Key aspects and technologies of future satellite communications are discussed toward multimedia era. Onboard processing called the switchboard in the sky and networking taking full advantage of features peculiar to satellite communications are pointed out as essential technologies to overcome a variety of big challenges for realizing future satellite communications. Several experimental and commercial systems are introduced as the first step toward multimedia era.

This paper presents fully digital high speed (17.6Mb/s) burst modem for Offset Quadrature Phase Shift Keying (OQPSK), which employs novel digital modem VLSICs. The modulator VLSIC directly generates modulated intermediate frequency (IF) signals in a fully digitalized manner. A newly proposed digital reverse-modulation and pre-filtered carrier filter-limiter scheme realizes low power consumption and stable operation in a low E_b/N_o condition. The demodulator VLSIC also achieves fast bit-timing acquisition in burst mode. Moreover, it supports stable initial burst acquisition by a novel automatic frequency control (AFC) acquisition detector and a digital burst detector. A digital burst automatic gain control (AGC) compensates burst-to-burst level differences without analog circutits. Performance evaluation results show that the new modem achieves satisfactory bit-error-rate performance in severe environments. The developed modem has been employed in a commercial portable earth station for ISDN services and reduces the hardware size to one third that of the conventional one.

This paper proposes a new simultaneous carrier and bit-timing recovery (CBR) scheme for offset quadrature phase shift keying (O-QPSK) for agile acquisition over satellite communication channels. The proposed simultaneous CBR scheme employs a preamble shared for the carrier and bit-timing recover, which has a specific bit-pattern designed so that its baseband signal alternates between two adjacent decision points at the symbol rate. Using the preamble, the proposed simultaneous CBR scheme estimates the carrier phase and the bit-timing, simultaneously and independently, by open-loop approach. For comparison, this paper also describes the performance and configuration of a joint carrier and bit-timing recovery scheme, which is expanded for O-QPSK from the one conventionally proposed for QPSK. This paper demonstrates with simulation results that the proposed simultaneous CBR scheme significantly improves the agility of acquisition: a mere 30-symbol preamble is sufficient for low-E_b/N_o channels typical of satellite communication systems. The proposed CBR scheme is also advantageous from the viewpoint of digital implementation: it processes at 2 samples/symbol and eliminates an analog voltage control clock (VCC). The proposed simultaneous CBR scheme is a strong candidate for TDMA systems that require the high data-transmission and frequency utilization efficiency.

This paper proposes a new AFC (automatic frequency control) circuit employing a double-product type frequency discriminator to enable fast acquisition in very-low CNR (carrier to noise power ratio) environments. The frequency step responses of the proposed AFC circuit are theoretically analyzed. In addition this paper evaluates the performance of the proposed AFC circuit by computer simulation in very-low CNR environments. The simulation results confirm that click noise at the frequency discriminator causes large frequency tracking error and that this error can be improved by increasing the delay time of the double-product type frequency discriminator. The frequency error can be also reduced by introducing the proposed frequency discriminator to modify the frequency error detection performance. The acquisition time of the proposed AFC circuit can be reduced by about 100 symbols compared to the conventional cross-product type AFC circuit.

This paper proposes a novel M-ary FSK demodulation scheme using the Short Time Discrete Fourier Transform (ST-DFT) analysis named Frequency Sequence Estimation (FSE) for low earth orbit (LEO) satellite-based personal multimedia communications. The FSE is a kind of the Viterbi algorithm, searching for the maximum likely frequency path using the instantaneous ST-DFT output as a metric. It is based on the fact that the discrete time-frequency representation of the received signal can be interpreted as a trellis diagram. The proposed method has the excellent transmission performance and spectral efficiency, as well as its own hardware simplicity and frequency offset insensitivity.

This paper evaluate the performance of a PN (pseudonoise) code acquisition for a direct-sequence/code-division-multiple-access (DS/CDMA) system in a mobile satellite environment. The acquisition scheme considered consists of a parallel matched-filter and a FFT processor. The uplink of mobile satellite channel is modeled as shadowed Rayleigh fading channel. The effects of power control error and shadowing are considered in the analysis of acquisition performance. It is shown that the power control error causes acquisition to be slower than the case of perfect power control, and for high SNR/chip, the effect of power control error becomes less significant. It is also shown that the case with heavy shadowing takes longer time to achieve acquisition than that with light shadowing. For the subinterval-based PN code search, the parallel MF scheme is thought to be more appropriate than the serial MF scheme at the cost of complexity. The analysis in the paper can be applied to the uplink of a DS/CDMA system for packet-type services in a mobile satellite channel.

As the demand for communications via satellite is rapidly increasing, techniques that produce large traffic capacity are becoming more and more appreciated. We present a class of block coded modulation (BCM) and multiple block coded modulation (MBCM) schemes in this paper. While the BCM scheme is directly derived from our previous work, the MBCM schemes are newly developed using a technique of multiple symbol transmission via a single trellis branch. This class of BCM and MBCM schemes is both power and bandwidth efficient. They also have an advantage in holding both a trellis and a block structure. Code structures, decoding trellises and the corresponding branch variables of these BCM and MBCM schemes are all derived. Their applications to satellite communications are discussed. Computer simulations are performed to verify coding gain performance.

A Reed-Solomon coded Type-I Hybrid ARQ scheme based on a Selective-Repeat (SR) ARQ with multicopy retransmission is proposed for mobile/personal satellite communication systems of a transmitter and a receiver both with the finite buffer. The performance of the proposed scheme on fading channels is analyzed. The basic idea of the strategy is the use of two modes; the SR mode and the multicopy mode. In the latter mode, erroneous blocks stored in the transmitter buffer are alternatively retransmitted multiple times when ν consecutive retransmissions in the SR mode are received in error. Numerical and simulation results for ν1 show that the proposed scheme presents better performance than the conventional SR+ST scheme 2 of the 2N block buffer by Miller and Lin.

Periodic reservation allows periodic and random packets to share the same satellite random access channel efficiently. The periodic reservation protocol is particularly suitable for mobile satellite position reporting services, where some of the information messages, such as dispatch function, are classified as "periodic" and others, such as signaling, are classified as "random." When a new mobile terminal logs on to the system, Network Management Center (NMC) reserves subsequent time slots for transmitting periodic packets without contention. A mobile terminal recognizes each time slot as "reserved" or "unreserved (available)" according to the broadcast message received from NMC. Other random packets use the slotted ALOHA protocol to contend with other mobile terminals for an unreserved time slot. The performance results suggest that the use of the periodic reservation protocol can be regarded as a viable solution for mobile satellite position reporting services such as automatic dependent surveillance (ADS).

In this paper, we evaluate the throughput performance of CDMA Slotted ALOHA systems. To improve the throughput performance, we employ the Quasi-synchronous sequences and the Modified Channel Load Sensing Protocol as an access control procedure. As a result, we found a good throughput by the QS-sequences. By employing MCLSP, we can keep the maximum throughput even in high offered load and in the presence of a long access timing delay, which is one of the issue in satellite packet communication systems.

In a conventional client-server system using the satellite communications, the responsibility of the system to the client user is considerably degraded by the long transmission time between the satellite and the ground terminal as well as the relatively low data transmission rate in comparison with the ground transmission line as the Ethernet. In this paper, a new client-server control, VEEC, is proposed to solve the problem. As a result of the experimental performance studies, it is clarified that the responsibility in the client is remarkably improved when the pre-fetching mechanism of VEEC works efficiently.

This paper proposes a time-dependent gateway earth station (GES) assignment method for a user terminal in non-geostationary orbiting satellite systems. Time-dependent nature of the GES service area is first discussed for an example intermediate circular orbit system. Then, the time-dependent GES assignment method is proposed. Finally, the advantage of the proposed method is shown by several calculation results.

The link properties of double-layered constellation composed of inclined orbits for an advanced global satellite communications network connected by optical inter-satellite links (ISLs) have been evaluated. The constellation consists of lower layer satellites for mobile and personal satellite communications, and upper layer satellites for large-capacity fixed satellite communications and feeder links. Optical inter-satellite links, which can perform high-capacity communications with small terminals, are used for all inter-satellite data transmission. Although a satellite constellation using polar orbits in both layers offers the merit of simplicity in network configurations, it has disadvantages caused by the satellite consentration above high latitudes. The inclined orbit constellation offers the potential for reducing the required number of satellites improving ling properties, and enhancing the coverage in middle and low latitudes, by selecting the optimum orbital inclinations. The link properties between the satellites and terminals on the ground, optical ISL properties, and required number of satellites were evaluated for constellations using inclined orbits, and compared with those of a polar orbit constellation. Three kinds of inclined orbit constellations achieving continuous double coverage, which is a minimum requirement for future advanced satellite communications applying satellite diversity, were assumed for each layer.

Recently, computer communications, especially Internet services, have become popular and as a result, high-speed network access circuits are now desired. NTT has developed an economical and high-speed multimedia computer network, combining satellite and terrestrial circuits. The satellite circuit transmission rate is approximately 30-Mbit/s. To select IP packets from such high-speed satellite circuits, this system employs the asynchronous transfer mode (ATM) in the satellite section and we have developed a new economical satellite circuit receive adapter (SRA) for the satellite section. This paper describes the system configurations and the key network control technologies for multi-link routing, high speed processing and broadcasting.

The DQDB MAC Protocol standardized by the IEEE 802.6 Committee is a single segment bandwidth reservation scheme that only reserves bandwidth for one segment in the distributed queue. Recently, multi-segment bandwidth reservation schemes that reserve bandwidth for not only one segment in the distributed queue but also a part of or all segments in the local node queue have been proposed. In this paper, we propose a new multi-segment bandwidth reservation protocol that can quickly react to changes in a node's traffic and can quickly allocate the bandwidth fairly and waste-free. We also evaluate the mean message transmission delay and throughput convergence performance by simulation. As a result, it is shown that the mean message transmission delay can be decreased and the throughput can be quickly converged to fair bandwidth allocation.

Banyan networks are used in multiprocessor computer applications for an ATM switching. In this paper, we study the continuous blocking of the first n-stage which makes the performance of the banyan networks decrease. We use the 2-dilated banyan networks into the banyan networks to remove the continuous blocking of the first n-stage. We call the new networks as the hybrid dilated banyan networks. We explain how to analyze the throughput of this networks at each stage. Based on the analysis of input rate and output rate at each stage, we can design the hybrid dilated banyan networks with the desirable output rate. The result of analysis shows the hybrid dilated banyan networks have higher performance and feasibility than the banyan networks.

A current ATM exchanger consists of ATM switch and ATM interface card is implemented with many LSIs. An investigation of an architecture of the ATM interface card is, therefore, important to decides ATM exchanger's functions and its flexibility. In this paper, we propose an architecture of the ATM interface card to contribute to improving the flexibility and reducing the cost for an ATM exchanger. The key feature of the proposed architecture is both physical layer and ATM layer functions at an interface point are executed by a general-purpose microprocessor and FIFOs. A realization of such architecture is discussed, especially, a software configuration of it is proposed because the physical layer functions have to be executed periodically and should not be interrupted. We developed an evaluation breadboard for such periodic software execution and evaluated the proposed ATM interface card architecture. The evaluation results indicate that a 50 MHz R3000 microprocessor and 5 MHz access speed FIFOs can realize the 6.3-Mbps cell relay interface card.

The design, fabrication, and testing of a highly multimode polymeric 88 star coupler is described. The design process allowed a comparison to be made of ray tracing and beam propagation methods for the design of such highly multimode waveguide devices. The results obtained with either of these two different methods agree well with actual measurements on a fabricated 88 multimode-input star coupler with a refractive index difference of Δn=0.0274 and a device length of L=4.25 cm. The reduction in the rms power fluctuation in the output guides with the choice of a higher refractive index difference is demonstrated.

A novel CDMA multi-user detection scheme, orthogonal decision-feedback detector (ODFD), is proposed for a synchronous CDMA system in this paper. It is robust for its near-far resistance and high multi-user detection efficiency with a performance similar to that of decorrelating decision-feedback detector (DDFD) but with a reduced complexity. The ODFD scheme employs a match-filter bank that matches a set of ortho-normal sequences. The ortho-normal sequences are generated by the Gram-Schmidt orthogonalisation procedure based on the spreading codes. The ODFD algorithm involves only with the ortho-normal coefficient-matrix which requires no frequent recalculations even when system parameters change. Successive decision-feedback detection is carried out immediately at the output of the ODFD match-filter bank without matrix inversion operations, resulting in a much simplified structure.

We have investigated the BER performance of TC 8PSK with 2 branch SC and MRC diversities on spatially correlated Rayleigh fading channel. The upper bounds using the transfer function bounding technique are derived several numerical results are shown. Although the correlation between branches causes signal-to-noise (SNR) loss (relative to uncorrelated fading case) for SC and MRC diversities, the diversity can lead to achieve the diversity gain compared to the system without diversity. It is found that the diversity gain of 4-state TC 8PSK is larger than 8-state TC 8PSK. It is also shown that the BER performance of TC 8PSK is decreased as the antenna separation is decreased.

We have proposed a digital beamforming (DBF) self-beam-steering array antenna which features maximal ratio combining enabling it to efficiently use the received power or to rapidly track the desired signal. The DBF self-beam-steering array antenna utilizes digital signal processing with an active array antenna configuration. ASIC implementation of the digital signal processor is inevitable for DBF antenna application in practical mobile communications environments. In this paper, we present a scheme for implementing a digital signal processor in ASICs using ten FPGAs (Field Programmable Gate Arrays) for the DBF self-beam-steering array antenna. Results of some experiments obtained in a large radio anechoic chamber are shown to confirm a basic function of the system.

A radio propagation experiment at the Okinawa Radio Observatory of the Communications Research Laboratory is investigating the feasibility of calibrating the spaceborne precipitation radar onboard the Tropical Rainfall Measuring Mission by using the path-averaged rainfall rate estimated from rain attenuation. Because this estimated rainfall rate has errors due to the spatial inhomogeneity of rainfall rate and the variability of raindrop size distribution, we used distrometer data to evaluate both of these errors by computer simulation.

This letter shows an architectural approach for analyzing real-time aspects of distributed multimedia processing systems. The results of this letter are 1) to propose the concept real-time supporting environments which consist of real-time traffic management/control environment and real-time application environment and 2) to analyze the real-time requirements of such environments.

A new simple cell spacing architecture that guarantees the peak cell interval and realizes preferential contention resolution is proposed. Scheduling the cell emission on departure of the previous cell, not arrival, allows the source peak cell interval to be regenerated without clumping. Priority control is also realized in the proposed spacer. A connection is scheduled either at the head or tail of the contention chain depending on its priority. The proposed method is applied to realize the UPC function. The proposed cell spacer eliminates the clumping effects of CDV completely and achieves high bandwidth efficiency.

A network with input and output buffer is proposed. It consists of several switching stages composed of 33 basic switching elements which are connected with perfect shuffle and horizontal connections. The proposed network reduces the required number of stages, and increases the fault tolerance due to its highly regular connection scheme. Its performance was evaluated with computer simulation under bursty traffic environment. For a 128128 switch with 11 switching stages, packet loss ratio of 10^-6 was obtained when the input load is 0.8 and the burstiness is 10.

We have developed an easily-assembled optical coupling device which consists of two multifiber array connectors and a single-mode planar waveguide chip whose ends are passively positioned in novel plastic plug components compatible with the multifiber array connectors. The assembled 18 splitter device exhibits a low exess loss of 0.8 dB.

In this letter, we propose a blind adaptation method for the decision feedback equalizer (DFE). In the proposed scheme, a DFE is divided into two parts: a front-end linear equalizer (LE), and a prediction error filter (PEF) followed by a feedback part. The coefficients of the filters in each part are updated using constant modulus algorithm and decision feedback prediction algorithm, respectively. The front-end LE removes intersymbol interference ISI, and the PEF with feedback part whitens the noise to reduce noise power enhanced by the LE. Pre-processing by the LE enables the DFE to equalize nonminimum phase channels. Simulation results show that the proposed scheme provides reliable convergence, and the resulting symbol error rate is much less than that of the conventional LE and very close to that of the DFE using a training sequence.