One of the reasons PTP 820 is so compelling is the way ODUs can be combined to create very high throughput link systems.
Two PTP 820S ODUs can be combined at each end of a link to create a 2+0 link.
Two PTP 820C ODUs can be combined at each end of a link to create a 2+2 link (a 2+0 link that’s protected).
Four PTP 820G RFU-Cs can be combined at each end of a link to create a 2x (1+1 HSB w/ SD) link.
When you “combine” ODUs with a mediation device (splitter, coupler, or OMT) or an antenna, be sure that the ODUs use the same sub-band.
Below is a hardware configuration for two 1+1 HSB w/ SD links that share antennas. One 1+1 HSB w/SD link uses the vertical ports, and the other link uses the horizontal ports.
The link was initially deployed with ODUs in different sub-bands.
Below we see what this looked like in the frequency domain.
The entire Lower 6 GHz band is drawn below, complete with Hi and Lo, Ch1w4 and Ch5w8 sub-bands, and the links’ assigned frequencies.
What’s the problem?
Well, it’s not easy to visualize, because you can’t see the transmitters and receivers separately.
To easily visualize any interference problems, split up the transmitters and receivers, and show what frequency they use at each end of each link.
Start with an example. Below is a Lower 6 GHz, 60 MHz channel, 1+0 link. The frequency pair is 5960.025 and 6212.065 MHz. The orange boxes represent the 60 MHz of transmit spectrum, and the blue boxes represent the 60 MHz of receive spectrum.
Next, let’s look at our deployed 2x (1+1 HSB w/ SD) link using a similar diagram.
The H’s and V’s are nicely separated by dual polarity antennas, which provide ~35 dB of cross polarization discrimination (XPD). Be careful, since that discrimination is between two receivers, one using the H port and one using the V port. XPD is not the same as port to port isolation!
Notice in the center of the band, at each end of the link, a transmitter shares the same antenna with a receiver, and there is very little frequency separation between the transmitter and receiver.
To test the isolation, mute one of the links, and observe the RSLs on all receivers.
Observe that when the V link is muted, Point A’s RSL on the V port (with the shared transmitter) is -77 dBm!
Observe that when the H link is muted, Point B’s RSL on the H port (with the shared transmitter) is -60 dBm!
How did this link perform?
In one word: insidious!
The link was installed and optimized according to specifications: all RSLs were within spec, and all MSEs were at or below -40 dB!
The link performed error-free for a day, but then it experienced outages from seconds to minutes throughout the day.
Then the link performed error-free for another day or two, and experienced outages from seconds to minutes throughout the day.
The process repeated.
What is going on here?
The receivers have sufficient power and signal to noise ratio (SNR) to maintain 2048QAM and high throughput. However, when the signal fades, the noise from the transmitter (on the shared antenna) does not fade.
This reduces the received signal's SNR, and depending upon how fast the fade occurs, the link reduces modulation mode or drops entirely!
When the fade goes away, the link starts up again.
The noise from the transmitter (on the shared antenna) reduces the link’s fade margin, which reduces the link’s availability by several nines.
Design and deploy the links so that the ODUs share the same sub-band. See the diagram below, and notice that there is sufficient frequency separation between all transmitters and receivers at each end of the link.
I hope this helps!
Dave
