Yongyu Chang

System-level analysis and evaluation of SF-DC transmit mode in HSPA+ system

Cooperated multi-point technique in HSPA+ system has not been paid attention until the Single Frequency Dual Cell (SF-DC) mode, where one user equipment (UE) may be served by dual NodeBs simultaneously in the single carrier frequency network, is proposed recently. In the previous research, the system-level performance with SF-DC operation had not been studied comprehensively. The main contribution of this paper is to exploit SF-DC transmit mode in the perspective of the system level and evaluate the system-level performance of SF-DC in HSPA+ system with different simulation scenes. The impact of SF-DC on legacy UEs and the load balancing capability of SF-DC in asymmetric loading case are investigated properly in different channel conditions. System-level simulation results show that SF-DC transmit mode performs better compared with single point transmission and the influence of SF-DC mode on legacy UEs is negligible. It also reflects that SF-DC has the ability to balance load in asymmetric loading case.

Performance of the Large-Scale Adaptive Array Antennas in the Presence of Mutual Coupling

In this paper, the performance of the large-scale adaptive array antennas in the presence of mutual coupling is explored. An expression for the output signal-to-interference-noise ratio (SINR) of the adaptive array in the presence of strong interference signals, taking into account the mutual coupling between the array elements, is derived. A low-complexity antenna selection algorithm is then proposed to select the optimal subset until the suitable performance is obtained with condition that the spacing between adjacent two antennas is fixed. The combined effect of reducing the distance between the antenna elements with increasing the number of elements in a very limited physical space is also investigated. The antenna array consisting of dipole antennas placed side by side in a linear pattern is assumed. The convergence speed of the adaptive algorithm by bounding and estimating the eigenvalues of its signal covariance matrix in the presence of the mutual coupling is further evaluated. Some novel theoretical study which may be very valuable for the actual deployment of the large-scale adaptive array antennas is made in comparison with previous studies. The theoretical analysis and simulation results show somewhat surprising results showing how the interelement spacing and the number of receiver antennas affect the performance of the large-scale adaptive array antennas with mutual coupling.

An energy-efficiency aware sleeping strategy for dense multi-tier HetNets

With the rapid development of mobile communication, data traffic is exploding. To meet the ever-increasing traffic challenge, Heterogeneous Networks (HetNets) gain extensive attention. Hyper-dense HetNets consisting of large number and high density multi-tier small cells are regarded as a key part of the future networks. However, large-scale and dense deployment of small cells can also bring about huge energy consumption and increase the operating cost to an unendurable price. In this paper, we take (i) the overlapping situation between small cells, (ii) the different power consumption profiles for multi-type small cells, (iii) the load condition of small cells into consideration, propose an energy-efficiency aware (EEA) sleeping strategy to improve the system power saving performance. The strategy introduces the energy-efficiency self-evaluation mechanism to the small cells, screens out the energy-inefficient small cells and switches them off. The simulation results show that EEA can achieve a substantial power saving performance compared to existing strategies.

Contention window size adaptation algorithm for LAA-LTE in unlicensed band.

Fair coexistence between Licensed-Assisted Access using LTE (LAA-LTE) and Wi-Fi in the unlicensed spectrum of 5 GHz is a primary challenge. Listen-Before-Talk (LBT) scheme is required to ensure the fairness among different operators in unlicensed band. In this paper, we focus on the downlink performance of LAA and Wi-Fi with different load intensity. To achieve not only channel access fairness but also higher throughput gain, a contention window (CW) size adaptation algorithm based on LBT Category 4 channel access scheme is proposed. In the analysis, the behavior of LAA eNodeB (eNB) is modeled as a Markov chain to depict the procedure of LBT Category 4 and the throughput can be calculated. Compared with the fixed contention window size mechanism, simulation results show that our algorithm can achieve at least 5% LAA-LTE performance gain in average user throughput with different load intensity.

A Universal Predictive Mobility Management Scheme for Urban Ultra-Dense Networks With Control/Data Plane Separation

In recent years, a new concept of network architecture with control/data plane separation is introduced to the future network, which can effectively improve the mobility robustness and reduce the handover failure by providing the data plane services under the umbrella of a macro cell coverage layer. However, in the ultra dense network, frequent data plane handovers would still introduce huge signaling exchanges and latencies. In this paper, we propose a universal predictive mobility management scheme for urban ultra-dense networks to speed up the data plane handover process. We utilize the probability suffix tree model to save and analyze the transition relationships between small cells in terms of variable markov chains, and pre-configure a cluster of small cells with larger handover probabilities for the users. To accommodate different versions of the users, a compatible network-controlled predictive mobility management procedure and an advanced user-autonomous predictive mobility management procedure are proposed to support the proposed predictive mobility management scheme. The simulation results show that the proposed scheme can significantly improve the prediction accuracy with a lower redundant configuration cost and can effectively speed up the data plane handover process compared with the traditional mobility management.

A novel resource allocation scheme for D2D communication underlaying small cell networks

Small cell deployment and device-to-device (D2D) communications can significantly enhance the system performance by offloading core network traffic and reducing the overall energy consumption, especially in the dense network. However, the interference emerging in it is severe and complicate. In this paper, we develop a model aiming to achieve the maximal system throughput and simultaneously ensuring the quality of service (QoS) of each user accessed into the heterogeneous network (HetNet). Further, we solve the optimization formulation in three steps. First, we find a feasible radio resource allocation profile of small cell networks without D2D communications. Then we optimize the D2D resources based on convex optimization theory. Finally, we get the maximal HetNet capacity according to the theoretical analysis by comparing different resource allocation profiles of small cell networks. Simulation results demonstrate that our approach is feasible and effective to achieve the maximal HetNet throughput simultaneously ensuring each users QoS.

Performance of two-way amplify-and-forward relaying with adaptive modulation over Nakagami-m fading channels

Abstract This paper investigates the performance of a two-way amplify-and-forward (AF) relay system with adaptive modulation over independent and non-identical Nakagami- m fading channels. The tight closed-form cumulative distribution function (CDF) expression of the instantaneous end-to-end signal-to-noise ratio (SNR) is provided. Further, approximate closed-form expression for the average spectral efficiency of the two-way AF system with adaptive modulation is obtained. Then, a tight lower bound of outage probability is derived. Finally, we use numerical simulations to verify the tightness of our analytical results.

Novel reliability-based HARQ scheme for irregular LDPC codes

In this paper, we propose a new and practical reliability-based hybrid automatic repeat request (RB-HARQ) scheme for irregular low-density parity-check (LDPC) codes. Irregular LDPC codes have an unequal error protection (UEP) property. Using this property, we find that the middle degree message nodes of irregular LDPC codes are important for HARQ systems. If we pick out the low reliable message nodes of them for retransmission, we will reduce the feedback. Moreover, the system performance of this scheme is as well as that of the original RB-HARQ scheme.

An efficient delay-restricted scheduling algorithm for multi-carrier TD-HSDPA systems

As a mainstream technology in the B3G TDD systems, TD-HSDPA (LCR TDD HSDPA, High-Speed Downlink Packet Access of Low Chip Rate TDD) adopts some key techniques to achieve high system throughput and high peak data rate, such as AMC (Adaptive Modulation and Coding) and HARQ (Hybrid Automatic Repeat reQuest). However, feedback delay is inevitable in practical TD-HSDPA systems due to the conventional scheduling mechanism, and its impact on wireless scheduling is critical. In this paper, the joint system performances of the key techniques are studied; furthermore, a new efficient scheduling algorithm is proposed. Compared to the conventional scheduling, the delay-restricted scheduling can reduce the feedback delay to improve the system capacity remarkably. Detailed network-performance results are shown from a comprehensive dynamic system-level simulator, and these presented results are obtained for a macrocellular scenario with best effort packet data services to reflect the expected improvement from introducing delay-restricted scheduling in a multi-carrier TD-HSDPA system.

Methods for downlink performance enhancement in HST SFN

With the rapid development of the high-speed-trains (HSTs), companies are paying more and more attention on HST wireless communication system. Although some schemes have been proposed to enhance the system performance, there are still many challenges, e.g. high penetration loss, large Doppler shift, low data rate and so on. Single frequency networks (SFNs) can offer many advantages including better coverage, less interference, less power, and higher reliability. Under the SFN scenario of HST, multiple links will lead to different frequency offsets of data from different remote radio heads (RRHs) when the downlink signals arrive at user equipment (UE). Thus, it will be found that the legacy UE performance under SFN scenario will degrade significantly, especially for the higher order modulation scheme. In this paper, in order to solve this issue, we proposed two methods which are the base station (BS) frequency pre-compensation and the UE receiver enhancement respectively. The simulation results show the significant downlink performance improvements can be achieved by the two solutions especially for high-speed users in SFN.