I have been monitoring power of a P25 control channel (at about 850MHz). For awhile, 24 hours a day. I graphed the power over a couple of weeks and it is almost sinusoidal, decreasing significantly at night, and ramping back up in the day. This is just power, not SNR. This seems to be the case on other frequencies I've been monitoring as well. What causes this?

I'm in an urban area, south US.

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    $\begingroup$ If you could roughly indicate your geographic location, this may help to receive a suitable answer. $\endgroup$ – Glenn W9IQ Nov 21 '18 at 17:07
  • $\begingroup$ @GlennW9IQ - Urban area, south US. $\endgroup$ – horse hair Nov 21 '18 at 18:35
  • $\begingroup$ How are you monitoring the power? Could your method be indirectly affected by changes in unrelated signals (e.g. AM broadcast stations that change their own broadcast power at night)? Do you know what the source of the signal is, like a cell tower or what — nearby or distant, etc.? $\endgroup$ – natevw - AF7TB Nov 21 '18 at 18:46
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    $\begingroup$ @natevw-AF7TB monitoring power with an SDR raw power output (hackrf). It's a P25 control channel $\endgroup$ – horse hair Nov 21 '18 at 18:54

That's a GSM band. If there's still GSM service in that band, it's very common to see Mobile Network Operators (MNOs) turn of base stations at night, effectively enlarging the remaining cells. I've witnessed that very drastically in a German city center, where telefonica used to completely switch of one of their 3G base stations.

This is done because power consumption is the nr. 1 cost driver in established cellular networks for MNOs.

850 MHz is by now also used as LTE/4G band by sprint. LTE is even more flexible in all aspects of power management.

In any case, you can probably simply observe the amount of power the base station observes as uplink from the cellular phones – more phones doing data or voice means more power in the air. It's really that simple. Every bit you transport needs fractions of a Joule. Transport more bits, see more power.

Downlink is just the same; when there's fewer phones that get data, you can more generously assign spectrum to each user, meaning that you need less power density to service these users with the same level of service. In a deep-night scenario, it's thoroughly possible that a vast amount of the time/frequency plane is simply unused. You have to buy your spectrum allocations for the peak volume you want to service. That means you underutilize it when there's noone to request that.

  • $\begingroup$ In one case this is a P25 trunked radio system, but I can see it in different places. It's not an on/off transition, but a sort of sinusoidal transition where the power ramps down slowly until about 4AM, then ramps back up. $\endgroup$ – horse hair Nov 21 '18 at 18:54
  • $\begingroup$ well, a) that would be something that's crucial to have in a question about "why do I see this system behaviour", if you know what kind of system that is ;) b) exactly the same applies: many P25 UEs on the air -> high power, few on the air -> low power. Basically, a room full of working washing machines is loud as hell. A room where only one washing machine is currently tumbling for its dear life is a lot silenter. $\endgroup$ – Marcus Müller Nov 21 '18 at 18:57
  • $\begingroup$ If I need to address every of my 300 students in a lecture hall by their name and tell them whether I have data for them every 5 minutes, I'm going to be loud all the time. If only two of them are awake, anyways, it's going to be a lot quieter. $\endgroup$ – Marcus Müller Nov 21 '18 at 18:58
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    $\begingroup$ @MikeWaters lol, no. I just hold a couple of lecture-accompanying exercises ("Saalübungen" in German) at university for the EE program; basic stochastics theory, communications technology 101 (mandatory exams for all >300 EE students each year) and a really small comms lab and applied information theory exercise. These translations are extremely rough :) $\endgroup$ – Marcus Müller Nov 21 '18 at 22:49
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    $\begingroup$ So, yep, I'm teaching, no, not as a lecturer, and no, I'm not a professor (uni prof in Germany is roughly equivalent to tenured professor in the US in terms of power and position, and even if I set my carreer and mind on that track, I'd be decades away from that title); nor a Ph.D.; I'm a M.Sc. in EE. $\endgroup$ – Marcus Müller Nov 21 '18 at 22:52

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