Choosing IoT connectivity technology needs careful consideration of multiple technical and commercial factors linked closely to individual use cases. Different applications favour different technologies so what the product is designed to do is a critical factor in the decision. This series of articles discusses what a successful LPWAN technology looks like.
Last time out we considered network QoS; today we are turning our attention to one of the parameters that is almost always discussed when developers consider wide area network technology, particularly those outside of the home and office – range.
Our intention with these articles is to provide guidance on which connectivity technology is right for your application. No single technology is right for every product or use case so we’re not aiming to advocate a particular commercial proposition. Instead we’re discussing the implications of the technology choices in terms of particular parameters.
It’s a complex decision – there are a lot of features and benefits, often conflicting, to weigh in the balance. So these articles distil the key characteristics that define an IoT connectivity technology to identify those important to your particular use case. Specifically, we are discussing IoT connectivity technology in terms of the following eight parameters.
- Capacity
- Quality of Service
- Range
- Reliability
- Battery life
- Security
- Cost
- Proprietary vs Standard
Range
It’s one of those factors that most technology vendors are seemingly keen to discuss and yet ironically it isn’t something directly needed by the end user. Users of IoT devices want connectivity at the lowest possible cost. Long range can assist with this by reducing the number of base stations an operator needs to deploy in order to provide the required coverage. That’s great – but range is the feature, low cost is the benefit – developers really want to focus on the payoff. However, range is not the only factor impacting the number of base stations as more may be needed to provide sufficient capacity. This will become increasingly true as the number of IoT devices increases. Indeed, for some wide band spread spectrum technologies, network capacity limitations will manifest themselves quickly as the number of nodes, payload size and transmission frequencies increase. In these circumstances, range is unlikely to even come into play as a factor. This is a familiar phenomenon – in cellular systems range is rarely discussed these days because most cells are now smaller than the maximum in order to deliver the needed capacity. So a solution that delivered long range but low capacity might end up requiring more base stations than a more balanced system.
Why range might not matter
Range may also be of less relevance in some cases such as smart cities. Here, city wide coverage can often be achieved with a handful of base stations, and reducing this by one or two because of greater range does not make any material difference. It is only where rural coverage is required that longer range becomes important.
Quoting range is a little like quoting car fuel consumption. Unless the test conditions are identical, comparing two cars, or technologies, will not be meaningful. Range is impacted by many factors from antenna height, to terrain, to frequency, to interference levels. A better approach is to quote the maximum path loss that can be tolerated which is more comparable. In practice, most competing systems will have similar parameters. All systems have the same constraints in terms of transmit power allowed by the regulator and noise floor imposed by physics. All can make the same trade-offs of lower data rate for greater range using techniques such as spreading or ultra-narrowband emissions. Most will allow for flexibility so different terminals can select different combinations of range and data rate. The only variable that makes a material difference is the bandwidth with systems deployed in broad bands having more bandwidth they can trade against range. This is why TV white space appeared so attractive, but unfortunately failed to become widely available.
So it is reasonable to assume that in practice all well-designed LPWAN technologies will have a similar range. The number of base stations is then more constrained by capacity than anything else. This is the factor that those evaluating a technology should concentrate more on. Readers might like to review the issues surrounding capacity here.
The reality behind the marketing
LPWAN technologies are commonly promoted with varying claims with respect to range but the reality is that all technologies operating in sub-GHz unlicensed spectrum are subject to the same conditions, regulations and laws of physics. Range is ultimately determined by signal path, link budget, antenna size, quality, position and location, data rate and transmission power. Lower data rates with channel coding provide for a similar link budget to alternative LPWAN technologies and so achieve a typical range of 2km in an urban environment. In reality it is only possible to offer a typical range – a more accurate claim would require actual modelling in the specific environment. Weightless-P is specified with a realistic range in a dense urban environment which is closer to that achievable from all licence exempt LPWAN technologies than is commonly claimed.
A more sophisticated approach
As we noted above, it is more useful to reference a network’s ability to tolerate path loss than to quote a parameter recorded in units of linear distance. So what about individual path loss variance between nodes – perhaps continually because of, say, location inside a building or dynamically because of, say, the transient movement of vehicles into the signal path. A network technology with full bidirectional capability can, optionally, permit 100% acknowledgement of every uplink transmission. With this acknowledgement, both the data rate and the Tx power can be configured dynamically and automatically by the network to optimise link budget. Where the path loss is less, either a lower Tx power or a higher data rate can be set. Lower Tx power improves battery life (lower active radio instantaneous power consumption), capacity and network performance by minimising self interference. Higher data rate improves battery life (less active radio time) and network performance. Weightless-P offers dynamic Tx power and data rate control capabilities.
