This paper proposes a novel modulo loss suppression scheme employing a theoretical BER formula for multi- user multiple-input multiple-output (MU-MIMO) Tomlinson-Harashima Precoding (THP) systems. The main feature of the proposed approach is to control the constellation distance by using the simple theoretical bit error rate (BER) formula in order to overcome the modulo loss. Behind this proposal, there is an interesting fact that the BER performance is dependent on the influence of both the thermal noise and the modulo loss and that its influence is determined by the constellation distance. In the proposed approach, we newly derive a theoretical simple BER formula for MU- MIMO THP and then find out the optimal constellation distance to suppress the modulo loss. The effectiveness of the proposed scheme is demonstrated in comparison to the traditional MU-MIMO THP with fixed distance by means of computer simulations.

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 user's 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.

Multi-user MIMO (MU-MIMO) employing Tomlinson Harashima Precoding (THP) is a powerful technique to enhance the system capacity in multi-user mobile communications systems. However, MU-MIMO has an inherent drawback in the degradation of the transmission performance due to the time variant fading channels which are caused by the mobility of the mobile station (MS). Considering that MU-MIMO is mainly applied to mobile communications systems, the performance degradation due to this terminal mobility has to be solved. This paper proposes a novel MU-MIMO THP transmission scheme for the sake of alleviating the performance degradation due to the terminal mobility. The feature of the proposed approach is to incorporate the concept of time division multiplexing (TDM) into MU-MIMO THP system, which enables the transmission of high mobility user to take precedence over that of the lower mobility user. Thanks to the use of TDM, the proposed scheme exploits the space diversity benefit because TDM decreases the number of MSs within a time slot. The effectiveness of the proposed scheme is demonstrated in comparison with the traditional MU-MIMO THP scheme by means of computer simulations.

This paper proposes an inter-symbol interference (ISI) suppression scheme using the periodic signal waveform for fixed-rate coded OFDM systems. In this scheme, the periodic signal waveform composed of the even-numbered sub-carrier group is used for extending the guard interval, and the odd-numbered sub-carrier group is also enable to be available by means of frequency conversion in order to realize the sub-carrier group selection. This paper mainly focuses on the combining effect of the sub-carrier group selection and the channel coding scheme in the inter-symbol interference (ISI) condition. Computer simulation results show that the proposed scheme has the great potential to create the combined frequency diversity effect without the error floor even in the severe ISI condition.

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.

Orthogonal frequency division multiplexing (OFDM) systems avoid the effect of multipath fading by absorbing delayed waves into guard interval. But as the effect of delayed waves over guard interval becomes stronger, it causes the bit error rate (BER) performance in OFDM systems to be more degraded. This paper proposes the approximate derivation method of the BER in DQPSK/OFDM systems over frequency selective fading channels. In this study, we will derive the approximate equation of the BER with differential coding/differential detection in the time domain. We confirm that the BER derived from the proposed approximate equation coincides with that by computer simulation even when such parameters as the Doppler frequency shift, the delay spread, the Rician factor and so on are varied.

The transmission quality in mobile wireless communications is affected not only by additive white Gaussian noise, but also by multipath fading, which drastically changes the amplitudes and phases of wireless signals. It is one of the key themes in direct-sequence code-division multiple-access systems to evaluate average bit error rate (BER) performances as parameters of the Doppler frequency, the delay profile, the number of simultaneous access users, and so on. This paper proposes the approximate equation for easily calculating the average BER over fading channels (multiray fading models). The performance evaluations are carried out in the two-ray model, the IMT2000 model, and the COST207 model. It is confirmed from the coincidence of approximate results with computer simulation results that the proposed approach is applicable to a variety of parameters.

An algorithm has been developed to evaluate the benefits of intersatellite links (ISL's) 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 ISL's 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.

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.

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 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 pre- and 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. The authors' 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.

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.