This paper presents a novel wireless autoencoder (WAE) consisting of neural network (NN) modulator and probabilistic data association detector with the aid of deep unfolding (PDA-DU) in coded multi-input multi-output (MIMO) systems. When using error correction code (ECC), the signal constellation that maximizes the minimum Euclidean distance, such as quadrature amplitude modulation (QAM), is not always optimal. For example, set partitioning is often utilized in higher-order modulation schemes. On the other hand, nonlinear iterative signal detection of PDA has been shown to efficiently suppress interference and to reduce the bit error rate (BER) compared to traditional minimum mean square error (MMSE) spatial filtering. However, PDA may degrade detection performance due to outliers in the expected value of transmitted signals. To solve these complicated issues, the proposed WAE retrieves the best signal constellation using the NN modulator, and simultaneously mitigates outliers in the expected value by introducing scaling factors optimized by DU in the PDA detector.

Global navigation satellite system (GNSS) positioning, including the global positioning system (GPS), is susceptible to multipath effects in urban areas and between tall buildings, which degrade positioning accuracy. In this study, to address this issue, we complement GNSS positioning with information obtained through communication with LTE base stations and use it as features for position estimation. Using this information, we propose a long short-term memory (LSTM)-based neural network (NN) model with a parallel structure. In this model, GNSS positioning information and long term evolution (LTE) signal information are combined and input into LSTM with different structures. Features obtained from multiple information sources are used in a complementary manner to improve the accuracy of position estimation. Furthermore, the positioning accuracy of test data is evaluated to confirm the effectiveness of the proposed method. This evaluation takes into account positioning at unknown locations that are different from those used during training.

This paper proposes deep unfolding (DU)-aided blind signal separation (BSS) for short data block communications of multi-user multi-input multi-output (MU-MIMO) transmission. Independent component analysis (ICA) is a BSS technique that does not require a pilot signal for channel estimation. However, when applying ICA to short data length, inaccurate non-Gaussianity evaluations deteriorate the performance of BSS. To compensate for this drawback, we design a novel DU-aided BSS algorithm as trainable ICA (TICA) that incorporates a data-driven design based on deep learning and learning optimization of internal trainable parameters. Considering that the weight matrix obtained as the outcome of ICA does not converge to an appropriate value for short data length, we introduce two trainable parameters: one for correcting the non-Gaussian evaluation and the other for damping to suppress oscillations. Numerical results show that the proposed algorithm substantially improves the bit error rate (BER) performance with trainable parameter optimization using DU techniques.

In this paper, considering its application in Bluetooth low energy (BLE), we propose a novel self-interference (SI) cancellation method for in-band full-duplex (IBFD) communications in the presence of fractional delay paths. Traditional SI cancellation methods, such as the active SI canceller (ASIC), generate and subtract a replica of the SI signal from the received signal. However, generating this replica requires accurate channel state information (CSI), which increases communication overhead and reduces spectral efficiency. Therefore, the differential active SI canceller (DASIC) has been proposed, which does not require prior CSI information. Nonetheless, the presence of fractional delay paths results in residual SI, making cancellation insufficient. In this paper, we propose a method to generate a replica of the residual SI from DASIC outputs, resolving the residual SI issue. The efficacy of the proposed method is confirmed through computer simulations by comparing its bit error rate (BER) performance with that of ASIC.

This paper proposes a novel blind self-interference (SI) canceller with adaptive differential delay for in-band full duplex (IBFD) communications, even in the presence of a fractional delay path. As an SI cancellation method, active SI canceller (ASIC) that generates and subtracts a replica of the SI signal from the received signal has been proposed. However, the replica generation requires accurate and periodic channel state information (CSI) acquisition, causing system performance degradation due to increased communication overhead. To avoid this inconvenience, differential active SI canceller (DASIC) without prior channel estimation of SI has been proposed. However, due to the effects of a fractional delay path, DASIC cannot completely cancel SI, resulting in residual SI. The proposed method adaptively controls the differential delay to eliminate the residual SI, according to the occurrence pattern of its own transmission signal. In addition, an appropriate maximum a-posteriori probability (MAP) detector is designed based on the DASIC outputs. Finally, computer simulations confirm the efficacy of the proposed method with and without error correction code (ECC) in terms of bit error rate (BER) performance.

We are developing a radio wave sensing technology focusing on antenna impedance. Since this sensing method uses physical phenomena different from conventional sensing technology, it may be able to complement conventional sensors. In the studies, we investigated a sensing method that covers a range of less than several tens of centimeters by measuring the complex voltage reflectance and confirmed its validity by FDTD analysis. However, the sensor circuit for this method had not yet been developed. In this paper, a prototype sensing circuit corresponding to the proposed sensing method was fabricated and evaluated.

This paper investigates the application of interference alignment (IA) to inter-vehicle communications for further performance improvement. In conventional IA in the spatial domain, the number of antennas of each terminal limits the number of terminals, and therefore the sum rate performance is upper-bounded. This paper shows that when IA is applied to inter-vehicle communications at an intersection, it is possible to exceed the sum rate of the conventional IA by taking into account the effect of path loss. To this end, we consider multiple-input multiple-output interference channels with two IA groups, where all interference signals from the same group are aligned at each receiver by virtue of IA, but interference signals from the other group are not. Simulation results show that two IA groups can geographically co-exist by utilizing high propagation loss at a corner of the intersection to mitigate inter-group interference, and thus the achievable sum rate can be significantly improved, compared with that of single- group IA.

In this paper, an energy detection scheme based on the pseudo bit error rate (BER) estimation is investigated. In the proposed scheme, the basic strategy is to use the pseudo BER to estimate the signal-noise-ratio (SNR) of primary signal for energy detection at the cognitive users. Threshold and sensing length are updated with the estimated SNR afterward. We have shown that this modified energy detector outperforms traditional energy detector with the noise uncertainty and reliably alleviate the SNR wall effect.

Multipath delay profile models for ITS wireless communications are presented. Assuming roadside-to-vehicle communications and vehicle-to-vehicle communications, propagation measurement was carried out using 700MHz spectrum and delay profile models are established based on the measured data. In the paper, the method to extract the delay profile models from the measured data is firstly presented. The established models are presented for LOS and NLOS environments. The simulated transmission performance using the model is also shown assuming RC-006 specifications.

We consider secret key agreement based on radio propagation characteristics in a two-way relaying system where two legitimate parties named Alice and Bob communicate with each other via a trusted relay. In this system, Alice and Bob share secret keys generated from measured radio propagation characteristics with the help of the relay in the presence of an eavesdropper. We present four secret key agreement schemes: an amplify-and-forward (AF) scheme, a signal-combining amplify-and-forward (SC-AF) scheme, a multiple-access amplify-and-forward (MA-AF) scheme, and an amplify-and-forward with artificial noise (AF with AN) scheme. In these schemes, the basic idea is to share the effective fading coefficients between Alice and Bob and use them as the source of the secret keys. The AF scheme is based on a conventional amplify-and-forward two-way relaying method, whereas in the SC-AF scheme and the MA-AF scheme, we apply the idea of physical-layer network coding to the secret key agreement. In the AF with AN scheme, the relay transmits artificially generated noise, as well as channel information signal, in order to conceal the latter. Simulation results show that the MA-AF scheme outperforms the other schemes in Rayleigh fading channels, whereas the AF with AN scheme is suitable for Rician fading channels.

The 31 papers in this special issue focus on using the physical layer for securing the next generation of communication systems.

We consider secret key agreement based on radio propagation characteristics in two-way relaying system where two legitimate parties named Alice and Bob communicate with each other via a relay. In this system, Alice and Bob share secret keys generated from their radio propagation characteristics by the help of the relay. In this paper, we present two secret key agreement schemes: an amplify-and-forward (AF) scheme and a multiple-access amplify-and-forward (MA-AF) scheme. In both of the schemes, the basic idea is to share the effective fading coefficient between Alice and Bob and use it as the source of secret keys. The AF scheme is based on a conventional amplify-and-forward two-way relaying method, while the MA-AF scheme utilizes the inherent combining of signals provided by simultaneous transmissions over a multiple-access channel in order to share the secret keys more securely efficiently. We analyze eavesdropping strategy in terms of eavesdropper's location and show that if the eavesdropper is located near the relay and can receive signals from the relay without multipath fading and noise, the AF scheme is not secure. Simulation results show that the MA-AF scheme is more secure and efficient than the AF scheme.

We consider information reconciliation in secret key agreement from a fading channel where two legitimate parties utilize channel estimates of their fading channel as correlated random variables to generate a secret key. The information reconciliation is a process of correcting the discrepancies between the legitimate parties' keys by public discussion in the presence of eavesdropper, where the amount of the information revealed in the public discussion is needed to be as little as possible. In this paper, we propose information reconciliation protocols using the reliability values of channel estimates in order to correct errors effectively. The proposed information reconciliation protocols are modified versions of a protocol used in quantum key distribution called sliced error correction using Cascade. Simulation results show that the proposed protocols can correct errors with less the number of disclosed bits and less the number of communications than the conventional sliced error correction using Cascade.

As a countermeasure for eavesdroppers in wireless communication, a secret key agreement scheme using a variable directional antenna named ESPAR antenna was developed. In this paper, we describe an improved key generation method using multi-level quantization and parity check in the secret key agreement scheme. It enhances key agreement probability in legitimate parties efficiently without reducing the security. There was a trade-off between the key agreement probability and the security. Simulation result shows that the proposed method realizes higher security than the conventional one in the key agreement system using ESPAR antenna.

The paper proposes a new multiuser interference cancellation system in MC-CDMA transmissions. In the proposed system, spreading codes having sinusoidal waveforms are used. It is shown that, by using the spreading scheme, the spreading and multiplexing processes are accomplished by IFFT in the transmitter and FFT in the receiver. Using the unique property of the spreading scheme, the effect of the multiuser interference due to multipath can be limited only between specific combinations of the spreading codes, therefore MLSE by the Viterbi algorithm can reasonably be applied. Simulation results indicate that significant improvement is obtained by the proposed system when compared with conventional MC-CDMA systems using the Walsh codes and MMSEC (minimum mean sequence error combining).

Several positioning systems to obtain precise location information have been researched. However, they have scalability and flexibility problems because they need completely configured space with a large number of sensors. To avoid the problems, we have proposed a self-organizing location estimation method that uses ad hoc networks and Self- Organizing Maps and needs no prepared space with a large number of sensors. In this paper and our demonstration, the proposed method and its characteristic are presented.

Our major contribution in this paper is to devise a MAC protocol that exploits the advantages of directional antenna in ad hoc networks for improved system performance. In order to implement effective MAC protocol in this context, a node should know how to set its transmission direction to transmit a packet to its neighbors and to avoid transmission in other directions where data communications are already in progress. In this paper, we are proposing a receiver-centric approach for direction and communication-aware MAC (DCA-MAC) protocol, so that, nodes become aware of its neighborhood and also the direction of the nodes for communicating directionally. Our proposed directional MAC protocol can be effective in both ITS (intelligent transportation system), which we simulate in string and parallel topology, and in any community network, which we simulate in random topology. The performance evaluation on QualNet network simulator clearly indicates the efficiency of our protocol. In addition, under the fading channel conditions, it's difficult for each node to perceive the status of the neighbors. In AWGN, Ricean and fading environments, we compare and analyze the performance of omnidirectional MAC, conventional directional MAC and our proposed directional MAC

This paper proposes a multihop mobile communications system using an MC-CDMA transmission technique in the forward links. In the system, it is assumed henceforth that each mobile station has a relaying function where a frequency band is used both in the 1st and 2nd hops. Then, we examined a radio resource allocation method in the forward links. In this method, an orthogonal codeword is assigned for each group that consists of one or more mobile stations and the resources of time and frequency are distributed to each link within a group. To clarify the characteristics of this method, we compared the proposed system and a conventional one without relaying, in terms of the base station connection rate, throughput, and so on. Simulation results indicate that the proposed system improves the connection rate without further orthogonal codeword consumption, and user throughput in an appropriate mobile station density.

We measured path loss in a macro cellular environment in 5 GHz band. In this paper, existing propagation models for path loss prediction in UHF band are applied to 5 GHz band, and the estimates are compared with the measured data. From the results of the comparison, we describe the differences between the measured and the estimated data.

The effect of both temporal and spatial dispersion on the performance of a combined configuration of a RAKE and a phased array system is analyzed theoretically. The mean power improvement due to an antenna array system, and fading reduction by means of the RAKE configuration, are described in the presence of the spatial and temporal dispersion of multipath waves.

The mechanism and fundamental nature of the "cycle slip phenomenon" in clock recovery, which triggers the occurrence of burst-type irreducible errors in frequency-selective fading conditions, are theoretically clarified for PSK transmission systems with a coherent detection scheme. The appropriateness of the theory is identified through measurement by means of a fading simulator and a 64 kb/s QPSK modem. Moreover, a simple formula for predicting the frequency of cycle slip occurrence in Rayleigh-fading environments is developed, the accuracy of which is confirmed by means of computer simulation.

In CDMA mobile radio communication systems, degradation of instantaneous signal to interference power ratio (C/I), which causes impairment of frequency utilization factor, will be significant in multipath fading environments. In this paper, a hybrid diversity scheme for fading reduction combining effects of space diversity and path diversity (represented by the RAKE method) is investigated. A quantitative evaluation of fading reduction effects of the hybrid diversity is performed, comparing them with those of space diversity only and path diversity only. The hybrid diversity scheme is promising in environments where the delay spread of a transmission path is 1 /spl mu/s or less.>

Based on the theoretical method developed by Clarke for the analysis of correlation characteristics in Rayleigh fading conditions, we develop a general model for envelope correlation statistics of line-of-sight fading known as Nakagami-Rice fading. The model developed here has a considerable degree of universality and can be applied to the analysis of correlation statistics in frequency and spatial domains for microcellular mobile radio, maritime and aeronautical mobile-satellite communications. Using this model, we examine the frequency correlation statistics of two typical Nakagami-Rice fading environments.