The intra-and/or inter-cell interference have been the major obstacle that constrain the performance of cellular systems. In this regard, there have been approaches to mitigate the inherent interference in the cellular systems, which support multiple user equipments (UEs) distributed in multiple adjacent cells. However, the previous approaches to cancel the interference at the receiver were mostly available at the cost of additional system information of the interference from the neighboring cells. In this paper, we propose an advanced receiver algorithm that efficiently suppresses interferences by utilizing statistical characteristics of interference signals, hence no need for additional information to detect the interference. It is shown that the proposed receiver suppresses inter-cell interference well and outperforms conventional receivers when applied to typical LTE systems.

With the increase of the cell loadings, the interference among the adjacent cells becomes severer acting as the limiting factor for the cell-edge user performance. In this regard, there have been many standard activities in 3GPP (3rd Generation Partnership Project) to mitigate the ICI (Inter-Cell Interference). The eNB (Enhanced Node-B)-centric ICI mitigation having its own limit, the recent approaches incorporate the interference cancellation of a dominant interference from the adjacent cell at the receiver. However, for the LTE (Long Term Evolution) uplink, the SC-FDMA (Single-Carrier Frequency Division Multiple Access) structure prohibited the eNB to use a similar interference cancellation scheme as for the OFDMA (Orthogonal Frequency Division Multiple Access) UE (User Equipment) receiver. In this paper, we propose a blind interference cancellation receiver for LTE eNB, which performs ICI cancellation blindly without prior information of the adjacent cell interferences. It is shown that the proposed blind interference cancellation receiver for SC-FDMA provides significant performance gain against the conventional receiver.

Massive multi-input multi-output (MIMO) is shown to significantly increase spectral efficiency by exploiting a large number of antennas to support high-order multiuser MIMO. In 3GPP Release-13, a full-dimension MIMO (FD-MIMO) technology was introduced to address practical aspects for massive MIMO in cellular systems; extensive simulations show 2–4 times capacity gain compared with current LTE systems. FD-MIMO has been identified as one of the key 5G technologies and is being continuously improved in 3GPP new radio standards. However, several practical challenges such as interference mitigation among MIMO streams for a large number of users with limited channel feedback, hardware limitations, such as calibration errors that limit the precoding capabilities need to be addressed carefully. A proof of concept (PoC) base-station and user equipment (UE) prototype has been designed to validate the potential of FD-MIMO technology and demonstrate commercial implementation feasibility. In this paper, we present theory and architecture behind an FD-MIMO prototype and share the field test results with LTE-based UEs in a multi-user MIMO setup. We also analyze the impact of transmitter and receiver calibration errors on the performance of the FD-MIMO system.

Multiuser multiple-input multiple-output (MU-MIMO) transmission techniques have been popularly used to improve the spectral efficiency and user experience. However, due to the coarse knowledge of channel state information at the transmitter, the quality of transmit precoding to control multiuser interference is degraded, and hence, co-scheduled user equipment may suffer from large residual multiuser interference. In this paper, we propose a new channel estimation technique employing reliable soft symbols to improve the channel estimation and subsequent detection quality of MU-MIMO systems. To this end, we pick reliable data tones from both desired and interfering users and then use them as pilots to re-estimate the channel. In order to jointly estimate the channel and data symbols, we employ the expectation maximization algorithm, where the channel estimation and data decoding are performed iteratively. From numerical experiments in realistic MU-MIMO scenarios, we show that the proposed method achieves substantial performance gain in channel estimation and detection quality over conventional channel estimation approaches.

Uniform planar antenna (UPA) structures are expected to become popular for massive multiple-input multiple-output (MIMO) systems because they enable deployment of a large number of antennas in limited space. In frequency division duplexing (FDD) systems, quantized channel state information (CSI) should be fed back from users to base stations to make adaptive transmission possible. However, it is difficult to accurately quantize massive MIMO channels due to their large dimensions. In this paper, we propose practical CSI quantizers for full-dimension (FD) MIMO systems employing UPAs. We focus on quantizing and combining dominant beams in channels by keeping realistic channel properties and antenna structures into account. To scan a channel space, we also develop a multi-round beam search approach. Numerical results verify that the proposed quantizers show better quantization performance than the previous channel quantization techniques.

Multiuser multiple-input multiple-output (MU-MIMO) transmission technique based on orthogonal frequency division multiplexing (OFDM) system has been received great deal of attention in recent years due to its potential higher spectral efficiency. However, because of the accuracy of channel state information, co-scheduled mobile users may suffer large residual multiuser interference in MU-MIMO system. In this paper, we propose a new channel estimation technique using expectation and maximization (EM) algorithm with reliable soft symbols for the MU-MIMO systems in LTE- Advanced. In the proposed scheme, we choose reliable data tones from both desired and interfering signals and improves the channel estimation quality using iterative process between channel estimation and data detection in frequency domain. We show that the proposed method achieves substantial performance gain over conventional channel estimation approaches.

A concept of full-dimension MIMO (FD-MIMO) adopting 2-dimensional (2D) antenna array has been proposed with intent to cope with practical challenges of massive MIMO systems. In this paper, we investigate an FD-MIMO operation that can be transparently applied with current 3GPP LTE specifications, and propose a novel method that facilitates an accurate estimation of channel quality indicator (CQI) at base station (BS). We first introduce a feedback scheme in which horizontal and vertical dimension channel information are separately reported. To further improve accuracy of modulation and coding scheme (MCS) selection to maximize system throughput, we propose an enhanced feedback scheme that is helpful to more accurate CQI estimation at the BS. In the evaluation results, we show performance improvement of the proposed scheme over the conventional MIMO scheme.

Owing to the high cost of radio frequency (RF) chains and power consumption of broadband signals, it is suitable to include both analogue and digital processing for millimetre wave (mmWave) beamforming. In this study, hybrid beamforming (HBF) schemes are considered for multi‐user downlink mmWave systems. RF beamforming schemes are proposed for HBF transmission with perfect channel state information at transmitter (CSIT) and limited feedback information in the multi‐user mmWave channel. Simulation results show that proposed HBF schemes enhance sum rate performance than conventional HBF schemes in both cases of perfect CSIT and limited feedback.

Recently, as asynchronous heterogeneous network scenario becoming one of the key features for next generation wireless communications, superior spectrum confinement as well as higher spectral efficiency compared to cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM) has been taken into consideration for future radio access technologies. In this paper, we propose a new quadrature amplitude modulation based filter-bank multi-carrier (QAM-FBMC) system that provides with an inherent spectral efficiency gain against CP-OFDM, which comes from reduction of the redundancies such as cyclic prefix (CP) and guard-band, while keeping the symbol rate same as in CP-less OFDM. We propose a new transceiver structure consisting of at least two different filter-bank bases at both transmitter and receiver sides. Practical algorithms including channel estimation and equalization to mitigate multi-path fading channel without CP are proposed. Various evaluation results show that the proposed system performs comparable to the CP-OFDM system even without CP and guard-band reduction is also available from the well-confined spectrum.

Massive multiple-input multiple-output (MIMO) systems hold the potential to be an enabling technology for 5G cellular. Uniform planar array (UPA) antenna structures are a focus of much commercial discussion because of their ability to enable a large number of antennas in a relatively small area. With UPA antenna structures, the base station can control the beam direction in both the horizontal and vertical domains simultaneously. However, channel conditions may dictate that one dimension requires higher channel state information (CSI) accuracy than the other. We propose the use of an additional one bit of feedback information sent from the user to the base station to indicate the preferred domain on top of the feedback overhead of CSI quantization in frequency division duplexing (FDD) massive MIMO systems. Combined with variable-rate CSI quantization schemes, the numerical studies show that the additional one bit of feedback can increase the quality of CSI significantly for UPA antenna structures.

To support downlink full-dimension (FD-MIMO) in FDD scenarios, an accurate yet reasonably efficient channel-state-information (CSI) feedback scheme is required to harness the potential beamforming gain offered by the use of a large number of antenna elements in a 2D active antenna array. In this paper, a reduced-spaced channel quantization and feedback scheme which exploits slow variation of the angle-of-departure (AoD) distribution is proposed. By perceiving the instantaneous spatial channel to be a linear combination of slowly varying subset of matrices, it is demonstrated that the proposed scheme offers up to 62.5% dimensionality reduction with negligible throughput loss in 16-antenna FD-MIMO system. When used together with vector quantization, the proposed scheme offers substantial gain over that supported by 3GPP LTE.

As a promising candidate to replace cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM), a new quadrature amplitude modulation filter-bank multi-carrier (QAM-FBMC) has been proposed for enabling spectral efficiency increase and spectrum confinement while keeping near orthogonality. QAM transmission can combat intrinsic interference of conventional offset-QAM (OQAM)-FBMC, which is main bottleneck in channel estimator and multiple antenna transceiver design. Compared to existing single filter based QAM-FBMC, the new QAM-FBMC has more optimization freedom in multiple prototype filters to increase self signal-to-interference ratio (SIR). In this paper, the possibility of per-tone multiple input multiple output (MIMO) reception in the new QAM-FBMC system is exploited. Our MIMO receiver is constructed with per-tone MIMO equalization in oversampled signal domain and its soft-demapper in symbol domain. Not only the separate log likelihood ratio (LLR) calculation per stream but also joint LLR calculation (i.e., soft maximum likelihood (ML) detection) is implemented with pre-processing. Simulation results show that the per-tone MIMO reception in QAM-FBMC is comparable to that of CP-OFDM.

Recent works on massive multiple-input multiple-output (MIMO) have shown that a potential breakthrough in capacity gains can be achieved by deploying a very large number of antennas at the base station. In order to achieve the performance that massive MIMO systems promise, accurate transmit-side channel state information (CSI) should be available at the base station. While transmit-side CSI can be obtained by employing channel reciprocity in time division duplexing (TDD) systems, explicit feedback of CSI from the user terminal to the base station is needed for frequency division duplexing (FDD) systems. In this paper, we propose an antenna grouping based feedback reduction technique for FDD-based massive MIMO systems. The proposed algorithm, dubbed antenna group beamforming (AGB), maps multiple correlated antenna elements to a single representative value using predesigned patterns. The proposed method modifies the feedback packet by introducing the concept of a header to select a suitable group pattern and a payload to quantize the reduced dimension channel vector. Simulation results show that the proposed method achieves significant feedback overhead reduction over conventional approach performing the vector quantization of whole channel vector under the same target sum rate requirement.

Recent works on massive multiple-input multiple-output (MIMO) have shown that a potential breakthrough in capacity gains can be achieved by deploying a very large number of antennas at the basestation. Although transmit-side channel state information (CSI) can be obtained by employing channel reciprocity in time division duplexing (TDD) systems, explicit feedback of CSI from the user to the basestation is required for frequency division duplexing (FDD) systems. In this paper, we propose an antenna grouping based feedback reduction technique for FDD-based massive MIMO systems. The proposed algorithm, dubbed antenna group beamforming (AGB), groups antenna elements using pre-designed patterns. The proposed method introduces the concept of using a header of overall feedback resources to select a suitable group pattern and the payload to quantize the effective channel vector. Simulation results show that the proposed method achieves significant feedback overhead reduction over conventional approach.

Along with the attention drawn to the millimeter-wave frequency bands for the drastically increased data rate support for the next generation (5G) mobile communications, the hybrid beamforming architectures combining analog beamforming and digital precoding have been investigated as potential candidates to provide good compromise between the complexity and the performance. Among the different hybrid beamforming architectures, the architecture with so-called shared array antenna is analyzed with respect to the peak-to-mean envelope power ratio over antenna elements and the total average transmission power constraint upon multi-beam transmissions. It is shown that the non-trivial normalization coefficients should be applied to the hybrid beamforming architecture under consideration to achieve the average transmission power within the total power constraint. Taking into account the total power normalization for the hybrid beamforming architecture with shared array antenna, various simulation results are provided to show the performance of the multi-beam transmission diversity against the single-beam analog beamforming.

The recent strong demands for higher data rate support to cope with the explosive mobile data crunch has initiated research on the next generation (5G) wireless mobile communication technologies that could provide with drastic capacity increase. One of the promising candidates is to use wide spectrum in the mmWave bands, where a breakthrough to overcome the unfavorable channel properties needs to be preceded. In this paper, we propose a novel hybrid beamforming scheme that jointly combines RF beamforming and baseband precoding as a key enabling technique to make efficient use of mmWave channel. The link level simulation results show that the proposed scheme can almost achieve the optimal performance with noticeably reduced implementation complexity. Furthermore, system level simulation results are provided pointing out the possibility of tens of Giga-bits/sec data rate support with the proposed scheme.