We consider the problem of time domain (TD) channel estimation for wideband millimeter wave systems with a hybrid architecture. TD channel estimators can exploit both angular and delay domain sparsity, and can perform better than frequency domain (FD) estimators exploiting only angular domain sparsity; however, the former usually require a heavier computational load than the latter. To overcome this difficulty, we propose an efficient, two-step TD channel estimator: in the first step, an effective channel incorporated with beamforming, array responses, and pulse shaping is estimated by the least squares (LS) method; in the second, the desired channel is estimated via an orthogonal matching pursuit (OMP) algorithm. It is analytically shown that the proposed two-step method can yield the same outputs as the original single-step TD estimator when an identity pulse shaping matrix and a unitary pilot matrix are employed. The proposed algorithm is simpler to implement than the other estimators based on OMP, because its sensing matrix is block-diagonal. The simulation results demonstrate that complexity reduction of the proposed method is achieved without (or with minor) performance degradation.

This paper presents a multi-mode transmission technique that can increase the T-R distance of UCA OAM systems. The proposed technique uses dual polarized UCA antennas to increase the number of simultaneously transmitable modes at a specific T- R distance by exploiting the two basis of the polarized dipole antenna elements, and further extends the reaching distance of the multi-mode signals by using the inter-mode spreading between the OAM modes belonging to the same polarization domain. Simulation results show that the proposed scheme improves the T-R distances by at least 2 times as much as the conventional scheme in transmitting the same number of OAM modes with the same symbol error rate, and also that the proposed technique can transmit twice as many modes as the conventional method with the SNR loss of less than 3 dB at the same T-R distance.

We propose the use of uniform circular array of subarrays (UCA-SAs) for millimeter wave (MMW) communication systems over line of sight (LoS) channels and design beamforming coefficients for each subarray. Specifically, for a UCA-SA consisting of Nssubarrays, which are M × M square arrays, the beamforming coefficients are designed so that the Ns× Nseffective channel becomes circulant. To this end, we first show that the UCA-SA can be decomposed into M2concentric UCAs having Nsantenna elements. Then, based on this result, we show that the set of beamforming coefficients making the effective channel circulant consist of identical values. The proposed UCA-SA system employs this type of beamformers, called the constant beamformers. The effective channel of the proposed UCA-SA system can be diagonalized by the discrete Fourier transform (DFT) precoder and the inverse DFT (IDFT) combiner, without the knowledge of channel state information (CSI) at the transmitter. This indicates that constant beamformers are optimal beamformers maximizing the Ns× Nseffective channel's mutual information. Simulation results show that the proposed UCASA can outperform the conventional UCA with NsM2antenna elements which are equally spaced on a ring.

We propose an efficient open-loop channel estimator for a millimeter-wave (mm-wave) hybrid multiple-input multiple-output (MIMO) system consisting of radio-frequency (RF) beamformers with large antenna arrays followed by a baseband MIMO processor. A sparse signal recovery problem exploiting the sparse nature of mm-wave channels is formulated for channel estimation based on the parametric channel model with quantized angles of departures/arrivals (AoDs/AoAs), called the angle grids. The problem is solved by the orthogonal matching pursuit (OMP) algorithm employing a redundant dictionary consisting of array response vectors with finely quantized angle grids. We suggest the use of non-uniformly quantized angle grids and show that such grids reduce the coherence of the redundant dictionary. The lower and upper bounds of the sum-of-squared errors of the proposed OMP-based estimator are derived analytically: the lower bound is derived by considering the oracle estimator that assumes the knowledge of AoDs/AoAs, and the upper bound is derived based on the results of the OMP performance guarantees. The design of training vectors (or sensing matrix) is particularly important in hybrid MIMO systems, because the RF beamformer prevents the use of independent and identically distributed random training vectors, which are popular in compressed sensing. We design training vectors so that the total coherence of the equivalent sensing matrix is minimized for a given RF beamforming matrix, which is assumed to be unitary. It is observed that the estimation accuracy can be improved significantly by randomly permuting the columns of the RF beamforming matrix. The simulation results demonstrate the advantage of the proposed OMP with a redundant dictionary over the existing methods such as the least squares method and the OMP based on the virtual channel model.

Hybrid multiple input multiple output (MIMO) systems consist of an analog beamformer with large antenna arrays followed by a digital MIMO processor. Channel estimation for hybrid MIMO systems in millimeter wave (mm-wave) communications is challenging because of the large antenna array and the low signal-to-noise ratio (SNR) before beamforming. In this paper, we propose an open-loop channel estimator for mm-wave hybrid MIMO systems exploiting the sparse nature of mm-wave channels. A sparse signal recovery problem is formulated for channel estimation and solved by the orthogonal matching pursuit (OMP) based methods. A modification of the OMP algorithm, called the multi-grid (MG) OMP, is proposed. It is shown that the MG-OMP can significantly reduce the computational load of the OMP method. A process for designing the training beams is also developed. Specifically, given the analog training beams the baseband processor for beam training is designed. Simulation results demonstrate the advantage of the OMP based methods over the conventional least squares (LS) method and the efficiency of the MG-OMP over the original OMP.

This study presents an orthogonal design of space‐time block code (STBC) that is single symbol decodable and enables full diversity and full rate transmission in a quasi‐static channel environment. In the proposed scheme, full diversity is achieved by use of constellation rotation and co‐ordination interpolation as is the case for the co‐ordinate interleaved orthogonal design, whereas the transmission matrix of the proposed code consists of orthogonal columns with all non‐zero elements, alleviating the concern about high peak‐to‐average power ratio. In particular, the proposed code offers lower complexity of channel estimation, and exhibits lower symbol error rate than the conventional quasi‐orthogonal STBC when an maximum‐likelihood decision‐directed channel estimator is assumed. The advantages of the proposed code over the conventional codes are demonstrated through computer simulations. In addition, the selection of the angle of rotation for the proposed design is also presented, regarding the impact of channel estimation error.

In this paper, we deal with user scheduling algorithms for a codebook based precoding of a multiuser multiple-input multiple-output (MIMO) downlink system considered in IEEE 802.16m and 3rd Generation Partnership Project Long-Term Evolution (3GPP LTE) standardization. We especially focus on per-user unitary rate control (PU2RC) and semi-orthogonal user selection schemes. Also, we examine the disadvantage of the codebook based precoding with these schemes - a loss of sum-capacity when the codebook size is larger than the number of total users who request the communication service to a base station. Finally, we propose two user scheduling algorithms to improve the sum-capacity when the number of total users is smaller than the codebook size. First, we introduce feedback information retransmission algorithm in which each user sends a new codeword again depending on a request of the base station. Second, we address the adaptive feedback algorithm where users transmit one or two codeword indices called preferred matrix index (PMI) depending on their channel condition. The improvement of the sum-capacity is verified through the simulation.

In this paper, we propose a new limited channel feedback algorithm for Orthogonal Frequency Division Multiple Access (OFDMA) systems. The proposed method reports channel information of relative maximum and minimum points of the channel gain from users to base station (BS). Based on the feedback information, BS reconstructs the channel for all the subcarriers using linear interpolation. Compared with the full CSI reporting, the proposed scheme requires only 1/60 feedback information at the expense of 1% throughput loss. Simulation results show that the proposed scheme provides an attractive tradeoff between uplink feedback amount and downlink system performance compared to the conventional limited feedback techniques.

This paper presents the achievable capacity expressions with analytical closed forms of spatial multiplexing transmission (SMT) and single antenna transmission (SAT) for multicell downlink distributed antenna systems (DAS). Considering that SMT and SAT yield high transmission gains of the center and the edge of the cell, respectively, we also propose switching algorithms depending on a user location. The transmission scheme decides a favorable transmission technique between SMT and SAT through instantaneous or ergodic capacity comparison. Simulation results show that the analytical closed forms well match to the empirical expressions. Furthermore, the proposed transmission schemes are superior to SMT and SAT.

Cooperative multiple input multiple output (MIMO) system is capable in mitigating co-channel interference (CCI) that limits the downlink capacity of cellular network. Among the joint transmission designs for base station (BS) cooperation, the linear joint transmissions with per-base power constraint is preferred due to its computational simplicity and reality. However, since the transmissions with per-base power constraint sometimes make BSs cannot fully utilize their usable power, it leads considerable capacity loss. In this paper, we propose a BS selection technique for MMSE joint transmitter in downlink cooperative MIMO system to relive the inefficient usage of power under per-base power constraint. To find the BS group which yields the highest capacity, we compare the system capacity of all the possible BS groups. Simulation results show that the BS selection significantly improves the system capacity compared to the conventional linear joint transmissions by overcoming the aforementioned power allocation problem.

A cell planning and resource allocation scheme for improving channel capacity and for maintaining QoS (quality of service) over a downlink OFDMA (Orthogonal Frequency Division Multiple Access) system is proposed. The frequency overlay is applied to improve link capacity and the sectorization is reduce the influence of carrier collision. This consideration enhances link capacity of system with a proper outage probability.

This paper investigates the problem of MAI (multiple access interference) occurrence when timing misalignment occurs in the downlink channel of OFDM-based networks. The adaptive guard band allocation scheme is mainly designed to achieve timing synchronization while reducing guard carrier redundancy and maintaining high channel throughput in asynchronous OFDM-based networks. Thus, the proposed scheme reduces the effect of MAI for users receiving multiple transmission signals from surrounding BSs (Base Stations). In a simulation, it is noteworthy that this adaptive carrier allocation is very effective in reducing MAI for asynchronous multi-cell network.

In this paper, we evaluated effects of ICI (inter-cell interference) on an OFDM (orthogonal frequency division multiplexing) based wireless communication system's reverse link capacity and suggested macroscopic diversity oriented resource allocation scheme to reduce interference and enhance reverse link capacity. For this, the exact amount of ICIs in the system is measured and represented as a closed form.