A Software Development Framework Based on C++ OOP Language for Link-level Simulation Tools

A Software Development Framework Based on C++ OOP Language for Link-level Simulation Tools Carlos H. M. de Lima, Elvis M. G. Stancanelli, Emanuel B. Rodrigues, Jean M. da S. Maciel and Francisco R. P. Cavalcanti Abstract This article introduces a software development framework, which amasses Object-Oriented Programming (OOP) concepts and designed procedures, intended to systematize the implementation of link-level simulation tools. This development framework is fully implemented in C++ programming language providing modularity and reusability (improving the coding activity). This framework then constitutes remarkable tool for quickly creating link-level applications. Aiming to evaluate the applicability of this software structure, the conversational service class transmitting at 12.2 kbps and over the Wideband CDMA (WCDMA) Downlink (DL) Dedicated Channel (DCH) is addressed as a case-study and therefore potential benefits of utilizing such development library are identified. It is shown that the development framework provides flexibility for the software development of link-level simulators remarkably reducing the design, programming and testing stages. Index Terms Development Framework, OOP, C++, IT++, WCDMA, DL, DCH. I. INTRODUCTION Independent of the business area and performed activity, the accomplishment of goals by employing a minimum amount of resources (time and investments) is the primary concern. Therefore, considering the present-day competitiveness, the employment of expensive procedures to conceive, develop and produce end-products is undesirable and even unacceptable. Nevertheless, the utilization of high fidelity models makes practical a detailed investigation of processes related to an actual system. Thus, costs are minimized, errors are foreseen and strategic decisions that avoid imminent failure are supported. The employment of such models throughout simulations constitutes a powerful tool to comprehend the operation of modeled systems, and besides allows to optimize the performance and to assure the reliability. Generally speaking, simulations mimic real systems, for instance, emulates the stock market tendencies, the dynamics of a machine, the assembly line of a product, or the behavior of a cellular system. In this way, the evaluation of cellular systems, addressing its operational principals, functionalities and feasibility, by means of models, which are conveniently incorporated into a simulator, constitutes an appropriate manner of representing the reality. A cellular system model comprises stochastic components and processes whose random nature need to be appropriately captured. The modeling of cellular networks, which are dynamic and complex systems, by means of analytical The authors are with the Wireless Telecom Research Group - GTEL, Federal University of Ceará - UFC, Fortaleza, Brazil. Email: {carlos, emiguel, emanuel, jean, rodrigo}@gtel.ufc.br approach demands so many simplifications that makes its applicability inappropriate to describe the related processes. A simulation is a controlled execution of models, which can be appropriately gathered and represented by means of a computer program, whose objective is to provide information about the assessed system. Traditionally, link-level applications are developed employing the functional programming paradigm that achieves computational tasks similarly to the evaluation of mathematical functions. Commonly, general-purpose computational tools are used, for instance, the MATLAB R that is a procedural programming language (with limited object-oriented programming capabilities) and provides numerical computing environment. Link-level Software Development Framework (LSDF) is a software framework fully implemented in C++ OOP language and using commonly known OOP design patterns [1]. By using LSDF, the implementation effort is significantly reduced due to the inherent modularity and reusability of this framework. The LSDF provides a structure for supporting the software development activity, especially as a skeletal support used as the basis for a link-level simulator being constructed. This framework allows for rapid implementation and simplified maintenance of link-level simulation tools. Further, it is an extensible system designed to easily allow the addition of new modules at a later date. Its modular structure easily enables parallel development of different functional modules in a collaborative approach between different programmers. The goal of LSDF is to provide a robust and automated framework for software development upon which link-level simulation can be easily implemented. LSDF simplifies the development activity reducing both codification time and effort, decreasing costs and avoiding reimplementation of either functionalities or entities making the coding activity as intuitive as possible. This article is divided into the following sections: Section II presents general aspects of the LSDF library. Section III presents the performance evaluation of a case-study based on the WCDMA DCH for the downlink direction. Section IV presents main observations, final conclusions and perspectives for the LSDF development framework. II. GENERIC SIMULATOR ARCHITECTURE Fundamentally, the functional architecture of each component, which is developed using the LSDF framework, is divided between an operational implementation and a generic interface. The operational implementation represents SBrT 337

the functional procedures executed by each building module, while the generic interface enables interactions between distinct building modules. By connecting several building modules together by means of the generic interface, a specific transmission chain entity is integrated in a functional structure. Mathematical functions, signal processing procedures, speech processing, and communications specific classes and functions are primarily provided by the IT++ library [2]. The external library IT++ provides functional procedures for the great part of currently used building modules. However, very specific functionalities of modeled radio networks, for instance rate matching, which are not provided by IT++, are therefore implemented in the LSDF framework. The generic interface essentially provides modularity and reusability, which are the most remarkable features of the LSDF structure. This software development structure was conceived, designed and implemented by the Wireless Telecommunications Research Group (GTEL) programming team. Each functional module has a multiple inheritance from a generic interface and from an operational implementation, which constitutes its functional part. The generic interface is dealt with in Sections II-A and II-B, while the implementation details (functional aspects of a building module) is addressed in Section III by means of a case-study: conversational service at 12.2 kbps over WCDMA DL DCH channel. A. Generic simulator logical structure The hierarchical structure of the LSDF link-level framework is fundamentally composed of three parts: building modules, data blocks (exchange blocks) and one transmission chain entity. Figure 1 illustrates the building module. Each functional module, independent of the performed task (e.g., coding, segmentation, etc.), is a building module itself. Therefore, a generic and common interface can be established among several modules in a transmission chain entity. A unique entity executes the transmission and reception updates. However, updates are controlled by the transmission chain entity that triggers the overall update chain; as a result, each building module sequentially performs its own update as well. The updates are sequential because the position of the modules in the stack determines the order to perform an update. During the update execution, a building module changes its state and also performs its functional procedure, for instance, a Cyclic Redundancy Check (CRC) module has to insert the CRC attachment during the transmission and verify the CRC integrity during the reception. The transmission chain entity connects successive building modules through the previous and next connectors. Connections between two distinct building modules are established during the insertion procedure of the building modules in the stack. A building module also encloses a list of data blocks, which in turn convey the information block that should be propagated through the transmission and reception chains. Data blocks Fig. 1. Previous Data Block Data Block Data Block Next Illustration of a generic building module. are propagated through the transmission chain entity as a specialized information block named exchange block. Figure 2 illustrates the structure of the transmission chain entity. This transmission entity controls the overall update of the building modules currently compounding the stack of modules. Moreover, any information that should be transmitted through the transmission chain entity is inserted and removed by using push and pop functionalities, respectively. Notice that the first and the last modules in the stack are connected to the transmission chain entity by the previous and next connectors, respectively. A transmission entity is itself a modified building module. Fig. 2. Transmission Chain Entity Transmission Reception Illustration of a generic transmission chain entity. connectors SBrT 338

B. Generic simulator flowchart Figure 3 succinctly illustrates a typical iteration of a generic link-level simulation tool. Each iteration starts by randomly generating data payload that should be transmitted throughout the transmission and reception chains. After that, the information is inserted in the stack of modules by using the push functionality. Next, the transmission chain entity performs an overall update, and each building module is updated in the chain. Notice that the transmission entity only is in charge of the update of the stack of modules. However, the exact task performed is implementation specific of each functional module. After propagating the information down in the stack during the transmission update and up during the reception update, the information is retrieved from the transmission/reception chain and relevant metrics can be calculated. During a transmission update, the radio frame is assembled and transmitted through the air interface, while during a reception update the transmitted information is recovered. Finally, the metrics are calculated and made available for on-line evaluation or for post-processing after a complete simulation run is finished. Fig. 3. Begin Generate Data Push to Stack Update Stack Pop from Stack Calculate Metrics End Illustration of an iteration of a generic link-level simulation tool. III. CASE-STUDY: CONVERSATIONAL SERVICE AT 12.2 KBPS OVER WCDMA DL DCH CHANNEL This section provides details about the implementation of a simulation tool by using the LSDF. For the downlink, the physical and the radio link layers of the WCDMA DCH transport channel was implemented according to the 3rd. Generation Partnership Project (3GPP) specifications [3], [4]. The complete multiplexing and coding chain for the WCDMA DCH was implemented by utilizing the GTEL link-level library. This library is completely written in C++ OOP language and has both modularity and reusability as great concerns. The modularity provides flexibility and independence to the programming activity, while the reusability allows time-saving during the implementation stage, since building modules previously developed can be re-utilized. In the following, sections III-A and III-B describe the structure of the DL DCH, and the coding/multiplexing and spreading/modulation chain of DL WCDMA, respectively. Finally, section III-C shows the illustrative results of Block Error Rate (BLER) performance for the simulation scenario considered in this case-study. A. Downlink DCH Channel Structure The transport channels characterize the manner and with which characteristics the data is transferred. They are divided in two groups: Dedicated Channels and Common Channels. The main difference between them is the fact that a common channel is a cell resource that is divided among a group of users (maybe all of them), while a dedicated channel resource, identified by a given code or frequency, is reserved for only one user [5]. The case-study presented here focus on dedicated transport channels that are used for the provision of conversational services (speech service using Adaptive Multirate (AMR) codec with 12.2 kbps). The DCH modeling is based on the 3GPP Release 99 (R99) specifications. The DCH carries all the information addressed to a given user that comes from layers above the physical layer, including service data (i.e. speech frames) and control information (e.g. handover commands or measurement reports). Moreover, the dedicated transport channel is characterized by functions such as fast power control, soft(er) handover, fast rate adaptation, and the possibility to support adaptative antenna techniques. The dedicated transport channel is mapped on two physical channels. The Dedicated Physical Data Channel (DPDCH) transports higher layer information including user data, while the Dedicated Physical Control Channel (DPCCH) transports control information necessary for the physical channel [6]. The Dedicated Physical Channel (DPCH) structure is depicted in Figure 4. In this model each pair of bits represents a Quadrature Phase-shift Keying (QPSK) symbol. The frame structure consists of a sequence of radio frames; one radio frame corresponding to 15 slots (10 ms or 38 400 chips); and one slot corresponding to 2 560 chips (0.667 ms), which equals one power control period. In the downlink, the dedicated physical channel (downlink DPCH) consists of a downlink DPDCH and a downlink DPCCH time-multiplexed with complex scrambling. Therefore, the dedicated data generated at higher layers carried on DPDCH is time-multiplexed with pilot bits, Transmit Power Control (TPC) commands, and Transport SBrT 339

Format Combination Indicator (TFCI) bits (optional) generated by the physical layer. The DPCH may or may not include the TFCI; if the TFCI bits are not transmitted, the Discontinuous Transmission (DTX) is used in the corresponding field. The I/Q branches of the modulator have equal power and the spreading factors range from 512 (7.5 ksps) down to 4 (960 ksps) [7]. The spreading factor for the highest transmission rate determines the channelization code to be reserved from the given code tree. TRANSPORT BLOCK CRC ATTACHMENT TRBK CONCATENATION & CODE BLOCK SEGMENTATION CHANNEL CODING DPDCH DPCCH DPDCH DPCCH RATE MATCHING DATA TPC TFCI DATA PILOT Fig. 4. 2 560 chips SLOT 0 SLOT 1 SLOT K SLOT 13 SLOT 14 Structure of the DPCH. FRAME =10ms TRANSPORT CHANNEL PROCESSING 1 st INSERTION OF DTX INDICATION 1 st INTERLEAVING RADIO FRAME SEGMENTATION OTHER TRANSPORT CHANNELS In the present case-study, which evaluates the provision of speech service using the AMR codec with 12.2 kbps, the DPCH was implemented in the WCDMA DL DCH simulator concerning the following features: The DPCH Transmission Time Interval (TTI) has fixed duration of 20 ms (two radio frames); the DPCH conveys only conversational service class users with data rate of 12.2 kbps; it was considered a fixed spreading factor of 128; only the first AMR transport channel (there are three in total) was simulated. This was due to the fact that only this transport channel performs CRC checksum, and therefore is crucial for block error detection. The other two transport channels do not degrade the final voice quality (intelligibility) severely in case of block reception error. B. WCDMA coding/multiplexing and spreading/modulation chain This subsection briefly presents the transmission chain of the WCDMA DL DCH. More details about the transmission chain can be obtained from the 3GPP technical specifications [3], [4]. Such a chain is composed by channel coding and data modulation functionalities. Additionally, some interleaving, segmentation and Transport Channels (TrCHs) multiplexing functionalities are performed. Figure 5 illustrates the transmission chain composition. At the beginning of this transmission chain, a CRC is attached to the transport block. At the end of reception chain, this CRC is checked so that the occurrence of errors is verified for each detected block. The possible sizes for the CRC are 0, 8, 12, 16 and 24, which are signalled from higher layers. The correction of errors should be accomplished as well. Either a convolutional encoder with 9-constraint length or a turbo encoder [8] can be employed. For the former, 1/3 or 1/2 coding rates are possible, while for the last only 1/3 can SPREADING/MODULATION CHAIN TRCH MULTIPLEXING 2 nd INSERTION OF DTX INDICATION PHYSICAL CHANNEL SEGMENTATION 2 nd INTERLEAVING PHYSICAL CHANNEL MAPPING MODULATION MAPPER SPREADING SCRAMBLING PHYSICAL CHANNELS Fig. 5. Transport channel processing and spreading/modulation chain for WCDMA DL DCH. be adopted. WCDMA turbo encoder consists of two recursive systematic convolutional encoders parallel concatenated and of a multistage internal interleaver between them [3]. The corresponding turbo decoding operates in an iterative manner, comprising two Soft-Input/Soft-Output (SISO) decoders. Maximum A Posteriori Probability (MAP)-based [9] and Soft-Output Viterbi Algorithm (SOVA) [10] algorithms are alternatives for the SISO decoder. MAP-based turbo decoder is the most accurate, while the SOVA-based is attractive due to its low complexity and relatively high speed in spite of its SBrT 340

very small accuracy degradation, when compared to the MAP algorithm. By using either puncturing or repetition of bits, a rate matching stage is performed in the chain aiming to match the transport channel instantaneous bit to the physical channel bit rate. The WCDMA rate matching can deals with many transport channels, setting up a common point of operation in order to differentiate Quality of Service (QoS) among them. Both E b /N 0 matching and unequal error protection control are accomplished [11]. The rate matching can be controlled through a semi-static parameter provided by higher layers. Additionally, DTX is implemented to accomplish lower transmission rates for the downlink. In accordance with the usage of fixed or flexible positions, the DTX indication insertion is performed in different points of the chain (such points are indicated in Figure 5). Throughout the chain, bits are interleaved, blocks are segmented and channels are multiplexed. These functionalities are performed for each transport channel processing. Subsequently, the data modulation functionalities (i.e, modulation mapping, spreading and scrambling) are performed to make possible the communication through the radio channel. However, it is worthy to notice that, regarding computational effort issues, the WCDMA is modeled in equivalent base-band signal representation. The scrambling for the WCDMA downlink uses 38 400 chips of a complex-valued long code that is built from a Gold sequence generated from 18-degree polynomials; the scrambling code has period of 10 ms radio frame. On the other hand, the spreading is based on the Orthogonal Variable Spreading Factor (OVSF) channelization codes; the Spreading Factor (SF) range is 4 to 512 for downlink Frequency Division Duplex (FDD) and does not vary on time. Typically, only one scrambling code and one spreading code tree are used per sector. Regarding multicode transmission for one user, the parallel code channels have different channelization codes, but the SFs are kept identical. Only QPSK modulation is considered on WCDMA downlink. DTX indications must be suitably treated during demodulation. C. Numerical Results Table I gathers the set of configuration parameters utilized throughout the simulations. TABLE I CONFIGURATION PARAMETERS. Parameter Value Transport block size 81 Transport block set sizes 81 Number of Iterations per E chip /N 0 15 000 Channel Model AWGN Modulation Scheme QPSK CRC size 12 Transport Channel DCH Channel Coding Convolutional Coding Coding Rate 1/3 TTI Period 20 ms Spreading Factor 128 Figure 6 illustrates the performance results in terms of BLER for the evaluated case study: WCDMA DL DCH considering the conversational service at 12.2 kbps. BLER Fig. 6. 10 0 10 1 10 2 10 3 30 29 28 27 26 25 24 23 22 21 20 E /N [db] chip 0 BLER for the WCDMA DL DCH transport channel. IV. CONCLUSION AND PERSPECTIVES In this article, an innovative software development framework intended to automate and simplify the implementation of link-level simulation tools was introduced. The programming framework is completely implemented in C++ language based on OOP concepts. This framework provides a solid structure on which the transmission chain of a Radio Access Network can be constructed: connection between adjacent functional modules, data information encapsulation and transmission throughout the complete chain, among others are provided by the LSDF. By using the LSDF framework, the modularity and reusability of functional modules are of paramount importance. As a consequence, modules can be implemented and tested independently. This fact facilitates and reduces the implementation effort considerably. Another remarkable characteristic of the LSDF framework is the flexibility to take advantage of distinct implementations (e.g., external mathematical and telecommunications library), since only the common interface among data blocks and building modules should be respected. In this way, external libraries can effectively and easily wrapped into the LSDF environment. An interactive Graphical User Interface (GUI) is foreseen to facilitate the construction of transmission chains for cellular systems. This envisaged GUI may have a SIMULINK R -like appearance, where graphical objects are manipulated in order to construct targeted systems. ACKNOWLEDGMENTS This work was supported by a grant from Ericsson of Brazil - research branch under ERBB/UFC.17 technical cooperation contract. Francisco R. P. Cavalcanti was partly funded by CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico, grant no. 304477/2002-8. SBrT 341

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