Fumio Takahata

A PSK Group Modem for Satellite Communications

A PSK group modem which modulates and demodulates multiple PSK signals en bloc will play an important role in realizing a cost-effective satellite communications system based on the narrowband multicarrier PSK/FDMA technique. This paper briefly discusses its possible applications, and presents a developed PSK group modem including its algorithm, hardware design, and implementation. In the algorithm, filtering of respective PSK signals is effectively carried out by digital signal processing techniques applied to the digital transmultiplexer. Furthermore, in order to compensate for the difference between the operation timing of the PSK group modem and the symbol timing of each input/output data signal, digital signal processings based on a table-look-up method using ROM and a rate-conversion filter with adaptive weighting are proposed for the modulator and demodulator, respectively. An experimental 4-phase PSK group modem has been developed, and experiments have been conducted under various conditions. The measured performance of the developed hardware is also demonstrated.

Development of a TDM/FDM Transmultiplexer

Recently, digital systems such as digital telephone switching systems and digital transmission paths are being introduced more and more into communications networks in which signals are handled with Time Division Multiplex (TDM). There still exists, on the other hand, a number of analog transmission paths in which signals are multiplexed with Frequency Division Multiplex (FDM). Therefore, it is important to develop a scheme to interconnect efficiently TDM and FDM systems. Transmultiplexer, which is an equipment directly converting PCM.TDM signals and SSB.FDM signals with each other, is expected to provide an efficient interconneetion between TDM and FDM systems. Authors have developed an experimental transmultiplexer which can provide mutual conversion between a 24-channel PCM primary group (1.544 Mbits/s) and two 12-channel FDM basic groups (60-108 kHz). The developed equipment is designed so as to achieve reduction of the computation rate and simplicity of the hardware by the following techniques: (1) The FFT (Fast Fourier Transform) technique is used to optimize the filtering and multiplexing processes. (2) The spectrum reversion of odd-numbered channel signals is achieved by inverting the sign bits of the PCM signals every other frame. (3) The bandpass sampling technique is applied to obtain SSB.FDM signals directly without frequency shifts at analog FDM stages. In order to evaluate the characteristics of the developed transmultiplexer, experiments have been conducted by connecting it with FDM terminal equipments. The experimental results showed that the transmultiplexer can offer better performance than the tandem connection of standard PCM multiplex equipments and standard FDM terminal equipments with reasonable hardware.

Ku-band long distance site-diversity (SD) Characteristics using new measuring system

This paper deals with the short (10 km) and long distance (300-1400 km) site-diversity (SD) characteristics by using a newly developed measuring system. In the proposed measuring system, six earth stations transmit 14-GHz band QPSK signals, and one measuring earth station receives 12-GHz band signals and processes them to determine SD characteristics. As a result, easy operation and maintenance, low-cost measuring system construction and highly accurate data have been obtained. By comparing those measured results with the SD joint probability approximation equation in ITU-R Rec.P.618-7, a good agreement can be obtained. Furthermore, the effect of typhoons on SD characteristics were measured.

Long distance site-diversity (SD) characteristics by using new measuring system

This paper deals with the short and long distance (10-1400 km) site-diversity (SD) characteristics by using a newly developed measuring system. In the proposed measuring method six Earth-stations transmit QPSK signals and one measuring Earth-station receives signals and processes them for site-diversity characteristics. As a result, easy operation and maintenance, low cost measuring system construction and high accuracy data can be obtained. By comparing these measured results with the site-diversity joint probability approximation equation in CCIR Rep. 564-4, relatively good agreement can be found.

Scheduling algorithm based on sender buffer backlog for real-time application in mobile packet networks

The demand for high-speed data communication and the use of real-time applications in mobile networks is becoming increasingly high and communication systems that specialise in downlink packet transfer are being developed and deployed. In these high-speed downlink packet networks, scheduling at the base station is one of the key technologies to accomplish such high speed. A scheduling algorithm that is based on the proportional fair criterion is widely used in commercial networks, but the problem with such an algorithm is that it does not schedule real-time application traffic efficiently. This paper proposes a new scheduling algorithm for real-time applications that does not need substantial change in the existing system and coexists efficiently with the proportional fair criterion. We evaluate the characteristics of the proposed algorithm through computer simulation and show that the efficiency of the proposed algorithm is better than that of existing algorithms.

Long distance site diversity characteristics measurement results via JCSAT by using one Earth station measuring method

Satellite communication technologies are now highly developed and high quality, distance-independent-services have expanded over very wide areas. In particular, low cost very small aperture terminal (VSAT) services for data, voice and video communications have expanded widely with new services and new applications. The Hokkaido Integrated Telecommunications (HIT) network has been examined with satellite communication systems and newly-built optical fiber or low cost SHF band terrestrial radio networks. By using the HIT network, economical and highly functional services, including expansion of the network to rural areas can be realized. As for the system design of the HIT network, it must be constructed as economically as possible. Furthermore, to apply the 14/12 GHz band VSAT system, it must first overcome rain attenuation discontinuity of the satellite links, which is most difficult problem to the 14/12 GHz band. To resolve those problems, several propagation experiments were carried out; (1) determination of the rain margin measurement for optimum design of the HIT network, and (2) site diversity characteristics measurement. The propagation characteristics for 14/12 GHz band are measured via the JCSAT-1 transponder by using a newly developed measuring method.

An Optimum Traffic Loading to Intersatellite Links

An algorithm has been developed to evaluate the benefits of intersatellite links (ISLs) quantitatively in satellite communications networks. The developed algorithm derives the minimum satellite capacity, under the condition that the same type of satellites are located onto the geostationary orbit, by assigning traffic among earth stations to the satellites effectively. Some example calculations are presented, based on a current INTELSAT database. The calculated results indicate that the benefits of ISL depend strongly on the distribution pattern of traffic in the network. They demonstrate, furthermore, that ISLs over long distances have significant potential advantages in the Atlantic and Pacific Ocean regions for accommodating a large volume of traffic, and enabling the effective utilization of satellite resources at high elevation angles.

International Connection of Plesiochronous Networks via TDMA Satellite Link

Digital transmission techniques are expected to spread throughout the fixed-satellite service in the near future. The introduction of digital satellite systems will lead to a new requirement regarding how to connect the satellite link with terrestrial digital networks. This paper presents a satellite/terrestrial digital interface that has been developed to interconnect 2.048 Mbit/s PCM terrestrial and INTELSAT TDMA links which are in the plesiochronous relationship. Major functions of the interface include frame slip control, compensation for path length variation, and rate and format conversions. This paper also describes the design and implementation of the developed equipment, and experimental results for signal impairment due to the frame slips and PCM multiplex standard conversion. The experimental results demonstrate that a large humber of bit errors are induced by frame slips for voiceband data transmission above 4800 bits/s.

A Digital 60 Channel Transmultiplexer: Algorithm Minimizing Multiplication Rate and Hardware Implementation

A TDM/FDM conversion algorithm for realizing transmultiplexers with an FFT processor and a set of digital subfilters is proposed which provides a significant saving in multiplication rate. In the proposed algorithm in which an FDM signal is directly sampled, the dimension of the FFT processor is reduced by provision of preand postprocessing and the multiplication rate in radix-3 is reduced by refinement of the processing. In addition, the multiplication rate in the subfilters is reduced by the adoption of symmetrical coefficients. Although the direct sampling of an FDM signal has been considered to increase the multiplication rate compared with the sampling with frequency shift of FDM signal, our estimation shows that the increase of multiplication rate in the proposed algorithm keeps within 20 percent compared with that in a typical algorithm based on sampling with frequency shift of FDM signal. Furthermore, it shows that the proposed algorithm reduces the multiplication rate by 30 percent compared with a previously proposed algorithm based on direct sampling of FDM signal. Based on the proposed algorithm, a digital 60 channel transmultiplexer has been implemented. System configuration of the developed equipment is outlined and its measured performance is described.

Vector perturbation using signal space conversion assisted vector ranking algorithm

Nonlinear precoding multi-user multiple-input multiple-output using vector perturbation (VP MU-MIMO) can provide good frequency efficiency at the cost of computational complexity, which exponentially increases as the number of spatial multiplexed users increases. This is because the original VP precoding requires to exhaustively search the optimum transmit signal vector among all the combinations of each users transmit signal candidates. To reduce the computational complexity, in this paper, we propose signal space conversion assisted vector ranking algorithm, which exploits the equivalency between the search of optimum transmit signal vector in the VP precoding and the signal detection from received signal in the general signal transmission. And, its transmission performance and computational complexity are evaluated by computer simulation.