In this letter, we explored the usage of spatio-temporal information in one unified framework to improve the performance of multichannel speech recognition. Generalized cross correlation (GCC) is served as spatial feature compensation, and an attention mechanism across time is embedded within long short-term memory (LSTM) neural networks. Experiments on the AMI meeting corpus show that the proposed method provides a 8.2% relative improvement in word error rate (WER) over the model trained directly on the concatenation of multiple microphone outputs.

In this letter, we present a new expression of ergodic capacity for two-wave with diffuse power (TWDP) fading channels. The derived formula is relatively concise and consists of well-known functions even in infinite series form. Especially, the truncated approximate expression and asymptotic formula are also presented, which enable us to obtain useful and physical insights on the effect of TWDP fading on the ergodic capacity for various fading conditions.

Our previous work proposed a semi-blind single antenna interference cancellation scheme to cope with severe inter-cell interference in heterogeneous networks. This paper extends the scheme to allow multiple-receive-antenna implementation. It does not require knowledge of the training sequences of interfering signals and can cancel multiple interfering signals irrespective of the number of receive antennas. The proposed scheme applies an enhanced version of the quantized channel approach to suboptimal joint channel estimation and signal detection (JCESD) during the training period in order to blindly estimate channels of the interfering signals, while reducing the computational complexity of optimum JCESD drastically. Different from the previous work, the proposed scheme applies the quantized channel generation and local search at each individual receive antenna so as to estimate transmitted symbol matrices during the training period. Then, joint estimation is newly introduced in order to estimate a channel matrix from the estimated symbol matrices, which operates in the same manner as the expectation maximization (EM) algorithm and considers signals received at all receive antennas. Using the estimated channels, the proposed scheme performs multiuser detection (MUD) during the data period under the maximum likelihood (ML) criterion in order to cancel the interference. Computer simulations with two receive antennas under two-interfering-stream conditions show that the proposed scheme outperforms interference rejection combining (IRC) with perfect channel state information (CSI) and MUD with channels estimated by a conventional scheme based on the generalized Viterbi algorithm, and can achieve almost the same average bit error rate (BER) performance as MUD with channels estimated from sufficiently long training sequences of both the desired stream(s) and the interfering streams, while reducing the computational complexity significantly compared with full search involving all interfering signal candidates during the training period.

Channel capacity is a useful numerical index not only for grasping the upper limit of the transmission bit rate but also for comparing the abilities of various digital transmission schemes commonly used in radio-wave propagation environments because the channel capacity does not depend on specific communication methods such as modulation/demodulation schemes or error correction schemes. In this paper, modeling of the noncoherent capacity in a highly underspread WSSUS channel is investigated using a new approach. Unlike the conventional method, namely, the information theoretic method, a very straightforward formula can be obtained in a statistical manner. Although the modeling in the present study is carried out using a somewhat less rigorous approach, the result obtained is useful for roughly understanding the channel capacity in doubly selective fading environments. We clarify that the radio wave propagation parameter of the spread factor, which is the product of the Doppler spread and the delay spread, can be related quantitatively to the effective maximum signal-to-interference ratio by a simple formula. Using this model, the physical limit of wireless digital transmission is discussed from a radio wave propagation perspective.

Based on imperfect channel state information (CSI), the energy efficiency (EE) of downlink distributed antenna systems (DASs) with multiple receive antennas is investigated assuming composite Rayleigh fading channels. A new EE is introduced which is defined as the ratio of the average transmission rate to the total consumed power. According to this definition, an optimal power allocation (PA) scheme is developed for maximizing EE in a DAS subject to the maximum transmit power constraint. It is shown that a PA solution for the constrained EE optimization does exist and is unique. A Newton method based practical iterative algorithm is presented to solve PA. To avoid the iterative calculation, a suboptimal PA scheme is derived by means of the Lambert function, which yields a closed-form PA. The developed schemes include the ones under perfect CSI as special cases, and only need the statistical CSI. Thus, they have low overhead and good robustness. Moreover, the theoretical EE under imperfect CSI is derived for performance evaluation, and the resulting closed-form EE expression is obtained. Simulation results indicate that the theoretical EE can match the corresponding simulated value well, and the developed suboptimal scheme has performance close to optimal one, but with lower complexity.

In this paper, we provide a novel solution to mitigate pilot contamination in massive MIMO technology. In the proposed approach, we consider seven copilot cells of the first layer of interfering cells of a cellular network. We derive and formulate the worst-case signal-to-interference power ratio (SIR) of a typical user in both downlink and uplink of a pilot contaminated cell. Based on the formulated SIR and other considerations of the system, the total pilot sequence length, the reliability of channel estimation within the cell, the spectral and energy efficiencies are derived and formulated in downlink. The user's transmit power and the achievable sum rate are also derived and formulated in uplink. Our results show that when the cell size is reduced the pilot contamination is significantly mitigated and hence the system performance is improved.

Motivated by the observation that certain convolutional channels of a Convolutional Neural Network (CNN) exhibit object specific responses, we seek to discover and exploit the convolutional channels of a CNN in which neurons are activated by the presence of specific objects in the input image. A method for explicitly fine-tuning a pre-trained CNN to induce object specific channel (OSC) and systematically identifying it for the human faces has been developed. In this paper, we introduce a multi-scale approach to constructing robust face heatmaps based on OSC features for rapidly filtering out non-face regions thus significantly improving search efficiency for face detection. We show that multi-scale OSC can be used to develop simple and compact face detectors in unconstrained settings with state of the art performance.

In recent years, since Turbo and LDPC codes are very close to the Shannon limit, a great deal of attention has been placed on the capacity of AWGN and fading channels with arbitrary inputs. However, no closed-form solution has been developed due to the complicated Gaussian integrations. In this paper, we investigate the capacity of AWGN and fading channels with BPSK/QPSK modulation. First, a simple series representation with fast-convergence for the capacity of AWGN is developed. Further, based on the series expression, the capacity of fading channels including Rayleigh, Nakagami and Rice fading can be obtained through some special functions. Numerical results verify the accuracy and convergence speed of the proposed expressions for the capacity of AWGN and fading channels.

In this paper, we present self-interference (SI) cancellation techniques in the digital domain for in-band full-duplex systems employing orthogonal frequency division multiple access (OFDMA) in the downlink (DL) and single-carrier frequency division multiple access (SC-FDMA) in the uplink (UL), as in the long-term evolution (LTE) system. The proposed techniques use UL subcarrier nulling to accurately estimate SI channels without any UL interference. In addition, by exploiting the structures of the transmitter imperfection and the known or estimated parameters associated with the imperfection, the techniques can further improve the accuracy of SI channel estimation. We also analytically derive the lower bound of the mean square error (MSE) performance and the upper bound of the signal-to-interference-plus-noise ratio (SINR) performance for the techniques, and show that the performance of the techniques are close to the bounds. Furthermore, by utilizing the SI channel estimates and the nonlinear signal components of the SI caused by the imperfection to effectively eliminate the SI, the proposed techniques can achieve SINR performance very close to the one in perfect SI cancellation. Finally, because the SI channel estimation of the proposed techniques is performed in the time domain, the techniques do not require symbol time alignment between SI and UL symbols.

In this paper, we investigate the capacity performance of an in-band full-duplex (IBFD) amplify-and-forward two-way relay system under the effect of residual loop-back-interference (LBI). In a two-way IBFD relay system, two IBFD nodes exchange data with each other via an IBFD relay. Both two-way relaying and IBFD one-way relaying could double the spectrum efficiency theoretically. However, due to imperfect channel estimation, the performance of two-way relaying is degraded by self-interference at the receiver. Moreover, the performance of the IBFD relaying is deteriorated by LBI between the transmit antenna and the receive antenna of the node. Different from the IBFD one-way relay scenario, the IBFD two-way relay system will suffer from an extra level of LBI at the destination receiver. We derive accurate approximations of the average end-to-end capacities for both the IBFD and half-duplex modes. We evaluate the impact of the LBI and channel estimation errors on system performance. Monte Carlo simulations verify the validity of analytical results. It can be shown that with certain signal-to-noise ratio values and effective interference cancellation techniques, the IBFD transmission is preferable in terms of capacity. The IBFD two-way relaying is an attractive technique for practical applications.

IEEE 802.11ah is a new wireless standard for large-scale wireless connectivity in IoT and M2M applications. One of the major requirements placed on IEEE 802.11ah is the energy-efficient communication of several thousand stations with a single access point. This is especially difficult to achieve during network initialization, because the several thousand stations must rely on the rudimentary approach of random channel access, and the inevitable increase in channel access contention yields a long association delay. IEEE 802.11ah has introduced an authentication control mechanism that classifies stations into groups, and only a small number of stations in a group are allowed to access the medium at a time. Although the grouping strategy provides fair channel access to a large number of stations, the presence of several thousand stations and limitation that only a group can use the channel at a time, causes the association time to remain excessive. In this paper, we propose a novel block association method that enables simultaneous association of all groups. Our experiments verify that our block association method decreases the total association time by many folds.

Cognitive radio (CR) is considered as the most promising solution to the so-called spectrum scarcity problem, in which channel sensing is an important problem. In this paper, the problem of determining the period of medium access control (MAC)-layer channel sensing in cognitive radio networks (CRNs) is studied. In our study, the channel state is statistically modeled as a continuous-time alternating renewal process (ARP) alternating between the OFF and ON states for the primary user (PU)'s communication activity. Based on the statistical ARP model, we analyze the CRNs with different SU MAC protocols, taking into consideration the effects of practical issues of imperfect channel sensing and non-negligible channel sensing time. Based on the analysis results, a constrained optimization problem to find the optimal sensing period is formulated and the feasibility of this problem is studied for systems with different OFF/ON channel state length distributions. Numerical results are presented to show the performance of the proposed sensing period optimization scheme. The effects of practical system parameters, including channel sensing errors and channel sensing time, on the performance and the computational complexity of the proposed sensing period optimization scheme are also investigated.

Device-to-device (D2D) networks are expected to play a number of roles, such as increasing frequency spectrum efficiency and improving throughput at hot-spots. In this paper, our interest is on the potential of D2D on reducing delivery latency. To enable fast D2D network forming, quick device discovery is essential. For quickening device discovery, we propose a method of defining and using common channel and group channels so as to avoid the channel scan uncertainty faced by the conventional method. Rules for using the common channel and group channels are designed. We evaluate and compare the discovery performance of the proposed method with conventional method by using the superframe structure defined in IEEE 802.15.8 and a general discovery procedure. IEEE 802.15.8 is a standard under development for fully distributed D2D communications. A Netlogo simulator is used to perform step by step MAC simulations. The simulation results verify the effectiveness of the proposed method.

The next generation wireless LAN standard IEEE 802.11ax aims to provide improved throughput performance in dense environments. We have proposed an efficient channel sounding mechanism for DL-MU-MIMO that has been adopted as a new sounding protocol in the 802.11ax standard. In this paper, we evaluate the overhead reduction in the 802.11ax sounding protocol compared with the 802.11ac sounding protocol. Sounding is frequently performed to obtain accurate channel information from the associated stations in order to improve overall system throughput. However, there is a trade-off between accurate channel information and the overhead incurred due to frequent sounding. Therefore, the sounding interval is an important factor that determines system throughput in DL-MU-MIMO transmission. We also evaluate the effect of sounding interval on the system throughput performance using both sounding protocols and provide a comparative analysis of the performance improvement.

In this study, the reference pure metal-oxide semiconductor field-effect transistors (MOSFETs) and low-voltage (LV) and high-voltage (HV) MOSFETs with a super-junction (SJ) structure in the drain side were experimentally compared. The results show that the drain-side engineering of SJs exerts negative effects on the electrostatic discharge (ESD) and latch-up (LU) immunities of LV n-channel MOSFETs, whereas for LV p-channel MOSFETs and HV n-channel laterally diffused MOSFETs (nLDMOSs), the effects are positive. Compared with the pure MOSFET, electrostatic discharge (ESD) robustness (It2) decreased by approximately 30.25% for the LV nMOS-SJ, whereas It2 increased by approximately 2.42% and 46.63% for the LV pMOS-SJ and HV nLDMOS-SJ, respectively; furthermore, LU immunity (Vh) decreased by approximately 5.45% for the LV nMOS-SJ, whereas Vh increased by approximately 0.44% and 35.5% for the LV pMOS-SJ and HV nLDMOS-SJ, respectively. Thus, nMOS-SJ (pMOS-SJ and nLDMOS-SJ) has lower (higher) It2 and Vh, and this drain-side SJ structure of MOSFETs is an inferior (superior) choice for improving the ESD/LU reliability of LV nMOSs (LV pMOS and HV nLDMOS).

In this paper, a new and an accurate symbol error probability's analytical model of Rectangular Quadrature Amplitude Modulation in α-µ fading channel is presented for single-user single-input multi-output environment, which can be easily extended to generalized fading channels. The maximal-ratio combining technique is utilized at the receiving end and unified moment generating functions are used to derivate the results. The fading mediums considered are independent and non-identical. The mathematical model presented is applicable for slow and frequency non-selective fading channels only. The final expression is presented in terms of Meijer G-function; it contains single integrals with finite limits to evaluate the mathematical expressions with numerical techniques. The beauty of the model will help evaluate symbol error probability of rectangular quadrature amplitude modulation with spatial diversity over various fading mediums not addressed in this article. To check for the validity of derived analytical expressions, comparison is made between theoretical and simulation results at the end.

This paper presents experimental results of our proposed null-space expansion scheme for multiuser massive multiple-input multiple-output (MIMO) in time varying channels. Multiuser MIMO transmission with the proposed scheme can suppress the inter-user interference (IUI) caused by outdated channel state information (CSI). The excess degrees of freedom (DoFs) of massive MIMO is exploited to perform additional null-steering using past estimated CSI. The signal-to-interference power ratio (SIR) and spectral efficiency performances achieved by the proposed scheme that uses measured CSI is experimentally evaluated. It is confirmed that the proposed scheme shows performance superior to the conventional channel prediction scheme. In addition, IUI can be stably suppressed even in high mobility environments by further increasing the null-space dimension.

In this paper, we consider a two-hop communication system with an amplify-and-forward (AF) relay under channel estimation errors. According to the channel quality of the link between the base station (BS) and the relay, we investigate two typical relay scenarios. We study the capacity performance for both In-Band Full-Duplex (IBFD) and Half-Duplex (HD) transmission modes. Moreover, we consider two operation modes of the user equipment (UE) for each scenario. Closed-form expressions of ergodic capacities with channel estimation errors are obtained for scenario-1. And we derive accurate approximations of ergodic capacities for scenario-2. Numerical experiments are conducted to verify the analytical results and show that our theoretical derivations are perfectly matched with the simulations. We show that with practical signal-to-noise ratio values and effective interference cancellation techniques, IBFD transmission is preferable in terms of capacity.

This paper proposes a novel method of reducing channel state information (CSI) feedback by using transmit antenna selection for downlink multiuser multiple input multiple output (DL-MU-MIMO) transmission in dense distributed antenna systems. It is widely known that DL-MU-MIMO transmission achieves higher total bit-rate by mitigating inter-user interference based on pre-coding techniques. The pre-coding techniques require CSI between access point (AP) and multiple users. However, overhead for CSI acquisition degrades the transmission efficiency of DL-MU-MIMO transmission. In the proposed CSI feedback reduction method, AP first selects the antenna set that maximizes the received power at each user, second it skips the sequence of CSI feedback for users whose signal to interference power ratio is larger than a threshold, and finally it performs DL-MU-MIMO transmission to multiple users by using the selected antenna set. To clarify the proposed method, we evaluate it by computer simulations in an indoor scenario. The results show that the proposed method can offer higher transmission efficiency than the conventional DL-MU-MIMO transmission with the usual CSI feedback method.

In order to utilize higher frequency bands above 6GHz, which is an important technical challenge in fifth generation mobile systems, radio propagation channel properties in a large variety of deployment scenarios should be thoroughly investigated. The authors' group has been involved in a fundamental research project aimed at investigating multiple-input-multiple-output (MIMO) transmission performance and propagation channel properties at microwave frequency above 10GHz from 2009 to 2013, and since then they have been conducting measurement and modeling for high frequency bands. This paper aims at providing comprehensive tutorial of a whole procedure of channel modeling; multi-dimensional channel sounding, propagation channel measurement, analysis, and modeling, by introducing the developed MIMO channel sounders at high frequency bands of 11 and 60GHz and presenting some measurement results in a microcell environment at 11GHz. Furthermore, this paper identifies challenges in radio propagation measurements, and discusses current/future channel modeling issues as future works.