This paper presents the characteristics of path loss produced by traffic sign blockage. Multi frequency bands including high frequency bands up to 40 GHz are analyzed on the basis of measurement results in urban microcell environments. It is shown that the measured path loss increases compared to free space path loss even on a straight line-of-sight road, and that the excess attenuation is caused by the blockage effects of traffic signs. It is also shown that the measurement area affected by the blockage becomes small as frequency increases. The blocking object occupies the same area for all frequencies, but it takes up a larger portion of the Fresnel Zone as frequency increases. Therefore, if blockage occurs, the excess loss in high frequency bands becomes larger than in low frequency bands. In addition, the validity of two blockage path loss models is verified on the basis of measurement results. The first is the 3GPP blockage model and the second is the proposed blockage model, which is an expanded version of the basic diffraction model in ITU-R P.526. It is shown that these blockage models can predict the path loss increased by the traffic sign blockage and that their root mean square error can be improved compared to that of the 3GPP two slope model and a free space path loss model. The 3GPP blockage model is found to be more accurate for 26.4 and 37.1GHz, while the proposed model is more accurate for 0.8, 2.2, and 4.7GHz. The results show the blockage path loss due to traffic signs is clarified in a wide frequency range, and it is verified that the 3GPP blockage model and the proposed blockage model can accurately predict the blockage path loss.

This paper proposes a path loss model for crowded outdoor environments that can consider the density of people. Measurement results in an anechoic chamber with three blocking persons showed that multiple human body shadowing can be calculated by using finite width screens. As a result, path loss in crowded environments can be calculated by using the path losses of the multipath and the multiple human body shadowing on those paths. The path losses of the multipath are derived from a ray tracing simulation, and the simulation results are then used to predict the path loss in crowded environments. The predicted path loss of the proposed model was examined through measurements in the crowded outdoor station square in front of Shibuya Station in Tokyo, and results showed that it can accurately predict the path loss in crowded environments at the frequencies of 4.7GHz and 26.4GHz under two different conditions of antenna height and density of people. The RMS error of the proposed model was less than 4dB.

This paper proposes a new scheme that can evaluate the cell throughput performance of wireless local area network (LAN) systems, which use carrier sense multiple access/collision avoidance (CSMA/CA) and multiple transmission bit-rates (multi-rate). We extend the interference model of the conventional scheme in order to deal with interference more accurately in multi-cell environments. Unlike the conventional scheme, the proposed scheme is able to handle multi-rate systems. We use the proposed scheme to evaluate the IEEE 802.11a system and systems whose signal-bandwidth is expanded from that of the IEEE 802.11a system. We find that a system with 35(75) MHz signal-bandwidth achieves about 1.3(1.25) times higher cell throughput than the IEEE 802.11a system. Furthermore, the system with 35(75) MHz signal-bandwidth is also shown to have the potential to achieve up to 1.5(1.8) times higher cell throughput performance than the IEEE 802.11a system if the transmission efficiency on the media access control (MAC) layer is assumed to be ideal. It is concluded that the proposed scheme confirms that the approach to expand the signal bandwidth of the IEEE 802.11a system is effective to improve the cell throughput performance. This result is virtually impossible to derive with the conventional scheme.

This paper proposes a scheme that reduces residual self-interference significantly in the analog-circuit domain on wireless full-duplex relay systems. Full-duplex relay systems utilize the same time and frequency resources for transmission and reception at the relay node to improve spectral efficiency. Our proposed scheme measures multiple responses of the feedback path by changing the direction of the main beam of the transmitter at the relay, and then selecting the optimal direction that minimizes the residual self-interference. Analytical residual self-interference is derived as the criterion to select the optimal direction. In addition, this paper considers the target of residual self-interference power before the analog-to-digital converter (ADC) dependent on the dynamic range in the analog-circuit domain. Analytical probability that the residual interference exceeds the target is derived to help in determining the number of measured responses of the feedback path. Computer simulations validate the analytical results, and show that in particular, the proposed scheme with ten candidates improves the residual self-interference by approximately 6dB at the probability of 0.01 that the residual self-interference exceeds target power compared with a conventional scheme with the feedback path modeled as Rayleigh fading.

The IEEE 802.11 distributed coordinated function (DCF) adopts carrier sense multiple access with collision avoidance (CSMA/CA) as its medium access control (MAC) protocol. In a wireless local area network (WLAN) stations (STAs) congested situation, the performance of the WLAN system is significantly degraded due to a collision between the STAs. In this paper, we propose a simple method that decreases the number of frame collisions. After a successful transmission, the proposed method refrains from transmission during certain time which is defined as post-inter-frame space (Post-IFS). This mechanism improves the system performance including the throughput characteristics and access delay by reducing the number of competing STAs. The length of the Post-IFS is a key factor in improving the system performance for the proposed method. If the access point (AP) can estimate the optimal value of the Post-IFS, collision-free operation similar to that in centralized control is performed. Even if the optimal Post-IFS is not estimated, the number of competing STAs and the collision probability are decreased. Computer simulations verify that the proposed method achieves 40% higher system throughput compared to the conventional CSMA/CA for a network with 50 STAs.

This paper proposes a reduced-complexity signal detection scheme for Orthogonal Frequency Division Multiplexing with Space Division Multiplexing (OFDM/SDM) systems that utilize Zero-Forcing (ZF) and K-best algorithms. It is known that Maximum Likelihood Detection (MLD) with exhaustive search achieves mathematically optimal performance for SDM signal detection. However, it also suffers from exponential computational complexity against the number of transmit antennas and modulation order. In order to reduce the computational complexity of MLD, we apply the K-best algorithm for signal detection. It is known that the K-best algorithm itself inherently reduces the computational complexity of MLD because it avoids exhaustive search. In this paper, we propose the modified K-best algorithm, which exploits the ZF algorithm for initial symbol estimation. This initial symbol estimation improves the decoding accuracy of the original K-best algorithm. We evaluate the performance of the proposed scheme through computer simulations. The computer simulation results show that the performance degradation from the MLD algorithm is suppressed to just 1 dB or so in terms of the required Eb/N0 for packet error rate (PER) = 10-2, When either 16 Quadrature Amplitude Modulation (16QAM) or 64QAM is applied with three transmit and three receive antennas. In these cases, 87% and 99% fewer metric computations are required than the MLD algorithm. It is confirmed that the proposed MLD algorithm offers a significant reduction in the computational complexity from the MLD algorithm while suppressing the performance degradation.

This paper proposes a novel sequential coherent preambleless demodulator that uses phase signals instead of complex signals in the automatic frequency control (AFC) and carrier recovery circuits. The proposed demodulator employs a phase-combined frequency error detection circuit and dual loop AFC circuit to achieve fast frequency acquisition and low frequency jitter. It also adopts an open loop carrier recovery scheme with a sample hold circuit after the carrier filter to ensure carrier signal stability within a packet. It is shown that the frame error rate performance of the proposed demodulator is superior, by 30%, to that offered by differential detection in a frequency selective Rayleigh fading channel. The hardware size of the proposed demodulator is about only 1/10 that of a conventional coherent demodulator employing complex signals.

This paper proposes a new multi-task synchronization scheme for packet mode orthogonal frequency division multiplexing (OFDM) signals in multi-input multi-output (MIMO) transmission systems; it targets high-rate wireless LANs that offer over 100 Mbit/s. In addition, this paper introduces a packet format for MIMO-OFDM signals that ensures backward compatibility with IEEE 802.11a. The proposed synchronization scheme has simple open-loop construction and consists of automatic frequency control (AFC), symbol timing detection, MIMO channel estimation, and phase tracking. AFC and symbol timing detection are carried out in the time-domain. After OFDM demodulation, the proposed scheme performs MIMO channel estimation and phase tracking in the frequency-domain. Considering all of the above synchronization tasks, we evaluate the packet error rate (PER) performance using the IEEE 802.11 TGn-defined channel model-D and model-E. In channel model-D with the RMS delay spread = 50 ns, the proposed scheme shows superior performance; it suppress the required Eb/N0 degradation to within 0.4 dB with 1000 byte packets compared to the performance achieved if only MIMO channel estimation is considered. Moreover, in channel model-E with the RMS delay spread = 100 ns, it is found that the proposed scheme degrades the required Eb/N0 only by approximately 1.5 dB compared to the MIMO channel estimation only case, even if the packet length is 1000 bytes with 64QAM and coding-rate = 7/8.

This paper describes a simple packet combining scheme with maximum likelihood detection (MLD) for multiple-input multiple-output with orthogonal frequency division multiplexing (MIMO-OFDM) in relay channels to construct reliable wireless links in wireless local area networks (LANs). Our MLD-based approach employs the multiplexed sub-stream signals in different transmit slots. The proposed scheme uses an additional combining process before MLD processing. Moreover, the proposed scheme sets the cyclic shift delay (CSD) operation in the relay terminal. We evaluate the performance of the proposed scheme by the packet error rate (PER) and throughput performance in the decode-and-forward (DF) strategy. First, we show that the proposed scheme offers approximately 4.5dB improvement over the conventional scheme in the received power ratio of the relay terminal to the destination terminal at PER =0.1. Second, the proposed scheme achieves about 1.6 times the throughput of the conventional scheme when the received power ratio of the relay terminal to the destination terminal is 3dB.

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.

In this paper, the spread spectrum code hopping (CH) system, which has some analogy to frequency hopping systems, is described. The CH system has robustness to code interference that restriction of kinds of PN matched filters (MFs) will cause. The mean acquisition time is shown by theoretical analysis and computer simulation. The acquisition rate results under a single code interference, which seriously affects direct sequence systems, and an asynchronous two-user channels are obtained. Moreover, using theoretical analysis and computer simulation, the bit error rate (BER) performance under single code interference is evaluated. It is shown that CH systems perform better than conventional ones under single code interference.

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 describes analytical results obtained for floor penetration loss characteristics and their frequency dependency by measurements in multiple frequency bands, including those above 6GHz, in an indoor office environment. Measurement and analysis results confirm that the floor penetration loss depends on two dominant components: the transmission path through floors, and the path traveling through the outside building. We also clarify that these dominant paths have different path loss characteristics and frequency dependency. The transmission path through floors rapidly attenuates with large inter-floor offsets and in high frequency bands. On the other hand, the path traveling through outside of the building attenuates monotonically as the frequency increases. Therefore, the transmission path is dominant at short inter-floor offsets and low frequencies, and the path traveling through the outside is dominant at high number of floors or high frequency. Finally, we clarify that the floor penetration loss depends on the frequency dependency of the dominant path on the basis of the path loss characteristics of each dominant path.

This paper considers a wireless LAN system operated in a multiple-cell environment with universal frequency reuse. A key technical goal is to increase cell-capacity within a cell. A very high-rate wireless LAN system, maximum data rates of over 100 Mbit/s, is proposed that offers an expanded signal-bandwidth compared to that specified in IEEE802.11a. The system employs OFDM and MC/CDMA signals in packet mode. It falls back from OFDM signals with low subcarrier modulation orders to MC/CDMA signals. A link level performance comparison shows that OFDM has superior performance to MC/CDMA at over 32 Mbit/s. Under 16 Mbit/s, however, MC/CDMA can establish wireless link connections unlike OFDM. Thus the fallback technique, which is triggered by the CIR environment, should select OFDM if the data rate exceeds 32 Mbit/s. It should fallback to MC/CDMA if the rate is less than 16 Mbit/s. We also evaluate the proposed scheme in a multiple-cell environment with universal frequency reuse, where the severe co-channel (other-cell) interference is present. We derive a cell capacity criterion for wireless LAN systems, and show that the proposed scheme offers a 2.2 times larger available transmission distance than the OFDM-only scheme. In addition, it is found that the proposed scheme offers a 1.3 times improvement in cell capacity compared to the MC/CDMA-only scheme, even if all other-cell interference is considered.

We have developed a direct spectrum division transmission (DSDT) technique that can divide a single-carrier signal into multiple sub-spectra and assign them to dispersed frequency resources of the satellite transponder to improve the spectrum efficiency of the whole system. This paper summarizes the satellite experiments on DSDT over a single and/or multiple satellite transponders, while changing various parameters such as modulation schemes, roll-off ratios, and symbol rates. In addition, by considering practical use conditions, we present an evaluation of the performance when the spectral density of each sub-spectrum differed across transponders. The satellite experiments demonstrate that applying the proposal does not degrade the bit error rate (BER) performance. Thus, the DSDT technique is a practical approach to use the scattered unused frequency resources over not only a single transponder but also multiple ones.

A propagation experiment on an actual channel is conducted to confirm the effectiveness of the 1-tap time domain beamforming (TDBF) technique we proposed in previous work. This technique offers simple beamforming for the millimeter waveband massive multiple-input multiple-output (MIMO) applied wireless backhaul and so supports the rapid deployment of fifth generation mobile communications (5G) small cells. This paper details propagation experiments in the 75GHz band and the characteristics evaluations of 1-tap TDBF as determined from actual channel measurements. The results show that 1-tap TDBF array gain nearly equals the frequency domain maximal ratio combining (MRC) value, which is ideal processing; the difference is within 0.5dB. In addition, 1-tap TDBF can improve on the signal-to-interference power ratio (SIR) by about 13% when space division multiplexing (SDM) is performed assuming existing levels of channel estimation error.

This paper proposes a simple and feasible decision-feedback channel tracking scheme for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems designed for wireless local area networks (LANs). In the proposed scheme, the channel state matrix for each subcarrier is tentatively estimated from a replica matrix of the transmitted signals. The estimated channel matrices, each derived at a different timing, are combined, and the previously estimated channel matrices are replaced with the latest ones. Unlike conventional channel tracking schemes based on a Kalman filter, the proposed scheme needs no statistical information about a MIMO channel, which makes the receiver structure quite simple. The packet error rate (PER) performances for the proposed scheme are evaluated on computer simulations. When there are three transmit and receive antennas, the subcarrier modulation scheme is 64 QAM, and the coding rate is 3/4, the proposed scheme keeps the SNR degradation at PER of 1e-2 less than 0.1 dB when the velocity of receiver is 3 km/h in an indoor office environment at 5 GHz band. In addition, compared to the conventional channel tracking scheme based on known pilot symbols, the proposed scheme improves throughput performance by 13.8% because it does not need pilot symbols. These results demonstrate that the proposed channel tracking scheme is simple and feasible for implementation in MIMO-OFDM systems based on wireless LANs.

In rich scattering environments, multiple antenna systems designed to accomplish spatial multiplexing have enormous potential of lifting the capacity of corresponding multiple input multiple output channels. In this paper, we present a new low complexity algorithm for decision feedback equalization detector in the SM scheme. The basic idea is to reduce the joint optimization problem to separate optimization problems to achieve better performance-complexity tradeoffs. Concretely, we separately optimize the detection order and the detector filters so that the complexity of the entire signal detection task is reduced. The new order search rule approximates the optimal Bell Labs layered space time (BLAST) approach from a geometrical perspective, and the detector filters are derived using a Cholesky based QR decomposition. The new algorithm is able to switch from zero forcing to minimum mean square error without additional operations and the computational effort is a small fraction of that in the optimal BLAST algorithm. Despite its low complexity, the error performance of new detector closely approximates that of the standard BLAST.

Path loss in high frequency bands above 6GHz is the most fundamental and significant propagation characteristic of IMT-2020. To develop and evaluate such high frequency bands, ITU-R SG5 WP5D recently released channel models applicable up to 100GHz. The channel models include path loss models applicable to 0.5-100GHz. A path loss model is used for cell design and the evaluation of the radio technologies, which is the main purpose of WP5D. Prediction accuracy in various locations, Tx positions, frequency bands, and other parameters are significant in cell design. This article presents the prediction accuracy of UMa path loss models which are detailed in Report ITU-R M.2412 for IMT-2020. We also propose UMa_A' as an extension model of UMa_A. While UMa_A applies different equations to the bands below and above 6GHz to predict path loss, UMa_A' covers all bands by using the equations of UMa_A below 6GHz. By using the UMa_A' model, we can predict path loss by taking various parameters (such as BS antenna height) into account over a wide frequency range (0.5-100GHz). This is useful for considering the deployment of BS antennas at various positions with a wide frequency band. We verify model accuracy by extensive measurements in the frequency bands from 2 to 66GHz, distances up to 1600 m, and an UMa environment with three Tx antenna heights. The UMa_A' extension model can predict path loss with the low RMSE of about 7dB at 2-26.4GHz, which is more accurate than the UMa_A and UMa_B models. Although the applicability of the UMa_A' model at 66GHz is unclear and needs further verification, the evaluation results for 66GHz demonstrate that the antenna height may affect the prediction accuracy at 66GHz.

We propose two simple weight calculation methods (primary method and enhanced method), that estimate approximated phase plane from a few antenna phase and calculate weights of all antenna elements, for wireless backhaul systems that utilize millimeter wave band massive antenna arrays. Such systems are expected to be used instead of optical fiber for connecting many small cell base stations (SCBSs) to the core network, and supporting the rapid deployment of SCBSs. However, beamforming with massive antenna arrays requires many analog-digital converters (ADCs) and incurs the issue of implementation complexity. The proposed methods overcome the problem by reducing the number of ADCs. Computer simulations clarify the appropriate layout and the number of ADCs connected to antenna elements; the effectiveness of the proposed methods is confirmed by evaluations with measured channel state information (CSI) in propagation experiments on a wireless backhaul system. Experimental verifications on the case of calculating the weight of 200 elements from the phases of just 9 elements show that the array gain degradation from ideal (the case in which the phases of all elements are used estimation) with both methods is less than 0.4 dB in the direct wave dominant environment. In addition, the enhanced method holds the array gain degradation to under 0.8dB in an environment existing reflected waves. These results show that the proposed methods can attain high accuracy beamforming while reducing ADC number.