Turning Back RF Technology to Take a Step Forward

Blog Post created by benz on Oct 14, 2016

Originally posted Dec 28, 2015


New activity on familiar terrain

High-profile developments in wireless networking usually involve ever-wider bandwidths and ever-higher operating frequencies. These support the insatiable need for increased wireless capacity, and they parallel mobile developments such as 5G cellular. And if the prognosticators are correct, more users generating more traffic will compete with increasing traffic from non-human users such as the Internet of things (IoT) and machine-to-machine (M2M) communications.

The increasing need for data capacity is undeniable, but the focus on throughput seems a bit narrow to me. I’m probably a typical wireless user—if there is such a thing—and find myself more often dissatisfied with data link availability or reliability than with capacity.

For example, Wi-Fi in my family room is mostly useless when the microwave in the kitchen is on. Sure, I could switch to a 5.8 GHz wireless router, but those signals don’t travel as far, and I would probably relocate the access point if I made the change. Another example: The 1.9 GHz DECT cordless phone in the family room will cover the front yard and the mailbox, but the one in my downstairs office won’t. A phone doesn’t demand much data throughput for voice, but it must provide a reliable connection. Yes, I can carry my mobile phone and forward to it, but I sometimes appreciate the lack of a tether.

I often think about the digital cordless phone I had a dozen years ago, operating on the 900 MHz ISM band with a simple 12-bit PN code for spread spectrum. Its range was hundreds of yards with obstructions and over half a mile in the open.

I’ve been reading a little about the proposed new 802.11ah wireless networking standard in that same 900 MHz band, and thinking about the implications. Two important technical factors are the limited width of the band—902 to 928 MHz—and improved signal propagation compared to the 2.4 and 5.8 GHz bands. In the technical press you’ll frequently see a diagram similar to this one:

Lower frequencies generally propagate better, and the difference can be significant in terms of network coverage in a house or office space. Of course, practical range depends on many other factors as well.

Lower frequencies generally propagate better, and the difference can be significant in terms of network coverage in a house or office space. Of course, practical range depends on many other factors as well.

The diagram is certainly oversimplified, in particular neglecting any band-crowding, interference or obstruction issues. Nonetheless, the potential range benefits are obvious. Some claim that real-world distances of more than a kilometer are feasible, and the 900 MHz band may allow higher effective transmit power than 2.4 or 5.8 GHz.

Throughput, however, is modest compared to other WLAN standards. Data can be sent using a down-scaled version of the 802.11a/g physical layer for data rates ranging from 100 Kb/s to more than 25 Mb/s. Significantly, the standard supports power-saving techniques including predefined active/quiescent periods.

As usual, the standard has many elements, supporting a variety of potential uses, and a few are likely to dominate. Those mentioned most often relate to IoT and M2M. Compared to existing Wi-Fi, 802.11ah should be better optimized for the required combination of data rate, range and power consumption.

Although that presumption seems reasonable, recent history tells us that attractive combinations of spectrum and PHY layer will be bent to unanticipated purposes. I think there are many situations in which users would be happy to trade transfer speed for a high-reliability link with longer range.

From an RF engineering standpoint, improved propagation is a double-edged sword. Current WLAN range relates well to the scale of homes and small businesses, naturally providing a degree of geographic multiplexing and frequency reuse due to lower interference potential. The combination of propagation and transmit power in the narrower 900 MHz band will change tradeoffs and challenge radio designers.

The 802.11ah standard is expected to be ratified sometime in 2016, and RF tools are already available. Keysight’s WLAN measurement applications for RF signal generators and signal analyzers already support the standard, and vector signal analysis is supported with pre-stored settings in the custom OFDM demodulation of the 89600 VSA software Option BHF.

With established alternatives ranging from ZigBee to 802.11ac, some are skeptical about the success of this effort in the relatively neglected 900 MHz ISM band. It’s a fool’s errand to try to predict the future, but it seems to me this band has too much going for it to remain under-occupied.