未來研究方向

I. 多種服務類型之考量與建模

為了簡化推導及模型複雜度，本研究假設macrocell 及 femtocell 基站使用者皆 接取同一類型服務，因此僅需考慮單一頻寬的連結流量需求，然在UMTS 系統與 HSDPA 系統中，能提供之服務基本上有四種類型，每種類型亦有不同的頻寬與服 務品質需求，而對於固網容量的規劃亦會呈現出更複雜的行為，諸如macrocell 語 音數據容量配置，數據又可分會話、互動等等類型，我們希望將來能考慮此種多 種數據服務流量分析，以設計多種服務(multi-service)頻寬配置的機制，考慮兩種 系統用戶的數據優先權，並分析其效益以及所提升的使用者服務品質。

II. 加入訊號干擾之因素

本研究考量有線端之容量限制對於此共存系統之影響，然本研究並未考慮無 線頻帶資源因素以及加進多個femtocell 基站對原本 macrocell 系統的干擾，是否彼 此干擾有可能影響原先macrocell 之無線訊號品質？且 femtocell 本身亦提供使用者 接取的能力，使得系統雖會提升整體干擾量，然而是否也可藉由 femtocell 的接取 能力而增加系統可同時接取之人數？未來能考慮此整體系統之架構，包含無線與

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有線端之流量與資源分析，以及所能提升的使用者服務品質。

III. 4G 技術的考量

本研究考量在UMTS 系統所提供之行動通訊數據服務，每種服務類型有不同 的頻寬與服務品質需求，然而因UMTS 實體層採用 WCDMA 技術，而 4G 實體層 則使用 OFDMA 技術，基本上傳輸信號的方式不同，用戶需求建模方法與資源分 配的方式與本研究方法有所差異，因此希望未來能加進考慮4G，以做為未來共存 系統中行動通訊數據服務參考建模的依據。

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附錄 A：指數分佈機率求解 

於 3.3 節討論之連結持續時間與用戶在室內之比例等隨機變數皆假設為具有 指數分佈的性質，在此根據指數分佈之特性推導求解：

考慮兩隨機變數 A 與 B 為指數分佈，分別具有平均值^E

 

^{A a及E}

 

^{B b，則}

t 及 為任意正整數， f_A

 

x  1e^ax及 f_B

 

t  1e^bx為 A 與 B 之機率密度函數 (i) 令^{Y min}

 

^A,B ，則

     

^{ay by}

Y y A y B y e e

F Pr  Pr   ^  ^

因此，Y 為之阻隔機率密度為 _Y

 

^Y

  

a b



e ^{a by} dy

y y df

f    ^{ } ，而

 

Y a b E  1

。 (A.1)

82

 

83

附錄 B：M/

Mb

/1//r

₁

-PS 阻隔機率公式推導 

推導參考[Coh79]對於有限人口(finite source population)下，對於 M/M_b/1//r1-PS 阻隔機率求解。Macrocell 連結有一最低頻寬之需求，假設為 bw1，固網接口之容 量為 BWIu-ps，連結數上限 r1 = BWIu-ps /bw1。計算M/M_b/1//r1-PS 穩態機率可直接藉 由[Coh79]的 M/Er/R/∞-PS 模型之結果求得，只是其狀態空間(state space)被限縮為 僅有最多 r2個連結可被服務，當系統中已存在 r1個連結佔用macrocell 固網端之容 量時，新進連結將會被阻隔掉。

先藉由[Coh79]對於一 Erlang 分佈(Erlang service distribution)的分析推導結果 如下，再修改為本論文討論之M/M_b/1/r1-PS 隊列，首先定義參數如下：

P為單一連結所需之服務層級(service phase)

M 為所有連結總數

K為具有相同特性之連結類別數 M 為第k k個類別之連結數

 

j

x_k 為第k個類別之連結數，且目前處於第k層服務層級

 ^j

 

^

Dk 為第k個類別之連結數，且目前處於第k層服務層級之連結持續時間之 分布

 ^j

 

^

dk 為Dk^{ j}

 

^ 之第一階moment

h j

k

 , 為第k個類別之連結數，且目前處於第 j層服務層級之連結的馬可夫鏈

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85

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在文檔中 Macrocell與Femtocell共存系統數據流量建模與阻隔機率計算 (頁 95-108)