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Multiuser hybrid precoding in 5G NR mmWave communication system faces significant challenges such as establishing accurate directional radio links and maintaining links for mobile stations (MSs) moving in outdoor environments. A conventional solution relies on finite codebook-based beam sweeping for initial first-stage beam acquisition and subsequent second-stage beam tracking by sweeping adjacent beam pairs. However, such a conventional solution has inevitable residual AoA/AoD errors even after the best beam pair is established in the first stage and incurs nonnegligible overheads to sound adjacent beam pairs for maintaining the best beam pair in the second stage. To overcome these problems, a novel codebook-based two-stage solution that combines a novel beam tracking protocol with a low sounding overhead, a unique receiver structure employing a beam scheduler and a beam tester, and a fine accuracy residual AoA/AoD error estimation algorithm based on the monopulse ratio concept is proposed. The solution is unique because of the receiver structure for the residual AoA/AoD error estimation that exploits the cyclic prefix in OFDM systems, inspired by the monopulse ratio in radar systems. Moreover, it can be applied to MSs with a single RF chain. This solution, using the proposed receiver structure and algorithm, can establish a more accurate directional beam pair right after the initial beam sweeping in the first stage. For beam tracking in the second stage, it estimates the residual AoA/AoD errors of the current best beam pair rather than sweeping adjacent beam pairs, thereby reducing beam tracking overheads. Numerical evaluation and computer simulations show that the proposed solution offers more accurate beam acquisition (i.e., average array gain improvement of several dB) and costs considerably reduced beam sounding overheads compared to the conventional solution. Lastly, a ray-tracing tool is used to demonstrate that our solution is effective in practical channel parameters for outdoor environments.

Large-scale cloud radio access network (LS-CRAN) is a highly promising next-generation cellular network architecture whereby many base stations (BSs) equipped with a massive antenna array are connected to a cloud-computing based central processor unit via digital front/backhaul links. This paper studies the asymptotic behavior of downlink (DL) performance of a LS-CRAN. As an asymptotic performance measure, the scaling exponent of the signal-to-interference-plus-noise-ratio (SINR) is derived for interference-free (IF), maximum-ratio transmission (MRT), and zero-forcing (ZF) operations. Our asymptotic analysis reveals four fundamental operating regimes and the performances of both MRT and ZF operations are fundamentally limited by the UL transmit power for estimating user's channel state information, not the DL transmit power. We obtain the conditions that MRT or ZF operation becomes interference-free, i.e., order-optimal. As higher UL transmit power is provided, more users can be associated and the data rate per user can be increased simultaneously while keeping the order-optimality.

In this study, an energy-efficient self-organized framework for sub-channel allocation and power allocation is presented for ultra-dense small cell networks, which can operate in both licensed and unlicensed bands. In order to protect legacy WiFi devices (operating in unlicensed bands), we consider the Long-Term Evolution (LTE) operation in unlicensed bands based on Carrier Sense Adaptive Transmission (CSAT), in which ’ON’ and ’OFF’ duty cycle approach is utilized. On the other hand, there are severe interference management problems among small cells (operating in licensed and unlicensed bands) and between macro cells and small cells (operating in licensed bands) due to co-channel and ultra-dense deployment of small cells. This article proposes a self-organized optimization framework for the allocation of sub-channels and power levels by exploiting a non-cooperative game with the objective to maximize the energy efficiency of dual-access small cells without creating harmful impact on coexisting network entities including macro cell users, small cell users, and legacy WiFi devices. Simulation results show that the proposed scheme outperforms (6 and 11 percent) and (8 and 18 percent) the round-robin and the spectrum-efficient schemes, respectively, for two different small cell scenarios. In addition, it is shown that for less channel state information (CSI) estimation errors $\varsigma = 0.02$ ς = 0 . 02 , the maximum performance degradation of the proposed scheme is reasonably small (5.5 percent) as compared to the perfect CSI.

A 5G communication system includes massive machine-type communication, enhanced mobile broadband, and ultra-reliable low-latency communication. To meet the requirements of 5G applications, researchers have considered various scalable subcarrier spacings and transmission time intervals. In this article, we introduce the concept of waveform multiplexing, which is a resource management system for numerology multiplexing that employs appropriate waveforms to various numerologies. It also possesses both a dynamic cyclic prefix and a minimum guard band, which are the key features for achieving high spectral efficiency. We verify the significant gain produced by proposed waveform multiplexing by performing extensive 3D ray-tracing- based system-level evaluations in realistic 3D environments. Finally, we provide guidelines on waveform multiplexing design by considering the time-domain and frequency-domain characteristics of the new radio waveform candidates.

In conventional radiography, image visibility is often limited mainly due to the superimposition of the object’s structure under investigation and scattered x-rays. Several methods, including the antiscatter grid technique, the air-gap technique, and scatter correction methods using measurements, mathematical-physical modeling, or a combination of both, have been extensively investigated in an attempt to overcome these difficulties. However, these methods require special equipment, geometry, and extra work to measure the scatter characteristics. In this study, we propose a new image restoration method based on a simple radiographic scattering model in which the intensity of the scattered x-rays and the direct transmission function of a given object are estimated from a single x-ray image by using the dark-channel prior. We implemented the proposed algorithm and performed a systematic experiment by using a 450-kV industrial x-ray inspection system to demonstrate its viability for nondestructive testing. Our results indicated that the structure of the examined object was much more clearly visible in the restored image, considerably improving the radiographic visibility.

Emerging 5G wireless networks need to support sufficient connectivity for a large number of machine-type communication devices, which may have poor radio-frequency (RF) circuit quality due to cost- and energy-efficient design. In this paper, the connectivity is analyzed for a training-based large-scale antenna system employing both non-orthogonal pilots and dirty-RFs on both the transmitter and receiver sides. By considering the effects of imperfect hardware and interference caused by non-orthogonal pilots, the performance of the linear minimum mean-square error (MMSE) channel estimator is derived, and the corresponding average rate of the maximum ratio combiner (MRC) is obtained. From these results, the connectivity maximization problem is formulated and the closed-form solutions for the optimal training length and the optimal number of simultaneously served users are provided. Asymptotic analysis further reveals that allowing only orthogonal pilots limits both the connectivity even when a large number of antenna is employed. However, by allowing non-orthogonal pilots, both the connectivity and the energy efficiency can be improved significantly, even when dirty-RFs are taken into account.

Signal occlusion by building blockages is a double-edged sword for the performance of millimeter-wave (mmW) communication networks. Buildings may dominantly attenuate the useful signals, especially when mmW base stations (BSs) are sparsely deployed compared to the building density. In the opposite BS deployment, buildings can block the undesired interference. To enjoy only the benefit, we propose a building-aware association scheme that adjusts the directional BS association bias of the user equipments (UEs), based on a given building density and the concentration of UE locations around the buildings. The association of each BS can thereby be biased: (i) toward the UEs located against buildings for avoiding interference to other UEs; or (ii) toward the UEs providing their maximum reference signal received powers (RSRPs). The proposed association scheme is optimized to maximize the downlink average data rate derived by stochastic geometry. Its effectiveness is validated by simulation using real building statistics., Comment: This work has been submitted to the IEEE for possible publication

In this paper, a joint user association (UA) scheme with JP-CoMP using a hybrid self-organizing network (SON) is proposed for a practical clustered heterogeneous cellular network (cHCN) to maximize the network-wide proportional fairness among users. The cell range expansion and the enhanced intercell interference coordination have been considered as key items in the long-term evolution-advanced to offload macrocell users to small-cell base stations (sBSs). However, in a cHCN where sBSs are not distributed at random but are clustered instead, the coverage of inner sBSs in a small-cell cluster would be hardly expanded and an increased bias may result in much poor link quality as well as much higher load in outer sBSs. Thus, the load-balancing capability becomes much lower than expected in a cHCN. In order to cope with such a problem, a network architecture and protocol for the cHCN is suggested, and a feasible suboptimal iterative algorithm for determining the joint UA solution of the proposed hybrid SON is provided. It is shown that the proposed hybrid SON scheme with the proposed joint UA solution is very effective in handling the load balancing in a practical cHCN not only improving the performance of the inner sBS users by reducing the intercell interference, especially for intratier offloaded users, but also enabling more aggressive intertier offloading by effectively improving the link quality of cluster edge users without causing an unnecessary resource waste.

In this paper, a novel accumulate-repeat-accumulate-check-accumulate (ARACA) code is proposed as a subclass of protograph-based low-density parity-check (LDPC) codes. The key feature of the proposed ARACA code is represented by the outer connection doping in the protograph. This feature can provide the linear minimum distance growth (LMDG) property at a good iterative decoding threshold while maintaining an efficient encoder structure. The effect of the outer connection doping on the typical minimum distance, the iterative decoding threshold, and the LMDG property is discussed and analyzed by comparing case examples and using the asymptotic protograph ensemble weight enumerator. Some good ARACA code protographs are provided for a wide range of code rates. In addition, an efficient and universal encoding procedure and the corresponding encoder structure are provided for them. The performance of the proposed ARACA code is evaluated and compared with well-known good LDPC codes. The simulation results confirm the superiority of the proposed ARACA codes in terms of encoding complexity and frame error rate performance, especially at low-rates in an ultra-reliable regime.

In this paper, the uplink semi-persistent scheduling policy problem of minimizing network latency is considered for a training-based large-scale antenna system employing two simple linear receivers, a maximum ratio combiner and a zero-forcing receiver, while satisfying each user's reliability and latency constraints under an energy constraint. The network latency is defined as the air-time requested either to serve all users with a minimum quality-of-service, including reliability constraints and minimum throughput levels, or to maximize the spectral efficiency. Optimal non-orthogonal pilots are used to decrease the network latency. An optimization algorithm for determining the latency-optimal uplink scheduling policy using binary-integer programming (BIP) with an exponential-time complexity is proposed. In addition, it is proven that a linear programming relaxation of the BIP can provide an optimal solution with a polynomial-time complexity. Numerical simulations demonstrate that the proposed scheduling policy can provide several times lower network latency in realistic environments than conventional policies. The proposed optimal semi-persistent scheduling policy provides critical guidelines for designing 5G and future cellular systems, particularly for their ultra-reliable low-latency communication services.

Device to device (D2D) communication brings numerous benefits for future heterogeneous cellular networks. However, an energy-efficient design of such D2D communications is a critical challenge due to the cochannel deployment and limited power of users. In this paper, we present an energy-efficient self-organized cross-layer optimization scheme, which aims to maximize the D2D communication energy-efficiency without jeopardizing the quality of service (QoS) requirements of other tiers. Specifically, we model the cross-layer optimization, which includes resource block (RB) and power allocation using a noncooperative game. In the proposed scheme, each D2D transmitter user, which is a player in the game, operates in a self-organizing manner and selects the RBs and the power levels for enhancing its energy efficiency while maintaining the QoS requirements of other heterogeneous parties. Concerning the computationally intense nature of the global optimization problem, we decompose the problem into two subproblems: the RB allocation and the power allocation, and solve them iteratively in a game-theoretic manner. Simulation results demonstrate superior energy efficiency performance of the proposed scheme over conventional schemes. In addition, it is also shown via simulation that the performance of the proposed scheme degrades if the channel state information is not precisely available.

In order to implement an optimized solution for cell range expansion (CRE) and enhanced intercell interference coordination (eICIC) schemes in long-term evolution-advanced (LTE-A) heterogeneous cellular networks (HCNs) and to realize good load-balancing performance in existing LTE-A systems, a practical tessellation-based algorithm is proposed. In this algorithm, a globalized cell-specific bias optimization and a localized almost blank subframe (ABS) ratio update are proposed. The proposed scheme does not require major changes to existing protocols. Thus, it can be implemented in existing LTE-A systems with any legacy user equipment (UE) with only a partial update to the BSs and core networks. From simulation results, it is shown that the tessellation formed by the proposed approach is quite consistent with the optimal one for various realistic scenarios. Thus, the proposed scheme can provide a much better load-balancing capability compared with the conventional common bias scheme. Owing to the improved load-balancing capability, the user rate distribution of the proposed scheme is much better than that obtained from the conventional scheme and is even indistinguishable from that of the ideal joint user association scheme.

A design method for rate-compatible (RC) accumulate repeat accumulate check accumulate (ARACA) codes as protograph-based RC low-density parity-check codes is presented. The proposed RC-ARACA codes provide not only good iterative decoding thresholds with the linear minimum distance growth property but also efficient encoder/decoder structures with low complexities, so that they are appropriate to ultra-reliable and low latency communication services for fifth generation.

This paper comprehensively studies a content-centric mobile network based on a preference learning framework, where each mobile user is equipped with a finite-size cache. We consider a practical scenario where each user requests a content file according to its own preferences, which is motivated by the existence of heterogeneity in file preferences among different users. Under our model, we consider a single-hop-based device-to-device (D2D) content delivery protocol and characterize the average hit ratio for the following two file preference cases: the personalized file preferences and the common file preferences. By assuming that the model parameters such as user activity levels, user file preferences, and file popularity are unknown and thus need to be inferred, we present a collaborative filtering (CF)-based approach to learn these parameters. Then, we reformulate the hit ratio maximization problems into a submodular function maximization and propose two computationally efficient algorithms including a greedy approach to efficiently solve the cache allocation problems. We analyze the computational complexity of each algorithm. Moreover, we analyze the corresponding level of the approximation that our greedy algorithm can achieve compared to the optimal solution. Using a real-world dataset, we demonstrate that the proposed framework employing the personalized file preferences brings substantial gains over its counterpart for various system parameters., 21 pages, 10 figures, 1 table, to appear in the IEEE Transactions on Mobile Computing

This paper presents novel ultrareliable and low-latency communication (URLLC) techniques for URLLC services, such as Tactile Internet services. Among typical use cases of URLLC services are teleoperation, immersive virtual reality, cooperative automated driving, and so on. In such URLLC services, new kinds of traffic such as haptic information including kinesthetic information and tactile information need to be delivered in addition to high-quality video and audio traffic in traditional multimedia services. Furthermore, such a variety of traffic has various characteristics in terms of packet sizes and data rates with a variety of requirements of latency and reliability. Furthermore, some traffic may occur in a sporadic manner but requires reliable delivery of packets of medium to large sizes within a low latency, which is not supported by current state-of-the-art wireless communication systems and is very challenging for future wireless communication systems. Thus, to meet such a variety of tight traffic requirements in a wireless communication system, novel technologies from the physical layer to the network layer need to be devised. In this paper, some novel physical layer technologies such as waveform multiplexing, multiple-access scheme, channel code design, synchronization, and full-duplex transmission for spectrally efficient URLLC are introduced. In addition, a novel performance evaluation approach, which combines a ray-tracing tool and system-level simulation, is suggested for evaluating the performance of the proposed schemes. Simulation results show the feasibility of the proposed schemes providing realistic URLLC services in realistic geographical environments, which encourages further efforts to substantiate the proposed work.

Although small cell base stations (BSs) have been considered to increase network capacity in dense user scenario, it may cause some problems when density of the small cell becomes too much. And then applying full-duplex to a small cell system would be able to provide users with a more consistent quality than just increasing the number of small cell BSs. Conventional system-level evaluation of full-duplex was obtained from average of 1000 runs under randomly dropping small cells. However, in a practical small cell environment, the coverage of each BS vary according to surrounding environment. Therefore, performance analysis based on digital map in realistic scenario is required for accurate evaluation. In this paper, we propose map-based system-level simulation (SLS) methodology for evaluating the performance of full-duplex communications. we used 3D ray-tracing tool (wireless insite) for map-design/network-setup. For realistic simulation, we reflected real-time wireless traffic scenario. Finally, the performance of full-duplex system is compared against of half-duplex system by using proposed map-based SLS.

To improve spectral efficiency in Next-generation WLAN environment, a Joint-Spatial-Division and Reuse method is proposed. With the hope to use this high spectral efficiency user selection and scheduling method, we calculate the interference distribution situation at both Tx position and Rx position when using JSDR method. We provide a statistical interference distribution analysis to prove the proposed JSDR method can be realized in realistic environment.

In millimeter wave (mmWave) systems, analog beam steering (i.e., direction of arrival (DOA) estimation) is an essential operation for subsequent channel estimations. In this paper, we propose a framework of incorporating a monopulse ratio (MR) concept, originally developed for radar systems, into an orthogonal frequency division multiplexing (OFDM) mmWave systems with cyclic prefix (CP) for DOA estimation. It is shown that CP can be used for DOA estimation in general OFDM mmWave systems by employing the MR concept. Based on performance analysis and simulation results, the proposed method can be considered as a practical and robust DOA estimation method for single RF chain analog beamforming systems.

As the under 6 GHz band suitable for mobile communication is saturated, it is very difficult to secure a new frequency. Also, a method of retrieving and rearranging a low-utilization frequency band is difficult to be practiced due to a cost limit. To solve this problem, research on frequency sharing is actively in underway overseas. The frequency sharing model which has recently been drawing attention is citizens broadband radio service (CBRS) in U.S. and licensed shared access (LSA) in Europe. The ultimate goal of these frequency sharing models is to increase the spectral efficiency by using the band utilized for other uses in common with mobile communication. In this paper, we propose a four-layer next generation frequency sharing model from a conceptual point of view larger than CBRS and LSA. The proposed frequency sharing model is not limited to managing only specific bands, but covers the entire wireless communication spectrum and numerous systems as a whole. For the four-layer system, we diversify the license type to mitigate the protection from interference from other layers against the two-layer like LSA or three-layer such as CBRS and increase the coexistence probability. We discuss the architecture for four-layer frequency sharing systems and the requirement of the main function of the spectrum management system (SMS), the main server of the proposed frequency sharing model. We also discuss nonexclusive license types, system operations, and the use case suitable for the actual situation of domestic frequency usage.

Heterogeneous cellular networks (HCNs) where conventional macro cells are being overlaid with low-powered small cells and the millimeter wave (mmW) frequencies are considered as a way to efficiently enhance the spectral efficiency per area with the availability of huge bandwidths. However, how to efficiently manage small cells and mmW frequencies is still an open question. In this paper, a load balancing problem for 5G HCNs is discussed with different band operation modes and a joint user association (UA) scheme using a hybrid self-organizing network (SON) is suggested for a practical clustered HCN.

In this paper, user detection performance of a grant-free uplink transmission in a large scale antenna system is analyzed, in which a general grant-free multiple access is considered as the system model and Zadoff-Chu sequence is used for the uplink pilot. The false alarm probabilities of various user detection schemes under the target detection probabilities are evaluated.

For fifth generation (5G) communications, full-duplex radio is one of the key technologies to increase the spectral efficiency. To apply full duplex to commercial wireless communication systems, it is crucial to adopt effective self- interference cancellation (SIC) techniques, which are compatible with the standards. In this paper, we focus on applying full duplex to the current long term evolution (LTE) standard. We introduce three LTE-compatible SIC methods and these are investigated in terms of reference signal, SIC performance, and stability. Numerical results confirm that, in certain conditions, full duplex can be applied and improve LTE systems especially in small-cell environments.

The increasing wireless data traffic demands have driven the need to explore suitable spectrum regions for meeting the projected requirements. In the light of this, millimeter wave (mmWave) communication has received considerable attention from the research community. Typically, in fifth generation (5G) wireless networks, mmWave massive multiple-input multiple-output (MIMO) communications is realized by the hybrid transceivers which combine high dimensional analog phase shifters and power amplifiers with lower-dimensional digital signal processing units. This hybrid beamforming design reduces the cost and power consumption which is aligned with an energy-efficient design vision of 5G. In this paper, we track the progress in hybrid beamforming for massive MIMO communications in the context of system models of the hybrid transceivers’ structures, the digital and analog beamforming matrices with the possible antenna configuration scenarios and the hybrid beamforming in heterogeneous wireless networks. We extend the scope of the discussion by including resource management issues in hybrid beamforming. We explore the suitability of hybrid beamforming methods, both, existing and proposed till first quarter of 2017, and identify the exciting future challenges in this domain.

Distributed relay scheme for wireless ad hoc multi-hop multicast network composed of low-power and low-complexity wireless devices with high density is proposed. The proposed relay scheme is shown to be better than flooding, which is the distributed relay scheme applied to ZigBee, in the outage probability and the multicast transmission rate by simulations.

In this paper, joint spatial division and reuse (JSDR) scheme is proposed for maximizing network throughput in densely-deployed wireless local area networks equipped with massive antenna array. The proposed JSDR scheme divides the massive spatial space into two subspaces: one is for suppressing the interference from the neighboring access points and another is for sensing the carrier sensing and transmitting the information-bearing signals to intended stations. By using computer simulation, the proposed JSDR can provide 133% higher network throughput, compared to the carrier sensing technique defined in the IEEE 802.11 standard so that the proposed JSDR is suitable for the next generation WLAN systems. ※ This work was supported by ICT RD Revised March 25, 2015; Accepted March 25, 2015 The Journal of Korean Institute of Communications and Information Sciences '15-03 Vol.40 No.03

In this paper, an uplink (UL) enhanced intercell interference coordination (e-ICIC) scheme using synchronously configured low-power almost blank subframes is proposed for providing as efficient UL load balancing as in the downlink (DL) of a self-organized heterogeneous cellular network. An analytical framework is provided as a theoretical background for a successful load balancing, and it is shown that the derived SIR distributions are quite close to the true ones in a wide range of network parameters so that it can be utilized for a successful load balancing. From the simulation results, it is shown that the proposed UL e-ICIC scheme can provide similar load balancing capability as the DL counterpart.

In this paper, the uplink (UL) signal-to-interference ratio (SIR) distributions in a two-tier heterogeneous cellular network (HCN) are analyzed by considering the power control effect and approximating the UL interferer locations as a non-uniform Poisson point process (PPP). Approximated closed-form SIR distributions are provided according to the tier, association rule, power control strategy for each tier, and resource partition (common or dedicated) and are shown to be quite close to the true distributions in a wide range of network parameters.

It is a challenging task to design a random access protocol that achieves the optimal throughput in multi-cell random access with decentralized transmission due to the difficulty of coordination. In this paper, we present a decentralized interference-aware opportunistic random access (IA-ORA) protocol that enables us to obtain the optimal throughput scaling in an ultra-dense multi-cell random access network with one access point (AP) and a number of users. In sharp contrast to opportunistic scheduling for cellular multiple access where users are selected by base stations, under the IA-ORA protocol, each user opportunistically transmits with a predefined physical layer (PHY) data rate in a decentralized manner if not only the desired signal power to the serving AP is sufficiently large but also the generating interference leakage power to the other APs is sufficiently small (i.e., two threshold conditions are fulfilled). As a main result, it is shown that the optimal aggregate throughput scaling (i.e., the MAC throughput of $\frac{1}{e}$ in a cell and the power gain) is achieved in a high signal-to-noise ratio regime if the number of per-cell users exceeds some level. Additionally, it is numerically demonstrated via computer simulations that under practical settings, the proposed IA-ORA protocol outperforms conventional opportunistic random access protocols in terms of aggregate throughput., 20 pages, 10 figures, 3 tables. Published in the IEEE Access

Fifth generation (5G) communication systems include enhanced mobile broadband, massive machine-type communication, and ultra-reliable low-latency communication. To support various 5G applications, researchers have considered multiple scalable subcarrier spacing and transmission time interval (TTI). In this paper, we introduce a concept of waveform multiplexing for frequency multiplexing of numerologies. Waveform multiplexing consists of different waveforms on subbands with scalable subcarrier spacing and TTI operating on one frequency band. The Waveform multiplexing also possesses dynamic cyclic prefix and minimum guard band, which are the key features for the high spectral efficiency. We verify the performance and the potential of the waveform multiplexing using 3-dimensional (3D) ray-tracing based system-level evaluation in a realistic environment and a 3D channel.

In an Ultra-dense network (UDN) where there are more base stations (BSs) than active users, it is possible that many BSs are instantaneously left idle. Thus, how to utilize these dormant BSs by means of cooperative transmission is an interesting question. In this paper, we investigate the performance of a UDN with two types of cooperation schemes: non-coherent joint transmission (JT) without channel state information (CSI) and coherent JT with full CSI knowledge. We consider a bounded dual-slope path loss model to describe UDN environments where a user has several BSs in the near-field and the rest in the far-field. Numerical results show that non-coherent JT cannot improve the user spectral efficiency (SE) due to the simultaneous increment in signal and interference powers. For coherent JT, the achievable SE gain depends on the range of near-field, the relative densities of BSs and users, and the CSI accuracy. Finally, we assess the energy efficiency (EE) of cooperation in UDN. Despite costing extra energy consumption, cooperation can still improve EE under certain conditions., Comment: 6 pages, 8 figure, to appear in EuCNC 2017

This letter provides the distributions on the association region of a small-cell BS (sBS) and that of a macro-cell BS (mBS) in a two-tier heterogeneous cellular network (HCN) as functions of the power and density ratios between tiers. Computer simulation results confirm that the proposed distributions are quite accurate for a wide range of network parameters. By using the association region distributions, the load distribution of a BS in each tier is derived. From the results, it is shown that the load is more dispersive in tier s than in tier m, which can be used for devising improved association, resource allocation and load balancing schemes in an HCN.

In this letter, an adaptive multi-antenna channel estimation scheme is proposed for uplink multiuser environments such as LTE-A systems to accurately estimate time-varying channels within an affordable complexity. It is confirmed that the proposed channel estimator can achieve accurate channel tracking performance even when various time-varying channel environments and traffic patterns are provided. Ⅰ. 서 론

In this letter, a distributed self-organized cell association scheme is proposed for heterogeneous cellular networks. In which each small cell uses a common self-organization strategy and takes its environment into account to establish its own cell association scheme. It is confirmed that the proposed cell association improves the user throughput and areal spectral efficiency compared to joint association.

An efficient multi-user multiple-input–multiple-output (MIMO) protocol is proposed and verified to be significantly advantageous over the conventional IEEE 802.11ac protocol for next-generation wireless local area network systems employing massive MIMO and orthogonal frequency division multiple access.

In this letter, a DFT-based channel estimator is proposed for the uplink of LTE-A systems to solve the leakage and enhance the spectral efficiency. It is confirmed that the proposed estimator can significantly improve user and cell spectral efficiencies compared to conventional estimators.

In this paper, a joint cell grouping and user association technique proposed for a semi-dynamic grouped network MIMO in a clustered heterogeneous cellular network (HCN). With the conventional macro BSs, small cells are being overlaid to increase a spectral efficiency per area and these small cells are expected to be concentrated to support exponentially increasing data traffic in hot spot areas. The main culprits of performance degradation in the clustered HCN are interference and load imbalance. The proposed scheme jointly handles them to maximize a proportional-fair metric. It is shown that the proposed technique can largely improve user average rate and proportional fairness among users than any other conventional schemes in the clustered HCN.

In conventional single-tier networks, downlink based association is the best association scheme for the uplink association because all macro base stations have the same physical specification. However, in uplink heterogeneous cellular networks, a downlink based cell association cannot be the best for uplink any more because of the difference of physical specification between the different base station. In this paper, we will propose a uplink based cell association scheme, and devise performance metric for describing a uplink performance in heterogeneous cellular networks. Then, we will discuss the necessity of the uplink based association by observing outage probability, delay constraint outage probability, delay constraint outage capacity.

In this paper, we consider delay-constrained wireless multi-hop ad hoc networks where a packet should be delivered to the destination within the maximum allowed delay while satisfying the target outage probability. The proposed performance metric for analyzing networks is the delay-constrained random access transport capacity (D-RATC), which quantifies the maximum end-to-end (e2e) link achievable rate per unit area of a delay-constrained network using a random access protocol. The scaling of the D-RATC is obtained for various slotted ALOHA (SA) protocols and it is shown that the SA protocol is order-optimal for delay-constrained random networks when interference control is used with an additional feature such as rate control or admission control. If interference control is not used, the SA protocol suffers from the negatively infinite scaling exponent except the case of using rate control where a finite but suboptimal scaling exponent may be achieved. Also, it is shown that multi-hop control does not affect the scaling exponent but just improves the D-RATC pre-constant.

Distributed multi-hop relay scheme to reduce delay-constrained outage probability for broadcast network is proposed. We consider multi-hop relay scheme, which is similar to distributed beamforming, where multiple nodes simultaneously relay packets, and we propose channel access control and power control for relaying nodes to satisfy energy constraint. Compared with flooding which is multi-hop relay scheme used for ZigBee, the proposed scheme is better in terms of outage probability and average reception throughput.

In this paper, massive multiple-input multiple-output (massive MIMO) and full-duplex communications (FDC) are considered together for high efficiency next generation wireless LAN (WLAN) systems such as IEEE 802.11ax or beyond. The proposed scheme allocates different carrier sensing thresholds by applying the joint spatial division and reuse (JSDR) scheme and is able to enhance the efficiency of multi-user MIMO (MU-MIMO) protocols by reducing the protocol overhead. Finally, FDC is applied to improve the spectral efficiency of the WLAN systems.

In this paper, the best-M feedback, where each user reports the channel quality indicators (CQIs) on its M best resource blocks (RBs), for orthogonal frequency division multiple access (OFDMA) systems is analyzed and optimized. First, the closed-form expression of the average sum-rate is derived as a function of M and the number of bits for a signal to noise ratio (SNR) quantization B. Then, M and B are jointly optimized to minimize the feedback overhead such that the desired system performance can be achieved. Numerical results confirm that the proposed analysis is quite well matched to the exact result so that our work is useful to optimize M and B instead of time-consuming computer simulations, and that the required feedback overhead to achieve a given performance decreases as the number of users and the average SNR increase.

In this paper, partial feedback schemes for a multiple input multiple output orthogonal frequency division multiple access system using a random beamforming are analyzed and optimized. For partial feedback schemes, the partial channel quality indicator feedback schemes and the partial channel quality rank indicator feedback schemes are considered. For these schemes, we first derive the effective downlink spectral efficiencies by considering the required uplink resource for feedback together over block fading channels. Then, by using the analysis, the amount of feedback overhead per user is optimized to maximize the effective downlink spectral efficiency according to the system and channel parameters. From the analysis and numerical examples, we show that the partial channel quality rank indicator feedback scheme provides better performance than the partial channel quality indicator feedback scheme unless the channel rapidly varies owing to its feedback efficiency, and the proposed adaptive control of the feedback overhead can improve the performance of practical systems, such as the 3rd generation partnership project 3GPP long term evolution, in practical scenarios. Copyright © 2012 John Wiley & Sons, Ltd.

SUMMARY In this paper, an efficient partial incremental redundancy (P-IR) scheme is proposed for an H-ARQ using block type low density parity check (B-LDPC) codes. The performance of the proposed P-IR scheme is evaluated in an HSDPA system using IEEE 802.16e B-LDPC codes. Simulation results show that the proposed H-ARQ using IEEE 802.16e BLDPC codes outperforms the H-ARQ using 3GPP turbo codes.

A cooperative communication in a two-way relay channel is a bidirectional communication between two terminals with the aid of a common relay. In order to improve the performance of the cooperative communication in a two-way relay channel, a network coding can be used to exploit the broadcasting nature of wireless transmission. In [8], it was shown that each individual link can achieve its own capacity when the length of the codewords is the same. However, the length of the codewords can be different in practical systems since the two-way relay channel is naturally asymmetric. Thus, in this paper, we show a practical network coding scheme with a given modulation and coding set by employing a rate-compatible puncturing technique. Simulation results show that the proposed scheme can provide at least as good performance as that obtained in individual link without using special encoder/decoder.

In this letter, we propose an adaptive transmission method for an OFDMA system supporting both band-AMC and diversity modes in a frame, simultaneously. In the proposed method, users are classified into the two groups preferring the band-AMC mode or the diversity mode based on their channel parameters. Then the BS performs resource allocation to maximize the throughput. It is observed that the proposed adaptive transmission method can reduce the feedback overhead with negligible performance loss.

This paper proposes an efficient multiuser adaptive modulation and coding (AMC) scheme that considers inevitable feedback delay by employing short-term and long-term channel state information (CSI) in time-varying frequency-selective fading channels. By taking the statistic of the true signal-to-noise ratio (SNR) at a given predicted SNR value into account, the required transmit power to meet the target packet-error-rate (PER) can be obtained and used for user selection, power allocation, and modulation and coding set (MCS) selection. In addition, a simple and useful approximation method of obtaining the required transmit power is proposed. The performance of the proposed scheme is shown to be much better than that of conventional schemes without considering the feedback delay or the prediction error. The proposed scheme can also reduce the feedback resource while maintaining the system throughput by allocating different feedback resources to different users according to their prediction error variances.

Various space time code (STC) designs have been proposed to obtain full diversity at full rate in multiple-input multiple-output (MIMO) channels for uncoded systems. However, commercial wireless systems typically employ powerful channel codes such as turbo codes and low density parity check (LDPC) codes together with an STC. For these applications, an STC optimized for uncoded systems may not provide the best performance. In this paper, an STC with relatively good performance over a wide range of code rates is proposed. Simulation results show that the performance of the proposed robust STC is very close to the best performance of the SM and the Golden code in various code rates.