As the demand of wireless personal communications networks increases, detailed chracteristics of indoor radio propagation in UHF band are required. In this paper, multipath fading characteristics at 1 GHz in an indoor mobil (walking speed) channel are investigated. By using a computer-controlled antenna scanner, signal strength was measured at 1400 points with a quarter wavelength resolution, which forms a two-dimensional fading map. The fading characteristics were found to be mainly dominated by the signals through the LOS path and the reflected paths due to two side-walls and one front-wall. It is analytically shown that middle-scale (over three wavelengths) fading is caused by the reflection from the side-walls, and periodical small-scale (equal to or less than a wavelength) fading is caused by the reflection from the front-wall. A software simulator based on geometric optics was developed in order to predict the measured fades. A modified algorithm for the two-dimensional ray launching technique which removes the necessity of checking the "multiple-counted rays" is presented. Comparison between measurements and predictions shows good agreement highlighting the usefulness of the two-dimensional simulator as a tool for channel design.

The performance of an M-ary spread-spectrum multiple-access (M-ary/SSMA) scheme in the presence of carrier frequency offset is discussed in this paper. The influence of carrier frequency offset on the non-coherent reception of M-ary/SSMA signals is examined and it is shown that the carrier frequency offset degrades the performance remarkably, yet. this influence has a distinctive property. Making use of this property, we propose a new M-ary/SSMA scheme that can mitigate the influence of the carrier frequency offset. The scheme is based on the assignment of two distinctive Hadamard codes to in-phase and quadrature components of the transmitted signal. The effect of simultaneous transmission is evaluated in terms of bit-error-rate performance with the carrier frequency offset. As the result, it is observed that the satisfactory bit-error-rate performance can be achieved in the presence of carrier frequendy offset.

Orthogonal frequency division multiplexing (OFDM) has been receiving a lot of attention in the field of broadcasting because of its ruggedness under multipath environments. One of important issues to realize high quality reception of OFDM signals is to correct frequency and timing offsets between the transmitter and receiver so that orthogonality of the carriers can be maintained. This paper discusses a frequency and timing period acquisition technique for OFDM systems. A new offset estimation technique is introduced that detects both the frequency and timing peirod offsets at the same time by using only one pilot symbol with its suitable frequency assignment. A pseudo noise (PN) sequence is also introduced to assign these frequencies of the pilot symbol so that the frequency acquisition range can be widened. Numerical examples are given to show the estimate variances of the proposed frequency and timing period estimator over both additive white Gaussian noise (AWGN) and multipath fading channels. Also the bit error rate (BER) performance for an open loop acquisition system is examined.

In future mobile radio, high-speed transmission and efficient spectrum utilization will be important. However, multipath propagation with large delay difference and cochannel interference are obstacles to the advanced mobile communication system. An adaptive antenna can suppress multipath signals and cochannel interference signals. This paper reviews basic performance of multipath fading reduction and cochannel interference suppression using the adaptive antenna. After a brief explanation of adaptive antenna concepts, we show simulation and experimental results of the fading reduction. It is pointed out that the adaptive antenna cancels multipath signals with large delay difference strongly. This feature is very important for high-speed TDMA systems. Moreover, it is shown from simulation results that the adaptive antenna improves the spectrum efficiency by suppressing the cochannel interference signals.

Analyzing multipath propagation structure is important to develop anti-fading techniques for high-speed digital radio systems. Several techniques have been employed to measure delay profiles and/or arrival angles. This paper presents a simultaneous estimation method of delay times and arrival angles of indoor multipath waves. We obtain frequency-domain data at different receiving antenna positions using a network analyzer. We estimate the propagation parameters by means of a two-dimensional MUSIC algorithm. In order to obtain reliable results, a two-dimensional discrete inverse Fourier transform and a gating technique are employed before the MUSIC algorithm. Simulation and experimental results show that the proposed method can estimate the propagation parameters properly.

This paper proposes a symbol rate controlled adaptive modulation/TDMA/TDD for future wireless personal communication systems. The proposed system controls the symbol rate according to the channel conditions to achieve wide dynamic range of the modulation parameter control as well as to improve the delay spread immunity. The main purpose of the proposed system is to increase the data throughput with keeping a certain transmission quality, especially in frequency selective fading environments. For this purpose, the proposed system predicts the C/N0 (carrier power-to-noise spectral density ratio) and the delay spread separately, and selects the optimum symbol rate that gives the maximum bit rate within a given bandwidth satisfying the required BER. The simulated results show that the proposed system can achieve higher transmission quality in comparison with the fixed symbol rate transmission system in both flat Rayleigh and frequency selective fading environments. The results also show that the proposed system is very effective to achieve higher bit rate transmission in frequency selective fading environments.

OFDM (Orthogonal Frequency Division Multiplexing) is a useful digital modulation method for terrestrial digital broadcasting systems, both for digital TV broadcasting and digital audio broadcasting. OFDM is a kind of multicarrier modulation and shows excellent performance especially in multipath environments and in mobile reception. Other advantages are its resistance to interference signals and its suitability for digital signal processing. When each carrier of the OFDM signal is modulated with DQPSK, we call it DQPSK-OFDM. DQPSK-OFDM is a basic OFDM system, which is especially suitable for mobile reception. This paper describes how a DQPSK-OFDM system works and shows several experimental and simulation results. The experimental results mainly concern the performance of the DQPSK-OFDM system relative to various disturbances such as multipath (ghost) signals, nonlinearity of the channel, and interference from analog signals. The transmission characteristics of DQPSK-OFDM are investigated and the basic criteria for the system design of DQPSK-OFDM are discussed.

This paper proposes an adaptive modulation/TDMA scheme to achieve high capacity personal multi-media communication systems. TDMA is employed to cope with various bit rate for multi-media services. The modulation scheme is selected from 1/4-rate QPSK, 1/2-rate QPSK, QPSK, 16QAM and 64QAM according to the received C/IC (power ratio of the desired signal to the co-channel interference) and the delay spread. The spectral efficiency is evaluated by using the simulated bit error rate (BER) performance as well as the cumulative distribution of the C/IC with parameters of cell configurations. The results show that the spectral efficiency of the proposed scheme is 3.5 times higher than that of the conventional QPSK systems at the outage probability of 10%, and the effect is more remarkable at lower outage probability. The results also show that the proposed adaptive modulation is effective in improving delay spread immunity.

Stored channel simulation for mobile radio channel can be the common base of the development of future world wide personal radio communication systems, especially for high bit-rate digital system. This paper proposes a mobile radio channel database which is suitable for the laboratory channel simulation using a simple stored channel simulator, also proposed by the author. The database enables the establishment of a mobile radio channel database containing worldwide channel data in a few discs of compact disc.

A receiving system suitable for multipath fading channels with co-channel interference is described. This system is equipped with both an M-sectored directional antenna and an adaptive equalizer to mitigate the influence due to multipath propagation and co-channel interference. By using directional antennas, this receiving system can separate desirable signals from undesirable signals, such as multipath signals with longer delay time and co-channel interference. It accepts multipath signals which can be equalized by maximum likelihood sequence estimation, and rejects both multipath signals with longer delay time and co-channel interference. Based on computer simulation results, the performance of the proposed receiving system is analyzed assuming simple propagation models with Rayleigh-distributed multipath signals and co-channel interference.

The performance of selection diversity combined with decision feedback equalizer for reception of TDMA carriers is investigated in this paper. The second generation digital land mobile communication systems standardized in the U.S., Japan, and Europe employ TDMA carriers at transmission bit rates up to several hundreds kbit/s. In order to provide higher quality of mobile communications services to the user with employing TDMA carriers, the systems would require both diversity and equalization techniques to combat attenuation of received signal power level due to Rayleigh fading and intersymbol interference resulting from time-variant multipath fading, respectively. This paper proposes a novel integration method of selection diversity and decision feedback equalization techniques which provides the better bit error rate performance than that for the conventional selection diversity method with decision feedback equalizer. The feature of proposed method is that selection diversity and decision feedback equalization techniques are integrated so as to interwork each other. We call the proposed method by the Decision Feedback Diversity with Decision Feedback Equalizer. The detailed algorithm of the proposed method is first presented, and then the system parameters for the method are evaluated based on the computer simulation results. Finally the computer simulation results for the performance of the proposed method are presented and compared to those for the conventional Selection Diversity with Decision Feedback Equalizer and the conventional Dual Diversity Combining and Equalization method under the typical mobile radio environments, in order to demonstrate the validity of the proposed method.

Upper bounds on the bit error probability and repeat request probability, and lower bounds on the throughput are derived for a Hybrid-ARQ scheme that employs trellis-coded modulation on a fading dispersive channel. The receiver employs a modified Viterbi algorithm to perform joint maximum likelihood sequence estimation (MLSE) equalization and decoding. Retransmissions are generated by using the approach suggested by Yamamoto and Itoh. The analytical bounds are extended to trellis-coded modulation on fading dispersive channels with code combining. Comparison of the analytical bounds with simulation results shows that the analytical bounds are quite loose when diversity reception is not employed. However, no other analytical bounds exist in the literature for the trellis-coded Hybrid ARQ system studied in this paper. Therefore, the results presented in this paper can provide the basis for comparison with more sophisticated analytical bounds that may be derived in the future.

A stored channel simulator for digital mobile radio enviroments is proposed, which enables the field tests in the laboratory under identical conditions, since it can reproduce the actual multipath radio channels by using the channel impulse responses (CIR's) measured in the field. Linear interpolation of CIR is introduced to simplify the structure of the proposed simulator. The performance of the proposed simulator is confirmed by the laboratory tests.

The bit error performance of a Direct Sequence Spread Spectrum Communication system in actual land mobile satellite channel is evaluated with experiments. Field test results with the ETS-V satellite in urban and suburban environments at L-band frequency show that this land mobile satellite channel of 3MHz bandwidth can be seen as a non-frequency selective Rician fading channel as well as shadowing channel. The bit error performance can be estimated from signal power measurement as in the case of narrow band modulation signals.

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.

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.

A superresolution technique is considered for use in antenna gain measurements. A modification of the MUSIC algorithm is employed to resolve incident signals separately in the time domain. The modification involves preprocessing the received data using a spatial scheme prior to applying the MUSIC algorithm. Interference rejection in the antenna measurements using the fast Fourier transform (FFT) based techniques have been realized by a recently developed vector network analyzer, and its availability has been reported in the literature. However, response resolution in the time domain of these conventional techniques is limited by the antenna bandwidth. The MUSIC algorithm has the advantage of being able to eliminate unwanted responses when performing antenna measurements in situations where the antenna band-width is too narrow to support FFT based techniques. In this paper, experimental results of antenna gain measurements in a multipath environment show the accuracy and resolving power of this technique.

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.

In this paper, some misconceptions about "multipath propagation" are discussed for those propagation engineers, who are not familiar with the close relationship between multipath propagation and a communication system in a mobile/portable radio communication environment. It is shown that believed facts about multipath propagation are not always true. Namely, it is well-known that multipath propagation is undesirable if a conventional sample-and-decision receiver is assumed. It is not well-recognized that it can be a desirable phenomenon if a sophisticated communication system uses adaptive equalization, anti-multipath modulation, or spread spectrum communication, for example. On the other hand, it is widely accepted that root mean square (rms) multipath delay spread is a good measure of bit-error-rate performance, i.e., as rms delay spread gets larger, bit-error-rate generally gets worse. However, it is pointed out that this is not always true, especially in propagation conditions with very long-delayed multipath signals. In short, it is the purpose of this paper to show examples that the facts believed to be true sometimes turn out to be false, unless we pay attention to both aspects of propagation and system design in the field of mobile/portable radio communications. In fact, for highly efficient communication systems design, propagation, antenna and system factors should be taken into account simultaneously.

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.