This research focuses on multiple-input multiple-output signal separation in environments with changing channels, such as mobile communications. When separating signals by combining weights fixed in a packet, the orthogonality between interfering signals and combining weights is broken due to the movement of the receiver, resulting in interference leakage. To mitigate such degradation, a null-space expansion technique in the frequency-domain is employed, assuming that there are extra antennas at the receiver side. This technique improves channel time-varying immunity by using extra receiving antennas to form a null-space that suppresses interference between streams. In this paper, the performance advantage of the null-space expansion technique is revealed through in-lab and field measurement campaigns.

For 6G, the use of AI/ML is one of the key technologies and its application to the air interface is being widely studied. As an effective use of AI/ML for air interface, a deep learning-based pilotless transmission scheme has been proposed where AI/ML is applied to functions such as channel estimation, equalization and demodulation. Several papers have shown through simulation evaluations that the application of AI/ML to the air interface can improve spectral efficiency. However, in all AI/ML applications, it is important to verify how well they perform in real environments. In this paper, a proofof-concept for the deep learning-based pilotless transmission scheme is constructed and experimental results in real indoor environments are shown. The results of the conducted experiments at fixed and walking speeds verify that the scheme improves throughput performance by 6.5-18.2 %.

This research focuses on multiple-input multiple-output signal separation in environments with changing channels, such as mobile communications. When separating linear signals by fixing combining weights to packets, the orthogonality between interfering signals and combining weights is broken due to the movement of the receiver, resulting in interference leakage. To suppress such degradation, we use the null-space expansion technique in the frequency-domain, assuming that there are extra antennas at the receiver side. This technique improves channel time-varying immunity by using extra receiving antennas to form a null space that suppresses interference between streams. Experiments evaluated the performance of the fluctuation tolerance when combining weights were applied to the entire packet.

The transmission performance deteriorates if the combining weights for interference suppression are fixed within a packet during signal separation in linear MIMO reception, since the propagation path changes with time in mobile communications. To solve this problem, we apply the null-space expansion technique, which improves transmission performance by forming additional nulls to suppress inter-stream interference using antenna degrees of freedom. In this study, we consider acquiring the past channel information for the null-space expansion technique from multiple positions in a packet. Error rate performance is evaluated using computer simulations when combining weights generated by this technique are applied to the entire packet.

This paper experimentally demonstrates a novel blockage prediction based on exhaustive beam-pair scans in millimeter wave (mmWave) networks toward 6G. In particular, the prediction uses the signal strength of all transmit/receive beam-pairs through a two-sided scan rather than that of all transmit beams through the one-sided scan assumed in existing studies. It is also possible to predict future line-of-sight (LOS) blockages. The mapping from the signal strength of all beam pairs to future blockages was learned with LightGBM. The experimental results confirm that the blockage prediction through a two-sided scan of all beam pairs is more accurate than that through a one-sided scan.

This paper discusses the device-free localization and communication in intelligent reflecting surface (IRS)-assisted integrated sensing and communication (ISAC) systems. In particular, the localization accuracy and the communication performance are dependent on the configuration of IRS, that is, the direction of IRS-reflected beam, which mitigates the blockage effect in millimeter-wave (mmWave) communications. Localization is realized by using the angular power profile (APP) obtained through beam search. Results of experimental evaluations show that high localization accuracy is achieved by steering the IRS-reflected beam toward the sensing area. In contrast, high communication performance is obtained by adjusting the IRS-reflected beam toward the receiver. These results confirm that in order to achieve high localization accuracy, the IRS-reflected beam should be reconfigured to be pointed in a different direction from that when communicating.

This paper considers the performance improvement of MIMO signal separation using a linear spatial filter in a time-varying channel such as in a mobile environment. The transmission performance deteriorates due to the movement of the receiving station if the combining weight that suppresses interference signals in a packet during linear signal separation is kept fixed. In this paper, the null-space expansion technique is applied to suppress such degradation. This technique improves the robustness against the time-varying channel by additionally forming nulls that suppress inter-stream interference by utilizing the antenna degrees of freedom. The robustness against intra-packet channel fluctuation is studied by computer simulations and the improvement of error ratio performance is confirmed.

This paper proposes handover control based on positioning information extracted from communication signals. The use of high-frequency bands requires the dense deployment of access points to compensate significant distance attenuation and diffraction loss. Dense deployment of access points raises station confusion by increasing candidate access point number. To select the access point, reference signals must be transmitted. Because reference signals cannot include payload, transmission capacity is degraded with more access point candidates and thus reference signals. Therefore, a simple base station selection scheme is needed. High-frequency wireless systems enable station position to be determined by its wideband and highly directional signal. This paper evaluates the handover control proposal by experiments in an ultra-high speed (293 km/h) mobility environment.

The optimal geometric configuration of the cells of a wireless network using multiple antennas for each base station and a macrodiversity technique is investigated. The simplest model of a cell of an existing wireless network is disk shaped, and the cell deployment is arranged in a honeycomb tiling pattern. This model has been used as the first-order approximation for designing and evaluating wireless networks. However, the cells of a network using multiple antennas and macrodiversity are no longer disk shaped. This study investigated a network with a cell-and-antenna deployment pattern that covers a given service area using the minimum number of cells. The objective of this paper is to offer a first-order approximation model for a cell-and-antenna deployment pattern of such a network. For this objective, first, by imposing practical conditions, cell-and-antenna deployment patterns are classified. Then, the asymptotic minimum coverage problem is formulated as an optimization problem with a constraint for a set of deployment patterns. To easily obtain the first-order approximation model, a simplified formulation and model are proposed. Numerical examples show that the proposed deployment pattern covers the service area with nearly half the cells required by the existing heuristic pattern.

Wide area ubiquitous networks should have the capability to handle more than 10,000 wireless terminals in a large wireless cell with the radius of several kilometers. However, the functions of wireless terminal are severely limited due to low power consumption and low cost requirements. Therefore, there is a need for a sophisticated wireless access system that employs smart wireless signal processing such as the site diversity technique with maximal-ratio-combining at the wireless access point. This paper presents the site diversity technique that employs digitized radio over fiber technology and field test that confirms the effect of site diversity.

The multi-target constant modulus algorithm (MT-CMA) can blindly separate multiple co-channel signals captured by an antenna array. It sequentially calculates the unknown weight vectors by executing CMA on a subspace spanned by these vectors, and estimates the weight vectors of all users. Thus, it needs to precisely estimate the subspace by using the calculated weight vectors. The conventional estimation method is to estimate the subspace assuming that the array output signals are orthogonal. However, when the signal-to-noise ratio (SNR) is low, the subspace estimation accuracy is degraded due to the failure of orthogonality. To overcome this problem, the paper proposed a new subspace estimation method for MT-CMA. It estimates the subspace by using eigenvalue decomposition of the received signals' covariance matrix, which is independent of signal orthogonality. Computer simulation results show that the proposed algorithm achieves a 10% improvement of user capacity or 5.0dB improvement in required CNR. Moreover, the BER performance of the proposed algorithm is close to that of MMSE.

This paper proposes a simplified decision-feedback channel tracking scheme for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems designed for wireless LANs. In the proposed scheme, the channel matrix for each subcarrier is tentatively estimated from a replica matrix of the transmitted signal. A number of the estimated channel matrices, each determined for a different timing, are combined, and the previously estimated channel matrices are replaced by the latest ones. The packet error rate (PER) performances for the proposed scheme are evaluated by computer simulations. Assuming 5 GHz band operation, the proposed scheme keeps the PER degradation at PER=10-2to less than 1 dB when the velocity of transmitter is less than 11.8 km/h. The result represents the proposed channel tracking scheme provides sufficient mobility for wireless LAN systems. We also evaluate the computation complexity of the proposed scheme. The proposed scheme requires less than 1% of the complex multipliers needed by the conventional channel tracking scheme based on Kalman filters.

This paper proposes a new signal detection scheme that combines zero-forcing (ZF) and K-best algorithms for orthogonal frequency division multiplexing with space division multiplexing (OFDM/SDM) systems. Among various signal detection algorithms for SDM signals, maximum likelihood detection (MLD) is one of applicable approaches, which achieves the optimal performance. However, it suffers from exponential computational complexity against the number of transmit antennas and modulation order. Thus, we consider the K-best algorithm for the reduction of the computational complexity. We proposed the modified K-best algorithm, which exploits the ZF algorithm for initial symbol estimation. The initial symbol estimation improves the decoding accuracy of the original K-best algorithm. Therefore, the proposed scheme achieves both the reduction of the MLD algorithm's computational complexity and the improvement of the original K-best algorithm's decoding accuracy. Computer simulation results show that the performance degradation from the MLD algorithm is suppressed to just 1 dB or so in the required E/sub b//N/sub o/ for packet error rate (PER) = 10/sup -2/, when either 16 quadrature amplitude modulation (16QAM) or 64QAM is applied with three transmit and three receive antennas. In these cases, about 87% and 99% fewer metric computations are required than the MLD algorithm, respectively. It is presented that the proposed algorithm offers significant reduction of the computational complexity compared to the MLD algorithm.

This paper describes an SDM-COFDM (space division multiplexed-coded orthogonal frequency division multiplexing) prototype employing a hierarchical ICI (inter-channel interference) canceller for MIMO (multi-input multi-output)-based broadband wireless access systems. The prototype has two types of ICI cancellers: multiplicative and subtractive. The multiplicative canceller multiplies the received signals by the estimated inverse propagation coefficient matrix. In addition, the subtractive one generates ICI replicas, subtracts the replicas from the received symbols, weights the subtracted signals by the estimated propagation coefficients, and combines them to obtain diversity gain. Experimental results show that the newly developed prototype with 2/spl times/2 antennas obtains a 1.4 dB improvement in required CNR at the PER of 0.1 compared to that achieved with only the multiplicative canceller. It achieves more than 100 Mbit/s per 18 MHz bandwidth by using 64QAM with the coding rate of 3/4 in frequency selective fading environments.