For the Hata model, the difference between path loss at 470MHz and 800MHz at 1.5km (our expected cell radius in urban environments) is about 6 dB. So, 470MHz offers better penetration, but goes further for a given transmit power. As i mentioned in the last post, our low-power end-points will have slightly lower RF output power than LTE UE's, so the LTE analogy is actually pretty consistent. As the Weightless system is targetted specifically at the IoT (whereas LTE is designed for people), we're able to trade data rate and latency for processing gain, allowing us to operate below the noise floor and connect to devices deep within buildings, where LTE might struggle.
Advanced signal processing in the receiver can significantly reduce co-channel interference, allowing us to get the penetration benefit while reducing problems introduced by reuse.
Another point that needs clarification is the number of devices one would expect to address in a cell. There is the obvious link between total cell capacity and bandwidth, where our system is sufficiently flexible to take advantage of the bandwidth available in a given region. It's worth also noting that in an FDMA combined with TDMA approach (as with Weightless), maximum per terminal throughput can be traded to some extent for total cell capacity, i.e. using NB uplink channels (this has to be bounded of course, going too NB makes terminal mobility problematic and it can get harder to overcome flat fading). Channel bonding can be used to add further flexibility.
Despite all this, zillions of IoT devices in a 1.5km cell would be a struggle. With the kind of application parameters we're seeing for launch applications, somewhere between 100k and 1M devices per cell should be feasible.
I was responding to the comment about no requirement for the user to enter an authorization code. Of course, anyone can make their WiFi network wide open. Not an issue at all. So whether you use TV spectrum or WiFi, or other schemes like Bluetooth, that question of authorization is orthogonal. For some uses, you do want the owner to key in some sort of code.
As to cell planning, it's a question of making consistent arguments. A cellular system that operates at 800 MHz will have an easier time of creating small cells than a system operating at 470 MHz. And even easier is a cell system that operates in the L band. I'm not saying the 470 MHz scheme can't work. I'm merely saying, it's inconsistent to first argue that you want long range, and then argue that you want to access zillions of IoT devices in a small area.
The ability of the owner of a device to control access to it can be entirely independent of the communications technology. Some may decide to buy a fridge which can be controlled by their energy company and they may be incentivised for doing so, whereas you and I might decide to buy a sprinkler system which we alone could control from anywhere in the world on our smartphones. Weightless aims to service both types of application (and many more).
Cell planning is complex, but not insurmountable. As per my last post, with appropriate protocol, adaptive modulation schemes, link budget, power control, base station and endpoint design, these things are acheivable. LTE at 800MHz (and higher power output than our endpoints) manages it, and with very good cell capacity. We target similar cell sizes to GSM.
Regarding TVWS & IoT, i think we're in agreement- there may be several uses for TVWS, one of which might be an IoT network. Equally, a Neul IoT network may use frequency spectrum other than TVWS, as i mentioned in my last post.
That all depends on the devices we're talking about. I certainly don't want others to determine when my lawn sprinklers come on, or when my self-cleaning oven starts cleaning itself. I also don't want the entire neighborhood using my printer. Depending on what IoT device we're talking about, you may or may not want to have the owner of the device limit access to it.
So again, this all needs to be more specific and well defined to be convincing to me. For example, let's say that we're looking for an RF link that can easily make it through walls, as you say. Well, such a link will also propagate longer distances outside those walls. So that means, that RF link will be more difficult to deploy in small cells (which promote large frequency reuse), and as a consequence, that RF scheme will be more limited in the number of devices it can scale up to serve. You can't have it both ways.
I don't doubt that there will be that subset of IoT devices that indeed might work well at those frequencies. My own tack would be to NOT tie use of TV spectrum so tightly to IoT, but instead make IoT just a possible use of it.
Part of the problem we're trying to solve is simplicity. The 802.15.4 based mesh / short range comms standards certainly have their place, but they're plagued by interoperability issues and require the end-users to take on a certain degree of network planning. For the IoT market to really take off, it's got to be super-simple: just turn your end device on and it's connected with the network, none of this putting down concentrators or working out how to get WiFi access codes into your products.
I'm not the expert on Alljoyn, but from what i know this is targeting the application layer and could quite possibly sit on top of a Neul / Weightless data pipe. If it makes things simple, and the big consumer electronics guys sign up, i could well imagine a whole ecosystem of developers building solutions on top of our technology.
Thanks for the comments, it's great to get the discussion going!
As i'm sure you're aware, there is a balance to be struck between good propagation and frequency reuse for cell planning. The fact that existing cellular standards use sub-GHz frequencies should provide evidence that with appropriate modulation schemes and protocol design, co-channel interference can be managed- and with reasonable reuse factors.
When it comes to antenna's, the same argument applies- all of our phones manage with small form factor antennas. You're absolutely right though, these tend to be relatively inefficient, but that's the same challenge which has been fought en masse for a variety of sub-GHz technologies for years (not just cellular), there being some innovative solutions out there right now. The real problem with TVWS antenna's is the dynamic spectrum access element. Having to be able to operate over such a wide frequency range, and re-tune on the fly as and when a channel allocation is changed, makes antenna design for the transmitter rather difficult (the receiver is possible- look at the small low cost DVBT antenna's out there). Large fractional bandwidth means large volume antenna's for transmitting, as you rightly say "you can't fool mother nature". So, as you can see, the key to solving the transmit problem would be to reduce the fractional bandwidth- which, without going in to too much detail is just part of what we do.
It's worth noting that although this article refers to Neul as "Television white-space spectrum pioneer", which is absolutely correct, we are much more than TVWS. We have built a frequency & bandwidth agile system which can operate over nearly all of the sub-GHZ UHF spectrum. In some regions, this might well be TVWS, in others it could be small chunks of licensed spectrum or even license exempt bands. Our PHY and MAC have been designed to be adaptable to all sorts of interesting spectrum! And wherever we operate, the system will deliver on the key requirements we see as intrinsic to the IoT market- long battery life, good in-building penetration and ultra-low cost end-points.
When it comes to applications, i don't want to give too much away for our launch applications- but it's actually pretty straight forward to think of business services which would benefit hugely in terms of operational efficiency by being able to gather data from devices in the field- battery life and end-point cost is preventing these applications from taking off today with 2G/3G/4G solutions.
Once the network is out there, consumer devices will follow. WiFi may be a suitable solution when the consumer knows that they want a service from that device (e.g. to stream content to it), but it's difficult to get consumers to attach and maintain the connection to their devices when the service element that is generated is not so obvious.
Our plan is to open up these networks as soon as possible to third party application developers and use the creativity of the masses! Will keep the world informed as we progress this journey!
I kind of feel like saying, "Mark my words," and then revisit in the next decades.
TV spectrum may come in useful, for IoT, but only as a sub-optimal slice of supposedly available spectrum. I say sub-optimal because its propagation qualities are exactly what you do not want, to achieve the levels of frequency reuse required for connecting to a huge number of devices. Also sub-optimal in terms of antenna requirements. You can't fool mother nature (i.e. physics, in this case). Longer wavelengrths require larger antennas, for a given amount of gain (or even negative gain, as would most likely be the case here).
The other point is, what "things" are we talking about, anyway, that would presumably require this long range RF link "to the Internet"? I'd love to see a list of devices people have in mind, and then see why this long range RF link to the Internet makes sense. For instance, are we talking electronic billboards along rural roads, or are we talking your in-home thermostat and lawn sprinkler? Makes a huge difference.
Everything you read on this, instead, sounds vague and thoroughly unconvincing.
Blog Make a Frequency Plan Tom Burke 17 comments When designing a printed circuit board, you should develop a frequency plan, something that can be easily overlooked. A frequency plan should be one of your first steps ...