When the multipath delay difference exceeds the guard interval (GI), the performance of MIMO-OFDM transmission suffers severely from both the inter-symbol interference (ISI) from the adjacent OFDM symbols and the inter-carrier interference (ICI) within the same symbol. This paper therefore proposes a MIMO-OFDM receiver employing the low-complexity turbo equalization. The proposed receiver initially separates the data streams and suppresses ICI by linear processing. In the iterative processing, it cancels the other data streams as well as ISI and ICI. The MIMO-OFDM turbo equalizer consists of an ISI canceller, an ICI canceller, an optimal detection filter, and a MAP detector. The proposed receiver can improve the transmission performance by exploiting the log-likelihood ratio that the decoding process produces for canceling both ISI and ICI and separating of the spatially multiplexed streams. Computer simulations, which apply the wireless LAN to MIMO, demonstrate that the proposed receiver can provide excellent performance in the severe multipath channels where the delay difference is greater than GI.

This paper reviews various switch architectures for Asynchronous Transfer Mode, which have been proposed and developed so far in Japan. The switch fabrics can be classified, owing to the arrangement of switch matrices and buffer memories, into four categories, input buffer switch, output buffer switch, shared buffer switch and crosspoint buffer switch. Those switches have their own advantages and disadvantages, which require additional effort to implement the switches for the practical network. This paper introduces examples of each category switch fabric and additional technical modifications to make it practical. Other general issues to construct ATM switch fabrics, such as non-blocking characteristics and path assignment within a multi-stage switch network, are also addressed. Furthermore, future directions in the ATM switch fabric is discussed.

This paper proposes a suboptimal algorithm for the maximum likelihood detection (MLD) in multiple-input multiple-output (MIMO) communications. The proposed algorithm regards transmitted signals as continuous variables in the same way as a common method for the discrete optimization problem, and then searches for candidates of the transmitted signals in the direction of a modified gradient vector of the metric. The vector is almost proportional to the direction of the noise enhancement, from which zero-forcing (ZF) or minimum mean square error (MMSE) algorithms suffer. This method sets the initial guess to the solution by ZF or MMSE algorithms, which can be recursively calculated. Also, the proposed algorithm has the same complexity order as that of conventional suboptimal algorithms. Computer simulations demonstrate that it is much superior in BER performance to the conventional ones.

A low-noise 2-GHz-band VCO for frequency synthesizers used in mobile/portable radio sets is designed and implemented. This frequency band has become increasingly important for mobile/portable radio to alleviate the present frequency congestion problem below 1 GHz. So far, however, low noise VCOs which satisfy the severe requirements of mobile radio systems operating above 1 GHz have not been investigated. The design specifications are summarized, and it is shown that the VCO phase noise reduction requires a rigorous designed oscillator. Oscillator noise characteristics are reviewed, and design principles are clarified. In order to suppress the 1/f noise, a Clapp oscillator circuit and a microstrip resonator are adopted. A Si BJT and GaAs MES FET are compared under optimum bias conditions. The noise level of the Si BJT oscillator is 13 dB less than to that of the GaAs MES FET at a 25-kHz offset frequency from the oscillation frequency, and was thus adopted in our VCO implementation. The following performance levels were obtained: 28 MHz variable frequency range with control voltage from 1 to 5 volts, 112 dB single sideband noise to carrier ratio in 1-Hz bandwidth at 25 kHz offset, and 37.5 dB noise ratio from 5 Hz to 20 kHz with respect to signal.

This paper proposes a maximum likelihood sequence estimation (MLSE) for the differential space-time block code (DSTBC) in cooperation with blind linear prediction (BLP) of fast frequency-flat fading channels. This method that linearly predicts the fading complex envelope derives its linear prediction coefficients by the method of Lagrange multipliers, and does not require data of decision-feedback or information on the channel parameters such as the maximum Doppler frequency in contrast to conventional ones. Computer simulations under fast fading conditions demonstrate that the proposed method with an appropriate degree of polynomial approximation is superior in BER performance to the conventional method that estimates the coefficients by the RLS algorithm using a training sequence.

This paper proposes joint maximum a posteriori (MAP) detection and spatial filtering for MIMO-OFDM mobile communications; it offers excellent receiver performance even over interference-limited channels. The proposed joint processor consists of a log likelihood generator and a MAP equalizer. The log likelihood generator suppresses cochannel interference by spatially filtering received signals and provides branch metrics of transmitted signal candidates. Using the branch metrics, the MAP equalizer generates log likelihood ratios of coded bits and performs channel decoding based on the MAP criterion. In the first stage, the log likelihood generator performs spatio-temporal filtering (STF) of the received signals prior to the fast Fourier transform (FFT) and is referred to as preFFT-type STF. Estimation of parameters including tap coefficients of the spatio-temporal filters and equivalent channel impulse responses of desired signals is based on the eigenvalue decomposition of an autocorrelation matrix of both the received and transmitted signals. For further improvement, in the second stage, the generator performs spatial filtering (SF) of the FFT output and is referred to as postFFT-type SF. Estimation of both tap coefficients of the spatial filters and channel impulse responses employs the recursive least squares (RLS) with smoothing. The reason for switching from preFFT-type STF into postFFT-type SF is that preFFT-type STF outperforms postFFT-type SF with a limited number of preamble symbols while postFFT-type SF outperforms preFFT-type STF when data symbols can be reliably detected and used for the parameter estimation. Note that there are two major differences between the proposed and conventional schemes: one is that the proposed scheme performs the two-stage processing of preFFT-type STF and postFFT-type SF, while the other is that the smoothing algorithm is applied to the parameter estimation of the proposed scheme. Computer simulations demonstrate that the proposed scheme can achieve excellent PER performance under interference-limited channel conditions and that it can outperform the conventional joint processing of preFFT-type STF and the MAP equalizer.

This paper proposes a new MIMO-OFDM precoding technique that aims to minimize a bit error rate (BER) upper bound of the maximum likelihood detection (MLD) in mobile radio communications. Using a steepest descent algorithm, the proposed method estimates linear precoding matrices that can minimize the upper bound of BER under power constraints. Since the upper bound is derived from all the pairwise error probabilities, this method can effectively optimize overall Euclidean distances between signals received by multiple antennas and their replicas. Computer simulations evaluate the BER performance and channel capacity of the proposed scheme for 22 and 44 MIMO-OFDM systems with BPSK, QPSK, and 16 QAM. It is demonstrated that the proposed precoding technique is superior in terms of average BER to conventional precoding methods including a precoder which maximizes only the minimum Euclidean distance as the worst case.

In this paper, we propose a simple and accurate transfer function model of the power amplifiers for mobile communications. Detail analysis yields a generalized model for AM/AM characteristics in classes AB, B, and C. The analysis includes the effect of drain current variation with input level variation. This model introduces a loadline variation ratio to indicate the change of drain current and to represent the operation classes in a small signal region. Further discussion leads to simplified approximate equations for the AM/AM characteristics, and the estimation procedures for the simplified model parameters. Using the derived procedures, an efficient power amplifier employing pseudomorphic high electron mobility transistor (PHEMT) is fabricated for the 2 GHz band. Finally, the various characteristics given by the model, simulator and measurements are compared and found to agree well in the range of 20 dB below the saturated output level. The model is very effective for characterizing the power amplifiers that are used in linear compensation techniques such as predistortion methods, due to its severe nonlinearity of AM/AM and AM/PM characteristics.

This paper describes the results of a series of laboratory experiments for performance evaluations of our proposed Maximum Likelihood Sequence Estimation (MLSE) based interference canceller, the Interference Canceling Equalizer (ICE), which can cancel both co-channel interference (CCI) and inter-symbol interference (ISI). To verify the feasibility of ICE for the Japanese cellular communications system, a standard of which has been released under the name of Personal Digital Cellular (PDC) system, a prototype system was constructed using 27 TI TMS320C40 Digital Signal Processor (DSP) chips. The ICE prototype works in real-time on the PDC air interface, major specifications of which are π/4 QDPSK 21 k symbols/s 3-channel time-division multiple-access (TDMA). Two-branch diversity reception is used to enhance the signal detection performance of ICE. In the experiments, BER performances were evaluated using the prototype system. Under a single-path Rayleigh fading and a single CCI condition, the ICE receiver attains the BER of less than 310-2 with the negative values of the average CIR: for fD = 5 Hz and 40 Hz, the average CIR more than -20 dB and -10 dB, respectively. Under a double-path Rayleigh fading and a single CCI condition, the ICE receiver attains the BER of less than 1.510-2 with the negative values of the average CIR: for fD = 5 Hz and 40 Hz, the average CIR more than -20 dB and -10 dB, respectively. The laboratory test results suggest that the ICE receiver has potential for system capacity enhancement.

This paper proposes a new approach for the joint processing of signal detection and channel estimation based on the expectation-maximization (EM) algorithm in orthogonal frequency division multiplexing (OFDM) mobile communications. Conventional schemes based on the EM algorithm estimate a channel impulse response using Kalman filter, and employ the random walk model or the first-order autoregressive (AR) model to derive the process equation for the filter. Since these models assume that the time-variation of the impulse response is white noise without considering any autocorrelation property, the accuracy of the channel estimation deteriorates under fast-fading conditions, resulting in an increased packet error rate (PER). To improve the accuracy of the estimation of fast-fading channels, the proposed scheme employs a differential model that allows the correlated time-variation to be considered by introducing the first- and higher-order time differentials of the channel impulse response. In addition, this paper derives a forward recursive form of the channel estimation along both the frequency and time axes in order to reduce the computational complexity. Computer simulations of channels under fast multipath fading conditions demonstrate that the proposed method is superior in PER to the conventional schemes that employ the random walk model.

This paper proposes a new correction technique for a linear amplification with nonlinear components (LINC) transmitter. The technique, which is based on the minimum mean squared error (MMSE) criterion, estimates the gain and phase imbalance between the two amplifier branches. With information on the estimation, the imbalance is offset by controlling the amplitude and phase of the input signal that is fed into one of the two amplifiers. Computer simulations with a DS-CDMA system demonstrate that this method can compensate for the imbalance and sufficiently suppress the out-of-band distortion spectrum.

This paper proposes an iterative maximum a posteriori probability (MAP) receiver for multiple-input-multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) mobile communications. For exploiting the space, time, and frequency diversity, the low-density parity-check code (LDPC) is used as a channel coding with a built-in interleaver. The receiver employs the expectation maximization (EM) algorithm so as to perform the MAP symbol detection with reasonable computational complexity. The minimum mean square error (MMSE), recursive least squares (RLS), and least mean square (LMS) algorithms are theoretically derived for the channel estimation within this framework. Furthermore, the proposed receiver performs a new scheme called backward symbol detection (BSD), in which the signal detection uses the channel impulse response that is estimated one OFDM symbol later. The advantage of BSD, which is explained from the viewpoint of the message passing algorithm, is that BSD can exploit information on the both precedent and subsequent OFDM symbols, similarly to RLS with smoothing and removing (SR-RLS) [25]. In comparison with SR-RLS, BSD reduces the complexity at the cost of packet error rate (PER) performance. Computer simulations show that the receiver employing RLS for the channel estimation outperforms the ones employing MMSE or LMS, and that BSD can improve the PER performance of the ones employing RLS or LMS.

In communication systems such as mobile telecommunication systems and the Internet, resource sharing among coexisting real-time and non-real-time services is extremely important to provide multimedia services. This paper analytically investigates the performance of the packet data control algorithm proposed in. This algorithm efficiently uses radio resources by utilizing the remaining capacity that is not used by real-time services. The state probability vectors and transition probability matrices of both the real-time and non-real-time services are first derived and then the delay characteristics, the outage probability of voice users, and the outage probability of data users are evaluated. A performance analysis with high bit-rate non-real-time services is also presented.

An important aspect of traffic safety is the development of aids that extend the driver's time and motion perception. One promising candidate is the compact, lightweight millimeter-wave FM-CW radar now being widely studied. Although the homodyne FM-CW radar is well known form its simplicity, it has a relatively low S/N ratio. This paper describes the principles behind our newly-developed heterodyne FM-CW radar and it's evaluation results. The heterodyne FM-CE radar generates sidebands by switching a front-end amplifier and also uses the heterodyne detection technique for gaining sensor sensitivity. The heterodyne FM-CW radar's signal to noise ratio was 19.5 dB better than previously designed homodyne FM-CW radar.

This paper proposes an iterative maximum a posteriori (MAP) receiver for orthogonal frequency division multiplexing (OFDM) mobile communications under fast-fading conditions. The previous work in [21] developed a MAP receiver based on the expectation-maximization (EM) algorithm employing the differential model, which can allow correlated time-variation of channel impulse responses. In order to make such a MAP receiver more robust against time-variant channels, this paper proposes two new message-passing algorithms derived from factor graphs; subcarrier removal and partial turbo processing. The subcarrier removal estimates the channel frequency response by using all subcarriers other than the targeted subcarrier. Such channel estimate can be efficiently calculated by removing information on the targeted subcarrier from the estimate of the original EM algorithm that uses all the subcarriers. This modification can avoid the repetitive use of incorrectly detected signals for the channel estimation. On the other hand, the partial turbo processing performs symbol-by-symbol channel decoding by using a symbol interleaver. Owing to this process, the current channel estimate, which is more accurate due to the decoding gain, can be used as the initial channel estimate for the next symbol. Computer simulations under fast multipath fading conditions demonstrate that the subcarrier removal and the partial turbo processing can improve the error floor and the convergence speed, respectively, compared to the conventional MAP receiver.

This paper applies iterative multiuser detection employing a new channel estimation scheme to multicarrier interleave-division multiple access (MC-IDMA), called OFDM-IDMA, which is expected to offer improved spectral efficiency in mobile communications. The MC-IDMA transmitter uses both a low-rate channel code and an individual chip interleaver for each user. The MC-IDMA receiver, which this paper focuses upon, repeats the iterative multiuser detection and soft decision-directed channel estimation (SDCE) by exploiting log-likelihood ratios (LLRs) of the coded bits which the maximum a posteriori (MAP)-based channel decoders for all users provide. SDCE estimates channel impulse responses of all users by the least-mean-square (LMS) algorithm, which aims to minimize the mean squared error between the received signal and its replica. This paper investigates the performance of MC-IDMA employing SDCE and compares it with those of three MC-CDMA techniques. Computer simulations demonstrate that MC-IDMA employing SDCE outperforms time-spread MC-CDMA and frequency-spread MC-CDMA, and that it can achieve almost the same bit error rate performance as chip-interleaved MC-CDMA while requiring lower complexity.

This paper proposes a MIMO-OFDM precoder that can minimize a BER upper bound of the maximum likelihood detector (MLD) under a non-cooperative downlink multicell co-channel interference (CCI) environment. Since there is no cooperation among base stations (BSs), it is assumed that information on the interference can be estimated at a mobile station (MS) and then fed back to the desired BS for the precoder. The proposed scheme controls its precoding parameters under a transmit power constraint so as to minimize the BER upper bound, which is derived from the pairwise error probability (PEP) averaged with respect to CCI plus noise. Computer simulations demonstrate that the proposed precoder can effectively improve BER performance of cell edge users and is superior in terms of BER to the eigenmode and the minimum mean squared error (MMSE) precoded transmissions which aim to maximize the channel capacity and to minimize MSE, respectively.

This paper proposes a multipacket-per-slot reservation-based random access protocol with multiuser detection (MD) and automatic repeat request (ARQ), called MPRMD, and analyzes its performance by computer simulations. In MPRMD, before data packet (DP) transmission, a user terminal (UT) transmits a small access request packet (AP) that is composed of an orthogonal preamble sequence and a UT identifier (UT-ID) in a randomly selected minislot during a short dedicated period. Even when several APs collide, a base station (BS) distinguishes them by matched filtering against the preamble part and then extracts the UT-IDs after separating each AP by MD. If the APs are not successfully detected, a small number of minislots are additionally arranged to retransmit them. Thus, by using MD under AP crowded conditions, BS can maximally detect the access requests in a short period, which results in reducing the overhead. Furthermore, in the assignment of a slot, BS intentionally assigns one slot to multiple UTs in order to enhance the efficiency and separates UT's DPs by MD. Since MPRMD can detect a multitude of access requests by utilizing MD in the short period and efficiently assign the slot to separable DPs by MD, it can enhance the system throughput. Computer simulations are conducted to demonstrate the effectiveness of MPRMD. It is shown that the maximum throughputs of MPRMD with the average SNR of 30dB reach 1.4 and 1.7 packets/slot when a data packet is 10 times and 50 times as long as a control packet, respectively.

Joint signal detection and channel estimation based on the expectation-maximization (EM) algorithm has been investigated for multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) mobile communications over fast-fading channels. The previous work in [20] developed a channel estimation method suitable for the EM-based iterative receiver. However, it remained possible for unreliable received signals to be repetitively used during the iterative process. In order to improve the EM-based iterative receiver further, this paper proposes spatial removal from the perspective of a message-passing algorithm on factor graphs. The spatial removal performs the channel estimation of a targeted antenna by using detected signals that are obtained from the received signals of all antennas other than the targeted antenna. It can avoid the repetitive use of unreliable received signals for consecutive signal detection and channel estimation. Appropriate applications of the spatial removal are also discussed to exploit both the removal effect and the spatial diversity. Computer simulations under fast-fading conditions demonstrate that the appropriate applications of the spatial removal can improve the packet error rate (PER) of the EM-based receiver thanks to both the removal effect and the spatial diversity.