For channels that are doubly selective, delay-Doppler (DD) modulation, commonly called orthogonal time frequency space (OTFS) modulation, allows for the correction of both delay and Doppler shifts at the same time. However, the signal using OTFS has a high peak-to-average power ratio (PAPR) due to the precoding in the delay-Doppler domain. To overcome this problem, we propose a single-carrier transmission for delay Doppler shifts (SC-DD) in the framework of OFDM with DFT precoding, where a single time-domain pilot symbol is transmitted in each block with a sufficient separation from the modulated data symbols. At the receiver, the channel delay-Doppler response is estimated on a slot-by-slot basis consisting of multiple blocks. By utilizing the band matrix structure induced by the insertion of the pilot symbol, the low-complexity minimum mean square error (MMSE) equalization can be employed in the time domain. Through computer simulations, the PAPR distribution and bit error rate (BER) performance of the proposed system are compared with those of conventional OTFS, OFDM, and single-carrier transmission with frequency-domain equalization (SC-FDE). It is shown that the proposed SC-DD achieves much lower PAPR than OTFS, while it exhibits slightly higher PAPR compared to SC-FDE due to the pilot symbol insertion. As a result, our proposed SC-DD demonstrates superior BER performance compared to OTFS, taking into account the nonlinear distortion caused by the power amplifier.

We compare the performance of orthogonal frequency division multiplexing (OFDM) system with high-order quadrature amplitude modulation (QAM) and single-carrier (SC) system with amplitude-phase shift keying (APSK) in conjunction with practical bit-interleaved coded modulation (BICM) considering their peak to average power ratio (PAPR) property. BICM-OFDM with high-order QAM is commonly adopted in several recent high-speed wireless communications standards, whereas APSK in conjunction with single-carrier signaling is adopted in satellite communications due to its low PAPR property compared to OFDM systems. In order to make a fair comparison, SC system is implemented as a discrete Fourier transform (DFT) precoding of OFDM such that the spectral efficiency of the two schemes is identical. Furthermore, clipping and filtering (CAF) is applied to the two systems so as to compare them under the same PAPR constraint. Simulation results over a frequency-selective Rician fading channel indicate that if the fading is moderate, SC with APSK can achieve significant gain over OFDM with QAM at the cost of increasing transmitter/receiver complexity.

We propose a practical covert communication technique for IoT networks based on serially concatenated systematic coded (SCSC) wireless steganography, where the secret information is transmitted by orthogonal frequency division multiplexing (OFDM) signal. Instead of sending the Gaussian artificial noise (AN) so as to prevent the detection by the warden, the secret OFDM signal is hidden into the cover DFT-spread OFDM (DFT-s-OFDM) signal as a public signal since it is suitable for low-cost IoT devices due to its low peak-to-average power ratio (PAPR) property. In our proposed system, the public signal is generated from the parity bits of the secret information by utilizing systematic encoding. As a result, the legitimate receiver improves the error rate performance by concatenating the likelihood corresponding to both secret and public signals. On the other hand, since the parity bits are broadcast as the public signal, there is a risk of the secret information being decoded by the unintended receiver, warden. Therefore, we derive the conditions for systematic codes so as to achieve secure covert communications. Through computer simulation, we demonstrate that the coded bit error rate (BER) performance of our proposed scheme significantly outperforms that of the conventional system, and this enhancement can be achieved without the expense of the secrecy evaluated in terms of the probability of event detection error at the warden.

We investigate the spectral efficiency of non-orthogonal frequency division multiplexing (NOFDM). NOFDM is regarded as a frequency-domain faster-than-Nyquist (FTN) signaling with block transmission, where each subcarrier is compressed in the frequency domain by utilizing the discrete fractional Fourier transform (DFRFT). Since pulse shaping filters are not employed, the effective bandwidth of the NOFDM signal should take into account the resulting out-of-band (OOB) radiation. Therefore, we derive a closed-form expression for the power spectral density (PSD) of the NOFDM signal with windowing for OOB reduction. Through numerical comparison, it is shown that NOFDM outperforms OFDM in view of the effective signal bandwidth if they are compared under the same number of subcarriers. On the other hand, by the capacity analysis based on the asymptotic spectral efficiency with a large number of subcarriers, we elucidate that NOFDM may not outperform OFDM from an information-theoretic perspective.

We propose a power efficient non-orthogonal multiple access (NOMA) scheme suitable for Internet of Things (IoT) and device-to-device (D2D) communications. OFDM-based power-domain NOMA (PD-NOMA) is one of the promising approaches to achieve simultaneous connections of massive devices. Nevertheless, the problem of high peak-to-average power ratio (PAPR) is a major challenge, since power amplifier (PA) efficiency is a critical factor for IoT and D2D devices consisting of low-cost hardware components. This paper overcomes this problem by introducing time-frequency domain NOMA (TF-NOMA), where OFDM and DFT-spread OFDM (DFT-s-OFDM) are superposed over the time-frequency domain. By utilizing the low PAPR property achieved by DFT-precoding, TF-NOMA can efficiently reduce the PAPR of the downlink signal transmitted by the base station (BS) as well as the uplink signals of the devices located far from BS. As a result, the proposed TF-NOMA can operate with higher PA efficiency than the conventional OFDM-based PD-NOMA. Furthermore, the insertion of DFT-precoding with the aim of PAPR reduction also enhances the error propagation resistance due to the resulting Gaussian mixture signal property. Throughout the mathematical analysis and computer simulations, we demonstrate that TF-NOMA achieves better error rate performance than the conventional PD-NOMA with significant improvement of PA efficiency.

Toward further enhancement of the fifth generation (5G) mobile communication system, referred to as 5G Evolution (5GE), more stable high data-rate transmission should be required even in high-mobility environments such as high-speed trains. By using 28 GHz-band experimental equipment with beamforming and beam tracking, a 5GE experimental trial had been conducted in the actual high-speed bullet train running at a speed of 283 km/h. However, it is important to evaluate transmission performances and robustness against the higher mobile speed in sub-6 GHz band in addition to millimeter-wave band. Thus, we conducted 4.85 GHz-band 5GE experimental trial aboard Shinkansen test train running at 360 km/h, which is the fastest experiment trial in Japan. This trial deployed two base station antennas along the Shinkansen railways, and a mobile station was mounted in the rolling stock of the Shinkansen test train. This paper introduces 4.85 GHz-band 5GE experimental equipment, an overview of the experimental trial, and measured transmission performances.

We propose a new interference cancellation (IC) approach designed for coded uplink multiple-input multiple-output (MIMO) systems that support a massive number of Internet-of-Things (IoT) devices. The performance of MIMO spatial multiplexing depends on the symbol detection schemes, which in general offer a trade-off between the error rate performance and computational complexity. Previous studies have shown that a simple matched-filter (MF)-based detector with powerful channel coding can achieve near-optimal performance as the number of base station (BS) antennas increases even without IC. However, if the number of BS antennas is the same order as that of the IoT devices supported by the BS, its achievable rate will be significantly degraded. Furthermore, channel estimation errors due to the lack of orthogonal pilot sequences should also become a dominant factor. This paper overcomes this problem by introducing a MF detector with quadrature interference cancellation (MF-QIC), which can improve error performance and spectral efficiency. Taking polar codes as our channel coding example, we propose a code design based on mutual information tailored for MF-QIC. Our simulation results demonstrate that the proposed low-complexity polar-coded MF-QIC outperforms other conventional approaches even in the presence of imperfect channel state information.

Spatial modulation (SM) exploits multiple transmit antennas for index modulation and thus alleviates the complexity issues inherent in the conventional MIMO schemes. Even though a significant amount of studies have been devoted to SM, its coded performance combined with low-complexity linear detectors has not been well investigated. In this work, we propose a practical low-complexity SM approach based on the combination of the scaling matched-filter (MF) detector and capacity-approaching channel codes. The scaling MF detector is sub-optimal but can be implemented with significantly less complexity than the maximum likelihood (ML) detector. The major challenge in the design of detectors for coded SM is how to calculate the soft output corresponding to the coded bits associated with index modulation that will be processed by the subsequent binary channel decoder, and we develop suitable soft output metrics based on the theoretical analysis of the associated scaling MF detector output. Through computer simulations, the effectiveness of our proposed coded SM system is demonstrated.

Among various MIMO detectors, the matched filter (MF) detector is known as the least complex receiver. It has been shown that with powerful channel coding, MF detector works well under the low SNR regime with diversity-rich environment, such as ergodic fading channels with a large number of antennas. On the other hand, if the diversity order is not sufficient, the performance of MF detector becomes poor. In this paper, we first review the fact that the per-antenna mutual information of MF detector will decrease as the number of both transmit and receive antennas increases over an ergodic Rayleigh fading channel. Since the use of powerful channel codes is essential for the MIMO system with MF detector, we apply polar codes with CRC-assisted successive cancellation list decoder and demonstrate its superiority over the conventional turbo codes. Focusing on such a practical scenario, we further investigate the use of interference cancellation (IC) with MF detector as a low- complexity alternative. After performance comparison of several IC schemes, we also propose an adaptive IC switching approach that can achieve the best performance over a wide range of SNR.

The matched-filter (MF) detector is the lowest complexity linear detector in practical MIMO spatial multiplexing systems. Its performance is significantly affected by the number of available antennas and an operating channel SNR. In this paper, we analyze the exact performance of the MF detector without any restrictions on the numbers of the transmit and receive antennas. To this end, we first develop the exact, closed-form expression of the MF detector output over uncorrelated Rayleigh fading channels with an arbitrary number of antennas. Based on this result, we derive the exact closed-form bit error rate expressions for uncoded BPSK, QPSK, and M-ary QAM signaling, which can be seen as the generalized forms of the conventional receiver diversity based on maximum ratio combining. We also elucidate the conditions on the numbers of antennas and channel SNR, where the use of MF detector should become effective. Furthermore, in the case of coded MIMO systems, we develop the optimal metric derived from the exact MF output. The corresponding mutual information justifies the simulated performance of LDPC-encoded MIMO spatial multiplexing with the MF detector over ideally interleaved Rayleigh fading channels. Finally, we extend our analysis to more practical systems with imperfect channel estimation.

We theoretically analyze the performance of the matched filter (MF) detector for multiple-input multiple- output (MIMO) systems with an arbitrary number of transmit and receive antennas over uncorrelated Rayleigh fading channels. As is the case with the MIMO systems with a large number of antennas, the MF detector is known as the simplest practical linear detector and thus is of significant practical interest when the complexity of the receiver is severely limited. Despite its poor uncoded performance in high signal to noise ratio (SNR) region, with the help of powerful channel codes, it has been shown that near optimal performance can be approached in low SNR with substantially low computational complexity. Nevertheless, its exact analysis in terms of bit error rate (BER) even in the case of uncoded scenarios has not been well established. In this work, we analyze the distribution of the output of MF detector, and develop the exact BER expressions for the uncoded MIMO systems with an arbitrary number of transmit and receive antennas. Furthermore, we develop an optimal soft-output of MF detector for the coded scenario. The accuracy of our derived expressions is verified by the corresponding simulation results.

With a large number of antennas at the transmitter and receiver, massive multiple-input multiple-output (MIMO) systems can achieve substantially high spectral efficiency as well as high stability and reliability. Nevertheless, one of the major challenges for its practical use is its computational complexity for symbol detection that grows exponentially with the number of antennas. In this work, we focus on a low complexity implementation of massive MIMO detector based on matched filter (MF) detection with parallel interference cancellation (PIC). In general, MF-based detector performs poorly in the high SNR region. Nevertheless, with the help of powerful channel coding such as low-density parity-check (LDPC) codes and the well-designed PIC, the resulting error performance of the proposed MF-PIC can approach or even outperform the conventional minimum mean square error (MMSE)-based detector with much less complexity. Furthermore, due to its parallel structure, low latency implementation based on parallel architecture is possible and thus is suitable for practical massive MIMO systems.