Karthik Upadhya

Superimposed pilots: An alternative pilot structure to mitigate pilot contamination in massive MIMO

Superimposed pilots are proposed as an alternative to time-multiplexed pilot and data symbols for mitigating pilot contamination in massive multiple-input multiple-output systems. Provided that the uplink duration is larger than the total number of users in the system, superimposed pilots enable each user to be assigned a unique pilot sequence, thereby allowing for a significant reduction in pilot contamination. Channel estimation performance in the uplink is further improved using an iterative data-aided algorithm. Based on approximate expressions for the uplink signal-to-interference-plus-noise ratio, it is shown that superimposed pilots provide a better performance when compared with methods that use time-multiplexed data and pilots. Numerical simulations are used to validate the approximations and the improved performance of the proposed method.

Superimposed Pilots Are Superior for Mitigating Pilot Contamination in Massive MIMO

In this paper, superimposed pilots are introduced as an alternative to time-multiplexed pilot and data symbols for mitigating pilot contamination in massive multiple-input multiple-output (MIMO) systems. We propose a non-iterative scheme for uplink channel estimation based on superimposed pilots and derive an expression for the uplink signal-to-interference-plus-noise ratio (SINR) at the output of a matched filter employing this channel estimate. Based on this expression, we observe that power control is essential when superimposed pilots are employed. Moreover, the quality of the channel estimate can be improved by reducing the interference that results from transmitting data alongside the pilots, and an intuitive iterative data-aided scheme that reduces this component of interference is also proposed. Approximate expressions for the uplink SINR are provided for the iterative data-aided method as well. In addition, we show that a hybrid system with users utilizing both time-multiplexed and superimposed pilots is superior to an optimally designed system that employs only time-multiplexed pilots, even when the non-iterative channel estimate is used to build the detector and precoder. We also describe a simple approach to implement this hybrid system by minimizing the overall inter- and intracell interference. Numerical simulations demonstrating the performance of the proposed channel estimation schemes and the superiority of the hybrid system are also provided.

An array processing approach to pilot decontamination for massive MIMO

We address the problem of pilot decontamination for massive MIMO systems for finite dimensional channels, wherein the channel vectors between the users and the base station are composed of a finite number of discrete paths. The pilot decontamination problem is addressed using high-resolution parametric spectral estimation methods, such as root multiple-signal classification (root-MUSIC), for resolving paths based on their angle-of-arrivals. The paths corresponding to the desired user are identified based on their amplitudes and then the paths are used to form the channel estimate. Our simulations show an improved performance in terms of bit-error rate compared to the existing approaches in certain critical scenarios.

Downlink performance of superimposed pilots in massive MIMO systems in the presence of pilot contamination

In this paper, we analyze the downlink (DL) performance of superimposed pilots in time division duplexing massive multiple-input multiple-output (MIMO) systems, and show that superimposed pilots offer an increased resilience to pilot contamination with respect to time-multiplexed pilots and data. Based on a closed form expression for the DL signal-to-interference-plus-noise ratio (SINR) at the user terminal, we show that the DL SINR increases without bound with an increasing number of antennas at the base station (BS). In addition, we derive the Cramér-Rao lower bound (CRLB) for the channel estimator that uses superimposed pilots. The CRLB is compared with the mean-squared error and we show that the estimator achieves the CRLB asymptotically in the number of antennas at the BS. Simulation results validate our closed-form expressions and the performance of the proposed method.

A risk minimization framework for channel estimation in OFDM systems

We address the problem of channel estimation for cyclic-prefix (CP) Orthogonal Frequency Division Multiplexing (OFDM) systems. We focus on situations wherein knowledge of channel statistics are not available a priori, and model the channel as a vector of unknown deterministic parameters. Since computing the mean-square error (MSE) is not practicable in such a scenario, we propose a novel technique using Steins lemma to obtain an unbiased estimate of the mean-square error, namely Steins unbiased risk estimate (SURE). The channel estimate is obtained from noisy observations using linear and nonlinear denoising functions, whose parameters are chosen to minimize SURE. Based on computer simulations, it is shown that using SURE-based channel estimates for equalization offers an improvement in signal-to-noise performance over existing channel estimation schemes. HighlightsSURE based Channel Estimation using preamble symbol for CP-OFDM systems is proposed.We obtain the channel estimate by minimizing an unbiased estimate of the MSE.The proposed approach does not require a priori knowledge of the channel statistics and outperforms existing methods.

Time-multiplexed / superimposed pilot selection for massive MIMO pilot decontamination

In massive multiple-input multiple-output (MIMO) systems, superimposed (SP) and time-multiplexed (TM) pilots exhibit a complementary behavior, with the former and latter schemes offering a higher throughput in high and low inter-cell interference scenarios, respectively. Based on this observation, in this paper, we propose an algorithm for partitioning users into two disjoint sets comprising users that transmit TM and SP pilots. This selection of user sets is accomplished by minimizing the total inter-cell and intra-cell interference, and since this problem is found to be non-convex, a greedy approach is proposed to perform the partitioning. Based on simulations, it is shown that the proposed method is versatile and offers an improved performance in both high and low-interference scenarios.