Archive for January, 2009

Common Misconceptions #2: SkyPilot’s products are less applicable in EIRP-restricted regions like Europe

Tuesday, January 27th, 2009

Given a particular link and radio technology, the primary variable components of the link budget are:

  • 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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Common Misconceptions #1: The technology’s main benefit is long links

Monday, January 12th, 2009

A common misconception from the early days of SkyPilot is that our products’ high link budget is only applicable to long-range communications. At the time, we had customers spanning mountain tops across state boundaries, so it was understandable that users didn’t see how the equipment would apply to denser deployments like municipal Wi-Fi or smart metering networks. But the core technology of scheduling communications with high-gain directional antennas on both sides of the link has several benefits beyond long range, specifically in the areas of interference avoidance and capacity.

Regarding interference, transmitting with a directional antenna reduces the interference caused to other devices. Receiving with a directional antenna reduces the amount of interference received from other devices. Both of these compound and allow for many simultaneous devices in an extremely dense area such as municipal and utility smart grid networks. It is common to have two pairs of nodes communicating simultaneously just a few blocks apart.

Capacity is also increased since multiple communications can take place simultaneously within the same network. For instance, while a gateway is communicating to one first hop node, other first hop nodes in the same network can communicate to second hop nodes. In this way, the gateway is always active and overall system capacity is increased, which is important for very dense networks with many users in a small area.

And since both long range communications and dense node clustering are enabled by the same underlying technology, these characteristics can coexist in a single network. Many of our customers’ networks contain a wide range of node densities and it is common to see long range links connecting dense pockets of users in downtown areas or apartment complexes, along with individual paths to office buildings or power substations – all in a single network. Since our protocol dynamically schedules communications and controls the pointing of directional antennas, it applies equally well to many levels of network density.

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