Adaptive downlink and uplink channel split ratio determination for TCP-based best effort traffic in TDD-based WiMAX networks

In this paper, we study the determination of downlink (DL) and uplink (UL) channel split ratio for Time Division Duplex (TDD)-based IEEE 802.16 (WiMAX) wireless networks. In a TDD system, uplink and downlink transmissions share the same frequency at different time intervals. The TDD framing in WiMAX is adaptive in the sense that the downlink to uplink bandwidth ratio may vary with time. In this work, we focus on TCP based traffic and explore the impact of improper bandwidth allocation to DL and UL channels on the performance of TCP. We then propose an Adaptive Split Ratio (ASR) scheme which adjusts the bandwidth ratio of DL to UL adaptively according to the current traffic profile, wireless interference, and transport layer parameters, so as to maximize the aggregate throughput of TCP based traffic. Our scheme can also cooperate with the Base Station (BS) scheduler to throttle the TCP source when acknowledgements (ACKs) are transmitted infrequently. The performance of the proposed ASR scheme is validated via ns^-2 simulations. The results show that our scheme outperforms static allocation (such as the default value specified in the WiMAX standard and other possible settings in existing access networks) in terms of higher aggregate throughput and better adaptivity to network dynamics.     

Flexible UL-DL Switching Point in TDD Cellular Local?Area Wireless Networks

An advantage of time division duplex (TDD) wireless networks over frequency division duplex (FDD) is that the UL-DL switching point may be flexibly adapted to asymmetric traffic loads. This enables more efficient spectrum use, but on the other hand may lead to harmful cross-link interference between cells. As a result, the net gain (or loss) from flexible TDD depends on the traffic characteristics and network scenario. In this paper we address the problem in local area packet data networks, such as small- or femto-cell network, where high fluctuation in short term traffic loads is expected. We show through analysis and system level simulations that in such a scenario a significant gain in effective user throughput and packet delays may be achieved under low traffic loads. At high load the gain becomes smaller as packets accumulate in queues and there is both UL and DL traffic to transmit with high probability. When the UL and DL transmit powers are not balanced and switching point adaptation is applied the link direction with lower power may provide degraded performance. We show that introducing interference awareness at the scheduler provides more balanced performance and lowers the packet delays further.     

Dynamic traffic split scheme in an integrated LTE and HSDPA networks

In the future, the wireless communication networks can be visualized as the integration of different radio access technologies （RATs）, which are referred to as heterogeneous wireless networks （HWNs）. In this paper, the traffic split scheme in the HWNs integrating the long term evolution （LTE） and the high speed downlink packet access （HSDPA） networks is investigated. Assuming that the networks can support multi-homing access and the user can be served by both networks simultaneously, the traffic split problem is described as an optimization problem with the aim of maximizing the throughput. By solving the problem, the dynamic traffic split scheme is proposed. The split ratios in the scheme should be proportional to the transmission rates in theory, which are hard to be described in the closed forms. Then the adaptive algorithm is proposed to obtain the split ratios. Simulation results show that the scheme with the adaptive algorithm provides better performance than the scheme without it over the additive white Gaussian noise （AWGN） fading channel and Rayleigh fading channel.     

Towards the performance evaluation of 4th generation wireless communication standards: LTE versus Mobile WiMAX

Advances in wireless communication protocols and networking toward support of the next generation of mobile and radio broadband technologies have contributed to a strong competition among various telecommunication standards in particular Long Term Evolution (LTE) and Mobile Worldwide Interoperability for Microwave Access (WiMAX). In this paper, we provide an in‐depth comparison analysis of LTE and Mobile WiMAX at the physical (PHY) layer by studying the most similar PHY configuration scenarios for these two technologies. Our study includes a throughput analysis of downlink (DL) and uplink (UL) transmissions in time division duplex with the least overhead possible and different antenna schemes as well as modulation and code rates. This study also performs an overhead analysis in both protocols to provide a more in‐depth understanding of the PHY layer capacity in various PHY layer configurations. Our simulation results generally show higher performance for LTE in both DL and UL transmission with 7 Mbps in DL and 5 Mbps in UL, when using one antenna port. However, by increasing the number of antennas for multiple‐input/multiple‐output configurations, the results illustrate a reduction in the performance of LTE compared to Mobile WiMAX. This arises from the increase in reference signal overhead in LTE from 4.7% in single‐input/single‐output (SISO) to 14.28% in 4 × 4 multiple‐input/multiple‐output (MIMO). Copyright © 2013 John Wiley & Sons, Ltd.     

System Design Issues and Performance Evaluations for Adaptive Modulation in New Wireless Access Systems

This paper discusses system design strategies and performance evaluation for adaptive modulation techniques used in new and up-and-coming wireless access systems. After a brief discussion on basic modulation parameter (MP) settings as well as functionalities specific for adaptive modulation systems, this paper discusses how to design a narrowband time-division multiple-access/time-division duplex (TDMA/TDD)-based adaptive modulation system and confirms through laboratory experiments that the narrowband TDMA/TDD-based adaptive modulation system dramatically enhances system robustness to multipath fading and flexibility in throughput and transmission quality control. Next, as an extension to many fields of application for adaptive modulation, this paper discusses subcarrier-level adaptive modulation for orthogonal frequency-division multiplexing (OFDM)-based one-cell reuse broadband cellular systems. One key result is that nonsubcarrier transmit power control (TPC) applied adaptive modulation systems can achieve almost the same performance as subcarrier TPC applied adaptive modulation systems provided that the required signal-to-noise-plus-interference power ratio increment in the MP sets is designed to be sufficiently small. Finally, this paper explains the dynamic parameter controlled OFDM/TDMA system as an example of a practical scheme for one-cell reuse broadband wireless access systems. The analysis confirms that this adaptive system can achieve a spatial reuse efficiency defined by (average throughput in multicell conditions)/(average throughput in single cell conditions) of 0.8 as well as an average media access control payload throughput of about 150 Mb/s using about 100 MHz of bandwidth.     

Quasi‐distributed Interference Coordination for HSPA HetNet

The heterogeneous network (HetNet) has been discussed in detail in the Long‐Term Evolution (LTE) and LTE Advanced standards. However, the standardization of High‐Speed Packet Access HetNet (HSPA HetNet) launched by 3GPP is pushing at full steam. Interference coordination (IC), which is responsible for dealing with the interference in the system, remains a subject worthy of investigation in regard to HSPA HetNet. In this paper, considering the network framework of HSPA HetNet, we propose a quasi‐distributed IC (QDIC) scheme to lower the interference level in the co‐channel HSPA HetNet. Our QDIC scheme is constructed as slightly different energy‐efficient non‐cooperative games in the downlink (DL) and uplink (UL) scenarios, respectively. The existence and uniqueness of the equilibrium for these games are first revealed. Then, we derive the closed‐form best responses of these games. A feasible implementation is finally developed to achieve our QDIC scheme in the practical DL and UL. Simulation results show the notable benefits of our scheme, which can indeed control the interference level and enhance the system performance.     

LTE后续演进中基于频谱聚合的协同通信

大带宽无线传输有两个实现途径,一是设计频带更宽的单一系统,一是进行不同系统间的协同设计。前者是在同一种空口内采用基于多载波的频谱聚合,后者是在不同空口之间采用基于多载波并行传输的频谱聚合。通过多载波并行传输实现系统内和系统间频谱聚合,可以充分利用现有射频技术和器件,降低设备的研制和运营成本。而系统间的频谱聚合,特别是异构系统间的频谱聚合,除了可以扩展传输带宽的基本功能之外,还可以解决频分复用系统对非对称业务的支持以及时分双工保护带的利用问题,对运营商更具有商业上的现实意义。     

Resource allocation algorithm for LTE networks using fuzzy based adaptive priority and effective bandwidth estimation

In this paper, we propose a scheme to allocate resource blocks for the Long Term Evolution (LTE) downlink based on the estimation of the effective bandwidths of traffic flows, where users’ priorities are adaptively computed using fuzzy logic. The effective bandwidth of each user traffic flow that is estimated through the parameters of the adaptive β-Multifractal Wavelet Mode modeling, is used to attain their quality of service (QoS) parameters. The proposed allocation scheme aims to guarantee the QoS parameters of users respecting the constraints of modulation and code schemes (modulation and coding scheme) of the LTE downlink transmission. The proposed algorithm considers the average channel quality and the adaptive estimation of effective bandwidth to decide about the scheduling of available radio resources. The efficiency of the proposed scheme is verified through simulations and compared to other algorithms in the literature in terms of parameters such as: system throughput, required data rate not provided, fairness index, data loss rate and network delay.     

Adaptive radio resource management for maximizing reward and balancing loads in 4G hybrid universal mobile telecommunications system and long term evolution communications

The universal mobile telecommunications system (UMTS) and long term evolution (LTE)/LTE‐advanced specifications have been proposed to offer high data rate for the forwarding link under high‐mobility wireless communications. The keys include supporting multi‐modes of various coding schemes (e.g., VSF‐OFCDM, OFDM, OFDMA), multiple‐input multiple‐output, relay networks, and so on. To balance loads among different communication interfaces is one of the most important issues that should be addressed for achieving efficient radio resource allocations. In a shared packet service, the 3GPP UMTS adopts the VSF‐OFCDM interface to allocate orthogonal codes of an OVSF code tree in two‐dimension (2D) spreading at the time and frequency domains. However, it easily leads to high packet loss rate and high bandwidth waste rate, because it does not consider interference and the adaptive modulation and coding scheme simultaneously. Conversely, although the LTE/LTE‐advanced interface offers a high data rate, it suffers from unbalanced loads and moderate reward. This paper thus proposes an adaptive radio resource allocation for balancing loads between the UMTS and LTE/LTE‐advanced interfaces according to various interference and mobility environments. In addition, an adaptive multi‐code allocation is proposed for the UMTS to minimize the bandwidth waste rate while guaranteeing quality of service. Numerical results indicate that the proposed approach outperforms other approaches in fractional reward loss and system utilization. Copyright © 2013 John Wiley & Sons, Ltd.     

LTE系统中自适应调制编码技术的研究

根据标准研究了LTE系统的自适应调制编码技术,并针对该系统提出了自适应调制编码技术的实现方案。在LTE系统环境下仿真了提出的实现方案的性能,结果表明：无论在高信噪比还是在低信噪比条件下,自适应技术均比单一的编码调制方案吞吐量有明显的提升,该自适应实现方案在保证一定的通信可靠性的前提下,能够更有效地利用频谱资源和提高系统吞吐量。     

Distributed timeslot allocation with crossed slots in CDMA‐TDD systems

Code division multiple access system with time division duplex (CDMA‐TDD) is a promising solution to cope with traffic asymmetry of downlink (DL) and uplink (UL) in multimedia services. When a rate of asymmetry is different in each cell, CDMA‐TDD system may employ crossed slots, where a timeslot is used for different links in cells. However, it may suffer from base station (BS)‐to‐BS and mobile station (MS)‐to‐MS interference problem. Zone division scheme is an efficient way to tackle the crossed slot interference by dividing a cell into inner and outer zones and restricting communication in crossed slots only to inner zone. In this paper, we propose distributed crossed slot resource allocation with zone division in multi‐cell CDMA‐TDD system. Two conditions for crossed slot resource allocation are defined and the bound on the size of inner zone is analyzed mathematically based on the conditions. Relationship between the capacity of crossed slot and the size of inner zone is also analyzed. Then, numerical results of the mathematical analysis are presented; showing that the proposed crossed slot allocation is effective for traffic asymmetry problem. Copyright © 2009 John Wiley & Sons, Ltd.     

Spectral Efficiency of Dynamic Time Division Duplex Fixed Wireless Cellular System for Asymmetric and Dynamic Multimedia Traffic

We analyze spectral efficiency of dynamic time division duplex in a fixed wireless cellular network. Conventionally, spectral efficiency has been analyzed for static and fully loaded systems. In this paper, we investigate how asymmetric and dynamic traffic affect the spectral efficiency of Time-Division-Duplex (TDD) systems. Recently, dynamic TDD (D-TDD) has gained much attention as an efficient duplex scheme for high-speed data communications, because adaptive switching ability enables the system to obtain statistical multiplexing gain by exploiting dynamic and asymmetric data traffic. However, it has been noted that a rather strong co-channel interference can be present due to adaptive switching in a cellular network that uses frequency reuse. Thus, benefits of dynamic TDD may not be justifiable unless a proper countermeasure is employed. To suppress the effect of strong co-channel interference, we employ time slot allocation (TSA) strategy along with sector antenna layouts, as proposed in our previous work Jeong and Kavehrad (IEEE and Transactions on communication, Vol. 50, no.10 pp. 1627–1636, 2002). We note that higher spectral efficiency is obtained in D-TDD systems by employing TSA strategy. We also evaluated spectral efficiency of D-TDD system employing adaptive modulation, assuming that traffic is delay tolerant. It is observed that five times higher spectral efficiency can be obtained by employing adaptive modulation. The effect of variance of ratio of offered load on uplink and downlink is also evaluated. Our computer simulation results show that spectral efficiency of D-TDD system with time slot allocation algorithm is more than that of static TDD (S-TDD) over a large range of traffic variation. In conclusion, D-TDD system can take advantage of statistical multiplexing gain of dynamic traffic by adaptively positioning the boundary to the varying traffic bandwidth in its two-way transmissions when TSA strategy is employed for suppression of strong co-channel interference.     

Throughput and cost‐efficient interference cancelation strategies for the downlink of spectrum‐sharing Long Term Evolution heterogeneous networks

In a heterogeneous network (HetNet), small cells such as femtocells considered in this work are deployed jointly with macrocells. This new cells' layer, when added to the network, generates interference, which could hamper neighboring macro‐user equipment (MUE) and femto‐user equipment (FUE) transmissions. In fact, this interference results in degradation of the network performance. In this paper, we propose a downlink interference cancelation (DL‐IC) strategy for spectrum‐sharing Long Term Evolution (LTE) HetNet. This DL‐IC strategy aims to reduce the interference impact on users by optimizing their received signal to interference plus noise ratio (SINR) using new utility functions for both FUEs and MUEs. These utility functions allow relaxation of the cancelation ratios in order to reduce implementation complexity while maximizing SINR, QoS, and throughput. We support by different system‐level simulations that both global network performance and user experience in terms of total throughput and received SNR or link‐level throughput, respectively, are significantly enhanced. Throughput gains achievable by the new DL‐IC strategy can reach as much as 200% against a homogeneous LTE network without IC along with an extra 48% per additional femtocell base station against a basic spectrum‐sharing LTE HetNet without IC. These performance figures are shown to surpass those achieved by interference avoidance techniques using either power or frequency resource allocation. Copyright © 2014 John Wiley & Sons, Ltd.     

Efficient Transmission Scheme for AWG-Based DWDM Millimeter-Wave Fiber-Radio Systems

We propose and demonstrate an upstream transmission scheme using a semiconductor optical amplifier (SOA) for arrayed waveguide grating (AWG)-based dense wavelength-division-multiplexed (DWDM) millimeter-wave fiber-radio systems and show improved link performance. In our scheme, unused optical carriers from the cyclic AWG in the downlink (DL) are tapped for uplink (UL) transmission. An SOA in conjunction with the AWG simultaneously amplifies the DL RF subcarriers and UL optical carrier, thus improving carrier-to-sideband ratio in the DL while also yielding an improved power budget for the UL. Our experimental results show that the proposed scheme can be a practical solution for future bidirectional wavelength interleaved DWDM transmission systems     

基于LTE资源块感知的自适应无线流媒体系统

自适应流媒体技术用户体验的好坏,很大程度上由流媒体应用对端到端带宽估计的准确度决定,而在高度动态的LTE网络中,对带宽的估计极具挑战。设计并实现了一种基于LTE资源块感知的自适应无线流媒体系统,系统中的LTE资源块监测模块监测当前蜂窝小区范围内的物理层资源占用情况;速率映射机制据此将视频分片下载速率映射为当前可用带宽;码率自适应算法结合当前LTE网络中潜在可用带宽和当前视频缓存状况,选择最合适的码率版本,以实现视频质量和播放卡顿之间的折中。在原型系统上的测试实验中,与两种基线码率自适应算法进行对比,所提方法在保持极低的播放卡顿率的情况下获得了最高的平均视频码率,有效提升了用户的视频观看体验。     

UL CoMP在LTE智能立体网络中的应用及外场测试结果

文章分析UL CoMP(上行多小区协作)原理及其在智能立体网络中的应用,在此基础上介绍了UL CoMP在爱立信北京LTE试验网中的测试结果。结果表明通过UL CoMP移动终端可实现从低功率RRU上获得最好的上行接收,同时从高功率RRU上获得最好的下行传输,从而解决智能立体网络中上下行功率不均衡的问题,显著改善小区边缘用户体验。     

基于多门限预留机制的自适应带宽分配算法

针对异构无线网络融合环境提出了一种基于多门限预留机制的自适应带宽分配算法,从而为多业务提供QoS保证。该算法采用多宿主传输机制,通过预设各个网络中不同业务的带宽分配门限,并基于各个网络中不同业务和用户的带宽分配矩阵,根据业务k支持的传输速率等级需求和网络状态的变化,将自适应带宽分配问题转化为一个动态优化问题并采用迭代方法来求解,在得到各个网络中不同业务和用户优化的带宽分配矩阵的同时,在带宽预留门限和网络容量的约束条件下实现网络实时吞吐量的最大化,以提高整个异构网络带宽的利用效率。数值仿真结果显示,所提算法能够支持满足QoS需求的传输速率等级,减小了新用户接入异构网络的阻塞概率,提高了平均用户接入率并将网络吞吐量最大提高40%。     

Resource Allocation for Real Time Services in LTE Networks: Resource Allocation Using Cooperative Game Theory and Virtual Token Mechanism

The LTE specifications provide QoS for multimedia services with fast connectivity, high mobility and security. However, 3GPP specifications have not defined scheduling algorithms to exploit the LTE characteristics to support real time services. In this article we propose a two level scheduling scheme composed by cooperative game theory, a virtual token mechanism, and the well known algorithms EXP-RULE and Modified-Largest Weighted Delay Firs (M-LWDF) in downlink system. By using cooperative game theory such as bankruptcy game and Shapley value, the proposed mechanism works by forming coalitions between flow classes to distribute the bandwidth fairly among all of them. Both algorithms EXP-RULE and M-LWDF have been modified to use a virtual token mechanism to improve their performance, giving priority to real time flows. By taking the arrival rate of packets into account, the proposed mechanism partially included in previous schedulers has been adapted to this work to increase remarkably the performance of the resource allocation for real time flows. The performance evaluation is conducted in terms of system throughput, Packet loss ratio, total cell spectral efficiency, delay and fairness index.     

Cubic metric improvement of aggregated carriers for downlink transmission in LTE-advanced

To meet long term evolution-advanced(LTE-A) requirements of wider bandwidth and maintain backwards compatibility with LTE release 8(Rel-8) simultaneously,carrier aggregation has been agreed by the third generation partnership project(3GPP).High cubic metric(CM) values and peak-to-average power ratio(PAPR) of aggregated component carriers(CCs) exists as an unpleasant issue for both uplink and downlink(DL) transmission.For DL transmission,to use distinct cell ID per DL CC to generate DL reference signal(RS) sequence can mitigate this problem but will cause some other issues on the corresponding uplink carrier.This article proposes two approaches to solve the problem of high CM/PAPR by introducing new variables to change the cell-specific DL RS sequence.In both methods,same cell identifier(cell ID) is allocated to all DL CCs.Simulation results show the performance improvements and prove the effectivity of our proposed approaches.     

Joint admission control,cell association,power allocation and throughput maximization in decoupled 5G heterogeneous networks

Downlink (DL)-uplink (UL) coupled cell association scheme was adopted in 3rd generation (3G) homogeneous network and 4th generation (4G) heterogeneous network (HetNet). In the coupled cell association scheme, a user is associated to single base station (BS) in DL & UL based on the strongest received signal power (SRSP) in DL from a macro base station (MBS) and multiple small base stations (SBS) in HetNet. This is a sub-optimal solution for cell association as most of the users are associated to a MBS due to dominant transmit power and brings challenges like multiple interference issues and imbalanced user traffic load which leads to a degraded throughput in HetNet. In this paper, we investigate downlink uplink decoupled cell association scheme to address these issues and formulate a sum-rate maximization problem in terms of admission control, cell association and power allocation for MBS only, coupled and decoupled HetNet. The formulated optimization problem falls into class of a mixed integer non linear programming (MINLP) problem which is NP-hard and requires exhaustive search to find the optimal solution. However, computational complexity of the exhaustive search increases exponentially with the increase in number of users. Therefore, an outer approximation algorithm (OAA) is proposed as a solution to find near optimal solution with less computation complexity. Extensive simulations work has been done to evaluate the decoupled cell association scheme in HetNet vs the coupled cell association scheme in traditional MBS only and HetNet. Results show the effectiveness of decoupled cell association scheme in terms of KPIs, such as throughput, addressing user traffic load imbalances and number of users attached.