第三代UMTS系統於異質系統交接中基於系統容量的壓縮模式控制方法

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(1)國立交通大學電子工程學系電子研究所碩士班碩士論文. 第三代 UMTS 系統於異質系統交接中基於系統容量的壓縮模式控制方法. Capacity-based Compressed Mode Control Algorithm for Inter-System Handover in UMTS. 研究生：張正達指導教授：黃經堯. 博士. 中華民國九十四年七月.

(2) 第三代 UMTS 系統於異質系統交接中基於系統容量的壓縮模式控制方法. Capacity-based Compressed Mode Control Algorithm for Inter-System Handover in UMTS. 研究生:. 張正達. 指導教授: 黃經堯. Student: Hsiao-Chiang Chuang Advisors:. ChingYao Huang. 國立交通大學電子工程學系電子研究所碩士班碩士論文. A Thesis Submitted to Department of Electronics Engineering & Institute of Electronics College of Electrical Engineering and Computer Science National Chiao Tung University in partial Fulfillment of the Requirements for the Degree of Master of Science in Electronics Engineering July 2005 HsinChu, Taiwan, Republic of China. 中華民國九十四年七月.

(3) 第三代 UMTS 系統於異質系統交接中基於系統容量的壓縮模式控制方法. 研究生: 張正達. 指導教授: 黃經堯博士. 國立交通大學. 電子工程學系電子研究所碩士班. 摘要. 第三代手機的開發正值巔峰，而佈建之後於其他異質系統(GSM,WLAN) 的交接轉換將成為一個重要的議題。由於不同系統的射頻介面皆不一致，通用行動通訊系統提出一種方法稱為壓縮模式，將中斷連線製造一段期間來量測其他系統的強度。執行壓縮模式需要提升傳輸功率來加強加速傳輸資料的可靠性。然而此提升的功率將會導致系統的效能嚴重影響，此篇論文則提出一個基於負荷量的壓縮模式，在基於能成功量測異質系統的前提下，將壓縮模式對負荷量所造成的影響降到最低。此篇內容亦建立一個通用行動通訊系統的模擬平台來模擬實際對異質系統交接的情況。而此模擬平台也成功地驗證提出的新壓縮模式演算法能確實的減少輸出功率的消耗並改善負荷量的降低。而且在量測異質系統載子的品質也都能達到令人滿意的成果。所以若實際採用此提出的演算法將可以大大提升壓縮模式的效能及效率。. i.

(4) Capacity-based Compressed Mode Control Algorithm for Inter-System Handover in UMTS. Student: Cheng Ta Chang. Advisor: Dr. ChingYao Huang. Department of Electronic Engineering & Institute of Electronics National Chiao Tung University. Abstract The multimedia services and the high data rate transmissions have become more popular. Therefore, the third generation is developed to overlap with the existing second generation systems. To have a smooth migration in between two systems, the Inter-system handover is one of the key features in the third generation systems. The compressed mode, with variable transmission gaps and power levels, is standardized to support the inter-frequency/system handover. To minimize the use of system resources while maintaining the border-cell handover quality, a capacity-based compressed mode algorithm is proposed. Considering the tradeoff between the capacity and the connection quality, the algorithm can adaptively manages the compressed mode operation based on the potential impacts on the capacity and the efficiency of the compressed mode measurement. A simulation platform is established based on 19 UMTS cells surrounded by GSM Sea to evaluate the performance. The simulation results shows that appropriate scheduled compressed mode can reduce the power consumption and enhance the capacity while maintaining the quality of the measurements of other systems.. ii.

(5) 誌謝. 轉眼間兩年的研究生生涯呼之而過，回想兩年前毅然決然的從控制的領域轉出，去探尋有趣的新領域，很幸運地能遇到黃經堯教授，帶領我進入這熱門的無線網路聖殿，一路上從基礎重新打紮，並一步一步往高深的學問鑽研，如今我願意將我小小的成果分享給所有在這學問大道上的先賢後進。在初進入實驗室時，多虧明原、慧源、文嶽、振坤、振哲、宜霖等學長發揮其廣泛的知識，熱情地照顧指導我這全然不懂的新進人員。感謝他們讓我能有完善的支援來加深我的學問。另外還要感謝與我同屆的裕隆、建銘、彥翔、雲懷、勇嵐、宜鍵、大瑜及諸位後進的學弟們，有你們的陪伴使我的研究生生活能多采多姿。在遇到疑難時，也都能經由實驗室夥伴們的討論並提供出許多寶貴的意見，讓我的問題順利解決。感謝上天能讓我幸運的遇到你們，願這珍貴的友情能歷久不衰。研究生的過程中使我獲益最多的契機，便是參與聯發科技所資助的研究小組。在每一週的會議報告中，充分訓練我組織報告的能力並從其他組員那獲得不少有用而難得的資訊。在黃經堯、王國禎、陳健三位教授的帶領指導下，使得此研究小組能順利進行並維持一定的品質。在此，這篇論文特別感謝聯發科技與交通大學的金錢資助，讓我能無後顧之憂的專心進行研究，並且在與聯發科技各部門經理報告交流的過程中，更加深了我論文的正確性與完整性。最後，最需要感謝的毫無疑問是我親愛的家人，有你們細心的扶持與照顧，讓我能順利的完成學業。每當我遇到挫折時，總能感受到你們在我背後默默的關懷，讓我有勇氣的去面對一切的困難。在我取得碩士學位的這個時刻，我願將此榮耀與你們共同分享。如今我碩士論文終於圓滿完成，謹以此畢業論文獻給你們，感謝你們豐富我的人生與我的未來。. 張正達. 謹誌. 2005 年 7 月,Wintech lab,交大,新竹,台灣. iii.

(6) Contents. Chapter 1 Introduction................................................................................................................1 1.1 Overview of the Modern Cellular Systems ..................................................................1 1.2 Motivation ....................................................................................................................3 1.3 Overview of UMTS ......................................................................................................4 1.3.1 Spreading ...........................................................................................................4 1.3.2 Handover ...........................................................................................................6 1.3.3 Power Control....................................................................................................9 1.4 Overview of Quality of Service..................................................................................11 1.5 Thesis Organization....................................................................................................12 Chapter2 Overview of the Compressed Mode .........................................................................13 2.1 The Compressed Mode Architecture..........................................................................13 2.1.1 The Gap Generation Methods..........................................................................14 2.1.2 The Transmission Gap Position and the Frame Structure ...............................15 2.2.3 The Parameters of the Compressed Mode.......................................................19 2.2 Power Control on the Compressed Mode...................................................................21 2.2.1 Uplink Power Control......................................................................................21 2.2.2 Downlink Power Control.................................................................................22 2.3 Measurements of GSM carriers..................................................................................23 2.4 The Performance Impacts...........................................................................................25 Chapter 3 Capacity-Based Compressed Mode .........................................................................27 3.1 The Prior Works .........................................................................................................27 3.2 The Concept of the Capacity-Based Compressed Mode ............................................28 3.3 The Compressed Mode Format ..................................................................................30 3.4 The Algorithm of the Capacity-Based Compressed Mode.........................................33 Chapter 4 Simulation Platform.................................................................................................37 4.1 Simulation Environment and the Mobility Model......................................................37 4.2 Uplink Link Budget and the Propagation Model........................................................38 4.3 The Platform Capability .............................................................................................41. iv.

(7) Chapter 5 Experimental Results ...............................................................................................42 5.1 The Performance of the Capacity-Based Compressed Mode.....................................42 5.2 The Performance of the Factor k ................................................................................46 Chapter 6 Conclusions..............................................................................................................48 6.1 Contributions ..............................................................................................................48 6.2 Future Works ..............................................................................................................48 REFERENCES .........................................................................................................................49. v.

(8) List of Figures Figure1- 1. The evolution of cellular systems ............................................................................2 Figure1- 2. UMTS system architecture ......................................................................................3 Figure1- 3. Spreading and dispreading in DS-CDMA ...............................................................5 Figure1- 4. The OVSF code tree ................................................................................................5 Figure1- 5. Important issues involved in the handoff mechanism .............................................7 Figure1- 6. UMTS soft handover algorithm...............................................................................8 Figure1- 7. UMTS power control procedure ............................................................................10 Figure1- 8. UMTS close loop power control scheme...............................................................10. Figure2- 1. The compressed mode transmission ......................................................................13 Figure2- 2. Transmission gap position .....................................................................................16 Figure2- 3. Frame structure in uplink compressed transmission..............................................16 Figure2- 4. Frame structure types in downlink compressed transmission ...............................17 Figure2- 5. Illustration of the compressed mode pattern parameters .......................................19 Figure2- 6. Power increasing scenario in the Compressed Mode ............................................23. Figure3- 1. The capacity with different interference ................................................................29 Figure3- 2. The pilot Ec/Io decay curve in center and border cells..........................................31 Figure3- 3. The pilot Ec/Io distribution function with different londing .................................32 Figure3- 4. (a) GSM control channel (b) Compressed Mode gap pattern................................32 Figure3- 5. The compressed mode scheme (a) Normal compressed mode at simultaneous time (b) Separate the position of transmission gap (c) Schedule to suspend the compressed mode 33 Figure3- 6. The relationship of Pilot RSCP and distance .........................................................34 Figure3- 7. The relationship of RRSCP and distance ..................................................................34 Figure3- 8. The flow chart of capacity-based compressed mode .............................................36. Figure4- 1. The Simulation Platform........................................................................................37 Figure4- 2. The mobility scenario ............................................................................................38 Figure4- 3. Propagation geography model ...............................................................................40 Figure4- 4. Base station cell geometric profile.........................................................................40. vi.

(9) Figure5- 1. Average base station transmit power .....................................................................42 Figure5- 2. Average base station transmit power in the compressed frame .............................43 Figure5- 3. The best pilot Ec/Io distribution function for 45 users ..........................................43 Figure5- 4. The suspension ratio with loading .........................................................................45 Figure5- 5. The number of hand-down RSSI samples with loading (k = 2) ............................45 Figure5- 6. The number of hand-down RSSI samples with different k (User = 45) ................46 Figure5- 7. The relationship of the suspend probability and RSCP with different k ...............47. vii.

(10) List of Tables Table1- 1. The channel rate with different spreading factors .....................................................6 Table1- 2. UMTS QoS classes.................................................................................................. 11 Table2- 1. Uplink DPDCH fields .............................................................................................16 Table2- 2. Uplink DPCCH fields..............................................................................................17 Table2- 3. Downlink DPCH fields ...........................................................................................18 Table2- 4. Parameters for combined UL/DL compressed mode ..............................................19 Table2- 5. Power control in power recovery mode ..................................................................22 Table2- 6. ∆Pi_compression value with different methods ......................................................23 Table2- 7. The gap length and GSM carrier RSSI measurement .............................................24 Table2- 8. The gap length and maximum time difference for BSIC verification.....................24 Table3- 1. Compressed mode triggering threshold...................................................................31 Table4- 1. Uplink Link Budget.................................................................................................39 Table4- 2. The parameter of the propagation model.................................................................39 Table4- 3. The simulation parameter ........................................................................................41. viii.

(11) Chapter 1 Introduction 1.1 Overview of the Modern Cellular Systems In Taiwan, the most popular second generation cellular system is Global System for Mobile Communications (GSM) system [1]. GSM is also widely used even in the world, especially in Europe and Asia. The principal technology in the specification is the air interface based on the hybrid frequency-division / time-division multiple access. Each GSM carrier has 200 kHz bandwidth and occupies one of eight time slots. The maximum transmission rate is 9.6 kbps and the services can only support speech and short message service (SMS). With the demands of multimedia services, the advanced systems are designed to improve the GSM system. Figure 1-1 shows the evolution of cellular systems [2]. High Speed Circuit Switch Data (HSCSD), General Packet Radio Service (GPRS), and Enhanced Data-rate for GSM Evolution (EDGE) are sequentially proposed based on the GSM air interface and infrastructure. Universal Mobile Telecommunications System (UMTS) changes the air interface to code division multiple access (CDMA) and then enters the third generation age. Moreover, High Speed Downlink Packet Access (HSDPA) is suggested for Beyond 3G (B3G) to enhance the transmission rate over the UMTS system. HSCSD improves the speech coder to support new speed of 14.4 kbps. By using multiple time slots transmission, the maximum transmission rate can extend to 57.6 kbps. GPRS transmit the packet switch data to replace the old circuit switch date. The packet switch connection doesn’t require any dedicated end-to-end connection. It only uses network resources and bandwidth when data is actually being transmitted. This means that a given amount of radio bandwidth can be shared efficiently and simultaneously among many users. GPRS transmit a packet data service using TCP/IP and X.25 to offer speeds up to 45-160 kbps. EDGE reuses the GSM carrier bandwidth and time slot structure and changes the modulation scheme. The transmission rate substantially increases to 384 kbps at most. The advantages of EDGE include fast availability, reuse of existing GSM infrastructure, and enabling existing 2G system to deliver 3G service in existing spectrum bands.. 1.

(12) UMTS implements Wideband CDMA (WCDMA) over 5 MHz and increases the transmission rate up to 384 kbps. With combining many physical data channel, the maximum data rate can support up to 2 Mbps. HSDPA is a packet-based data service and use enhanced technologies. including. Adaptive. Modulation. and. Coding. (AMC),. Multiple-Input. Multiple-Output (MIMO), Hybrid Automatic Request (HARQ), fast cell search, and advanced receiver design. Thus, the transmission rate can up to 10 Mbps over the same bandwidth in UMTS.. Figure1- 1. The evolution of cellular systems The UMTS system [3] architecture is depicted in Figure1-2. The UMTS Terrestrial Radio Access Network (UTRAN) handles all radio-related functionality. It consists of two elements, Node B and Radio Network Controller (RNC). The RNC owns and controls the radio resources. The Core Network (CN) is responsible for switching and routing calls and data connections to external networks. Home Location Register (HLR) is the database located in the user’s home system that store the user’s service profile. Mobile Switching Center (MSC) and Visitor Location Register (VLR) are the switch and database that serves the mobile in its current location for circuit switched services. Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) are the switch and the gateway used for packet switched services. MSC is upgraded from the existing GSM network backbone and connects to circuit switched networks, Public Switched Telephone Network (PSTN) or Integrated Service Digital Network (ISDN). SGSN and GGSN are upgraded from GPRS network backbone and connect to packet data networks. UMTS only separates the architecture by UTRAN for the different air interface. Other elements are based on an evolved GSM core. 2.

(13) network, provided backward compatibility the GSM in terms of network protocol and interface. The core network can support both GSM and UMTS services in handover or roaming.. Figure1- 2. UMTS system architecture. 1.2 Motivation In order to support more applications and high transmission rates, 3G technologies have been developed. Since UMTS, one of 3G systems, is the extension of GSM system and provides backward compatible with GSM. UMTS has been deployed to overlap with the existing GSM system. In the early deployment, UMTS will be deployed mostly at urban area to save the initial capital cost. However, it needs to handover to GSM system in the border cells to ensure a seamless connection. To achieve that, there are three methods for handover as below: a. Blind Handover The mobile is not requested to perform measurements, and the target cell is chosen autonomously by the UTRAN. This speeds up the handover procedure but results in a lower handover success rate. For the inter-system handover procedure, it based on GSM measurement to guarantee a reliable handover success rate. b. Dual Transceiver The mobile is continuously transmitting and receiving on the carriers. In order to perform measurement, the mobile uses two transceivers for each carrier at the same time.. 3.

(14) This method requires additional complexity and cost for the mobile. However, it might not enough to measure the third frequency band, like GSM 1800 band. The hardware architecture will extend endlessly. c. Compressed Mode In order to save the hardware complexity, the mobile will stop receiving on one carrier and receive temporarily another one. The UMTS proposes a mechanism called “Compressed Mode” [4], which interrupts the current connection to measure the carriers of other systems. The compressed mode is chosen both for reliable handover and less hardware overhead in advanced cellular system. The UMTS specification has set up the regulars about the compressed mode. The specification suggests three methods to generate the transmission gap: (1) Reducing the spreading factor by two, (2) Puncturing, (3) Higher layer scheduling. These methods either speed up the transmission rate in physical layer or delay the data transmission in higher layer. During the compressed frame, more power is required to guarantee the quality of enhanced transmission rate. The increasing power will impact the performance, such as the capacity and coverage. If we want to measure more GSM carriers, the compressed mode is required frequently. However the more frequent compressed mode, the more impacts are suffered in performance. A capacity-based compressed mode is then proposed to tradeoff between the successfully handover and the performance.. 1.3 Overview of UMTS UMTS is a wideband Direct-Sequence Code Division Multiple Access (DS-CDMA) system. The user information bits are spread over a wide bandwidth by multiplying the user data from the spreading factor. The chip rate of 3.84 Mcps used leads to a carrier bandwidth of approximately 5 MHz. Each frame consists of 15 slots over 10 ms. The fast power control based on slot information is supported with 1.5 kHz frequency in both uplink and downlink. Then, some of the Radio Research Management (RRM) schemes [5] related to the compressed mode are introduced.. 1.3.1 Spreading DS-CDMA [5] spreads out the narrowband data by multiplying from the wideband. 4.

(15) spreading code. Each user has its own spreading code which is orthogonal to all other codes. Only the same spreading code data can be despreaded as the initial data from all other code data. Figure 1-3 depicts the basic operation of spreading and dispreading for a DS-CDMA system.. Figure1- 3. Spreading and dispreading in DS-CDMA. c 4 ,1 = (1 ,1 ,1 ,1 ). C 8 ,1 C 8 ,2. c 4 ,2 = (1 ,1 ,-1 ,-1 ). C 8 ,3 C 8 ,4. c 2 ,1 = (1 ,1 ). c 1 ,1 = (1 ) c 4 ,3 = (1 ,-1 ,1 ,-1 ) c 2 ,2 = (1 ,-1 ) c 4 ,4 = (1 ,-1 ,-1 ,1 ) SF = 1. SF = 2. SF = 4. Figure1- 4. The OVSF code tree CDMA users can transmit at higher data rates by using either multiple number of orthogonal constant spreading factor (OCSF) codes or orthogonal variable spreading factor (OVSF) code. The former supports higher data rates by assigning a multiple of OCSF codes to one call. It requires multiple transceiver units, thus resulting in increased hardware complexity. The later supports higher data rates by using lower OVSF. It may lead to a higher code blocking rate for higher data rate users. The OVSF code is suggested in UMTS standard [7] as depicted in Figure 1-4. The spreading code is derived from the lower spreading factor code. In IS-95 [8], the walsh code is generated from the former walsh code by equation 1-1. ⎡W Wn ⎤ Wn +1 = ⎢ n ⎥ ⎣Wn − Wn ⎦. (1-1). 5.

(16) Although the generation methods are different, the basic concepts and the generated codes are almost the same. The reason is only to avoid the IS-95 patents. Based on the UMTS chip rate 3.84 Mcps, the channel rate equals to the chip rate divided by the spreading factor. The corresponding channel rates are listed in Table 1-1. When achieving higher data rate or reducing the spreading factor by two in the compressed mode, the shorter OVSF code is needed. How to assign them with avoiding the code blocking is another research. The existed methods are static code assignment [9], dynamic code assignment [10], region division OVSF code assignment [11], and hybrid OVSF code assignment [12]. All of these algorithms separate the longer code from the shorter code and do their best effort to enhance the code capacity.. Table1- 1. The channel rate with different spreading factors DPDCH spreading factor 256 128 64 32 16 8 4 4,with 6 parallel codes. DPDCH channel rate (kbps) 15 30 60 120 240 480 960 5740. User data rate with 1/2 coding rate (approx.) 7.5 kbps 15 kbps 30 kbps 60 kbps 120 kbps 240 kbps 480 kbps 2.3 Mbps. 1.3.2 Handover When a mobile moves away from a base station, the signal level degrades. The weak signal strength can’t support the original services, so there is a need to switch communication to another base station. Handover is the mechanism that supports the mobility of the user terminals. There are many important issues related to handover in Figure 1-5 [13]. These issues can divide into five parts: control, methodology, metric, parameter, and performance. The handover control scenario consists of network-controlled, mobile-assisted, and mobile-controlled. The base station has the responsibility to do the measurement and decide the handover operation in network-controlled system. On the contrary, the mobile station has the responsibility in mobile-controlled system. In the mobile-assisted system, the mobile measures the strength of the base stations and then reports to the serving base station. The. 6.

(17) serving base station makes the final decision with the handover execution. Based on the system architecture, it chooses the proper handover control method. The handover methodologies include hard, soft, and softer handover. “The mobile connects to the new base station after it breaks the origin base station” is called hard handover. The mobile has only one connection at the same time. The hard handover saves the usage of the radio resource but may cause a connection dropping. For guaranteeing the connection quality, the simultaneous connection is suggested. In soft handover there are multiple cells simultaneously support a call, and in softer handover there are multiple sectors simultaneously support the call. With the soft and softer handover, the call dropping rate is apparently reduced and the received signals have more confidence by the handover gain. The handover algorithm divides into handover metrics and handover parameters. The algorithm can use pilot Ec/Io, RSCP, RSSI, SIR … etc. as the handover triggering metrics. The most popular criterion in CDMA system is the pilot Ec/Io, because it can reflect the influences of the noise. Other metrics are also used in the other radio interface systems. The handover parameters consist of hysteresis margin, dwell timer … etc. The hysteresis margin tolerates the rapidly fluctuation of the measured signal. The dwell timer avoids the ping-pong effect which means switch between two systems too frequently. At last, the handover performance reflects on call blocking, call dropping, delay … etc. According to these performances, the quality of the handover algorithm can be judged.. Figure1- 5. Important issues involved in the handoff mechanism. 7.

(18) For example of UMTS, it is a mobile-controlled soft handover system. The soft handover uses pilot Ec/Io as the handover measurement quantity. The measured base stations are divided into below three sets: Active Set: The cells form a soft handover connection to the mobile user. Monitored Set: The cells are that the mobile user continuously measures, but whose pilot Ec/Io are not strong enough to be added into the active set. Remaining Set: Other cells are not in above two sets and whose pilot Ec/Io are usually weak. ∆T. ∆T M easurement Quantity. ∆T. Pilot Ec/Io of cell 1. As_Th + As_Th_Hyst. AS_Th – AS_Th_Hyst. As_Rep_Hyst. Pilot Ec/Io of cell 2 Pilot Ec/Io of cell 2. A ⇒ Add Cell 2. ⇒ Cell 1 Connected. B. C. Time. ⇒ Replace Cell 1 ⇒ Remove Cell 3 with Cell 3. Figure1- 6. UMTS soft handover algorithm The soft handover algorithm in standard [7] as described in Figure 1-6 is as follows: ¾. If the measured signal is greater than (the best measured signal in active set - As_Th + As_Th_Hyst) for a period of ∆T and the Active Set is not full, then the cell is added to the Active Set.. ¾. If the measured signal is less than (the best measured signal in active set - As_Th As_Th_Hyst) for a period of ∆T, then the cell is removed from the Active Set.. ¾. If Active Set is full and the best measured cell in the Monitored Set is greater than (the worst measured cell in the Active Set + As_Rep_Hyst) for a period of ∆T, then the weakest cell in the Active Set is replaced by the strongest Monitored Set.. 8.

(19) Where: AS_Th: Threshold for macro diversity (reporting range); AS_Th_Hyst is the hysteresis for the above threshold. AS_Rep_Hyst is the replacement hysteresis. ∆T: Time to Trigger. The soft handover algorithm can be easily modified the parameter or add more condition to enhance the performance. The thresholds are modified and the performance is evaluated in [14]. With the soft handover, the seamless connection can be guaranteed and the connection quality also can be maintained.. 1.3.3 Power Control Power control procedures determine the power levels for transmission to and from the terminal. The effective power control can enhance the battery life and avoid the near-far problem which means the signal of near terminal may cover the far ones. If the terminal moves closer to the serving base station, the system may reduce the power levels in order to reduce interference to other calls. After processing power control, all received power levels are desired to be equal. The UMTS power control procedure [15] is depicted in Figure 1-7. When the User Equipment (UE) initially accesses to the network in uplink, it executes the open loop power control to achieve the target power level. After the connection has been set up, the close loop power control is then executed to adjust the power level. The close loop power control includes of inner loop and outer loop power control. The outer loop power control set the target signal to interference ration, SIRtarget according to the connection quality (in terms of FER). With the basis of the SIRtarget, the inner loop power control increases or reduces the power level. The inner loop power control increases the power level if SIR is larger then SIRtarget and decreases the power level otherwise. There are two alternative algorithms which the base station can instruct the mobile for modifying the power level. “Algorithm 1” is designed for use when the mobile speed is sufficiently low for inner-loop power control to act against the fading. The mobile simply increases and reduced power by 1 dB. “Algorithm 2” is designed. 9.

(20) to emulate the effect of using a step size smaller than 1 dB and is effective to avoid the rapid short-term fluctuations. The mobile increases and reduced power by 1dB only if the power control commands are continuously consistence in 5 times. Algorithm 2 gives better performance than algorithm 1 for stationary users and for high-speed users.. Figure1- 7. UMTS power control procedure Unlike the 1.5 kHz fast inner loop power control, the frequency of the outer loop power control is typically 10-100Hz. While the frame error appears and the received data quality is less than the expected value, the network increases SIRtarget to enhance the signal strength. Otherwise the network decreases SIRtarget to reduce the interference. The whole close loop power control scheme is depicted in Figure 1-8. With this power control procedure, the received signal strength is always allocated at the proper range and the connection quality can then be satisfied.. Figure1- 8. UMTS close loop power control scheme. 10.

(21) 1.4 Overview of Quality of Service The multimedia services and the file download services are become more and more popular. However the different services have different constrains. In order to satisfy all users with different services, the Quality of Service (QoS) needs to be defined. The classic QoS attributes include the following kinds: ¾. The traffic characteristics specified (in terms of bandwidth): Ex: The peak rate, the minimum acceptable rate, the average rate, the maximum burst size …. ¾. The reliability requirements of the connection: Ex: Bit Error Rate (BER), Frame Error Rate (FER), the maximum loss rate …. ¾. The delay requirements: Ex: The maximum tolerated delay, the maximum tolerated jitter … In [16], the author also proposed two advanced QoS attributes for the wireless. communication. The Seamless Service Descriptor (SSD) describes the type of seamless service the user is requesting, and the Service Degradation Descriptor (SDD) describes how much the user is willing to get a degraded service. Both of these two attributes are according to the user profile. The user profile may determine by the bill paying.. Table1- 2. UMTS QoS classes Traffic class Conversational class. Streaming class. Interactive class. Background class. Fundamental characteristic Conservative real-time No ARQ (retransmission) High sensitivity to delay and jitter Streaming real-time No ARQ High sensitivity to jitter Medium sensitivity to delay Interactive best effort ARQ High sensitivity to round trip delay High sensitivity to BER Low sensitivity to delay Background best effort ARQ High sensitivity to BER No delay sensitivity. 11. Application Voice, video-telephony Streaming multimedia. Web browsing, network games. Background download, e-mail.

(22) In UMTS standard, four traffic classes have been identified [17]. Two real time services (conversational, streaming) and two best effort service (interactive, background) are suggested. The fundamental characteristic and the application of each class are described in Table 1-2. The real time services emphasize on the delay requirements and the best effort services consider about the reliability requirements. According to the different service class, the system gives the different research and creates the maximum satisfaction.. 1.5 Thesis Organization The organization of this thesis is described as follows: Chapter 2 introduces the detail operations of the compressed mode mechanism. The physical behaviors are studied and the performance impacts are also included. In chapter 3, the problem of existed compressed mode is addressed and the enhanced control algorithm is also proposed. Chapter 4 discusses the simulation platform capability. In chapter 5, some experimental results based on the proposed compressed mode algorithm are shown. The performances of power consumption and measurement efficiency are illustrated to verify the proposed algorithm. Finally, chapter 6 gives the contributions and the advanced future works.. 12.

(23) Chapter2 Overview of the Compressed Mode This chapter introduces the detail operations of the compressed mode in the physical layer. Based on this architecture, the metrics of the GSM measurement are also addressed. Finally, the performance impacts caused by the compressed mode are discussed.. 2.1 The Compressed Mode Architecture In wireless communication systems, in order to keep a seamless connection, the handover scheme is essential. From 2nd generation, the mobile assisted handover is applied, and the mobile has the responsibility to measure other base stations’ power strength. In GSM/GPRS which are time division multiple access (TDMA) systems, the mobiles can use the slots non-transmitting to measure other carriers. In contrast to CDMA/UMTS which are code division multiple access (CDMA) systems, the mobiles transmit continuously. The measurement of other carriers becomes a problem. In UMTS, the compressed mode, with variable transmission gaps, measures the other carriers’ strength and is depicted in Figure 2-1.. (Power Level on DPCH). One frame (10 ms). Transmission gap available for inter-frequency measurements. Figure2- 1. The compressed mode transmission For the inter-system handover in UMTS, the compressed mode is operated as follows: First, the mobiles in UMTS cells need to measure the carrier strength of other systems during. 13.

(24) the transmission gap. Then, the mobiles acquire the control channel message of the measured system based on the associated carrier strength. When the information of the control channel has been collected, the handover operation will then be performed. The compressed frames can occur periodically or requested on demand. The frequency and type of the compressed mode depends on the measurement requirements, and is decided by the network.. 2.1.1 The Gap Generation Methods The UMTS implements the compressed mode to execute the inter-system handover. In UMTS standard, following three methods are suggested to prevent the data lost [4]: ¾. Reducing the spreading factor by two The first method is to double the data rate by reducing the spreading factor by two. The time saved from accelerating the data transmission rate is used for the transmission gap. This method is not supported for SF=4, because 4 is the minimum value of allowed spreading factor. Besides, more power is also required for supporting the higher transmission rate.. ¾. Puncturing The second method is to puncture the redundancy bits made by channel coding in the transport channel to allow the system to generate the gaps. This method modifies only the channel coding rate but keeps the existing spreading factor and bit rate unchanged. Due to the limitation of the physical channel format and the channel coding rate, this method applies to downlink transmission and short transmission gap only.. ¾. Higher layer scheduling Besides of increasing the speed of the physical layer transmission, the data can be processed at the higher layer for non-real time data services or voice services with enough silence time. Since the number of bits can be rescheduled, a gap can then be generated for a non-schedule period. This method does not affect the physical transmission rate so it has the least impacts on the performance but is only suitable for non-real time services. Due to the limitations of implementing the “puncturing” and “higher layer scheduling”. schemes, in this thesis “reducing the spreading factor by two” is chosen for the rest studies to. 14.

(25) satisfy the entire transmission scenarios. Beside these three methods, the system can easily give other alternatives to increase the transmission rate and then leave time for transmission gap. For example, the formula of the transmission bit rate, Rb, is defined as equation (2-1) in [18]:. Rb = (1 − γ ) ⋅ N ⋅. Rc ⋅ R ⋅ M SF. (2-1). where γ (the compressed mode ratio) is defined as the fraction of the frame left idle during the compressed mode, N is the number of parallel physical CDMA code channels used, Rc is the chip-rate, R is the channel encoding rate, M is the modulation order, and SF is the spreading factor. Base on this equation, the transmission bit can easily increased by modify the parameter in the left part. Then the system can propose four methods to generate the transmission gap [18]: a. Variable Spreading Factor Compressed Mode (VCF-CM) b. Code-Rate Increased Compressed Mode (CRI-CM) c. Multi-Code Compressed Mode (MC-CM) d. Higher Order Modulation Compressed Mode (HOM-CM) However, some methods, like method c and method d, change the radio interface architecture and then increase the hardware complexity. Other methods, like method b and method d, compact the signal constellation and then decrease the reliability of transmitted data. All these methods will be chosen only if the QoS constraints are satisfied.. 2.1.2 The Transmission Gap Position and the Frame Structure In UMTS, there are 15 slots in one frame and each frame spans 10 ms. The compressed mode has many types of Transmission Gap Length (TGL) and the TGL formats are equal to only 3, 4, 5, 7, 10, 14 slots [4]. The gap position is depicted as Figure 2-2, where Nfirst is the first slot of the transmission gaps and Nlast is the last slot of the gaps. In order to ensure the power control successfully, there are at least 8 slots transmitted in each radio frame. The transmission gap should be divided into two types, the single frame method and double frame method. Thus Nfirst and TGL must be chosen to correspond with this criterion.. 15.

(26) T ra n s m is s io n g a p. #0. R a d io fra m e. # N la s t + 1. # N firs t -1. #14. (1 ) S in g le -fra m e m e th o d. T ra n s m is s io n g a p. F irs t ra d io fra m e. #0. # N firs t -1. S e c o n d ra d io fra m e. # N la s t + 1. #14. (2 ) D o u b le -fra m e m e th o d. Figure2- 2. Transmission gap position The framing structures of the uplink and downlink are depicted in Figure 2-3 and Figure 2-4. In uplink, the Dedicated Packet Data CHannel (DPDCH) and the Dedicated Packet Control CHannel (DPCCH) are transmitted separately. The details of frame formats are listed in Table 2-1 and Table 2-2 [19]. In the uplink DPDCH fields, the data is transmitted with BPSK and the network modifies the spreading factor to change the transmission rate. In the uplink DPDCH fields, the data rate keeps the minimum rate, 15 kbps, to guarantee the reliable of the control signals. There are two possible compressed slot formats labeled as A and B in DPCCH. Label A has smaller gaps for the compressed mode and its number of transport format combination indicator (NTFCI) bits is also less for easy combination.. Slot # (Nfirst – 1). Slot # (Nlast + 1). transmission gap. Data. Data Pilot. Pilot. TFCI FBI TPC. TFCI FBI TPC. Figure2- 3. Frame structure in uplink compressed transmission Table2- 1. Uplink DPDCH fields Slot Format #i. Channel Bit Rate (kbps). Channel Symbol Rate (ksps). SF. Bits/ Frame. Bits/ Slot. Ndata. 0 1 2 3 4 5 6. 15 30 60 120 240 480 960. 15 30 60 120 240 480 960. 256 128 64 32 16 8 4. 150 300 600 1200 2400 4800 9600. 10 20 40 80 160 320 640. 10 20 40 80 160 320 640. 16.

(27) Table2- 2. Uplink DPCCH fields Slot Format #i. Channel Bit Rate (kbps). Channel Symbol Rate (ksps). SF. Bits/ Frame. Bits/ Slot. Npilot. NTPC. NTFCI. NFBI. Transmitted slots per radio frame. 0 0A 0B 1 2 2A 2B 3 4 5 5A 5B. 15 15 15 15 15 15 15 15 15 15 15 15. 15 15 15 15 15 15 15 15 15 15 15 15. 256 256 256 256 256 256 256 256 256 256 256 256. 150 150 150 150 150 150 150 150 150 150 150 150. 10 10 10 10 10 10 10 10 10 10 10 10. 6 5 4 8 5 4 3 7 6 5 4 3. 2 2 2 2 2 2 2 2 2 1 1 1. 2 3 4 0 2 3 4 0 0 2 3 4. 0 0 0 0 1 1 1 1 2 2 2 2. 15 10-14 8-9 8-15 15 10-14 8-9 8-15 8-15 15 10-14 8-9. In downlink transmission, the data channel and the control channel are interlacing into Dedicated Packet CHannel (DPCH). There are two types of frame structure labeled as A and B. In type A, the Pilot (PL) field of the last slot in the transmission gap is transmitted for synchronization. In type B, an additional Transmit Power Control (TPC) field of the first slot in the transmission gap is transmitted for optimizing the power control. The detail numbers of downlink DPCH bits are listed in Table 2-3 [19]. The data in downlink is transmitted with QPSK and the network modifies the spreading factor to change rate. There are also two possible compressed slot formats labeled as A and B. The slot format B shall be used by “reducing the spreading factor by two”, and the slot format A shall be used by “puncturing” or “higher layer scheduling”.. Slot # (Nfirst - 1) T TF Data1 P CI C. Data2. Slot # (Nlast + 1). transmission gap. T TF PL Data1 P CI C. PL. Data2. PL. (a) Type A: maximize the transmission gap length. T TF Data1 P CI C. Data2. Slot # (Nlast + 1). transmission gap. Slot # (Nfirst - 1). PL. T TF PL Data1 P CI C. T P C. Data2. (b) Type B: optimized the power control Figure2- 4. Frame structure types in downlink compressed transmission. 17. PL.

(28) Table2- 3. Downlink DPCH fields Slot Format #i. 0 0A 0B 1 1B 2 2A 2B 3 3A 3B 4 4A 4B 5 5A 5B 6 6A 6B 7 7A 7B 8 8A 8B 9 9A 9B 10 10A 10B 11 11A 11B 12 12A 12B 13 13A 13B 14 14A 14B 15 15A 15B 16 16A. Channel Channel Bit Rate Symbol (kbps) Rate (ksps) 15 15 30 15 30 30 30 60 30 30 60 30 30 60 30 30 60 30 30 60 30 30 60 60 60 120 60 60 120 60 60 120 60 60 120 120 120 240 240 240 480 480 480 960 960 960 1920 1920 1920. 7.5 7.5 15 7.5 15 15 15 30 15 15 30 15 15 30 15 15 30 15 15 30 15 15 30 30 30 60 30 30 60 30 30 60 30 30 60 60 60 120 120 120 240 240 240 480 480 480 960 960 960. SF. Bits/ Slot. DPDCH Bits/Slot. DPCCH Bits/Slot. NData1 NData2 NTPC NTFCI NPilot 512 512 256 512 256 256 256 128 256 256 128 256 256 128 256 256 128 256 256 128 256 256 128 128 128 64 128 128 64 128 128 64 128 128 64 64 64 32 32 32 16 16 16 8 8 8 4 4 4. 10 10 20 10 20 20 20 40 20 20 40 20 20 40 20 20 40 20 20 40 20 20 40 40 40 80 40 40 80 40 40 80 40 40 80 80 80 160 160 160 320 320 320 640 640 640 1280 1280 1280. 0 0 0 0 0 2 2 4 2 2 4 2 2 4 2 2 4 2 2 4 2 2 4 6 6 12 6 6 12 6 6 12 6 6 12 12 12 24 28 28 56 56 56 112 120 120 240 248 248. 4 4 8 2 4 14 14 28 12 10 24 12 12 24 10 8 20 8 8 16 6 4 12 28 28 56 26 24 52 24 24 48 22 20 44 48 40 96 112 104 224 232 224 464 488 480 976 1000 992. 2 2 4 2 4 2 2 4 2 2 4 2 2 4 2 2 4 2 2 4 2 2 4 2 2 4 2 2 4 2 2 4 2 2 4 4 4 8 4 4 8 8 8 16 8 8 16 8 8. 0 0 0 2 4 0 0 0 2 4 4 0 0 0 2 4 4 0 0 0 2 4 4 0 0 0 2 4 4 0 0 0 2 4 4 8* 16* 16* 8* 16* 16* 8* 16* 16* 8* 16* 16* 8* 16*. 4 4 8 4 8 2 2 4 2 2 4 4 4 8 4 4 8 8 8 16 8 8 16 4 4 8 4 4 8 8 8 16 8 8 16 8 8 16 8 8 16 16 16 32 16 16 32 16 16. Transmitted slots per radio frame, NTr 15 8-14 8-14 15 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14 8-14 15 8-14. From the above discussions, the related parameters are summarized in Table 2-4. It lists the difference transmission time reduction method between uplink and downlink, and the related transmission gaps and frame formats.. 18.

(29) Table2- 4. Parameters for combined UL/DL compressed mode TGL. DL Frame Type. Idle length [ms]. Transmission time Reduction method. 3. 1.47 – 1.73. 4. 2.13 – 2.39. DL: Puncturing, Spreading factor division by 2 or Higher layer scheduling. 5. Spreading Factor. DL: 512 – 4. 2.80 – 3.06. UL: 256 – 4. 4.13 – 4.39. A or B 7 10. 6.13 – 6.39. 14. 8.80 – 9.06. UL: Spreading factor division by 2 or Higher layer scheduling. Idle frame Combining (S) (D) =(1,2) or (2,1) (S) (D) =(1,3), (2,2) or (3,1) (S) (D) = (1,4), (2,3), (3, 2) or (4,1) (S) (D)=(1,6), (2,5), (3,4), (4,3), (5,2) or (6,1) (D)=(3,7), (4,6), (5,5), (6,4) or (7,3) (D) =(7,7). 2.2.3 The Parameters of the Compressed Mode In response to request the compressed mode from higher layers, the UTRAN shall signal to the UE the compressed mode parameters [20]. Parts of parameters indicate the position of the transmission gap pattern and are depicted in Figure 2-5. The position parameters include Transmission Gap Starting Slot Number (TGSN), Transmission Gap Length (TGL), Transmission Gap start Distance (TGD), Transmission Gap Pattern Length (TGPL), Transmission Gap Pattern Repetition Count (TGPRC), and Transmission Gap Connection Frame Number (TGCFN). Based on these parameters, the connection generates two transmission gap patterns to measure carriers of other systems.. #1. #2. #3. #4. #5. T G pattern 1. T G pattern 2. T G pattern 1. T G pattern 2. T G pattern 1. #T G P R C T G pattern 2. TGCFN. T G pattern 1. T G pattern 2. T ransm ission T ransm ission. T ransm ission. T ransm ission. gap 1. gap 2. gap 1. TGSN. gap 2. TG SN T G L1. T G L1. T G L2. T G L2 TGD. TG D T G P L1. T G P L2. Figure2- 5. Illustration of the compressed mode pattern parameters. 19.

(30) Besides the parameters defining the positions of transmission gaps, the physical behaviors of the compressed mode also characterized by following parameters: ¾. UL/DL compressed mode selection: This parameter specifies whether the compressed mode is used in uplink only, downlink only or both uplink and downlink. In most cases compressed frames are used in the downlink. If they are necessary in downlink and uplink, they must need time-aligned with some time offset.. ¾. UL/DL compressed mode method: The methods for generating the uplink or downlink compressed mode gap are puncturing, spreading factor division by two or higher layer scheduling. The method is decided by the requested services and channel condition.. ¾. Downlink frame type: This parameter defines if frame structure type 'A' or 'B' shall be used in downlink compressed mode. The frame structure type A maximizes the transmission gap and the type B is optimized for power control.. ¾. Scrambling code change: This parameter indicates whether the alternative scrambling code is used for the compressed mode method 'reducing the spreading factor by two'.. ¾. RPP: Recovery Period Power control mode specifies the uplink power control algorithm applied during recovery period after each transmission gap in the compressed mode. The recovery period enlarges the control step sizes and accelerates the power control.. ¾. ITP: Initial Transmit Power mode selects the uplink power control method to calculate the initial power after the gap. The initial transmit power control resumes the power to initial level immediately. Higher layers will base on those parameters to execute the compressed mode. It should. ensure that the compressed mode gaps do not overlap and the performance is satisfied the QoS constraints.. 20.

(31) 2.2 Power Control on the Compressed Mode Since there is no power control during the transmission gaps, it needs higher power to maintain the connection quality (BER, FER, etc.). In the compressed mode, the power level is increased and the power control scheme is different from the normal situation [15].. 2.2.1 Uplink Power Control The basic formula for uplink power control is in equation (2-2), where ΔDPCCH is the change of power level in DPCCH, ΔTPC is the step size per change, and TPC_cmd is the power control command to decide whether to increase or decrease the transmitted power. ∆DPCCH = ∆TPC x TPC_cmd. (2-2). If the estimated Signal to Interference Ratio SIRest is greater than the preset target SIR SIRtarget, then TPC_cmd is equal to -1 and the power level is decreased toward SIRtarget on next slot. On the contrary, if SIRest is less than SIRtarget, then TPC_cmd is equal to 1 and the power level is increased toward SIRtarget. During the compressed mode, the power level might need to be increased to ensure the quality, and the target SIR increases as SIRcm_target depicted in equation (2-3). The frame which exists of transmission gap and the next frame should increase DeltaSIR and DeltaSIRafter for connection quality. From Table 2-2, it can be observed that pilot bits in uplink compressed mode will be decreased, and the synchronization ability will be degraded. The mobile should increase addition power level to achieve synchronized. SIRcm_target = SIRtarget+∆PILOT+ DeltaSIR + DeltaSIRafter. (2-3). In additional after the transmission gap, UMTS proposes two optional mechanisms as the power resume mode and the power recovery mode to accelerate power control. The power resume mode sets the mobile initial transmit power after a transmission gap. This can take one of two values: either the same power as immediately before the gap, or a value equal to the approximated average of the transmit power by the recursive function listed in equation (2-4). The power resume mode resumes the initial power and restarts the power control.. 21.

(32) δi = 0.9375δi-1 – 0.96875 x (Most_Recent_power_change) δi-1 = δi. (2-4). The second mechanism provided to optimize power control after the transmission gaps is the power recovery mode. The power recovery mode controls the power control step size and algorithm as listed in Table 2-5 for a number of slots after each transmission gap. This mode replaces the slower power control algorithm 2 to the faster algorithm 1, and enlarges the step size. By operating power recovery mode, the mobile can accelerate the power control and speed up to acquire the correct power level. Table2- 5. Power control in power recovery mode Outside Recovery Period Alg. 1, 1dB step Alg. 1, 2dB step Alg. 2, 1dB step. In Recovery Period Algorithm Step Size (dB) 1 2 1 3 1 1. 2.2.2 Downlink Power Control The basic formula for downlink power control is listed in equation (2-5), where PTPC(k) is the kth power adjustment due to the inner loop power control, and Pbal(k) is a correction according to the downlink power control procedure for balancing radio link powers towards a common reference power. P(k) = P(k - 1) + PTPC(k) + Pbal(k). (2-5). During the compressed mode, the base station also needs to increase power level by adding a term PSIR(k) for ensuring the quality and is modified to equation (2-6). This added term can be formulated as equation (2-7). The frame which exists of transmission gap and the following frame should increase DeltaSIR and DeltaSIRafter. With different compressed method, different ∆Pi_compression values have been applied as Table 2-7. P(k) = P(k - 1) + PTPC(k) + PSIR(k) + Pbal(k). (2-6). PSIR(k) = Δ( ∆Pi_compression + DeltaSIR + DeltaSIRafter ). (2-7). When the reducing the spreading factor by two method has been applied, the most data rate is increased. And thus the largest power, 3 dB, is increased. When the puncturing method. 22.

(33) is applied, the increased power is according to the transmission gap length. When the higher layer scheduling method is applied, the data rate maintains the same value. And thus power rising is not needed. Table2- 6. ∆Pi_compression value with different methods ∆Pi_compression. Method. 3 dB. Reducing the spreading factor by two. 10 log (15*Fi / (15*Fi - TGLi)) dB. Puncturing. 0 dB. High layer scheduling or Non-compressed frame. Finally, the power increasing scenario is simply summarized in Figure 2-6, where the transmission rate could be increased to compensate the “no transmission” during the gaps. With higher transmission rate and no power control during the transmission gaps, the system should increase the transmission power to overcome unpredictable channel variance at the compressed frame and next frame by DeltaSIR and DeltaSIRafter. Besides, the mobile increases the power to compensate the reduced pilot bits used for synchronization in the uplink as ∆PILOT . In the downlink, a base station also increases the corresponding power by ∆Pi_compression to ensure the quality of increased data rate. UL : ΔPILOT DL : ΔPi_compression. DeltaSIR1 DeltaSIR2. Power Level DeltaSIRafter1 DeltaSIRafter2. 10ms. R. 2R. R. Data Rate. Figure2- 6. Power increasing scenario in the Compressed Mode. 2.3 Measurements of GSM carriers The system can measure FDD, TDD, or GSM carriers during transmission gap. Now, the following will talk about the GSM measurements with three steps [21]: GSM carrier Received Signal Strength Indicator (RSSI) measurement, Initial Base Station Identity codes (BSIC) identification, and BSIC re-confirmation. ¾. GSM carrier RSSI measurement A UE supports GSM measurements using the compressed mode shall meet the. 23.

(34) minimum number of GSM RSSI carrier measurements specified in Table 2-7. The UE measures parts of the samples of carriers in the first transmission gap, and measures the remaining samples in the next gaps until complete measuring all carriers. The gaps should synchronize to transmitting frames in GSM, such as Frequency Correlation CHannel (FCCH) or Synchronization CHannel (SCH), to ensure detecting the GSM carrier RSSI successfully. For example [22]: If the pattern with 7 slot gap every 3rd frame is applied, and assuming the maximum 32 carriers, which need 3 samples for each carrier in GSM neighbor list. The UE needs (3*32)/6 * 30ms = 480ms to complete the measurement. Note that 480ms is equal to the time that one Slow Associated Contorl Channel (SACCH) message has been transmitted in GSM. The timing of measurement interval is almost the same. Table2- 7. The gap length and GSM carrier RSSI measurement TGL 3 4 5 7 10 14. ¾. Number of GSM carrier RSSI samples in each gap. 1 2 3 6 10 15. Initial BSIC identification The initial BSIC identification includes searching for the BSIC, which is transmitted in SCH, and decoding the BSIC for the first time when there is no knowledge about the relative timing between the FDD and GSM cell. The UE shall trigger the initial BSIC identification within the available transmission gap in Table 2-8. The UE shall use the last available GSM carrier RSSI measurement results and arrange GSM cells in signal strength order for performing BSIC identification. For GSM cells that are requested to decode the BSIC with 8 strongest carriers. If the UE has not successfully decoded the BSIC within a preset abort time, the UE shall abort the BSIC identification attempt and searching the second strong carrier. Table2- 8. The gap length and maximum time difference for BSIC verification Gap length [slots]. Maximum time difference [µs] ± 500 ± 1200 ± 2200 ± 3500. 5 7 10 14. 24.

(35) ¾. BSIC re-confirmation The BSIC re-confirmation is tracking and decoding the BSIC of a GSM cell after initial BSIC identification is performed. The UE shall trigger the BSIC re-confirmation within the available transmission gap. If the UE fails to decode the BSIC after two successive attempts or if the UE has not been able to re-confirm the BSIC within a preset abort time, the UE shall abort the BSIC re-confirmation attempts and then move to the initial BSIC identification procedure again.. 2.4 The Performance Impacts Although the compressed mode could help on the inter-system handover, some system performance besides of the signal quality (such as Bit Error Rate or Frame Error Rate) will also be impacted: ¾. Coverage Because more power is needed for the same QoS constraint during the compressed mode, the maximum path loss budget is reduced. As a result, the distance that the mobile can transmit is reduced, thus the uplink coverage is reduced. In [5], the path loss will be reduced by 2.3dB if compressed frame is operated every second frame.. ¾. Capacity Since the power control cannot work during compressed frame, the transmitted data needs higher Eb/N0 requirement to maintain the quality. The capacity will be decreased by the higher Eb/N0 requirement. The capacity can be reduced by about 2% even only 10% of users are operating the compressed mode every third frame [5].. ¾. Code Shortage Problem When the “reducing the spreading factor by two” method is applied, the blocked code is twice than before and the available code space is reduced. There might not have enough codes to support all active users especially for high data rate users (low spreading factor). There are three proposed solutions for the code shortage problem: 1. Using Non-orthogonal scrambling codes. 2. Avoid the transmission gaps from different mobiles overlap in one frame, the spreading factor would be used in fair distribution [23]. 3. Base station assigns a dynamic common channel to a mobile station performing inter-frequency/system handover [24].. 25.

(36) Here, the challenges are how to find a robust mechanism to get the best efficiency and performance by allocating the transmission gaps in various fading and the relative uplink/downlink power control before and after the transmission gaps for different fading condition. It is critical to have an adaptive compressed mode operation to minimize the impacts on system performance.. 26.

(37) Chapter 3 Capacity-Based Compressed Mode The performance impacts of the compressed mode are studied. To reduce the impacts and maintain the compressed mode efficiency, a capacity-based compressed mode is proposed in this chapter. The concepts and the implement of the proposed control algorithm are discussed here.. 3.1 The Prior Works So far, many articles discuss the compressed mode performance. However, they mostly modify compressed mode algorithms to enhance the border-cell handover success rate without concerning the potential impacts caused by the compressed mode. Besides of three compressed mode methods suggested by 3GPP standard [4], Gustafsson et al. [18] provide the formula of the transmission bit rate and relative parameters to generate the transmission gap. Moreover, the issues of triggering criteria for the compressed mode are researched. Ying et al. [25] compare the periodic and event-triggered compressed mode. With extra overhead cost, the period-triggering algorithm has higher handover success rate. Zhang [26] considers the base line quality and suggests that the pilot Ec/Io is suitable for high traffic load and the pilot received signal code power (RSCP) is suitable for low traffic load. The combined pilot Ec/Io and pilot RSCP triggering method is then proposed to guarantee good system performance under all traffic loads. The relationship of the compressed mode gap pattern and GSM carriers measurement are considered in [22][27]. The required handover time for different transmission gap patterns is studied. During each compressed frame, more power is suggested to guarantee the quality of increased transmission rate. The interference caused by the increasing power will impact the performance in either capacity or coverage. Holma and Toskala [5] quantify the degradation of the capacity and coverage. Some discussions in 3GPP TSGR4 meetings also address the impact scenarios [28-30]. Although the above articles point out the performance impacts, no proper solution is proposed. To resolve that, a capacity-based compressed mode is proposed. 27.

(38) to balance the tradeoff between the handover success rate and the capacity. If the mobiles in the border of UMTS cells want to hand down to GSM cells, it uses the compressed mode to measure GSM carriers. However the GSM measurement is often not enough in current implementation. The critical problem is that measure at wrong time when GSM doesn’t transmit signals. With the wrong time measurement, not only measure no signal but also waste power without any profit. It prefers that all mobiles measure at the right time simultaneously and then use the proposed capacity-based compressed mode to limit the interference level. It benefits both effective measurements and performance maintenance.. 3.2 The Concept of the Capacity-Based Compressed Mode The increasing power is to compensate the influence of the transmission gaps, but it also impacts the performance on capacity or coverage. In this thesis, the fixed cell size is calculated by the maximum path loss, so only the capacity impacts are considered. To calculate the uplink capacity, the required bit energy to interference ratio (Eb/I0) can be computed in equation (3-1). For simplicity, the same traffic services and the same received power at the base station are assumed for all mobiles.. Eb S ⋅ PG ≈ I o FN thW + α [(1 + β )( N − 1) S + N CM ∆S ]. (3-1). where Eb is the received bit energy, Io is the total interference, S is the received power at the base station, PG is the processing gain, F is the noise figure, Nth is the thermal noise power density, W is the transmission bandwidth, α is the voice activity, β is the adjacent cell interference factor, N is the number of users, NCM is the number of users in the compressed mode, and ΔS is the average increasing power for the compressed mode. The capacity, Nc, can then be calculated in equation (3-2), where. ratio. The capacity will be deducted by NC ≈. ⎛ Eb ⎜⎜ ⎝ Io. ⎛ N CM ⋅ ∆S ⎞ ⎜⎜ ⎟⎟ ⎝ (1 + β )S ⎠. ⎞ ⎟⎟ ⎠ t arg et. is the target bit energy to interference. for the compressed mode.. FN thW PG M∆S +1− − α (1 + β ) ( Eb I o ) t arg et α (1 + β ) S (1 + β ) S. (3-2). The more users in the compressed mode the more capacity will be degraded. So a capacity-based compressed mode is proposed to reduce the impact.. 28.

(39) According to equation 3-2, Figure 3-1 shows the capacity with varied interference. The capacity with the compressed mode is obviously lower than the capacity without the compressed mode. Initially the scenario of many users with the compressed mode is located at point a. The interference exceeds the maximum tolerated value and the capacity is only CCM. In order to enhance the capacity, the number of users with the compressed mode is reduced and the relation curve tends to the non-compressed mode curve gradually (the operating point towards b). The interference is then reduced and the capacity can increase toward CNCM (as point c). Either the number of users in the cell is reduced (as point d), and the interference is low enough to restart the suspended compressed mode. With the suspended compressed mode users operating the compressed mode again, the operating point tends toward point e. Last, when the number of users increases, the operating point backs to point a. This plot announces that arranging the compressed mode operation can effectively improve the capacity.. Number of users. w/o CM CNCM CCM. with CM. c b d. a. e Imax. Interference. Figure3- 1. The capacity with different interference When concerning about the downlink capacity, the downlink capacity is decided from the maximum power budget as equation (3-3): ⎡ ⎤ ⎛E ⎞ ) ⋅ M i (CM )⎥ ≤ Pmax Ptot = POH + ∑ ⎢α ⋅ Pi (⎜⎜ b ⎟⎟ N ⎢ ⎥⎦ ⎝ I o ⎠ t arg et ⎣. (3-3). where Ptot is the total base station transmitted power, POH is the overhead power, N is the number of users in the cell, α is the voice activity, Pi is the base station transmitted power for single user, Mi is the multiplier of compressed mode power increase, and Pmax is the maximum power budget. The Pi is decided by the target bit energy to interference ratio. 29.

(40) ⎛ Eb ⎜⎜ ⎝ Io. ⎞ ⎟⎟ ⎠ t arg et. and the Mi is decided by whether the compressed is operated or not. Mi is equal to 1. when the compressed mode is not operated. When operating the compressed mode Mi is equal to the multiple of power increase. The total transmitted power is required to be less than the maximum power budget. It can be observed that the more power increases in the compressed mode the less capacity could be supported. Whether the uplink or the downlink, the capacity is decreased when the number of the compressed mode users is increasing. Consequently, a capacity-based compressed mode is required for both uplink and downlink transmission.. 3.3 The Compressed Mode Format The compressed mode format is composed of gap generation method, triggering criteria and gap pattern. In the simulation, these factors are considered as below: a. Gap generation method As discuss in chapter 2: Puncturing cannot be used in uplink, and can only generate small gaps. Scheduling cannot apply to real time service. Only reducing the spreading factor by two is chosen into simulation among the three methods and can be suitable for all 3G services. b. Compressed mode triggered criteria The pilot Ec/Io is the typical threshold for the UMTS soft handover. However the pilot Ec/Io is not suitable for the compressed mode triggering due to following two reasons. The first reason is the border cell effect which means the Ec/Io decays smoothly when the mobile away from the base station. In border cells, the pilot signal and the interference are under the same fading condition, thus the pilot Ec/Io will hold the value until hitting the noise limit, where the total interference is dominated by the background noise. Figure 3-2 shows the different Ec/Io degrade scenario between the center cell and the border cell. It can obviously see the curve keeps consistence in the border cell and the pilot Ec/Io can not reflect the signal strength. The border cell effect is also illustrated in [31] and the curve also follows the same trend. The second reason is that the pilot Ec/Io is influenced by the different loadings. Figure 3-3 shows the pilot Ec/Io varies with different loadings. In the center cell, the UMTS uses relative threshold to trigger handover. However in the border cell, the mobile might measure only one pilot signal. As a result, the algorithm hardly finds a proper absolute threshold adapt to all the different loading. In this case, the pilot Ec/Io is not suitable for being the compressed mode triggering and handover decision.. 30.

(41) To ensure in-time measurement and avoid unnecessary compressed mode triggering, the event triggering by pilot RSCP is chosen. For a baseline performance of measurement, the simulation starts the compressed mode when the pilot RSCP is smaller than -95 dBW and stop the compressed mode when the pilot RSCP is larger than -90 dBW. If the mobile keeps on going out, it will hand-down to GSM cells. The handover triggers when the RSCP is smaller than -118 dBW for 500 ms. The trigger timer is used for avoiding the ping-pong effect. The above parameters are listed in Table 3-1. The threshold settings are according to the distance from the base stations with zero fading.. Table3- 1. Compressed mode triggering threshold Threshold. Value. Distance. Compressed mode start threshold for pilot RSCP. -104 dBW. 0.5*radius. Compressed mode stop threshold for pilot RSCP. -108 dBW. 0.6*radius. Handover triggering threshold for pilot RSCP. -118 dBW. radius. Time to trigger handover. 500 ms. Figure3- 2. The pilot Ec/Io decay curve in center and border cells. 31.

(42) Figure3- 3. The pilot Ec/Io distribution function with different londing c. Gap pattern to measure GSM carrier In GSM, only Frequency Correlation Channel (FCCH), Synchronization Channel (SCH) and Broadcast Channel (BCH) are transmitted at all time. To be useful, the measurement of GSM carriers and Base Station Identity codes (BSICs) [14] should only on FCCH and SCH. However, the gap patterns in UMTS specification [14] are not guaranteed to match with the GSM timing structure. Ideally, the measurement gap should be 9.2ms for every 46ms, but the formula does not match UMTS format. In the simulation, the gap time of 14 slots for every 5 frame (9.3ms for every 50ms) is chosen to approach the GSM control frame structure. The GSM channel scenario is depicted in Figure 3-4(a) and the corresponding gap pattern is depicted in Figure 3-4(b). F S BBBBP P P P F S P P P P P P P P F S P P P P P P P P F S P P P P P P P P F S P P P P P P P P X (a) (b). 46ms. 46ms. 46ms. 46ms. 50.6ms. Figure3- 4. (a) GSM control channel (b) Compressed Mode gap pattern. 32.

(43) 3.4 The Algorithm of the Capacity-Based Compressed Mode When a mobile needs to measure other systems such as GSM, the mobile can measure only at few measurable time slots. As a result, all the users in border cells will execute the compressed mode simultaneously to match the measurable time slots as depicted in Figure 3-5(a). The increasing aggregate power could cause a serious impact on the capacity. To resolve the power problem, two methods are suggested and are depicted in Figure 3-5(b) and 3-5(c). The first one is to separate the position of transmission gaps and spreads out the aggregate increasing power. The second one is to schedule the order of the execution of the compressed mode. However, the first one moves the transmission gap to the adjacent time slot but there is no guarantee that the new measurement interval can match with the actual transmission slots of other systems. As a result, the second method is chosen for the proposed algorithm. User 1 (a) User 2 ․ ․ ․. ․ ․ ․. User 1 (b) User 2 ․ ․ ․. ․ ․ ․. User 1 (c) User 2 ․ ․ ․. ․ ․ ․. Figure3- 5. The compressed mode scheme (a) Normal compressed mode at simultaneous time (b) Separate the position of transmission gap (c) Schedule to suspend the compressed mode First, the relationship of pilot RSCP versus the distance from the base station is depicted in Figure 3-6. When the mobile is close to the base station, the curve shows the exponential increase of the RSCP. When the mobile is away from the base station, the curve tends to stay linear. By using the linear relationship, the critical pilot RSCP ratio, RRSCP, to represent the distance that needs performing the compressed mode is calculated in equation (3-4).. 33.

(44) RRSCP =. Tstop − RSCP. (3-4). Tstop − Tho. where Tstop is the threshold to stop the compressed mode, and Tho is the threshold to trigger border-cell handover. According to the previous compressed mode format, the compressed mode operates only in between Tstop and Tho (RRSCP is ranged in between 0 to 1). The compressed mode stops when RRSCP equals to 0. The hand-down to GSM occurs when RRSCP equals to 1. The relationship between RRSCP and RSCP is shown in Figure 3-7. According to RRSCP, it can estimate the proportion of the effective distance for the compressed mode operating.. Figure3- 6. The relationship of Pilot RSCP and distance Tstop Tstart. Tho. RSCP Status. Non-compressed mode. RRSCP. Compressed mode. 0. Figure3- 7. The relationship of RRSCP and distance. 34. 1.

(45) The proposed capacity-based compressed mode limits the power level in the compressed frame by suspending low priority users from operating the compressed mode. Thus it can ensure the capacity while maintaining the proper hand-down priority. The following steps of this algorithm are described as follows: (1) Every frame, the suspend factor Fi(n) base on the pilot RSCP ratio RRSCP, the aggregate measured GSM samples Nmeas(n), and the record of last time compressed mode Rsuspend is calculated in equation (3-5). Suspend factor : Fi (n) =. N meas ,i (n − 1) RRSCP ,i (n) k. (3-5). × Rsuspend ,i (n). ⎧1, last compressed mod e has been suspended Rsuspend ,i (n) = ⎨ ⎩2, otherwise. (3-6). As shown, the algorithm prefers to suspend the users close to the base station (small RRSCP). The users keep larger sampled GSM carriers, Nmeas, will easily be suspended. The. schedule algorithm wants to make each user have the same. N meas RRSCP k. ; it can balance the. aggregate measured samples with the effective distance for the compressed mode operating. Besides, there is a tunable factor k to modify the schedule algorithm. The factor k modifies the dominant degree of the distance. Finally, the Rsuspend equals to 1 when the last time compressed mode operation have been suspended, otherwise it equals to 2 as equation (3-6). This term halves the suspend priority for users who just has been suspended and is designed to reduce the chance of the continuous suspension of the same user. The continuous suspension may delay the measurement efficiency and it might be delayed to handle the emergency handover. (2) The second step is to observe the base station transmitted power and set a suspend threshold of transmitted power Pthr, which is smaller than the maximum transmitted power budget. If the estimated base station transmitted power, PBSest, doesn’t exceed the threshold Pthr and then the system can operate the compressed mode without any suspension to guarantee the measurement efficiency. If the PBSest exceeds Pthr, then the system suspends the compressed mode according to the priority order based on Fi(t) until the transmitted power below Pthr or there is no other compressed mode available. The flow chart of the capacity-based compressed mode is depicted as Figure 3-8. The base station collects the information of the compressed mode and computes the suspend factor for. 35.