GSM系統之無線電資源分配

Abstract

全球式行動通訊系統（GSM；Global System for Mobile Communications）提供電信以及資料傳輸服務給行動使用者。GSM系統為一蜂巢式數位系統，此系統是由Group Special Mobile of CEPT（Conference Europeenne des Postes et Telecommunications）及其前身歐洲電信標準協會所發展。為提升使用者滿意度及增加網路的利用率，GSM Phase 2+ 的版本定義一些先進的服務。這些服務包括先進的語音服務，一般封包式無線電服務（GPRS；General Packet Radio Service），以及短訊服務（SMS；Short Message Service）。 在GSM Phase 2+中，為解決因為障礙物阻擋而導致的無線電聯結斷訊（此現象稱為通話中斷現象），制定了重建通話服務。此服務的運作方式為－當無線電通道被中斷時，網路端啟動一通話中斷計時器並保留幹線，以及選擇性的保留無線電通道給被中斷的通話。若在中斷計時器逾時或是對方將電話掛上前中斷現象能消除，此中斷的通話便可以經由重建通話服務來恢復通話。否則，這通電話便被強迫斷話。 GSM Phase 2+ 提出了半速（Half-Rate）語音通道來增加系統的容量。半速語音通道傳輸率為5.6 kbps。GSM在每個載頻上依時間軸切割為時框（Time Frame），每個時框具8個時巢（Time Slot）。也就是在一載頻上最多可以支援8個全速（Full-Rate）語音使用者或是16個半速語音使用者，或是任意的組合。經由引進半速的語音通道將可以增加GSM系統容量。 GPRS分享GSM的無線電資源來提供可改變傳輸率的封包式資料服務。與其他的GSM服務（短訊服務除外）相較，GPRS並不會長時間的佔住GSM系統的無線電資源。這樣的方式將可以提昇無線電資源的利用率。GPRS服務的使用者將享有迅速上網以及高速的資料服務。 與傳統的GSM迴路式語音服務相較，這些新服務為GSM系統新增了新型式的網路流量（例如，半速通話流量、GPRS封包流量）。因此如何將無線電資源有效率的分配給這些新服務與傳統服務使用者用將是一個很重要的課題。在本論文中，我們為這些服務提出了無線電資源分配的機制來分配無線電資源。同時我們亦建置了模擬模組與分析模組來評估每個機制的效能。經由訂出GSM系統網路的工作量，我們的研究明確指出每個機制的實際效能，這些結果提供了分配GSM系統無線電資源所需的指標。

Global System for Mobile Communication (GSM) provides telecommunication and data services to moving users, which is a digital cellular system developed by Group Special Mobile of Conference Europeenne des Postes et Telecommunications (CEPT) and its successor European Telecommunications Standard Institute (ETSI). To increase end user satisfaction and network quality perception, GSM is evolved to GSM phase 2+ for advanced services. These services include advanced speech services, group call and related services, General Packet Radio Service (GPRS) and Short Message Service (SMS). In GSM phase 2+, call re-establishment service is specified to allow a mobile station (MS) to resume a call in which the radio link has been temporarily interrupted due to interference or bad signal which is referred as a call interruption phenomenon. In this mechanism, if a communication channel is interrupted, the network still reserves the trunk and/or the channel for the interrupted call, and an interrupt ion timer is triggered. If the timer expires or the remote party hangs up the phone before the interrupt ion period is over, the interrupted call is actually forced terminated. Otherwise, the interrupted call is resumed by the call re-establishment service. GSM phase 2+ also provides a good compromise between the cost of the system and the quality of the services to the users. The half-rate speech codec (5.6 kbps) has been standardized to achieve this goal. The GSM half-rate speech codec can reasonably provide the same quality of service as the GSM full-rate speech codec in most cases. In GSM, the frequency carrier is divided into eight time slots per frame. Thus, mixing full- and half-rate calls in a frequency carrier results in eight full-rate calls, 16 half-rate calls, or any feasible combinations. Better frequency band utilization can be expected. Based on the GSM radio architecture, General Packet Radio Service (GPRS) provides users data connections with variable data rates and high bandwidth efficiency. In contrast to the other services (except SMS), GPRS will not permanently allocate separate radio resource of the GSM. This reduces the waste of bandwidth when bursty communications are supported over a continuous circuit. Users of GPRS benefit from shorter access times and higher data rates. Compared with the voice circuit-switched services provided by GSM, the three services mentioned above generate new type of traffic (e.g., half-rate call traffic and GPRS packet data) to occupy the radio resource. Thus, it is important to allocate the radio channels to both the new service users and the traditional service (i.e., circuit-switched voice) users in an efficient way. In this dissertation, we propose channel allocation schemes for these services. Analytical models are built to investigate the performance of the channel allocation schemes. Simulation experiments are conducted to validate the analytical model. Based on the workload to the GSM system, our studies show the performance of each scheme, which provides guidelines for radio resource allocation for GSM. Acknowledge Contents List of Figures List of Tables Notation 1 Introduction 2 Improving GSM Call Completion by Call Re-Establishment 3 Channel Assignment for GSM Half-Rate and Full-Rate Traffic 4 Channel Allocation for GPRS 5 Conclusions and Future Work