High-Altitude Platform Station (HAPS) provides communication services from an altitude of 20km via a stratospheric platform such as a balloon, solar-powered airship, or other aircraft, and is attracting much attention as a new mobile communication platform for ultra-wide coverage areas and disaster-resilient networks. HAPS can provide mobile communication services directly to the existing smartphones commonly used in terrestrial mobile communication networks such as Fourth Generation Long Term Evolution (4G LTE), and in the near future, Fifth Generation New Radio (5G NR). In order to design efficient HAPS-based cell configurations, we need a radio wave propagation model that takes into consideration factors such as terrain, vegetation, urban areas, suburban areas, and building entry loss. In this paper, we propose a new vegetation loss model for Recommendation ITU-R P.833-9 that can take transmission frequency and seasonal characteristics into consideration. It is based on measurements and analyses of the vegetation loss of deciduous trees in different seasons in Japan. Also, we carried out actual stratospheric measurements in the 700MHz band in Kenya to extend the lower frequency limit. Because the measured results show good agreement with the results predicted by the new vegetation loss model, the model is sufficiently valid in various areas including actual HAPS usage.

In recent years, High-Altitude Platform Station (HAPS) has become the most interesting topic for next generation mobile communication systems, because platforms such as Unmanned Aerial Vehicles (UAVs), balloons, airships can provide ultra-wide coverage, up to 200km in diameter, from altitudes of around 20 km. It also offers resiliency to damage caused by disasters and so ensures the stability and reliability of mobile communications. In order to further integrate HAPS with existing terrestrial mobile communication networks in providing mobile services to users, radio wave propagation models such as terrain, vegetation loss, human shielding loss, building entry loss, urban/suburban areas must be taken into consideration when designing HAPS-based cell configurations. This paper proposes a human body shielding propagation loss model that considers the basic signal attenuation by the human body at high elevation angles. It also analyzes the effect of changes in actual urban/suburban environments due to the arrival of multipath radio waves for HAPS communications in the frequency range of 0.7 to 3.3GHz. Measurements in actual urban/rural environments in Japan and actual stratospheric base station measurements in Kenya are carried out to confirm the validity of the proposed model. Since the measured results agree well with the results predicted by the proposed model, the model is good enough to provide estimates of human loss in various environments.

With the advances in computer processing that have yielded an enormous increase in performance, numerical analytical approaches based on electromagnetic theory have recently been applied to mobile radio propagation analysis. One such approach is the ray-tracing method based on geometrical optics and the uniform geometrical theory of diffraction. In this paper, ray-tracing techniques that have been proposed in order to improve computational accuracy and speed are surveyed. First, imaging and ray-launching methods are described and their extended methods are surveyed as novel fundamental ray-tracing techniques. Next, various ray-tracing acceleration techniques are surveyed and categorized into three approaches, i.e., deterministic, heuristic, and brute force. Then, hybrid methods are surveyed such as those employing Physical optics, the Effective Roughness model, and the Finite-Difference Time-Domain method that have been proposed in order to improve analysis accuracy.

Automatic modulation classification (AMC) involves extracting a set of unique features from the received signal. Accuracy and uniqueness of the features along with the appropriate classification algorithm determine the overall performance of AMC systems. Accuracy of any modulation feature is usually limited by the blindness of the signal information such as carrier frequency, symbol rate etc. Most papers do not sufficiently consider these impairments and so do not directly target practical applications. The AMC system proposed herein is trained with probable input signals, and the appropriate decision tree should be chosen to achieve robust classification. Six unique features are used to classify eight analog and digital modulation schemes which are widely used by low frequency mobile emergency radios around the globe. The Proposed algorithm improves the classification performance of AMC especially for the low SNR regime.

A cognitive radio will have to sense and discover the spectral environments where it would not cause primary radios to interfere. Because the primary radios have the right to use the frequency, the cognitive radios as the secondary radios must detect radio signals before use. However, the secondary radios also need identifying the primary and other secondary radios where the primary radios are vulnerable to interference. In this paper, a method of simultaneously identifying signals of primary and secondary radios is proposed. The proposed bandwidth differentiation assumes the primary and secondary radios use orthogonal frequency division multiplexing (OFDM), and the secondary radios use at the lower number of subcarriers than the primary radios. The false alarm and detection probabilities are analytically evaluated using the characteristic function method. Numerical evaluations are also conducted on the assumption the primary radio is digital terrestrial television broadcasting. Result showed the proposed method could achieve the false alarm probability of 0.1 and the detection probability of 0.9 where the primary and secondary radio powers were 2.5 dB and 3.6 dB higher than the noise power. In the evaluation, the reception signals were averaged over the successive 32 snapshots, and the both the primary and secondary radios used QPSK. The power ratios were 4.7 dB and 8.4 dB where both the primary and secondary radios used 64QAM.

We address the problem of multiuser co-channel interference scheduling in multicell interference-limited networks. Our target is to optimize the network capacity under the SIR-balanced power control policy. Since it's difficult to optimize the original problem, we derive a new problem which maximizes the lower bound of the network capacity. Based on the analysis of this new problem, we propose an interference matched scheduling algorithm. This algorithm considers the caused co-channel interference and the channel conditions to schedule the "matched" users at the same time. We prove that this interference matched scheduling algorithm optimizes the lower bound of the network capacity for any arbitrary numbers of cells and users. Moreover, this scheduling method is low-complexity and can be implemented in a fully distributed fashion. Simulation results reveal that the performance of the proposed algorithm achieves near optimal capacity, even though it does not optimize the network capacity directly. Finally, the proposed algorithm holds a great gain over formerly proposed round robin and power matched scheduling method, especially when the scale of the network is large.

In order to reduce the amount of interference to neighboring cells in cellular systems, we generally use base station (BS) antennas that have sharp beam patterns in the vertical plane; however, the distribution of incoming waves at the BS affects the effective gain of the BS antennas which have directional pattern. Therefore, we have to clarify the characteristics of the distribution of the incoming waves. A recent trend is decreasing the cell radius; therefore, clarifying the distribution of the incoming waves at the BS when mobile stations (MSs) are located within 1 km from the BS is important. In this report, we evaluate the effective gains of the BS antennas, which are calculated using the measured vertical power angle profile (PAP). Moreover, we examine the application of a simple incoming wave model to the evaluation of the antenna effective gains. In the model, the average power of the incoming waves is set to the Laplacian function and each wave is changed to a lognormal distribution. The antenna effective gain calculated using the model agrees well with that calculated using the measured PAP.

A site-general type prediction formula is created based on the measurement results in an urban area in Japan assuming that the prediction frequency range required for Fourth -- Generation (4G) Mobile Communication Systems is from 3 to 6 GHz, the distance range is 0.1 to 3 km, and the base station (BS) height range is from 10 to 100 m. Based on the measurement results, the path loss (dB) is found to be proportional to the logarithm of the distance (m), the logarithm of the BS height (m), and the logarithm of the frequency (GHz). Furthermore, we examine the extension of existing formulae such as the Okumura -- Hata, Walfisch -- Ikegami, and Sakagami formulae for 4G systems and propose a prediction formula based on the Extended Sakagami formula.

This paper proposes an OFDM mobile radio packet system that employs a new protocol of automatic repeat request (ARQ) for nonlinear multiuser detection (MUD) with log likelihood ratio combining (LC) on the appropriate bits. The conventional metric combining (MC) MUD separates collided packets by using nonlinear MUD, accumulates the Euclidian distance metrics of the received subcarrier symbols in the packets, and then achieves throughput improvement. However, when MC-MUD detects a packet error, it makes user terminals retransmit the same packets so as to reproduce the collision of the same packets. The proposed LC-MUD scheme simplifies the ARQ protocol and requires no reproduction of the same packet collision. The computer simulations demonstrate the superior throughput of LC-MUD to that of MC-MUD, and further improvement due to adaptive modulation and coding (AMC) optimized for the nonlinear MUD in LC-MUD.

This paper proposes a new blind adaptive MLSE equalizer for frequency selective mobile radio channels. The proposed equalizer performs channel estimation for each survivor path of the Viterbi algorithm (VA), and restricts the number of symbol candidates for the channel estimation in order to reduce prohibitive complexity. In such channel estimation, autocorrelation matrices of the symbol candidates are likely to become singular, which increases the estimation error. To cope with the singularity, the proposed equalizer employs a recursive channel estimation algorithm using the Moore-Penrose generalized inverse of the autocorrelation matrix. As another problem, the blind channel estimation can yield plural optimal estimates of a channel impulse response, and the ambiguity of the estimates degrades the BER performance. To avoid this ambiguity, the proposed equalizer is enhanced so that it can take advantage of the fractional sampling. The enhanced equalizer performs symbol-spaced channel estimation for each fractional sampling phase. This equalizer combines separate channel estimation errors, and provides the sum to the VA processor as the branch metric, which tremendously reduces the probability that a correct estimate turns into a false one. Computer simulation demonstrates the effectiveness of the proposed equalizers in the frequency selective fading channels.

We introduce an adaptive subchannel, bit, and power allocation (ASBPA) algorithm to maximize the bandwidth efficiency of the mobile communication system that use orthogonal frequency division multiplexing (OFDM). We propose a suboptimal rate adaptive ASBPA algorithm that guarantees fairness in resource allocation and overcomes inherent co-channel interference (CCI) in the cellular system. Furthermore, we evaluate the maximum possible bandwidth efficiency of the cellular OFDM system achieved by the ASBPA algorithm which is practical to implement. Our simulation results show that the proposed algorithm outperforms the existing ones and achieves the cellular bandwidth efficiency of up to 5 b/s/Hz/cell. We also investigate some of the conditions that govern the bandwidth efficiency of the cellular OFDM system using the proposed ASBPA algorithm.

Multiple-input multiple-output (MIMO) eigenbeam space division multiplexing that uses adaptive modulations for substreams is a promising technology for improving transmission capacity. A fundamental drawback of this approach is that the modulation levels determined from the carrier-to-noise ratio at each substream are sometimes overly optimistic so the use of these modulation levels results in transmission errors and diminished transmission performance. A novel method of determining substream modulation levels is proposed that alleviates this degradation. In the proposed method, the expected bit error rates for possible modulations of each substream are calculated from delay profiles. Simulation results indicate that transmission capacity is improved by 30% using the new method compared with the conventional method.

Future broadband mobile communication systems are necessary to achieve the bit rates of 100 Mbit/s. Orthogonal Frequency Division Multiplexing (OFDM) transmission is an attractive technology because it can remove the influence of frequency selective fading in broadband transmission by adding a suitable guard interval to each OFDM symbol. However, peak-to-average power ratio (PAPR) is very large in OFDM transmission. In this paper, we propose a new PAPR reduction method which can be applied even when unusable bands are inside the system band. In the proposed method, peak reduction signals are generated by iterative signal processing only in the usable frequency band, and filtering to remove out-of-band components of the peak reduction signals is incorporated into the iterative signal processing. The results of computer simulation show that the proposed method can effectively reduce peak power without expanding the spectrum both outside the system band and into unusable bands inside the system band. By using the proposed method, the broadband mobile communication system with low peak power and high flexibility of frequency band use can be realized.

In this paper, we improve the performance analysis of the Rake receiver for the DS-CDMA forward link using long random spreading sequences (RSS's) by more accurately evaluating the correlation between the various interference terms. We also extend the analysis to the case of short (periodic) RSS. The accuracy of the expressions obtained in our analysis is verified by computer simulation. We show that for a given normalized spreading factor, the bit error rate (BER) performance of the Rake receiver is the same for BPSK and QPSK data modulation. We also show that when the channel delay spread is smaller than a data symbol duration, the CDMA receiver has similar BER performance for long and short RSS's. However, for large delay spread, the employment of short RSS's may result in severe performance degradation.

In this Letter, a frequency-domain pre-rake transmission is presented for a direct sequence spread spectrum with time division duplex (DSSS/TDD) system under a frequency-selective fading channel. The mathematical relationship between frequency-domain and time-domain pre-rake transmissions is discussed. It is confirmed by the computer simulation that, similar to the time-domain pre-rake transmission, frequency-domain pre-rake transmission can improve the bit error rate (BER) performance. The frequency-domain pre-rake transmission shows only slight performance degradation compared to the frequency-domain rake reception for large SF.

We have demonstrated radio-over-fiber transmission of wireless signals at millimeter-wave bands. The system incorporated 25 km of an optical intermediate frequency feeder and 60 GHz OFDM signal transmission at 155 Mbps with a BER of less than 10-6 was achieved within the system cell of a radius of 2.6 m under the channel condition of Line-of-Sight.

To improve the DS-CDMA signal transmission performance in a frequency-selective fading channel, the frequency-domain equalization (FDE) can be applied, in which simple one-tap equalization is carried out on each subcarrier component obtained by fast Fourier transform (FFT). Equalization weights for joint FDE and antenna diversity combining based on maximal ratio combining (MRC), zero-forcing (ZF), and minimum mean square error (MMSE) are derived. The conditional bit error rate (BER) is derived for the given set of channel gains in a frequency-selective multipath fading channel. The theoretical average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation. Performance comparison between DS- and multi-carrier (MC)-CDMA both using FDE is also presented.

We have developed a code-division-multiplexing (CDM) transmission scheme for future cellular communication systems, which uses cyclic shifted-and-extended (CSE) codes generated from an M-sequence to enable seamless communication in highly mobile environments. Because the correlation characteristics of CSE codes are determined by the M-sequence, the cross-correlation values are accumulated as a result of combining transmitted signals with opposite polarities in parallel channels. The accumulated cross-correlation values significantly degrade transmission performance, especially with multi-level modulation schemes such as quadrature amplitude modulation (QAM). We thus propose a cancellation technique to eliminate the accumulated cross-correlation values. We have evaluated the transmission performance of the CDM transmission scheme with the proposed technique by computer simulation. The new scheme enables high-quality data transmission in fast-fading channels.

In this paper, the space time transmit diversity (STTD) decoding combined with minimum mean square error (MMSE) equalization is presented for MC-CDMA downlink and uplink in the presence of multiple receive antennas. The equalization weights that minimize the MSE for each subcarrier are derived. From computer simulation, it was found that the BER performance of STTD decoding combined with MMSE equalization and Mr-antenna diversity reception using the weights derived in this paper provides the same diversity order as 2Mr-antenna receive diversity with MMSE equalization but with 3 dB performance penalty and is always better than that with no diversity. The uplink BER performance can also be improved with STTD, but the error floor still exists. However, with 2-receive antennas in addition to 2-antenna STTD, the BER floor can be reduced to around 10-5 even for the uplink.

This paper presents time slot assignment algorithms applicable to uplink of space division multiple access (SDMA)/time division multiple access (TDMA) systems with adaptive antennas. In the time slot assignment process for a new terminal in a cell, we consider not only the signal quality of the new terminal but also that of active terminals in the same cell. Intra-cell hand over is performed for an active terminal when its signal quality deteriorates. We evaluate the blocking and forced termination probabilities for pure TDMA systems, sectorized systems, and SDMA/TDMA systems in cellular environments by computer simulations. The simulation results show that the SDMA/TDMA systems have much better performance than the pure TDMA and sectorized systems.