How Aruba Optimizes Performance of Dual 5 GHz APs
How Aruba Optimizes Performance of Dual 5 GHz APs
“Know the rules well, so you can break them effectively.”
—Dalai Lama XIV
Surely, the Dalai Lama didn’t have wireless access point engineering in mind when he said that, but the principle still applies. There’s been a lot of noise in the industry about delivering an AP that has two 5 GHz radios in the same chassis, and from reading those opinions, it would be easy to conclude that designing a great dual 5 GHz AP is a simple matter of slapping two radios into a box and moving on.
While this approach might seem intuitive, it could not be more incorrect for two reasons:
- It is important to understand the mechanics and physics of that design decision.
- It is important to understand the impact on the wider system since access points usually do not exist in a vacuum.
A radio generates energy over a wide frequency spectrum. It can be broken into three pieces:
- Desired modulation: The actual useful signal that conveys information between the AP and the end user. It has a well-defined channel width and energy level.
- Spectral regrowth: Created by the reality that APs are constrained by how much power they can consume, their physical size and operating temperature.
- A completely linear radio is not achievable in practice, as it would consume 10x to 15x the power that a typical AP uses.
- The transmit power amplifiers in a completely linear device would be very large, very power hungry and run very hot.
- The 802.11 standard and other radio specs account for spectral regrowth by defining the error vector magnitude (EVM), which is a representation of the amount of distortion that an AP is allowed to generate.
- Higher-order modulations (256 QAM, 1024 QAM) need more linearity so they tend to be operated at lower power.
- The regrowth meaningfully occurs over a channel span that is seven times wider than the actual signal. So for example, a 20 MHz channel will have substantive regrowth over a span of 140 MHz.
- Wideband noise: Largely due to the fact that ideal signals in the radio chipset have to be converted to analog signals by imperfect means.
- The digital-to-analog (DAC) conversion process has a limited number of bits available to achieve this conversion.
- The result is quantization noise or energy that is the error between the digital and analog representations.
- It is not large in magnitude, typically 60 dB less than the desired signal, but it is VERY wide in frequency. It’s effectively DC to daylight.
Figure 1 shows the desired signal in red, the spectral regrowth in blue and the wideband noise in green.
The authors of the 802.11 standards included all of these real radio aspects when the requirements were drafted.
Now consider that a radio transmitting 20 dBm of conducted power will have wideband noise at -40 dBm. So even if you have your first radio on channel 165, the wideband noise is present on channel 36 at -40 dBm. While the radio on channel 165 is transmitting, the radio on channel 36 is trying to receive. How does the radio on channel 36, with a noise floor of -100 dBm, receive a signal in the presence of -40 dBm emissions?
The simple answer: It doesn’t. You would need at LEAST 60 dB of isolation to keep the receiver impact below 3 dB.
Now that we know the RF challenges, we set out to deliver an access point in the Aruba 340 Series that can truly allow customers to boost the 802.11ac performance by increasing 5GHz utilization without introducing interference that is inherent in other vendors’ poorly designed dual-5GHz APs.
So, how could we make this work?
- We can use separate antennas. Using separate antennas provides about 25 dBm of isolation and gets the noise down to -65 dBm. Still not enough.
- We can reduce the power on one of the radios. While this helps to keep the low power radio from interfering with the high power radio, it does nothing in the opposite direction. So that approach does not work either.
- We can try polarization, similar to what you see in the solution from one of the dual 5 GHz vendors. Aruba measured the other vendor’s solution and was only able to show 35 dB of isolation. Still not enough.
- The reality is that to get the wideband noise down to a non-impacting level, we need to dramatically increase the distance between the antennas on the two radios, which is not really practical in a single AP chassis. Or we can physically filter the signal between the two radios.
Until the release of the Aruba AP-340 Series, no vendor had introduced any filtering between the two radios when both are operating in the 5 GHz range.
It turns out that the gap between the 5.15 GHz to 5.35 GHz and 5.47 GHz to 5.850 GHz ranges is just enough to get a bandpass filter on the upper and lower halves and divide the spectrum into two chunks.
802.11ac Channel Allocation (North America)
Using a bandpass filter provides a number of desirable product attributes:
- Nothing unnatural needs to be done with the antenna subsystem. As a result, both radios provide EXACTLY the same EIRP and coverage area.
- A true doubling of capacity. Two users at the same distance from the AP-344 or AP-345 will have the same signal strength and throughput on either of the two radios.
- The two radios operate independently without impairing the operation of the other and are completely asynchronous.
- An intelligently designed system can load balance the two radios to maximize throughput for a given group of users.
Now, what about the system level? As Aruba has stated before, a network cannot simply use two 5 GHz radios on every AP.
- Frequency planning is still a factor in a system design.
- Doubling the number of radios with a given channel set will decrease the distance between reuses by about a factor of two.
- This can subsequently increase the number of “visible” radios on the same channel by a factor of four. Those radios will then share the channel and can result in a net reduction in system capacity.
- Dropping 2.4 GHz entirely, which is three channels of useful capacity, is not really a desirable outcome. 2.4 GHz should be managed aggressively and exploited as much as possible.
- When a radio is established as better allocated to 5 GHz, it should be aware of the larger system impact.
- The system needs to have the ability to manage user connections.
- Load balancing across the available radios to maximize the utility of all frequency bands.
- Moving users between 5 GHz and 2.4 GHz.
- Be aware of the client capabilities and manage those as part of finding optimal network solutions.
Aruba provides these capabilities in the AirMatch feature suite to rightsize the number of radios switched to 5 GHz operation. AirMatch intelligently manages radios on 2.4 GHz, 5 GHz and those that are placed in monitoring or test functions to ensure that changes are to the net benefit of all users on the system.
Aruba also provides Client Match to ensure that the radio resources deployed are matched to the user demands. Since Aruba APs that are in dual 5 GHz mode have matched coverage and throughput capabilities, Aruba can provide a superior result compared to the mismatched coverage of a competitive solution, and not have to deal with the RF impairments incurred by the other competitive dual 5 GHz solutions.
Aruba delivers all of this capability on a platform that is cost-effective. The AP-345 is at a $1,395 US list reference price point, supports multi-gigabit wired interfaces and leverages the best-in-class ArubaOS operating system to maximize end-user value and function.
Read the AP 340 Series data sheet.
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