UNB, narrow band or spread spectrum – which works for the IoT?
Lots of things matter to a user, developer or operator of a wireless IoT network – security, QoS, reliability, energy consumption etc. All of these are important but one parameter consistently makes the top of this list – cost. It’s arguably the one that has done more to hold back the realisation of the lofty projections for IoT than any other. And a major contributing factor to this cost has consistently gone unrecognised – network capacity.
Network capacity
The primary costs in a wireless network are obviously associated with the hardware at both ends of the wireless link – the base station and the terminal device. Additional costs such as data processing, storage etc. are not considered here since we’re only comparing the OTA link technology. End device BoM costs for all three technologies are reasonably equivalent, at least the differences are essentially negligible, so we’re not considering these here either.
Network capacity is arguably the most significant parameter to factor into our cost calculation. It determines cell size and consequently the number of base stations in the network. And base station count is the primary factor in a viable total cost of ownership calculation.
Normally we think of network capacity as a measure of the number of end devices connected simultaneously to a single base station so we’ll keep with this convention and see how MAC total throughput, transmission frequency and data payload all factor into capacity calculation below.
Figure 1 shows the MAC throughput for three popular LPWAN technologies – LoRa, Sigfox and Weightless. These numbers pertain to EU regulations. LoRa is a spread spectrum technology, Sigfox uses ultra narrow band and Weightless-P is a narrow band technology.
| SIGFOX | LORA | WEIGHTLESS-P | |
| MAC throughput bits/s | 1404 | 93 | 4923 |
- Weightless-P adaptive data rate with 10dB margin, PER target 0.1%
- Scheduled up-link capacity is calculated
- Mean data rate determined by throughput for randomly positioned nodes with properly assigned data rate
- Weightless-P: -134dBm sensitivity for 0.625kbps, EU Tx power is 14dBm and US Tx power is 27dBm
- Sigfox and Weightless-P MAC throughput based on urban Hata model (BST antenna height 30m and ED height 0.5m)
- Coverage for Weightless-P is 1.5km for EU and 3.8km for US
- LoRa MAC throughput based on Ingenu white paper but removed over-conservative assumptions on repetition rate
- Capacity loss due to slot granularity is absorbed by assuming 50% protocol overhead and 50% UL half duplex ratio
In any wireless system the data throughput determines the achievable network capacity. Higher data throughput enables larger data packets, more frequent transmissions and a greater number of end points. These fundamental parameters are the key factors in the scalability debate. Increase any one of these and you are stress testing the scalability of the network. Let’s look at a typical scenario.
Smart Meters
In the utility metering sector a 15 minute reading interval is the accepted default frequency of uplink transmissions. And a data packet of 200 bytes would be considered normal. What does this mean for Sigfox, LoRa and Weightless?
First up for this example we could discount Sigfox based on the payload limitation of 8 bytes but I’m using this as the basis for comparison of network capacity so let’s keep going. 200 bytes every 15 minutes is 800 bytes/hour or, expressed in bits per second, 1.78 BPS. The MAC throughput divided by the end device data throughput will define for us the number of nodes that can be serviced – this is how data rate and capacity are linked.
Weightless-P can handle 2769 end points per base station with these uplink characteristics. LoRa can manage 52 and Sigfox can accommodate 789 end points.
Impact on cost of ownership
The CAPEX on an IoT base station might be in the region of USD$5k – let’s not get too hung up on the exact numbers. Ancillary equipment might cost USD$4k. Site engineering another USD$7k. In terms of OPEX, site rental around USD$2.5 – 4k per year and backhaul and comms another USD$2.5 – 4k per year. Over a 10 year timespan the lifetime cost of the base station will be in the region of USD$50 – 80k. Bottom line, the BST hardware BoM cost is virtually irrelevant when calculating the total cost of ownership.
Taking the lower end of these figures, USD$50k, we can calculate the cost to cover a typical city. Let’s use San Diego, CA, as our example, with a population of just over 3 million people. The typical San Diego household has 2 people deriving 1.5 million households consuming energy – each with, let’s say, one utility meter.
For a Sigfox network (assuming that Sigfox’ technology was suitable for this use case) servicing 1.5 million households approximately 1500000/789 base stations would be required. That is about 1900 base stations with an approximate cost of USD$9.5 million per year.
For LoRa the same calculation derives 1500000/52. This equates to around 29000 base stations. That’s an equivalent cost of approximately USD$14.4 million per year.
The same calculation for Weightless-P. 1500000/2769 equates to 542 base stations with an approximate, equivalent cost of USD$2.7 million per year.
The difference between these costs needs hardly to be pointed out and the conclusion is clear – network capacity matters. Whilst for some use cases it might be possible to absorb the additional cost of multiple base stations many other applications far more price sensitive and commercially unsustainable.
