Time slot allocation in CDMA/TDD systems for mobile multimediaservices

An important issue in wireless multimedia communications is to cope with the traffic asymmetry between uplink and downlink. The asymmetrical slot allocation in the code division multiple-access systems with time-division duplex mode (CDMA/TDD systems) can be a good solution for this problem. However, the level of traffic asymmetry can be significantly different from cell to cell. In this letter, we calculate the utilization of a CDMA/TDD system for the multicell model. In addition, we suggest an optimal slot allocation that maximizes the frequency utilization     

Comparison of time slot allocation strategies for CDMA/TDD systems

The traffic (load) asymmetry between uplink and downlink is a remarkable traffic characteristic in cellular mobile multimedia communications. The code division multiple access system with time division duplex mode (CDMA/TDD system), adopting unbalanced slot allocation between uplink and downlink, is a good solution for this traffic asymmetry. However, the level of traffic asymmetry may be significantly different from cell to cell. In this paper, we investigate a slot allocation strategy (DA strategy), by which each cell has its own slot allocation according to the level of traffic asymmetry. We compute the system capacity with DA strategy and find out the optimal slot allocation for the system. We also compare the maximum capacity to that with another strategy (SA strategy), by which all cells have the same slot allocation. As a result, this paper shows that the system with DA strategy outperforms the system with SA strategy in the aspect of capacity     

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.     

Suppressing opposite-direction interference in TDD/CDMA systems with asymmetric traffic by antenna beamforming

One of the key advantages for the time-division duplex (TDD) system is the capability to deliver asymmetric traffic services by allocating different numbers of uplink and downlink time slots. However, in a TDD/code-division multiple-access (CDMA) system, asymmetric traffic may result in severe opposite-direction interference because downlink transmitted signals from neighboring base stations may interfere with the uplink received signals of the home cell. In this paper, we investigate the effect of four-antenna beamforming schemes from the perspective of suppressing the opposite-direction interference. We compare the uplink bit energy-to-interference density ratio of a traditional beam-steering technique (Scheme I) with that of the minimum-variance distortionless-response (MVDR) beamformer (Scheme II). Furthermore, Scheme III applies the conventional beam-steering technique for both downlink transmissions and the uplink reception. In Scheme IV, we implement beam-steering for downlink transmissions, while adopting the MVDR beamformer to process the uplink signals received at base stations. Our numerical results indicate that Scheme IV outperforms all the other three schemes, which can effectively suppress the strong opposite-direction interference in TDD/CDMA systems. While keeping low implementation costs in mind, employing the simpler Scheme III in a sectorized cellular system can also allow every cell to provide different rates of asymmetric traffic services.     

海面的红外被动成像

本文在研究海面的红外成像特性中,建立了基于刚体的双尺度成像模型,从被动成像方面讨论了海面辐射特性的变化规律.该模型仿真结果很好地说明了风向对海面红外被动成像特性的影响.     

Design and implementation of a simple and efficient medium accesscontrol for high-speed wireless local area networks

This paper deals with the medium access control (MAC) subsystem which has been implemented in the MEDIAN project (wireless broadband LANs for professional and residential multimedia applications) belonging to the European Union's (EU's) advanced communications technologies and services (ACTS) program. The proposed MAC aims at providing a low complexity and efficient transport for asynchronous transfer mode (ATM) cells from a base station (BS), interfaced with an ATM local exchange (LEX), to ATM portable stations (PSs) and vice versa. A time division duplex (TDD) scheme is adopted in which uplink and downlink frame durations can be varied frame-by-frame, in order to fit the present uplink/downlink traffic mix. Moreover, in the uplink, a contention-free based technique is considered in which the slots are assigned to the calls depending on the present load of the PS uplink buffers     

Interference considerations for the time division duplex mode ofthe UMTS Terrestrial Radio Access

The air interface of the UMTS Terrestrial Radio Access (UTRA) covers both a frequency division duplex (FDD) part for the paired bands and a time division duplex (TDD) part for the unpaired bands of the UMTS spectrum. The Universal Mobile Telecommunication System (UMTS) is the 3rd-generation mobile communication system. This paper presents an interference evaluation of the UTRA TDD. Since both uplink and downlink share the same frequency in TDD, the signals of the two transmission directions can interfere with each other. This interference can occur between two mobile stations or between two base stations within one carrier or between two operators. The interference between uplink and downlink is evaluated by system simulations. Synchronization and coordination requirements of UTRA TDD are evaluated based on the results     

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.     

TDD场景下小基站蜂窝网络上下行交叉干扰建模及性能分析

针对小基站网络立体分布特性,提出了一种TDD制式下小基站蜂窝网络三维随机几何模型并进行了性能分析。首先,本文根据3-D PPP模型下小基站的空间分布特性推导了TDD制式下小基站网络上下行覆盖概率的数学表达式。接下来,基于覆盖概率的表达式和网络频谱效率的定义,分别推导了上下行交叉干扰场景下,小基站网络上下行频谱效率的一般表达式。仿真结果在验证了本文模型的合理性的同时还说明,随着下行小区占比和功率控制因子的增大,网络的频谱效率都呈下降趋势。除此之外,室内环境也会对网络的覆盖性能产生影响。      

Erlang capacity of a TDD-TD/CDMA architecture supportingheterogeneous traffic

This letter determines the capacity of a TDD-TD/CDMA architecture which supports different classes of subscribers and adopts an interference-driven admission control policy. The blocking probabilities of the system users are evaluated under various traffic conditions for several uplink/downlink configurations of the time slots, demonstrating that the time division full duplex approach needs careful tuning in order to maximize system capacity     

Optimum Joint Transmission (JT) Scheme in TD-SCDMA Downlink

1　Introduction　TD CDMAhasbeenselectedby 3GPPastheairin terfacestandardfortheTimeDivisionDuplexing(TDD)modeofthe 3GsystemInternationalMobileTelecommunications 2 0 0 0 (IMT 2 0 0 0 ) [1] .AlsotheChinese 3GairinterfaceTD SCDMAreliesontimeslot tedCDMAandTDDandiscloselyrelatedtoTD CD MA .TD SCDMAhasbeendecidedasthestandardforTDDmodeofthe 3rdGenerationmobileradiosystembyITU ,itisaimportantbreakthroughinacenturycom municationhistoryofChina .　InFDDmode ,Itneedawideenoughint…     

Network Performance Analysis of TCP-Based 3GPP LTE Time Division Duplex Systems

This paper deals with the determination of downlink (DL) and uplink (UL) channel split ratio for time division duplex (TDD) based long term evolution (LTE) networks. In a TDD system, UL and DL transmissions are carried out at different time intervals, but share the same frequency band. The TDD framing in LTE is adaptive in the sense that the DL to UL bandwidth ratio may vary with time. This paper proposes an adaptive split ratio (ASR) scheme for LTE networks to automatically adjust the bandwidth ratio of DL to UL, according to the current traffic profile, wireless interference, and transport layer parameters. This provides the maximum aggregate throughput in LTE systems. The performance analysis shows that ASR scheme outperforms static allocation in terms of higher aggregate throughput and better adaptively to network dynamics. Further, it is also observed that the ASR scheme performs well for LTE, compared to worldwide interoperability for microwave access (WiMAX) system.     

Supporting Asymmetric Traffic in a TDD/CDMA Cellular Network via Interference-Aware Dynamic Channel Allocation and Space–Time LMMSE Joint Detection

In a time-division duplexing (TDD)/code-division multiple-access (CDMA) cellular network with asymmetric data traffic, dynamic channel allocation (DCA) enhances resource utilization compared with fixed channel allocation (FCA). However, it also induces base-to-base and mobile-to-mobile crossed-slot intercell interference, which can severely degrade system performance. To deal with this problem, a decentralized scheme is proposed, which combines an interference-aware DCA algorithm with space–time linear minimum-mean-square-error (LMMSE) joint detection at the base and mobile stations. The former assigns active links to timeslots in a way that crossed-slot interference is mitigated, while the latter suppresses the remaining intercell interference (along with intersymbol and intracell interference), exploiting its spatio-temporal autocorrelation statistics. The performance of this scheme is evaluated in terms of downlink and uplink signal-to-interference-plus-noise ratio (SINR) outage and average throughput via analytical approximations and Monte Carlo simulations, and it is compared with that of benchmark random DCA (RDCA) and FCA schemes. The cases of single- and dual-antenna reception with perfect and imperfect channel state information are examined. It is shown that the proposed scheme achieves higher average throughput than FCA (particularly for dual-antenna reception) as well as RDCA (for heavy traffic loads). These throughput gains are more significant in uplink than in downlink.      

智能天线在CDMA2000 1x EV-DO中的实现方案

讨论了智能天线技术及其在CDMA2000 1x EV—DO标准中的应用,否定了有关智能天线应用的传统观点：智能天线最适合用于上下行链路同频点的TDD（时分双工）系统。依据1x EV—DO标准支持上下行链路工作于FDDCDMA／TDMA模式和智能天线（SA）的定向收发信原理,对智能天线技术在CDMA2000 1x EV—DO标准中的实现提出了两种实现方案。两种方案需要估计出上行链路用户的DOA后,即可用一副SA实现单一载波上多个CDMA用户的定向接收,再将该DOA及时用于下行高速数据载波多个TDMA用户的定向发信,可大幅度提高上下行链路的频谱和功率的利用率。     

Robust beamforming for multicell downlink TDD massive MIMO system based on QoS

In view of multicell downlink time division multiplexing (TDD) massive multiple-input multiple-output (MIMO) systems which had imperfect channel state information (CSI),the beamforming problem that minimized the total transmit power and signal leakage power based on quality of service (QoS) was studied.First,the objective problem was approximated as a standard convex optimization problem.Then,by using the duality of uplink and downlink,an inner and outer layer iterative algorithm was proposed.Numerical results show that,comparing with other typical downlink multicell massive MIMO beamforming algorithms,the proposed algorithm has obvious advantages in terms of complexity and energy efficiency.     

TDD-CDMA for the 4th generation of wireless communications

This article discusses an evolutionary TDD mode of CDMA-based path for 3.5G/4G systems. This technology has already been the basis of two 3G standards: TD-CDMA and TD-SCDMA with a synchronous uplink. Several techniques have been developed that allow TDD-CDMA-based systems to transmit at rates usually associated with 3.5G to 4G, up to 10-20 Mbit/s with wide area coverage. The present TDD-CDMA-based 3G standards are evolving to provide these higher rates, while fully retaining the coverage and mobility associated with, and expected from, the present mobile communications systems. This article discusses how TDD-CDMA specific techniques facilitate delivering services at data rates associated with 3.5G and 4G. We review how TDD-CDMA can provide for asymmetric uplink and downlink transmissions, facilitate deployment of advanced antennas for improved downlink coverage, and enable provision of advanced receiver techniques in base stations and mobiles. We also discuss how these techniques affect systems capacity in full packet-switched IP-based systems. We then discuss 4G TDD CDMA systems: those with different modulation techniques for uplink and downlink communications. These are generally based on a multicarrier mode of CDMA, and may incorporate OFDM technique.     

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.     

A multicarrier CDMA architecture based on orthogonal complementarycodes for new generations of wideband wireless communications

This article is a review of our ongoing research effort to construct a new multicarrier CDMA architecture based on orthogonal complete complementary codes, characterized by its innovative spreading modulation scheme, uplink and downlink signaling design, and digital receiver implementation for multipath signal detection. There are several advantages of the proposed CDMA architecture compared to conventional CDMA systems pertinent to current 2G and 3G standards. First of all, it can achieve a spreading efficiency (SE) very close to one (the SE is defined as the amount of information bit(s) conveyed by each chip); whereas SEs of conventional CDMA systems equal 1/N, where N denotes the length of spreading codes. Second, it offers MAI-free operation in both upand downlink transmissions in an MAI-AWGN channel, which can significantly reduce the co-channel interference responsible for capacity decline of a CDMA system. Third, the proposed CDMA architecture is able to offer a high bandwidth efficiency due to the use of its unique spreading modulation scheme and orthogonal carriers. Lastly, the proposed CDMA architecture is particularly suited to multirate signal transmission due to the use of an offset stacked spreading modulation scheme, which simplifies the rate-matching algorithm relevant to multimedia services and facilitates asymmetric traffic in up- and downlink transmissions for IP-based applications. Based on the above characteristics and the obtained results, it is concluded that the proposed CDMA architecture has a great potential for applications in future wideband mobile communications beyond 3G, which is expected to offer a very high data rate in hostile mobile channels     

Integrated load‐based power saving for real‐time and nonreal‐time traffic in LTE‐TDD

Long‐Term Evolution (LTE) is a 4G wireless broadband technology developed by the Third Generation Partnership Project. Two duplex modes, namely, frequency division duplex and time division duplex (TDD), are defined in LTE for transmission in both downlink and uplink directions simultaneously. Power saving mechanisms for LTE‐frequency division duplex were proposed in the authors' previous work. Applicability of the previously proposed mechanisms to LTE‐TDD is investigated in this paper, and the idea of “virtual time” associated with the mapping mechanism from the virtual time domain to the actual time domain for different TDD configurations is proposed. With the help of the mapping mechanism, 3 revised power saving schemes are proposed to support real‐time user equipments and nonreal‐time user equipments in LTE‐TDD. Simulation study demonstrates the effectiveness of the mapping mechanism as well as the benefit of the proposed schemes in power saving efficiency and real‐time support in comparing with the standard‐based mechanism.     

Traffic and Interference Adaptive Scheduling for Internet Traffic in UMTS

In this paper we propose a scheduling strategy for the radio resources management when transmitting Internet traffic over third-generation systems. More precisely, we consider the UMTS Terrestrial Radio Access Network (UTRAN) Time Division Duplex (TDD) mode standardized by ETSI. UTRAN TDD uses a hybrid solution of code and time division multiple access, called TD-CDMA. In UMTS systems a key issue in developing access methodologies for the available spectrum is an optimal management of the rare radio resources. In this paper we present a fair and efficient scheduling algorithm that adapts its behavior to traffic and interference conditions. Specifically, our scheduling algorithm is able to manage the radio resources taking into account both the traffic fluctuations in the uplink and downlink direction and the variations of the system interference. The goal of our scheduler is the data-throughput maximization for an efficient utilization of available radio resources. The effectiveness of our scheduling algorithm is shown by exploiting analytical tools.