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1、.外文原文Spectral efficiency of a single-cell multi-carrierDS-CDMA system in Rayleigh fadingP. Varzakas, G.S. TombrasLaboratory of Electronics, Department of Physics, University of Athens, GR-157 84 Athens, GreeceReceived 8 June 2005; received in revised form 12 September 2005; accepted 11 March 2006Abs
2、tract: The spectral efficiency of a multi-carrier direct-sequence code-division multiple-access (MC/DSCDMA) system operating in a Rayleigh fading environment is investigated and evaluated in terms of the theoretically achievable channel capacity (in the Shannon sense) per user, estimated in an avera
3、ge sense.。This short paper covers operation of the considered system over broadcast communication randomly time-varying channels as applicable to wireless radio networks and single-cell indoor mobile systems and leads to the derivation of a closed-form expression for the achieved spectral efficiency
4、. Furthermore, the relation between the number of the employed sub-carriers and the achieved spectral efficiency is revealed.Keywords: Spectral efficiency; Channel capacity; Multi-carrier modulation; Code-division multiple access; Rayleigh fading;1. IntroductionBased on orthogonal frequency-division
5、 multiplexing (OFDM), multi-carrier direct sequence code-division multiple-access (MC/DS-CDMA) has been proposed and investigated in the context of high data rate communication over time-variant channels 1. In such a MC/DS-CDMA system, the data sequence, multiplied by a spreading sequence, modulates
6、 a set of N orthogonal frequency carriers rather than a single one, as it is the case in conventional DS-CDMA. In this paper, we consider the operation of a single-cell MC/DS-CDMA system in a Rayleigh fading environment and evaluate its spectral efficiency in terms of the theoretically achievable ch
7、annel capacity (in the Shannon sense) per user, on the condition that this is estimated in an average sense. Such an approach is justified by the fact that the spectral efficiency achieved by any multiple access communication system depends on the physical model of the fading radio environment, i.e.
8、, on what is known about the particular fading channels. For example, if nothing is known for the fading statistics of a channel then its countable capacity (in bits/s) will be dictated by the minimum fade signal-to-noise ratio (SNR), , and will, thus, tend to zero as on the other hand, if fading st
9、atistics is known, then average capacity formula can be derived after the distribution of the fading SNR for a fixed transmission rate 2-4.The presented analysis refers to the downlink transmission (broadcast channel) for a fixed number K of simultaneous users, reflecting a static model of operation
10、. Then, following the method described in 5-7, channel capacity is estimated assuming an equal power case, meaning that all users receive equal average signal power when an appropriate power control scheme is applied, in combination to the path-diversity achieved by a conventional coherent maximal-r
11、atio combining (MRC) RAKE receiver and the physical frequency diversity potential provided by frequency-division multiplexing on a set of orthogonal carriers. However, this approach does not deal with the problem of the “capacity region”, i.e., the set of information rates at which simultaneously re
12、liable communication of each user messages is possible 8,9; it is merely based on estimating the average channel capacity per user, considering the systems inherent diversity potential in conjunction to the path-diversity obtained by MRC RAKE reception. Hence, the derived spectral efficiency express
13、ion does not indicate the systems maximum value in (bits/s/Hz), but it represents an optimistic upper bound, in average sense, for practical modulation and coding schemes 5-7.2. System description and channel capacity per userIn MC/DS-CDMA, each users data symbol is transmitted in parallel over N or
14、thogonal carrier frequencies (sub-carriers), multiplied by a spreading sequence unique to each user. Then, the received signal will be the output sum of all these N “branches”, while the totally allocated system bandwidth, , assuming no guard band between adjacent frequency bands and a strictly band
15、-limited “chip” sequence with bandwidth = will be equal to = (1)Where is the processing gain applied for direct-sequence (DS) spread transmission and W is the user unspread signal bandwidth. However, reliable transmission of each user signal over each of the N sub-carriers clearly depends on the lev
16、el of cooperation among the K users of the system, i.e., the multiple access interference (MAI) power, while inter-carrier interference (ICI) is considered minimum. Thus, the nth channel capacity per user, , i.e. each users () conditional channel capacity (in the Shannon sense) over the nth sub-carr
17、ier, (), will be given by the Shannon Hartley theorem when arbitrarily complex coding and delay is applied and the total MAI power, caused by even a small number of interfering users, tends to be Gaussian distributed 10. In this respect, considering the profound resemblance to the operation of a spr
18、ead spectrum system with K simultaneous users transmitting with the same average power and the channel capacity per user expression in a non-fading additive white Gaussian noise (AWGN) environment 4,6,it can be readily written that (2)Where is the noise power spectral density, is the average receive
19、d power over the considered nth channel, and (3)is the Gaussian distributed MAI power (for ,) over this channel. Then, considering the total of the N sub-carriers over which, the user i information data (spread by ) is transmitted, we conclude that the totally assigned by the MC/DSCDMA system channe
20、l capacity per each user i will be (4)中文译文瑞利衰落环境下单蜂窝多载波DS-CDMA系统的频谱效率希腊·雅典大学物理系电子实验室收稿日期：2005 年6月8 日；收到修改稿日期：2005 年9月12 日；接受日期：2006 年3月11 日 摘要：对运行在瑞利衰落环境下的多载直接序列码分多址(MC/DSCDMA)系统的频谱效率进行了研究，以及在一般意义下，对理论上单用户可达到的信道容量(香农意义)进行了评价。本文涵盖了适用于无线网络和室内移动系统的随机时变广播通信信道在系统中的运行情况；推导出了频谱效率封闭表达式的偏差。此外，还揭示了应用的多载波
21、数目与能达到的频谱效率之间的关系。关键词：频谱；信道容量；多载波调制；码分多址；瑞利衰落；1. 介绍在正交频分复用(OFDM)的基础上，提出了多载波直接序列码分多址(MC/DS-CDMA)，并对通过时变信道中高速通信进行了研究1。在这个MC/DS-CDMA系统中，数据序列与扩频序列相乘，去调制N个正交频率载波，而不像在传统DS-CDMA中调制单个载波。在本文中, 我们考虑运行在瑞利衰落环境下的单蜂窝MC/DS-CDMA系统的运行情况，根据香农理论上可能达到的单用户信道容量来评估频谱效率，并且假设是在平均意义上进行评价。该方法被这样的事实证明了，多径通信能够达到的频谱效率都依赖于无线衰落环境下的
22、物理模型，依赖于对那些特殊衰落信道的了解。例如，如果对一个信道的衰落统计值一无所知，那么可数容量被强行规定为最小信噪比，所以理所当然容量趋进于零，即0。另一方面，如果知道衰落统计值，那么可以根据固定传输速率2-4下衰落信噪比的分布推导出平均容量公式。上面的分析涉及到K个用户同时使用下行链路传输(广播信道)，反映出一个运行的静态模型。然后，根据5-7描述的方法，信道容量是在假定功率相等的情况下被估计的采用适当的发射功率控制方案，由传统相干MRC RAKE分集接收机接收分路信号，由一系列正交载波频分复用提供物理频分电压，使所有用户接收到的信号平均功率相等。但这种方法不能解决区域容量的问题，即在各用
23、户同时进行可靠通信时设置传信率是可能的8-9；它仅考虑由MRC RAKE接收到的分路信号的内在差异，在此基础上估计每用户的平均信道容量。因此，这样得到的频谱效率(bits/s/Hz)表达式不能表明系统的最大值，但它代表了一般意义上实用模块化和编码方案的最优上界5-7。2. 系统说明和单用户信道容量在MC/DS-CDMA中，各用户的数据码乘以各自唯一的扩频序列，在N个正交载波频率上传送(副载波)。然后, 当系统带宽完全被分配，接收信号将是所有这些N的"分支"的总和，假设相邻频带之间没有隔离带，并且序列片带宽没有严格的带限，那么公式=将等价于：= (1)其中是直接序列扩频的处理
24、增益，W是用户扩频前的信号带宽。但是，在N个子载波上的每个用户信号的可行性传输很明显取决于系统中K个用户之间合作的程度，即当载体相互干涉(ICI)被认为最小时多径干涉(MAI)的功率。因而，每用户第n通路容量，即各用户()在第n个()载波上时的通路容量(在香农意义内)将由香农哈特莱定理当任意地编码和延迟被采用和由很小数量干涉的用户造成的总MAI功率趋向于高斯分布10时给出。在扩频系统中，K用户以相同平均功率同时传送和在一个无衰落加性白色高斯噪声(AWGN)环境里，每用户通路容量表达式4，6，可以写为 (2)这里n0 是噪声功率谱密度，是在理想信道时的平均接收功率， (3)是高斯信道中的表达式(,)。然后,就用户i信息数据在总N载波(扩展后)被发送,我们认为,指定用MC/DSCDMA系统时，每个用户的通路容量公式将是 (4) :