This paper discusses the employment of adaptive array antennas at the base station of a Multi Processing Gain (MPG) CDMA system. It is shown that the adaptive array antenna with the weight control scheme based on the signal before despreading procedure does not increase but even decreases the performance than that with an omni-directional antenna, and the cause of this serious performance degradation is revealed. Then it is shown that the performance with the weight control scheme based on the signal after despreading procedure is always better than that with an omni-directional antenna. Furthermore, the possibilities of performance improvement by the combination of adaptive array antenna and interference cancellation techniques are mentioned.

In this paper, we propose the introduction of space diversity techniques to the code acquisition of a direct-sequence spread-spectrum signal. In this scheme, both a transmitter and a receiver have multiple antennas and the signals corresponding to all the combinations of the transmitter and receiver antennas are combined at the acquisition circuit of the receiver. The performance is evaluated for an indoor packet radio communication system from the viewpoints of the average time for acquisition, the probability of success of acquisition, and the necessary preamble length. As the result, we show great performance improvements by the proposed scheme under slow and flat Rayleigh fading environment.

This paper discusses factorization of bent function type complex Hadamard matrices of order pn with a prime p. It is shown that any bent function type complex Hadamard matrix has symmetrical factorization, which can be expressed by the product of n matrices of order pn with pn+1 non-zero elements, a matrix of order pn with pn non-zero ones, and the n matrices, at most. As its application, a correlator for M-ary spread spectrum communications is successfully given, which can be simply constructed by the same circuits with reduced multiplicators, before and behind.

This paper presents a quadriphase sequence pair, whose aperiodic auto-correlation functions for non-zero shifts and cross-one for any shift take pure imaginary values. Functions for pairs of length 2n are formulated, which map the vector space of order n over GF(2) to Z4. It is shown that they are bent for any n, such that their Fourier transforms take all the unit magnitude.

This paper proposes an adaptive list-output Viterbi equalizer (LVE) with fast compare-select operation, in order to achieve a good trade-off between bit error rate (BER) performance and processing speed. An LVE, which keeps several survivors for each state, has good BER performance in the presence of wide-spread intersymbol interference. However, the LVE suffers from large processing delay due to its sorting-based compare-select operation. The proposed adaptive LVE greatly reduces its processing delay, because it simplifies compare-select operation. In addition, computer simulation shows that the proposed LVE causes only slight BER performance degradation due to its simplification of compare-select operation. Thus, the proposed LVE achieves better BER performance than decision-feedback sequence estimation (DFSE) without an increase in processing delay.

This paper describes a digital delay-lock Loop (DLL) to which delta-sigma (Δ Σ) modulation technique is applied in order to reduce circuit elements. The DLL is evaluated in both transient and steady-state behavior by theoretical analysis, computer simulations and circuit experiments. Not deteriorated by the internally generated Δ Σ-modulation noise, the DLL shows good tracking performance in transient response and steady-state RMS jitter of phase error against additive white Gaussian noise. Using the proposed DLL most parts of receiving circuits are realized by digital integrated circuits. After realizing the circuit, power-line communication system with spread spectrum is possibly expected in a small size with low cost.

Recently portable computers and mobile communications have been developed rapidly. Along with the developments of mobile computing environment, the demands of transmitting images over wireless channels are growing. However, high efficiency image coding techniques are not designed for the wireless communication channels. The use of variable length codes (VLCs) in those coding techniques makes a receiver hard to synchronize with the codewords on the noisy channel. On the other hand, a variable length code transmission technique utilizing multicode CDM (Code Division Multiplex) has been proposed. In this paper, the multicode CDM technique is applied to wireless transmission of images compressed by the H. 263 algorithm. Especially, the proposed technique is employed for motion vector transmission. In consequence, the proposed transmission technique reduces the required Eb/N0 as compared with the conventional transmission technique.

This paper addresses an important issue on the spreading bandwidth of direct sequence code division multiple access (DS-CDMA) cellular mobile radio systems: does wider spreading bandwidth provide larger capacity? And if so, to what extent? The capacity of the perfect power controlled reverse link is evaluated by computer simulation for 1.25 MHz and 5 MHz spreading bandwidths under various sets of propagation channel parameters (path loss decay factor, shadowing standard deviation, shadowing correlation, number of resolved propagation paths) and antenna diversity reception.

This paper proposes a novel spread spectrum (SS) modem for random access satellite communication systems that employs digital matched filters. The proposed modem employs a parallel structure to ensure detection of packet arrival. Code timing detection with a combination of a coarse detector and a fractional error detector reduces the sampling rate while maintaining the BER performance. An in-symbol pilot multiplexing scheme is also proposed for fast and stable carrier synchronization with a simple hardware. A performance evaluation shows that the proposed modem achieves the UW miss-detection probability of 10-4 at the Eb/No of 0 dB. The overall BER performance achieved in experiments well agrees simulation.

In this paper, a type of multi-carrier direct sequence code division multiple access (MC-DS-CDMA) system which uses frequency spread coding is proposed and investigated for the down-link. An MC-DS-CDMA system is a combined system of CDMA and multi-carrier modulation. This system is often categorized as a "serial to parallel (S/P) type" system because serial to parallel converted data symbols are transmitted. They use different sub-carriers which are narrow-band DS waveforms. In this system, benefits of path or frequency diversity can not be obtained because of the narrow-band transmission of each data symbol. In order to benefit from the diversity, we propose to adopt frequency spread coding in an MC-DS-CDMA system. The proposed system exploits frequency diversity without additional redundancy, i. e. , no frequency or time redundancy is required to improve the performance. Computer simulation is carried out in a frequency selective fading channel and the results show its effectiveness in terms of average bit error rate (BER). Furthermore, the proposed system is compared with a multi-carrier (MC-) CDMA system which is often categorized as a "copy type" system and a single-carrier (SC-) DS-CDMA system using a RAKE receiver.

In this letter, the bit error probability of a hybrid DS/SFH-SS (Direct-Sequence/Slow-Frequency-Hopped Spread-Spectrum) system is derived for a Nakagami fading channel in the presence of multiple tone jamming. At a low JSR (jamming-to-signal power ratio), a pure DS-SS system can achieve lower bit error probability than a hybrid DS/SFH-SS system. But at a high JSR, a hybrid DS/SFH-SS system is shown to be superior to a pure DS-SS system. Especially, the worst case performance of a hybrid DS/SFH-SS system is almost equal to that of a pure DS-SS system.

Wireless communication systems are affected by several factors in the indoor environment. The complexity of this environment, however, has hampered the development of methods for analyzing it. Reported here is our investigation of the relationship between the propagation characteristics and performance of a 2.4-GHz ISM-band wireless LAN in various indoor environments. Our objective was to develop guidelines for designing ideal indoor environments for wireless LANs. A booth constructed of a ceiling, floor, and wall materials that could be changed was used for our investigation. The transmission loss and delay spread were measured for four environments; they were calculated by using a ray-tracing method to verify the effectiveness of the ray tracing calculation. The throughput and BER characteristics were measured for the same environments. The following results were obtained. (1) The transmission loss and delay spread could be estimated by using this ray tracing method because the deviations between the calculated and measured data were within 5 dB for the transmission loss and within 10 ns for the delay spread. (2) Reflections from the walls caused a serious interference problem: throughput was 0.0 at more than 30% of the positions along the center line of the booth when the walls were constructed of high-reflection-coefficient material. (3) The throughput and BER were closely correlated with the delay spread; the number of positions meeting a certain throughput was estimated by the method based on the delay spread calculated using the ray tracing method. It was within 10% of the number measured. The results obtained can be used to design ideal indoor environments for 2.4-GHz ISM-band LAN systems.

This paper proposes applications of a code-division multiplexing technique to VLSI systems free from interconnection problems. We employ a pseudo-random orthogonal m-sequence carrier as a multiplexable information carrier to achieve efficient data transmission. Using orthogonal property of m-sequences, we can multiplex several computational activities into a single circuit, and execute in parallel using multiplexed data transmission with reduced interconnection. Also, randomness of m-sequences offers the high tolerance to interference (jamming), and suppression of dynamic range of signals while maintaining a sufficient signal-to-noise ratio (SNR). We demonstrate application examples of multiplex computing circuits, neural networks, and spread-spectrum image processing to show the advantages.

This paper presents a new technique for the synthesis of sets of low-peak sequences exhibiting low peak cross correlation. The sequences also have flat power spectra and are suitable for many applications requiring such sets of uncorrelated pseudo-white-noise sources. This is a new application of the ta-sequence (trigonometric function aliasing sequence), which itself is a very new technique that uses the well-known "Reed-Solomon code" or "One coincident code" to generate these sets of low-peak-factor pseudo-white-noise exhibiting low peak cross correlation. The ta sequence method presented here provides the means for generating various sequences at the lengths required for such applications as system measurement (needing uncorrelated test signals), pseudo-noise synthesis (for spread spectrum communication), and audio signal processing for sound production (for enhancing spatial imagery in stereo signals synthesized from mono sources) and sound reproduction (for controlling unwanted interference effects in multiple-loudspeaker arrays).

Two classes of nonlinear feedforward logic (NLFFL) pseudonoise (PN) code generators based on the use of AND and majority logic (ML) gates are compared. Cross-correlation and code-division multiple-access (CDMA) properties of properly designed NLFFL sequences are found to be comparable with the properties of well-known linear PN codes. It is determined that code design employing ML gates with an odd number of inputs is easier compared with designing with AND gates. This is especially true when the degree of nonlinearity is large, since the nonbalance problem, e. g. , at the output of an AND gate, can be avoided. ML type sequences are less vulnerable to correlation attack and jamming by the m-sequence of an NLFFL generator

We have developed a new type of phase interpolation direct digital synthesizer (DDS) with a symmetrically structured delay generator. The new DDS is similar to a sine output DDS in that it produces lower spurious signals, but it does not require a sine look-up table. The symmetrically structured delay generator reduces the periodic jitter in the most significant bit (MSB) of the DDS accumulator. The symmetrical structure enables the delay generator to produce highly accurate delay timing and eliminates the need to adjust the circuit constants. Experimental results confirm frequency synthesizer operation in which the spurious signal level is reduced to less than that of the accumulator.

There is currently a need for development of a new frequency band to enable creation of next-generation mobile communication systems. Of the potential bands, the 3 GHz and over microwave band holds the greatest promise. Experimental studies on the delay characteristics of multipath propagation must be conducted in order to achieve high-speed transmission in the microwave band. We have developed a system for measuring the microwave broadband propagation delay profile over 100 MHz spread bandwidths in the 3, 8 and 15 GHz bands. Our experiments confirmed system performances of 20-ns resolution, 40-µs maximum measurable delay, relative amplitude error of within 3 dB and dynamic range of over 60 dB. We used our system to measure delay profiles on an urban area with line of sight, particularly, in terms of the effects of mobile antenna height. Typical examples are presented. Analysis showed that delay spreads increased with transmit/receive distance and decreased with the higher antenna height.

Effects of the model order estimation error in the TLS-ESPRIT algorithm were investigated. It was found that if the model order is overestimated true signal parameters are preserved even though spurious signals of which power values are negligibly small appear, whereas if the model order is underestimated some signals degenerate to each others, resulting in the erroneous estimates.

In this paper, we discuss the initial acquisition of the code division multiplexed DS/SS down-link signals at a user terminal of multiple LEO mobile satellite communication systems. In LEO systems, a receiver generally receives signals from plural satellites for soft hand-off and for satellite diversity as a countermeasure to shadowing. In this situation, the signal from each satellite becomes the interference to the signals from other satellites. In addition to this inter-satellite interference, we have to consider the intra-satellite interference from user channels to a pilot channel because of the loss of orthogonality of channels at initial acquisition stage especially under frequency offsets due to Doppler effect. Thus in this paper, we analytically evaluate the performance of an initial acquisition scheme, taking the intra/inter-satellite interference under Doppler shift into account.

In a multi-bandwidth CDMA system, the performance of a multiple order selection combining rake receiver is analyzed according to the spreading bandwidth of the system and the delay spread of a Rayleigh fading channel. The results for various channel environments indicate a tradeoff between total received signal energy and multipath fading immunity. Increasing the occupied bandwidth of the system (wide-bandwidth spreading) gives better performance for small delay spread environments, while gathering more energy (narrow-bandwidth spreading) gives better performance for large delay spread environments. It is also shown that the performance difference between low and high order selection combining grows larger as the spreading bandwidth is increased. It is noted that performance degrades by increasing the bandwidth above a certain point and the optimum spreading bandwidth for each channel environment decreases as the delay spread of the channel increases.