communication systems, it is conventional to assume the absence of a line of sight (LoS) path between the users and theirassociated access points (APs). This assumption however becomes ...questionable in the context of recent developments in the direction of cell-free (CF) massive MIMO systems. In the CF massive MIMO, the AP density is assumed to be comparable with the user density, which increases probability of existence of an LoS path between the users and their associated APs. In this paper, we analyze the performance of an uplink CF massive MIMO system, with a probabilistic LoS channel model. Here, we first derive the effective statistics of this channel model, and argue that their behaviour is fundamentally different from that of the conventional rich scattering channels. Utilizing these statistics, we next compare the rates achievable by CF massive MIMO systems, under both stream-wise and joint decoding at the central processing unit. Following this, we also discuss the centralized MMSE based data detection to obtain a complexity/ performance trade-off. Finally, using detailed Monte-Carlo simulations, we validate our analytical results, and evaluate the performance of the three data detection schemes.
Exact analytical expressions for the average output amplitude gains in the very long-time limit for two coupled oscillators governed by fractional-order intrinsic and external damping, under the ...influence of a multiplicative quadratic asymmetric dichotomous noise affecting the two potential parameters and the coupling factors, and subjected to a noise-free or noise-modulated external periodic force with same frequency, have been derived. The trustworthiness of the analytical expressions has been checked by comparing the numerical results obtained using these for some typical cases with corresponding findings based on numerical simulations. The numerical simulations have also been used to investigate the time-evolution of a representative system in the transient state by studying the probability density and displacements of the two oscillators at different times and for different values of noise intensity. Analytical expressions have been used to obtain plots for gains versus noise intensity to study the effect of (i) variation in mass parameter of one oscillator keeping that of other the same, and (ii) change in the relative values of the two coupling coefficients. Furthermore, the special case where both the potential parameters are taken to be zero, which corresponds to rectilinear motion of the two particles in the absence of fluctuations, has been examined under the influence of the second-order noise and stochastic resonance has been found to occur at lower frequencies of the applied force. This highlights the importance of nonlinear term in the noise, which makes the rectilinear motion to be oscillatory. Also, the key role played by the coupling in this has been brought out.
•Two-coupled fractional oscillators with random potential and coupling parameters are studied.•Amplitude gains due to noise-free and noise-modulated applied periodic forces are compared.•Effect of variation in mass and in coupling coefficients on stochastic resonance is investigated.•SR in two-coupled particles in rectilinear motion under quadratic noise is analysed.•Ab initio numerical simulations have been used to verify accuracy of the analytical results and to study time-evolution of the system in the transient state.
In this letter, we study the performance of a time division duplex (TDD) hybrid massive multiple input multiple output (MIMO) system with imperfect calibration of the transmit and receive radio ...frequency chains. Using deterministic equivalent (DE) analysis, we establish the loss in the achievable rates due to calibration imperfections. We then discuss limited downlink training to estimate the effective downlink channel, and compare the relative merits of this method against frequency division duplexed (FDD) operation. Finally, via detailed numerical simulations, we evaluate the relative performance of no downlink training, limited downlink training, and FDD mode operation in the presence of reciprocity calibration errors.
The time varying nature of the wireless propagation channel causes a mismatch between the true channel at the time of data transmission and its available estimate based on previously received pilot ...symbols, and is known to impair the performance of the massive multiple input multiple output (MIMO) systems. In this paper, we develop and evaluate adaptive data aided channel tracking and data detection algorithms to counter the effects of channel aging for uplink and downlink massive MIMO systems. We first present a recursive least squares (RLS) algorithm for tracking the matrix uplink channel at the base station (BS), and derive bounds on its MSE performance. We also derive a linear complexity stochastic gradient descent (SGD) algorithm for tracking the uplink channel, along with its performance bounds. Following this, we develop RLS and SGD based algorithms for tracking the scalar effective downlink channel at each UE, and derive their performance guarantees. Finally, via Monte Carlo simulations, we validate the efficacy of the algorithms in terms of their mean squared error performance, and demonstrate the gains achievable by channel tracking in the form of the improvement in the symbol error rates.
A primary design goal of the cell-free (CF) massive MIMO architecture is to provide uniformly good coverage to all the user equipments (UEs) connected to the network. However, it has been found that ...this requirement may not be satisfied in case the channels between the access points (APs) and the UEs are mixed LoS/NLoS. In this paper, we try to address this issue via the use of appropriate power control in both the uplink and downlink of a CF massive MIMO system under mixed LoS/NLoS channels. We find that simplistic power control techniques, such as channel inversion-based power control perform sub-optimally as compared to exhaustive search based max–min power control. As a consequence, we propose a particle swarm algorithm (PSA) based power control algorithm to optimize the performance of the system under study. We then use numerical simulations to evaluate the performance of the proposed PSA based solution and show that it results in a significant improvement in the fairness of the underlying system while incurring a lower computational complexity.
We consider a dynamic time division duplex (DTDD) enabled cell-free massive multiple-input multiple-output (CF-mMIMO) system, where each half-duplex (HD) access point (AP) is scheduled to operate in ...the uplink (UL) or downlink (DL) mode based on the data demands of the user equipments (UEs), with the goal of maximizing the sum UL-DL spectral efficiency (SE). We develop a new, low complexity, greedy algorithm for the combinatorial AP scheduling problem, with an optimality guarantee theoretically established via showing that a lower bound of the sum UL-DL SE is sub-modular. We also consider pilot sequence reuse among the UEs to limit the channel estimation overhead. In CF systems, all the APs estimate the channel from every UE, making pilot allocation problem different from the cellular case. We develop a novel algorithm that iteratively minimizes the maximum pilot contamination across the UEs. We compare the performance of our solutions, both theoretically and via simulations, against a full duplex (FD) multi-cell mMIMO system. Our results show that, due to the joint processing of the signals at the central processing unit, CF-mMIMO with dynamic HD AP-scheduling significantly outperforms cellular FD-mMIMO in terms of the sum SE and 90% likely SE. Thus, DTDD enabled HD CF-mMIMO is a promising alternative to cellular FD-mMIMO, without the cost of hardware for self-interference suppression.
In this paper, we consider the problem of channel state information (CSI) acquisition in massive MIMO systems with the users exhibiting non-identical channel aging profiles. For this purpose, we ...derive the per user achievable rates, and the corresponding CSI outage times for different users. We then use these derived results to argue that scheduling the training of all the users, with respect to the outage time of the fastest moving user is suboptimal, and results in unnecessary training overhead. Therefore, we present and evaluate the performance of two simple schemes for scheduling the training of users with different mobilities. Via numerical results, we show that training a subset of users based on the ages of their channel estimates can significantly increase the spectral efficiency of a massive MIMO system.
