# How to indicate which side of the PtP (600/650 integrated) link is misaligned?

Is it possible tell which radio is misaligned in a point to point link by looking at the Tx/Rx power and modulation mode of each radio?

For the example below, I've been told that the side with higher TX capacity is properly aligned and the opposite radio should to be adjusted.

Example: PtP600 integrated

Side A Master - - Tx Mod 256 QAM - Rx Mod 64 QAM | Rx -70 dBm

Side B Slave - - Tx Mod 64 QAM - Rx Mod 256 QAM  | Rx -65 dBm

The explanation I receive is based on Side A is transmitting at maximum capacity because it's center beam is focused directly at the remote radio, thus properly aligned. The remote radio is not transmitting at max capacity because it's center beam is not pointing back directly back at the Side A radio, so Side B is misaligned.

However, I disagree with that logic and argues that Side A radio is misaligned since it's received power is less than Side B. Side A is transmitting at maximum capacity because Side B is aligned correctly and thus receiving the maximum power from A.

The purpose of this thread is only to get some clarification on the topic, rather than to solve a specific problem.
I understand there are other variables not included, but for the sake of alignment standpoint we'll simplify.

Hi,

It's actually quite difficult to find out which end of a PTP link is misaligned without realigning both ends.

Can you tell us:

• Is the link using the same RF channel in the two directions (Master to Slave, and Slave to Master)?
• Is this an unobstructed line of sight link?

If you have a LINKPlanner project, compare the Total Path Loss predicted for this link with the Link Loss reported by each of the ODUs.

Here's an example of the performance report in LINKPlanner. In this example, Total Path Loss is 138.24 dB.

Here's an example of the Link Loss (147.0 dB) reported for this link in PTP 650:

If the link is unobstructed and correctly aligned, the two figures should agree within 5 dB. In my example, the measured loss is about 9 dB worse than the prediction, so we suspect some problem with misalignment.

Mark

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Also, can you tell us what Maximum Transmitter Power is configured for these ODUs?

When the link is misaligned, the impact on link budget for both direction are the same, right?

That's right. When an antenna is misaligned it affects both directions equally. The measured receive signal strength depends on the transmitter power in the remote ODU, the antenna gain at the remote ODU including any gain reduction arising from misalignment, the free space path loss, and the antenna gain at the near end ODU including any gain reduction arising from misalignment. The reverse direction consists of the same gains and losses, but added up in a different order.

If the link is unobstructed and the transmit and receive frequencies are the same, it would be temping to assume that the 5 dB difference is simply the result of one ODU that is measuring a bit high and one that is measuring a bit low.

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Hi Mark,

Really appreciate the response. Just wondering if there is a way to empirically determine which end is misaligned based only on tx/rx stats and power levels.

Both radios using a different channel and configured max power is 27 dBm.

There are treelines in between the link. Not sure if this could affect one side more than the other.

Cheers

Hi,

If I understand correctly, you are thinking that the ODUs might not be aligned because the received signal level is different at the two ends.

The 5 dB difference might be caused by measurement errors in the ODUs. Accuracy is normally better than 5 dB, but it might be that one ODU is reporting too high and the other is reporting too low.

If the two directions are on different frequencies, and there are obstructions, it is possible that the excess loss cause by the obstruction is greater at one frequency than the other.

Another possibility is that the antenna gain is different at the two frequencies. This will be particularly significant if an antenna is misaligned, as the off-axis respons is likely to be more uneven than the on-axis response.

Finally, consider that the transmitter power depends on modulation mode when the ODU is configured with high transmitter power. The maximum power available is 27 dBm at BPSK, 26 dBm at QPSK, 25 dBm at 16QAM, 24 dBm at 64QAM and 23 dBm at 256QAM. For this reason, it is often more useful to look at the Link Loss value reported, which is compensated for the actual Tx power. In your case, 1 dB of the difference is explained by the different modulation mode.

I recommend doing:

• Temporarily operate the two directions on the same frequency

Mark

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Thanks for the clarification, Mark.

Never knew about that relationship between power level and modulation.

Hi,

The power back off is quite common in wireless products like PTP 600/650/670. The higher order constellations (for example 256QAM) require low distortion, which often means we have to minimise saturation of the power amplifier in the transmitter. If you configure power at less than the backed-off power (23 dBm in this case) then the transmitter power is constant over modulation mode changes.

If you look at the System Status page of the web-based interface, you should see that Maximum Transmit Power is reporting the 27 dBm you configured, while Transmit Power is reporting the actual power.

Mark

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