A field trial was conducted to evaluate the technical performance of land mobile message communication in different environments. The OmniTRACS system and the Ku-band JCSAT satellite were utilized as the mobile communications system and the satellite, respectively. The trial took place in September 1990 at different areas in Japan. Data collected correspond to about 65 hours of operation, during which a large number of messages were sent via the satellite. Two land mobile terminals operated simultaneously, each terminal having a function of generating messages automatically which simulates a large volume of traffic corresponding to about 50 terminals. Thus, the system was evaluated under the condition that 100 mobile terminals were in operation. Obtained data have been analyzed with a particular focus on the message transmission correlating with actual environments. The analysis was done by classifying environments into five categories: overall condition, type of roads, terrain, areas and weather conditions. The average transmission count per message experienced under all conditions is equal to 1.432 for forward messages transmitted from the hub station to mobiles, and 1.157 for return messages transmitted from mobiles to the hub station. With respect to the classification by the type of roads, for enample it becomes obvious that the performance is generally good except along roads of North-South orientation through dense urban areas. It is concluded that the message communications from/to mobiles are feasible in a wide range of environments, with the performance of success essentially depending on the visibility of satellite.

New method for deriving the bit error rate (BER) of the BPSK signal in the cochannel interference is proposed, which utilizes the eye pattern of the interference signal, and is different from the conventional method based on the conversion of the interference components to thermal noise. The validity of the proposed derivation method is quantitatively evaluated in terms of the BER performance and is confirmed by comparing with the results obtained by the computer simulation.

This paper proposes an inter-symbol interference (ISI) suppression scheme using only the even-numbered sub-carriers for the fixed-rate OFDM systems with the 2-dimensional modulation. The proposed scheme is based on the principle that the first half of the waveform in the time domain is the same as the second half when an OFDM symbol is composed of only the even-numbered sub-carriers. The feature of the proposed scheme is that, in the case of the maximum multipath delay beyond the duration of the guard interval, the OFDM symbol with only the even-numbered sub-carriers is transmitted in order to generate the extended virtual guard interval and that the high-level modulation with the sub-carrier power enhancement is applied to achieve the constant data rate. In addition, at the receiver, only the second half of the OFDM symbol is used for the FFT processing to avoid the ISI. Moreover, the condition of the maximum multipath delay is notified to the transmitter by using the feedback channel. Numerical results given by computer simulation showed that the proposed scheme provides far better bit error rate (BER) performance than the traditional OFDM transmission using all sub-carriers under the multipath delay beyond the duration of the guard interval.

Considering that the application of the direct-sequence slotted spread ALOHA communications system to access/control channel is effective in communications systems with traffic channels operating at CDMA mode, the spread ALOHA communications system is discussed in terms of the system configuration and transmission efficiency. The transmission efficiency of the spread ALOHA communications system using a unified spread code is derived by means of two methods. One is based on the simulation of demodulation algorithm, and the other is based on the approximation by modeling. It becomes obvious from quantitative evaluation in terms of the probability of packet success and the throughput performance that the approximated results coincide with the simulated results, and that the modeling is very effective to estimate the transmission efficiency of the spread ALOHA communications system.

This paper proposes a frequency diversity scheme using only even-numbered samples for single-carrier transmission with frequency-domain equalization (SC-FDE). In the proposed scheme, a periodical frequency spectrum generated by using only even-numbered samples in the time domain provides the frequency redundancy, which is utilized for frequency diversity. Moreover, in order to avoid the data rate reduction due to the decrease in the samples within one block, the high-level modulation is applied to each sample and the transmitting power of each sample can be doubled for the equivalent power transmission instead. Computer simulation results show that the proposed scheme provides a steeper BER curve than the typical SC-FDE over frequency selective fading channels, while the typical SC-FDE is more favorable than the proposed scheme over flat fading channels. Moreover, the proposed scheme still retains its characteristic even when channel estimation and channel coding are additionally taken into account.

This letter presents an iterative decision feedback channel estimation scheme for burst mode COFDM transmission. The feature of the proposed scheme is that the channel estimation using metrics comparison is applied to the initial stage of the iterative mechanism, which makes it possible to provide a reliable data stream at the initial stage. Computer simulation results show that the proposed approach provides better BER than the traditional iterative decision feedback channel estimation scheme irrespective of the number of iterations.

The paper proposes a transmitter diversity scheme with a desired signal selection for the mobile communication systems in which the severe cochannel interference (CCI) is assumed to occur at the base station. The feature of the proposed scheme is that the criterion of the downlink branch selection is based on the desired signal power estimated by the correlation between the received signal and the unique word at the matched filter. Moreover, the unique word length control method according to the instantaneous SIR is applied to the proposed scheme, taking account of the uplink transmission efficiency. Computer simulation results show that the proposed scheme provides the better performance than the conventional transmitter diversity in the severe CCI environments, and that the unique word length control method applied to the proposed scheme decreases the unique word length without the degradation of the transmission quality, comparing with the fixed unique word length method.

This paper proposes a 2-dimensional linear propagation prediction (LPP) in maximal ratio combining (MRC) transmitter diversity for orthogonal frequency division multiplexing (OFDM) time division multiple access--time division duplex (TDMA/TDD) systems in order to overcome the degradation of the transmission performance due to the fast fading or the TDD duration. In the proposed scheme, the downlink channel condition of each sub-channel is predicted by interpolating the uplink fading fluctuation with both the amplitude and phase, and the predicted downlink channel condition is used for the weighting factor to employ MRC transmitter diversity. Numerical results obtained by the computer simulation show that the proposed 2-dimensional LPP with the second-order Lagrangeis interpolation predicts the downlink channel condition accurately under the fast fading or the long TDD duration. Moreover, in such a condition, the proposed LPP provides far better performance than the conventional 1-dimensional LPP.

The transmission quality in mobile wireless communication systems is affected by not only AWGN but also multi-path fading. Particularly, the Doppler frequency, the delay spread and the Rician factor have a great influence upon the quality over fading channels. This letter proposes the approximate equation for easily calculating the BER in DS-CDMA systems over multi-path fading channels. The validity of the approximate equation is confirmed from the fact that the BER calculated by the equation coincides with that by the computer simulation.

This paper presents the transmission performance of a class-IV partial-response signaling SSB system and proposes a method that can improve its power efficiency. A line code that has no dc component has been used in the SSB transmission of digital signals. The type of line code, such as a partial-response signaling, increases the modulation states, and as a result, decreases the power efficiency. To overcome this obstacle, a new demodulation method called "re-filtering and combining" is proposed on the assumption of orthogonal phase detection. The demodulated quadrature channel is re-filtered by a Hilbert filter and is combined with the in-phase channel. It is confirmed by computer simulations that the new demodulation method improves the BER performance and a 3 dB improvement of the power efficiency is obtained.

This paper discusses and evaluates, from the viewpoints of definition, analysis, and performance, frequency assignment schemes that enable the efficient assignment of multiple-bandwidth carriers on the transponder in SCPC/FDMA systems with demand assignment operation. The system considered handles carriers of two different bandwidths, and assigns only consecutive slots on the transponder band to broadband carriers. Three types of frequency assignment schemes are proposed, each of which incorporates one or both of two assignment concepts: (1) pre-establishment of assignment priorities on the transponder band, and (2) establishment of broadband slots to guide broadband carrier assignment. Following a definition of the schemes, equations are derived to theoretically analyze performance factors such as call loss for the narrowband and broadband carriers, and system utilization efficiency. Finally, theoretical performance calculated for various traffic and system conditions are presented and evaluated, for the purpose of comparison between the three schemes. Computer simulation results are also presented, to demonstrate the accuracy of the derived equations and to supply data for models too large for theoretical computation. Main results obtained are as follows. (1) Regardless of traffic or system conditions, the assignment scheme incorporating both assignment priorities and broadband slots shows the best performance in terms of broadband call loss and system utilization efficiency. (2) The establishment of broadband slots improves performance when the ratio of broadband traffic to the total traffic volume is high, but worsens performance when the narrowband traffic ratio is higher. (3) All aspects of performance improve with the increase of the total number of assignable slots on the transponder band.

The transmission quality in mobile wireless communications is affected by not only the thermal noise but also the multipass fading which changes drastically an amplitude and a phase of received signal. The paper proposes the theoretical and approximate methods for deriving an average bit error rate in DS-CDMA systems under the Rician fading environment on the assumption of the frequency non-selective fading, as parameters of the number of simultaneous access stations, the maximum Doppler frequency and so on. It is confirmed from the coincidence of theoretical and approximate results with simulation ones that the proposed approach is applicable to a variety of system parameters.

Transmitter diversity is a powerful technique to improve the transmission quality of downlink in microcellular mobile communications systems. Under cochannel interference (CCI) at the base station (BS), the transmitter diversity is not necessarily effective, because the desired-plus-interference signal power used as a criterion of downlink branch selection is not always relative to the downlink propagation condition. This paper proposes the theoretical derivation of bit error rate (BER) performance in the transmitter diversity under CCI occurring at BS, as parameters of average SIR at BS, normalized Doppler frequency, and so on. It is confirmed from the correspondence of theoretical results with simulation results that the proposed theoretical approach is applicable to the CCI environments at BS.

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.