Filtering in 3 Domains: Why PrismStation is the Greatest Product Ever Brought to the WISP Industry.


Back in the early days, I was working with a customer to replace an older, slower 802.11b 2.4GHz mini-PCI radio with Ubiquiti’s latest SuperRange2 802.11g hi-power mini-PCI module.  Response to the SuperRange mini-PCI cards had been overwhelmingly positive, but this user was experiencing significantly worse performance post-upgrade.  At first I thought it was bad hardware and shipped another card, but the results did not change.  I then worked with him, trying to improve performance through several attempts at design modifications but also to no avail.

At that point, I bought a sample of the older 802.11b card at issue for myself, which was based on the Prism 2.5 chipset from Intersil.  And like the “Prism” name implies, I quickly saw that this older card although slower in max speed, had a radio design with a superior “selectivity” — the ability to filter out neighboring channels.

But, how could this be the case? The Super Range (Atheros 802.11a/b/g based) radios were the latest technology and I assumed it would outperform the older 802.11b technology in all areas. After taking apart the Intersil Prism radio, things became clearer.

The Intersil radio was based on a true “superheterodyne” receiver architecture where the carrier was down-converted to an Intermediate Frequency (IF) and filtered with a dedicated discrete filter. Meanwhile, the Atheros radio, was a completely integrated CMOS chipset without any off-chip IF filtering.


So what does this all mean? The ability to filter a radio signal largely depends on 2 things:


1. The fractional bandwidth:

Filtering out a 1MHz channel at 1GHz (.1% fractional bandwidth) is much harder than filtering out a 10MHz channel at 100MHz (10% fractional bandwidth)


2. The filter effectiveness:

A dedicated specialized filter is typically far superior than a filter integrated into an IC.


In the case of the Prism radio, it optimized both areas. By having a down-converted IF of 384MHz, it was able increase the filtering fractional bandwidth. And with a dedicated off-chip SAW (Surface Acoustic Wave) filter, it had a much more effective filter. In comparison, the Atheros radio was built for complete low-cost IC Integration and had neither. It might have performed well indoors, but in outdoor WISP applications, the Prism radio could survive in RF environments where the Atheros based radio had no chance.



Filtering in the Frequency Domain

This experience would plant the seed for what we ironically call our “Prism Technology” at Ubiquiti. We wanted a way to leverage the speeds of the latest WiFi chipset technology but also retain the great “selectivity” of the original Intersil Prism radios.

What we patented would be counterintuitive to most. We essentially put our own radio in front of the WiFi chipset radio. How does this exactly improve performance? The diagram below helps explains the concept

Our Prism technology receives the unlicensed band spectrum (5GHz or 2.4GHz), down converts to an intermediate frequency, applies specialized hi-selectivity filtering on the area of interest, and up converts the channel back to appear magically “clean” to the WiFi Radio.

We have proven selectivity improvements of up to 30dB. To put this in a linear perspective, our Prism Technology reduces noise seen by the Wifi radio by up to 1,000 times!



Filtering in the Spatial Domain

Horn antenna technology has been around for over a century, but only recently have they been attractive and proven to be successful in WISP applications. In the early days of this industry, antenna gain was most valued and traditional sectors and reflectors were best suited for deployments. Fast-forward to today, with billions of unlicensed radios in use worldwide, the ability of antenna isolation to mitigate noise is becoming more valuable. We want our antennas to only hear and talk in a single direction and “ignore” all other directions. Horn antennas do this exceptionally well.

The challenge for our antenna team at Ubiquiti was how do we take advantage of the RF isolation advantages of horns, but still maintain enough antenna gain required for high performance links?

The answer is what we call “asymmetrical horns” and we believe they are the future of WISP deployments



Filtering in the Time Domain

The 802.11 WiFi standard uses something called CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance). It is a contention based protocol which means if all clients on a network can “hear” each other, everything can work well. But, in the case of outdoor networks and isolated directional links, most of the clients cannot hear each other and end up talking over one another. To solve this we introduced a TDMA (Time Division Multiple Access) protocol where clients are assigned organized time windows to talk so they do not interfere with each other.

This was the essence of our AirMax TDMA protocol we have improved throughout the years. While this works well for clients connected to a single AP, what about interference issues with multiple APs co-located together?

Our all-new GPS synchronization protocol specifically addresses this challenge. PrismStation uses GPS to provide a global synchronization timer for potentially every deployment in the world. What this means is multiple BaseStations can work seamlessly on a single tower or neighboring towers and even using the same spectrum. And we can also achieve synchronization between AirMax and AirFiber Basestations (including our upcoming LTU technology)


We believe the culmination of technologies driving 3-Domain Filtering (Frequency, Spatial, Time) will enable the next stage of high-performance, high-density AirMax networks throughout the world.  We are really proud to bring this product to market and hope it will be the weapon that operators can use to fight and build higher performance networks even in the presence of increasing amounts of RF noise in the unlicensed bands.

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