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I wonder how a mobile phone (handset) sends signals to the Base Transceiver Station (BTS). The BTS is obviously quite large and, thus, possesses much more power than a phone does. So, how is a phone able to send information?

Receiving is quite clear but I don't get the other way around. I couldn't find any detailed explanation on that. The only hint I've discovered is that the signals are overlapped to a carrier frequency. But, as far as I understand, this does not touch or even explain the mentioned "power"-issue. I often read that most mobile phones use the same antenna for receiving and sending. If that's possible then why are the BTSes so large when such small ones like in our handsets are sufficient?

My questions are intentionally naive, since they are fundamental questions. And because I do not really know it better.

Rodrigo de Azevedo
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Ben
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    I think that by "antenna" you mean "base station". Both the base station and phone have an antenna. And by "how is a phone able to send information" you mean "how is the base station able to receive the relatively weak signal from the phone". – Transistor Apr 28 '20 at 06:49
  • Yes, but the "base station" is finally an antenna, or? – Ben Apr 28 '20 at 13:35
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    No, the base station is *attached* to an antenna. The antenna is part of the transmit / receive circuit and plays a part in the power transmitted and on the sensitivity to the received signals. – Transistor Apr 28 '20 at 13:38
  • If you really want to understand that you have to dig into RF. But I think I know what your considerations are. Note that a good transmission can be achieved by TX-power as well as antenna gain. So what you call BTS in most cases has a big and nifty antenna. – Ariser Apr 28 '20 at 17:45
  • @Transistor What exactly is the base station? The entire "building"? – Ben Apr 29 '20 at 07:13
  • @Ariser-reinstateMonica Diving into antenna electronis is on my todo list for a while. Hope to follow that topic somewhen.. What is TX-power? – Ben Apr 29 '20 at 07:14
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    The base station consists of all the electronics and antennae needed to handle the calls. This includes the station to station handover, the multiplexing of multiple channels, rapid switching between transmit (TX) and receive (RX) and the antennae themselves. See http://www.emfexplained.info/?ID=25196 for a short explanation. You should be able to find plenty more. – Transistor Apr 29 '20 at 07:36
  • @Ben we are not your dictionary. Consider our time valuable. Don't ask us for any word which can be retrieved by any search engine in seconds. I will answer such questions after you pay $15 each to my PayPal address eBay_11@pilotomie.de. Thank you. – Ariser Apr 29 '20 at 09:13
  • The size of the receiving antenna affects the strength of the received signal. – Hot Licks Apr 29 '20 at 16:18

6 Answers6

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There is no "power issue." In the simplest case, base station (in the cell tower) transmits to your phone with about the same power level that your phone transmits.

The base station also transmits to other phones at the same time so the total transmitted power is higher. It "talks" to each phone no "louder" than necessary, though.

You're thinking of the base station like a public address speaker - big and loud to get the message out as far as possible, and you wonder how the little phone can match that for distance. In reality, it's more like an intercom - no louder than the "speaking" voice of those who can hear it. The base station and the phone "hold a conversation" at the volume of the "speaking voice" of the phone.

The base stations use large antennas because it helps with transmitting and receiving. The base station uses the same antenna to transmit as well as to receive. A big, efficient antenna helps the base station to "hear" your phone better. At the same time, the big, efficient antenna makes the base station's "voice" louder so your phone can "hear" it better.

The actual power transmitted from the base station towards your phone is about the same as the power your phone transmits towards the base station.

Cell phone towers often have more than one antenna, so it looks like there's an even bigger difference in antenna size than there really is.

The multiple antennas on a cell phone tower are arranged to cover different areas, and may have multiple base stations behind them to handle the traffic.

Each phone channel can only handle a certain number of users at once, and there is only a certain number of channels available. Since you can't change those two things, you use directional antennas and divide the area around the tower into sectors - each sector is then covered by an antenna and base station. You can get more users on the system that way.

The big antennas are both an ear trumpet and a megaphone at the same time.

JRE
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  • So my mobile phone is capable of sending signals without any further magic directly to the base station? – Ben Apr 28 '20 at 13:37
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    Both have antennas. Both have receivers. Both have transmitters. What "magic" would you expect? – JRE Apr 28 '20 at 13:40
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    @Ben yes, the antenna and power in your mobile phone are strong enough to send a signal that reaches the base station. Basically, your phone is weak of hearing and voice - but the base station hears very well (so the weak voice of the phone is enough) and speaks loudly (so the bad hearing of the phone is enough). Your phone might not be able to send a signal to another phone-sized antenna at the same distance (because then bad hearing would try to understand a weak voice), but base station to phone and phone to base station works. – Syndic Apr 29 '20 at 05:13
  • @Syndic That's interesting. So the base station provides a much better SNR? Is it only because of the antenna or are there "just" more advanced electronical components involved? – Ben Apr 29 '20 at 07:16
  • @Ben I'm sadly not that knowledgeable, only an layperson with wide interests, so take anything I say with a grain of salt. But I assume that the larger antenna helps a lot. – Syndic Apr 29 '20 at 08:10
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    @Ben: It's mostly the bigger antennas that makes the difference. Not **just** that they are bigger. They are built differently to be more effective, which requires more elements in the antenna, which makes them bigger. Just being big doesn't help. Putting a potato on your ear doesn't help you hear better, but putting your (approximately) potato sized hand behind your ear can help you hear better. – JRE Apr 29 '20 at 08:14
  • I have to admit that I totally lack any knowledge about antennas. I always thought the sizes do matter a lot. I spent some minutes on Google but couldn't find a simple statement underlying my assumption. I only found out that the size of the antenna seems to be a hot topic :) However, what I conclude is, it doesn't harm the bigger the antenna is (somewhen it is just not efficient anymore) so the question is just "How big does it have to be at least?". – Ben Apr 29 '20 at 13:14
  • The size of the elements of an antenna are dictated by the frequency of the radio signals. They are usually sized by particular fractions of the wavelength of the radio waves. Higher frequencies means shorter wavelengths means smaller antenna elements. More elements means a bigger antenna for the same wavelength. – JRE Apr 29 '20 at 13:24
  • In other words, the base station doesn't just have a big mouth; it also has big ears. – Asteroids With Wings Apr 29 '20 at 15:09
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Radio is a lot more effective than people think. Radio telescopes can listen to distant galaxies. Shortwave propagates around the world; amateurs with medium-sized sets compete to find the furthest away they can make a link.

The larger size base station antenna doesn't just give it greater transmit power, but greater receive "power" as well. There are multiple antennas each aimed in a different direction off the base station. So it has a high "antenna gain".

The answer given by orfruit also points the antenna reciprocity theorem. The efficiency of a radio channel is the same in both directions, so a small antenna transmitting to a big one is just as effective as the other way round.

Mobile handsets also have quite a high transmit power for a small radio. They transmit in short bursts to economise on power and keep the channel clear for other handsets, but those bursts may be in the region of 2W.

pjc50
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  • Additionally, early analogue phones could have transmitters capable of 15 Watts of output - more common in car kits, but that's a lot. – Criggie Apr 29 '20 at 20:49
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In a ideal condition (say free space) the energy (RF power) needed in one way = the power needed in the other way around, it's pure physics :)

But this assumption is wrong in practice, because even antennas are passive elements (both base and phone a a system), the receive electronics in mobile is much worse (as sensitivity) compared to base. This is obvious due to cost reasons, space available, consumed power, etc.

These are main reasons that makes base to transmit 10x more power from the beginning.

But links are smart now (3G, 4G and upwards, I believe even 2G). It means that once link is established, the mobile device inform the base also about it's RSSI (signal strength) so base can adjust (scale down) it's power to not use more than necessarily.

BTW, it doesn't matter how many receivers are in system :) they will not "load" the base antenna at all.

yo3hcv
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some numbers here

The noise floor for electronics is (we'll use logs here) -174 dBm/rootHertz at room temperature. [In non-log non-dBm, we multiply Boltzmann Constant by the Kelvin Temperature; for system design we use 290 degree Kelvin, and we find the K * T product is 4.0000 * 10^-21 Watts/Hertz. Converting to milliWatts, we have 4.0000 * 10^-18 milliWatts/Hertz. Take log10 of that, and scale by 10, and you get the very useful -174dBm/rtHz.]

A cell phone needs about 1MHz bandwidth (yes, I know that number may be different for your system).

That means a faster datalink is needed than just one Bit Per Second; we'll increase the noise floor by 1,000,000X or by 60dB, thus -174 + 60 = -114dBm.

Now I know (computed this many years ago, using P = V^2/R) that 0dBm is 0.632 volts peak_peak (in 50 ohm system which is standard for radio systems).

And I know that -120dBm is 10^-12 weaker than 0dBm in power, and 10-6 weaker in voltage.

Thus -120dBm (yes, we want -114dB, be patient) is 10^-6 weaker than 0.632 volts PP.

We just computed a realistic voltage for our radio signal ------ 0.632 microVolts.

Now the transmitters operate on volts and tens of volts to drive energy TO the antenna.

And we just computed we need only 0.632 microVolts in our receivers.

Life is good.

Ohhhhh not done yet. We have -114dBm as our goal. That is 6dB stronger. That means 2X the voltage.

Thus our -114dBm is 2 * 0.632 microvolts PeakPeak. Life is even better.

analogsystemsrf
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There is, both at the base station and at the cell phone, a transmitter and a receiver working off a common antenna.

They utilise a full-duplex communication system in which simultaneous two-way transmission/reception is possible using different transmit and receive frequencies and a device called a duplexer, which isolates the receiver from the transmitter at both ends.

There is no question of the base station antenna overpowering the cell phone antenna. In fact, high gain of the base station antenna actually facilitates effective communication between the base station and the cell phone.

vu2nan
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Base stations do NOT use the same antenna for transmitting and receiving. Usually there are 2 receive antennas and 1 transmit antenna. This means the receiving end at the base station is more sensitive, thus it helps balance out the discrepancy (or imbalance) between the transmitter power of the mobile handset and the base station. In layman's terms, the base station has a big voice and big ears, the handset a small voice and small ears.

Keith
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