Fiber-optic passive double star (PDS) network is described as an access network for microcellular radio communication systems. The intrinsic characteristics of the PDS network, reduction in the optical fiber count and flexible access capability, are examined. A unit cell structure is introduced which enables the PDS network to be effectively incorporated into the access portion of microcellular radio communication systems. The reduced total fiber length in the unit cell structure based on the PDS network is discussed in comparison with the conventional architecture. Calculations show that there is an optimum splitting ratio that minimizes the total fiber length. When the microcell radius and service area radius are 100m and 10km, respectively, the total fiber length of the PDS network is reduced to only about 9% of that of the conventional single star (SS) network for a splitting ratio of 34. Resource sharing and handover between microcells in a unit cell are performed by using the dynamic channel allocation function of the PDS system. Substantial performance improvement for loaded traffic can be obtained by resource sharing. When the splitting ratio is 32, the available traffic of a base station (BS) increases from 0.9 [erl/BS] to 3.4 [erl/BS] by adopting dynamic channel allocation for the lost call probability of 0.01.

We have developed a new HIGFET structure achieving an extremely high K-value of 13.3 S/Vcm with a gate length of 0.15 µm. Self-aligned ion implantation is excluded to suppress a short-channel effect. An i-GaAs cap layer and an n+-GaAs contact layer are employed to reduce source resistance. The threshold voltage shift is as small as 50 mV when the gate length is reduced from 1.5 µm to 0.15 µm. Source resistance is estimated to be 53 mΩcm. We have also developed a new fabrication process that can achieve a shorter gate length than the minimum size of lithography. This process utilizes an SiO2 sidewall formed on the n+-GaAs contact layer to reduce the gate length. A gate length of 0.15 µm can be achieved using 0.35 µm lithography.

This paper proposes M-ary/SSMA using co-channel interference cancellation techniques and presents comparisons with conventional DS/SSMA and other systems. First, ideal models of DS/SSMA and M-ary/SSMA using co-channel interference cancellation techniques are analyzed. In the cancellation circuit of DS/SSMA, when an error bit of other user's data arises, the received signal is degraded by "voltage addition" of the error sequence. While, in M-ary/SSMA, it is degraded by only "power addition" of the error code. Therefore, though the circuits are complicated, bit error rate of the proposed system can be improved considerably. Further, improvement of spectral efficiency in these systems are shown for several bit error rate and chip waveforms.

In this paper we present a new tracking scheme using two tracking modes which are based on the concept of Delay Lock Loop (DLL). Under the multipath fading channels, a conventional DLL has problems of jitter performance degradation, lock-off and delay offset. It is necessary to solve these problems, because mobile communications have increased drastically. We propose the combination of a coarse tracking mode and a fine tracking mode. The former mode is employed for reducing the possibility of losing lock, the latter mode is used for suppressing the jitter of delay error and the delay offset in the presence of multipath fading. The both modes utilize the power of delay paths shown in the auto-correlation function of the received signal at the DLL. Computer simulation results show that our proposed scheme is extremely useful comparing with a conventional scheme over the multipath fading channels.

A time division duplex (TDD) direct sequence spread spectrum communication (DS-SS) system is proposed for operation in channels with Rayleigh fading characteristics. It is shown that using the TDD method is advantageous because the devices can be designed more simply, the method is more frequency efficient and as a result the systems will be less costly and less power consuming. It is also shown that an efficient power control method can be implemented for the TDD systems. In contrast to the traditional access techniques such as frequency division multiple access (FDMA) and time division multiple access (TDMA) that are mainly frequency limited, the code division multiple access (CDMA) method which uses the DS-SS technique is interference limited. This means that an efficient power control method can increase the capacity of the DS-SS communications system. Computer simulations are used to evaluate the performance of the TDD power control method. Performance improvement of order of 12 to 17dB at bit error rate (BER) of 10-3 can be obtained for different methods of power control. The advantages of the TDD technique for the future DS-SS systems operating in the Industrial, Scientific and Medical (ISM) band are explained in an appendix to this paper.

Indoor radio communications is an important component of the emerging personal communication systems service. It is also the basis for wireless local area networks. The indoor radio channel is characterized by fading multipaths as well as noise. Direct sequence spread spectrum (DSSS), with its inherent resistance to multipath interference is an attractive technique for this environment. To allow multiple users within the limited bandwidths available, code division multiple access is needed. This paper analyzes the performance of a DSSS scheme employing random orthogonal codes over fading multipath indoor radio channels using actual channel measurements from five different locations. A RAKE receiver is used to study the effects of power control, code length and receiver structure. The average probability of error as a function of signal-to-noise ratio or as a function of the number of simultaneous transmitters is used as the performance criteria.

An adaptive multichannel IIR lattice predictor for k-step ahead prediction is constructed and the effectiveness of the proposed predictor is evaluated using digital simulations.

In this paper, an optimized wideband channel sounder designed for measuring the time variant impulse response of outdoor radio channels in the frequency range 1800-2000 MHz is presented. Prior to hardware implementation the system was first modelled on a high performance supercomputer to enable the system designer to optimize the digital signal processing algorithms and the parameters of the hardware components by simulation. It is shown that the proposed measuring system offers a significantly larger amplitude resolution, i.e. dynamic range, than conventional systems applying matched filtering. This is achieved by transmitting digitally generated periodic spread spectrum test signals adjusted to amplifier non-linearities and by applying optimum unbiased estimation instead of matched filtering in the receiver. A further advantage of the hardware implementation of the proposed system compared to conventional systems [5]-[7] is its high flexibility with respect to measuring bandwidth, period of the test signal and sounding rate. The main features of the optimized system are described and first measurement results are presented.

A new method of multipath diversity combination is proposed for Direct Sequence Spread Spectrum (DS-SS) mobile communications. In this method, the transmitted signal from the base staion is the sum of a number of the same spread signal, each one delayed and scaled according to the delay and the strength of the multipaths of the transmission channel. As a result the received signal at the mobile unit will already be a Rake combination of the multipath signals. This new method is called Pre-Rake diversity combination because the Rake diversity combination process is performed before transmission By this method the size and complexity of the mobile unit can be minimized, and the unit is made as simple as a non-combining single path receiver. A theoretical examination of the Signal to Noise Ratio (SNR) and the Bit Error Rate (BER) results for the traditional Rake and the Pre-Rake combiners as well as computer simulations show that the performance of the Pre-Rake combiner is equivalent to that of the Rake combiner.

Algorithms for computing channel capacity have been proposed by many researchers. Recently, one of the authors proposed an efficient algorithm using Newton's method. Since this algorithm has local quadratic convergence, it is advantageous when we want to obtain a numerical solution with high accuracy. In this letter, it is shown that this algorithm can be extended to the algorithm for computing the constrained capacity, i.e., the capacity of discrete memoryless channels with linear constraints. The global convergence of the extended algorithm is proved, and its effectiveness is verified by numerical examples.

This paper addresses onboard processing architecture employing direct regeneration. The advantage of direct regeneration is its hardware simplicity, even though the bit error rate performance is slightly inferior to that of demodulation-remodulation scheme with coherent detection. The channel filtering schemes as well as achievable capacities are examined by computer simulation. It is found that the system with direct regeneration has advantage in channel capacity and transmit earth station e.i.r.p. for small earth stations. A possible configuration of direct regeneration onboard in future satellite systems is proposed.

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.

We propose a direct-sequence spread-spectrum multi-access (DS/SSMA) receiver that incorporates multipath diversity combining and multistage co-channel interference (CCI) cancellation. This receiver structure which is more resistant to the near/far problem essentially removes more and more of the CCI with each successive cancellation stage. With the assumption that perfect channel estimates have been obtained, we analyze the bit error rate (BER) performance of this system when received powers are unequal. Results show that the BER can approach that of a single-user case as the number of CCI cancellation stages increases.

This paper proposes and investigates an adaptive equalizer with diversity-combining over a multipath fading channel. It consists of two space-diversity antennas and a Ts/2-spaced decision-feedback-equalizer (DFE). Received signals from the two antennas are alternatively switched and fed into the feed forward-filter of DFE. We call this structure a Switched Input Combining Equalizer with diversity-combining (SICE). By using an SICE, the receiver structure for combining diversity equalization can be simplified, because it needs only two receiver sections up to IF BPF. The bit error rate (BER) performance of SICE was evaluated by both computer simulation and experiment over a multipath fading channel. We experimentally confirmed the excellent BER performance, around 1% of BER over a multipath fading channel at 160Hz of maximum doppler fading frequency. Therefore, the proposed SICE is applicable to highly reliable transmission in the 1.5-GHz-band mobile radio.

A Decision Feedback Equalizer (DFE) structure with a varying number of tap lengths was used with a recursive least squares (RLS) algorithm to determine tradeoffs between equalizer size and performance in mobile and portable digital radio systems. A mobile channel simulator, SMRCIM, was used to demonstrate how much an equalizer can improve the BER in real world urban channels. The results show that at 850MHz, the DFE is unable to improve the BER when the mobile terminal exceeds speeds of 115km/h for U.S. Digital Cellular systems. The performance of adaptive equalization for indoor high data rate systems was evaluated using the indoor channel simulator SIRCIM, and we found that DFEs have excellent performance for indoor radio channels. For simple structures, the BER is less than 10-3 at 15dB Eb/NO using coherent QPSK modulation. Finally, an equalizer structure for non-coherent π/4 DQPSK modulation was developed and simulation results are presented.

Two types of multicast error control protocols based on a product code structure with or without interleaving are considered. The performances of these protocols are analyzed on burst error channels modeled by Gilbert's two-state Markov chain. The numerical results reveal that the interleaving does not always succeed in improving the performance of the protocol proposed in Part .

The paper deals with the evaluation of channel allotment criteria in a mixed cellular environment composed by a regular grid of macrocells, plus a number of microcells deployed in the most congested areas. The optimum allotment of the resources between microcells and macrocells, which is a key issue for future personal communication systems, has to be tailored on the basis of their different functionalities. The approach is quite innovative since the analysis is carried out considering real traffic statistics, which are characterised by peaks and fluctuations resulting in uneven traffic loads on different cells. Different propagation models for macrocells and microcells have been adopted. Finally, the impact of the allotment of frequency resources to microcells and macrocells is analysed.

A multicast error control protocol proposed by Metzner is generalized and the performance of the proposed protocol on random error channels (binary symmetric channels) is analyzed. The proposed protocol adopts an encoding procedure based on a product code structure, whith enables each destined user terminal to decode the received frames with the Reddy-Robinson algorithm. As a result, the performance degradation due to the re-broadcasting of the replicas of the previously transmitted frames can be circumvented. The numerical results for the analysis and the simulation indicate that the proposed protocol yields higher throughput and less degradation of throughput with an increase of the number of destined terminals.

The demand for mobile communications is continuing to grow, but there is a limit on the radio frequency resources. Micro cellular systems are a strong solution to this problem. However, Forced Call Termination (FCT) and Channel Changing (CC) occur frequently in these systems because of their small cell size. This paper proposes a new Dynamic Channel Assignment (DCA) strategy which uses information of moving direction of Mobile Stations (MSs) to reduce FCT and CC. This strategy, the MD (Moving Direction) strategy, is compared with other major DCA strategies by simulating a one-dimensional service area covering a road, such as an expressway. The simulation shows that the MD strategy performs better than the other strategies with regard to FCT, CC, and carried load. FCT is an especially important factor in the quality of service. The MD strategy reduces FCT and has the largest carried load of the strategies, which means that it has the most efficient channel usage. This is an attractive characteristic of the MD strategy for micro cellular systems.

This paper describes voice communication connection controls in digital public land mobile networks (D-PLMNs). Voice communications in the D-PLMNs are carried at about 10 kbit/s over narrow-band TDMA channels with highly efficient cellular voice encoding schemes. Extensive research is being carried on half-rate voice encoding schemes that will effectively double radio resources. We first outline the configuration of voice communication connection between a cellular phone in the D-PLMN and a telephone in a fixed network, and we describe the optimum position for the CODECs that transform cellular voice codes to the conventional voice codes used in the fixed network, and vice versa. Then we propose a CODEC-bypassed communication control scheme that improves the quality of voice communication between cellular phones. And we propose a cellular voice code negotiation scheme in the D-PLMN which supports different cellular voice encoding schemes. We also propose an efficient channel reassignment scheme for effectively assigning TDMA channels to voice calls with two different bitrates (full-rate and half-rate), and we analyze this scheme's traffic capability. Finally, we describe a dual-tone multiple-frequency (DTMF) signal transmission scheme and estimate the number of DTMF signal senders required in the D-PLMN.