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

by Ubiquiti Employee 2 weeks ago - last edited a week ago

 

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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.

 

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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)

 

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2. The filter effectiveness:

 

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

 

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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

 

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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

 

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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)

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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.

Comments
by
2 weeks ago

We believe the culmination of technologies driving 3-Domain Filtering (Frequency, Spatial, Time)

 

Or, that line could read ... 3-Domain Filtering (Spectral, Spatial, Temporal) .... and then trademark Ubiquiti's SST (tm) filtering technology.

 

 

by
2 weeks ago

I have an unimportant question. Why was it named "PrismStation"? Station seems to indicate it is a client device like the nanostation, isostation, etc. instead of an AP or base... station... Ah, maybe that was why?

 

PrismAP seems like a better name of the radio, with some other name for the add-on horns.

 

by
2 weeks ago

@flipper- I think the marketing team just fell over themselves to get that to the relevant agencies.

 

Great read as these articles are.

by
2 weeks ago

The "-station" name comes from the IsoStation name - and NanoStation/PowerStation legacy names - Prism filtering (yes I like the SST thing too ) is mostly applicable for APs located at noisy sites and IsoStation Prism doesn't sound nearly so good... 

Jim

by
2 weeks ago

I love this.  It's really a genius idea filtering that way, and obviously works very, very well.

by
2 weeks ago

IsoPrism AP? Man Happy

 

by
2 weeks ago - last edited 2 weeks ago

Really admire your writing skills, can explain technical stuff so well for everyone to understand. 

by
a week ago

Looking forward to trying them out!

by
a week ago

@flipper Love it..lol

Great product design!

by
a week ago

So, what about 2.4GHz?

 

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by
a week ago
by
a week ago

Simply put, a 2.4GHz horn would be approx. double the size of the 5GHz units, too big to be practical in most cases.   But you still get the other benefits of the Prism radios with the Rocket2AC Prism.

Jim

by
a week ago
Sometimes the old tricks are the best especially with a new twist. Hearing that you are using superheterodyne techniques to acheive this awesome result just makes me feel warm all over. Great work Robert!
by
a week ago

Ehi Bob, I'm with you!

by
a week ago
I believe intersil patented the prism filter system for TV signals originally (wifi isnt the only crowded RF space). Im not sure who owns intersil nowadays.
by
a week ago

Bought by Renesas in Febuary of this year, but still operating under their own name.   I looked seriously at using their Prism chipset and reference desogn to build our own 802.11b gear back in 1999.   Really glad I let folks like Robert do it instead ;-)

Jim

by
a week ago

Horn's are nice, but not what I need...

 

I've been able to run with 120deg shielded sectors, 8pol filters, 10MHz channels and three AP's on a tower with minimal self interference.

 

To improve on my current system...

 

I need 2.4GHz with selectivity like the old Intersil gear...

I need working GPS...

I need backwards compatibility with my M clients, both XM and XW...

 

If I can sub these in for my existing Rocket 2M AP's and get those benefits, I am ready to buy!

 

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by
a week ago

Customers have quite a few expectations with this new generation v2, Congratulations Robert for the new product innovations

by
a week ago

I tried 2.4 frequency. There were many problems. Did this product solve the jamming problem?

by
a week ago

Define 'jamming problem' ...

by
a week ago
Congratulations Robert, Tesla would be proud of you!
by
a week ago

Robert,

 

What's the difference in size between a horn and parabolic antenna of equivalent gain (say 30db)?

 

Thanks,

 

Joe

by
a week ago

Congratulations for discovering the SAW filters, other manufacturers have been using them for long time, but they don't call Airprism, just SAW filters.

 

Also if the GPS is finally working, 6 years late, let me know what to do with the Rocket M5+GPS I have installed in towers still waiting for a reliable firmware.

 

 

by
a week ago

What is SST filtering?

by
a week ago

The superheterodyne receiver was invented almost 100 years ago so hardly a new idea.
Are you using the filter on transmit (to reduce OOBE) as well?

by Ubiquiti Employee
a week ago

@roanwifi

 

@david_lewis

 

Ha...I will expand a bit for you skeptics:

 

 

-Integrating a local oscillator that must "talk" to the WiFi radio to track carrier changes (both center frequency and channel width) and TX/RX changes. To do this over the complete 5GHz band with low phase noise and down conversion design that minimize intermod spurs is not a trivial task

 

-Yes, we use SAW filters...actually multiple SAW filters for different channel widths...5MHz, 10MHz, 20MHz, etc.  So our "selectivity" actually becomes even greater as you reduce channel width.  There are no SAW filters that operate effecitvley at 5GHz.  Downconverting to an IF enables the use of SAW filtering and we then upconvert  to the original carrier whic feeds into the WiFi Radio

 

-This is our 3rd generation of our Prism tech which has improved significantly over the years

 

 

-

 

by
a week ago - last edited a week ago

The PrismStation, Rocket Prism AC Gen2, and Rocket 5AC Prism have filtering on both TX and RX.

 

SST was @flipper's rework of Frequency, Space, and Time into Spectrum, Spatial, Temporal (first comment above).

by
a week ago

this all sounds great but how many more years to we have to wait before these products are available at in the retail supply chain?

by
a week ago

Not a sceptic, just amazed that direct conversion (cheap and you get what you paid for) is still used for “professional” equipment.

 

Are you using a VCO/PLL or DDS for the 5GHz local oscillator? Guess image rejection is probably an issue at 5GHz with an 384MHz IF?

 

Surprised there isn’t a chip set designed from the ground up as a superhet as down converting and then up converting will add noise.

by
a week ago

As to availability, I'm installing them today...

Jim

by
a week ago

As david_lewis said, downconverting, then upconverting to feed the wifi chips  degrades IP3

 

 

 

by
a week ago

OFDM signal quality boils down to SNR which is degraded by;

  • Adjacent channel noise that is not filtered out
  • Noise in the frontend
  • Noise in the mixer
  • Noise from non-linearity (IP3)
  • Noise from the local oscillator(s)
  • Noise from the image

The issue to bear in mind is that a 6db improvement in SNR should double the range for the same speed.