Negatively Correlated Branches in Frequency Diversity Systems to Overcome Multipath Fading
Negatively Correlated Branches in Frequency Diversity Systems to Overcome Multipath Fading
复制标题
频率分集系统中的负相关分支克服多径衰落
DOI：
10.1109/tcom.1974.1092173
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发表时间：
1974
期刊：
影响因子：
--
通讯作者：
M. Noorchashm
中科院分区：
文献类型：
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作者：
F. Haber;M. Noorchashm
在多径环境中，其中在通过每个间接路径和直接路径到达的信号之间存在时间延迟差（TDD）值的紧凑集中，通过适当地选择频率差，可以设计频率分集系统，其中分集分支是负相关的。这导致在所有分支上的同时深度衰落的概率比用被设计为独立的分支获得的概率降低。在第一部分中的纸张的负相关的大小取决于间接反射器的数量，有助于总的间接信号，它是最大的间接信号时，从一个单一的反射器。还示出了如何使用TDD的概率密度函数来最大化负相关的幅度以建立频率分配。这是一个先驱的第二部分的文件，其中调查的错误概率的改善，明智的选择的频率差的报告。作为频率差的函数的错误概率被发现的特定情况下，非相干频移键控（FSK）在镜面多径信道使用平方律组合。对M阶分集得到了一般性的结果，但仅对二阶和三阶分集作了详细的研究。结果是均匀分布的TDD的各种范围和平均值，建议的参数时遇到的物理条件低飞的飞机是多径源或自己试图沟通。结果为其他密度的TDD，正常和伽马，引用。对于二阶情况，发现可以根据平均TDD来确定有利的频率间隔。对于三阶情况，两个频率差可用作设计参数。结果提出了一个规则的基础上选择频率差类似于在二阶的情况下使用，并发现实质性的改进。另一个规则调查，给出了更好的结果被引用。最后，错误概率的比较与系统没有多径，非分集系统与多径，和分集系统在多径与独立分支。对于所假设的条件，并为合理的水平的发射信号功率和接收机噪声，数量级的改善被发现获得独立的分支。
In multipath environments in which there is a compact concentration of the values of time-delay-difference (TDD) between signals arriving over each of the indirect paths and the direct path, it is possible, by suitably choosing frequency differences, to design a frequency diversity system in which the diversity branches are negatively correlated. This results in a reduced probability of simultaneous deep fades on all branches over that obtained with branches designed to be independent. In the first part of the paper the magnitude of the negative correlation is shown to depend on the number of indirect reflectors contributing to the total indirect signal and that it is maximum when the indirect signal arises from a single reflector. Also shown is how the magnitude of the negative correlation can be maximized using the probability density function of the TDD to establish the frequency assignment. This is a precursor to the second part of the paper in which an investigation of the improvement of the error probability by a judicious choice of frequency differences is reported. The error probability as a function of frequency differences is found for the specific case of noncoherent frequency-shift keying (FSK) in a specular multipath channel using square-law combining. A general result for M th order diversity is obtained but a detailed study is made for two and three order diversity only. Results are presented for uniformly distributed TDD's with various ranges and mean values, the parameters being suggested by the physical conditions encountered when low flying aircraft are the multipath source or are themselves trying to communicate. Results for other densities of TDD, the normal and the gamma, are cited. For the second order case, it is found that an advantageous frequency spacing can be determined in terms of the average TDD. For the third order case, two frequency differences are available as design parameters. Results are presented based on a rule for selecting frequency differences similar to that used in the second order case and substantial improvements are found. Another rule investigated that gave even better results is cited. Finally, error probability comparisons are made with systems having no multipath, nondiversity systems with multipath, and with diversity systems in multipath with independent branches. For the conditions assumed, and for reasonable levels of transmitted signal power and receiver noise, order of magnitude improvements are found to be obtained over independent branches.