16 th International Conference on AEROSPACE SCIENCES & AVIATION TECHNOLOGY, ASAT - 16 May 26-28, 2015, E-Mail: asat@mtc.edu.eg Military Technical College, Kobry Elkobbah, Cairo, Egypt Tel : +(202) 24025292 24036138, Fax: +(202) 22621908 Paper: ASAT-16-166-CM Signaling Protocol Encoding and Simulation for Third Generation CDMA Network Air-Interface G. M. Abdel Hamid *, E. K. Aboseif, M. E. Hammad Abstract: In cellular networks signaling protocol is responsible for processing and exchanging messages and orders between the base station and the mobile station. These messages and orders are necessary for registering the mobile station on the network, initiating and receiving calls and doing other functions like handoff and power control. In CDMA2000 mobile network upper layer (layer 3) is responsible for signaling. CDMA2000 Signaling Standard specifies in depth the complex procedures that occur between the mobile and the base station without mentioning how to encode these procedures to software or how to simulate them. This paper shows how standards are being interpreted and encoded and simulated where the registration process is chosen to be a case study using C programming language. Keywords: CDMA2000; air Interface; layer 3; signaling, protocol standard. 1. Introduction CDMA is a nonconventional multiple-access technique that immediately found wide application in modern wireless systems. In CDMA, the entire bandwidth is made available simultaneously to all signals. Signals are discriminated by the means of code sequences, which correspond to the physical channels. Each pair of transmitter receivers is allotted one code sequence with which a communication is established. At the reception side, detection is carried out by means of a correlation operation. Ideally, the best performance is attained with zero cross-correlation codes, i.e., with orthogonal codes. In theory, for a synchronous system and for equal rate users, the number of users within a given bandwidth is dictated by the number of possible orthogonal code sequences. In practice, orthogonality is very difficult to achieve. Even though the codes may be designed to present zero cross-correlation, the impairments imposed by the communication channel may introduce a certain level of correlation among the codes. To accomplish CDMA, spread spectrum techniques are used [8]. Spread spectrum is defined as a communication technique in which the signals are spread over a bandwidth in excess of the minimum bandwidth required for their transmission. It has its origin in the military scenario where robustness in communications is vital. Systems presenting difficult detectability of the signal by the enemy and easy-to-combat enemy introduced interference are referred to respectively, as low-probability of intercept (LPI) and anti-jam communications (AJM) systems. By allowing the signals to occupy a bandwidth in excess of the required bandwidth for their transmission, spread-spectrum signals can be given pseudorandom characteristics, which cause the signals to have a noise like appearance. The CDMA2000 standards are investigated by the number of standards [1, 2, 3, 4 &5], however more work is still required to investigate and understand how the protocol works and this will be the objective of this paper. The paper is organized as follows: Section II describes * Egyptian Armed Forces, Egypt, gmabrouk@hotmail.com. Egyptian Armed Forces, Egypt. Egyptian Armed Forces, Egypt, m3esawy@hotmail.com 1
the mobile communication standards. Section III describes the CDMA network architecture. Section IV describes CDMA interfaces and protocol architecture. Section V describes standard encoding and simulation. Section VI concludes the paper. 2. Mobile Communications Standards A. 3GPP2 The Third Generation Partnership Project 2 (3GPP2) was born out of the International Telecommunication Union's (ITU) International Mobile Telecommunications "IMT-2000" initiative, covering high speed, broadband, and Internet Protocol (IP)-based mobile systems featuring network-to-network interconnection, feature/service transparency, global roaming and seamless services independent of location. 3GPP2 is a collaborative third generation (3G) telecommunications specifications-setting project Comprising North American and Asian interests developing global specifications for ANSI/TIA/EIA-41 Cellular Radio telecommunication Intersystem Operations network evolution to 3G. B. CDMA2000 Family of Standards Scope The cdma2000 Family of Standards includes core air interface, minimum performance, and service standards. The cdma2000 air interface standards specify a spread spectrum radio interface that uses Code Division Multiple Access (CDMA) technology to meet the requirements for Third Generation wireless communication systems. The core air interface standards in the family are described in the following references [1, 2, 3, 4 & 5]. The technical requirements contained in cdma2000 form a compatibility standard for CDMA systems, where compatibility is understood to mean: any cdma2000 mobile station is able to place and receive calls in cdma2000 or IS-95 systems. Conversely, any cdma2000 system is able to place and receive calls for cdma2000 and IS-95 mobile stations. To ensure compatibility, both radio system parameters and call processing procedures are specified. The sequence of call processing steps that the mobile stations and base stations execute to establish calls is specified, along with the digital control messages. 3. CDMA Network Architecture Functions of the elements in the CDMA network are described in Table 1. CDMA network architecture is shown in the following figure 1. Figure 1: CDMA2000 network architecture 2
Table 1: Functions of the elements in the CDMA network Network Element BTS BSC MSC MGW VLR HLR PDSN HA AAA Function Description Transmits and receives radio signals to realize communication between the radio system and the MSs Controls and managing BTSs Implements call connection and disconnection Implementing power control Managing radio resources Providing reliable radio links for upper-layer services through soft and hard handoffs The MSC is responsible for: Call setup, route selection, radio resource allocation Mobility management Location registration The MGW provides the voice encoding and decoding function and processes voice services. The VLR is a dynamic database. It stores the information of the subscribers that roam to its MSC area. Physically, the VLR is integrated with the MSC. The HLR is a database used to manage mobile subscribers. The PDSN is a gateway used to connect the mobile network to the IP backbone network. The PDSN provides access to packet data services for mobile subscribers. The HA provides the interface between the mobile network and the Internet. The HA is an auxiliary node for mobile IP subscribers to access the Internet. It provides authentication, authorization, accounting, and data value-added services. 4. CDMA Interfaces and Protocol Architecture A. Interfaces of CDMA20001X network Logical interfaces of CDMA2000 network are shown in the following figure 2 Figure 2: CDMA2000 logical interfaces 3
B. Air interface protocol Architecture The IS-2000 standard calls out explicitly the functions of four different protocol layers. These layers are the physical layer, medium access control, signaling link access control, and upper layer [7]. The signaling entity is the one that effectively controls the operation of the entire IS-2000 system. The signaling entity also controls and executes those functions that are necessary for the setup, maintenance, and disconnecting the call [7]. To ensure compatibility, both radio system parameters and call processing procedures are specified. The sequence of call processing steps that the mobile stations and base stations execute to establish calls is specified, along with the control messages. The base station is subject to different compatibility requirements than the mobile station and both the MS and the BTS have different processing functions resulting in two different protocol stacks shown in figure (3) and figure (4) from the perspective of mobile station and from the perspective of the base station respectively [1]. Radiated power levels, both desired and undesired, are fully specified for mobile stations, in order to control the RF interference that one mobile station can cause another. Detailed call processing procedures are specified for mobile stations to ensure a uniform response to all base stations. Base stations are fixed in location and their interference is controlled by proper layout and operation of the system in which the station operates. Base station procedures which do not affect the mobile stations operation, are left to the designers of the overall land system. This approach of writing the compatibility specification is intended to provide the land system designer with sufficient flexibility to respond to local service needs and to account for local topography and propagation conditions. 5 Standard Encoding and Simulation A. Signaling protocol encoding The information of the required message must be collected from the standard to get the parameters required for signaling encoding and simulation (message length, message content (fields), values of each field and the meaning of each value), the following Table 2 shows an example of order message field lengths. Table 2: Order message fields and field lengths [5] Each field in the message is designed to be an array of characters (0s and 1s) having specific length according to the field length information collected from the standard, where each field has a value that corresponds to certain meaning according to the field type as shown in figure 5. Message body is formed by concatenating message fields in the order mentioned in the standard. 4
Figure 3: CDMA2000 protocol architecture (mobile station) 5
Figure 4: CDMA2000 protocol architecture (Base station) Order code (27) Additional record length (0) 0 1 1 0 1 1 0 0 0 Figure 5: Order message-body 6
The header is added to the message body as shown in figure 6, where channel type and the message type are selected to be the guiding headers for the receiver side (BTS or MS) to determine on which channel the message is sent and what type of message is received. Channel type Message type Message body Figure 6: Order message-body A format conversion function is applied to the message to convert from binary to hexadecimal and vice versa because sending short hexadecimal message is more convenient than sending long binary sequence. After receiving the message on the receiver side (BTS or MS) and converting hexadecimal message to binary digits sequence a pointer is used to read the header to determine message type according to Table 3 and to read the message bit by bit and decides the end of each field and the end of the message according to the length of each field and the length of the message and the value of each field is then stored in the memory allocated for that field. Table 3: Message types on r-csch channel [4] Message fields are processed at the receiver side (BTS or MS) according to lookup tables found in the standard to take the decisions in different cases and prepare the suitable reply to the received message order or request. The reply message is formed according to the same procedures mentioned above after adding the header where the message fields are filled with the suitable values for the existing situation. B. Power on Registration Simulation: When the mobile station is powered on a hardware test is initiated as shown in figure 7 to make sure that there is no problem with the hardware components. The mobile station starts power on registration process by forming a registration message (in binary format) inside the mobile station with registration type field equal to the value (1) which corresponds to power up registration as shown in Table 4, then the binary message is converted and represented in hexadecimal for convenience. The message is sent to the BTS simulator shown in figure 8 by means of socket programming where the BTS simulator is considered as a server and each mobile station simulator is considered as a client, where the BTS can handle multiple MSs at the same time and figure 9 shows socket programming concept. 7
Table 4: Registration types and corresponding values [5] The BTS simulator receives the hexadecimal message and converts it to binary message and the pointer function starts to determine the message header, the start and end of the message body, the start and end of each field. The first field in the header is the channel type and the second field is the message type and the rest is the message body (message fields). After the BTS simulator determines the received message type, it starts to read and store the message fields according to the field length for that message type. After reading the message fields and processing the field contents (checking requested registration type and accepting the registration request), registration accepted order PDU is prepared in binary format then converted to hexadecimal format and sent back to mobile station simulator by means of socket programming. When the mobile station receives the registration accepted order PDU it follows the same procedures (conversion- reading- storing- processing- deciding) to deal with the received message. The mobile station check the (order code) field and finds that its value is (27) then comparing this value with the value from the standard it finds that the requested power on registration procedure is accepted by the BTS. C. Tools used for encoding and simulation Eclipse is an integrated development environment (IDE) written mostly in Java, Eclipse can be used to develop applications in other programming languages: C, C++, JavaScript, etc. Cygwin is a Unix-like environment and command-line interface for Microsoft Windows providing native integration of Windows-based applications, data, and other system resources with applications and software tools. Socket programming allows applications to communicate using standard mechanisms built into network hardware and operating systems. Socket-based software usually runs on two separate computers on the network, but sockets can also be used to communicate locally (inter-process) on a single computer. 8
Figure 7: Mobile station simulator Figure 8: Base station simulator Figure 9: Socket programming server-client model 9
6. Conclusion Although CDMA20001X air interface standards mention the specifications needed for manufacturing mobiles and base stations, it doesn t mention the procedures and steps for doing this. In this paper these steps are demonstrated starting from how standards (including protocol stack structure, messages and orders format) are read and interpreted, how messages and orders encoded and decoded (as a software program) and ending with a simulation of the CDMA2000-1X registration process as a case study to show how standards are being interpreted and simulated. These simulations are useful for the design and the conformance tests of the air interface signaling protocols in 3 rd and 4 th generation mobile networks. References [1] 3GPP2 C.S0001-F v2.0 Introduction to cdma2000 Standards for Spread Spectrum Systems, May 2014. [2] 3GPP2 C.S0002-F v2.0 Physical Layer Standard for cdma2000 Spread Spectrum Systems, May 2014. [3] 3GPP2 C.S0003-F v2.0 Medium Access Control (MAC) Standard for cdma2000 Spread Spectrum Systems, May 2014. [4] 3GPP2 C.S0004-F v2.0 Signaling Link Access Control (LAC) Standard for cdma2000 Spread Spectrum Systems, May 2014. [5] 3GPP2 C.S0005-F v2.0 Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems, May 2014. [6] Serge Willenegger, Cdma2000 Physical Layer: An Overview, Journal of Communications and Networks, 2000. [7] Samuel C. Yang, 3G CDMA2000 wireless system engineering, 2004. [8] Michel Daouod, WIRELESS TECHNOLOGY Protocols, Standards, and Techniques, 2002. [9] Lee,J.S., Miller, L. E., CDMA Systems Engineering Handbook, Artech House Boston, 1998. [10] Knisely, D. N, Li, Q. and S. Ramesh, N. S., cdma2000: a third-generation radio transmission technology, Bell Labs Tech, J., 63 78, July September 1998. [11] Garg, V. K., IS-95 CDMA and cdma2000 Cellular/PCS Systems Implementation, Prentice-Hall, Upper Saddle River, NJ, 2000. [12] CDMA for Next Generation Mobile Communications Systems. IEEE Communications Magazine, vol. 36, no. 9, September 1998. [13] Dahlman, E., Gumudson, B., Nilsson, M., and Skold, J., UMTS/IMT-2000 Based Wideband CDMA, IEEE Communications Magazine, vol. 36, no. 9, September 1998. [14] Garg, V. K., Halpern, S., and Smolik, K. F., Third Generation (3G) Mobile Communication Systems, IEEE International Conference on Personal Wireless Communications, February 1999, Jaipur, India. [15] Knisley, D., Quinn, L., and Ramesh, N., Cdma2000: A Third Generation Radio Transmission Technology, Bell Labs Technical Journal, vol. 3, no. 3, J1, September 1998.s 10