microwave link
We are encounter a problem in 15Ghz link near to military base and it seams that the problem comes from an interferance from the military radar, we solve the problem by change the frequency from 15Ghz to 7 Ghz, my question is who can i solve this problem without change the frequency ?
note : this mircowave link was established to connect 2 bast trancever station for mobile communication
If very directive antennas do not help to reject the interferers, and if the interferers are in-band, probably the only option is to change the frequency. Sometimes proper space diversity between antennas it helps reducing the interferer, but probably you already tried this.
Also you can install for a while a jammer on that frequency, hoping that the military guys will change the frequency?just kidding
ok thank for your reply but do you think that we can use the diversity solution for microwave frequinces? i dont think so, i think it can be used in lower frequinces, that is used in GSM anttenas not in microwave links
RADAR is a low duty cycle signal but with high peak intensity.
If it is overloading your receivers, changing bands as you did is the only solution.
If it is just increasing your bit error rate, changing to another error correcting method, such as requesting retransmissions, should help.
Space diversity can be used successfully at microwave frequencies.
A combination of all these methods: space diversity, frequency diversity, antenna isolation (with good front-to-back ratio and side lobe suppression), different FEC, will improve definitely your BER. Specially when the interfere is a radar, like in your case.
There is another thing you can do if you have control over the data sent. Detect the presence of the radar pulse and use this to stop the data flow. This detector can be antenna-diode-video amplifier-comparator.
If the RADAR is overloading your receivers you have two further options.
1. Wait to transmit your data until a certain time after the RADAR pulse, this certain time being the overload recovery time of your receiver.
2. Disconnect the receiver from the antenna during the pulse. This can be as easy as changing the TR switch to the TX position (without activating your transmitter) during the presence of the RADAR pulse.
Changing polarisation will sometimes give an improvement.
Things I would do, in order of importance:
1) If you have headroom, bump up the transmit power on both ends.
2) change to high gain antennas on both ends and point them AWAY from the military base as much as possible while maintaining proper link margin. The higher antenna gain will give more off-boresight attenuation drop off.
3) Hook a spectrum analyzer to the antenna and figure out just what frequencies are jamminng you at the site. (Only do this if you think you will not get shot for being a spy!). IF the jamming frequencies are out of band, you could add a band-reject or bandpass filter to pass your desired frequencies while rejecting the jamming frequencies. It might be possible to move to a channel within your 15 GHz band that is jammed a little less than the other channels.
4) if it is a critical application and the above do not work, you could go to a frequency diversity system with a hitless switch.
And I would also try swaping antenna polarization, as suggested above. It MIGHT help, although a lot of radars are circular polarization (where you would be out of luck).
thank you friends but i think you cant change the polarization when ever you want this becoase that the type of polrization depend on many parameters, so before you select the type of polaraization you have to studdy the terrain and other barameter to avoid the refraction and reflection effects, other thing i want to mention on it that we cant stop taransmitting when the radar generates its pulse this because this microwave link carry out so many BTS's so if we are stop the transmitting this will lead to kill the mobile coverage in that area.
i think at the end the only thing that help us to avoid such case is to change the frequency band that we are use.