8

Do cell-phone base station antennas emit the same power of waves as from a cell phone?
I would assume so because both waves need to travel the same distance, right?

JobHunter69
  • 915
  • 1
  • 15
  • 26
  • 1
    Cell antennas favour some directions more than others, so a **cellphone** in different locations receive various power levels from their omni-directional internal antennas. – glen_geek Mar 12 '23 at 16:05
  • @glen_geek right so the max power an antenna can transmit is always about the same as the max power transmitted by a cellphone right? – JobHunter69 Mar 12 '23 at 16:14
  • 1
    I'm trying to make a distinction between *effective radiated power ERP* of a directional **cell-tower** antenna, and the simpler non-directional antenna inside a cellphone. I'd not like to guess at tower ERP. – glen_geek Mar 12 '23 at 16:48
  • 13
    You know that cell phones are variable-power, right? They tune down their transmit power as low as possible to still make their connection (first to save battery and second to avoid stomping on bandwidth any farther out than they have to). They're mind-numbingly complex. – Harper - Reinstate Monica Mar 12 '23 at 23:16
  • 1
    An important thing to understand is that phones have omnidirectional antennas while towers have directional antennas, so 1W from a cell tower will result in a stronger signal than the same 1W from a phone in the direction that the tower's antenna is facing, and that could be a factor of 10 or so. If you're not in the path of the antenna then the signal will be much weaker. So it's about the same amount of energy, but distributed differently. – Frog Mar 14 '23 at 19:46
  • @Frog What is the purpose of the cell tower sending out a stronger effective signal if the reply can't be seen as far away? – Michael Mar 14 '23 at 22:55
  • 1
    @Michael more likely the tower will transmit less power so the range is the same in both directions. But there’s no guarantee that the phone and tower have the same sensitivity. Typically there’s a negotiation where each device tells the other how far it can reduce transmit power and still maintain communications. – Frog Mar 15 '23 at 23:07

4 Answers4

27

In principle, this is symmetric, correct.

In practice, the tower will control both its and the connected phones' transmit power to get a working configuration, and the tower will also use beamforming techniques to direct power only in specific directions.

For reception to work, the tower needs to receive all connected phones at roughly similar levels, because the analog amplifier behind the antenna is shared for all connections, and division is made later, on digital data, which has a limited dynamic range, so the actual transmit power in phones is mostly dependent on distance and transmission properties to the base station it is connected to.

The transmit power at the tower depends on the reception capabilities of the phones the transmission needs to reach, largely how much gain is available in their RF amplifiers, and whether there are other transmissions that further limit the available gain.

For example, if I have one LTE modem downloading lots of data, and use my phone in the same room, the phone's antenna will also pick up the LTE traffic from the modem to the tower at a fairly high level, so it needs to reduce its receive amplifier gain to not overload the mixer stage. It will notify the tower of this, and the tower will then increase the transmit power to this phone to make up for this reduction in receive gain.

In short, how much is actually radiated depends on a lot of factors. In general, everyone will try to use the smallest possible setting, this will be considerably lower when base stations are placed closer together.

Simon Richter
  • 12,031
  • 1
  • 23
  • 49
  • Cell phones (digital) also make use of multipath reflections to increase signal strength. This also allows cell tower to reduce signal strength. In contrast, multipath fading can cause the recieved signal to dissappear entirely in analog radios (FM broadcast) – RussellH Mar 12 '23 at 19:55
  • 2
    "the phone's antenna will also pick up the LTE traffic from the modem to the tower at a fairly high level" No, this is why radio bands are duplexed, either in frequency or time. In a FDD system, which is the most popular, the uplink and downlink frequencies are separated by several MHz. A duplex filter in the phone blocks the uplink frequency from the receiver. In fact, the filter works so well, IS-95, WCDMA, LTE, and NR systems are full-duplex, a normal handset transmits and receives at the same time. In a TDD system, all UE will be listening at once. – user71659 Mar 13 '23 at 21:40
  • @user71659: This answer is talking about two separate devices in the same room interfering with each other, not the up and down sides of the same device. To be fair, the same frequency and/or time division that allows full-duplex transmit+receive can also help avoid interference between nearby devices, but for separate devices you don't have direct access to the antenna input of the other device to subtract it from what you're receiving. – Peter Cordes Mar 14 '23 at 20:15
  • 1
    @PeterCordes You misunderstand. The uplink and downlink frequencies in FDD systems are significantly separated and filtered exactly to prevent interference described. All stations transmit on a different frequency range than they listen to, never the same frequency. Think about the situation at a cell tower: there's multiple independent carriers putting out potentially kW. The base station radio needs to hear weak UE uplinks in that environment. That is the "same room" scenario described. – user71659 Mar 14 '23 at 20:32
  • 1
    @PeterCordes For example, if your phone listens at 1960 MHz (GSM 1900, WCDMA/LTE/NR Band 2), it would transmit 1880 MHz. The base station would listen at 1880 MHz and transmit at 1960 MHz. At the cell site, all transmitters would be in the 1930-1990 MHz range, and all receivers would be in 1850-1910 MHz. The 20 MHz gap between these two ranges is specifically provided for filter roll-off. This is how the vast majority of cell frequencies operate. – user71659 Mar 14 '23 at 20:46
  • @user71659: Ok, that makes more sense, with 20 MHz gaps allowing for just filtering based on frequency, vs. "several" MHz seemed so close I assumed something that only worked for a shared antenna. I guess a cell base station would ask nearby devices to use different frequencies specifically so it could filter by frequency and direction to minimize interference. Thus two devices in the same room would likely be far enough apart in frequency to work with minimal interference. – Peter Cordes Mar 14 '23 at 20:52
  • 1
    @Peter Cordes - It's not the total ERP (power) that's radiated by the cell phone tower, but rather the amount of power that's within the frequency band that the hand held cell phone is using that's the important parameter. Here's a made up, simplistic example. Say the cell phone tower/antenna is talking to 10 different users (cell phones) over 10 different channels. If the tower is radiating 10 W total, then only 1 W is available to each of the 10 channels. – SteveSh Mar 15 '23 at 13:11
  • 1
    @user71659, the worst part about the "same room" scenario is that the transmission of one UE saturates the receive amplifier of another, so the filtering needs to be on the analog side on the UE, before receive gain control. – Simon Richter Mar 15 '23 at 15:41
  • 1
    This doesn't account that when you have plenty of power, you can have better filters and thus better SNR. If the tower receiving SNR is 6db above the phone's receiving SNR, then it can instruct the phone to transmit at half the power and still decode the signal. The tower will have to compensate to emit at twice the power too for the phone to get the signal. This is largely asymmetric. – xryl669 Mar 15 '23 at 17:16
  • @SimonRichter Yes, so the front-end SAW filters on cell phones are the most expensive part of the transceiver, especially with the proliferation of bands. It's also why China pushed TDD-based systems and why WiMAX was TDD because the handset can share one filter between transmit and receive. At the base station, like xyrl669 mentions, they use very large tuned cavity filters for better filtering. – user71659 Mar 15 '23 at 18:52
16

The same order of magnitude - somewhat yes

The same exactly - not really.

The radio signal propagation is pretty much symmetrical between the base station and the cellphone antenna. This is independent on factors like beamforming, cell sectors, antenna gain and likes. But, their design constraints and their use cases are pretty much different.

  1. The base station has cheap and abundant power available so it can simply use more power if this could improve the connection. The phone has its finite-capacity battery, its SAR limits and its transmitter cooling limits that constrain this approach. Phones do vary their radio transmission power, but within their own limits.

  2. The base station also could use more sensitive (but bulky and power-hungry) receiving circuity so it can hear the weaker phone signal so it can negotiate with the phone lower transmission power on the phone side.

  3. The base station serves multiple (hundreds or thousands) phones at once. It has to beam the downstream data for all of the phones and the phones have to beam only their own upstream data.

  4. While voice connections are generally symmetric (upstream and downstream data rates are equal), data connections (e.g. used for browsing the web) are pretty much downstream dominated with down/up rates of 5-50. This translates to the base station having to beam as much more data packets on average, compared to the phone.

  5. The usual radio noise distribution favors the base station (it is usually away from radio noise sources other than its own gear). The phone, on the other hand, is usually near home, office and industrial equipment so the base station has to use more power in order to overcome the noise around the phone.

All this can be summed up to:

Phone radiates at most 1 watt (3G or above) or 2 watt (2G) pulses that average at few hundreds of miliwatts. In a good coverage conditions this goes as low as few miliwatts for the typical use case. If there is no data traffic and the phone is not moved, it does not transmit anything for as long as hours. This is how non-smart phones run for a week off an 2-4 Wh battery.

The base station can radiate as little as under 1 watt (for micro cells) or hundreds of watts (for the usual cells) on average. Some special base stations go well into kilowatt-range radio power.

If you happen to be the single user of a base station, the phone and the base station happen to be of the same technology generation, the distance is moderate and you only do a single voice call (no web browsing, etc...), chances are that the phone and the cell tower radiate the same amount of radio power. But this is usually pretty much not the case.

fraxinus
  • 8,726
  • 11
  • 34
  • Another factor is that in CDMA-type systems, the uplink is many-to-one, and is subject to frequency/amplitude/phase/timing errors of all the UE that are transmitting at once. The downlink is one-to-many, so it doesn't have the interference between multiple stations. – user71659 Mar 13 '23 at 21:56
  • All one-to-many systems, independent of the access model (not only CDMA) do have some collision and/or collision avoidance overhead. All parties bear some of this overhead, but I am not sure how much goes to the mobile device in CDMA. – fraxinus Mar 14 '23 at 10:21
  • 1
    Space probes are a good example of asymmetry of a very powerful transmitter and sensitive receiver (giant dish) at one end (earth) communicating with a much smaller and lower-power device at the other end. – Peter Cordes Mar 14 '23 at 20:09
  • 1
    @PeterCordes indeed. On the other hand, the asymmetry in power is not that much - e.g. 100-1000W at the probe and 1-20kW on Earth. On the third hand, the noise sources are impressive - e.g. the Sun... – fraxinus Mar 14 '23 at 20:39
  • +1 for the asymmetry description in power available. The cell tower doesn't need to match the phone's power if it has a better receiving gain amplifier and transmitter. If its SNR for the receiver after amplifier is twice the phone's SNR then it allows the phone to emit at half the power. It will have to emit twice stronger to compensate however. – xryl669 Mar 15 '23 at 17:14
9

Yes, approximately. As discovered with a simple Google search:

Note, though, that a cell phone tower handles many calls, each using that level of power, so the total power from the cell phone tower is much higher than that value. A cellphone handles a single call.

Davide Andrea
  • 16,164
  • 4
  • 33
  • 62
  • Yes, it is useless if two people are talking with each other and one of them is much louder. Because human #1 can hear what human #2 says, but human #2 can not hear what human #1 want to tell him. This communication would be only one-directional, so a phone call would not be possible. – MikroPower Mar 12 '23 at 18:07
  • 6
    @MikroPower The output power of a transmitter has nothing to do with how loud the received signal is. A typical two way radio system can have 2W portables, 30W mobiles and 100W repeaters and they all talk to each other just fine. – GodJihyo Mar 12 '23 at 18:34
  • 1
    @GodJihyo - That may be true for today's cell phones, but is not true in a general sense, particularly for old fashioned AM radio. In that technology, the received signal is amplified linearly from the antenna to the speaker. So a stronger input signal - from a higher power transmitter - results in a louder audio output, assuming the gain controls are kept the same. Also, a stronger signal gives you a better SNR, which could make all the difference between hearing anything or nothing. – SteveSh Mar 14 '23 at 13:19
5

The cell tower emits much more power than the "user equipment" (UE -- the phone in your pocket.)

The important thing to carry information is the power spectral density, which is a measure of how much power there is over a given frequency range. As the frequency range increases, more power is required to overcome the noise over a wider spectrum. Greater power spectral density results in greater signal/noise ratio which, for 4G and 5G, means more bits per symbol and so higher data rates.

The UE needs to transmit enough power to provide a useful uplink. The base station needs to transmit enough power to provide useful downlinks for all the UEs that are using the base station.

In practice, one of those antennas you see at the top of a mast is emitting tens of watts of RF. The mobile in your pocket is emitting tens or hundreds of milliwatts.

Simon EI3JDB
  • 51
  • 4