Coding scheme is discussed for M-Choose-T communication in which at most T active users out of M potential users simultaneously transmit their messages over a common channel. The multiple-access channel considered in this paper is assumed to be a time-discrete noiseless adder channel without feedback with T binary inputs and one real-valued output, and is used on the assumption of perfect block and bit synchronization among users. In this paper a new class of uniquely decodable codes is proposed in order to realize error-free M-Choose-T communication over the adder channel described above. These codes are uniquely decodable in the sense that not only the set of active users can be specified but also their transmitted messages can be recovered uniquely as long as T or fewer users are active simultaneously. It is shown that these codes have a simple decoding algorithm and can achieve a very high sum rate arbitrarily close to unity if exactly T users are active.

The access control method adopted by UNIX is simple, understandable, and useful. However, it is quite possible that unexpected information flows occur when we are cooperating with some group members on UNIX. Introducing notions such as "flow right," "maximal permission" and "minimal umask value", this note proposes a simple method, can be seen as a natural extension of UNIX, to control indirect information flows without losing availability and understandability of UNIX.

Different signature codes in an optical code division multiple access (CDMA) network have been known to demonstrate different performances. The performance of different signature codes in an optical CDMA network was analyzed here in this paper by including the performance evaluation for the synchronization process which was not considered previously. Both auto- and cross-correlation properties of the signature codes were found to be important. In addition, the performance comparison of (n, w1, 1, 1), (n, w2, 2, 1) optical orthogonal codes (OOC's), and (n, w3, w3, 1) extended prime code demonstrated that an (n, w2, 2, 1) OOC could accommodate more users than the other two.

Many methods, based on the concept of key-lock-pair have been proposed for access control in computer protection systems. However, the proposed methods still either lack of dynamic ability or need quite a lot of computation in performing requests of deleting users/files, inserting users/files, or updating access rights of users to files. In this paper we propose a two-key-lock-pair access control method that is based on the unique factorization theorem and a time stamp mechanism. Our method is dynamic and needs a minimum amount of computation in the sense that it only updates at most one key/lock for each access request, which has not been achieved before.

A uniquely decodable (UD) code pair (C, S) is considered for the two-user binary adder channel. For a class of linear codes C, the maximum independent set of the graph associated with C, which is the second code S, is evaluated. When the rate R1 of C is less than 0.5, there exist UD codes (C, S)'s such that the rate R2 of S exceeds the Khachatrian's and Guo's results in amount.

In this paper, we propose DSVMA (Data Steal into Voice Multiple Access) scheme for integration of voice and data in wireless information networks. By using speech activity detectors and effective downstream control signals, DSVMA enables data terminals to transmit multi-packet messages when voice terminals are in silent periods. The S-G (throughput versus offered load) performance of the DSVMA system and the blocking probabilities of both the second generation systems and the DSVMA systems are evaluated by the static analysis. A dynamic analysis of a system with finite number of terminals is also presented using an approximate Markov analysis method. Some numerical examples are given in the paper. As a result, it is shown that DSVMA can improve the channel utility efficiency of a circuit-switched TDMA (Time Division Multiple Access) wireless communication system and is directly applicable for second generation wireless information systems.

Advanced Traveller Information Systems (ATIS) and Advanced Traffic Management Systems (ATMS) require real-time traffic data to observe and control the trafic flow. Still, there is a lack of proficient traffic monitoring systems. One method to collect such data is using particular equipped vehicles, called probes, transmitting experienced travel times to base stations which in turn are connected to a traffic control center. In this paper we analyse the performance of a radio network for collecting real-time traffic data from probes. The results reveal that random transmission of traffic reports is a (spectrum) efficient, inexpensive and flexible method for collecting road traffic data that can provide reliable traffic monitoring.

This paper examines a novel digital modulation/multiple access technique called Multi-Carrier Code Division Multiple Access (MC-CDMA) where each data symbol is transmitted at multiple narrowband subcarriers. Each subcarrier is encoded with a phase offset of 0 or π based on a spreading code. Analytical results are presented on the performance of this modulation scheme in an indoor wireless multipath radio channel.

This paper studies the effect of frequency re-using patterns on the channel capacity in the forward link of orthogonal code division multiple access (CDMA) cellular systems. The received carrier-to-interference ratio (CIR) determined by computer simulation shows that re-using the same frequency channel on every third sector (3-sector layout) provides superior channel capacity than does every-sector re-use (1-sector layout).

In our preceding paper, I-ISMA (Idle Signal Multiple Access for Integrated services), a combination of ISMA and time reservation technique, was proposed to transmit an integrated voice and data traffic in third generation wireless communication networks. There, the channel capacity of I-ISMA was evaluated by the static analysis. To fully estimate performance of contention-based channel access protocols, however, we also need dynamic analysis to evaluate stability, delay, etc. Particularly, in systems concerning real-time voice transmission, delay is one of the most important performance measures. A six-mode model to describe an I-ISMA system is set up. With some assumptions for simplification, the dynamic behavior of the system is approximated by a Markov process so that the EPA (Equilibrium Point Analysis), a fluid approximation method, can be applied to the analysis. Then, numerical and simulation results are obtained for some examples. By means of the same analysis method and under the same conditions, the performance of PRMA is evaluated and compared briefly with that of I-ISMA.

Corresponding to the development of B-ISDN, integrated services for data, voice, etc. are imperatively required for the so called third generation wireless communication networks. In this paper, I-ISMA (Idle Signal Multiple Access for Integrated services) is proposed to transmit integrated voice and data traffic from dispersed terminals to a base station. In the system, data packets and the first packets of talkspurts of conversational speeches are transmitted using ISMA protocol over a shared channel while subsequent packets of talkspurts are sent with time reservation technique. The channel capacity of I-ISMA is evaluated and compared with that of PRMA. The region in which I-ISMA has larger capacity than PRMA is figured out. Generally speaking, I-ISMA has larger capacity than PRMA when the duration for transmitting and detecting an idle signal is not too long and the channel is not too congested by the reserved voice transmissions. When we concern real time voice transmission, delay is one of the most important performance measures. Only is a qualitative discussion on delay performance given here. The quantitative evaluation is obtained by the dynamic analysis in our succeeding paper.

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.

It is concluded from numerical examples for the well-known linear PN sequence families of a large range of periods that the mean-square cross-correlation value between sequences is the dominating parameter to the average signal-to-noise power ratio performance of an asynchronous direct-sequence (DS) code-division multiple-access (CDMA) system. The performance parameters derived by Pursley and Sarwate are used for numerical evaluation and the validity of conclusion is supported by reviewing the other related works. The mean-square periodic cross-correlation takes the equal value p (code period) for the known CDMA code families. The equal mean-square cross-correlation performance results from the basic results of coding theory.

This paper discusses the performance of asynchronous direct-sequence spread-spectrum multiple-access systems using binary or quaternary phase-shift keyed signals with the strict bandwidth-limitation by Nyquist filtering. The signal-to-noise plus interference ratio (SNIR) at the output from the correlation receiver is derived analytically taking the cross-correlation characteristics of spreading sequences into account, and also an approximated SNIR of a simple form is presented for the systems employing Gold sequences. Based on the analyzed result of SNIR, bit error rate performance and spectral efficiency are also estimated.

Assuming application to the mobile multiple-access communication, chip-asynchronous mobile-to-base performances of FH/FTH (Frequency-Time-Hopped)-MFTSK (Multi-level Frequency-Time Shift Keying) systems are investigated. Analytical expressions are obtained for the probabilities of false detection and missed detection of signal elements, assuming independent and asynchronous arrival of each of the signal elements with Rayleigh fading and optional AWG noise. Using the result or by simulation and employing dual-k coding, parameter optimization was carried out to obtain the maximum spectrum efficiency. The results of the noisy case analysis and simulation show high noise-robustness of the FTH systems. For a given value of information transmission rate the optimized FTH-MFTSK gives an effectively constant spectrum efficiency for a wide range of the number Kf of frequency chips. As a result, FTH-MFTSK well outperforms FTH-MFSK at any, especially small value of Kf. Relative to the overall optimum FH-MFSK, FTH-MFSK systems show typically around 20% of degradation in spectrum efficiency even with one-eighth of Kf. Compared with FH-MFSK, accordingly, FTH-MFTSK systems allow the designer to reduce, without any degradation in multiple-access performances, the number of frequency chips to the minimum value tolerated by the frequency selective fading characteristics and the time chip duration requirement imposed by the signal-to-noise ratio margin and the transmitter peak power rating.

Recently, a family of 4-phase sequences (alphabet {1,j,-1,-j}) was discovered having the same size 2r+1 and period 2r-1 as the family of binary (i.e., {+1, -1}) Gold sequences, but whose maximum nontrivial correlation is smaller by a factor of 2. In addition, the worst-case correlation magnitude remains the same for r odd or even, unlike in the case of Gold sequences. The family is asymptotically optimal with respect to the Welch lower bound on Cmax for complex-valued sequences and the sequences within the family are easily generated using shift registers. This paper aims to provide a more accessible description of these sequences.

In this paper we consider multiuser detection using a neural network in a synchronous code-division multiple-access channel. In a code-division multiple-access channel, a matched filter is widely used as a receiver. However, when the relative powers of the interfering signals are large, i.e. the near-far problem, the performances of the matched filter receiver degrade. Although the optimum receiver for multiuser detection is superior to the matched filter receiver in such situations, the optimum receiver is too complex to be implemented. A simple technique to implement the optimum multiuser detection is required. Recurrent neural networks which consist of a number of simple processing units can rapidly provide a collectively-computed solution. Moreover, the network can seek out a minimum in the energy function. On the other hand, the optimum multiuser detection in a synchronous channel is carried out by the maximization of a likelihood function. In this paper, it is shown that the energy function of the neural network is identical to the likelihood function of the optimum multiuser detection and the neural network can be used to implement the optimum multiuser detection. Performance comparisons among the optimum receiver, the matched filter one and the neural network one are carried out by computer simulations. It is shown that the neural network receiver has a capability to achieve near-optimum performance in several situations and local minimum problems are few serious.

In this paper, we propose a spread spectrum pulse position modulation (SS-PPM) system, and describe its basic performances. In direct sequence spread spectrum (DS/SS) systems, pseudo-noise (PN) matched filters are often used as information demodulation devices. In the PN matched filter demodulation systems, for simple structure and low cost of each receiver, it is desired that each demodulator uses only one PN matched filter, and that signals transmitted from each transmitter are binary. In such systems, on-off keying (SS-OOK), binary-phase-shift keying (SS-BPSK) and differential phase-shift keying (SS-DPSK) have been conventionally used. As one of such systems, we propose the SS-PPM system; the SS-PPM system is divided into the following two systems: 1) the SS-PPM system without sequence inversion keying (SIK) of the spreading code (Without SIK for short); 2) the SS-PPM system with SIK of the spreading code (With SIK for short). As a result, we show that under the same bandwidth and the same code length, the data transmission rate of the SS-PPM system is superior to that of the other conventional SS systems, and that under the same band-width, the same code length and the same data transmission rate, the SS-PPM system is superior to the other conventional SS systems on the following points: 1) Single channel bit error rate (BER) (BER characteristics of the SS-PPM system improve with increasing the number of chip slots of the SS-PPM system, and as the number of chip slots increases, it approaches Shannon's limit); 2) Asynchronous CDMA BER; 3) Frequency utilization efficiency. In addition, we also show that With SIK is superior to Without SIK on these points.

This paper investigates a multihopping scheme for MFSK (Multilevel Frequency Shift Keying) /FH-SSMA (Frequency Hopping-Spread Spectrum Multiple Access) system. Moreover, we propose and investigate a modified decoding scheme for the coded MFSK/FH-SSMA system. In this multi-hopped MFSK/FH-SSMA system, several hopping frequencies per chip are assigned and transmitted in parallel in order to improve its frequency diversity capability for a fading channel. We theoretically analyze the performance of the multihopped MFSK/FH-SSMA system in a Rayleigh fading channel. Moreover, in the coded MFSK/FH-SSMA system, we propose a modified scheme of the error and erasure decoding of an error-correcting code. The modified decoding scheme utilizes the information of rows having the largest number of entries in the decoded time-frequency matrix. Their BER (Bit Error Rate) performance is evaluated by theoretical analysis in order to show the improvement in user capacity.

The capacity of a cellular direct sequence code division multiple access system is investigated in situations with and without power control for both the reverse link (from mobile to base station) and the forward link (from base station to mobile). The capacity is defined as the number of simultaneous users per cell with a prespecified performance. A theoretical analysis of the effect of imperfect power control on the reverse link capacity is presented using an analytical model. To investigate the reverse link capacity without any form of power control, a general spatial user distribution is developed which is very suitable for analytical study of any multiple access system with the near-far effect problem. The performance of the reverse link of a CDMA system is also evaluated considering the users located in surrounding cells. Finally, the forward link capacity is studied considering multiple cells. Two possible forward power control schemes, namely carrier-to-interference ratio driven and distance driven systems, are discussed.