Multi-user multiple input multiple output (MU-MIMO) systems have attracted much attention as a technology that enhances the total system capacity by generating a virtual MIMO channel between a base station and multiple terminal stations. Extensive evaluations are still needed because there are many more system parameters in MU-MIMO than in single user (SU)-MIMO and the MU-MIMO performance in actual environments is still not well understood. This paper describes the features and effectiveness of a 1616 MU-MIMO testbed in an actual indoor environment. Moreover, we propose a simple adaptive modulation scheme for MU-MIMO-OFDM transmission that employs a bit interleaver in the frequency and space domains. We evaluate the frequency efficiency by obtaining the bit error rate of this testbed in an actual indoor environment. We show that 1644-user MU-MIMO transmission using the proposed modulation scheme achieves the frequency utilization of 870 Mbps and 1 Gbps (respective SNRs: 31 and 36 dB) with a 20-MHz bandwidth.

Efficient use of heterogeneous wireless access networks is necessary to maximize the capacity of the 5G mobile communications system. The wireless local area networks (WLANs) are considered to be one of the key wireless access networks because of the proliferation of WLAN-capable mobile devices. However, throughput starvation can occur due to the well-known exposed/hidden terminal problem in carrier sense multiple access with collision avoidance (CSMA/CA) based channel access mechanism, and this problem is a critical issue with wireless LAN systems. This paper proposes two novel schemes to identify starved access points (APs) and user equipments (UEs) which throughputs are relatively low. One scheme identifies starved APs by observing the transmission delay of beacon signals periodically transmitted by APs. The other identifies starved UEs by using the miscaptured beacon signals ratio at UEs. Numerous computer simulations verify that that the schemes can identify starved APs and UEs having quite low throughput and are superior to the conventional graph-based identification scheme. In addition, AP and UE management with the proposed schemes has the potential to improve system throughput and reduce the number of low throughput UEs.

This paper proposes a simple adaptive channel estimation scheme for orthogonal frequency division multiplexing (OFDM) in order to realize high-rate wireless local area networks (LANs). The proposed estimator consists of simple frequency-domain FIR filters, which are adaptively selected according to the difference vector between adjacent subcarriers and channel amplitude of the subcarrier. No precomputation or matrix signal processing is required in the derivation of these characteristics. Computer simulations show that the packet error rate performance of the proposed scheme is superior to that of the least-squares scheme by 1.1 dB in terms of required Eb/N0 at PER=0.1 in AWGN channels. They also show, for the same criterion, a 0.7 dB improvement in a frequency selective fading channel with delay spread values of 100 ns.

This paper proposes a fast synchronization scheme with a short preamble signal for high data rate wireless LAN systems using orthogonal frequency division multiplexing (OFDM). The proposed OFDM burst format for fast synchronization and the demodulator for the proposed OFDM burst format are described. The demodulator, which offers automatic frequency control and symbol timing detection, enables us to shorten the preamble length to one quarter that of a conventional one. Computer simulation results show that the degradation in required Eb/N0 due to the synchronization scheme is less than 1 dB in a selective Rayleigh fading channel.

This paper proposes a half-chip offset QPSK (Quadrature Phase Shift Keying) modulation CDMA (Code Division Multiple Access) scheme to allow the simple differential detection while realizing a compact spectrum in nonlinear channels for wireless LAN systems. The experimental results show the proposed scheme achieves excellent Pe (probability of error) performances in ACI (adjacent channel interference) and CCI (co-channel interference) environments. Moreover, by employing time diversity and high-coding-gain FEC (Forward Error Correction), the half-chip offset QPSK-CDMA scheme realizes an improvement of 3.0 dB (in terms of Eb/No at a Pe of 105) in Rician fading environments with a Doppler frequency fD of 10 Hz and a delay spread of 40 nsec.

Orthogonal frequency division multiplexing (OFDM) signals suffer nonlinear distortion within the transmitter, in the high-power amplifiers in particular. This paper proposes a nonlinear distortion compensation technique for OFDM signals, which incorporates a baseband predistortion with a constant peak-power OFDM (CP-OFDM). CP-OFDM fixes the envelope-peak amplitude to a constant value while maintaining the linearity of the inverse fast Fourier transform (IFFT) outputs; consequently, the baseband predistorter entirely compensates the nonlinear distortion. Simulation and experimental results verify that the proposed technique significantly reduces out-of-band power emission and adjacent channel interference (ACI) more than 10 dB respectively.

MIMO communication systems have been studied as a way to increase system capacity and to improve spectrum efficiency. This paper proposes a novel branch metric generation method for soft-decision Viterbi decoding in coded OFDM systems employing SDM (OFDM-SDM) systems, one form of MIMO communication, that employ the maximum likelihood decision (MLD) algorithm. In coded OFDM systems, the branch metric for forward error correction (FEC) is normally calculated based on received sub-carrier power level and the constellation point likelihood. This conventional approach fails in OFDM-SDM systems. In the proposed method, branch metric is derived based on the likelihood calculated by MLD algorithm, which improves FEC performance effectively. Then, availability of coded OFDM-SDM to realize wireless LANs with data rate above 100 Mbit/s in a bandwidth of 20 MHz is evaluated in office environments using required CNR calculated by computer simulation. The evaluation indicates that OFDM-SDM has the possibility of realizing 100 Mbit/s in office environments.

This paper proposes a novel space diversity reception scheme suitable for packet-based orthogonal frequency division multiplexing (OFDM) wireless access systems that achieves large diversity gain by improving the accuracy of both carrier frequency synchronization and phase tracking. Phase tracking compensates the phase rotation caused by residual carrier frequency error and phase noise and is necessary for high data rate OFDM systems that use coherent detection. In the proposed scheme, the accuracy of carrier frequency synchronization is improved by combining the information of the carrier frequency offset detected on all diversity branches; the accuracy of phase tracking is improved by using pilot signals whose signal to noise ratio (SNR) is raised by maximal ratio combining of the pilot signals extracted from all branches. Computer simulation results show that the proposed scheme effectively improves the diversity gain even in severe environments such as those with low carrier to noise ratios (CNR) and large delay spreads.

This paper proposes a channel capacity maximization method for Multiple-Input Multiple-Output (MIMO) antennas with parasitic elements. Reactive terminations are connected to the parasitic elements, and the reactance values are determined to achieve stochastically high channel capacity for the environment targeted. This method treats the S-parameter and propagation channel of the antenna, including the parasitic elements, as a combined circuit. The idea of the 'parasitic channel,' which is observed at the parasitic antenna, is introduced to simplify the optimization procedure. This method can significantly reduce the number of necessary measurements of the channel for designing the antenna. As a design example, a bidirectional Yagi-Uda array, which has two driven antennas at both ends of the linear array, is measured in an indoor environment. The resulting design offers enhanced channel capacity mainly due to its improved signal-to-noise ratio compared to the antenna without the parasitic antennas.

The implementation and experimental evaluations of distributed zero-forcing beamforming (DZFBF) for downlink multi-user multiple-input multiple-output (DL MU-MIMO) systems are presented. In DZFBF, multiple access points (APs) transmit to own desired stations (STAs) at the same time and using the same frequency channel while mitigating inter-cell interference. To clarify the performance and feasibility of DZFBF, we develop a real-time transmission testbed that includes two APs and four STAs; all are implemented using field programmable gate array. For real-time transmission, we also implement a simple weight generation process based on ZF weight using channel state information which is fed back from STAs; it is an extension of the weight generation approach used in DL MU-MIMO systems. By using our testbed, we demonstrate the real-time transmission performance in actual indoor multi-cell environments. These results indicate that DL DZFBF is more effective than DL MU-MIMO with time division multiple access.

Multiuser -- Multiple Input Multiple Output (MU-MIMO) techniques were proposed to increase spectrum efficiency; a key assumption was that the Mobile Terminals (MTs) were simple with only a few antennas. This paper focuses on the Block Diagonalization algorithm (BD) based on the equal power allocation strategy as a practical MU-MIMO technique. When there are many MTs inside the service area of the access point (AP), the AP must determine, at each time slot, the subset of the MTs to be spatially multiplexed. Since the transmission performance depends on the subsets of MTs, the user selection method needs to use the Channel State Information (CSI) obtained in the physical layer to maximize the Achievable Transmission Rate (ATR). In this paper, we clarify the relationship between ATR with SU-MIMO and that with MU-MIMO in a high eigenvalue channel. Based on the derived relationship, we propose a new measure for user selection. The new measure, the eigenvalue decay factor, represents the degradation of the eigenvalues in null space compared to those in SU-MIMO; it is obtained from the signal space vectors of the MTs. A user selection method based on the proposed measure identifies the combination of MTs that yields the highest ATR; our approach also reduces the computational load of user selection. We evaluate the effectiveness of user selection with the new measure using numerical formulations and computer simulations.

In this paper, we propose demodulators for the Golden and Alamouti codes in amplify-and-forward (AF) cooperative communication with one relay. The proposed demodulators output exact log likelihood ratios (LLRs) with recursion based on the Jacobian logarithm. The cooperative system with the proposed demodulator for the Golden code has the benefit of efficient data transmission, while the system for the Alamouti code has low demodulation complexity. Quantitative analyses of computational complexity of the proposed demodulators are conducted. The transmission performance for various relay location and power settings is evaluated on cooperative orthogonal frequency division multiplexing (OFDM)-based wireless local area network (LAN) systems. In evaluations, the optimal relay location and power settings are found. The cooperative system with the proposed demodulators for the Golden and Alamouti codes offers 1.5 and 1.9 times larger areas where 10.8 and 5.4Mbit/s can be obtained than a non-cooperative (direct) system in a typical office environment, respectively.

The adaptive phase tracking scheme for orthogonal frequency division multiplexing (OFDM) signals can provide superior PER performance in channels with varying phase noise power. It is an effective technique for achieving high-rate and high quality wireless transmission. This paper proposes a new simple adaptive phase tracking scheme for OFDM signals in order to realize high-rate wireless local area networks (LANs). The proposed scheme measures the integrated phase rotation in order to appropriately set the properties of the FIR filter in the phase tracking circuits. This scheme uses the fact that the integrated phase rotation is correlated to the phase noise power. Assuming an RMS delay spread of 100 ns, computer simulations show that the proposed scheme offers superior required Eb/N0 performance (with regard to the phase noise power) compared to the conventional fixed-tap scheme, where the phase noise to signal power ratios are below -18 dB. It also offers excellent PER performance at the packet length of 1000 bytes unlike the conventional schemes, which suffer degraded PER performance.

As wireless LAN systems become more widespread, the number of access points (APs) is increasing. A large number of APs cause overlapping cells where nearby cells utilize the same frequency channel. In the overlapping cells, inter-cell interference (ICI) degrades the throughput. This paper proposes an interference-aware multi-cell beamforming (IMB) technique to reduce the throughput degradation in the overlapping cells. The IMB technique improves transmission performance better than conventional multi-cell beamforming based on a decentralized control scheme. The conventional technique mitigates ICI by nullifying all the interference signal space (ISS) by beamforming, but the signal spaces to the user terminal (UT) is also limited because the degree of freedom (DoF) at the AP is limited. On the other hand, the IMB technique increases the signal space to the UT because the DoF at the AP is increased by selecting the ISS by allowing a small amount of ICI. In addition, we introduce a method of selecting the ISS in a decentralized control scheme. In our work, we analyze the interference channel state information (CSI) and evaluate the transmission performance of the IMB technique by using a measured CSI in an actual indoor environment. As a result, we find that the IMB technique becomes more effective as the number of UT antennas in nearby cells increases.

Combining heterogeneous wireless networks that cross licensed and unlicensed spectra is a promising way of supporting the surge in mobile traffic. The unlicensed band is mostly used by wireless LAN (WLAN) nodes which employ carrier sense multiple access/collision avoidance (CSMA/CA). Since the number of WLAN devices and their traffic are increasing, the wireless resource of the unlicensed band is expected be more depleted in 2020s. In such a wireless environment, the throughput could be extremely low and unstable due to the hidden terminal problem and exposed terminal problem despite of the large resources of the allocated frequency band and high peak PHY rate. In this paper, we propose user equipment (UE) centric access in the unlicensed band, with support by licensed band access in the mobile network. The proposed access enables robust downlink transmission from the access point (AP) to the UEs by mitigating the hidden terminal problem. The licensed spectrum access passes information on the user data waiting at the AP to the UEs and triggers UE reception opportunity (RXOP) acquisition. Furthermore, the adaptive use of UE centric downlink access is presented by using the channel utilization measured at the AP. Computer simulations confirm that licensed access assistance enhances the robustness of the unlicensed band access against the hidden terminal problem.

Over 100 Gbit/s/ch high-speed optical transmission is required to achieve the high capacity networks that can meet future demands. The coherent receiver, which is expected to yield high frequency utilization, is a promising means of achieving such high-speed transmission. However, it requires a high-speed Analog to Digital Converter (ADC) because the received signal bandwidth would be over several tens or hundreds of GHz. To solve this problem, we propose a band-divided receiver structure for wideband optical signals. In the receiver, received wideband signals are divided into a number of narrow band signals without any guard band. We develop a band-divided receiver prototype and evaluate it in an experiment. In addition, we develop a real-time OFDM demodulator on an FPGA board that implements 1.5 GS/s ADCs. We demonstrate that the band-divided receiver prototype with its real-time OFDM demodulator and 1.5 GS/s ADC can demodulate single polarization 12 Gbit/s OFDM signals in real-time.

Implicit feedback is an approach that utilizes uplink channel state information (CSI) for downlink transmit beamforming on multiple-input multiple-output (MIMO) systems, relying on over-the-air channel reciprocity. The implicit feedback improves throughput efficiency because overhead of CSI feedback for change of over-the-air channel responses is omitted. However, it is necessary for the implicit feedback to calibrate circuitry responses that uplink CSI includes, because actual downlink and uplink channel responses do not match due to different transmit and receive circuitry chains. This paper presents our proposed calibration scheme, weighted-combining calibration (WCC); it offers improved calibration accuracy. In WCC, an access point (AP) calculates multiple calibration coefficients from ratios of downlink and uplink CSI, and then combines coefficients with minimum mean square error (MMSE) weights. The weights are derived using a linear approximation in the high signal to noise power ratio (SNR) regime. Analytical mean square error (MSE) of calibration coefficients with WCC and calibration schemes for comparison is expressed based on the linear approximation. Computer simulations show that the analytical MSE matches simulated one if the linear approximation holds, and that WCC improves the MSE and signal to interference plus noise power ratio (SINR). Indoor experiments are performed on a multiuser MIMO system with implicit feedback based on orthogonal frequency division multiplexing (OFDM), built using measurement hardware. Experimental results verify that the channel reciprocity can be exploited on the developed multiuser MIMO-OFDM system and that WCC is also effective in indoor environments.

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.

This paper investigates an adaptive movable access point (AMAP) system and explores its feasibility in a static indoor classroom environment with an applied wireless local area network (WLAN) system. In the AMAP system, the positions of multiple access points (APs) are adaptively moved in accordance with clustered user groups, which ensures effective coverage for non-uniform user distributions over the target area. This enhances the signal to interference and noise power ratio (SINR) performance. In order to derive the appropriate AP positions, we utilize the k-means method in the AMAP system. To accurately estimate the position of each user within the target area for user clustering, we use the general methods of received signal strength indicator (RSSI) or time of arrival (ToA), measured by the WLAN systems. To clarify the basic effectiveness of the AMAP system, we first evaluate the SINR performance of the AMAP system and a conventional fixed-position AP system with equal intervals using computer simulations. Moreover, we demonstrate the quantitative improvement of the SINR performance by analyzing the ToA and RSSI data measured in an indoor classroom environment in order to clarify the feasibility of the AMAP system.