WiMAX | Solution to Wireless SCADA System
What is SCADA?
SCADA stands for Supervisory Control and Data Acquisition. SCADA systems are used in many industrial and infrastructure processes such as manufacturing, production and development, and electric power generation, transmission, and distribution. In electric power systems, SCADA is mainly used to monitor important system parameters such as frequency, voltage, current, demand, and energy. It is also used in monitoring and control substation electrical equipment such as power transformers, circuit breakers, and line equipment such as load break switches and reclosers. SCADA systems enable remote monitoring and control of an electric power system.
How does a SCADA system operate?
A SCADA system is typically composed of the following:
- Computer – Usually located in the control stations. These computers are responsible for the monitoring and control of field devices using a Human Machine Interface (HMI).
- Remote Terminal Unit (RTU) – this microprocessor-based electronic device enables the transfer of the process values from the field devices to the control station.
- Communication Infrastructure – the communication infrastructure allows the computers in the remote control station to connect to the RTUs for the whole SCADA system to actually work. Within cities, the communication infrastructure uses a network of fiber optic cables however for very remote areas, a wireless SCADA system is usually employed. These include cellular networks, IP radio, and WiMAX.
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Basic Network Design Principles for Electrical Engineers
Introduction to WiMAX
Wireless Broadband
Wireless broadband has been used extensively today to access the Internet for services such as VOIP, FTTH, VDSL, HDTV, VoD, and other multimedia applications. However, before the wireless technology was widely adopted, wired connection was the preferred choice. Broadband access was only possible through Digital Subscriber Line (DSL), which uses twisted-pair telephone wires, or cable modem, which uses coaxial cable TV. Because of the potential of wireless technology to fuel the demand for broadband and the convenience it can provide, it has since enjoyed its adoption in the broadband market.
Broadband wireless is about bringing broadband to a wireless context. There are two types of broadband wireless. The first is the fixed wireless broadband which uses wireless as the medium of transmission, second is the mobile broadband which offers the additional functionality of portability, nomadicity, and mobility.
Development of WiMAX
Narrowband Wireless Local-Loop Systems
WiMAX is one of the technologies developed for both fixed and mobile wireless broadband. It started from the narrowband wireless local-loop systems which were developed and deployed for voice telephony. One development was the “Project Angel” by AT&T but has short-lived offering because of cost run-ups and poor take-rate. Wireless internet provider (WISP) during that time deployed systems for the license-exempt 900MHz to 2.4GHz with speeds up to a few hundred kbps.
First Generation Line-of-Sight (LOS) Broadband Systems
The second development was the 1st generation line-of-sight (LOS) broadband systems. It was during this time that systems for higher frequencies were developed. Millimeter-wave frequency bands, 24GHz, and 39GHz were used but were rather short-lived. Systems for 2.5GHz, the MMDS band, were developed after the FCC allowed two-way communication in 1998. Fixed wireless broadband was developed using towers typically several hundred feet tall and required subscribers to install an outdoor antenna pointed toward the tower for a clear LOS transmission path. However, the system capacity was limited by a single tower serving a fairly large area.
Second Generation Non-Line-of-Sight (NLOS) Broadband Systems
The 2nd generation non-line-of-sight broadband systems were the next development to address the limitation of LOS broadband systems. During this time, technologies such as Orthogonal Frequency Division Multiplexing (OFDM), Code Division Multiple Access (CDMA), multiantenna processing was developed. It was also during this time that most solutions were proprietary.
WiMAX and The IEEE 802.16
Because most NLOS systems were mostly proprietary, a standards-based wireless broadband system was developed. A group known as the IEEE802.16 was tasked to create standards for Wireless Metropolitan Area Network (WMAN). WMAN-SC standard was developed with a physical layer using single-carrier modulation and a Media Access (MAC) layer with a burst time-division multiplexing (TDM) supporting frequency division duplexing (FDD) and time division duplexing (TDD). The WMAN-SC was initially based on LOS operating on a 10GHz-66GHz millimeter-wave band but was later amended to IEEE802.16a to work in both licensed and license-exempt frequencies, 2GHz-11GHz, to enable NLOS with OFDM schemes. Developments in the standard furthered with IEEE802.16e-2005 which specifies a scalable OFDM and modified MAC layer for high-speed mobility.
The WiMAX Forum
WiMAX emerged from the industry standard, IEEE802.16. The WiMAX Forum developed interoperability certification for this standard. WiMAX Forum’s Network Working Group is responsible for developing the end-to-end network requirements, architecture, and protocols for WiMAX, using IEEE 802.16e-2005 as the air interface. They developed a network reference model to serve as an architecture framework for WiMAX deployments and to ensure operability among various WiMAX equipment and operators.
What is WiMAX?
Worldwide Interoperability for Microwave Access (WiMAX) is a Wireless Metropolitan Area Network based on IEEE802.16 standards. It has an OFDM-based physical layer to enable NLOS. It can provide data rates of up to 74Mbps when operating at a 20MHz wide spectrum. Its physical layer is scalable which allows data rate to scale easily with available channel bandwidth. It supports adaptive modulation (AMC), automatic retransmission requests (ARQ) at the link layer, and FDD and TDD for low-cost system implementation. WiMAX also supports Orthogonal Frequency Division Multiple Access (OFDMA), and advanced antenna techniques such as beamforming, space-time coding, and spatial multiplexing. The WiMAX MAC is designed to support a large number of users, with multiple connections per terminal, each with its own QoS requirement. WiMAX also supports Advanced Encryption Standards (AES), secure seamless handovers for delay-tolerant full-mobility applications, and more importantly, WiMAX is based on an all-IP platform.
The Role of WiMAX in a Wireless SCADA System
Wireless SCADA system has been the choice of electric power distribution systems in the monitoring and control of remote devices situated in high vegetation and mountainous areas. However, a wireless SCADA system faces the challenge of the Line-Of-Sight (LOS) requirement of most radio systems. The introduction of WiMAX to the wireless SCADA system solves this problem by enabling Non-Line-of-Sight (NLOS) communication.
References
Andrews J., Ghosh A., Muhamed R., Fundamentals of WiMAX Understanding Broadband Networking, Pearson Education, Inc., USA, 2007.
SCADA – Wikipedia. https://en.wikipedia.org/wiki/SCADA
WiMAX – Wikipedia. https://en.wikipedia.org/wiki/WiMAX
Wimax Forum Website. http://wimaxforum.org/
