This diagram shows how a traditional single radio mesh system has its bandwidth reduced due to a large interference domain allowing only a single device to transmit at a time (note: the circles show the communication range, while the interference range will usually have a radius many times larger).

A multi-radio system could use several frequencies to allow multiple transmissions to take place at the same, reducing some of these interference conditions (however, not only does this require multiple clean channels, but there are some pitfalls that will be analyzed in a future post).

So an obvious question is, “How does SkyPilot’s dynamic antenna switching affect system capacity?” The answer is that even though the SkyPilot system uses a single backhaul radio, it can still provide 1/n the channel capacity simultaneously to each device due to the dynamic antenna switching.

In addition to all of the previously discussed benefits of dynamic antenna switching, such as higher link budget, interference avoidance and point-to-point power levels, the largest benefit is probably from something called “spectral re-use”. Basically, spectral re-use is a benefit of using dynamically switched high-gain antennas where multiple transmissions can take place simultaneously, on the same frequency, in very close proximity.

For example, the dynamic point-to-point link formed by the high-gain antennas allows a first-hop transmission to not interfere with a third-hop reception, even on the same channel. And while one first-hop device is relaying, spectral re-use allows many other devices to simultaneously communicate, such as allowing the gateway to transmit to another first-hop device. That is why we always recommend at least 2 first-hop devices. This allows the gateway, and most other devices within the mesh, to be continuously active, so the capacity of the overall system is equal to the capacity of the gateway radio.  This allows at least 1/n to be delivered to each device simultaneously, equivalent to the multi-radio mesh system and much higher than traditional single radio systems.

And by only consuming a single channel, additional channels can be employed in order to multiply overall system capacity (plus, it is often difficult to find the multiple clean channels that multi-radio architectures require). But, the use of multiple radios in context of traditional mesh networks and the SkyPilot system will be explored in a future post.

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• The radio’s transmit power
• The transmit antenna gain
• The receive antenna gain

For this discussion, we’ll ignore other common link budget parameters, such as cable and path losses, since we are looking at power and antenna gain on a particular link.

Most regions restrict a device’s EIRP (Effective Isotropic Radiated Power), which is essentially the radio’s transmit power plus the transmit antenna gain.  To comply with EIRP limits, each device must either reduce its radio’s output power, reduce its transmit antenna gain, or both.

Since SkyPilot’s products use high gain antennas, the radio’s output power must be reduced to comply with EIRP limitations.  Due to this, there is a common misconception that SkyPilot’s high-gain antennas are not beneficial in EIRP-restricted regions.

However, since the SkyPilot system uses high-gain directional antennas on both transmit and receive, the link budget is still increased due to the antenna gain.  Let’s look at an example by comparing two identical links, with different antennas, in a region restricted to 30 dBm EIRP:

• Link 1: radio output power of 24 dBm + transmit antenna gain of 6 dBi + receive antenna gain of 6 dBi = 30 dBm EIRP and comparable link budget of 36 dBm
• Link 2: radio output power of 12 dBm + transmit antenna gain of 18 dBi + receive antenna gain of 18 dBi = 30 dBm EIRP and comparable link budget of 48 dBm

So, even though the use of a high-gain antenna on link 2 resulted in the radio’s output power needing to be turned down by 12 dB in order to meet the 30 dBm EIRP limit, the actual link budget is 12 dB higher than link 1 due to the receive antenna gain.  In free-space, this would result in 4 times the range (or an increase in modulation, depending on how you want to spend the link budget).

To put it more simply, EIRP restrictions limit how big your mouth is, but not how big your ear is.  And of course there are still the other benefits of using directional antennas, such as causing less interference and being less susceptible to interference.

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