This paper presents a field experimental trial on frequency-domain multiplexing of a mixture of multiple numerologies. The goal is to support diverse applications and services in the fifth generation mobile communications (5G) system. Frequency-domain multiplexing of mixed numerology has been discussed for 5G in order to flexibly support the variety of requirements introduced by future applications. However, frequency-domain multiplexing of mixed numerology causes inter sub-band interference. In this paper, we apply filtered-orthogonal frequency division multiplexing (f-OFDM), which is a new waveform, in order to reduce the inter sub-band interference. Through the field trial, we show that f-OFDM realizes mixed numerology while reducing the impact from inter sub-band interference compared to conventional cyclic prefix OFDM. The field trial results show that f-OFDM has no throughput degradation when using more than one guard subcarrier.

The 5th generation of mobile communications system (5G) is nowadays expected to support very diverse applications, devices, and services such as enhanced mobile broadband (eMBB) and the Internet of things (IoTs). In order to meet the future requirements, new 5G candidate radio access technologies have been proposed and studied. For example, Massive multiple-input multiple-output (MIMO) is an attractive technology to further improve the spectrum efficiency and to boost system capacity. This paper presents a large scale field experimental trial of downlink time division duplex (TDD) Massive MIMO using the developed 5G test bed at the 4.5 GHz band. Specifically, the system performance of multi-user (MU) Massive MIMO with different numbers of sets of user equipment (UEs) and spatial layers is investigated, including the impact of different inter-UE separations and UE deployments, which have not been well investigated compared to the previous studies. The experimental results show that the throughput performance for a wide UE separation linearly increases as the number of layers and UEs increases, while the performance improvement is diminished when inter-UE separation narrows. In addition, the maximum total user throughput of 11.29 Gbps, which corresponds to a spectrum efficiency of 79.82 bps/Hz/cell, was achieved with 24 layers. The results indicate that TDD Massive MIMO using the developed 5G test bed greatly improves the spectrum efficiency for large scale MU-MIMO.

Recently, NTT DOCOMO and Huawei conducted a large-scale experimental trial of key technologies for the 5th generation (5G) mobile systems. Downlink multi-user (MU) transmission with massive MIMO in the time-division duplex (TDD) mode is one of the technologies evaluated in this trial. With linear and non-linear downlink precoding schemes, we investigated the practical performance of MU massive MIMO systems under different numbers of users, user distributions, and different radio frequency channel calibration settings with up to 24 users deployed. When linear precoding is used, the cell spectrum efficiency reaches 39 bit/s/Hz, and it reaches 43 bit/s/Hz when non-linear precoding is used. In term of the cell throughput, up to 1.35 Gbps cell throughput was observed with a linear precoder and 100 MHz bandwidth; and 343 Mbps cell throughput was observed with a non-linear precoder and 20 MHz bandwidth. Based on these investigations, we verified the feasibility and performance of a TDD massive MIMO system for 5G mobile systems, and studied the impact of key factors on the system performance.

In line-of sight (LOS) environments, the performance of Multiple-Input Multiple-Output (MIMO) systems depends largely on the difference of the phase difference of direct paths from transmit antennas to each receive antenna. When the phase difference of direct paths are close to each other, the spatial division multiplexing (SDM) channel is not orthogonal to each other so that the signal detection becomes difficult. In this paper, we propose a MIMO system with relative phase difference time-shift modulation (RPDTM) in Rician fading environments. The proposed scheme transmits independent signals from each antenna at each time slot where relative phase difference between signal constellations used by transmit antennas varies with a pre-determined pattern. This transmission virtually changes the phase difference of direct paths from transmit antennas to each receive antenna without lowering data rate and without knowledge of the channels. In addition, forward error correction coding (ECC) is applied to exploit the time slots where the receiver can detect the signals easily to improve the detection performance. From the results of computer simulation, we show that MIMO system with RPDTM can achieve the better bit error rate (BER) than the conventional MIMO system. We also show that the MEMO system with RPDTM is effective by about Rician factor K = 10 dB.

We focus the security and privacy threats in radio interface between evolved node B (eNB, "base station") and user equipment (UE). We identify new threats including several user tracking attacks by various information in MAC and RRC signalling messages, and an active attack with false buffer status reports. Finally, we propose a solution including confidentiality of RRC layer messages, periodic C-RNTI reallocation on one cell; discontinuous sequence number in RRC message, and one time access token for MAC buffer status report.

Array low density parity check (LDPC) codes are high-rate codes that can achieve good error rate performance in additive white Gaussian noise (AWGN) channels. However, array LDPC codes do not support arbitrary code lengths, because the code length of an array LDPC code with good error rate performance is limited to a multiple of a prime number. This paper proposes Size Compatible (SC)-array LDPC codes; they achieve good error rate performance while supporting arbitrary code lengths. We conduct computer simulations to evaluate the block error rate (BLER) performance of SC-array LDPC codes in AWGN channels. We also evaluate the corresponding performance of SC-array LDPC coded multiband OFDM systems for data transmission rates over 1 Gbps in the UWB multipath channel model CM 3. We show that if the submatrix size is a non- prime number, SC-array LDPC codes achieve better error rate performance than the conventional array LDPC codes in AWGN channels. We also show that the SC-array LDPC codes achieve better error rate performance than the punctured array LDPC codes in AWGN channels. We also show that if the submatrix size is a prime number, SC-array LDPC codes achieve the same error rate performance as conventional array LDPC codes in AWGN channels. Moreover, we show that for multiband OFDM systems with data transmission rates of over 1 Gbps, SC-array LDPC codes achieve better error rate performance than conventional array LDPC codes.

Among new technologies, ultra wideband (UWB) is gathering interests as a short-reach transmission system. At present, as one of the prevailing UWB systems, the multiband-orthogonal frequency division multiplexing (MB-OFDM) system has attracted much attention where convolutional code (CC) is used. The main contribution of this paper is to evaluate the bit error rate (BER) and decoding complexity performances of MB-OFDM with concatenated coding scheme and dual carrier modulation (DCM) on IEEE 802.15.3a UWB multipath channel model. We employ the CC for the inner code and the Reed Solomon (RS) code for the outer code in the concatenated coding scheme. From the results of our computer simulation, we show good combination of coding rates in the concatenated coding scheme at the data rates of about 320, 400 480 Mbps. We also show that MB-OFDM with concatenated coding scheme can achieve the better BER than MB-OFDM with only CC. In addition, we show that the BER improvement of MB-OFDM with concatenated coding scheme over that with only CC becomes larger as the data rate becomes higher.

This paper proposes a parallel recursive least square (P-RLS) channel estimator for the iterative maximum a posteriori (MAP) receiver. The targeted system is the low-density-parity-check (LDPC) coded multiple-input-multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) system. The proposed estimator, which can be theoretically derived from the message-passing algorithm, can track a fast fading channel, and allows the parallel implementation of the whole receiver. A simplified version of P-RLS (SP-RLS) that can reduce the computational complexity is also proposed. Compared to the RLS with smoothing and removing (SR-RLS), P-RLS can reduce the computational time by parallelization, and SP-RLS can further reduce the computational time and complexity. Computer simulations show that SP-RLS achieves almost the same packet error rate (PER) performance as that of P-RLS. It also achieves almost the same PER performance as that of SR-RLS with double iterations. Even in this case, the computational time and complexity of SP-RLS are smaller than those of SR-RLS.

This paper proposes novel channel estimation methods for an iterative maximum a posteriori (MAP) receiver. The targeted systems are low-density parity-check (LDPC)-coded multiple-input-multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) mobile communications. By reconsidering the joint processing of the iterative MAP receiver from the viewpoint of the message-passing algorithm in the factor graph, this paper theoretically derives the recursive least squares (RLS) algorithm with smoothing and removing (SR-RLS). The computational complexity of SR-RLS is about two to three times larger than that of the original RLS algorithm. Computer simulations under fast multipath fading conditions show that the MAP receiver using the proposed SR-RLS channel estimator outperforms the one using the conventional RLS channel estimator.

This paper proposes two new types of maximum a posteriori probability (MAP) receivers for multiple-input-multiple-output and orthogonal frequency-division multiplexing mobile communications with a channel coding such as the low-density parity-check code. One proposed receiver employs the expectation-maximization algorithm so as to improve performance of approximated MAP detection. Differently from a conventional receiver employing the minimum mean-square estimation (MMSE) algorithm, it applies the recursive least squares (RLS) algorithm to the channel estimation in order to track a fast fading channel. For the purpose of further improvement, the other proposed receiver applies a new adaptive algorithm that can be derived from the message passing on factor graphs. The algorithm exploits all detected signals but one of targeted time, and can gain a considerable advantage over the MMSE and RLS. Computer simulations show that the first proposed receiver is superior in channel-tracking ability to the conventional receiver employing the MMSE. Furthermore, it is demonstrated that the second proposed receiver remarkably outperforms both the conventional and the first proposed ones.