Hi Lo violation


what is the Hi Lo violation? and does LinkPlanner taking it in to consideration while planning a network of back to back links?



Our 5 GHz unlicensed Point To Point (PTP) radios use one frequency for transmit and receive. They are Time Division Duplex. When the near end transmits (on one frequency), the far end receives (on one frequency). After a time, the roles are reversed: the near end receives, whilst the far end transmits. The frequency is shared between the near and far ends. The near and far ends use two different times to transmit and receive.


Our licensed Point To Point (PTP) radios use two different frequencies for transmit and receive. The near end uses one frequency to transmit, and the far end uses the other frequency to transmit. This allows each end to transmit (and receive) simultaneously!

The two frequencies are separated by a transmit/receive spacing. For example, in the United States, Lower 6 GHz radios are separated by 252.04 MHz, 11 GHz can be separated by 5000 MHz, etc.

Hi and Lo

One of the frequencies is greater than the other: call this frequency, “Hi”.

One of the frequencies is less than the other; call this frequency, “Lo”.

For example, at 11 GHz, a frequency pair might be 11,000 MHz and 11,500 MHz.

11,000 MHz is the Lo frequency.

11,500 MHz is the Hi frequency.

A radio that transmits at 11,000 MHz is the transmit Lo radio, and a radio that transmits at 11,500 MHz is the transmit Hi radio.

Note that a transmit Lo radio receives at the Hi frequency, and a transmit Hi radio receives at the Lo frequency.

(In our example, the transmit Lo radio transmits at 11,000 MHz and receives at 11,500 MHz, and the transmit Hi radio transmits at 11,500 MHz and receives at 11,000 MHz.)

Hi Lo Violation

A “Hi Lo violation” happens when a hub site (with two or more links) has a mixture of Hi and Lo radios.

To see why this might be a problem, put a Hi radio and a Lo radio from two different links on the same tower.

The problems start when both of these links use the same frequency pair.

In our example, the Lo radio transmits at 11,000 MHz, and the Hi radio receives at 11,000 MHz. (Remember, the Hi radio transmits at 11,500, and receives at 11,000 MHz).

At our tower, the Lo radio transmits right into the Hi radio’s receiver at the same frequency!

The Hi radio transmits at 11,500 MHz, and the Lo radio receives at 11,500 MHz.

At our tower, both Hi and Lo radios are interfering with each other in each direction!

Why is this a problem?

Why is this a problem, since the Lo and Hi radios don’t use the same antenna? Can’t you get some isolation from antenna separation, front-to-back ratio, etc.?

If the antennas are pointed back to back (and mounted at the same height), you can use the antenna’s gain and front-to-back ratio to see how much the Lo radio interferes with Hi radio. Let's say each of the antennas has a 30 dBi gain and 60 dB front-to-back ratio. In this case, the Hi receiver sees a signal that’s 60 dB less than the Lo’s transmit power. If Lo transmits at 20 dBm, Hi receives the signal at -40 dBm! That might be higher than the signal that the Hi radio expects to receive from its far end!

So what can you do to avoid this problem?

Physical, Angular, and/or Frequency Separation

You can use physical separation, angular separation, and/or frequency separation.

Physical separation might not be sufficient, but mounting the antennas 50 ft apart from each other might not provide enough isolation.

Angular separation might not be sufficient. In our example, the combined front-to-back ratios only provided 60 dB isolation. Rarely will you have a choice over the amount of angular separation that's available, since angular separation is dictated by the physical network topology and where the sites are located.

Frequency separation is often the best answer: either choose to mount all Hi radios together and all Lo radios together, or use two different frequency bands where Hi Lo violations occur.

For example, if you’re designing a ring network, try to design the network with an even number of links. Assign one node to be Hi and work your way around the ring Lo, Hi, Lo, etc. You’ll find that all Hi radios are together at a given node, and all Lo radios are together at a given node.

If you’re designing a star network, put all the Hi radios in the center of the star, and put all the Lo radios at the outer nodes of the star.

Sometimes you don’t have a choice. The ring network has an odd number of links, and there isn’t enough spectrum to use a different band. When this is the case, choose the node with the greatest angular separation between links for the Hi Lo violation, and estimate the power that each radio will interfere with. If possible, physically separate the antennas from each other. With a careful analysis and planning, sometimes a Hi Lo violation can be overcome. (This is rare, which is why, in general, you should try not to mix Hi and Lo radios at the same location.)

Please let us know if this doesn’t answer your question!

If you have more questions, please ask!


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P.S. LINKPlanner does not prevent, identify, or resolve Hi Lo violations.

The end user is responsible for preventing, identifying, and resolving Hi Lo violations.

The better frequency coordinators will identify Hi Lo violations, but the end user should not always rely on their frequency coordinator for this important information.

I hope this helps!


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