This paper proposes a pilot-based channel state information (CSI) feedback from a terminal to a base station (BS), considering the terminal's co-channel interference in a time-division duplex/multi-input multi-output system. In the proposed method, the terminal determines a precoding matrix according to the terminal's co-channel interference characteristics and transmits the precoded pilot signals on uplink. Using the responses of the precoded pilot signals, the BS determines appropriate weight vectors for downlink transmit beams considering the terminal's interference characteristics. Furthermore, the BS can predict the terminal's output signal-to-interference-plus-noise power ratio (SINR) for the downlink data stream. Numerical results show that the BS can achieve efficient transmission and accurate SINR prediction using the proposed CSI feedback.

This paper proposes an efficient multiuser relay scheme in OFDMA systems. In the proposed scheme, multiple terminals transmit their data packets simultaneously in the same subband and multiple relay stations retransmit their received signals in different subbands after subband conversion. A base station (BS) extracts individual packets from received signals in the different subbands. In advance of data transmission, the BS selects appropriate terminals so that the BS can extract individual data packets successfully. Numerical results show that the proposed relay scheme achieves higher system throughput than the conventional relay scheme when scheduling is applied to a larger number of terminals than the number of relay stations.

This paper proposes a new control scheme in which the base station (BS) controls terminal's transmit beamforming in time-division duplex (TDD)/multi-input multi-output (MIMO) systems. In the proposed scheme, the BS transmits pilot signals using appropriate downlink beams to instruct a terminal on target transmit beamforming. Using responses of the downlink pilot signals, the terminal can perform transmit beamforming close to the target one. Our theoretical investigation reveals that the BS can control multiple terminals' transmit beamforming simultaneously. Furthermore, an efficient signal processing at the terminal is investigated to obtain precise weight of transmit beamforming in noise environments. Numerical results show that the terminal can perform precise transmit beamforming close to the target one in noise environments. It is also shown that the amount of downlink control signalling in the proposed scheme is much less than that in codebook-based approach.

We investigated the suppression of multiuser interference in uplink multicarrier CDMA systems using the minimum mean squared error combining (MMSEC) method. In MMSEC, many pilot symbols are required to converge the weight vectors, and if we use just a few pilot symbols, the performance cannot be improved very much. We therefore developed a method for calculating weight vectors for MMSEC that uses just a few pilot symbols. The impulse responses of all users are first estimated using the pilot symbols in the time domain and modulated by a discrete Fourier transform. Next, the correlation matrices and correlation vectors are estimated from the impulse responses and the spreading codes of all users. Finally, the weight vectors that are obtained from the correlation matrices and correlation vectors are multiplied by the received signal to suppress the multiuser interference. The results of computer simulations indicated that the bit-error-ratio performance obtained using this method was better than that obtained when using the conventional fading compensation scheme or when using conventional MMSEC with the recursive least squares algorithm.

In code division multiple access (CDMA) systems with adaptive antennas, the direction of terminals must be considered when controlling new call admission. This paper proposes a data access control algorithm based on estimated signal-to-interference-plus-noise ratio (SINR) at the output of adaptive antennas. The algorithm estimates SINR for new data call using a response vector of the request packet to determine acceptance or blocking of the new data call. Numerical results show that the combination of transmission technology of adaptive antennas and proposed data access control can effectively increase the capacity of CDMA systems.

We analyzed the performance of adaptive array antennas with a RAKE receiver by employing a common correlation matrix of the sample matrix inversion (CCM-SMI) algorithm in a multipath Rayleigh fading environment for W-CDMA reverse link. A common correlation matrix is usually used to provide adaptive weights for multiple users and multiple delay paths and can be used in packet communications transmitted using frame units. The proposed CCM-SMI algorithm had a better BER and SINR for lower computational complexity compared with the conventional SMI algorithm, even when using a RAKE receiver in multipath Rayleigh fading environment.

This paper presents efficient time slot assignment algorithms applicable to the uplink of SDMA system. A frame consists of one control time slot and multiple communication time slots where terminals in different angular positions share the same time slot. In the proposed algorithms, a time slot is assigned to a new terminal considering not only the signal quality of the new terminal but also the signal quality of active terminals. Simple calculation method for estimated signal-to-interference plus noise ratio (SINR) is employed to decrease the computational complexity. The performance of the proposed algorithms is evaluated by computer simulation and compared with sectorized systems to show the validity of the proposed algorithms.

This paper studies a multiband mobile communication system to support both high data rate services and wide service coverage, using high and low frequency resources with different propagation characteristics. In the multiband system, multiple frequency bands are managed by a base station and one of the frequency bands is adaptively allocated to a terminal depending on his channel quality. By limiting the low frequency resources to a terminal not covered by the higher frequencies, the presented multiband system can accommodate many terminals providing wide coverage area, as if all radio resources have low frequency. From numerical results, the multiband system can provide wide service coverage area for much larger number of terminals than conventional systems. It is also found that an appropriate balance of multiple frequency resources is essential to achieve high capacity.

This paper presents upper bounds on the average bit error rate (BER) of coherent detection of PSK and differential detection of DPSK with adaptive arrays in fading environments. A model where a line of sight (LOS) component and Rayleigh distributed scattering components arrive at a receiver with specific arrival angles is used considering the correlation of signal between multiple antennas. The upper bounds are expressed in a simple matrix form using signal and interference-plus-noise correlation matrices. Examples for the case of 4-element adaptive arrays are given to illustrate the tightness and the application of this upper bounds.

Cooperative diversity using space-time codes offers effective space diversity with low complexity, but the scheme needs the space-time coding process in the relay nodes. We propose a simple cooperative relay scheme that uses space-time coding. In the scheme, the source node transmits the Alamouti coded signal sequences and the sink node receives the signal sequence via the two coordinated relay nodes. At the relay nodes, the operation procedure is just permutation and forwarding of the signal sequence. In the proposed scheme, none of the relay nodes need quadrature detection and space-time coding and the simple relay process offers effective space diversity. Moreover, simulations show the effectiveness of the proposed relay process by some simulations.

This paper presents time slot assignment algorithms applicable to uplink of space division multiple access (SDMA)/time division multiple access (TDMA) systems with adaptive antennas. In the time slot assignment process for a new terminal in a cell, we consider not only the signal quality of the new terminal but also that of active terminals in the same cell. Intra-cell hand over is performed for an active terminal when its signal quality deteriorates. We evaluate the blocking and forced termination probabilities for pure TDMA systems, sectorized systems, and SDMA/TDMA systems in cellular environments by computer simulations. The simulation results show that the SDMA/TDMA systems have much better performance than the pure TDMA and sectorized systems.

This paper proposes a new antenna array calibration technique which uses frequency selection in orthogonal frequency division multiple access (OFDMA)/time division duplexing (TDD) systems. In the proposed method, subbands or frequencies of good channel conditions are initially selected for channel measurements. The relative calibration is performed at the selected subbands, which compensates for mismatch of analogue gains in multiple antennas using the measured uplink and downlink channel parameters. Furthermore, the calibration parameters are interpolated in the frequency domain for the whole bandwidth. The proposed calibration maintains accurate channel reciprocity for the whole bandwidth compared to the conventional calibration which does not use the frequency selection. The proposed calibration technique is effective in exploiting channel reciprocity at both base station and terminals with feasible amount of feedback and low-cost operation.

This paper proposes a new partial channel state information (CSI) reporting method for spatial scheduling in TDD/MIMO systems. In the proposed method, a terminal transmits pilot signals using transmit beams which have large channel gains between the base station (BS) and the terminal. Then, the BS can obtain partial CSI through responses of the pilot signals. Furthermore, adaptive allocation of pilot signals is proposed, in which pilot signals for CSI reporting are adaptively allocated to terminals depending on the number of terminals. We evaluate system throughput of spatial scheduling under the partial CSI reporting from multiple terminals. Numerical results show that the proposed method reduces uplink signalling for CSI reporting effectively, keeping high system throughput of spatial scheduling.

Recently, multiple-input multiple-output (MIMO) systems that realize high bit rate data transmission with multiple antennas at both transmitter and receiver have drawn much attention for their high spectral efficiency. In MIMO systems, space division multiplexing (SDM) has been researched widely. In SDM, the input data symbols are transmitted from multiple transmit antennas at the transmitter, and the output data symbols are extracted by the signal processing at the receiver. In recent wireless communications, the environments that the number of transmit antennas is larger than that of receive antennas often exist. Under such environments, the MIMO system that transmits independent data streams from each transmit antenna simultaneously cannot separate the received signals, and the signal quality deteriorates largely. Therefore, we need the scheme that attains high quality and high throughput data transmission under such environments. In this paper, we propose a throughput maximization transmission control scheme for MIMO systems. The proposed transmission control scheme selects a transmission scheme (a set of transmit antennas, modulation schemes, and coding rates) with maximum throughput based on output signal to interference and noise ratio (SINR) and output signal to noise ratio (SNR). We show that the proposed transmission control scheme attains high throughput by our computer simulation.

This paper proposes a novel pre-FFT type OFDM adaptive array antenna called "Eigenvector Combining." The eigenvector combining array antenna is a realization of a post-FFT type OFDM adaptive array antenna through a pre-FFT signal processing, so it can achieve excellent performance with less computational complexity and shorter training symbols. Numerical results demonstrate that the proposed eigenvector combining array antenna shows excellent bit error rate performance close to the lower bound just with 2 OFDM symbol-long training symbols.

This paper proposes pilot-based channel quality reporting for orthogonal frequency division multiple access/time division duplex (OFDMA/TDD) systems with cochannel interference. In the proposed method, a terminal reports his channel quality in multiple subbands to base station (BS) using channel reciprocity of TDD systems. The terminal transmits uplink pilot signals in the subbands with different transmit power which is inversely proportional to the subband-based interference power. The BS can obtain knowledge of the terminal's received signal-to-interference-plus-noise power ratio on subband basis, measuring the pilot signal power. In performance evaluation, accuracy of channel quality reporting and amount of uplink signalling are examined. From numerical results, it is found that the proposed method becomes effective as the number of subbands and terminals for channel quality reporting increases.

In wideband code division multiaccess (W-CDMA) uplink, immediate accommodation of high data rate packet causes power control error and makes active users' signal quality deteriorate in a beginning of a frame. To avoid the deterioration, we propose a new radio resource management (RRM) which accommodates high data rate traffic gradually in several frames. The proposed RRM reduces the signal quality deterioration in the beginning of the frame. We also propose an effective power control scheme, where a power increase command is sent to all users before a new high data rate packet is transmitted. Simulation results show that joint utilization of the proposed two methods is effective to keep signal quality good for all users.

Multi-carrier code division multiplexing (MC-CDM) is one of promising multiplexing techniques for fourth-generation mobile downlink communications systems, where high data rate services should be provided even for high speed-cruising mobiles. For MC-CDM-based packet communication, a frequency scheduling method, which adaptively assigns different sub-carriers to different users, is proposed. This paper proposes a frequency scheduling method, which utilizes pre-assignmented subcarriers in the frequency domain for the MC-CDM scheme. Furthermore, the performance of the proposed system in frequency selective fading channels is compared with that of a no-scheduled MC-CDM scheme by computer simulation in both single- and multi-cell environments. From the results, it is found that the proposed system achieves better bit error rate performance than the no-scheduled MC-CDM scheme and can control quality of service (QoS) for active users.

We propose 3-hop cooperative diversity using QOSTBC (Quasi-Orthogonal Space-Time Block Code), which offers 3-hop cooperative diversity without signal separation in relay nodes. The key of our proposed scheme is encoding signal sequence in different signal unit according to relay stage. This letter explains details of the proposed scheme and shows that it offers interference reduction among streams and space diversity gain by result of simulations.

It is well known that Orthogonal Frequency Division Multiplexing (OFDM) scheme is robust to frequency selective fading in wireless channels. However, once delayed signals beyond a guard interval of an OFDM symbol are introduced in a channel with large delay spread, inter-symbol interference causes a severe degradation in the transmission performance. In this paper, we propose a novel pre-Fast Fourier Transform (FFT) OFDM adaptive antenna array, which requires only one FFT processor at a receiver, for suppressing such delayed signals. We analytically derive the optimum weights for the beamformer based on the Maximum Signal-to-Noise-and-Interference power Ratio (SNIR) and the Minimum Mean Square Error (MMSE) criteria, respectively. Computer simulation results show its good performance even in a channel where Directions of Arrival (DoAs) of arriving waves are randomly determined.