In this paper, we compare two signal designs for uplink quasi-synchronous code division multiple access (CDMA) channels in order to optimize the trade-off between bandwidth efficiency and power efficiency. The design we call band-limited DS/CDMA design, is based on the time-domain assignment of Gold sequences, just as in the ordinary DS/CDMA, but with band-constrained cyclic chip interpolation functions, which is unlike the ordinary DS/CDMA. The other design, MC/CDMA design, is based on frequency-domain assignment of the sequences, as in the ordinary MC/CDMA. In both designs, we assume insertion of guard intervals at the transmitter and frequency-domain processing in reception. Assuming quasi-synchronous arrival of CDMA signals at the CDMA base station and FFT in the effective symbol interval, the intersymbol interference is evaded in both designs. First we identified the signal parameters that optimize bandwidth efficiency in each of the band-limited DS design and MC design. Second, we clarified the signal parameters that optimize the power efficiency as functions of frequency efficiency in each of the two designs. Finally, we derived and compared the trade-off between the bandwidth efficiency and power efficiency of band-limited DS and MC designs. We found a superiority of band-limited DS design over MC design with respect to the optimized trade-off.

In this paper, we propose a new algorithm to detect the presence of narrow band interference signals on the band of an Ultra Wide-Band (UWB) system when the UWB spectrum overlaps the bands of other narrow band wireless services. In our proposed algorithm for an UWB Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) system, an appropriate model is selected from the assumed interference models based on the Akaike Information Criterion (AIC) which is an explicit theoretic criterion and a measure of fit of the model. The proposed algorithm does not need a priori information on the interference signals except that we can reduce a computational complexity to implement the algorithm if we have knowledge of the bands of the interference signals. Furthermore, we introduce the Expectation Maximization (EM) algorithm to our algorithm in order to estimate the transmitted signals and the interference signals simultaneously. The proposed algorithm may not require the pilot symbols in the assumed UWB system to detect the presence of other systems. By computer simulations, we show that the proposed algorithm validly detects the presence of interference signals on the UWB band.

We have already proposed a carrier power control scheme of OFDM that makes it possible to improve the bit error rate characteristic. However, the data on the lost carriers by the severe fading or large additive noise can't be recovered by only this power control scheme. In this paper, a method of transmission power control of OFDM carriers combined with data spreading scheme in the frequency domain is proposed. In the proposed system, transmission power of each carrier is controlled so as to maximize the average signal to noise ratio of the data symbols after de-spreading the signals from DFT. As the results of computer simulations, it is confirmed that the proposed system can improve the bit error rate characteristics under multi-path channel.

It is well known that diversity performance of communication systems using signals with high dimensions in time, frequency and/or spatial domains depends on correlation of the channel characteristics along signal dimensions. On the other hand, it has not been payed due attention how the coherent receiver which combines the signals is greatly affected by the erroneous channel estimation which can undermine the diversity gain. In this paper, assuming that the estimator is given the a priori probability of the channel characteristics, we propose an optimum estimation scheme based on MAP criterion, in an uplink-MC/CDMA system on channels with frequency selective fading, with an array of antennas at the receiver. The MAP estimator effectively takes into account the correlation of the channel characteristics that the conventional estimator neglects. We also propose a signal design in pilot symbol periods that enables the receiver to separately obtain the sufficient statistic for estimating the channel characteristics without MAI. Using computer simulation, we obtained MSE error performances of the proposed estimator compared with the conventional estimator and their effect on BER performances of the diversity combining receiver. It was observed that using the conventional estimator for combining greater number of signals than the effective channel dimension deteriorated the BER performance while using the proposed estimator kept the optimum performance just as the error-free estimator did. Also obtained for MC/CDMA systems are BER performances of the single user matched filter and MMSE receivers using the proposed and the conventional estimators. A considerable improvement of the MMSE performance was achieved owing to the optimum estimator. It remains for the a priori probability of the channel characteristics to be properly assumed and dealt with in sequential estimation.

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.

This paper presents a new design of spread spectrum signals with the minimally sufficient dimension from the view point of frequency diversity. Letting the signature signal duration and the bandwidth be denoted by T and B, respectively, we can nominally represent a signature signal of either Direct Sequence (DS) or MultiCarrier (MC) spread spectrum system as the sum of N=BT sinusoidal signal units with their frequencies separated by 1/T or its multiples. In our design,assuming the maximum expected channel delay spread σd « T as usual, one signature signal viewed in the frequency domain is made up of the minimum number K 2πσdB of sinusoidal signal units which are arranged so as there is placed at least one unit in coherence bandwidth 1/(2πσd) in which the fading channel transfer function has strong correlation. Although the signature signal does not make use of all the units in the given frequency domain as in the ordinary spread spectrum systems, but uses only skipped units, it can be shown that almost the same frequency diversity effect is attained. And it is also shown that the immunity to the external interfering signals is by no means inferior. If every L=N/K T/(2πσd) consecutive sinusoidal signal units are assigned to the K signal units of a signature signal, L different signature signals are simultaneously available mutually orthogonal when the synchronous demodulation is performed in the same T period. We call each of the orthogonal sinusoidal signal sets a Frequency Devision (FD) signal set. Now, CDMA can be independently realized on each of the L FD signal sets provided the operation is synchronous or quasi-synchronous with respect to the symbol demodulation (or signature) period. Partitioning the simultaneous users among the FD sets, it is possible to decrease the number of CDMA users to be processed, retaining the total number of simultaneous users. Owing to this effect, the multiple access performance for the FD/CDMA system is shown to be superior to that of the ordinary DS or MC/CDMA system, assuming matched filter reception based on the complete estimation of the channel characteristics for the both cases. The decrease of the number of CDMA users per FD set makes it practical for the receiver to employ multiple access interference cancellation and even the maximum likelihood detection. Curiously, any FD signal set can be represented in the time domain as L repetition of a sequence with its period equal to K in the number of 1/B duration time chips.

The paper proposes and reports on pototyping a work bench system for novice Prolog programmers which consists of a visually-structured interactive tracer and a prototype-based programming support. The tracer actually is a simulated interpreter in Prolog. It is interpreted by a Prolog interpreter being embedded with facilities interfacing programs in Prolog and the objects programmed in C. It displays, by way of these objects, the past, current and future goals, highlights variable sharing and value substitution, and marks the current goals and backtrack choice points. It is at user's will to let the tracer show and hide subgoals as well as to let it backtrack when it failed, step back for redoing or terminate tracing. The programming support module first provides the programmer with structural prototype patterns and the roles of the constituent functions. We developed a support system for the 2 types of recursive definitions. After having selected the prototype, the user is requested to specify the data types and the names of variables to be put in the arguments, which propagate through the structure. The support module then offers a menu of primitive or user-registered constituent functions as may be useful in processing and/or obtaining user-specified types of data. Thirdly the system lets the user express his/her intention by sample input-output data instances in his/her task goals. It makes the values propagate through the structures thus motivating the user to design the constituent functions. At the goal recursion point, the user is allowed to creep into examining the definitions of the reduced versions of the instances, helping the user find the condition with which the recursion terminates. Finally the module assists the user to convert the structural descriptions into Prolog programs.

In this paper, we propose a new multiuser detection scheme using Maximum Likelihood (ML) criterion and the M algorithm for Multi Carrier (MC)-Code Division Multiple Access (CDMA) systems in the down-link channel. We first describe an implementation of ML detection separating In- and Quadrature-phase components and using well-known linear filters. In the proposed algorithm, we produce hypothesis symbol vectors in a tree structure by partly changing the sub-optimum hard decisions based on the linear filter output. At each stage, we adopt the best M likely paths with respect to the true log likelihood or distance function as survivors. We determine the symbol vector which minimizes the distance function at the final stage. Although the complexity of ML detector is exponentially increasing as a function of the number of users, the proposed scheme requires by far less complexity. We demonstrate that the proposed scheme achieves equivalent Bit Error Rate (BER) performance with lower complexity in comparison with ML detector by computer simulations. Moreover we compare the proposed detection scheme with QRD-M algorithm which is based on QR decomposition combined with M algorithm.

In this paper, we propose a technique to improve the gain of ultra wide-band (UWB) planar antennas by using low profile reflectors based on frequency selective surfaces (FSS). This technique not only enhances the gain of the planar UWB antennas but also guarantees a constant gain with weak variation across the entire UWB while keeping their attractive merits such as planar structure and easy fabrication. An UWB coplanar waveguide (CPW) fed antenna is installed above the proposed reflectors, to prove the effectiveness of the proposed technique. As a result, a constant gain is achieved across a very large bandwidth.

Orthogonal Frequency Division Multiplexing (OFDM) systems are very sensitive to the frequency offset of the local oscillator at the receiver while the symbol timing offset can be absorbed in the guard interval. For the same reason, estimation of the frequency characteristics, needed for OFDM to be adapted to the frequency selective fading, can only be carried out conventionally after the frequency offset has been compensated. And accurate estimation of large frequency offset certainly requires high precision estimate of the frequency characteristics. In this paper, we propose a new joint estimation method of the frequency offset and the channel frequency response using an Expectation-Maximization (EM) algorithm for OFDM systems. The proposed algorithm overcomes the limitation of the thus far proposed algorithm. By computer simulations, we show the proposed algorithm provides estimation accuracy close to its lower bound in a wide range of the frequency offset.

Currently, IEEE802.11p and ARIB STD T-109 are available as the typical inter-vehicle communication (IVC) standards. Carrier sense multiple access/collision avoidance (CSMA/CA) and orthogonal frequency division multiplexing (OFDM) are used in these standards. However, the performance degrades when there are hidden terminals. In this paper, IVC system that using a direct sequence spread spectrum (DS/SS) modulation scheme is discussed because it has code division multiple access (CDMA) capability. In DS/SS-IVC scheme, it is possible to avoid hidden terminal problem. On the other hand, near-far problem (NFP), multiple access interference (MAI) and interference by equivalent pseudo noise (PN) codes occurs in DS/SS communication. These problems cause performance degradation. In this paper, interference cancellation scheme and slotted ALOHA scheme with code sensing are applied so as to mitigate the impact of MAI, NFP and interference by equivalent PN code. By applying interference cancellation scheme and slotted ALOHA scheme with code sensing, the performance of DS/SS-IVC is improved. In this paper, location oriented PN code allocation is focused on as a method of PN code assignment. However, DS/SS-IVC scheme based on location oriented PN code allocation has a problem. Since each vehicle obtain PN code based on the position that is estimated by GPS, performance degrades when GPS positioning error occurs. Therefore, the positioning system of DS/SS-IVC scheme is also discussed in this paper. Elimination of ranging data that has large ranging error is proposed in addition to interference cancellation scheme and slotted ALOHA scheme with code sensing in order to improve the performance of positioning. From the simulation results, the positioning error can be mitigated by applying these proposed techniques.