Bachelor Thesis HSDPA CQI Mapping Optimization Based on Real Network Layouts

Transcription

1 Bachelor Thesis HSDPA CQI Mapping Optimization Based on Real Network Layouts Supervisor: Prof. Dr.-Ing. Markus Rupp Assistant: Dipl.-Ing. Martin Wrulich by María Elsa Feliz Fernández Institute of Communications and Radio-Frequency Engineering November October 2008

2 Acknowledgements I would like to thank all those people who have somehow supported me during my work in this Bachelor Thesis and also during all my career in Madrid and Vienna. First I want to express my gratitude to Professor Markus Rupp for his supervision and warm wellcome, and also to all the staff of the Institute who did my time in the university more comfortable. I am specially grateful to my assistant Martin Wrulich for his constant support, guidance and patience in the development of this thesis, for the interest he expressed since the first day and the general support in my abroad experience. I am deeply grateful to my parents, who have always expressed their love, support and interest and have given me the opportunity to study abroad, and also for visiting Vienna with me. I want specially thank to my brother for helping me everytime he could, and all my friends in Madrid and León. I want to thank also the friends that I met in Vienna who did my life there during six months really special and happy and showed their interest in my work, specially Corinna, Alberto, Cristina and the turkish girls. I thank their lovely frienship and support. I want to dedicate a special mention to Borja, for coming with me to the adventure of having an Erasmus experience in Vienna, for his constant support and love during this years, for his care and patience and for giving me courage in the difficults moments since we started our careers. Without his support and help this project would have never been possible. i

3 Abstract In this bachelor thesis, the SISO HSDPA simulator developed for Mobilkom Austria AG shall be extended in order to handle real network layout data. The Mobilkom Austria AG will provide measured path-loss matrices of a HSDPA cluster. This data has to be converted in a suitable form to be analyzable in the simulator. Furthermore, a memory efficient loading of the data has to be implemented. Based on this real network data, an optimization of the CQI mapping of HSDPA mobiles shall be performed in order to find the mapping which maximizes the overall cell throughput. This mapping can be used to implement a suitable CQI re-mapping at the Node-B, granting an optimum HSDPA network performance. The source-code has to be developed in MATLAB to ensure simple debugging and feature extendability. ii

4 Contents 1 Introduction Third Generation Services Technology Work environment Objectives Structure of the thesis HSDPA Principles Introduction HSDPA Standardization HSDPA vs Release 4 DCH Radio resource management architecture HSDPA operation principle HSDPA channels HSDPA new channels High-speed dedicated physical control channel HSDPA Simulator Introduction System model Simulation process Load settings Precalculations Simulation loop Results Real Network Layouts Introduction Files structure and information Network setting based on real layout data iii

5 5 CQI Mapping Optimizations CQI Basis in HSDPA CQI mapping proposal CQI Table CQI optimizations in the Simulator Source Code Enhancements Consequences Results Miscellaneous New scenarios More efficient implementation of the pathloss generation Further enhancements Outage users Throughput figures Conclusions Simulator enhancement Results Future enhancements iv

6 List of Figures 1.1 View coverage map about HSDPA deployment in the world, [1] Estimated cell throughput per sector, [2] Downlink data rates, [2] Fundamental properties of the DCH and HS-DSCH, [2] RRM architecture, [2] HSDPA Node B scheduling principle, [2] HSDPA operation channels HS-DSCH channel coding chain, [3] QAM and 4QAM constellations, [2] Relative timing between HS-SCCH and HS-DSCH, [2] Three main steps in the simulator: load settings, precalculations (i.e Node-B and users positions in order to prepare the network) and simulation loop to obtain the HSDPA data rate Network layout with 7 and 19 base stations Example of the grid positions generation in the serving cell Example of the users position in the serving cell Overview of the basic steps in the simulator Average data rates with RNC power control of the HS-DSCH Information extracted from the header of the files Read pathloss from data files given by Mobilkom AG Node-Bs positions and respective prediction files Provisional grid, main BS and sector shape Last step: user positions located randomly and uniformly within the limits of the generated sector (green points) Diagram of the overall handling real layouts data process High-Speed Dedicated Physical Control Channel that carries the uplink v

7 5.2 HS-DSCH link adaptation principle: (1) the UE reports lowquality channel information and the Node B allocates a low bit rate; (2) the UE reports high-quality channel information and the Node B allocates a high bit rate, [2] Block diagram showing the received signal at the HSDPA user and report of the CQI to the serving HS-DSCH cell, [4] CQI mapping Coarse view of the simulation including slope and shift Different values of the slope for the CQI mapping Different values of the shift for the CQI mapping Throughput as function of the slope value with shift = Throughput as function of the shift value with slope = Throughput as function of the slope and shift Four possible user positions configuration Snapshot and exhaustive snapshot scenarios Fixed angle and fixed distance scenarios Network with 19 base stations and 3 sectors model Variation of the user pathloss with the distance Variation of the user pathloss with the angle HSDPA outage users as function of the power Throughput as a function of the angle with two fixed BS - user positions distances (50 meters in the figure of the left and 250 meters in the right) Throughput as a function of the distances with two fixed angles between the users and the BS (80 o in the figure of the left and 40 o in the right) vi

8 Abbreviations 16QAM - 16-Quadrature Amplitude Modulation 3G - Third Generation 3GPP - Third Generation Partnership Project AMC - Adaptive Modulation and Coding ARP - Allocation and Retention Priority ARQ - Automatic Repeat Request AWGN - Additive White Gaussian Noise BLER - Block Error Rate BS - Base Station BTS - Base Transceiver Station CDMA - Code Division Multiple Access CmCH-PI - Common Transport Channel Priority Indicator CPICH - Common Pilot Channel CQI - Channel Quality Indicator CSI - Channel State Information DEM - Digital Elevation Models FCS - Fast Cell Selection FCSS - Fast Cell Site Selection FP - Frame Protocol GGSN - Gateway GPRS Support Node GSM - Global System for Mobile Communications HARQ - Hybrid Automatic Repeat Request HSDPA - High-Speed Downlink Packet Access HS-DPCCH - Dedicated High-Speed Physical Control Channel HS-DSCH - High-Speed Dedicated Shared Channel HSPA - High-Speed Packet Access HS-PDSCH - High-Speed Physical Downlink Shared Channel HS-SSCH - High-Speed Shared Control Channels HSUPA - High-Speed Uplink Packet Access IR - Incremental Redundancy ITU - International Telecommunication Union vii

9 Max C/I - Maximum Carrier to Interference MCS - Modulation and Coding Scheme MIMO - Multiple Input Multiple Output MS - Mobile Station PDP - Power Delay Profile PDU - Protocol Data Unit PF - Proportional Fair QoS - Quality of Service RLC - Radio Link Control RNC - Radio Network Control RR - Round Robin RRM - Radio Resource Management SAW - Stop And Wait SF - Spreading Factor SGSN - Serving GPRS Support Node SINR - Signal to Noise and Interference Ratio SISO - Single Input Single Output SNR - Signal to Noise Ratio SPI - Scheduling Priority Indicator TCP - Transmission Control Protocol TBS - Transport Block Size TTI - Transmit Time Interval UE - User Equipment UMTS - Universal Mobile Telecommunications System UTRAN - UMTS Terrestrial Radios Access Network WCDMA - Wideband Code Division Multiple Access WSS - Widesense Stationary viii

10 Chapter 1 Introduction 1.1 Third Generation Services During the last decades, the mobile communication market evolution has led to demands for higher data rates and larger system capacity. To successfully satisfy these requirements, Third Generation systems must increase their spectral efficiency and support high user data rates, especially on the downlink direction of the communication path due to its heavier load. For this purpose, the 3GPP has standardized in Release 5 a new technology called High Speed Downlink Packet Access (HSDPA) that represents an evolution of the WCDMA radio interface. These technological enhancements can allow operators to enable new high data rate services, improve the QoS of already existing services, and achieve a lower cost per delivered data bit. Consumers are expected to acquire mobile data services if their contents add value to the consumer s life by satisfying a concrete necessity or requirement. From the end user s interest, the value provided by the service contents contribute to his cost-effectiveness, time-efficiency, or simple entertainment; for instance, rich content services like video telephony, audio/video clips, and map based information, or fast Internet access for business users. 1.2 Technology High-Speed Downlink Packet Access, or also known as HSDPA, is a mobile telephone protocol in the High-Speed Packet Access (HSPA) family of third generation (3G) technologies designed to reduce the latency of the link and increase data transfer rates and the capacity of such networks through the transfer of data using a cellular phone. HSDPA is associated with the vari- 1

11 ous Universal Mobile Telecommunications System (UMTS) networks. Current HSDPA deployments support down-link speeds of 1.8, 3.6, 7.2 and 14.4 Mbps. The first phase of HSDPA has been specified in the 3rd Generation Partnership Project (3GPP) Release 5. The second phase of HSDPA is specified in the 3GPP Release 7 and has been named HSPA Evolved or also HSPA+; it can achieve data rates of up to 25 Mbps, [1]. As a difference with other WCDMA channels, the High-Speed Downlink Shared Channel (HS-DSCH) lacks two basic features - fast power control and variable spreading factor. Instead, it presents an improved downlink performance by using adaptive modulation and coding (AMC), fast packet scheduling and fast retransmissions at the base station, known as hybrid automatic repeat-request (HARQ), together with a shorter 2-ms Transmission Time Interval (TTI). Figure 1.1 shows the coverage map of deployed HSDPA technology around the world. Figure 1.1: View coverage map about HSDPA deployment in the world, [1]. 1.3 Work environment Due to the importance of the HSDPA technology, a SISO-HSDPA System level simulator was developed in a collaboration of the Institute of Communications and Radio Frequency Engineering and Mobilkom Austria AG. 2

12 The source-code is based on MATLAB and it simulates a mixed network in which both UMTS and HSDPA traffics are present. MATLAB is a high-level technical computing language and interactive environment for algorithm development, data visualization, data analysis, and numeric computation. It is an extended engineering tool and has enhanced the tradicional languages. It allows solving technical computing problems and a wide range of applications like signal and image processing, communications, easy matrix manipulation, plotting of functions and data, implementation of algorithms, creation of user interfaces, and interfacing with programs in other languages. The extension of the initial simulator is also developed in MATLAB, to ensure extendability and simple debugging. 1.4 Objectives Let me briefly sketch the motivation of this bachelor thesis. The first goal of the simulator was to evaluate the HSDPA throughput performance in the mixed traffic network; and based on this groundwork this thesis should develop two enhancements, namely: Extend the SISO HSDPA Simulator in order to handle real network layout data: this is the main functionality on which the work has been focused; the Mobilkom Austria AG provided measured path-loss matrices of a HSDPA cluster, including parameters like antenna gain patterns, the height of the antennas and the Nodes-B positions. This data has to be converted in a suitable form to be analyzable in the simulator. CQI mapping optimizations: an optimization of the CQI mapping of HSDPA mobiles shall be performed in order to find the mapping which maximizes the overall cell throughput. This optimization can result in a new mapping at the Node-B that grant an optimum HSDPA network performance. 1.5 Structure of the thesis This thesis report is organized as follows: Chapter 1: a short introduction about the technology and work environment, objectives and outline of the thesis is described. Chapter 2: provides a general overview of the HSDPA technology, like basis and key features. 3

13 Chapter 3: this chapter describes the initial SISO-HSDPA Simulator, including the system model and simulation process. Chapter 4: the conversions of the real data matrices into useful information for the simulator, the overall process and necessary new functionalities and modifications in the initial source-code for handling the new real data layout are explained in this section. Chapter 5: this chapter outlines the CQI basis in HSDPA, the current CQI mapping and the mapping optimizations investigated in the thesis, as well as the new functions including in the original simulator for this purpose. Chapter 6: includes a description of some investigations and new functionalities for the enhancement of the simulator that do not fit into the previous chapters. Chapter 7: summarises the main conclusions of the bachelor thesis. 4

14 Chapter 2 HSDPA Principles This chapter covers high-speed downlink packet access (HSDPA) principles for wide-band code division multiple access (WCDMA) - the new key feature included in Release 5 specifications and enhanced further in Release 6 specifications. HSDPA has been designed to increase downlink packet data throughput, compared to the rates provided by the Release 4 (also called Release 99) WCDMA specifications by means of fast physical layer (L1) retransmission and transmission combining as well as fast link adaptation controlled by the Node B. 2.1 Introduction HSDPA, also called 3.5G, is the evolution of the third generation (3G) and is considered the previous step before the fourth generation (4G), the future High-Speed Mobile Network. HSDPA and High-Speed Uplink Packet Access (HSUPA) are the components of the High-Speed Packet Access (HSPA) family. HSPA is an upgrade of the network infrastructure and it is part of the WCDMA 3G network. As an enhancement of UMTS, HSDPA was designed to improve the quality of service, increase the peak data rates (currently speeds supported by HSDPA are 1.8, 3.6, 7.2 and 14.4 Mbps). Also compared to UMTS, the spectral efficiency is significantly increased, and this allows more users being able to use high data rates on a single carrier. The fundamental techniques used in HSDPA to achieve this improvements are Adaptive Modulation and Coding (AMC), extensive multi-code operation and a fast and spectrally efficient retransmission strategy. The assignment of the HS-DSCH (High-Speed Downlink Shared Channel) among the users on 5

15 a TTI basis (1 TTI = 2 ms) is coordinated by a fast scheduler. Higher cell capacity and higher spectral efficiency are required to provide these higher data rates and new services with the current base station sites. Figure 2.1 illustrates the estimated cell capacity per sector per 5MHz with WCDMA, with basic HSPA and with enhanced HSPA in the macro-cell environment. Figure 2.1: Estimated cell throughput per sector, [2]. HSDPA is able to satisfy the most demanding multimedia applications such as attachments, Power Point presentations or web pages. An HSDPA 3.6 Mbps network can provide a 3MB music file in 8.3 seconds and a 5 MB video clip in 13.9 seconds. Speeds achieved by HSDPA reach 14.4 Mbps but currently most network operators provide speeds up to 3.6 Mbps, with the rollout of 7.2 Mbps quickly growing. It is important to note that the total available downlink speed within one sector is split among all the active users. Also, HSDPA can coexist with Release 4 in the same frequency band of 5 MHz. In Austria four HSDPA operators are giving service, Mobilkom Austria, Hutchison 3 Austria and ONE Austria serving HSDPA data rates of 7.2 Mbps, and Mobilkom Austria serving HSUPA (with a data rate of 1.4 Mbps). Currently only Telstra (in Australia) is serving HSDPA data rates of 14.4 Mbps. There are 185 commercial HSDPA networks in 92 different countries; the current deployment of HSPA networks in the world and the 6

16 HSDPA data rates supported are shown in Table 2.1. Data rate Networks Mbps Mpbs Mpbs 38 Table 2.1: Current HSDPA commercial networks and data rates, [1]. 2.2 HSDPA Standardization High-speed downlink packet access (HSDPA) was standardized as part of 3GPP Release 5 with the first specification version in March Highspeed uplink packet access (HSUPA) was part of 3GPP Release 6 with the first specification version in December HSDPA and HSUPA together are called high-speed packet access (HSPA). The first commercial HSDPA networks were available at the end of 2005, as we can see in Figure 2.2, and many improvements have been introduced in the Release 6, 7, and 8. Figure 2.2: Downlink data rates, [2]. 7

17 HSPA is deployed on top of the WCDMA network. Both of them can share all the network elements in the core network and in the radio network including base stations, Radio Network Controller (RNC), Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). WCDMA and HSPA are also sharing the base station sites, antennas and antenna lines. 3GPP creates the technical content of the specifications, based around work items, though small changes can be introduced directly as change requests against specification, but it is the organizational partners that actually publish the work. In addition to the organizational partners, there are also market representation partners, such as the UMTS Forum, part of 3GPP. With bigger items a feasibility study is done usually before rushing in to making actual changes to the specifications, [2]. A feasibility study for HSDPA was started in March 2000 in line with 3GPP principles, having at least four supporting companies. Motorola and Nokia supporting the start of the work from the vendor side and BT/Cellnet, T- Mobile and NTT DoCoMo from the operator side. The study was finalized for the TSG RAN plenary for March 2001 and there were issues studied to improve the downlink packet data transmission over Release 4 specifications. Physical layer retransmissions and BTS-based scheduling were studied as well as adaptive coding and modulation. The study also included some investigations for multi-antenna transmission and reception technology, titled MIMO (Multiple Input Multiple Output), and also Fast Cell Selection (FCS), [2]. 2.3 HSDPA vs Release 4 DCH In Release 4 specifications basically exist three different methods for downlink packet data operation: dedicated channel (DCH), forward access channel (FACH) and downlink shared channel (DSCH). The most interesting comparison is between Release 4 and HSDPA dedicated channel; the FACH is used either for small data volumes or when setting up the connection and during state transfers. In connection with HSDPA, the FACH is used to carry the signalling when the terminal has moved. However the DSCH has been replaced with the high-speed DSCH of HSDPA. The Release 4 based DCH is the key part of the system, and Release 5 8

18 HSDPA is always operated with the DCH running in parallel. If the service is only for packet data, then at least the signalling radio bearer (SRB) is carried on the DCH. In case the service is circuit-switched, then the service always runs on the DCH. In Release 5, uplink user data always go on the DCH (when HSDPA is active), whereas in Release 6 an alternative is provided by the Enhanced DCH (E-DCH) with the introduction of high-speed uplink packet access (HSUPA). In the case of multiple services, the reserved capacity is the sum of the peak data rate of the services. The main functionality for the DCH is the fast power control in addition to encoding the data packet provided by the RNC. Furthermore, soft handover is supported for the DCH. As a difference with Release 4, HSDPA introduces some methods for improving downlink packet data in terms of capacity and bit rates. The key differences between the HS-DSCH (HSDPA dedicated channel) and the Release 4 DCH-based packet data operation are as follows: Lack of fast power control. Instead, link adaptation selects the suitable combination of codes, coding rates and modulation to be used. Support of higher order modulation than the DCH. With 16-Quadrature Amplitude Modulation (16QAM) the number of bits carried per symbol is doubled in favourable conditions compared to the quadrature phase shift keying (4QAM) in Release 4. User allocation with base station based scheduling every 2ms, including fast physical layer signalling. With DCH the higher layer signalling from the RNC allocates semi-permanent code (and a spreading factor) to be used. The transmission time interval (TTI) is also longer with the DCH, allowing values such as 10, 20, 40 or 80 ms. (The longest is limited in the specific case of small data rates that have a spreading factor of 512). Use of physical layer retransmissions and retransmission combining, while with the DCH - if retransmissions are used - they are based on RLC level retransmissions. Lack of soft handover. Data are sent from one serving HS-DSCH cell only. Lack of physical layer control information on the HS-PDSCH. This is carried instead on the HS-SCCH for HSDPA use and on the associated DCH (uplink power control, etc). 9

19 Multicode operation with a fixed spreading factor. Only spreading factor 16 is used, while with the DCH the spreading factor could be a static parameter between 4 and 512. The DCH may use both turbo-coding or convolutional coding, while in HSDPA only turbo-coding is used. This was motivated by the fact that turbo codes make it possible to increase data rate without increasing the power of a transmission, or they can be used to decrease the amount of power used to transmit at a certain data rate. No discontinuous transmission (DTX) on the slot level. The HS-PDSCH is either fully transmitted or not transmitted at all during the 2-ms TTI. The main differences are summarised in Figure 2.3: Figure 2.3: Fundamental properties of the DCH and HS-DSCH, [2] Radio resource management architecture The radio resource management (RRM) functionality with HSDPA and HSUPA is significantly changed compared to Release 4. In Release 4 the scheduling control was purely based in the radio network controller (RNC) while in the base station (BTS or Node B in 3GPP terminology) mainly a power control related functionality (fast closed loop power control) was located. In Release 4 if there were two RNCs involved in the connection, the scheduling was distributed. The serving RNC (SRNC) - the one being connected to the core network for that connection - would handle the scheduling for 10

20 dedicated channels (DCHs) and the one actually being connected to the base transceiver station (BTS) would handle the common channels. Due to the BTS based scheduling, the overall RRM architecture changed. The SRNC will still retain control of handovers and is the one which will decide the suitable mapping for quality of service (QoS) parameters. With HSDPA the situation is simplified because, as there are no soft handovers for HSDPA data, the utilization of the Iur interface can be avoided by performing SRNC relocation, when the serving high-speed downlink shared channel (HS-DSCH) cell is under a different controlling RNC (CRNC). Thus, just a single RNC could be enough for the typical HSDPA scenario, [2]. Figure 2.4 shows the new RRM architecture. Figure 2.4: RRM architecture, [2]. 2.4 HSDPA operation principle HSDPA is based on a fast Node B scheduling where the Node B estimates the channel quality of each active HSDPA user on the basis of the physical 11

21 layer feedback received in the uplink. Scheduling and link adaptation are then conducted on a fast pace depending on the scheduling algorithm and the user prioritization scheme. The general HSDPA operation principle is shown in Figure 2.5. Figure 2.5: HSDPA Node B scheduling principle, [2]. The other new key technology is physical layer retransmission. In Release 4 when the data has not been received correctly, is necessary to retransmit it again from the RNC. In Release 4 there is no difference in physical layer operation, regardless if the packet is a retransmission or a new packet. With HSDPA the packet is first received in the buffer in the BTS. The BTS keeps the packet in the buffer even if has sent it to the user and, in case of packet decoding failure, retransmission automatically takes places from the base station without RNC involvement. So, the terminal can combine the transmissions, capturing the energy of both. Using a radio link control (RLC)-acknowledged mode of operation, RLC layer acknowledgement is provided in the RLC layer as would be done for Release 4 based operation. 12

22 2.5 HSDPA channels HSDPA new channels Several new channels have been introduced for HSDPA operation. For user data there is the high-speed downlink shared channel (HS-DSCH) and the corresponding physical channel. For the associated signalling needs there are two channels: high-speed dedicated physical control channel (HS-DPCCH) in the uplink direction and high-speed shared control channel (HS-SCCH) in the downlink. In addition to the basic HSDPA channel covered in Release 5 specifications, there is now a new channel in Release 6 specifications - the fractional dedicated physical channel (F-DPCH) - to cover for operation when all downlink traffic is carried on the HS-DSCH. The channels needed for HSDPA operation are shown in Figure 2.6. Figure 2.6: HSDPA operation channels. High-speed downlink shared channel The HS-DSCH is the transport channel that carries the actual user data. In the physical layer the HS-DSCH is mapped onto the high-speed physical 13

23 downlink shared channel (HS-PDSCH). An important property of the HS-DSCH is that it can dynamically allocate the resource. When the Node-B decides which user is going to be served, the data is sent continuously during the 2-ms TTI, so there is no discontinuous transmission (DTX) on the slot level like with the DCH. With DTX the downlink interference generated is reduced, but it keeps the code resource occupied according to the highest data rate possible on the DCH, because the code resource reservation is not changed when moving to a lower data rate; (the only way to reduce resource consumption is to reconfigure the radio link, but this takes time in reconfiguring the data rate to a new smaller value, and then a new reconfiguration to upgrade the data rate again). As a difference to DTX, with HS-DSCH, once there are no more data to be transmitted for that user, there is no transmission on the HS-DSCH again for the same user, but the resources in the according 2-ms are allocated to another user. Let s see the important technical apects: Adaptative Modulation and Coding: To cope with the dynamic range of the signal-to-noise ratio (Es/No) at the UE, HSDPA adapts the modulation, the coding rate and number of channelization codes to the instantaneous radio conditions. The combination of the first two mechanisms is denominated Adaptive Modulation and Coding (AMC). The channel coding is simpler than the corresponding DCH one, because in the HS-DSCH there is no need to deal with DTX or compressed mode, and there is only one transport channel active at a time because fewer steps in multiplexing/de-multiplexing are needed. The HS-DSCH channel coding chain is illustrated in Figure QAM : While the DCH only uses 4QAM modulation, the HS-DSCH may additionally use the higher order modulation 16QAM. HS-DSCH incorporates this modulation to increase the peak data rates for users served under favourable radio conditions. Support of 4QAM is mandatory for the mobile, despite the support of 16QAM is optional for the network and the UE. The inclusion of this high order modulation introduces some complexity challenges for the receiver terminal, which needs to estimate the relative amplitude of the received symbols, whereas it only requires the detection of 14

24 Figure 2.7: HS-DSCH channel coding chain, [3]. the signal phase in the 4QAM case. The turbo encoder is in charge of the data protection. The 16QAM constellation rearrangement depends on the transmission number, because the symbols in the constellation do not have the same error probability. The 16QAM and 4QAM constellations are shown in Figure 2.8. Figure 2.8: 16QAM and 4QAM constellations, [2]. 15

25 QAM is a digital modulation that transports data by changing the amplitude of two carrier signals. These two waves, generally sinusoidal, are in the same frecuency but with a phase difference of ninety degrees; both signal paths - I and Q - carry information. It is used for the data transmission with a high speed by channels with restricted bandwidth. By having more constellation points - 16 instead of 4 - now 4 bits can be carried per symbol instead of 2 bits per symbol with 4QAM. In reception the use of higher order modulation like 16QAM introduces additional decision boundaries, as shown in Figure 2.8. Signal quality needs to be better when using 16QAM instead of 4QAM. Because of this, with 16QAM it is not sufficient to figure out the phase correctly but also the amplitude needs to be estimated for more accurate phase estimate. The channel coefficients can be estimated from the common pilot channel (CPICH), which directly gives phase information. The offset of the HS-DSCH data channel to the CPICH however has to be signalled in order to estimate the amplitude information. This suggests that at the base station - during the 2-ms transmission - power changes should be avoided. In the system there can be other traffic that is consuming code space as well - such as circuit switched speech or video calls - which cannot be mapped on HSDPA. Thus, radio resource management will then determine the available code space for the scheduler at the BTS, [2]. Bit scrambling: The bit scrambling functionality was introduced to avoid long sequences repeating the same symbol, as long sequences of 0s or 1s. These could occur with some type of content, and especially when not using ciphering at higher layers. In such a case the terminal would have difficulties with HS-DSCH power level estimation and, thus, physical layer scrambling operation was introduced. Operation is the same for all users and is purely for ensuring good signal properties for demodulation, [2]. 16

26 HS-DSCH Link Adaptation HSDPA utilizes link adaptation techniques to substitute power control and variable spreading factor. The HS-DSCH link-adaptation algorithm at the Node-B is very dynamic, and adjusts the transmit bit rate on the HS-DSCH every 2-ms TTI. It is based on the physical layer CQI being provided by the terminal. Various sources contribute to the time-variant SINR at the user even though the HS-DSCH transmit power is assumed to be constant. The total transmit power from the serving HS-DSCH cell is time variant due to the transmission of the power controlled DCHs; the downlink radio channel is time variant if the user equipment is somehow moving; and finally, the experienced inter-cell interference at the user position is also time variant. For the purpose of HS-DSCH link adaptation, the user therefore periodically sends a CQI to the serving HS-DSCH cell on the uplink high-speed dedicated physical control channel (HS-DPCCH), [4]. Using link adaptation, the network will also gain from the limitation of power control dynamics in the downlink. As signals in the downlink cannot use a too large dynamic range to avoid the near-far problem between signals from the same source, the downlink power control dynamics is also limited. While in the uplink a 71-dB or more dynamic range is used, in the downlink only around 10 to 15 dbs can be utilized. The exact number depends on the implementation, channel environment and spreading factors applied. This means that for users close to the base station the power level transmitted is higher than necessary for reliable signal detection. Using link adaptation, there is a difference of a few decibels in the signal strength, just by changing from 4QAM to 16QAM; and by playing with the coding rates and the number of codes the total dynamic range can reach 30 db. Hybrid ARQ : HSDPA incorporates a physical layer retransmission functionality that adds robustness against link adaptation errors and improves the performance significantly. The Hybrid ARQ functionality consists of a two stage matching functionality which allows tuning two different retransmission types. These 17

27 two Hybrid ARQ strategies are: (1) identical retransmissions (also called soft combining) or (2) non-identical retransmissions (or so-called incremental redundancy ). The Hybrid ARQ technique is fundamentally different from the WCDMA retransmissions because the UE decoder combines the soft information of multiple transmissions of a transport block at bit level. Let us go a little bit more into detail: Soft combining : as proposed in [5] every retransmission is simply a replica of the coded first transmission. The same bits after rate matching operation are sent, for every retransmission of the same packet. The decoder at the receiver combines these multiple replicas of the transmitted packet weighted by the received SNR prior to decoding (so called soft combining ). This technique requires some memory on the mobile terminal, which must store the soft information of unsuccessfully decoded transmissions. The delay in the retransmissions and memory required shell be as small as possible. Incremental Redundancy (IR): it requires even more memory in the receiver user equipment capabilities. The retransmissions include additional redundant information that is incrementally transmitted if the decoding fails on the first attempt. That causes the effective coding rate to be increased with the number of retransmissions. Incremental Redundancy can be further classified in Partial IR and Full IR. Partial IR includes the systematic bits in every coded word, which implies that every retransmission is self-decodable, whereas Full IR only includes parity bits, and therefore its retransmissions are not self-decodable. If due to a signalling error that could fill the buffer with undesired data, due to a low coverage, or due to a change of the serving HS- DSCH cell, the number of physical layer retransmissions exceeds the maximum or the retransmissions fail, the radio link layer will handle further retransmissions. High-speed shared control channel (HS-SCCH) The HSDPA concept includes a Shared Control Channel (HS-SCCH) to signal the users when they are going to be served as well as the necessary information for the decoding process. Compared with the HS-DSCH, the HS-SCCH has two slots offset, as shown if Figure 2.9. This enables the HS- SCCH to carry time-critical signalling information which allows the terminal to demodulate the correct codes. A spreading factor of 128 allows 40 bits 18

28 per slot to be carried (with 4QAM modulation). The phase reference does not change when using HS-DSCH due to the lack of pilots or power control bits on the HS-SCCH. Figure 2.9: Relative timing between HS-SCCH and HS-DSCH, [2]. The HS-SCCH carries the following information, [6]: UE Id Mask: to identify the user to be served in the next TTI. Transport Format Related Information: specifies the set of channelization codes, and the modulation. The actual coding rate is derived from the transport block size and other transport format parameters [2]. Hybrid ARQ Related Information: such as if the next transmission is new or related to an earlier transmitted packet, and if it should be combined, the associated ARQ process, and information about the redundancy version, [7]. This control information solely applies to the UE to be served in the next TTI, which permits this signalling channel to be a shared one. The RNC can specify the recommended power of the HS-SCCH (offset relative to the pilots bits of the associated DPCH), [8]. The timing between the HS-SCCH and the HS-DSCH allows the terminal to have one slot time to receive the information which codes have to despread and with which to modulate. For the remaining parameters, a slot processing time is needed before a new 2-ms TTI starts. 19

29 When HSDPA is operated using the time multiplexing principle, then only one HS- SCCH can be configured. In this case only one user receives data at a time. When there is code multiplexing, then more than one HS-SCCH is needed. A single terminal may consider at most four HS-SCCHs; the system itself could configure even more. The use of code multi-plexing is not necessarily needed either when the carrier is shared with DCH traffic, or when there is a desire to have HSDPA data users operating with reasonable data rates -in the order of 384 kbps or more. In general, the data rate available for each user in different cases will depend on power allocation, the environment and the type of terminal being used. The channel coding is one-third convolutional coding (as turbo-coding does not make sense with such a small amount of information). In the second part there is a cyclic redundancy check (CRC) to make sure that there is no corruption of the information. A signalling error with, say, an HARQ process number would cause problems as it would cause buffer corruption; thus, a 16-bit CRC is used to ensure sufficient reliability, [2] High-speed dedicated physical control channel An uplink High Speed Dedicated Physical Control Channel (HS-DPCCH) carries the necessary control information in the uplink, namely, the ARQ acknowledgements, and the Channel Quality Indicator (CQI) reports. The CQI reports are deeply described in Section 5. To aid the power control operation of the HS-DPCCH an associated Dedicated Physical Channel (DPCH) is run for every user. This information from the terminal to the base station allows for the link adaptation and physical layer retransmissions. According to [8], the RNC may set the maximum transmission power on all the codes of the HS-DSCH and HS-SCCH channels in the cell. Likewise, the RNC determines the maximum number of channelization codes to be used by the HS-DSCH channel. By keeping the existing uplink DPCCH and DPDCH unchanged the active set can also accommodate Release 4 based base stations. The initial uplink DPCCH transmit power is set by higher layers. Subsequently the uplink transmit power control procedure simultaneously controls the power of a DPCCH and its corresponding DPDCHs (if present). The relative transmit power offset between DPCCH and DPDCHs is determined by the network. Any change in the uplink DPCCH transmit power shall take place immediately before the start of the pilot field on the DPCCH. The change in DPCCH 20

30 power with respect to its previous value is derived by the UE. The previous value of DPCCH power shall be that used in the previous slot, except in the event of an interruption in transmission due to the use of compressed mode, when the previous value shall be that used in the last slot before the transmission gap. During the operation of the uplink power control procedure the UE transmit power shall not exceed a maximum allowed value which is the lower out of the maximum output power of the terminal power class and a value which may be set by higher layer signalling. Uplink power control shall be performed while the UE transmit power is below the maximum allowed output power, [9]. As already mentioned, the uplink feedback information is carried on the HS- DPCCH. The HARQ feedback informs the base station whether the packet was decoded correctly or not. The CQI, respectively, tells the base station scheduler the data rate the terminal expects to be able to receive at a given point in time. Fractional DPCCH For Release 6, further optimization took place for the situation where only packet services are active in the downlink other than the signalling radio bearer (SRB). In such a case, especially with lower data rates, the downlink DCH introduces too much overhead and can also consume too much code space if looking for a large number of users using a low data rate service (like VoIP). The solution was to use an F-DPCH, which is basically a strippeddown version of DPCH that handles the power control. The code resource is time-shared, thus several users can share the same code space for power control information. Each user sees only the channel which has one symbol per slot for transmission power control (TPC) information and assumes there is no transmission in the rest of the symbols. With several users, the network configures each user having the same code but different frame timing and, thus, users can be transmitted on the single code source. Up to ten users can share one SF 256 code, thus reducing code space utilization for the associated DCH for users with all services mapped to the HS-DSCH,[2]. 21

31 Chapter 3 HSDPA Simulator In this chapter the initial simulator on which this bachelor thesis is based and all the work that was developed is explained. It contains the information about the initial simulator created by M.Wrulich, et al., in which I had to implement some new functionalities for the enhancement of the simulator. The chapter is organized as follows: Section 3.1 briefly sketchs the purpose and context of the simulator; a short description of the system-level model for the investigation is included in Section 3.2 and Section 3.3 explains the structure of the simulator. 3.1 Introduction Let me briefly introduce the work environment and goals of the initial SISO HSDPA Simulator. In the program, written in MATLAB, a mixed UMTS and HSDPA network is simulated. As described in Chapter 2, one of the advantages in a WCDMA network is that HSDPA can coexist within an existing 5MHz band of Release 4, allowing for sharing the power amplifier and spreading codes at the Node-B between the HS-DSCH and the Release- 4 dedicated channels. Even if HSDPA is widely installed, a mixed carrier operation is more cost-efficient for cells that are not fully loaded, [10]. One of the goals of the initial simulator was to deduce the optimum Node-B power split within the mixed scenario, by means of snapshot based network simulations, in order to maximize the overall cell throughput. The obtained results can be used for the cell operation planning by the network operators. 22

32 3.2 System model According to [10], the HSDPA performance is done with a system-level simulator in which the following aspects are modelled: Channel modeling: the channel coefficient used for the simulation allows us to model the radio propagation and it considers the macroscale pathloss, that depends on the the distance between the base station and the user equipment, the shadow fading and the fast fading with multiple paths and no time correlation. HSDPA modeling: this modeling represents the HSDPA transmission performance in an accurate way by using a simplified system level description in which they have modelled the channel quality observed by the user equipment and the bit error/decoding performance. The models are the so-called link-measurement model and link-performance model respectively. In the link-measurement model, the signal-to-noiseand-interference ration (SINR) is evaluated after Rake-combining and despreading for each user equipment in the cell. The link-performance model aims at an analytical approximation of the block error ratio (BLER), where it is assumed that the scheduler in the Node B decides to serve a specific user with an MCS as specified by the CQI mapping table [11]. Furthermore, it is assumed that the desired HSDPA user always gets the full available transmission power and there is enough data to transmit (full buffer assumption). Release 4 modeling: the Release 4 traffic is only coarsly modelled, since the main goal of the simulator was the prediction of the achievable user data rates with HSDPA within the existing Release 4 network. In [10] further information and the estimation of the number of DCH users that can be served can be found. Power split: the total intra-cell transmission power depends on the transmission power of the DCH and the HSDPA traffic, and in each cell, the total available transmit power is shared between DCH and HSDPA users. The power of HSDPA can be allocated in the Base station downlink power budget by means of two possibilities: By sending Node-B application part (NBAP) messages to the base station, the RNC can dynamically allocate the HSDPA power. This is kept at a fixed level by the base station, and the DCH power varies accordingly to the fast closed loop power control. 23

33 The base station is allowed to allocate all unused power for HS- DPA, instead of sending NBAP messages. The total intra-cell transmission power is calculated as folows: P intra = P DCH + P HS DSCH + P other (3.1) P other incorporates the power from the common pilot channel and other needed common channels. This explains the fact that the total intracell power depends on the DCH and HSDPA transmission power. 3.3 Simulation process After explaining briefly the system model of the simulator, now the initial SISO HSDPA Simulator itself is going to be treated in detail. Figure 3.1 illustrates the three main steps: Load settings: before starting the process, the function load settings is called. This settings file allows for the specification of the simulation, i.e. the kind of network, channel and user equipment. Precalculations: after the settings are loaded and before the simulation starts, there are some precalculations which allow us creating some necessary elements like user and Node-B positions, all the pathlosses for every users and PDP for serving links. Simulation loop: the last step is the simulation loop, which calculates different average data rates by means of multiple independent snapshots Load settings The settings are divided in: network, channel, user equipment and simulator, and by changing the parameters we can specify the kind of simulation. Network settings: the mixed traffic network is modelled according to the parameters: 24

34 Figure 3.1: Three main steps in the simulator: load settings, precalculations (i.e Node-B and users positions in order to prepare the network) and simulation loop to obtain the HSDPA data rate. R 99: contains the needed parameteres for Release 99, like the bandwith (5 MHz), the chiprate, the UMTS load in percent and the UMTS required Eb/No for the requested UMTS DCH bearer. Node-B: here, some variables like the distance between Node-Bs, the power level of each Node-B (the maximum power, the CPICH power and the common power), and the power distribution of the Node-B are specified. Power distribution: determines the power distribution among the neighboring Node-Bs, thus specifying the intercell-interference structure. HSDPA: in this part the HSDPA network is specified, thus the number of HSDPA users, the spreading factor of HSDPA transmissions (fixed at 16), the number of codes, the absolute HSDPA 25

35 power and the TTI value (usually 2 ms) are assigned. MAC-hs: this is used in an enhanced version of the simulator for scheduling variables. Other: variables like the grid density, which determines the number of grid points for user positioning within the cell, the G factor of the network or the other cells interferences are chosen here. Network structure: the number of base stations (7 or 19), the number of sectors for each base station (1 or 3) and the antenna gain pattern are specified. Channel settings: the channel modeling is done through three different fadings that can attenuate the signals in the communications between the base station and the users. Deterministic fading: first the model (COST231, Berger, fixed, exponent, tr25848 or none) is selected, and accordinglyly the necessary variables like frequency or antenna height are assigned. Shadow fading: there are two possibilities for the shadow fading: the lognormal model or the lognormal moving model, and accordinglyly different parameters can be selected. Fast fading: it can be modeled with a Rayleigh model or it can be omitted. Power Delay Profile: the oversampling factor, the model (pedestrian A or B, vehicular A or B, or none) and the chiprate are selected here. User Equipment settings: the parameters needed to model the user equipment are defined here. General: the user category class and different noise powers seen in the receiver, like the receiver noise figure or the thermal noise density are defined. Movement: contains the setting for speed of the user. Receiver: the reciver type is specified, and in case of a Rake receiver the number of fingers is also determined. Traffic: will be used in further developements of the simulator. Simulator settings: now, some parameters of the simulator are defined, although they do not belong to the communication scheme itself. 26