SkyPilot Blog » omnidirectional http://www.skypilot.com/blog This blog focuses on broadband wireless principally centered around a technical perspective from SkyPilot's engineering team. Wed, 25 Mar 2009 22:26:38 +0000 http://wordpress.org/?v=2.8.4 en hourly 1 Mesh Capacity (Part 1) http://www.skypilot.com/blog/system/mesh-capacity-part-1/ http://www.skypilot.com/blog/system/mesh-capacity-part-1/#comments Fri, 06 Feb 2009 02:28:42 +0000 Randy Frei http://www.skypilot.com/blog/?p=21 There has been an ongoing discussion in the mesh community about how much capacity is lost due to the relaying of data within a wireless mesh network. Proponents of multi-radio architectures have argued that they can deliver close to 1/n (where n is the number of hops) of the capacity of a radio simultaneously to each mesh device, while single radio architectures are closer to 1/2^n. For instance, a 4-hop path in a multi-radio system (assuming several clean channels are available) could deliver on the order of 1/4 the capacity of a radio simultaneously to all mesh devices, while a single-radio system may only be able to deliver 1/2^4, or 1/16, the capacity of a radio, due to multi-hop interference.

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).

Single Radio Mesh

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.

Dynamically Switched Directional Antennas

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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