Ether through the prism Or new Rocket AC AirPrism

Although the Ubiquiti equipment lineup is now several times wider than just a few years ago and includes switches , routers, and relay stations , the most popular and well-known product of ubiquiti is the AirMax and AirMax Ac equipment line. We have already considered Rocket AC - this is high-performance equipment, but, alas, when using even 40 MHz spectrum bands, traditional four-sector construction requires more than 200 MHz of pure ether for maximum modulation. It is very difficult to fulfill this condition, especially if the equipment is mounted on a common tower and there are interference from neighboring devices.
The reason for this is the insufficient steepness of Atheros chipset filters, which requires the use of guard intervals between sectors and leads to the defeat of the system outside of bandpass interference.
So let's take a closer look at why you need to use guard intervals in a multi-sector base station, and how to reduce broadcasting requirements, while maintaining bandwidth, thanks to the new model Rocket ac Prism (not to be confused with Rocket AC PTP and Rocket 5 AC PTMP) with AirPrism technology second generation and support of all relevant frequencies for us 5150-5875 MHz (fans of the range of 6-6.4 GHz can now re-read our material on the Nanostation M6 , however, we will soon have a new product for them).

Users of broadband radio have long been accustomed to the reconfiguration of the working frequency range of the equipment. By purchasing a device, for example, Rocket Ac lite, you can choose any working rating from the range of 5150-5875 MHz.
This is convenient, especially when it is not known in advance which frequencies the RFC will issue, or when it is necessary to organize, let's say, test equipment operation.
But there is a negative point in this possibility, namely, the insufficient steepness of the built-in filter of the system.
In the first approximation, to isolate the signal sectors from each other, they should be in non-intersecting channels. For example, in denominations of 5280 and 5300 MHz with bands of 20 MHz. However, everyone who tried such a construction in practice, certainly noted that capacity with such a construction suffers, there are repetitions, a decrease in the capacity of the sector.
The reason for this effect is the steepness of the Atheros chipset filters. In fact, working in the nominal 5280 with a band of 20 MHz, the device hears all other frequencies, just with a lower signal level, and the closer the frequency of interference to our nominal, the less it is attenuated.
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17. Bandwidth of the UHF receiver

Received Signal Classes	The bandwidth of the MHF receiver MHz level
	- 3 dB	- 30 dB	- ___ dB
one	2	3	four
20M0G7D	100	130	-
20M0D7D	100	130	-

Excerpt from the IDFS form - in fact, this means that when operating in the 20 MHz band, not only devices operating in our nominal will interfere with us, but also neighbors at 40 MHz in each direction (3 db. Of attenuation means almost nothing) .
The logical solution is to increase the guard interval to 40-80 MHz, however, spending frequencies on guard intervals is too wasteful outdoors, and the guard interval for complete suppression of out-of-band near noise should be significantly more than 5 MHz, as a result of which systems usually operate in neighboring frequencies interfering with each other.
Is there a solution to this situation?
Of course, yes, moreover, tested by relay stations.


The traditional relay station works on the principle of a frequency converter: a universal modem generates a relatively low-frequency signal (hundreds of MHz), and amplifiers specialized in frequency bands multiply the frequency to the desired nominal and amplify the signal.
That is, by changing external units, you can get different frequencies from one IDU and good isolation from out-of-band interference, since the IDU always works with a fixed nominal value and can have narrow-band filters with a high filtration coefficient.
Ubiquiti was able to place a similar system in a compact Rocket package (just a few centimeters longer than the Rocket Ac lite).

So consider two generations of AirPrism technology.

Generation	Models	MHz frequency range	Signal processing on the frequency converter	Operating frequency bands MHz	Max / Min power dbm modulation
The first	RocketAC ptp	5475-5875	Only reception	10,20,30,40,50,60,80	22/27
	Rocket AC pmp			10,20,30,40
The second	Rocket ac	5150-5875	Reception and transmission	10,20,30,40 in all modes, 50,60,80 only in the "point-to-point" mode	22/27

The differences are significant. In fact, without losing power or adding to the cost, we received filtering not only the receiving signal, but also the transmitting signal, as well as frequencies below 5,475 MHz, which were so lacking before. Not surprisingly, the early models of the Rocket AC are discontinued, and only the Rocket AC (with Prism 2) and the Rocket AC lite remain in the line.
According to the manufacturer, Prism technology allows to improve out-of-band filtering by 30 dB, respectively, the difference in mutual interference of the multi-sector base station on the Rocket AC and Rocket AC lite will be even greater.
Additional weakening of out-of-band interference allows not only to get more performance in current installations, but also to plan new installations without a guard interval and even install devices on a common three-sector antenna (of course, the devices do not work in one direction, but serve separate 30-degree sectors):
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Findings:
The new model Rocket Ac AirPrism got rid of the shortcomings of the first revision of devices, significantly expanding the frequency range and the range of applicability of the solution.
Filtration of both the received and transmitted signals should significantly reduce the interfering effects of the sectors on each other, as well as reduce the level of interference on neighboring devices.
That is, replacing sectors with new devices, you not only increase their productivity, but also improve the situation for neighboring equipment.
On the manufacturer's forum, you can find practical measurements of the influence of the presence of AirPrism technology on a channel under the interference conditions https://community.ubnt.com/t5/airMAX-Stories/Rocket-AC-ptmp-Ultimate-airPrism-Adjacent-Channel-Attack/ cns-p / 1188160 .
I will give only a short table with conclusions.
Stand layout: the channel between the rocket ac lite and the rocket ac ptmp (airprism 1) is exposed to interference from airFiber 5x with different guard intervals.
Measured modulation and useful bandwidth of the communication channel.

Frequency shift	AC lite Mbit performance	AC Ptmp performance mbps
No interference	92.4	137
Protective interval 2 MHz	104	134
Guard interval 3 MHz	91.4	139
Neighboring channels	34.7	122

Thanks to these tests, we see in practice the influence of out-of-band interference and the effect of its additional filtering.