In this paper, a beam-tracking technique is proposed for millimeter-wave (mmWave) cellular systems with an intelligent reflective surface (IRS). A preamble design technique for IRS-assisted cellular systems is proposed using the complex conjugate property of the Zadoff-Chu sequence and reflecting nature of IRS. Moreover, a beam-tracking technique for mmWave cellular systems with a uniform planar array (UPA) is also proposed to track the variation in the angle of departure using sub-panel structures of the UPA.

In this paper, a random-access preamble (RAP) for the user equipment (UE) experiencing high Doppler in millimeter-wave (mmWave) cellular systems is proposed. After demonstrating the inefficiency of the conventional long-term evolution (LTE)-based RAP for fast-moving UEs (FUEs) in mmWave systems, linear-frequency-modulation-based RAP (LFM RAP) is proposed for the detection of FUEs. The correlation of LTE-based RAP, and ambiguity and extended-ambiguity functions of LFM RAP are analyzed to examine their properties affected by the high Doppler. The performance of RAPs is evaluated through detection and false alarm probabilities in a high Doppler and multiuser environment. The results show that the LFM RAP is appropriate for FUEs in mmWave cellular systems because of its high detection probability in a high-Doppler environment.

Unmanned aerial vehicle (UAV) communications over cellular networks is currently gaining increased interest because it can enable real-time control of UAVs and wide-area coverage. We discuss various challenges for future UAV cellular networks, where cellular networks with down-tilted antennas optimized for terrestrial users can extend terrestrial cellular services to aerial users. To overcome challenges such as large channel variation and UAV power consumption in UAV cellular networks, we propose a technique of preamble design for these networks using scalable sequences. Specifically, three types of scalable sequences are proposed for efficient use of battery power in UAVs, namely, maximum-length scalable sequence, Zadoff-Chu scalable sequence, and linear frequency modulated (LFM) scalable sequence. The properties of these scalable sequences are analyzed and compared through simulations in different channel environments.

In this paper, a new cell selection technique for millimeter-wave (mmWave) cellular systems considering a hybrid beamforming structure in data transmission stage is proposed. Using the proposed technique with cell and beam synchronization signals (CBSS) transmitting simultaneously from multiple beams, the processing time required for channel estimation and cell selection can be reduced significantly. It is shown by simulation that additional gains in channel capacity can be achieved as compared to conventional technique.

In millimeter-wave (mm-wave) cellular systems, beamforming antennas are necessary at both the base station (BS) and mobile station (MS) to compensate for high attenuation in mm-wave frequency bands and to extend the transmission range. In space-division multiple access (SDMA) systems, the challenge is the inter-beam interference (IBI) caused by adjacent beams that are formed by the BS in the same cell and BSs in neighboring cells. The beams that are formed toward MSs in each cell may generate significant interference to MSs in neighboring cells, especially for MSs at the cell boundary. In this paper, we propose four different digital precoding techniques (Type-1, Type-2, Type-3 and Type-4) to reduce IBI in mm-wave cellular systems with a hybrid beamformer.

The use of beamforming techniques at the ground station is known to be effective in obtaining coverage extension or SNR gain for communication with unmanned aerial vehicles (UAVs). When a UAV moves, periodic beam tracking is necessary to maintain beam gain. To track beams for multiple UAVs, the ground station needs to receive different preambles from multiple UAVs. In this paper, a Hadamard sequence is considered for designing preambles for multiple UAVs because of its ideal cross-correlation property. A preamble appropriate for a GMSK-based communication system with multiple UAVs is proposed after analyzing the property of received signals in the system with the Hadamard sequence as the input.

A small-cell network (SCN) constructed by splitting a macro-cell into numerous small cells using an active antenna array system is studied. A synchronization signal appropriate for the SCN, virtually generated by a base station (BS) with 3D beamforming, is proposed for efficient handover in SCNs. The virtual cell synchronization signal (VCSS) carries a macro-cell ID (MCID) and virtual- cell ID (VCID) in a hierarchical manner, allowing us to distinguish between an intra-cell handover and inter-cell handover in SCNs. Using the signal metrics obtained by the VCSS, an efficient handover measurement technique is proposed which can significantly reduce the processing time and overhead by distinguishing between the intra- cell/inter-cell handovers.

Abstract A small‐cell network (SCN) constructed by splitting a macro‐cell into numerous small cells using an active antenna array system is studied. A synchronization signal appropriate for the SCN, virtually generated by an eNodeB with 3D beamforming, is proposed for efficient handover in SCNs. The virtual cell synchronization signal (VCSS) carries a macro‐cell ID (MCID) and virtual‐cell ID (VCID) in a hierarchical manner, allowing us to distinguish between an intra‐cell handover (virtual cell handover within a cell without changing the serving eNodeB) and inter‐cell handover (virtual cell handovers across cells while changing the serving eNodeB) in SCNs. Using the signal metrics obtained by the VCSS, an efficient handover measurement technique is proposed which can significantly reduce the processing time and overhead by distinguishing between the intra‐cell/inter‐cell handovers. The performance of the proposed technique is evaluated by simulating two different deployment scenarios of LTE‐based SCN with 3D beamforming. Copyright © 2017 John Wiley & Sons, Ltd.

In this study, the authors propose a signal detection technique of successive interference cancellation to reduce the effect of inter‐symbol interference (ISI), which is caused by the time difference of arrivals among distributed nodes in an asynchronous wireless mesh network based on filtered multi‐tone modulation. The proposed signal detection technique uses partial matrices of a transmission gain matrix to mitigate the ISI effect from the adjacent symbols. Under the assumption of perfect symbol time offset and channel estimation at each node, it is shown by simulation that the proposed technique can improve the bit error rate performance with lower complexity compared with the conventional technique.

In this paper, a beamforming and cell searching method is proposed for a mobile station (MS) with antenna array to compensate the high attenuation in a millimeter wave (mm-wave) OFDM-based cellular systems. A cell searching and direction-of-arrival (DoA) estimation method is proposed for a multipath environment, even with the existence of the symbol timing offsets (STOs) and carrier frequency offsets (CFOs). It is shown by simulation that the proposed method can successively estimate both cell IDs and DoAs from adjacent base stations (BSs) in a multipath environment and is robust to STO and CFO.

In this paper, a mobile relay station (MRS) for vehicles with a beamforming antenna is considered to increase the reliability of a transmission link, especially for the MRS at cell boundary. Cell searching and direction-of-arrival (DoA) estimation method for an MRS with a uniform linear array (ULA) is proposed for a multipath environment in OFDM-based cellular systems. Performances of the proposed method are evaluated by computer simulation with the standard profile of IEEE802.16e.

In this paper, the effect of time difference of arrival (TDoA) is investigated for distributed nodes in OFDMA-based wireless mesh networks (WMNs). In order to minimize the interferences caused by TDoA in WMNs, the optimal starting point of the FFT window at the receiver side of a node is derived by maximizing the effective SINR for each subcarrier. Also, a signal detection technique, called two dimensional ordered successive interference cancellation (TD-OSIC), is proposed for WMNs with TDoAs. It was shown via simulation that the proposed technique can achieve effective SINR and BER performances similar to the ideal case (no TDoA), even in WMNs with large TDoAs.

In this paper, a new handover technique for OFDM-based mobile mesh networks (MMNs) is proposed to reduce the handover overhead by distinguishing an inter-cluster handover event from an intra-cluster handover event using a hierarchical design concept of a preamble. The proposed handover procedure can simplify scanning procedure and skip/simplify network entry procedure, resulting in a significant reduction of handover overhead.

In this letter, a 2-D Direction-of-Arrival (DoA) estimation scheme with cell searching is proposed for a mobile relay station (MRS) with uniform circular array (UCA) at a handover region. The proposed scheme of cell searching and DoA estimation for the MRS with UCA is robust even when there exist symbol timing offsets (STOs). Compared with the previous scheme, the proposed scheme can reduce computational complexity and processing time for cell searching and DoA estimation. Simulation results are included to verify the performance of the proposed scheme under a mobile WiMAX environment.

This paper proposes two new signal detection methods, which are based on the ordered successive interference cancellation (OSIC) algorithm, for spatially multiplexed multiple input multiple output (MIMO) systems. By using an enhanced version of the OSIC algorithm, one of the proposed methods significantly improves the performance of the original OSIC algorithm, while slightly increasing the hardware complexity. The other proposed method combines the enhanced SIC algorithm with multiple ordering technique of the channel matrix. This new method is especially useful in soft decoding MIMO systems since it can generate the log likelihood ratio (LLR) values without using any LLR estimation methods. The simulation and implementation results show that the proposed detectors provide very good solutions for both hard and soft decoding MIMO systems.

In a cellular system, efficient power saving techniques for a mobile station (MS) are necessary because of its inherently limited battery of MS. The paging indicator (PI) transmission scheme in CDMA cellular systems is known to be an effective power saving strategy. However, in OFDM-based cellular systems, the MS has to operate FFT for PI symbol detection, resulting in a significant power consumption. In this paper, a reduced power consumptive PI transmission technique using the preamble in OFDM-based cellular systems, especially for mobile WiMAX systems, is proposed for the MS under power saving mode. Simulations indicate a 30~50% power saving result from our proposed PI transmission technique, at the expense of a slight increase of paging response delay.

In this paper, a Direction-of-Arrival (DoA) estimation scheme with cell searching is proposed for a mobile relay station (MRS) with uniform circular array (UCA) at handover region. The proposed scheme of cell searching and DoA estimation for the MRS with UCA is robust even when there exist symbol timing offsets (STOs). Compared with the previous scheme, the proposed scheme can reduce computational complexity and processing time for cell searching and DoA estimation. Simulation results are included to verify the performance of the proposed scheme under Mobile WiMAX environment.

In this paper, the effect of the time difference of arrival (TDoA) on synchronous types or asynchronous types of OFDMA-based multi-hop relay (MR) systems is investigated. Effective SINRs for downlinks and uplinks are defined to measure the degree of interference due to TDoA in asymmetric MR systems. A path selection technique based on the effective SINR is then proposed to determine the optimal paths for the uplink and downlink independently. It is shown that asymmetric routing can improve the performance of OFDMA-based MR systems when the proposed technique is applied to the path selection of the uplink taking into account the effect of the TDoA.

In this paper, we propose a new digital duplexing scheme, called synchronous digital duplexing (SDD), which can increase data efficiency and flexibility of resource by transmitting uplink signal and downlink signal simultaneously in wireless communication. In order to transmit uplink and downlink signals simultaneously, the proposed SDD obtains mutual information among subscriber stations (SSs) with a mutual ranging symbol. This information is used for selection of transmission time, decision on cyclic suffix (CS) insertion, determination of CS length, and re-establishment of FFT starting point. It is shown that in terms of efficiency and flexibility the proposed SDD is better equipped as a duplexing scheme for indoor wireless environments than conventional time division duplexing (TDD) and frequency division duplexing (FDD).

In this paper, we propose an efficient estimation technique for time-varying multipath channels by exploiting two dimensional delay-time nature in pilot inserted orthogonal frequency division multiplexing (OFDM) systems. The proposed technique consists of multipath snapshot estimation at periodic channel observation instances over an OFDM block and the linear interpolation of two consecutive multipath snapshots. Two decision-directed data utilizing schemes, data removed and data added correlation schemes, are proposed to enhance the performance of the multipath snapshot estimation.

In this paper, two receive diversity combining techniques are proposed for cooperative relay systems based on single-carrier frequency division multiple access (SC-FDMA) when DFT spreading sizes for a mobile station (MS) and relay station (RS) are different. A simplifled-MRC (S-MRC) technique performs diversity combining in the time-domain by using the estimated channel weights and initial estimates obtained by SC-FDMA signal detector. An interference rejection-MRC (IRMRC) technique performs diversity combining in the frequencydomain by adjusting DFT spreading size at the receiver. It is shown by computer simulation that the proposed receive combining techniques achieve a significant diversity gain over the conventional ones.

In this paper, a mobile relay station (MRS) for vehicles with beamforming antennas is considered to increase the reliability of transmission link, especially for the MRS at cell boundary. Joint methods for cell searching and DoA estimation are proposed to form a beam in the direction of target BS while nulling interferences from adjacent BSs, especially for IP- based cellular systems employing break-before-make handover or make-before-break handover. The proposed cell searching and DoA estimation methods are evaluated by computer simulation under the environment of IEEE 802.16e (WiBro).

In this paper, a new handover procedure for OFDM-based multi-hop relay systems is proposed to reduce handover overhead by distinguishing an inter-cell handover event from an intra-cell handover event at the level of the physical layer using a preamble with a hierarchical design. A subcell ID concept that identifies RS in a cell is proposed in the hierarchical design that works in conjunction with the existing cell ID that identifies BS. The proposed handover procedure can simplify the scanning procedure and skip/simplify the network re-entry procedure, resulting in a significant reduction of handover overhead.

In spatially multiplexed MIMO systems that enable high-data-rate transmission over wireless communication channels, spatial demultiplexing at the receiver is a challenging task. Thus, various demultiplexing methods have been developed by many researchers. In this paper, a novel signal detection method that achieves near-ML performance in a computationally efficient manner is proposed. The proposed method is based on the well-known minimum mean squared error (MMSE) ordered successive interference cancellation (OSIC) method. However, unlike the conventional MMSE-OSIC, a number of candidates are selected at each layer, hence, it has been termed MMSE-OSIC with candidates (MMSE-OSIC2). In this study, it is demonstrated via a series of computer simulations that the MMSE-OSIC2achieves near-ML performance while requiring a complexity comparable to that of the conventional MMSE- OSIC. Also shown here is that the log likelihood ratio (LLR) values are successfully generated via the proposed method.

In this paper, an iterative decoding technique for cooperative space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) systems is proposed to mitigate the intercarrier interference (ICI) caused by multiple carrier frequency offsets (CFOs) in cooperative transmission. The proposed iterative decoding technique is shown to mitigate the noise enhancement effect on the STBC-OFDM signals caused by multiple CFOs, and be effective in reducing ICI especially when the system has a large FFT size and a large multiple CFOs.

In this paper, a decision-directed expectation maximization (DEM) algorithm is proposed to improve the performance of channel estimation in OFDM-based cellular systems. The DEM algorithm enables a mobile station (MS) with multiple antennas, located at the cell boundary, to increase the performance of channel estimation using transmit data, without decreasing spectral efficiency. Compared with the expectation maximization (EM) algorithm, the DEM algorithm can greatly reduce computational complexity while improving the performance of channel estimation in fast fading channels.

In this paper, we propose a novel signal detection method that achieves the maximum likelihood (ML) performance but requires much less computational complexity than the ML detection. When the well-known linear decoding method is used for space-time block coded (STBC) OFDM systems in fast-fading channels, co-channel interference (CCI) as well as inter-carrier interference (ICI) occurs. A maximum likelihood (ML) method can be employed to deal with the CCI; however, its computational complexity is very high. In this paper, we propose a signal detection method for orthogonal space-time coded OFDM systems that achieves the identical error performance as the ML method, but requires much less computational complexity.

In this paper, a minimum mean square error (MMSE) channel estimation technique for an OFDM system with a transparent mobile multi-hop relay (MMR) is proposed by taking into account the effect of propagation delays due to relaying. It is demonstrated by computer simulation that the proposed MMSE channel estimation technique for OFDM systems with cooperative MMR is superior to the conventional technique in terms of the mean square error (MSE) and bit error rate (BER).

In this paper, we address the QRM-MLD signal detection method for spatially multiplexed MIMO systems. Recently, the QRM-MLD signal detection method that can achieve a 1 Gbps transmission speed for the next generation mobile communications has been successfully implemented by the industry when the mobile is moving at a pedestrian speed. In this paper, we propose a novel signal detection method, the "low complexity QRM-MLD", which achieves an identical error performance to that of the conventional QRM-MLD while significantly reducing the computational complexity. We have rigorously compared these two detection methods in terms of computational complexity to confirm the reduction of the computational complexity in the proposed method. We have also performed computer simulations to demonstrate that the two detection methods have identical error performances.

In this paper, the effect of a propagation delay resulting from the use of an OFDM system with a transparent mobile multi-hop relay (MMR) is initially analyzed. Additionally, a channel estimation technique for the OFDM system with throughput enhancement (TE) MMR or cooperative MMR is proposed. Secondly, the effect of a Doppler shift caused by a mobile MS is analyzed and a compensation technique for the carrier frequency offset (CFO) in the OFDM system with TE MMR is proposed.

In this paper, a linear decoding method for differential space-time block code (DSTBC) is derived for the cases of two transmit antennas or four transmit antennas. Also, a simple table lookup method is proposed to reduce the computation in the encoding/decoding process of DSTBC for the case of non-constant modulus constellations. It is shown by simulation that the BER performance of the proposed decoding method is comparable to that of ML decoding method at a much reduced computational complexity. The proposed decoding method is used to develop radio transmission technology for next generation digital multimedia broadcasting (DMB). Using the DMB MIMO channel model, developed by extending 3GPP MIMO model to DMB environments, performances of differential space block coding schemes are compared for various channel environments.

In this paper, a new method of inter-cell interference (ICI) cancellation in the downlink is proposed for users at the cell boundary in OFDM-based cellular systems, by using the concept of a subcarrier-based virtual multiple-input multiple-output (SV- MIMO) where multiple antenna techniques are performed on a set of subcarriers, not on the actual antenna array. In the proposed SV-MIMO, "subcarrier signatures," obtained at the mobile station (MS) with a single antenna, are exploited to separate the desired signal from ICIs. The concept of a virtual signature randomizer (VSR) is introduced to improve channel separability in the SV-MIMO approach. Also, the concept of a virtual smart antenna, in which the steering vector is formed by a set of subcarriers with the estimated symbol timing offset (STO) between BSs, is compared with conventional ICI reduction methods. It is shown by simulation that the proposed method is effective in reducing ICI and inter-sector interference for fully- loaded OFDM cellular systems with a frequency reuse factor equal to 1, compared with conventional methods.

In this paper, we propose a new detection method for an OFDM signal distorted by IQ imbalance, and a pilot pattern to estimate the channel associated with IQ imbalance. It is shown by computer simulation that the proposed method can achieve robust detection even when severe IQ imbalance exists in OFDM systems with an input of higher-order constellation.

In this paper, a concept of virtual smart antenna (SA) is introduced for OFDM-based cellular systems with a frequency reuse factor equal to 1. An estimation method of intercell symbol timing offsets (STOs) from received OFDM signals impinging on virtual antenna is proposed for users at a cell boundary. Also, adaptive beamforming methods for virtual SA are proposed to reduce intercell interference (ICI) from adjacent base stations (BSs).

A two-dimensional pilot-symbol aided (2-D PSA) channel estimation technique for coherent orthogonal frequency-division multiplexing (OFDM) systems with transmitter diversity is proposed. The 2-D PSA channel estimator and the corresponding orthogonality condition of the set of pilot symbols are derived under minimum mean-square error (MMSE) criterion for OFDM systems with transmitter diversity. The proposed estimator is shown to be accurate and effective for tracking variations of channels between multiple transmitter antennas and receiver antennas.

This paper presents an adaptive beamforming algorithm for MC-CDMA system with adaptive antenna array. The proposed adaptive beamforming algorithm for the MC-CDMA systems is derived by (1) calculating the error signals between the pilot symbols of desired user and the received pilot signals in the frequency-domain, (2) transforming the frequency-domain error signals into time-domain error signals, (3) updating the filter coefficients of the adaptive beamformer in the direction of minimizing the MSE. The convergence behavior and user-capacity improvement of the proposed approach are demonstrated through computer simulation by applying it to the MC-CDMA system in the presence of interference from other users.

We propose an efficient channel estimation technique for orthogonal frequency-division multiplexing (OFDM) systems with transmitter diversity. The proposed technique estimates uniquely all channel frequency responses needed in space-time coded OFDM systems using "comb-type" training symbols. The computational complexity of the proposed technique is reduced dramatically, compared with the previous minimum mean-squared error (MMSE) technique, due to the all the processing being done in the frequency-domain. Also, several other techniques for mitigating the random noise effect and tracking channel variation are discussed to further improve the performance of the proposed approach. The performances of the proposed approach are demonstrated by computer simulation for mobile wireless channels.

We propose a multicarrier code-division multiple-access (MC-CDMA) system employing an antenna array at the base station, and analyze the performance of the MC-CDMA system in a fading channel. An optimal beamformer not requiring explicit direction of arrival estimation or training signals is derived for the MC-CDMA system in the reverse link, and is shown to reduce interferences from other users significantly, thus increasing the system's user capacity. Also, it is shown that the MC-CDMA approach can achieve better performance than the direct-sequence CDMA approach when both approaches employ the code filtering technique for the antenna array.

A loss of subchannel orthogonality due to time-variant multipath channels in orthogonal frequency division multiplexing (OFDM) systems leads to interchannel interference (ICI) which increases the error floor in proportion to the Doppler frequency. A simple frequency-domain equalization technique which can compensate for the effect of ICI in a multipath fading channel is proposed. In this technique, the equalization of the received OFDM signal is achieved by using the assumption that the channel impulse response (CIR) varies in a linear fashion during a block period and by compensating for the ICI terms that significantly affect the bit-error rate (BER) performance.

This paper presents an efficient adaptive predistortion technique compensating for nonlinear distortions caused by a high-power amplifier (HPA) cascaded with a linear filter in an OFDM system. In the proposed approach, the memoryless HPA, preceded by a linear filter with memory in OFDM systems, is modeled by the Wiener system, which is then precompensated by the proposed adaptive predistorter with a minimum number of filter taps. It is confirmed by computer simulation that the proposed approach produces a faster convergence speed than the previous adaptive predistortion technique, and provides a small output backoff as low as 5.5 dB for an OFDM system employing an HPA with a linear filter.

In this paper, an equalization technique for OFDM and MC-CDMA systems in a time-varying multipath fading environment is described. A loss of subchannel orthogonality due to time-variation of multipath channels leads to interchannel interference (ICI) which increases the error floor in proportion to Doppler frequency. A simple frequency domain equalizer which can compensate the effect of time-variation in a multipath fading channel is proposed by taking into account only the ICI terms significantly affecting the error performance and by modifying the previous frequency-domain equalization techniques. The effectiveness of the proposed approach is demonstrated via computer simulation by applying it to OFDM system when the multipath fading channel is time-varying.

In this paper, an adaptive data predistortion technique which can compensate for the nonlinear effect of a TWT high-power amplifier (HPA) in orthogonal frequency-division multiplexing (OFDM) systems is proposed. It is shown that the convergence time and MSE of an adaptive data predistorter can be significantly reduced by using a broadcasting technique and by designing appropriate training signals.

Two methods (two-tone input approach and random process input approach) for the estimation of harmonic and intermodulation distortion of nonlinear systems are compared. The random input approach, where a nonlinear system is modeled by a second-order Volterra series, is examined in terms of its statistical properties, and its advantages and limitations over the classical two-tone input approach. Experimental results are shown where these two approaches are applied to evaluate second-order distortion of a loudspeaker and to compare the performance of these approaches in terms of Volterra kernels and distortion factors.>

The properties of higher-order moment sequences and higher-order spectral moments of an independent, identically distributed (i.i.d.) process up to fourth order are discussed. These properties are utilized to develop algorithms to identify time-invariant nonlinear systems, which can be represented by second-order Volterra series and which are subjected to an i.i.d. input, in both the time and frequency domain. A relatively simple solution for estimating the Volterra kernels, which requires neither the calculation of the moment sequences for various time lags (or higher-order spectral moments) of the input nor the calculation of the inverse matrix, is shown to exist, even though the second-order Volterra series is not an orthogonal model for an i.i.d. input (unless the input is a white Gaussian process).>

A digital spectral method of evaluating second-order distortion of a nonlinear system, which can be represented by Volterra kernels up to second-order and which is subjected to a random noise input, is discussed. A fundamental assumption that the input possesses Gaussian statistics is investigated. Deleterious effects and misleading results of assuming that the input is Gaussian when in fact it is not are demonstrated. The proposed approach is utilized to model, identify, and quantify the linear response and second-order distortion products of a loudspeaker in the low-frequency band by using higher-order coherence spectra.>