Single-carrier (SC) modulation has been proposed as an effective waveform for massive multiple-input multiple-output (MIMO) systems. Previous studies have shown that the time-reversal maximum ratio combining (TR-MRC) is a simple yet powerful method to be used for SC massive MIMO systems under low signal-to-noise (SNR) conditions. However, in moderate to high SNR conditions, especially in realistic scenarios when the number of base station antennas is finite, multiuser interference leads to a significant amount of performance degradation. This interference is due to the residual cross-talk channels between different users after application of TR-MRC. To resolve this problem, in this paper, we propose a frequency domain multiuser detection technique in addition to TR-MRC. Our proposed technique is based on the minimum mean square error (MMSE) criterion. The additional computational complexity of our proposed technique depends only on the number of user terminals and does not grow with the number of base station antennas. This complexity can be further reduced through mathematical approximations in expense of a negligible performance loss as shown through our numerical results. We have investigated the performance of our proposed technique through simulations and computational complexity analysis where we have shown the superiority of our proposed technique to the existing ones in the literature.

Future mobile networks not only envision enhancing the traditional link quality and data rates of mobile broad band (MBB) links, but also development of new control channels to meet the requirements of delay sensitive use cases. In particular, the need for ultra-reliable low-latency communications (URLLC) for many internet of things (IoT) users is greatly emphasized. In this paper, we present a novel spread spectrum waveform design that we propose for transmission of control signals to establish URLLC communications. These control signals are transmitted over the spectral resources that belong to the MBB communications in the network, but at a level that minimally affects these data channels. The proposed waveform, although a direct sequence spread spectrum (DSSS) technique, is designed to take advantage of symbol synchronization available to the OFDM broad band communications in the network. This, clearly, allows simple synchronization with the rest of the network. The proposed DSSS method can transmit single and multiple bits within each OFDM time frame and can serve many user equipment (UE) nodes simultaneously.

We perform an asymptotic study of the performance of filter bank multicarrier in the context of massive multi-input multi-output. We show that the effects of channel distortions, i.e., intersymbol interference and intercarrier interference, do not vanish as the base station (BS) array size increases. As a result, the signal-to-interference-plus-noise ratio (SINR) cannot grow unboundedly by increasing the number of BS antennas, and is upper bounded by a certain deterministic value. We show that this phenomenon is a result of the correlation between the multiantenna combining tap values and the channel impulse responses between the mobile terminals and the BS antennas. To resolve this problem, we introduce an efficient equalization method that removes this correlation, enabling us to achieve arbitrarily large SINR values by increasing the number of BS antennas. We perform a thorough analysis of the proposed system and find analytical expressions for both equalizer coefficients and the respective SINR.

We perform an asymptotic study on the performance of filter bank multicarrier (FBMC) in the context of massive multi-input multi-output (MIMO). We show that the signal-to-interference-plus-noise ratio (SINR) cannot grow unboundedly by increasing the number of base station (BS) antennas, and is upper bounded by a certain deterministic value. This is a result of the correlation between the multi-antenna combining tap values and the channel impulse responses between the terminals and the BS antennas. To solve this problem, we introduce a simple FBMC prototype filter design method that removes this correlation, enabling us to achieve arbitrarily large SINR values by increasing the number of BS antennas.

We study the possibility of removing the cyclic prefix (CP) overhead from orthogonal frequency division multiplexing (OFDM) in massive multiple-input multiple-output (MIMO) systems. We consider the uplink transmission, while our results are applicable to the downlink as well. The absence of CP increases the spectral efficiency in expense of intersymbol interference and intercarrier interference. It is known that in massive MIMO, the effects of uncorrelated noise and multiuser interference vanish as the number of base station antennas tends to infinity. To investigate if the channel distortions in the absence of CP fade away, we study the performance of the standard maximum ratio combining receiver. Our analysis reveals that in this receiver, there always remains some residual interference leading to saturation of signal-to-interference-plus-noise ratio. To resolve this problem, we propose using the time reversal (TR) technique. Moreover, in order to further reduce the multiuser interference, we propose a zero-forcing equalization to be deployed after the TR combining. We compare the achievable rate of the proposed system with that of the conventional CP-OFDM. We show that in realistic channels, a higher spectral efficiency is achieved by removing the CP from OFDM, while reducing the computational complexity.

This paper studies the possibility of eliminating the redundant cyclic prefix (CP) of orthogonal frequency division multiplexing (OFDM) in massive multiple-input multiple-output systems. The absence of the CP increases the bandwidth efficiency at the expense of intersymbol interference (ISI) and intercarrier interference (ICI). It is known that in massive MIMO, different types of interference fade away as the number of base station (BS) antennas tends to infinity. In this paper, we investigate if the channel distortions in the absence of the CP are averaged out in the large antenna regime. To this end, we analytically study the performance of the conventional maximum ratio combining (MRC) and realize that there always remains some residual interference leading to saturation of signal to interference (SIR). This saturation of SIR is quantified through mathematical equations. Moreover, to resolve the saturation problem, we propose a technique based on time-reversal MRC with zero forcing multiuser detection (TR-ZF). Thus, the SIR of our proposed TR-ZF does not saturate and is a linear function of the number of BS antennas. We also show that TR-ZF only needs one OFDM demodulator per user irrespective of the number of BS antennas; reducing the BS signal processing complexity significantly. Finally, we corroborate our claims, as well as analytical results, through simulations.

This paper presents a comparison of the conventional filter bank multicarrier (FBMC) with the generalized frequency division multiplexing (GFDM) and cyclic FBMC (C-FBMC) that have recently been proposed as waveforms candidates for 5G. We identify situations where FBMC outperforms GFDM/C-FBMC, as well as the cases where GFDM/C-FBMC provide more flexibility than FBMC.

Application of filter bank multicarrier (FBMC) as an effective method for signaling over massive MIMO channels has been recently proposed. This paper further expands the application of FBMC to massive MIMO by applying frequency spreading equalization (FSE) to these channels. FSE allows us to achieve a more accurate equalization. Hence, higher number of bits per symbol can be transmitted and the bandwidth of each subcarrier can be widened. Widening the bandwidth of each subcarrier leads to (i) higher bandwidth efficiency; (ii) lower complexity; (iii) lower sensitivity to carrier frequency offset (CFO); (iv) reduced peak-to-average power ratio (PAPR); and (iv) reduced latency. All these appealing advantages have a direct impact on the digital as well as analog circuitry that is needed for the system implementation. In this paper, we develop the mathematical formulation of the minimum mean square error (MMSE) FSE for massive MIMO systems. This analysis guides us to decide on the number of subcarriers that will be sufficient for practical channel models.

In underwater acoustic (UWA) communications, the mobility of communicating vehicles results in a change of the time scale in the received signal. This is called Doppler scaling. Doppler scaling, although small (usually 1%, or smaller), impacts the receiver performance significantly, if uncompensated. This paper suggests a novel method for compensating Doppler scaling in filter bank multicarrier (FBMC) systems. We show that the method of frequency spreading that have already been applied to the design of digital filters can be applied to each subcarrier in FBMC systems to compensate the impact of Doppler scaling. This novel approach compensates for Doppler scaling with a very small addition in the complexity.

This paper presents a study of the candidate waveforms for 5G when they are subject to timing and carrier frequency offset. These waveforms are: orthogonal frequency division multiplexing (OFDM), generalized frequency division multiplexing (GFDM), universal filtered multicarrier (UFMC), circular filter bank multicarrier (C-FBMC), and linear filter bank multicarrier (FBMC). We are particularly interested in multiple access interference (MAI) when a number of users transmit their signals to a base station in an asynchronous or a quasisynchronous manner. We identify the source of MAI in these waveforms and present some numerical analysis that confirm our findings. The goal of this study is to answer the following question, " Which one of the 5G candidate waveforms has more relaxed synchronization requirements?".

Underwater acoustic (UWA) channels are characterized by low availability of bandwidth, large propagation delays and fast varying multipaths. The fast variation of multipaths, in particular, hampers the bandwidth efficiency of UWA channels, as a significant percentage of transmission resources should be allocated to pilots. The problem magnifies further in the case of multiple-input multiple-output (MIMO) channels as the number of pilots increases linearly with the number of transmitting transducers. This paper introduces the concept of massive MIMO (a technology that has recently been proposed as a candidate for 5G wireless communication systems) for UWA channels and shows that the choice of filter bank multicarrier (FBMC) modulation for transmission in a multicarrier system removes the need for pilots, hence achieves a high level of bandwidth efficiency.

Pilot contamination problem in massive MIMO networks operating in time-division duplex (TDD) mode can limit their expected capacity to a great extent. This paper addresses this problem in cosine modulated multitone (CMT) based massive MIMO networks; taking advantage of their so-called blind equalization property. We extend and apply the blind equalization technique from single antenna case to multi-cellular massive MIMO systems and show that it can remove the channel estimation errors (due to pilot contamination effect) without any need for cooperation between different cells or transmission of additional training information. Our numerical results advocate the efficacy of the proposed blind technique in improving the channel estimation accuracy and removal of the residual channel estimation errors caused by the users of the other cells.