In this paper, we propose the code orthogonalizing filter (COF) based adaptive array antenna using sample matrix inversion with common correlation matrix (CCM-SMI) of time domain signals for multicarrier DS/CDMA systems. The conventional array antenna system calculates the weight using the correlation matrix of individual subcarrier's signals. On the other hand, our proposed system calculates the weight using the correlation matrix of time domain signals before FFT operation, so it can reduce the calculation time and the complexity of weight calculation than the conventional scheme, to maintain the system performance. Moreover, we consider the code orthogonalizing filter to reduce the demerit of adaptive array antenna system using sample matrix inversion algorithm with common correlation matrix that requires heavy computational complexity while the signal environment frequently changes. Our proposed system obtains more accurate channel response vector using COF than that of the conventional CCM-SMI based on the matched filter, without increasing the matrix size. The performance is evaluated in term of bit error probability. From the analysis and simulation results, it is shown that our proposed scheme achieves better BER performance than that of the conventional system.

The pre-Rake system is known as a technique in TDD DS/CDMA system to reduce the mobile complexity and achieve the same BER performance like Rake receiver. The pre-Rake system itself is not optimum, since the channel impulse responses of uplink and downlink are slightly different in TDD system, so the signal- to-noise ratio (SNR) can be maximized with a matched filter based Rake receiver, which has not been considered in the conventional pre-Rake system. Furthermore pre-Rake system is sensitive to the Doppler frequency. Even though the pre-Rake system has the ability to suppress other user interference, it is not efficient to maximize the received signal in high Doppler frequency. However, Rake combiner is utilized for the detection method in our proposed system. So the maximized signal can keep the orthogonality better than the pre-Rake system and our proposed system can compensate the Doppler frequency effect. From these reasons, our system achieves better BER performance than that of the pre-Rake system with increasing the number of users in high Doppler frequency.

This paper expands our previously proposed semi-blind uplink interference suppression scheme for multicell multiuser massive MIMO systems to support multi modulus signals. The original proposal applies the channel state information (CSI) aided blind adaptive array (BAA) interference suppression after the beamspace preprocessing and the decision feedback channel estimation (DFCE). BAA is based on the constant modulus algorithm (CMA) which can fully exploit the degree of freedom (DoF) of massive antenna arrays to suppress both inter-user interference (IUI) and inter-cell interference (ICI). Its effectiveness has been verified under the extensive pilot contamination constraint. Unfortunately, CMA basically works well only for constant envelope signals such as QPSK and thus the proposed scheme should be expanded to cover QAM signals for more general use. This paper proposes to apply the multi modulus algorithm (MMA) and the minimum mean square error weight derivation based on data-aided sample matrix inversion (MMSE-SMI). It can successfully realize interference suppression even with the use of multi-level envelope signals such as 16QAM with satisfactorily outage probability performance below the fifth percentile.

In AMS/OFDM systems, a base station controls the modulation level of each subcarrier with feedback information (FBI), and then, adaptive modulated packets are transmitted from the base station to the mobile station. In this case, the mobile station requires modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, so the throughput is degraded. To overcome this problem and increase the total throughput, in this paper, we propose superimposed frequency symbol based adaptive OFDM with frequency spreading and equalization. In the proposed system, each S/P transformed signal is spread by orthogonal spreading codes and combined. This means that each subcarrier holds several superimposed S/P transformed signals with the same power rate. In this case, the frequency-selective faded subcarriers obtain the same power rate for each S/P transformed signal. Therefore, the detected signals also obtain the same SINR, and as a result, we can assign the same modulation level for each frequency symbol spreading block. Hence, the proposed system requires only one piece of FBI and MLI for each frequency symbol spreading block, as compared with conventional adaptive OFDM.

Space-time block code (STBC) with complex orthogonal designs achieves full diversity with a simple maximum-likelihood (ML) decoding, however, do not achieve a full transmission rate for more than two antennas. To attain a higher transmission rate, STBC with quasi-orthogonal designs were proposed, whereas there are interference terms caused by relaxing the orthogonality. It has an impact on decoding complexity because a receiver needs to decode two symbols at a time. Moreover, QO-STBC does not achieve full diversity. In this paper, we propose a scheme which makes possible to decode symbols one by one, and two schemes which gain full transmission diversity by upsetting the balance of the transmit power and rotating constellation.

In mobile communication systems, the channel state information (CSI) is severely affected by the noise effect of the receiver. The adaptive subcarrier grouping (ASG) for sample matrix inversion (SMI) based minimum mean square error (MMSE) adaptive array has been previously proposed. Although it can reduce the additive noise effect by increasing samples to derive the array weight for co-channel interference suppression, it needs to know the signal-to-noise ratio (SNR) in advance to set the threshold for subcarrier grouping. This paper newly proposes adaptive zero padding (AZP) in the time domain to improve the weight accuracy of the SMI matrix. This method does not need to estimate the SNR in advance, and even if the threshold is always constant, it can adaptively identify the position of zero-padding to eliminate the noise interference of the received signal. Simulation results reveal that the proposed method can achieve superior bit error rate (BER) performance under various Rician K factors.

In OFDM systems, the pilot signal averaging channel estimation is generally used to identify the channel state information (CSI). In this case, large pilot symbols are required for obtaining an accurate CSI. As a result, the total transmission rate is degraded due to large number of pilot symbols transmission. To reduce this problem, in this paper, we propose time-frequency interferometry (TFI) for OFDM to achieve an accurate CSI.

Orthogonal space-time block code (OSTBC) can achieve full diversity with a simple MLD, but OSTBC only achieves 3/4 of the maximum rate if more than two transmit antennas are used. To solve this problem, a quasi-orthogonal STBC (QOSTBC) scheme has been proposed. Even though a QOSTBC scheme can achieve the full rate, there are interference terms resulting from neighboring signals during detection. The existing QOSTBC using the pairs of transmitted symbols can be detected with two parallel MLD. Therefore, MLD based QOSTBC has higher complexity than OSTBC. To reduce the detection complexity, in this paper, we propose the heterogeneous constellation based QOSTBC for improving the detection property of QRD-MLD with maintaining a simple decoding structure.

This letter expands the previously proposed High Time Resolution Carrier Interferometry (HTRCI) to estimate a larger amount of channel status information (CSI). HTRCI is based on a comb-type pilot symbol on OFDM and CSI for null subcarriers are interpolated by time-domain signal processing. In order to utilize such null pilot subcarriers for increasing estimable CSI, they should generally be separated in frequency-domain prior to estimation and interpolation processes. The main proposal is its separation scheme in conjunction with the HTRCI treatment of the temporal domain. Its effectiveness is verified by a pilot de-contamination on downlink two-cell MIMO transmission scenario. Binary error rate (BER) performance can be improved in comparison to conventional HTRCI and zero padding (ZP) which replaces the impulse response alias with zeros.

In AMS/OFDM systems, base station is in control of the modulation level of each subcarrier, and then, adaptive modulated packet is transmitted from the base station to the mobile station. In this case, the mobile station is required the modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, therefore, the throughput is degraded. In an OFDM, the channel response at a particular subcarrier frequency is not supposed to be totally different from its neighboring frequencies, and hence, they must have correlation which depends on the coherence bandwidth of the channel Bc. If we could assign the same modulation level for coherently faded subcarrier block, MLI is required only one time for each subcarrier block. Moreover, we can assign the data on the empty space of pilot signals for increasing the total transmission. In this paper, we propose an adaptive subcarrier block modulation with differentially modulated pilot symbol assistance for downlink OFDM using uplink delay spread.

In MIMO systems, the deployment of a multiple antenna technique can enhance the system performance. However, since the cost of RF transmitters is much higher than that of antennas, there is growing interest in techniques that use a larger number of antennas than the number of RF transmitters. These methods rely on selecting the optimal transmitter antennas and connecting them to the respective. In this case, feedback information (FBI) is required to select the optimal transmitter antenna elements. Since FBI is control overhead, the rate of the feedback is limited. This motivates the study of limited feedback techniques where only partial or quantized information from the receiver is conveyed back to the transmitter. However, in MIMO/OFDM systems, it is difficult to develop an effective FBI quantization method for choosing the space-time, space-frequency, or space-time-frequency processing due to the numerous subchannels. Moreover, MIMO/OFDM systems require antenna separation of 5 10 wavelengths to keep the correlation coefficient below 0.7 to achieve a diversity gain. In this case, the base station requires a large space to set up multiple antennas. To reduce these problems, in this paper, we propose the link correlation based transmit sector antenna selection for Alamouti coded OFDM without FBI.

MIMO (multiple-input multiple-output) transmission is a promising technology to improve the frequency usage efficiency in mobile radio communications. In this letter, MIMO transmission with eigenbeam space division multiplexing (E-SDM) is focused on and the site-dependent beamforming characteristics is examined to know the transmission characteristics. Site-dependent radiowave direction of arrival and impulse responses are obtained using ray tracing. Result shows that effect of radiowaves with longer delays is enhanced due to E-SDM beamforming, and rather more capability of treating a longer excess delay is necessary for time-domain multipath compensation.

This paper improves our previously proposed semi-blind uplink interference suppression scheme for multicell multiuser massive MIMO systems by incorporating the beamspace approach. The constant modulus algorithm (CMA), a known blind adaptive array scheme, can fully exploit the degree of freedom (DoF) offered by massive antenna arrays to suppress inter-user interference (IUI) and inter-cell interference (ICI). Unfortunately, CMA wastes a lot of the benefit of DoF for null-steering even when the number of incoming signal is fewer than that of receiving antenna elements. Our new proposal introduces the beamspace method which degenerates the number of array input for CMA from element-space to beamspace. It can control DoF expended for subsequent interference suppression by CMA. Optimizing the array beamforming gain and null-steering ability, can further improve the output signal-to-interference and noise power ratio (SINR). Computer simulation confirmed that our new proposal reduced the required number of data symbols by 34.6%. In addition, the 5th percentile SINR was also improved by 14.3dB.

Amplify-and-forward (AF) relay multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems can achieve high data rate and high quality communications. On the other hand, it has to estimate all channels between the source-relay and relay-destination nodes in the destination node. In MIMO/OFDM systems, high time resolution carrier interferometry (HTRCI) has been proposed to achieve an accurate channel estimation (CE) with a small number of pilot signals. However, since it has many interferences, an accurate CE is not obtained and the system performance is degraded in AF relay MIMO/OFDM systems. Therefore, in this paper, we propose the complex HTRCI (C-HTRCI) pilot signal and the enhanced minimum mean square error (E-MMSE) equalization to achieve an accurate CE and to improve the system performance for AF relay MIMO/OFDM systems.

In an AMS/OFDM system, base station is in control of the modulation level of each subcarriers, and then, adaptive modulated packet is transmitted from the base station to the mobile station. In this case, the mobile station is required the modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, therefore, the throughput is degraded. Moreover, it is necessary to have some transmission delay times and the processing time to make an adaptive modulation command (AMC) using feedback information (FBI). With the FBI delay and processing time, the system performance might be degraded. To reduce these problems, in this paper, we propose a differential modulated pilot symbol assisted adaptive OFDM for reducing the MLI with predicted FBI.

To achieve more high speed and high quality systems of wireless communications, orthogonal frequency division multiple access (OFDMA) has been proposed. Moreover, OFDMA considering the multiuser diversity (MUDiv) has been also proposed to achieve more high system performance. On the other hand, the conventional MUDiv/OFDMA requires large complexity to select the subcarrier of each user. To solve this problem, we have proposed a MUDiv/OFDMA based on the low granularity block (LGB). However, it degrades the system performance in the environment which contains many deep faded subcarrier channels. Therefore, in this paper, we propose a cooperative LGB-MUDiv/OFDMA to mitigate the influence due to the deep faded subcarrier channel.

In this letter, we propose an iterative decoding with LDPC based unitary matrix modulated OFDM with splitting the diagonal components over the coherence bandwidth. The proposed system can obtain a frequency diversity gain by splitting the diagonal components of unitary matrix modulated symbols, and also obtain large coding gain by using LDPC code.

MIMO-OFDM is considered a key technology in emerging high-data rate systems. With MIMO techniques, the transmission quality deteriorates due to inter-antenna interference (IAI). Several signal detection schemes have been proposed to mitigate this problem. However, it is impractical to use the conventional methods without reducing theirs computational complexity. Previously, we have proposed a parallel detection algorithm using multiple QR decompositions with permuted channel matrix (MQRD-PCM) for MIMO-OFDM to reduce the system complexity. This method achieves a good BER performance with low system complexity. However, since MQRD-PCM is a kind of parallel detection method, the wrong detection probability is increased due to the bad channel SINR of the transmitted signal. As a result, the average BER performance is influenced by the wrong detection probability of the bad channel SINR. To overcome this problem, in this paper, we propose the channel ranking based joint symbols detection for MQRD-PCM/MIMO-OFDM.

The combination of OFDM and multiple antennas in either the transmitter or receiver is attractive to increase a diversity gain. However, multiple antennas system requires an antenna separation of 5-10 λ to keep the correlation coefficient below 0.7 for the space diversity, so this may be difficult to implement in a mobile station with high mobility. Recently, the polarization transmit diversity is considered in a mobile station. However, polarization transmit diversity requires twice transmit powers to compare with the conventional transmit diversity, since only vertically polar antenna cannot receive the horizontal signal components. In this paper, we express the cross correlation of each polarization antenna and the cross polarization discrimination (XPD) of multiple polarization antennas with simple model, and we propose an wideband OFDM using Alamouti coded heterogeneous polarization antennas for reducing the previous problem. From the simulated results, the proposed system shows better BER performance than that of the conventional STBC/OFDM.

Multiple-input multiple-output (MIMO) eigenbeam space division multiplexing that uses adaptive modulations for substreams is a promising technology for improving transmission capacity. A fundamental drawback of this approach is that the modulation levels determined from the carrier-to-noise ratio at each substream are sometimes overly optimistic so the use of these modulation levels results in transmission errors and diminished transmission performance. A novel method of determining substream modulation levels is proposed that alleviates this degradation. In the proposed method, the expected bit error rates for possible modulations of each substream are calculated from delay profiles. Simulation results indicate that transmission capacity is improved by 30% using the new method compared with the conventional method.