This paper analyzes performances of trellis coded(TC) 8PSK modulation in Rician fading channels to optimize parameters of Viterbi decoders. The computer simulation clarifies that when carrier power to multipath power ratio (C/M) equals 10dB, Pe performance with reasonable length interleaving depends on free Euclidean distance (dfree) but not on effective code length(ECL) in a practical range of bit error probability (Pe= 10-3-10-4). In the case of C/M=5dB, the effect of ECL appears in the range of bit energy to noise power density ratio (Eb/No)C/M (=5dB). Therefore, for domestic mobile satellite communication systems for non-high latitude countries, only dfree must be considered for TC8PSK Viterbi decoder design. Moreover, to improve Pe performance in slow fading environment, pre-Viterbi-decoding maximal ratio combining is proposed. The proposed diversity scheme improves required Eb/No to achieve Pe=10-4 by more than 6dB compared with no interleaving and more than 2.5 dB compared with interleaving.

In this paper, we introduce the concept of a multi-operator mobile relay node (RN) for cellular networks on buses or trains. The installation of RNs improves spectral efficiency because an antenna with a higher gain than that of user equipment (UE) can be installed in an RN. However, installing different RNs for different operators is not efficient because of the large amount of space needed to install multiple RNs in a bus. Thus, sharing one RN among multiple operators is a more practical approach. When we use a multi-operator mobile RN, the required amount of resource for each operator varies independently as the RN moves. Consequently, we propose a system of shared-spectrum allocation among operators for RN-UEs communication. Shared bandwidth can be allocated to operators according to link quality in order to achieve effective utilization of radio resources. However, to introduce shared-spectrum allocation, fairness among the operators and the total efficiency of the system should be taken into consideration. Using computer simulations, we evaluate shared-spectrum allocation based on the Nash bargaining solution (NBS). The results, in terms of both fairness and efficiency, indicate that total throughput can be improved by approximately 20% compared with the situation where multiple operators install different RNs individually.

In densely deployed wireless local area network (WLAN) environments, the arbitrary deployment of WLAN access points (APs) can cause serious cell overlaps among APs. In such situations, the ability to realize adaptable coverage using transmission power control (TPC) is effective for improving the area spectral efficiency. Meanwhile, it should be guaranteed that no coverage holes occur and that connectivity between APs and wireless stations (STAs) is maintained. In this paper, the self-organization of coverage domains of APs using TPC is proposed. The proposed technique reduces the incidence of coverage overlaps without generating area coverage holes. To detect coverage holes, STAs and/or APs are used as sensors that inform each AP of whether or not the points at which they exist are covered by the APs. However, there is a problem with this approach in that when the density of STAs is not sufficiently large, the occurrence of area coverage holes is inevitable because the points at which the sensors do not exist are not guaranteed to be covered by APs. This paper overcomes the problem by focusing APs that belong to network's outer boundary (boundary APs) and prohibiting the APs from operating at low transmission power levels, the idea being that the coverage domains of such APs always include the region covered by only those APs. The boundary APs are determined by performing Delaunay triangulation of the set of points at which all APs exist. Simulation results confirm the effectiveness of the proposed TPC scheme in terms of its ability to reduce the total overlap area while avoiding the occurrence of area coverage holes.

This paper proposes a semi-autonomous frame synchronization scheme for a TDMA (Time Division Multiple Access)-TDD (Time Division Duplexing) personal communication system to realize accurate frame synchronization in a simple manner. The proposed scheme selects specific adjacent base stations by the station indicator (SID), carries out high resolution frame timing control, and compensates the propagation delay between base stations by using geographical data. This autonomously synchronizes all base stations to each other. Computer simulation and analysis results confirm the accurate and stable TDMA frame synchronization of all base stations even in fading environments.

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.

In this study, a contextual multi-armed bandit (CMAB)-based decentralized channel exploration framework disentangling a channel utility function (i.e., reward) with respect to contending neighboring access points (APs) is proposed. The proposed framework enables APs to evaluate observed rewards compositionally for contending APs, allowing both robustness against reward fluctuation due to neighboring APs' varying channels and assessment of even unexplored channels. To realize this framework, we propose contention-driven feature extraction (CDFE), which extracts the adjacency relation among APs under contention and forms the basis for expressing reward functions in disentangled form, that is, a linear combination of parameters associated with neighboring APs under contention). This allows the CMAB to be leveraged with a joint linear upper confidence bound (JLinUCB) exploration and to delve into the effectiveness of the proposed framework. Moreover, we address the problem of non-convergence — the channel exploration cycle — by proposing a penalized JLinUCB (P-JLinUCB) based on the key idea of introducing a discount parameter to the reward for exploiting a different channel before and after the learning round. Numerical evaluations confirm that the proposed method allows APs to assess the channel quality robustly against reward fluctuations by CDFE and achieves better convergence properties by P-JLinUCB.

Sharing perceptual data (e.g., camera and LiDAR data) with other vehicles enhances the traffic safety of autonomous vehicles because it helps vehicles locate other vehicles and pedestrians in their blind spots. Such safety applications require high throughput and short delay, which cannot be achieved by conventional microwave vehicular communication systems. Therefore, millimeter-wave (mmWave) communications are considered to be a key technology for sharing perceptual data because of their wide bandwidth. One of the challenges of data sharing in mmWave communications is broadcasting because narrow-beam directional antennas are used to obtain high gain. Because many vehicles should share their perceptual data to others within a short time frame in order to enlarge the areas that can be perceived based on shared perceptual data, an efficient scheduling for concurrent transmission that improves spatial reuse is required for perceptual data sharing. This paper proposes a data sharing algorithm that employs a graph-based concurrent transmission scheduling. The proposed algorithm realizes concurrent transmission to improve spatial reuse by designing a rule that is utilized to determine if the two pairs of transmitters and receivers interfere with each other by considering the radio propagation characteristics of narrow-beam antennas. A prioritization method that considers the geographical information in perceptual data is also designed to enlarge perceivable areas in situations where data sharing time is limited and not all data can be shared. Simulation results demonstrate that the proposed algorithm doubles the area of the cooperatively perceivable region compared with a conventional algorithm that does not consider mmWave communications because the proposed algorithm achieves high-throughput transmission by improving spatial reuse. The prioritization also enlarges the perceivable region by a maximum of 20%.

The present paper introduces a prototype design and experimental results for a multi-user MIMO linear precoding system. A base station and two mobile stations are implemented by taking full advantage of the software-defined radio. The base station consists of general purpose signal analyzers and signal generators controlled by a personal computer. Universal software radio peripherals are used as mobile stations. Linear spatial precoding and a simple two-way channel estimation technique are adopted in this experimental system. In-lab and field transmission experiments are carried out, and the bit error rate performance is evaluated. The impact of the channel estimation error under average channel gain discrepancy between two mobile stations is analyzed through computer simulations. Channel estimation error is shown to have a greater influence on the mobile station with the greater average channel gain.

This paper proposes an adaptive algorithm for adaptive arrays that minimizes the bit error rate (BER) of the array output signals in radio communication systems with the use of multilevel modulation signals. In particular, amplitude phase shift keying (APSK) is used as one type of multilevel modulations in this paper. Simultaneous non-linear equations that are satisfied by the optimum weight vector of the proposed algorithm are derived and used for theoretical analyze of the performance of the adaptive array based on the proposed algorithm. As a result of the theoretical analysis, it can be shown that the proposed adaptive array improves the carrier to interference ratio of the array output signal without taking advantage of the nulls. Furthermore, it is confirmed that the result of the theoretical analysis agrees with that of computer simulation. When the number of the received antenna is less than that of the received signals, the adaptive array based on the proposed algorithm is verified to achieve much better performance then that based on the least mean square (LMS) algorithm.

This paper proposes a new effective data transfer method for IEEE 802.11 wireless LANs by integrating priority control and multirate mechanism. The IEEE 802.11 PHY layer supports a multirate mechanism with dynamic rate switching and an appropriate data rate is selected in transmitting a frame. However, the multirate mechanism is used with the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol, low rate transmissions need much longer time than high rate transmissions to finish sending a frame. As a result, the system capacity is decreased. The proposed method assumes the same number of priority levels as the data rates, and a data rate is associated to a priority level. Priority of a transmission goes up with the used data rate. For this purpose, we have modified the CSMA/CA protocol to support prioritized transmission. By selecting the appropriate priority depending on the data rate and giving more transmission opportunities for high rate transmission, the system capacity is increased. The effect of the proposed mechanism is confirmed by computer simulations.

To improve the reliability and efficiency of multicast transmissions in wireless systems, a novel retransmission procedure is desired. In this paper, the representative acknowledgment scheme for reliable wireless multicast communications is proposed that offers quite a low packet loss rate. The proposed protocol carries out retransmissions in the datalink layer within the wireless region, and retransmissions do not affect the traffic in the wired region. The representative acknowledgment scheme employs both positive acknowledgment (ACK) and negative acknowledgment (NACK) to achieve reliable multicast transmissions and reduces the number of responses to be returned by forming groups of stations in the cell. One of the members in a group, called a representative station, returns a response for a received data frame while the others return a NACK if necessary. With this scheme, reliable multicast transmissions are enabled in wireless communications without spending much time as in conventional reliable multicast protocols. The performance of the proposed protocol is evaluated by numerical analyses and by computer simulation. The results show that 30% or more decrease in transmission time is achieved in a typical wireless environment.

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 new vector error measurement scheme for orthogonal frequency division multiplexing (OFDM) systems that is used to define transmit modulation accuracy. The transmit modulation accuracy is defined to guarantee inter-operability among wireless terminals. In OFDM systems, the transmit modulation accuracy measured by the conventional vector error measurement scheme can not guarantee inter-operability due to the effect of phase noise. To overcome this problem, the proposed vector error measurement scheme utilizes pilot signals in multiple OFDM symbols to compensate the phase rotation caused by the phase noise. Computer simulation results show that the vector error measured by the proposed scheme uniquely corresponds to the C/N degradation in packet error rate even if phase noise exists in the OFDM signals. This means that the proposed vector error measurement scheme makes it possible to define the transmit modulation accuracy and so guarantee inter-operability among wireless terminals.

This paper proposes a lattice-reduction-aided MIMO-OFDM receiver with virtual channels; the receiver enables an increase in the downlink transmission speed for a user where the number of transmit antennas is considerably higher than that of the receive antennas. However, the receiver has a higher computational complexity than conventional lattice-reduction-aided MIMO receivers. Accordingly, we also propose novel techniques to reduce the computational complexity for the lattice-reduction-aided MIMO receivers with virtual channels. The proposed MIMO receiver achieves superior performance in 102 MIMO-OFDM systems. Furthermore, the proposed techniques are shown to reduce the computational complexity to approximately 40% of the original configuration in the 102 MIMO-OFDM systems.

This paper proposes a novel scheme called as“frequency domain imbalance estimation” that estimates the imbalance of the Hilbert transformer in heterodyne multimode/band receivers with baseband automatic gain control (AGC). The proposed scheme uses correlation matrices in the frequency domain. This enables the receivers to keep high transmission performance in spite of the imbalance of the analog Hilbert transformer, by offsetting the imbalance. Moreover, the baseband AGC relaxes the requirement of the baseband A/D converter. The performance of imbalance estimation and imbalance cancellation is verified by computer simulation. As a result, it is shown that the proposed scheme not only estimates the imbalance of Hilbert transformer with extremely high precision, but also cancels the image-band interference such that it achieves the theoretical performance.

This paper proposes a suitable combination of the digital modulation schemes and the coding-rate of forward error correction (FEC) schemes for satellite digital video communication networks. The comparative study is carried out by computer simulation considering non-linearly amplified, narrow bandwidth satellite channels with adjacent channel interference signals. The proposed system employs an offset QPSK modulation scheme supported by the coding-rate of 7/8 convolutional encoding and Viterbi decoding to realize high-quality and compact spectrum characteristics in non-linear channels. By employing a 32Mbps DPCM video codec, the developed prototype system achieves a post demodulated S/N ratio of higher than 52dB. Moreover, it achieves high protection ratio against co-channel interference than conventional analog FM systems. The optimized digital video transmission system makes it possible to transmit high-quality NTSC video signals over non-linearly amplified narrow bandwidth satellite channels, for example 27MHz or 36MHz bandwidth transponders, with high-security digital encryption.

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.

Long-distance wireless local area networks (WLANs) are the key enablers of wide-area and low-cost access networks in rural areas. In a WLAN, the long propagation delay between an access point (AP) and stations (STAs) significantly degrades the throughput and creates a throughput imbalance because the delay causes unexpected frame collisions. This paper summarizes the problems caused in the medium access control (MAC) mechanism of the WLAN by a long propagation delay. We propose a MAC protocol for solving the delay-induced throughput degradation and the throughput imbalance between the uplink and the downlink in WLANs to address these problems. In the protocol, the AP extends NAV duration of CTS frame to protect an ACK frame and transmits its data frame to avoid delay induced frame collisions by piggybacking on the ACK frame transmission. We also provide a throughput model for the proposed protocol based on the Bianchi model. A numerical analysis using the proposed throughput model and simulation evaluation demonstrate that the proposed protocol increases the system throughput by 150% compared with that obtained using the conventional method, and the uplink throughput can be increased to the same level as the downlink throughput.

This paper proposes a new maximal ratio combining (MRC) frequency diversity automatic-repeat-request (ARQ) scheme suitable for high-speed orthogonal frequency division multiplexing (OFDM) systems that is based on the conventional packet combining ARQ scheme. The proposed scheme regularly changes the previously prepared subcarrier assignment pattern at each retransmission according to the number of retransmissions. This scheme sharply reduces the number of ARQ retransmissions and much improves the throughput performance in slow fading environments by virtue of the frequency diversity effect while it requires no complex adaptive operations. Computer simulation results show that the proposed scheme reduces the required number of retransmissions to about 3 at the accumulative correct packet reception rate (ACPRR) of 0.9.

This paper proposes a superposed SSMA (Spread Spectrum Multiple Access)-QPSK (Quadrature Phase Shift Keying) signal transmission scheme over high speed QPSK signals to achieve higher frequency utilization efficiency and to facilitate lower power transmitters for SSMA-QPSK signal transmission. Experimental results show that the proposed scheme which employs the coding-rate of one-half FEC (Forward Error Correction) and a newly proposed co-channel interference cancellation scheme for SSMA-QPSK signals can transmit twenty SSMA-QPSK channels simultaneously over a nonlinearly amplified high speed QPSK signal transmission channel and achieve as ten times SSMA channels transmission as that without co-channel interference cancellation when the SSMA-QPSK signal power to the high speed QPSK signal power ratio equals -30dB. Moreover, cancellation feasibility generation of the interference signals replica through practical hardware implementation is clarified.