Setting the Stage: Categories of Low-Power, Wide-Area Modulation Schemes
At first glance, the number of communication technologies being discussed for Low- Power, Wide-Area (LPWA) networks may be a bit overwhelming. What may be helpful is to look at the underlying technology from a fundamental perspective and tune out the marketing component. Note that marketing is an extremely important aspect of LPWA success, but since I am a CTO, I would like to push it to the side for this discussion.
Many of you know that Communication Theory is a very mature field going back many decades with a vast wealth of generated knowledge. Tens of thousands of books, articles, and papers have been published over this time. There are giants in the field – Claude Shannon, Harry Nyquist, Ralph Hartley, Alan Turing, and Andrew Viterbi (who has been an Ingenu strategic advisor from the beginning) whose work we can turn to for clarity. This great body of work gives us frameworks and vocabularies for comparison. It’s often a drier and less interesting world once the marketing innovation is subtracted out – but please, bear with me.
The table below shows four categorizations of the various categories of modulation schemes that are being discussed for LPWA. Bold denotes those technologies being branded as applicable to Low-Power, Wide-Area networking. Since this is a technology treatment, I am defining Local Area Network (LAN) and Wide-Network (WAN) by the underlying technology as opposed to how these approaches are being marketed. Note that the most well-known approaches in each category are the Sigfox™ technology, LoRa®, also known as Chirp Spread Spectrum (CSS), Narrow-Band IOT (NB-IOT), and Random Phase Multiple Access (RPMA®). These technologies tend to be those with the best marketing (yes, marketing is very important).
|Category||Local Area Network||Wide Area Network|
|Non-Coherent M-ary Modulation (NC-MM)||Bluetooth
|Direct Sequence Spread Spectrum (DSSS)||802.11
|Orthogonal Frequency Division Multiplexing
From a technology perspective, the definition of being appropriate as a WAN is whether the multiple-access considerations of coverage and capacity are taken into account:
- Coverage. If you want to build a WAN, you would like a single piece of network infrastructure (often on a tower or rooftop) to cover as much area as possible.
- Capacity. There’s not a lot of good in covering a massive area if you cannot support the data needs of all the devices in that footprint.
Giving a bit more color on the categories:
- Ultra-Narrow Band (UNB). The reason many companies have elected this approach is the advantage of low barrier to entry. Companies in this category can leverage commodity radios and skip any technology development. These companies tend to argue that no new technology is required. We disagree for many reasons including the inability to make the economics of LPWA work as discussed in Blog 5: The Economics of Receiver Sensitivity and Spectral Efficiency.
- Non-Coherent M-ary Modulation (NC-MM). This is a commonly used modulation in both LAN and WAN applications. Cellular 2G technology was based on GSM/GPRS which uses a modulation approach called Minimum Shift Keying (MSK) and is also being repurposed to Extended Coverage GSM (EC-GSM) which is cellular LPWA in 2G spectrum. The LoRa modulation (CSS) is a member of this category (as is justified in Blog 3: Chirp Spread Spectrum: The Jell-O of Non-Coherent M-ary Modulation. The “spreading” of CSS has no discernible advantage and indeed, as discussed in Blog 4: “Spreading” – A Shannon-Hartley Loophole?, has some significant drawbacks in terms of spectral efficiency.
- Direct Sequence Spread Spectrum (DSSS). We described LoRa as “spreading” for no discernible reason. Well it turns out NC-MM does not have the monopoly on this. DSSS has a couple of technologies that also spread for no discernible reason – IEEE 802.11 (the original 1 and 2 Mbps data rates) and Zigbee (based on IEEE 802.15.4). This is just one example to show you that standards bodies are less about technology and more about politics. I will discuss this in more depth in a future blog.
- Orthogonal Division Multiple Access (OFDM). This is the way you get extreme spectral efficiency. It’s great for voice high-speed data and has enabled LTE (4G) to become the dominant cellular standard. When you try to point this approach at LPWA (such as NB-IOT), significant problems emerge. I will discuss this in more depth in a future blog.
In the next blog, we visit the building blocks of communication and translate the perfectly good and intuitively understandable terms of coverage and capacity to nerd in Blog 2: Back to Basics – The Shannon-Hartley Theorem.
- Coverage translates to receive sensitivity, which is a function of something called Eb/No (energy per bit relative to thermal noise spectral density).
- Capacity translates into something called spectral efficiency and we need to go even one level deeper into nerd and assign it a Greek letter (of course) and that Greek letter is…. η.
Using this fundamental framework, in Blog 3: Chirp Spread Spectrum: The Jell-o of Non-Coherent M-ary Modulation, I’ll talk about a category of approaches that is very similar from a technology point of view and one very successfully marketed approach in this category called Chirp Spread Spectrum (also known as LoRa).
And while we’re on the subject of spreading, I’ll discuss how there a potential capacity loophole to take advantage of in Blog 4: “Spreading” – A Shannon-Hartley Loophole?.
And finally, let’s get back into the real world and test whether this discussion is even useful in Blog 5: The Economics of Receiver Sensitivity and Spectral Efficiency to attempt to quantify the commercial value of that which is discussed on these blogs.
At any point, if you’re interested in a more in-depth treatment of these topics, please download How RPMA Works: The Making of RPMA.