“A radio wave carries energy. Modulation gives that energy meaning. The strength of the signal determines whether it arrives, but the intelligence encoded within it determines what it has to say.” – MJ Martin
The Canadian RF Context
In Canada, wireless systems from municipal AMI networks to Wi-Fi operate within spectrum and power rules administered by Innovation, Science and Economic Development Canada. ISED distinguishes transmitter output power from radiated power and establishes limits by frequency band. For example, RSS-247 limits many licence-exempt digital systems in the 902 to 928 MHz band to 1 watt conducted power and 4 watts EIRP. (ISED Canada) Modulation does not create RF power. It determines how information is carried by that power.
Carrier, Modulation and Demodulation
An RF carrier is a sinusoidal radio signal at a particular frequency. Think of it as an empty delivery truck. Modulation loads information onto the carrier by deliberately changing its amplitude, frequency or phase. Demodulation is the reverse process at the receiver, extracting those changes and reconstructing the original information. Modern digital systems commonly manipulate the in-phase and quadrature, or I/Q, components of the signal to control amplitude and phase. (Rohde & Schwarz)
A subcarrier is an additional signalling component within the overall radio channel. In OFDM systems, for example, one wide channel is divided into many closely spaced subcarriers that simultaneously carry portions of the data.
FSK, QPSK and QAM
Frequency Shift Keying, or FSK, represents digital information by shifting between discrete frequencies. It is simple and comparatively robust, making it useful for telemetry and other lower-rate communications.
Quadrature Phase Shift Keying, or QPSK, uses four phase states, allowing each symbol to represent two bits. Quadrature Amplitude Modulation, or QAM, varies both amplitude and phase. A 16-QAM constellation represents four bits per symbol, 64-QAM represents six, and 256-QAM represents eight.
Higher-order modulation therefore increases spectral efficiency, but Keysight notes that these schemes also become more sensitive to signal impairments. (Keysight Technologies)
Low Order Versus High Order
Low-order modulation uses fewer, more widely separated signal states. The receiver can distinguish them more easily when signals are weak, noisy or fading, but fewer bits are carried per symbol.
High-order modulation packs more states into the same signalling space. It is like printing more words on the same postcard using smaller type. More information fits, but imperfections become more troublesome. High-order QAM therefore requires better signal-to-noise ratio, cleaner amplification and tighter modulation accuracy.
FEC: Protecting the Message
Forward Error Correction, or FEC, adds structured redundant information before transmission so the receiver can detect and correct some errors without requesting retransmission. IEEE describes FEC as a form of channel coding that trades additional overhead for improved reliability. (IEEE Technology Navigator)
FEC is comparable to including enough clues in a damaged sentence that missing letters can still be reconstructed. Stronger FEC improves reliability, but consumes part of the available communications capacity.
Receive Threshold, Transmit Power and Link Budget
Transmit power is the RF power produced by the transmitter, normally expressed in watts or dBm. It differs from EIRP because antenna gain and transmission-line losses affect the final radiated power.
Receive threshold is the minimum received signal level required to satisfy a defined performance target, typically a specified bit-error rate. It is closely related to receiver sensitivity. Analog Devices notes that sensitivity depends on factors including receiver noise figure, bandwidth and the signal-to-noise requirement of the modulation. (Analog Devices)
A link budget is the RF system’s financial statement. Start with transmit power, add antenna gains, subtract cable, connector, propagation, obstruction and fading losses, then compare the predicted received level with the receive threshold. The difference is the link margin. Analog Devices identifies these same elements as fundamental inputs to practical RF link-budget calculations. (Analog Devices)
For Canadian utility communications, the important question is therefore not simply, “Which modulation is fastest?” It is, “Which modulation provides the required throughput while preserving enough link margin to reach a basement, meter pit or remote endpoint under worst-case conditions?” Would you favour slower but exceptionally robust communications for AMI endpoints, or higher-order modulation where network capacity is the dominant concern? And how much fade margin should a Canadian utility demand when snow, foliage, buildings and terrain are part of the real RF environment?
About the Author:
Michael Martin is the Vice President of Technology with Metercor Inc., a Smart Meter, IoT, and Smart City systems integrator based in Canada. He has more than 40 years of experience in systems design for applications that use broadband networks, optical fibre, wireless, and digital communications technologies. He is a business and technology consultant. He was a senior executive consultant for 15 years with IBM, where he worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He is a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX).
Martin served on the Board of Directors for TeraGo Inc (TGO: TSX) and on the Board of Directors for Avante Logixx Inc. (XX: TSX.V). He has served as a Member, SCC ISO-IEC JTC 1/SC-41 – Internet of Things and related technologies, ISO – International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology. He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) [now Ontario Tech University] and on the Board of Advisers of five different Colleges in Ontario – Centennial College, Humber College, George Brown College, Durham College, Ryerson Polytechnic University [now Toronto Metropolitan University]. For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section.
He holds three master’s degrees – in business (MBA), communication (MA), and education (MEd). As well, he has three undergraduate diplomas and seven major certifications in business, computer programming, internetworking, project management, media, photography, and communication technology. He has completed over 80 next generation MOOC (Massive Open Online Courses) [aka Micro Learning] continuous education programs in a wide variety of topics, including: Economics, Python Programming, Internet of Things, Cloud, Artificial Intelligence and Cognitive systems, Blockchain, Agile, Power BI, Big Data, Design Thinking, Security, Indigenous Canada awareness, and more.
Martin is a volunteer, a photographer, a learner, a technologist, a philosophizer, and a romantic optimist.