Most operators find the 5.2GHz and 5.4GHz bands among the most difficult to operate in for Point to Multipoint. Even with 300MHz of spectrum available many operators will avoid these bands. The problem is the limited EIRP allowed by the FCC for both Downlink and Uplink. Often times it reduces the distance/range to an untenable level. However we believe this offers an opportunity for users with the knowledge of how to properly operate in these bands.
Problem:
To better understand the limitations of the 5.2/5.4 bands let’s take a quick comparison of these bands with the more commonly used 5.7GHz band.
In 5.7 GHz the FCC has different rules for both “PMP” and “PTP” radios. Interestingly in a PMP network the downlink (AP -> SM) is considered to be a “PMP” radio as the AP is communicating to multiple radios and falls into PMP rules while uplink (SM -> AP) is considered to be a “PTP” radio because the SM only ever communicates directly with a single radio (its associated AP).
FCC 5.7GHz rules can be roughly generalized*** as follows:
PMP: 36dBm Max EIRP (TxPower + Antenna Gain <= 36dBm)
PTP: Unlimited Max EIRP
- Max Tx Power = 30dBm
For the downlink 36dBm EIRP is equivalent to 4 Watts. With a PMP450b high gain the achievable EIRP is 44dBm (~25 Watts).
This extra power on the uplink allows us to counteract the noise/interference that APs typically see on the uplink. Typically APs are mounted higher and have antennas with wider beamwidths which make them more sensitive to noise than the SMs that typically sit lower (sometimes down in tree growth) and have directional/higher gain antennas.
When we look at the link budget for the 5.7 band what we typically find is that the band is downlink limited. We run out of range/distance on the downlink far before we run out of range/distance on the uplink. (Note: This does depend on the noise floor seen at the AP vs the SM and on the antenna selection for both, but most deployments in the 5.7 band will be downlink limited.)
In 5.2/5.4 GHz bands however the FCC rules change significantly. Both PMP and PTP radios must follow the same rules. Essentially both are limited to 30dBm of EIRP. This represents a 6dB decrease in downlink power (~half the distance/range). Uplink however is considerably worse. For example an Integrated PMP450i SM (23dBi) will lose 24dB of EIRP moving from the 5.7 to the 5.4 band. A PMP450b High Gain (23dBi) will lose 15 dB of EIRP. At this point most deployments become uplink range limited because of any noise that may be present on the channel. Additionally most operators use higher gain antennas on the SM compared to the AP. This makes the deficiencies with the uplink worse as the stronger gain SM antenna increases the downlink range but does nothing for the uplink.
Here is a quick range comparison using a 450i(AP) and a PMP450b High Gain(SM) in both the 5.7 and the 5.4 bands. Everything has remained the same except the band. They both consider 20MHz channel bandwidth with a clean channel, an 18dBi AP antenna, and a 23dBi SM antenna.
Band: 5.7 GHz (PMP450i)
|
Modulation |
DL Range(mi) |
UL Range(mi) |
|
8X MIMO-B |
2.6 |
3.4 |
|
6X MIMO-B |
6.2 |
8.3 |
|
4X MIMO-B |
12.1 |
16.3 |
|
2X MIMO-B |
27.0 |
32.8 |
|
1X MIMO-A |
44.8* |
59.1* |
Note: Downlink range limited. Uplink propagates farther than downlink.
*Theoretical-Max range is limited to 40 miles in the radio.
Band: 5.4 GHz (PMP450i)
|
Modulation |
DL Range(mi) |
UL Range(mi) |
|
8X MIMO-B |
1.1 |
0.7 |
|
6X MIMO-B |
2.9 |
1.6 |
|
4X MIMO-B |
5.9 |
3.5 |
|
2X MIMO-B |
11.7 |
6.8 |
|
1X MIMO-A |
19.7 |
12.6 |
Note: Uplink range limited. Downlink propagates farther than uplink.
The key thing to keep in mind in this example is that both downlink and uplink are required to maintain a connection. Typically when operators field test the 5.2/5.4 band they find that they cannot connect past a certain distance. It’s important to note that this distance limitation may only be uplink related. Downlink may have sufficient signal, but the RF link requires a connection in both directions to be established and maintained. Also notice that the range for the 5.4 on the downlink is not especially abysmal. It’s possible to achieve 2X modulation (QPSK MIMO-B) out to nearly 12 miles. It’s the uplink that limits the overall range of 2X modulation to just under 7 miles.
To make matters worse there is very little that can be done to overcome this problem. Increasing the antenna gain at the SM does not increase uplink link budget because the 30dBm EIRP cap requires the operator to reduce the power by 1dB for every 1dB of additional antenna gain. Increasing the gain of the AP antenna helps this equation, but it is very difficult to find high gain sector antennas that provide more than a couple dB of extra gain while maintaining the beamwidth that is desirable for a sector antenna.
Solution:
The hallmark of the PMP450m is the Massive MU-MIMO. It is a 14x14 radio that offers very fine beamsteering to allow the AP to communicate to up to 7 SMs at once. One of the additional benefits of this design is that it can (and does) massive beamsteering on the uplink with all 14 receivers. (7 on each polarity) This allows for an ~8dB increase in RxSensitivity and therefore link budget on the uplink. This extra 8dB dramatically changes the range calculations shown above.
Here they are again comparing a PMP450i with a PMP450m (with massive uplink beamsteering).
(Again this is considering a clean 20MHz channel with a 23dBi gain SM.)
Band: 5.4 GHz (PMP450i)
|
Modulation |
DL Range(mi) |
UL Range(mi) |
|
8X MIMO-B |
1.1 |
0.7 |
|
6X MIMO-B |
2.9 |
1.6 |
|
4X MIMO-B |
5.9 |
3.5 |
|
2X MIMO-B |
11.7 |
6.8 |
|
1X MIMO-A |
19.7 |
12.6 |
Band: 5.4 GHz (PMP450m)
|
Modulation |
DL Range(mi) |
UL Range(mi) |
|
8X MIMO-B |
1.2 |
1.6 |
|
6X MIMO-B |
3.3 |
3.5 |
|
4X MIMO-B |
6.6 |
7.3 |
|
2X MIMO-B |
13.2 |
14.5 |
|
1X MIMO-A |
22.1 |
25.2 |
Note: The slight difference in DL Range is caused by the FCC EIRP limit for each radio. PMP450i is FCC limited to 28dBm EIRP for the AP while PMP450m is FCC limited to 30dBm EIRP.
With the massive uplink beamsteering of PMP450m we are able to nearly double our distance that we see from the PMP450i. The ranges are still not what we see in the 5.7 bands, but they are not unreasonable.
Also for this comparison we considered clean channels for both the 5.7 and the 5.4 bands. Many times operators will find the 5.2/5.4 band to be cleaner than the 5.7 which helps to close this gap.
Additionally operators can purchase the PMP450m Limited with a significant cost savings which has all of the same features(except MU-MIMO) as the PMP450m Full. Massive uplink beamsteering is still included in the PMP450m Limited. And the Limited version has a 30 day start/stop trial of MU-MIMO and can be upgraded to a Full (with MU-MIMO) at any point in the future with the purchase of a license key.
***There are other FCC requirements as well but these largely fall on the manufacturer and limit the amount of out of band emissions the radios are allowed to produce. (The limits essentially define how “clean” the radio must be.)