Performance issues and algorithms for dynamic channel assignment

Using dynamic channel assignment (DCA) algorithms to select communications channels as needed, time-division multiple access (TDMA) or frequency-division multiple access (FDMA) systems can serve dynamic and nonuniform traffic demands without frequency planning as long as quality is sufficient and equipment is available. Here, performance issues and algorithms for DCA in a TDMA portable radio system are considered. A fixed number of traffic servers (time slots) per radio port is assumed: therefore, the system capacity is hard-limited by the equipment availability, and the DCA efficiency is compared mainly through the signal-to-interference ratio in both the uplink and downlink directions     

CDMA移动卫星通信系统容量的计算

本文在考虑了下行功率通量密度限制、遮蔽效应、多个系统和相邻波束同频复用、采用话音激活和极化隔离技术的前提下,分别计算和分析了不同下行功率通量密度限制方式、不同信道扩频带宽和不同移动用户接收系统品质因数(G/T)条件下CDMA移动卫星通信系统的容量,并与TDMA和FDMA的容量进行了比较,在最后给出了主要结论。     

THE CAPACITY CALCULATION FOR CDMA MOBILE SATELLITE COMMUNICATION SYSTEM

On the basis of considering downlink power flux density limit, shadowing effect frequency reuse in different systems and adjoining beams of same system, adopting voice activity and polarization isolation techniques, the capacity of CDMA mobile satellite communication system at different downlink power flux density limit modes, channel spread bandwidth and mobile user's quality factor of receiving system(G/T) are analysed and calculated, and compared with the ones of TDMA and FDMA. The conclusions are given at the end of this paper.     

A satellite switched CDMA system for fixed service communications

Satellite-switched code-division multiple access is a system proposed for geostationary fixed satellite service. SS/CDMA provides both multiple access and switching to a multibeam satellite. In this article we present the system architecture and performance. The SS/CDMA consist of the common air interface (CAI) and the onboard code switching mechanism. The CAI provides signaling control and traffic channels. Traffic channel access and modulation is based on a spectrally efficient CDMA scheme. The satellite code-division switch (CDS) routes calls from an uplink to a downlink beam without onboard demodulation and channel decoding. This system operates with demand assignment control; that is, channel bandwidth and switch connections are only assigned per user request. The system can offer a wide variety of bit rates with wireline quality of service. The services provided are circuit-switched calls for voice data and video, and packet-switched for data. As shown, the proposed SE-CDMA can achieve high capacity and very low bit error rates at low signal-to-noise ratio. Also, the innovative switching technique can provide low complexity and achieve high throughput for all services     

The performance evaluation of spatial–temporal algorithms in W‐CDMA systems

Differing from FDMA, TDMA, and CDMA, space division multiple access (SDMA) uses space resources to improve communication system performance. Utilizing the smart antenna system is an approach to realize the SDMA technique. Smart antenna systems using the beamforming technique can reduce the co‐channel interference and multipath fading to increase the channel capacity and communication quality. In this study the smart antenna system and rake receiver are integrated. The performance of spatial–temporal structure applied to the W‐CDMA system is evaluated. From the cumulative distribution function simulation results, W‐CDMA system with spatial–temporal algorithm can exactly provide SINR gain to improve the system performance and capacity. Copyright © 2005 John Wiley & Sons, Ltd.     

具有双层系统的分扇区CDMA小区上行链路容量

码分多址系统与传统的时分多址和频分多址系统相比,在各方面都具有优势,尤其是能提供更好的服务质量和更大容量,因而成为第三代移动通信系统的核心技术。通过在小区中划分扇区能进一步提升系统容量,在扇区中又引入双层系统,可以显著提高该扇区内容量。由于CDMA系统是噪声受限系统,因此以信干比为标准,分析具有双层系统的分扇区CDMA小区系统的上行链路容量。最后通过仿真结果证明了该系统上行链路容量相对普通小区系统上行链路容量的提升。     

Channel segregation-a self-organized dynamic channel allocationmethod: application to TDMA/FDMA microcellular system

Channel segregation was previously proposed as a self-organized dynamic channel assignment. Its performance was examined by applying it to frequency-division multiple access systems. Its applications to the TDMA/FDMA (time-division multiple access/frequency-division multiple access) or multicarrier TDMA system is discussed. The spectrum efficiency of the TDMA/FDMA cellular system deteriorates due to the problem of unaccessible channel: a call can be blocked in a cell even when there are idle channels because of the restriction on simultaneous use of different carrier frequencies in the cell. It is shown that channel segregation can resolve this problem with a small modification of its algorithm. The performance of the TDMA/FDMA system with channel segregation on the call blocking probability versus traffic density is analyzed with computer simulation experiments. The effect of losing the TDMA frame synchronization between cells on the performance is also discussed     

Capacity unbalance between uplink and downlink in spectrallyoverlaid narrow-band and wide-band CDMA mobile systems

In this paper, we discuss capacity unbalance between uplink (mobile-to-base) and downlink (base-to-mobile) in future code division multiple access (CDMA) radio networks where both narrow-band and wide-band CDMA systems are coexisted. Since the two links are not operated in an identical condition, their capacities are unequal and either of the links determines the whole system capacity. The purpose of this paper is to examine which link limits the system capacity and what are the limiting factors. To facilitate the examination, “transmission capacity” and “connection capacity” are defined, and simplified formulas are presented to compute those capacities, respectively for uplink and downlink. Signal quality required for each link, effectiveness of power control, spatial distribution of mobile users and other-cell as well as same-cell user interference are usually determining the limiting link. Besides, the traffic unbalance between the links imposed by specific service applications and the network evolution scenarios are also shown to be very influencing factors     

3G主流标准小区用户最大容量研究

文章根据CDMA系统多用户干扰产生原理导出小区上下行链路用户速率为Rb时的最大用户数公式,据此公式可求出WCDMA、cdma2000和TD-SCDMA标准能提供的小区上下行链路最大用户数,接着导出小区上下行链路可以提供的最大速率Rbmax。文章结果证明只有采用TDMA方式的cdma20001xEV-DO或EDGE方式才能提供合理的高速无线接入Rb。     

Delay and throughput characteristics of TH, CDMA, TDMA, and hybridnetworks for multipath faded data transmission channels

The new concepts of adaptive time hopping and variable frame code division (CDMA) multiple access are introduced. By a unified analysis, the probabilities of bit and packet errors in multipath fading environment for five time division (TDMA), code division, and time hopping (TH) related multiaccess networks are obtained; namely, TDMA, CDMA, CDMA/TDMA, Adaptive CDMA/TH, and variable frame CDMA/TDMA networks. The delay and useful throughputs of the five systems are also evaluated for data and voice traffic. All systems compared have the same channel power and bandwidth and support the same traffic. Though implementation issues are not covered, CDMA systems are put at a disadvantage (compared to cellular-type FDMA networks, for example) by ignoring such inherent advantages as voice silence utilizations and automatic frequency reuse. Nontheless, two CDMA systems outperform TDMA systems at low and medium input traffics     

A comparison of frequency-division systems to code-division systems in overloaded channels

In this paper, a frequency-division counterpart of joint power control and sequence design problem for code- division multiple-access (CDMA) systems is solved. Total transmit and receive power minimizations are considered for frequency- division multiplexing (FDM) and frequency-division multiple- access (FDMA) communications over overloaded channels. After the definition of channel overloading for CDMA systems is extended to the frequency-division systems, the user admissibility is characterized by a necessary and sufficient condition for the existence of the optimal solution under unequal signal-to- interference-plus-noise ratio constraints at the output of linear receivers and asymmetric data transmission rate constraints among users. The optimal signal power, bandwidth, transmit waveform, and receive waveform are derived for each user as the decision parameters of the optimization problem. It is shown that, if this solution is applied for the uplink users to minimize the total receive power, the optimal FDMA system performs the same as the optimal CDMA system. It is also shown that, if this solution is applied for the downlink users to minimize the total transmit power, the optimal FDM system always outperforms the code-division system that minimizes the extended total squared correlation. Numerical results suggest that the optimal FDM system and the optimal downlink code-division system achieve the same performance when the total transmit power is minimized.     

Performance of a cellular hybrid C/TDMA mobile radio systemapplying joint detection and coherent receiver antenna diversity

For future mobile radio systems, an appropriately chosen multiple access technique is a critical issue. Multiple access techniques presently under discussion are code division multiple access (CDMA), time division multiple access (TDMA), and hybrids of both. In the paper, a hybrid C/TDMA system using joint detection (JD-C/TDMA) with coherent receiver antenna diversity (CRAD) at the base station (BS) receiver is proposed. Some attractive features of the JD-C/TDMA system are the possibility to flexibly offer voice and data services with different bit rates, soft capacity, inherent frequency and interferer diversity, and high system capacity due to JD. Furthermore, due to JD, a cluster size equal to 1 can be realized without needing soft handover. The single cell E_b/N₀ performance and the interference situation in a cellular environment of the uplink of a JD-C/TDMA mobile radio system with CRAD is investigated in detail. It is shown that the cellular spectrum efficiency is remarkably high, taking values up to 0.2 bit/s/Hz/BS in the uplink, depending on the actual transmission conditions     

The capacity of a spread spectrum CDMA system for cellular mobileradio with consideration of system imperfections

There has been much interest in the use of spread spectrum code division multiple access (CDMA) techniques for cellular mobile radio. To date, spread spectrum has been used mainly for military applications, in which the inherent transmission security and immunity to deliberate jamming are important. Spread spectrum systems, however, possess various other features such as multiple access and multipath rejection capability, which make their use attractive within the mobile radio environment. However, the current interest has been principally motivated by the work of Gilhousen et al. (see IEEE Trans. Vehic. Technol., vol.VT-40, no.2, p.303, 1991) in which it is claimed that the CDMA option may offer capacity improvement over more conventional frequency and time division multiple access, FDMA and TDMA, techniques. Within this paper, the relative capacities of a basic FDMA and CDMA system are examined. It is shown that, in the absence of capacity-enhancing features such as voice activity detection and cell sectorization, the capacity of each system is comparable. The paper then assesses the sensitivity of the CDMA system to typical propagation conditions, power control errors, and realistic antenna patterns and shows that the capacity of a CDMA system may be significantly reduced under nonideal conditions     

SDMA系统FD方案阻塞率的仿真

现代无线移动通信系统广泛考虑采用空分多址（SDMA）多址接入技术来提高系统容量。比如在频分复用（FDMA）时分复用（TDMA）,码分复用（CDMA）。分析研究SDMA系统中的FD信道分配机制,给出了理论推导,并在此基础上进行了仿真。仿真结果表明：采用该机制以后,用户可以在同一时间共享同一信道。     

CDMA／SDMA系统性能分析

空分多址接入技术（SDMA）可以通过信号不同的空间传播路径来区分用户，从而提高了蜂窝移动通信系统容量．而此技术又可以和其他多址方式相互兼容，比如频分多址，时分多址，码分多址．CDMA／SDMA系统就是码分多址和空分多址两种接入技术相结合的系统．该系统可通过时空编码实现的．文中分析此种系统的性能，并与多用户环境下的CDMA系统进行了比较．     

CDMA/TDD system for wireless multimedia services with trafficunbalance between uplink and downlink

This paper proposes a code division multiple access (CDMA) time division duplex (TDD) system for wireless multimedia services with traffic unbalance between uplink and downlink. In the proposed system, the number of uplink time slots in a TDD frame differs from that of downlink. Moreover, the difference can be reset by the network operator according to the traffic pattern. We evaluate the performance of the proposed system under multimedia environment using Markov analysis and computer simulation. The results show that the frequency utilization is maximized even when the uplink and downlink traffic volumes are unbalanced. This, in turn, reduces drastically the blocking rate of multimedia calls (connections) in the proposed system compared with that in the traditional CDMA systems where the uplink and downlink use equal bandwidth     

Downlink channel covariance matrix (DCCM) estimation and itsapplications in wireless DS-CDMA systems

The downlink channel covariance matrix (DCCM) is of vital importance in determining downlink beamforming weights for base station (BS) antenna array systems. For the frequency-division-duplex (FDD) mode, DCCM is difficult to obtain due to a lack of direct measurement of downlink channel responses. In this paper, a novel technique is proposed for estimating DCCM using uplink channel responses only, which does not need direction-of-arrival (DOA) estimation and its association. The downlink beamforming scheme is then proposed for wireless DS-CDMA systems, using the obtained DCCM information together with the so-called virtual uplink beamforming and power control technique. Computer simulations show that using the BS antenna array together with this new beamforming technique can provide larger system capacity than traditional DOA-based approaches, which just direct the main beam toward the desired user     

Uplink user capacity in a CDMA system with hotspot microcells: effects of finite transmit power and dispersion

This paper examines the uplink user capacity in a two-tier code division multiple access (CDMA) system with hotspot microcells when user terminal power is limited and the wireless channel is finitely-dispersive. A finitely-dispersive channel causes variable fading of the signal power at the output of the RAKE receiver. First, a two-cell system composed. of one macrocell and one embedded microcell is studied and analytical methods are developed to estimate the user capacity as a function of a dimensionless parameter that depends on the transmit power constraint and cell radius. Next, novel analytical methods are developed to study the effect of variable fading, both with and without transmit power constraints. Finally, the analytical methods are extended to estimate uplink user capacity for multicell CDMA systems, composed of multiple macrocells and multiple embedded microcells. In all cases, the analysis-based estimates are compared with and confirmed by simulation results.     

Capacity Comparison of CDMA and TDMA in Vector Broadcast Channel

1　Introduction　MultipleInputMultipleOutput (MIMO)havereceivedagreatdealofattentionasamethodtoachievelargecapacityandhighreliabilityoverwire lesslink .Accordingtovectorbroadcastchannel,thebasestationanduserinthissystembothhavemulti pleantennas.Costa[1 ] hadpointedoutthatifthein terferencesignalwasindependentoftransmitsignalanditwasknownatthetransmitterthenthechan nelcapacitywiththeinterferenceisthesameasiftheinterferencewasnotexist.　Thoughthepreconditionofcosta sresultisscalarchannel.The…     

Traffic engineering of the radio interface for cellular mobilenetworks

The paper reviews traffic-related issues in the operation of the radio interface in cellular mobile networks operating with FDMA, TDMA, or CDMA technology. The emphasis is on issues which affect the traffic performance of the network, rather than on transmission technology, or transmission quality measures such as bit error rate. The author reviews the important traffic performance measures for cellular systems and the issues of channel assignment, handoff, access control, traffic modeling, and traffic management. Traffic modeling for all types of networks is considered in some depth, both by drawing analogies with fixed-wire networks, and by considering cellular-specific modeling. From the modeling results, he concludes that reuse-based networks such as FDMA and TDMA possess quite different traffic characteristics from CDMA networks