In this paper, a frequency domain adaptive antenna array (FDAAA) algorithm is proposed for broadband single-carrier uplink transmissions in a cellular system. By employing AAA weight control in the frequency domain, the FDAAA receiver is able to suppress the multi-user interference (MUI) and the co-channel interference (CCI). In addition, the channel frequency selectivity can be exploited to suppress the inter-symbol interference (ISI) and to obtain frequency diversity (or the multi-path diversity). Another advantage of the FDAAA algorithm is that its performance is not affected by the spread of angles of arrival (AOA) of the received multi-path signal. In this study the structure of FDAAA receiver is discussed and the frequency domain signal-to-interference-plus-noise-ratio (SINR) after weight control is investigated. The performance of the FDAAA algorithm is confirmed by simulation results. It is shown that, the optimal FDAAA weight to obtain the best BER performance is that which fully cancels the interference when single-cell system is considered; On the other hand, when multi-cell cellular system is considered, the optimal FDAAA weight depends on both the cellular structure and the target signal to noise ratio (SNR) of transmit power control (TPC).

In this paper, we propose a novel scheme of cooperative relaying network based on data exchange between relays before forwarding their received data to destination. This inter-relay data exchange step is done during an additional middle-slot in order to enhance the transmit signals from relays to the destination under low transmit power condition. To reduce the propagation errors between relays as well as the required transmit power during this data exchange, only the relay possessing the highest SNR is engaged into exchanging data by forwarding its received signal to the other relays. As for the remaining non-selected relays, i.e. with low SNR, the transmitted signal is estimated by using both signals received separately at different time slots (i.e., 1st and 2nd slot) from source and the 'best' selected relay, respectively, emulating virtual antenna array where appropriate weights for the antenna array are developed. In addition, we investigate distributed transmit beamforming and maximum ratio combining at the relays and the destination, respectively, to combine coherently the received signals. At the relay optimal location and for low SNR condition, the proposed method has significant better outage behavior and average throughput than conventional methods using one or two time slots for transmission.

MC-CDMA is an attractive multi-access method for the next generation high-speed mobile communication systems. The uplink transmission performance is limited by the multi-access interference (MAI) from other users since all users share the same bandwidth. Adaptive antenna array can be used to suppress the MAI and to improve the uplink transmission performance. In this paper, we propose a frequency-domain adaptive antenna array for multi-code MC-CDMA. The proposed frequency-domain adaptive antenna array uses a simple normalized LMS (NLMS) algorithm. Although the NLMS algorithm is used, very fast weight convergence within one MC-CDMA symbol duration is achieved since the weight updating is possible as many times as the number of subcarriers within one MC-CDMA symbol duration.

We propose an adaptive beamforming scheme for the combination of orthogonal frequency division multiplexing (OFDM) and adaptive antenna array. The combinational scheme is characterized by the sample matrix inverse (SMI) algorithm, frequency-to-time pilot transform and pre-FFT combination. For every OFDM block containing both data and pilot symbols, we transform the frequency-domain pilot symbols to the corresponding time-domain components. One of the obvious advantages of this transform is that the time interval of the antenna weight vector update can be reduced to only one OFDM sample interval, from one OFDM block interval of the conventional beamforming scheme in which the transform is not applied. This feature can greatly accelerate the convergence of SMI beamforming. The simulation results verify that the proposed beamforming scheme is capable of improving the convergence behavior significantly.

We have proposed two space division multiple access (SDMA) approaches for OFDM signals: "Virtual Subcarrier Assignment (VISA)" and "Preamble Subcarrier Assignment (PASA)," both of which can enhance the system capacity without significant change of transmitter/receiver structures for already-existing OFDM-based standards such as IEEE802.11a. In order to investigate the performance of the proposed approaches in real wireless scenarios, we conducted a measurement campaign to obtain real channel state data at 5-GHz band in an indoor environment. Using the measured channel data, we can make the performance evaluation realistic. In this paper, after the brief overview of the two proposed SDMA approaches, we describe our measurement campaign in detail. Furthermore, we evaluate the performance of VISA-based system and PASA-based system by computer simulations using the measured channel state data and present a comparative study on the performance of the two proposed SDMA approaches in the realistic wireless environment.

A novel signal enhancement scheme using the rotation of signal subspace (RSS) and Toeplitz matrix approximation (TMA) methods to enhance the performance of an adaptive antenna array in multirate DS/CDMA systems is proposed. The basis of RSS is to find a transformation matrix in order to recover the desired complex array covariance matrix from a sampled complex array covariance matrix which is contaminated by an interference-plus-noise component, which is the total noise. Also, the objective of TMA is to change the output matrix of RSS into a matrix having the theoretical properties of a total noise-free signal. Consequently, the proposed signal enhancement scheme using RSS and TMA methods can greatly improve the performance of an adaptive antenna array by reducing the undesired total noise effect from the sampled complex array covariance matrix of the pre-correlation received signal vector that is used to calculate a weight vector of an adaptive antenna array. It is shown through various simulation results that the system performance using the proposed signal enhancement scheme is much superior to that of the conventional method.

Efficient schemes to enhance the performance of the optimum beamforming for DS/CDMA systems are proposed. The main focus of the proposed schemes is to enhance the practical estimation of an array response vector used at the weight vector for the optimum beamforming. The proposed schemes for the performance enhancement of the optimum beamforming are the Complex Toeplitz Approximation (CTA) and the real Toeplitz-plus-Hankel Approximation (RTHA) which have the theoretical property of an overall noise-free signal. It is shown through several simulation results that the performance of the optimum beamforming using the proposed schemes is much superior to that of a system using the conventional method under several simulation environments, i.e., the number of users, the SNR value, the number of antenna elements, the angular spread, and Nakagami fading parameter.

In this letter, we investigate the performance of using subband adaptive loading for the combination of orthogonal frequency division multiplexing (OFDM) and adaptive antenna array. The frequency-domain adaptive loading is very effective to deal with the frequency-selective fading which is inevitable in broadband wireless communications. However, almost all of the existing adaptive loading algorithms are based on "subcarrier-to-subcarrier" mode which may results in awfully large signaling overhead, since every subcarrier needs its own signaling loop between the transmitter and receiver. We investigate the performance of the combination of OFDM and adaptive antenna array when a subband adaptive loading algorithm is used to decrease the signaling overhead. It is shown by simulation results that at the cost of some tolerable performance loss, the signaling overhead of adaptive loading can be greatly reduced.

Adaptive antenna array is a promising technique to increase the link capacity in mobile radio communications systems by suppressing multiple access interference (MAI). In the mobile radio, the received signal consists of discrete paths, each being a cluster of many irresolvable paths arriving from different directions. For large arrival angle spread of each cluster of irresolvable paths, antenna array cannot form a beam pattern that sufficiently suppresses MAI even in the presence of single interference signal and hence, the transmission performance may degrade. In this situation, the use of antenna diversity may be a better solution. It is an interesting question as to which can achieve a better performance, antenna diversity reception or adaptive antenna array. In this letter, we study the impact of the arrival angle spread on the DS-CDMA transmission performances achievable with adaptive antenna array and antenna diversity reception. It is pointed out that the arrival angle spread is an important parameter to determine the performances of adaptive antenna array and antenna diversity.

This paper investigates based on laboratory experiments the multiuser interference suppression effect of the coherent adaptive antenna array diversity (CAAAD) receiver employing an optical fiber feeder in the intermediate frequency (IF) stage, aiming at the practical use of adaptive antenna array beam forming techniques based on the W-CDMA air interface. We employed a configuration in which the optical fiber conversion, i.e., electrical-to-optical (E/O) conversion (vice versa (O/E)), is performed on a received signal amplified by an automatic gain control (AGC) amplifier in the IF stage, to abate the impact of the noise component generated by the E/O (O/E) converters. We first show by computer simulation the superiority of the optical fiber conversion in the IF stage to that in the radio frequency (RF) stage based on the achievable bit error rate (BER) performance. Furthermore, experimental results elucidate that the loss in the required transmit signal energy per bit-to-background noise power spectrum density ratio (Eb/N0) of the implemented CAAAD receiver at the average BER of 10-3 employing the optical fiber feeders in the IF stage compared to that with coaxial cables is within a mere 0.2 dB (six antennas, three users, two-path Rayleigh fading channel model, and the ratio of the target signal energy per bit-to-interference power spectrum density ratio (Eb/I0) of the desired user to that of the interfering users for fast transmission power control (TPC) is ΔEb/I0=-15 dB).

An efficient performance enhancement scheme using the rotation of signal subspace (RSS) and Toeplitz matrix approximation (TMA) methods to enhance the performance of an adaptive antenna array in DS/CDMA systems is proposed. The basis of RSS is to find a transformation matrix in order to recover the desired complex array covariance matrix from a sampled complex array covariance matrix which is contaminated by an interference-plus-noise component. Also, the objective of TMA is to change the output matrix of RSS into a matrix having the theoretical properties such as Toeplitz structure matrix or a positive semidefinite matrix. Consequently, the proposed scheme using RSS and TMA methods can greatly improve the performance of an adaptive antenna array by reducing the interference-plus-noise effect from the sampled complex array covariance matrix of the pre-correlation received signal vector that is used to calculate a weight vector of an adaptive antenna array. Simulation results demonstrate the effectiveness of the proposed scheme.

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.

This paper presents laboratory and field experimental results of the coherent adaptive antenna array diversity (CAAAD) receiver employing receiver antenna-weight generation common to all Rake-combined paths (hereafter path-common weight generation method) in the W-CDMA reverse link, in order to elucidate the suitability of the path-common weight generation method in high-elevation antenna environments such as cellular systems with a macrocell configuration. Laboratory experiments using multipath fading simulators and RF phase shifters elucidate that even when the ratio of the target Eb/I0 of the desired to interfering users is Δ Eb/I0=-12 dB, the increase in the average transmit Eb/N0 employing the CAAAD receiver coupled with fast transmission power control (TPC) using outer-loop control from that for Δ Eb/I0=0 dB is within only 1.0 dB owing to the accurate beam and null steering associated with fast TPC. Furthermore, field experiments demonstrate that the required transmission power at the average block error rate (BLER) of 10-2 employing the CAAAD receiver with four antennas is reduced by more than 2 dB compared to that using a four-branch space diversity receiver using maximum ratio combining (MRC) with the fading correlation between antennas of 0 when Δ Eb/I0=-15 dB and that the loss in the required transmission power of the CAAAD receiver in the same situation as that in a single-user environment is approximately 1 dB. The field experimental results in an actual propagation environment suggest that the CAAAD receiver is effective in suppressing multiple access interference, thus decreasing the required transmission power when the gap in the direction of arrival between the desired user and interfering users is greater than approximately 20 degrees.

This paper investigates an accurate channel estimation method using the common pilot channel (CPICH) in addition to a dedicated pilot channel (PICH) when the fading correlation between the dedicated PICH and CPICH is high, and clarifies the area in which the proposed channel estimation method is effective for adaptive antenna array transmit diversity (AAA-TD) in the forward link. Computer simulation results elucidate that although a more precise channel estimation is possible by using the primary-CPICH (P-CPICH) transmitted from an omni-directional antenna in addition to the dedicated PICH for the area where the distance, d, between a base station and a mobile terminal is longer than approximately 200 m, no improvement is obtained for the area where the value of d is shorter than approximately 200 m. Meanwhile, by employing the secondary-CPICH (S-CPICH) transmitted with several directional beams in addition to the dedicated PICH, the required average received Eb/N0 at the average BER of 10-3 is decreased by approximately 0.4 (0.2-0.4) dB compared to the channel estimation method using only the dedicated PICH regardless of the value of d when the number of antennas is 4 (8).

This paper investigates the optimum adaptive antenna array beam forming (AAA-BF) configuration considering the diversity effect provided by transmit diversity (TD) in a multipath fading channel in the W-CDMA forward link. Computer simulation results show that the required transmit signal energy per bit-to-background noise power spectrum density ratio (Eb/N0) at the average block error rate (BLER) of 10-2 using AAA-BF coupled with TD is decreased by approximately 1.0 dB compared to that of AAA-BF assuming the identical number of total antennas when the capacity, i.e., the number of simultaneously accommodated users with the transmission power proportional to the symbol rate, normalized by processing gain, Pg, is below approximately 20%. However, we find that in an interference-limited channel, when the capacity normalized by Pg is above approximately 30%, AAA-BF employing all antennas accommodates a larger capacity compared to AAA-BF coupled with TD because of a sufficient interference suppression effect due to a much narrower beam width despite the absence of the antenna diversity effect. This paper also elucidates in a multi-cell model that AAA-BF employing all antennas can accommodate approximately 1.5 times more users with the average BLER of 10-2 and with the outage probability of 5%, compared to the case with AAA-BF coupled with TD, when the total number of antennas is 8.

This paper proposes adaptive antenna array transmit diversity (AAA-TD) in the W-CDMA forward link with frequency division duplexing (FDD), based on adaptively-generated receiver antenna weights in the reverse link, which only track the changes in the average signal-to-interference power ratio (SIR) and direction of arrival (DOA) but with the calibration of the phase/amplitude variations of the parallel RF receiver/transmitter circuits corresponding to the number of array antennas. The laboratory and field experimental results exploiting AAA-TD are presented to show the strong multipath interference (MPI) suppression effect especially from high-rate users with large transmission power. Laboratory experiments elucidate that by using AAA-TD with four antennas, the required transmitted SIR before multiplying the transmitter antenna weights at the average BER of 10-3 is decreased by approximately 13 dB compared to that with one omni-directional antenna transmitter. Field experiments also show that although an error floor above 10-2 is observed with one omni-directional antenna transmitter when the transmitted SIR is -12 dB due to severe MPI, no error floor is observed when employing 4-antenna AAA-TD and the loss of the required received signal power at the average BER of 10-3 from the single-user case is suppressed to below approximately 5 dB. Therefore, we show that AAA-TD is very effective in suppressing severe MPI especially from high rate users with large transmission power due to its adaptive main lobe and null steering.

In this paper, the efficient signal enhancement scheme for an adaptive antenna array under the Nakagami fading channel is proposed. The proposed signal enhancement scheme is the modified linear signal estimator with Toeplitz Matrix Approximation (TMA) method. The underlying principle of the proposed signal enhancement scheme is to reduce a noise component using not only the Lagrangian method of the constrained minimization but also a signal-plus-noise subspace method. TMA is also used to have the theoretical property of noise-free signal. These functions greatly enhance the performance of an adaptive antenna array by removing the all undesired noise effects from the post-correlation received signal. The proposed signal enhancement scheme is applied at the Wiener Maximal Ratio Combining (MRC) method which has been widely used as the conventional adaptive antenna array. Also, we investigated the effect of the power control error (PCE) for the proposed scheme over the Nakagami multipath fading channel. Several computer simulation examples are provided for illustrating the effectiveness of the proposed scheme.

This paper investigates the optimum transmit-antenna-weight generation method for adaptive antenna array transmit diversity (AAA-TD) in the W-CDMA forward link: AAA-TD with beam and null steering (BNST), AAA-TD with beam steering (BST), or switched beam transmit diversity with fixed weights (SBTD-FW). The achievable BER performance after carrier frequency calibration in the transmit beam pattern is compared among the three methods assuming a different carrier frequency in a 2-GHz band with the carrier separation of 184.5 MHz based on computer simulations. The simulation results show that the achievable BER performance in the forward link using AAA-TD with BNST is almost identical to that using AAA-TD with BST when there are many more interfering users than there are array antennas, except for the special case when a small number of higher rate users exists in the reverse link. This is because by performing carrier frequency calibration, the directions of the beam nulls are shifted from the real directions of arrival (DOAs) of the interfering users. However, we also show that the required transmit Eb/N0 at the average BER of 10-3 using AAA-TD with BST is decreased by approximately 1.0 to 1.2 dB compared to that using SBTD-FW with 12 beams.

This paper compares the path detection probability for Rake combining and the BER performance of packet transmission with the length of TPKT = 10 (20) msec using the coherent adaptive antenna array diversity (CAAAD) receiver with those using a multi-beam receiver employing fixed antenna weights both with four antennas in a multipath fading channel in the W-CDMA reverse link. Laboratory experimental results elucidate that the required average received signal energy per bit-to-background noise spectrum density (Eb/N0) at the detection probability of 90% of at least one path and of two paths for the fading maximum Doppler frequency of fD = 5 (80) Hz using a multi-beam receiver with the number of beams equal to NBeam = 12 was decreased by approximately 1.0 (1.0) dB and 2.0 (2.0) dB, respectively, compared to that using the CAAAD receiver with the step size of µ= 10-2 for the average received SIR of the desired user of -12 dB in a 2-path Rayleigh fading channel with average equal power in a 5-user environment. We also found that the required average received Eb/N0 at the average BER of 10-3 using the multi-beam receiver was decreased by approximately 5.0 (2.5) dB compared to that of the CAAAD receiver with µ= 10-2, and the loss of the required average received Eb/N0 compared to that of CAAAD with sufficiently converged receiver antenna weights was approximately 2.0 (1.0) dB for TPKT = 10 (20) msec when the average received SIR = -12 dB in a 5-user environment.

In wideband direct sequence code division multiple access (W-CDMA), employing an adaptive antenna array is a very promising technique to reduce severe multiple access interference (MAI) especially from high rate users. This paper proposes a fast and accurate two-step beam tracking algorithm implemented in a pilot symbol-assisted coherent adaptive antenna array diversity (CAAAD) receiver and evaluates its performance both by computer simulation and laboratory experiments. In the proposed scheme, the receiver antenna weights are updated by using both the signal-to-interference power ratio (SIR) measurements employing multiple sets of antenna weights (MSAW) and an adaptive algorithm based on the minimum mean square error (MMSE) criterion, in which other sets except for a original set of antenna weights are simply generated by a original set. Computer simulation results show that antenna weights of a four-antenna CAAAD receiver using the proposed beam tracking algorithm tracks changes in the direction of arrival (DOA) of the desired user at up to 34.3 degrees/sec, which corresponds to 215 km/h at 100 m from a base station. We also confirm based on the experiments in a radio anechoic room that the generated antenna weights track the DOA changes up to 12.3 degrees/sec.