I am seeking guidance on the best design practices to avoid/reduce interference in a microwave network consisting of multiple PTP and PMP links.
The network intends to use PTP 670 with 45MHz channel bandwidth for the point to point links and 450i PMP with 40MHz bandwidth.
Point to point links: 2 + 0 configuration by using two 670 and external switch. Am I correct to design each link on a different band (for example link A on 4.7Ghz and link B on 4.9GHz) or will the DSO functionality be sufficient to avoid interference? I am using a channel bandwidth of 45MHz, if my math is correct one band will only have 2 non-overlapping channels.
Where there are multiple PTP links from one tower into different directions, what is the minimum separation angle required between two PTP links so that both links can use the same channel without interference?
At one location, the PTP overlaps with the PMP sector (450i), again here I assume different bands are required to avoid interference.
I look forward to your advice and thoughts on this.
That's an interesting point. The TDD frame is generally extended somewhat in TDD synchronization, and this results in a small reduction of capacity. However, without TDD sync you will need very careful antenna placement, and a wide antenna separation. Even so, interference between collocated links is likely to limit use of higher efficiency modulation modes. In most cases, loss of the higher efficiency modulation modes is a bigger factor than the extended TDD frame duration, so TDD sync actually improves overall capacity.
Thanks for your insights. Does the degraded performance only apply to a scenario where the same frequency is re-used? If the links are on different frequency bands, would you not expect the system without TDD synchronisation to achieve higher throughput?
If the two links are not synchronized, we need to consider isolation between the transmitter in one link and the receiver in the neighbour link. This isolation must be sufficient to avoid saturating the receiver in the "victim" link.
The largest interferer we can tolerate in the victim receiver before degrading sensitivity is about -40 dBm. The isolation between two vertically-separated antennas 1 m apart on the same tower is not reliably greater than 45 dB, limiting the transmitter power in the interfering (neighbour) link to 5 dBm. That 45 dB isolation can be improved either by using high performance shielded antennas or by increasing the separation of antennas on the tower. You could experiment with this by installing your 2+0 links unsynchronized (but on different channels) and checking the level of the interference in the Spectrum Expert display. Note the peak interference level, and aim to get this below -40 dBm. Note that the range of the display is only to -40, so a display of -40 might indicate a larger signal.
If you can achieve the required isolation, then you need to ensure enough frequency separation between the two links. A separation of three channels provides about 60 dB of rejection, and that should be sufficient.
Please let us know how this goes if you decide to experiment.
Of course, if you choose to synchronize then isolation is larely unimportant.
At the moment I am at the planning/design phase working with Linkplanner to determinet he best setup.
I've been playing around with the TDD sync option to get to an acceptable link throughput. How will that impact the freqnecy planning? I'm still looking at using different bands for the PTP 670's at the same mast.
As we said in earlier messages, if we don't use TDD synchronization, we need to make sure that we have enough isolation to keep the interference from the neighbour link below -40 dBm to avoid desensitizing the receiver. That part of the solution is not needed if we use TDD synchronization correctly. LINKPlanner doesn't help you to estimate this isolation; we need to consider the antenna pattern at 90 degrees, the separation on the mast, and the transmitter power. Some experimentation might be the way forward.
If we don't use TDD synchronization, and we achieve the -40 dBm target, we still need about three bandwidths between channels to ensure that the interference doesn't appear in the receiver passband. This is because we need to ensure that the large interferer doesn't adversely affect the wanted signal.
If we synchronize, we need less isolation between channels, because we're only concerned with the interference received from the remote end of the link. That's normally a similar level to the wanted signal. In this case, two channels separation is enough.
There's generally nothing special about using channels in different frequency bands. Of course, this helps to put some frequency difference between interferer and wanted signal, which is needed as described above, but PTP 670 does not have dedicated RF filters for each of the frequency bands. The only band with a dedicated filter is 5.8 GHz.