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It seems confusing that my phone receives -87 dbm LTE signal and shows a full 4 bars with the speeds of

UPLOAD: 20Mbps

DOWNLOAD:13.6Mbps

But my WiFi is showing 1 bar at -89 dbm and I get disconnected as soon as I move slightly away, and speeds are very low. Why does this happen? It happens with all my phones.

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closed as too broad by winny, Dmitry Grigoryev, awjlogan, Nick Alexeev Sep 25 '18 at 16:54

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Just to make sure you are clear, -89dBm is only 67% of the power of -87dBm, which is quite a difference. It's not the answer to your question, but worth noting. \$\endgroup\$ – Tom Carpenter Sep 6 '18 at 8:34
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    \$\begingroup\$ But at -92 dbm LTE shows full bars on my phone. \$\endgroup\$ – ObsessionWithElectricity Sep 6 '18 at 9:52
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    \$\begingroup\$ The "bars" are basically a nice fiction that bears only a very loose relationship to actual reception. \$\endgroup\$ – pjc50 Sep 6 '18 at 14:15
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    \$\begingroup\$ @ObsessionWithElectricity No, data speeds have nothing to do with connection quality on WiFi. \$\endgroup\$ – Mast Sep 6 '18 at 16:05
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    \$\begingroup\$ Ok, come on, my connection drops by just moving a few steps away from the router. Remember, I just wanna know why WiFi has poor connection at -89dbm while LTE seems to be fine even at -95dbm \$\endgroup\$ – ObsessionWithElectricity Sep 6 '18 at 16:10
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As with any radio receiver, if it can handle a higher data rate, then it is usually burdened with having a higher RF bandwidth and this inevitably means more received background noise i.e. a wider BW lets in more noise and hence, you need a higher received signal level to operate with a decent SNR (signal to noise ratio).

Therefore WiFi is at a significant disadvantage because it has a wider RF bandwidth than LTE (normally) and needs a higher signal level to operate at a decent bit-error-rate (BER). This is embodied in the following empirical but commonly-found relationship.

Power (dBm) needed by a receiver is -154 dBm + \$10log_{10}\$(data rate)

For example, if the WiFi data rate is ten times your LTE data rate, then you need 10 dB more signal to operate at the same SNR. Basically if you double the RF bandwidth you "collect" 3 dB more noise. This means that WiFi is usually the first to suffer as signal levels drop (compared to LTE data rates).

Why WiFi has shorter range than LTE?

This is related to the Friis transmission equation but, more simply, you can think about the same effect with light bulbs; consider a 1000 watt lamp and the distance you could see this at night time - you would probably see it fairly clearly from 10 km away and, if you walked a further 100 metres, it wouldn't look significantly dimmer.

Compared with a small 1 watt lamp, you might see it glowing at 100 metres but, if you walked away a further 100 metres, it would be noticeably dimmer.

There are a bunch of other factors too such as operating frequency - WiFi can operate at a higher carrier frequency and the Friis transmission equation informs you that as frequency rises, the path-loss increases: -

Path loss (dB) = 32.45 + \$20log_{10}\$(F in MHz) + \$20log_{10}\$(D in kilometres).

In other words at ten times the frequency, the path loss increases by 20 dB.

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    \$\begingroup\$ My phone carrier is at LTE B40 - 2300-2400Mhz . So I guess there is not much difference in frequencies. I think the reciever of WiFi routers is less sensitive than receiver of BTS of LTE \$\endgroup\$ – ObsessionWithElectricity Sep 6 '18 at 9:58
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    \$\begingroup\$ Also, you don't tend to go and mount your WiFI access point at optimum height on a huge honking pole, unlike commercial mobile carrier equipment :) \$\endgroup\$ – rackandboneman Sep 6 '18 at 12:11
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    \$\begingroup\$ The wifi frequency is irrelevant to a discussion about band width. \$\endgroup\$ – pjc50 Sep 6 '18 at 12:19
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    \$\begingroup\$ @rackandboneman: And if you do mount a Wi-Fi access point like that, decent equipment (with a directional antenna like LTE, not omnidirectional like cheap home "routers") can easily reach several km. Long range point-to-point Wi-Fi links are rather common. \$\endgroup\$ – grawity Sep 6 '18 at 12:26
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    \$\begingroup\$ Phone antenna's are generally not heavily directional, or you would expect your phone to lose signal as you changed its orientation. The base stations can be more directional, but they generally still a broad region, potentially doing software based beamforming. \$\endgroup\$ – mbrig Sep 6 '18 at 20:47
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in addition to Andy's answers, WIFI is usually limited in power, 30dBm in north America, lower levels in most of the world. LTE usually can transmit up to 4W (36dBm) And the towers transmit at a much higher power.

Also, LTE has much better network management capabilities (automatically finding the best channel and data rate), the towers have much better clock sources than WIFI (this affects receiver sensitivity) and the towers have higher placed antenna (10-30m) than the common wifi router.

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  • \$\begingroup\$ But I heard that LTE transmit upto 200mW in phone's. For towers I don't know. Which is correct 4 W or 200mW? \$\endgroup\$ – ObsessionWithElectricity Sep 6 '18 at 11:43
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    \$\begingroup\$ I'm fairly certain it's 200mW. GSM (2G) used to be 2W, but that was dropped first to 1W and then 500mW. Battery life at 4W would be atrocious. Note that even GSM would use these figures as absolute maximums, and dynamically lower it where possible. \$\endgroup\$ – MSalters Sep 6 '18 at 12:18
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    \$\begingroup\$ Yes, it's 200mW. This paper is interesting: ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7879218 - the average power of the 95th percentile of transmissions was 4mW! \$\endgroup\$ – pjc50 Sep 6 '18 at 12:23
  • \$\begingroup\$ It is up to. The limit is regulatory, not technical. Obviously, you want to use as little power as possible. If you are in an urban setting, with lots of towers, sure your phone will transmit few mW. If you are out in the wilderness, with the nearest tower 5km away from you, your phone will transmit at maximum power. \$\endgroup\$ – Lior Bilia Sep 6 '18 at 13:31
  • \$\begingroup\$ How much is power emitted by tower of LTE? \$\endgroup\$ – ObsessionWithElectricity Sep 6 '18 at 15:52
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For comparison, these are the transceiver part of a LTE network(from two different vendors)(antennas are mounted on towers/poles and connected via cables to the eNodeB), equivalent to the antenna part plus the transceiver circuit of a wifi AP.

https://www.motorolasolutions.com/en_xl/products/lte-broadband-systems/broadband-systems-equipment/enhanced-node-b/rbs6101.html#tabproductinfo

https://www.scribd.com/document/204866576/RBS-6000-Spec-Sheet

A LTE eNodeB can handle hundred of users(depending on configuration) in a controlled manner. How many can a wifi AP handle??? You cannot really compare Wifi to LTE. It is two completely different kind of system meant to be used in quite different scenarios.

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