Dynamic TDD Fixed Cellular Systems Using Smart and Sectored Antennas

There are many benefits in using time division duplex (TDD) instead of frequency division duplex (FDD) schemes in fixed wireless cellular systems. For example, channel reciprocity for a single carrier frequency used on both uplinks and downlinks will allow easy access to channel state information, reduced complexity of RF design, much higher flexibility in handling dynamic traffic, simpler frequency plan, etc. However, there exists a serious limiting factor in using dynamic TDD (D-TDD) in cellular systems. This is due to a steady interference on an uplink in any cell caused by downlink transmissions in other cells. Simulation results show in D-TDD cellular systems, performance is unacceptable, when an omnidirectional antenna is used at base stations. Simulation results also suggest great potential for smart antennas in achieving substantial performance improvement in fixed D-TDD bandwidth-on-demand wireless systems.     

Dynamic TDD and fixed cellular networks

There are many benefits in using time division duplex (TDD) instead of frequency division duplex (FDD) schemes in fixed wireless cellular systems. To name a few; channel reciprocity for the single carrier frequency used on both up and downlinks will allow easy access to channel state information, reduced complexity of RF design, much higher flexibility in handling dynamic traffic, simpler frequency plan, etc. However, there exists a serious limiting factor in using dynamic-TDD (D-TDD) in cellular systems. This is due to a steady interference on uplinks in a cell, caused by downlink transmissions of other cells. Simulation results show in D-TDD cellular systems, performance is unacceptable, when omni-directional antennas are used at base stations. Simulation results have also demonstrated a great potential for smart antennas in fixed D-TDD bandwidth-on-demand wireless systems     

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.     

Error performance of differentially coherent detection of binary DPSK data transmission on the hard-limiting satellite channel

The error performance of differentially coherent detection of a binary differential phase-shift keying (DPSK) system operating over a hard-limiting satellite channel is derived. The main objective is to show the extent of error rate degradation of a DPSK system when a power imbalance exists between the two symbol pulses that are used in a bit decision interval. Consideration is also given to the DPSK error rate performance for the special case of {em uncorrelated} uplink and {em correlated} downlink noises at the sampling instants in adjacent time slots. Error probabilities are given as functions of uplink signal-to-noise ratio (SNR) and downlink SNR with different levels of SNR imbalance and different downlink SNR and uplink SNR as parameters, respectively. Our numerical results show that 1) as long as the symbols are equiprobable, the error probability is not dependent upon the downlink noise correlation, regardless of whether there is a power imbalance; 2) error performance is definitely affected by the power imbalance for all cases of symbol distributions; and 3) the error probability does depend upon downlink noise correlation for all levels of power imbalance if the symbol probabilities are not equal.     

Cochannel Interference in Cellular Fixed Broadband Access Systems with Directional Antennas

The Cellular Fixed Broadband Access System for LMCS (Local Multipoint Communication Services) is aiming to provide broadband services to fixed subscribers through wireless access. A representative LMCS scenario involves a cellular system design with the entire allocated bandwidth being used in each cell and with a highly directional antenna employed at the subscriber. The LMCS system designs rely on assumed information regarding radio propagation phenomena in the anticipated coverage areas. The research reported here, therefore, has the objective of verifying such information by theoretical modeling and simulation. Simulation and analysis have been used to study the effects of a number of factors on the system performance and interference characteristics of such a LMCS system. The investigated factors include beamwidth and gain ratio of directional antennas, sectorization of hub antennas, propagation exponent, lognormal shadowing, site diversity and transmitter power control. Both models of LOS (line of sight) and NLOS (non-line of sight) have been investigated in this work. The simulation results demonstrate that highly directional antennas at the subscribers can dramatically improve the system performance for both uplink and downlink. The system design with frequency reuse of one is examined, and it is confirmed that an outage of 0.9% for a NLOS system could be achieved by using narrow beam antennas of 3 degrees with site diversity and power control techniques employed. Under the condition of LOS available for the desired links, the outage can be expected to be as low as 0.3%.     

Analysis of smart antenna outage in UTRA FDD networks

The outage occurring from beamformer-based smart antennas is analysed for an antenna array deployed in an urban macro-cell environment. Spatio-temporal channel data obtained from a small, urban cell and from within the UTRA frequency allocation has enabled beamforming performance to be determined from the measured uplink and downlink frequency bands. The analysis shows a sub-optimum uplink beamformer performance is observed for 8% of occasions and a 25% downlink beamforming outage for an eight-element uniformly spaced linear array     

Analysis of a multiple-cell direct-sequence CDMA cellular mobileradio system

The performance of a multiple-cell direct-sequence code division multiple-access cellular radio system is evaluated. Approximate expressions are obtained for the area-averaged bit error probability and the area-averaged outage probability for both the uplink and downlink channels. The analysis accounts for the effects of path loss, multipath fading, multiple-access interference, and background noise. Two types of differentially coherent receivers are considered: a multipath rejection receiver and a RAKE receiver with predetection selective combining. Macroscopic base station diversity techniques and uplink and downlink power control are also topics of discussion     

A unified wireless LAN architecture for real-time and non-real-timecommunication services

This paper addresses how to support both real-time and non-real-time communication services in a wireless LAN with dynamic time-division duplexed (D-TDD) transmission. With D-TDD, a frequency channel is time-shared for both downlink and uplink transmissions under the dynamic access control of the base station. The base station (1) handles uplink transmissions by polling mobiles in a certain order determined on a per-connection (per-message) basis for transmitting real-time (non-real-time) traffic from mobiles and (2) schedules the transmission of downlink packets. To handle location-dependent, time-varying, and bursty errors, we adopt the channel-state prediction, transmission deferment, and retransmission. We consider the problems of scheduling and multiplexing downlink packet transmissions, and polling mobiles for uplink transmissions depending on the channel state. We also establish conditions necessary to admit each new real-time connection by checking if the connection's delivery-delay bound can be guaranteed as long as the channel stays in good condition without compromising any of the existing guarantees. Last, the performance of the proposed protocol is evaluated to demonstrate how the protocol works and to study the effects of various parameters of the protocol     

System Performance Enhancement in CDMA Mobile System with Uplink Adaptive Power Control Based on Fuzzy Logic

In this paper, an uplink adaptive SIR (signal-to-interference)-based powercontrol mechanism fora CDMA mobile system using fuzzy logic techniques is developed and analyzed.The proposed algorithm providesbetter system performance in terms of outage probability, response time, andstability than current power controlalgorithms and has the advantage of easy implementation.     

阴影Nakagami-m衰落信道条件下分布式天线系统中断分析

This paper presents an outage analysis of distributed antennas system (DAS) suffering from shadowed Nakagami-m fading environment where the desired signal also suffers from cochannel interference. The desired signal and interfering signal are subjected to path loss, multipath and shadowing fading. Based on Wilkinson's method, the signal to interference ratio (SIR) probability density function ( PDF) of fixed DAS is obtained. Some numerical results of outage probability with different parameters are analyzed. The analysis results can provide sufficient precision for evaluating the outage performance of DAS     

中断概率在TD-LTE信道资源分配中的应用

TD-LTE(时分同步码分多址-长期演进)系统业务呈非对称状态,对下行数据率的要求要超过上行数据率,因而对系统信道上下行资源分配的要求更为严格。文章利用中断概率接纳控制算法对系统信道资源进行了智能动态分配,通过系统仿真证明了该策略能够灵活分配系统时隙资源。     

多天线两小区蜂窝系统的低开销线性干扰对齐算法

在蜂窝系统中,由于干扰的存在,用户性能受到影响,特别是对于小区边缘用户,其通信质量较差。干扰对齐作为一种能够消除干扰、提高系统容量的技术,近年来受到广泛关注。针对多天线两小区蜂窝系统的边缘用户,本文给出了一种系统开销小、需要天线数少的线性干扰对齐算法。该算法利用发送端预编码矢量消除小区内干扰,利用接收端干扰抑制矢量消除小区间干扰。采用本文算法后,在每个小区有 个用户、基站有 根发送天线、用户有 根接收天线的情况下,只需 和 就可以实现上下行的干扰对齐,整个系统可以达到 的自由度,并且在下行链路中不需要小区间反馈,而在上行链路中只需要较小的小区间反馈。仿真结果表明,本文提出的算法能够以较小的开销实现比以往的算法更好的性能。      

SIR-based call admission control for DS-CDMA cellular systems

Signal-to-interference ratio (SIR)-based call admission control (CAC) algorithms are proposed and studied in a DS-CDMA cellular system. Residual capacity is introduced as the additional number of initial calls a base station can accept such that system-wide outage probability will be guaranteed to remain below a certain level. The residual capacity at each cell is updated dynamically according to the reverse-link SIR measurements at the base station. A 2^k factorial experimental design and analysis via computer simulations is used to study the impact of the parameters used in the algorithms. The influence of these parameters on system performance, namely blocking probability and outage probability, is then examined via simulation. The performance of the algorithms is compared together with that of a fixed call admission control scheme (fixed CAC) under both homogeneous and hot spot traffic loading. The results show that SIR-based CAC always outperforms fixed CAC even under overload situations, which is not the case in FDMA/TDMA cellular systems. The primary benefit of SIR-based CAC in DS-CDMA cellular systems, however, lies in improving the system performance under hot spot traffic     

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.      

Partial Processing Satellite Relays for Frequency-Hop Antijam Communications

Military satellite communications systems may be implemented with varying levels of satellite on-board processing. For frequency-hop (FH) spread-spectrum signals two options are the dehop-rehop transponder (DRT) and the symbol regenerative processor (SRP). This paper considers FH,M-ary FSK modulation in the presence of full band and worst case partial-band noise jamming. The relationships among end-to-end signal-to-noise ratio (SNR) performance or probability of bit errors and the uplink and downlink SNR are derived. For a DRT, an additional parameter is the satellite filter bandwidth Wssince this may differ from the despread modulation bandwidth Wrfor a particular user access under TDMA variable-data-rate operation. The SNR performance in this case depends upon the ratioW_{s}/W_{r}a well as the uplink and downlink SNR. The resulting penalty in the link budget of the weaker of the uplink or downlink is much less than the mismatched ratioW_{s}/W_{r}and diminishes as the links become unbalanced. The SRP slightly outperforms the DRT but it is more complex and does not as easily accommodate variable data rates.     

带内全双工多用户MIMO系统中的用户选择

考虑一种多用户MIMO的传输设计,配置发射天线阵列和接收天线阵列的蜂窝基站可以工作在带内全双工传输模式。在该全双工通信方案中,基站的下行发射信号对基站的上行接收产生显著的干扰,即自干扰。这里,下行预编码处理和上行发射协方差矩阵处理将被依次进行,以简化全双工的设计。其次,为了进一步改善上、下行信道的和速率性能,我们提出一种尝试性的下行用户选择方案,其基本思想是:当某一个下行用户的信道矩阵的范数较小时,关闭该下行用户的数据流。计算机仿真结果表明,在基站下行总发射功率受限时,在低的和中等的下行信噪比区域,用户选择有助于提高下行和速率;在高的上行信噪比区域,简化的用户选择使得上行和速率明显提高。      

Performance Analysis of Maximum SNR Scheduling with an Infrastructure Relay Link

We consider infrastructure-based amplify-and-forward (AF) relaying for extending downlink and uplink coverage areas of a cellular base station. The base station serves multiple mobile users via a multi-hop backhaul relay link by sharing out access link channel resources with maximum signal-to-noise ratio (SNR) scheduling. We analyze the performance of the system by deriving closed-form expressions for outage probability, outage capacity, ergodic capacity, average end-to-end SNR and amount of fading (AoF). These measures show that maximum SNR scheduling of multiple users in a cellular relay link offers significant diversity, capacity and SNR improvement over single-user transmission and round robin scheduling. We also relate performance of the relay link to that of a distributed antenna system (DAS), and show that the noisy wireless backhaul relay link induces tolerable performance deterioration compared to deploying a cable-connected distributed antenna.     

基于双向中继和网络编码的NOMA网络性能分析

针对实际场景中存在的具有上下行双向传输任务的通信系统,本文提出了一种双向中继协作非正交多址接入(NOMA, non-orthogonal multiple access)传输方案,基于解码转发(DF, Decode and Forward)协议研究信号的上行和下行双向传输技术,与现有NOMA方案不同,本方案为近端用户分配较大的功率,利用网络编码(NC, network coding)原理在两个时隙内实现基站和用户之间的双向信息交换。进一步考虑不完美信道状态信息(CSI, Channel State Information)条件,分析系统的传输性能并推导了系统中断概率以及遍历和速率闭合表达式。仿真结果表明,在完美CSI和不完美CSI条件下,相比于现有文献所提方案、单向中继(OWR,One-Way Relay)和正交多址(OMA, Orthogonal Multiple Access)网络,本文所提方案有效降低了系统的传输中断概率,提高了系统的遍历和速率以及系统吞吐量。      

Resource allocation for outage performance in heterogeneous networks: a matching game approach

In heterogeneous cellular networks (HCNs), the deployed small cell base stations share the same spectrum resource with that of macro base station, which causes cross-tier interferences and the performance depreciation of the system. This paper investigates the downlink outage performance of HCNs and its resource allocation problem to mitigate interferences. Specifically, we first derive closed-form expressions of overall outage probability of the system. Then, the resource allocation problem is proposed to reduce the outage probability of HCNs. This problem is formulated as a matching game, and a stable matching is considered to be the solution. Finally, to address this matching problem, a distributed algorithm is proposed which can find a stable matching. Both analytical results and simulations show that the proposed algorithm improves the overall performance of HCNs.     

Bridging the gap between dynamic and static methods for cellplanning

Frequency reuse distance is a crucial parameter in cellular system design that determines the service quality, which is usually measured in terms of the signal-to-interference ratio (SIR). The instantaneous SIR is a random variable; theoretically, its characterization is completely provided by the outage probability. In cellular engineering, however, frequency reuse planning is usually based on a simple static design, without taking into account the dynamic propagation effects. The static method has the advantage of simplicity, but failing to indicate the system dynamic performance. In this paper, we determine a simple mathematical expression between the static and dynamic SIRs whereby a new cell planning technique is developed. The new technique makes it possible to incorporate dynamic considerations into a static design thereby bridging the gap between the two methods. We also use the new results to study the system capacity, revealing the dependence of the system capacity on the minimum required outage probability