最大比宏分集下的CDMA系统反向容量分析

本文综合考虑快衰落,阴影衰落和路径衰减的影响,建立了最大比宏分集下CDMA系统反向信道分析模型并推导出了中断率(outage probability)表达式.数值结果表明,采用宏分集后反向容量随参与宏分集基站数的增加而大大提高了,而且移动台的反向性能和其位置有关,在小区边界处性能最佳.     

CDMA系统中的空时发射分集方案及其在MIMO MC-CDMA中应用的研究

 本文结合了空时发射分集技术能够有效地对抗信道衰落、提高系统容量和多载波码分多址(MC CDMA)具有高效频谱利用率、对频率选择性信道鲁棒性的优点,提出了CDMA系统中两种新的空时发射分集方法,研究了这两种空时发射分集方法在MIMO MC-CDMA系统中的应用,并与基于空时分组编码的发射分集方法分别在MIMO CDMA和MIMO MC-CDMA两种系统下进行了仿真比较.     

TD-SCDMA集群系统中部分宏分集组网前向容量分析

TD-SCDMA集群小区使用宏分集技术可以有效的克服大尺度衰落,保证信号质量。本文首先分析了在没有任何措施的前提下进行宏分集组网,但前向容量会随着宏分集基站数的增多而减少。在分析容量损失原因的基础上,我们进一步提出在终端利用Rake接收机对各路信号进行同步、相位调整,再进行最大比值合并,然后发送给用户,由此,前向容量大大提高。     

CDMA2000中的发送分集技术

在第三代移动通信系统中,发送分集技术被用来提高前向链路的接收性能,因而介绍了在CDMA2000系统中使用的分集传送方式0TD和STS,分析了分集的原理和合并的方法,并给出了在三种典型信道下采用不同分集技术时系统性能的仿真结果.     

前向正交发送分集技术OTD与STS

OTD和STS技术是在CDMA2000中采用的前向正交发送技术，用来提高前向链路的接收性能。本文分析了其分集原理并提出统一的Walsh空间合并方法，最后给出在三种典型信道下普通方式和分集方式时系统性能的仿真结果。     

TD-SCDMA集群通信系统中二基站宏分集性能分析

TD-SCDMA 集群通信系统可以承载不同类型的集群多媒体业务,并能够为用户提供服务质量（QoS）保证。集群小区使用宏分集技术可以有效地克服大尺度衰落,保证信号质量。本文提出一种二基站宏分集方法,旨在对处在小区边缘的移动台进行信号的最大比合并,保证边缘小区用户的信号质量,同时降低传统宏分集对所有信号进行合并而增加的复杂度,且调整参与宏分集的基站,提高系统的前向容量。     

OFDM系统中信道有限反馈的门限设置研究

基于多用户下行OFDM系统资源分配及有限反馈理论的研究,该文推导了基于门限设置的信道有限反馈的容量及容量损失的数学表达式,并提出了3种信道有限反馈的门限设置方案。这3种方案利用多用户分集,从反馈中断概率、容量损失及反馈效用3个角度来设置门限。数值计算结果表明,通过适当的门限设置,可以在相对较少的容量损失下,较大程度地减少反馈量,使得下行多用户OFDM资源分配算法在实际中可以有效应用。     

改善系统中断速率的一种新虚拟接收天线方案

在发射分集系统中,使用虚接收天线技术将时间分集转化为虚接收天线分集,以提高信道的秩,可以有效提高系统中断速率,这十分适用于某些需要能够在较低的中断概率下以一定的传输速率进行通信的系统。已有的研究是将同样的数据在两个时隙内重复发送以形成虚接收天线,而这使得传输效率下降了一半。为改善性能,提出了一种新的虚接收天线方案,不再简单地重复发送,而用一个时隙发送前两个时隙数据的和,从而提高传输效率。分析和仿真表明,新方案可以提供与重复发送方案相同的分集阶数,而且相对于重复发送方案,新方案可以更有效地提高系统中断速率,并且遍历容量也相应提高。     

分布式移动通信系统中多用户分集分析

分析了分布式移动通信系统中,Rayleigh-lognormal复合信道下,发送分集对下行链路多用户分集的影响,发现基于最大比发送(MRT)和基于选择发送分集(STD)调度算法获得了几乎相同的多用户分集增益。该结论表明,在分布式移动通信系统中,采用基于发送分集的调度方案并不能有效地提高系统的吞吐量。     

衰落信道下Turbo码在宽带CDMA系统中的应用研究

本文在研究Turbo码在衰落信道下典型性能分析的基础上,提出了在宽带CDMA系统中结合RAKE接收机实现路径分集后Turbo码的性能分析方法.在高斯和衰落信道下,采用计算机仿真进行了实际系统的模拟,结果表明宽带CDMA系统中Turbo码对抗衰落的性能有了很大提高.     

Capacity analysis in CDMA distributed antenna systems

In this letter, the effect of maximal ratio combining (MRC)-based macrodiversity on the reverse-link and forward-link capacity in code division multiple access (CDMA)-distributed antenna systems is analyzed. The concept of virtual cell is illustrated, and the analytical outage probability expressions are derived. The present investigation shows that on the reverse link, the interference can be suppressed greatly with macrodiversity, which leads to a significant increase in capacity. However, on the forward-link, it is proven that if simulcasting is used in CDMA-distributed antenna systems, the forward-link capacity cannot increase with macrodiversity whatever power allocation scheme is adopted. Based on the analysis of the cause of capacity loss, a new transmission scheme is further presented and the optimal power allocation scheme is derived. It is shown that, in this case, the forward-link capacity increases rapidly with the number of involved distributed antennas.     

Effect of soft and softer handoffs on CDMA system capacity

The effect of soft and softer handoffs on code-division multiple-access (CDMA) system capacity is evaluated for unsectorized and sectorized hexagonal cells according to an average bit energy-to-interference power spectral density, which corresponds to a bit-error rate (BER) of 10^-3. The effect of imperfect sectorization on sectorization efficiency is also considered. On the reverse link, there is no capacity loss as no extra channels are needed to perform soft handoff, while the macrodiversity provided by soft handoff can improve the reverse-link quality and extend the cell coverage. On the forward link, when soft handoff is employed in unsectorized cells, the capacity loss due to two traffic channels assigned to a user in the handoff zone is 0.2% or 1.1% for a voice activity factor of 3/8 or 1/2, respectively. As the forward-link capacity is higher than that of the reverse link, this small capacity loss does not affect the system capacity. For sectorized cells having three sectors per cell, there are overlapping coverage areas between sectors, where mobiles in these areas are subjected to an increase in cochannel interference. For an overlapping angle of 5°, the sectorization efficiency is 0.96 and 0.7 for the reverse-link and forward-link systems, respectively. When soft and softer handoffs are employed, the forward-link sectorization efficiency is improved to 0.97. We find the application of soft and softer handoff improves not only the forward-link capacity, but also the signal-to-interference ratio (SIR) for mobiles near the cell and sector boundaries     

Forward-link capacity of a DS/CDMA system with mixed multiratesources

Some studies have been done on capacity of a code division multiple access (CDMA) system with mixed multirate sources. However, a vast majority of these studies have concentrated on the reverse-link. This trend comes from the fact that the capacity of a CDMA system is reverse-link limited. However, the forward-link can be a limiting link because emerging data services are likely to require higher data rates in the forward-link than in the reverse-link. In this paper, we analyze and simulate the forward-link capacity of a CDMA system with mixed multirate sources in a multipath fading channel. The outage probability of the forward-link is derived for a CDMA system with mixed multirate sources. By introducing a forward-link power factor, the forward-link Erlang capacity is obtained in a closed form. The forward-link capacity is analyzed in terms of the number of multipaths, the number of RAKE fingers in a mobile station, closed-loop power control, and impact of soft handoff. The results in this paper can be applied to overall system design of a CDMA system with multimedia services in future mobile communication systems     

A simple performance/capacity analysis of multiclass macrodiversityCDMA cellular systems

Multiclass CDMA systems, which support multiple services with various quality of service (QoS) requirements, are studied. The focus is on the reverse link capacity and application of macrodiversity with maximal ratio combining (MMRC), where the signals from each mobile station (MS) are received by multiple base stations (BSs) and coherently combined. A simple analytical solution is first derived for the multiclass reverse link carrier-to-interference ratio (CIR). Using this CIR solution, a simple capacity analysis is developed in terms of the QoS requirements. Finally, the analysis is fully supported by simulation results     

Macrodiversity power control in hierarchical CDMA cellular systems

Hierarchical code division multiple access (CDMA) cellular systems, consisting of macrocells with underlying microcells, are studied. We seek power control schemes which will allow both hierarchical layers to share the same spectrum. For the reverse link, hierarchical maximal ratio combining (HMRC) is applied where each mobile station (MSs) is received and coherently combined by base stations (BSs) in both layers. For the forward link, selective transmit diversity (STD) is applied where each BS provides multiple transmit paths for MSs to choose. We show that both HMRC and STD are effective in hierarchical CDMA architectures. We conclude that hierarchical architectures are a viable solution for improving CDMA cellular system capacity, and a significant performance gain can be achieved without assigning disjoint spectrum between the layers, by utilizing macrodiversity schemes such as HMRC and STD     

Impact of macrodiversity on capacity and coverage of cellular network

This paper proposes an analytical study of downlink macrodiversity. Considering a mobile in macrodiversity with two base stations, we first characterize and compute explicitely the load induced in a cell by downlink macrodiversity. We show that in most cases this load is positive, increasing the total cell load. We moreover show that the contribution of the macrodiversity to the cell load can be negative in the sense that the macrodiversity can increase the cell’s capacity. Afterward, we propose a new framework for the study of cellular networks called the fluid model which allows to calculate analytically that load. The key idea of the fluid model is to approximate the discrete base stations (BS) entities by a continuum of transmitters which are spatially distributed in the network. This allows us to obtain simple analytical expressions of the main characteristics of the network. In this paper, we focus on the downlink other-cell interference factor f, which we define here as the ratio of outer cell received power (i.e. the power received from other cells) to the inner cell received power. This fluid model allows calculating the influence of interference on any mobile in a cell, whatever its position. The fluid analysis we develop enables to calculate the load of a cell analytically, and to quantify the macrodiversity impact. We generalize our analysis, considering a macrodiversity with a great number of base stations of the network.     

Frequency overlay of GSM and cellular B-CDMA

The maximum capacity gain of a cellular broad-band direct-sequence code-division multiple-access (DS-CDMA) overlay on top of a cellular global system for mobile communications (GSM) system is estimated. Using geometrical arguments and a standard propagation model, all relevant contributions to the carrier-to-interference ratios (CIRs) of the GSM and CDMA systems are numerically evaluated. It is assumed that the base stations (BSs) of both systems are unilocated and that the power control in both systems is perfect. Furthermore, in the CDMA transmitters and receivers, ideal notch filtering is assumed around the occupied GSM frequencies of the same cell. Adapting standard limits for the CIRs of 5-7 dB for CDMA and 9-12 dB for GSM, the total system capacity can be increased considerably over the GSM-only case: the capacity is increased by a factor of 1.8-2.9 if no macrodiversity is employed and by a factor of 1.9-3.3 if macrodiversity in the CDMA downlink is applied     

Power control at the combiner output to maximize the uplinkcapacity on a cellular spread spectrum system

The benefits of applying maximal ratio combining instead of selection combining when using macrodiversity in a cellular code-division multiple access (CDMA) system were studied. In particular, using maximal ratio combining with power control made at the combiner output, and assuming equal interference at every base station, we provide a simple proof to show that we can achieve on the uplink the same capacity per cell as with an isolated cell. On extending the study to a practical case with a limited number of cells in macrodiversity in comparison with the unrestricted theoretical case, we found that the results on cell capacity with maximal ratio combining were quite close to the results with an isolated cell