1、 NOTICE OF COPYRIGHT This is a copyright document and may not be copied or distributed in any form or manner without the permission of ISA. This copy of the document was made for the sole use of the person to whom ISA provided it and is subject to the restrictions stated in ISAs license to that pers
2、on. It may not be provided to any other person in print, electronic, or any other form. Violations of ISAs copyright will be prosecuted to the fullest extent of the law and may result in substantial civil and criminal penalties. TECHNICAL REPORT ISA-TR100.00.01-2006 The Automation Engineers Guide to
3、 Wireless Technology Part 1 The Physics of Radio, a Tutorial Approved 29 December 2006 ISA-TR100.00.01-2006 The Automation Engineers Guide to Wireless Technology Part 1 : The Physics of Radio, a Tutorial ISBN: 978-1-934394-09-0 Copyright 2006 by ISA. All rights reserved. Not for resale. Printed in t
4、he United States of America. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of the Publisher. ISA 67 Alexander Drive P. O. B
5、ox 12277 Research Triangle Park, North Carolina 27709 USA 3 ISA-TR100.00.01-2006 Preface This preface, as well as all footnotes and annexes, is included for information purposes and is not part of ISA-TR100.00.01-2006. This document has been prepared as part of the service of ISA towards a goal of u
6、niformity in the field of instrumentation. To be of real value, this document should not be static but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms and asks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alex
7、ander Drive; P. O. Box 12277; Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288; E-mail: standardsisa.org. The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Unit
8、s (SI) in particular, in the preparation of instrumentation standards. The Department is further aware of the benefits to USA users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this en
9、d, this Department will endeavor to introduce SI-acceptable metric units in all new and revised standards, recommended practices, and technical reports to the greatest extent possible. Standard for Use of the International System of Units (SI): The Modern Metric System, published by the American Soc
10、iety for Testing wireless real time field-to-business systems (e.g., wireless equipment interfacing work order systems, control LAN, business LAN, voice). This covers all industries including fluid processing, material processing, and discrete parts manufacturing environments. Equipment cost will be
11、 an important factor. It is easy to imagine new and useful monitor and control applications which, although not feasible today because of the cost of the wired connections, will become practical when the cost of wireless networks has been reduced sufficiently. The broad scope of these applications s
12、uggests that there is unlikely to be an optimum “one size fits all” wireless solution. The bandwidth, throughput, signal latency (transmission delay), bit error ratio, availability and reliability requirements of a wireless local area network carrying large volumes of file transfer or voice traffic
13、or a link used for real-time control of critical temperature or pressure parameters in a process tank are very different from those of a link used to monitor the water level in a remote reservoir, where only a few bytes may need to be transmitted three or four times a day. This technical report focu
14、ses on radios using the unlicensed industrial, scientific and medical (ISM) and Unlicensed National Information Infrastructure (UNII) bands. In this technical report where there are references to the ISM bands the UNII bands should also be considered to be included. These bands have certain aspects
15、in common: They are required to comply with regulations set by government agencies, such as the Federal Communications Commission (FCC) in the United States. For example, both transmitter output power and radiated power density are capped, to limit the potential for interference. In general, the reg
16、ulations are very flexible and provide ample scope for developing innovative solutions to wireless communications problems. They are exposed to the constant threat of radio frequency interference from other equipment operating in the same bands. Copyright 2006 ISA. All rights reserved. ISA-TR100.00.
17、01-2006 10 This technical report presents a general tutorial on the basic principles of radio communications, and then discusses the performance of spread spectrum radio systems in the presence of interference from other ISM band users and non-ideal propagation conditions, such as may be expected in
18、 the industrial plant environment. Its objective is to give readers a realistic understanding of how radio links can complement and/or replace wired connections, the factors influencing link range, and the pitfalls for the unwary. Industrial applications for wireless communications range from the re
19、latively non-critical, such as asset monitoring to support preventive maintenance programs, all the way to the highly-critical, involving plant, personnel and public safety. The technical report will show that well-designed radio systems can satisfy these varying needs, but that the associated trade
20、offs of performance vs. security and reliability result in different system solutions for different applications. In general, the radio links analyzed in the technical report are assumed to operate in the ISM bands, and to carry information in the form of digital bit streams. Discussions are limited
21、 to the physical layer and error-control portions of the data-link layer of the seven layer open systems interconnection (OSI) model. Wireless networking concepts are touched on only superficially, in the context of spectrum sharing and spectrum management; further details may be incorporated in a f
22、uture publication. Where examples are given, they are based on the 2.4 GHz ISM band and the rules in effect in North America. Section 2 of the technical report deals first with the performance of a single, one-way radio link in ideal “free space” conditions, assuming unobstructed line-of-sight propa
23、gation of the carrier waves and no sources of interference (such as other transmitters, electrical machinery or electronic equipment), the only sources of impairment being the thermal noise generated in the receiver and the reduction in signal strength due to path losses. Section 3 of the technical
24、report covers various aspects of radio wave propagation, including reductions in signal strength due to increasing distance from the transmitter and transmission through “lossy” (partially-conducting) media, reflection of electromagnetic waves at conducting, non-conducting or partially-conducting su
25、rfaces, multipath signal fading caused by reflections and techniques for mitigating these effects, and the design, selection and deployment of antennas. Section 4 of the technical report deals with the “real world” issue of how multiple radio systems can “co-exist” without suffering unacceptable lev
26、els of radio frequency interference from other users sharing the same frequency bands. Topics discussed include the traditional regulatory approach to spectrum management, the constraints applicable to the unlicensed frequency bands, sources of interference and the effects of interference on radio p
27、erformance, and techniques to mitigate the effects of interference. The final section of the technical report addresses the issue of wireless communication standards and how they might be rationalized to facilitate the secure and reliable deployment of radio links in the industrial environment for a
28、pplications in all categories from non-critical to highly critical. 2 Free space communications basics 2.1 Elements of a radio link Radio (or wireless) links make use of electromagnetic radiation covering a band of frequencies made up of channels. The width of a channel used is determined by the amo
29、unt of information to be transmitted in a given time and the modulation and filtering design of the radio. A higher data rate generally requires a wider channel when using a particular modulation format. Modulation is the method by which the data is impressed onto the radio signal. Modulation techni
30、ques which impress more data onto a given band are generally more complicated to implement and less immune to propagation impairments and interference. Regulations set the acceptable channel widths and in some cases even the modulation types that may be used in a given band. Narrow band radios norma
31、lly stay on a single center frequency and occupy only the bandwidth required to accommodate Copyright 2006 ISA. All rights reserved. 11 ISA-TR100.00.01-2006 their data transmission requirements. In order to more effectively share the frequencies, radios operating in unlicensed bands typically use ad
32、ditional techniques to avoid generating interference and improve their resistance to interference. Two of the more popular methods of operation within the ISM bands are frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). Frequency hopping radios move a narrow band ar
33、ound to different frequencies. Direct sequence radios modulate a narrowband signal with an additional wide band random noise-like signal that has the effect of spreading it over a wider band of frequencies. Figure 2.1 Block Diagram of a Generic Radio Link There are newer techniques such as Ultra Wid
34、e Band and Chirp transmission that are emerging and in the future there will undoubtedly be continuing development and innovation. Another popular technique is Orthogonal Frequency Division Multiplexing (OFDM) which generates a large number of narrow signals adjacent to each other sharing the overal
35、l data payload. Figure 2.1 shows the block diagram of a generic radio link, which we will assume to be operating in one of the ISM bands. The modules shown as non-shaded will typically be found in both narrow-band and spread spectrum radios. By inserting the shaded modules “PN Code” and “Spreader” o
36、n the transmit side and “Synchronizer”, “PN Code” and “Despreader” on the receiver side, we can convert this narrow-band radio into a DSSS radio. If we instead insert the shaded modules “PN Code” and “Frequency Hopper” on the transmitter side and “Synchronizer”, “PN Code” and “Frequency Dehopper” on
37、 the receiver side, we convert it to a frequency-hopping spread spectrum (FHSS) radio. Sections 2.1 through 2.5 deal primarily with the general case of a radio link carrying digital data. Direct sequence and frequency-hopping spread spectrum technologies will be discussed in section 2.6. 2.1.1 A not
38、e about filters Filters will be a recurring topic in this technical report. They are extensively used in radio communications, and generally perform one of two basic functions: Copyright 2006 ISA. All rights reserved. ISA-TR100.00.01-2006 12 to allow certain frequencies to pass (the “pass band”) whi
39、le rejecting (or attenuating) other frequencies (the “stop” band or “attenuation band”); to control the shape of the transmitted signal spectrum. Figure 2.2 Common Filter Types Filters in the first of these categories are usually described in terms of their characteristics, as shown in Figure 2.2: “
40、bandpass”, “lowpass”, “highpass”, and “bandstop” or “notch”. The five filters shown in Figure 2.1 usually will be implemented as bandpass filters. We will discuss these filters in more detail later in the technical report. For many radio applications, the rectangular characteristics shown in Figure
41、2.2 would be ideal, but cannot be realized in practice, although complex filters are capable of achieving quite close approximations. Typically, the “cut-off characteristic will be a smooth “roll-off”, the stop band attenuation will be finite, and both the pass band and the attenuation band will exh
42、ibit ripples. See Figure 2.3. Figure 2.3 Filter Terminology An important characteristic of a bandpass filter is its “quality factor” or “Q”, which is defined as the center frequency of a filter divided by the bandwidth, where the bandwidth is defined as the frequency of the upper 3 dB* roll-off poin
43、t minus the frequency of the lower 3 dB roll-off point. For example, a Copyright 2006 ISA. All rights reserved. 13 ISA-TR100.00.01-2006 filter with a center frequency of 2.400 GHz and a 3 dB bandwidth of 100 kHz has a Q of (2,400/0.1) = 24,000. Such a high-Q filter would be extremely difficult to de
44、sign and implement. When we come to discuss the receiver design shown in Figure 2.1, we will see how radio designers have traditionally dealt with this problem. (* For an explanation of dB (decibel) power measurement, please see the power amplifier section below.) We will discuss spectrum shaping fi
45、lters in the “Modulation and Spectrum Bandwidth” section below. 2.1.2 Transmitter Although it is not mandatory under the regulations, most transmission standards used in the ISM bands divide the total frequency range of the band into several narrower, equally-spaced “channels”. The radio transmitter
46、 generates a radio frequency (RF) electromagnetic wave (the “carrier”) centered on one of these channel frequencies and superimposes the data signal on the carrier by a process known as “modulation”. The transmitter must ensure that the carriers transmitted power, frequency and spectrum bandwidth co
47、mply with the specifications set by the regulatory agencies. 2.1.2.1 Data signal coding In Figure 2.1, in the absence of the DSSS spreader, the input data signal is assumed to be applied directly to the modulator. In reality, it is usually subjected to some preliminary “coding” to add “overhead” dat
48、a bits for receiver synchronization, error correction, and error checking. In this technical report, we will generally ignore such overhead bits. We will use the symbol “R” to represent the data signal bit rate in bit/s. 2.1.2.2 Pseudo noise (PN) code generator This module generates a pseudorandom d
49、igital sequence used to spread the bandwidth in DSSS systems and to generate the hop frequencies in FHSS systems. Detailed discussion of this module is deferred to section 2.6. 2.1.2.3 Frequency reference Although the regulators do not require ISM band radios to comply with specific frequency plans, the center frequency of the transmitted carriers must be accurate enough to allow the transmitters and receivers to “find” one another. Requirements are more stringent for radios specified over a wide temperature range and for systems operating at
