3G及Wifi頻譜使用率與涵蓋範圍的量測與分析:以台大校園為例

Abstract

第三代行動通訊系統(3G)用戶需求近年來不斷地攀升，門號於2012年3月突破2300萬，台灣各大電信公司雖致力於建設基地台以提昇良好的訊號涵蓋範圍，但在建築林立的都市或較為偏遠的郊區容易因訊號受到阻隔或距離基地台較遠的因素而導致訊號涵蓋不良的情形發生，因此積極以佈建短距離傳輸為主的WiFi熱點來改善行動上網訊號涵蓋狀況，並對3G接取流量進行分流，例如台北市至2012年全市已達到超過4000個WiFi熱點。 然而僅依靠繁密的配置WiFi熱點，無透過適當的頻道干擾機制設計來決定合適的熱點擺放位置，則訊號間可能因不同時間受到人為、頻道重疊干擾嚴重，導致該地區頻譜無法有效的被利用。另一方面，訊號涵蓋範圍因著不同的傳播環境及電信公司基地台、熱點設置的地點不同也有極大的差異，因此透過使用者的角度進行量測可直接觀察該地區頻譜被使用與訊號涵蓋的真實情形。 本論文以台大校園為場域進行研究，研究目的如下: 一、 學理上了解頻道以不同技術接取時頻譜使用情形的變化及不同頻率訊號在環境中衰減的情況，確保所進行量測可真實掌握到無線訊號受採用技術的影響。 二、透過對台大校園3G及WiFi頻譜使用率與訊號涵蓋範圍進行量測與估計來 2.1) 檢視數據頻譜使用率定義，並了解實際使用的情形，及 2.2) 評估室內、外環境因素對WiFi訊號涵蓋的影響、室外3G和WiFi間涵蓋的搭配情形、以及進行與使用模式的關聯分析。 在頻譜使用率量測的研究方面，本論文研究使用頻譜分析儀、全向性天線組成頻譜使用率量測硬體設備，分別對兩種使用率的量測問題探討如下: 1) 補強文獻上3G頻譜使用率定義的不足，包含(1)當頻道功率大於臨界值後，使用者數目增減，頻譜使用率不會跟著變動, 及(2)未明確定義頻譜使用率指標為上行或下行頻段。本論文研究以使用者數量的多寡及量測位置分別對上下行頻段使用率造成的影響佐證，提出定義3G頻譜使用率以下行頻段使用率為頻譜使用率指標，並藉由不同使用者數量於單一頻道上進行通訊與數據接取產生的碼功率大小值，來分辨使用者人數變化對頻譜使用率的影響和以下行頻段為使用率指標可減少量測誤差的原因。針對WiFi頻譜使用率與三個考量因素:頻道、時間、地點的對應關係，進行頻譜使用率實地量測。頻譜使用率量測實驗獨立變數包含頻道、時間與地點，所得數據依本論文研究定義的使用功率臨界值準則，以Matlab軟體進行比較與分析。量測結果顯示: (r1)使用率最高的頻道為CH1、CH6和CH11; (r2) WiFi在不同地點整體頻帶使用率約介於3%~60%之間，以圖書館為五個量測地點中的最高(57.54%)，校門口為最低(3.6%); (r3)使用率變化可由離峰時段的24%到尖峰時段的67%; (r4)因台大校園WiFi配置繁密(1904個AP)，師生採用WiFi進行數據接取普遍，故量得使用率明顯高於早年文獻中量測結果。 2) 在訊號涵蓋範圍量測的研究方面，本論文研究專注於探討室內、外訊號強度的分佈情形，並利用頻譜分析儀與智慧型手機搭配APP軟體”WiFi-分析儀”為量測硬體設備:室內WiFi訊號量測的獨立變數包含頻道、地點。所得的量測數據以克利金(Ordinary Kriging)法估算未量測地點之訊號，並結合實際量測的訊號以Matlab繪圖軟體探討(3)已知的WiFi熱點位置與平均功率強度的對應關係、(4)自行配置WiFi熱點位置與平均功率強度的對應關係、(5)樓層間溢散的WiFi訊號對平均功率的影響。量測結果顯示: (r5)館舍管理單位所配置的熱點位置與訊號強度分佈圖並無直接對應關係; (r6)各實驗室是否有自行配置熱點、配置數量皆不易掌握，有越靠近實驗室區則訊號強度越強的趨勢; (r7)樓層彼此間溢散訊號的影響低。 3) 室外WiFi/3G訊號涵蓋範圍量測探討議題包括(6)室內WiFi訊號與室外WiF訊號間的對應關係、(7)周圍環境(系館、空地等)與3G平均功率強度的對應關係、(8)周圍環境與WiFi平均功率強度間的對應關係、及(9)3G訊號與WiFi訊號的平均訊號強度比較。量測結果顯示: (r8)室內溢散出的訊號強度經過衰減後遠低於室外訊號，因而對與室內建築物鄰近的室外區域訊號的影響低；(r9)周圍系館分布較鬆散的區域，其3G訊號強度(約-64dBm)遠高於以建築物為主體的區域(約-84dBm); (r10)在較為空曠的地區WiFi訊號相對的低(約-92dBm); (r11)室外3G訊號強度(-64~-84dBm)普遍優於WiFi(-84~-92dBm)。 綜合以上分析與討論，從頻譜使用率的量測結果可知在WiFi熱點配置繁密的環境之下，大部分的使用者會選擇以WiFi進行數據接取服務的可能性高。以台大校園為例，不同的地點頻譜使用率變化差異可達50%，尖峰與離峰時段頻譜使用率變化約達30%，顯示數據頻譜需求不斷上升下，廣設WiFi AP提高ISM段頻譜使用率可提高對3G接取流量分流的機會。而從訊號涵蓋的觀點來看，3G室外涵蓋訊號強度約分佈在-64~-84dBm，遠比WiFi室外訊號最高僅達-84dBm高出許多，由此可推斷大部分使用者在戶外環境仍以3G接取為優先考量。

User demands for broadband mobile data access via the third generation (3G) mobile communication systems have constantly risen in recent years, and the total 3G phone numbers has grown to 23 million and up. Taiwan's major telecommunication companies have been facing the challenge of increasing deployment of base stations for good signal coverage and throughput. To overcome the poor signal coverage caused by blockage of buildings or long distance from accessible base stations, short distance transmission-based WiFi hotspots have been widely deployed as a complimentary solution to offload 3G access traffic. For example, there have been over 4,000 WiFi hotspots in Taipei city as of 2012. It is well known that dense deployment of WiFi hotspots does not necessarily lead to good coverage and effective spectrum utilization without proper design of hotspot locations and channel interference avoidance. Signal coverage and spectrum utilization may largely vary with communication environments and locations of base stations and hotspots. Therefore, direct measurements provide, from consumers’ point of view, understanding of spectrum utilization and signal coverage in an area. This thesis conducts measurement research over National Taiwan University campus. Research objectives are as follows: 1. To study in theory how spectrum utilization varies with different access technologies and how signals of different frequencies are attenuated with environment so that the measurement techniques adopted truly capture the effect of access technologies on wireless signals; 2. To conduct measurements and estimation of 3G and WiFi spectrum utilization and signal coverage over National Taiwan University campus 2.1) to investigate the definition of spectrum utilization by data access and the actual utilization, and 2.2) to conduct evaluation of the effect of indoor and outdoor environment factors on WiFi signal coverage, the joint coverage between 3G and WiFi and correlation analysis between the two usage modes. The measurement system of spectrum utilization consists of a spectrum analyzer, an omni-antenna, a PC and a MatlabTM based analyzer. The measurement research first addresses the deficiencies of in defining 3G spectrum utilization by the existing literature, studies via measurements how the number of users and measurement locations affect 3G spectrum utilization, and defines the downlink frequency band utilization as spectrum utilization indicator. This thesis (1) recommends that the measured code powers under different number of users in a single channel be used to identify the impact of user number on spectrum utilization and (2) explains why the use of downlink spectrum utilization as an indicator may reduce measurement error. This thesis also performs and analyzes measurements of WiFi spectrum utilization over the three dimensions of channel, time, and location. In measuring signal coverage, this thesis focuses on the distribution of indoor and outdoor signal power level. The measurement system consists of a spectrum analyzer and smart mobile phones equipped with APP software “WiFi-Analyzer” as hardware measurement devices. The independent variables of indoor WiFi signal measurement include channel and location. Collected data is used to estimate the signals at the unmeasured location by application of the Ordinary Kriging method, and then validated by further measurements. Items of analyses are (3) the relationship between the known WiFi hotspots and the average power level, (4) the relationship between the self-configured WiFi hotspots and the average power level, (5) the impact of WiFi signals dispersed between floors on the average power. The issues of outdoor WiFi/3G signal coverage measurement include (6) the relationship between the indoor and outdoor WiFi signals, (7) the relationship between the surroundings (department building, open space, etc.) and the 3G average power level, (8) relationship between the surroundings and the average WiFi power level, and (9) comparison between the 3G signal and the average WiFi signal power levels. The measurement results of spectrum utilization show that in an environment of dense WiFi hotspot configuration, most users will choose to use WiFi for data access service. For example, the difference between the changes in spectrum utilization at different locations over National Taiwan University campus may reach up to 50%, wherein the spectrum utilization changes from the off-peak time interval utilization to the peak-time interval utilization reaches about 30%. Therefore, with the rising demand for mobile broadband access, the opportunities for 3G data access may increase by providing WiFi AP widely and improving the spectrum utilization of the ISM band. From the viewpoint of signal coverage, the power level of outdoor 3G signal outdoor coverage is distributed approximately in -64~-84 dBm, which is much higher than outdoor WiFi signal of only up to -84 dBm. It can be inferred that in the outdoor environment, most users would take 3G access as a priority.