How can the mobile station transmit its signals?

I was wondering about the the ability of the antenna of mobile to transmit the signals to the BTS. It is known that the BTS uses high powers to provide its antennas with large distances of signal transmissions (more than 20 km ) so that we see that one BTS can cover very big area with a huge number of mobile stations and hence transmits its signals to all the mobile stations, but in the opposite case when the mobile try to transmit its signals to the BTS and the distance between them is very big (say more the 10 km), how can the antenna of mobile station transmit its signals with very small power taken from the mobile battery?

Cell Phones have low power transmitters in them and base stations are also transmitting at low powers due to following reasons

1 The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, 2 different cells can reuse the same 56 frequencies. Hence, the same frequencies can be reused extensively across the city.
2 The power consumption of the cell phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible.

thanks nomigoraya
what i meant that how the antenna of mobile can transmit its signal for long distances (more than 10 km) with very low power (the power of mobile battery)?
but i have read from before that the mobile stations (uplink and downlink ) are low frequencies (MHz) so that the wavelength will be very high and hence the signal can move long distances, but also it needs more power, doesn't it??

but i have read from before that the mobile stations (uplink and downlink ) are low frequencies (MHz) so that the wavelength will be very high and hence the signal can move long distances, but also it needs more power, doesn't it??

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Depends on what you mean by 'low frequencies', all voice/2G/3G at at frequencies higher than 920MHz and 4G is at frequencies higher than about 600MHz, most people would consider then to be high frequencies. The frequency (or wavelength) plays little part in the distance the signal will travel although it does influence the efficiency of the antenna that can be practically used.

Base stations have to be slightly higher power than mobile stations or they would not have the extra range needed to control cell handover. To ensure uninterrupted calls while mobile, the BTS has to overlap it's neighbors slightly or there would be a break in the link as you passed from one to another. The power in the cell phone is automatically adapted under control of the BTS to keep it's reception at constant level and to maximize battery life. If the BTS picks up a strong signal, it tells the phone it can reduce power and save battery consumption.

Brian.

I vaguely .. recall monitoring all the mobile bands in Atlanta, Ga and all the uplink bands were 20 MHz lower than the down links for a slight advantage on Friis losses to the mobile transmitter.

Thanks Brian
I know that the BTS sends control signals to the mobile station every time to adapt its power relating the distance with the BTS, but this is not the solution, the power feeder of the antennas in BTS are greater so BTS can control the Hand-overing, Tx-Rx and so on, but the mobile has very low power.
We can look at the question from another perspective, How any antenna can transmit its signal for long distances?? does it depend on the power fed?
Also as long as the wavelength doesn't play big role in the distance, so how the microwave antennas are used to connect between two BSCs or BTSs?
what i am wondering every time is what does the antenna depend on to transmit signals very long distances?

In almost all cases, the BTS are linked by copper landline or another "non cell" data link such as fiber optic or dedicated microwave link. A call from one phone to another is generally:
phone ---> local BTS ---> landline ---> remote BTS --->remote phone.

The landline is an "always on" link, it doesn't dial a number to connect you, it just carries the data from one BTS to another through a switching network and the remote number is then routed to the local phone line or the cell station they are currently registered with. Where a microwave link is used to connect BTS, they use dish antennas to provide very high gain in a narrow beam width so the power could be very low. I have used 10GHz microwave links over more than 200Km with less than 10mW RF power by using 1m dishes at each end!

For the most part, the distance any antenna can send or receive, over an unobstructed path, depends on the power used at the transmitter, the gain of both antennas and the presence of interference at the receiver. An antenna with 'gain' doesn't actually produce more signal, there is no amplification but by concentrating the existing signal in one direction it can reach further. As for most point-to-point purposes both antennas are at fixed locations it makes sense to concentrate all the available signal between them rather than radiating it in all directions.

The example I use to explain this to students is to visualize it as a stick and a loop of string. The stick is placed upright on the ground and the loop is placed around it. The length of the string represent the transmitter power and the distance from the stick represents how strong it will be received, further away meaning a weaker signal. If the antenna is omni-directional the loop is a circle with the stick in the middle but the distance to the loop is as small as it is possible to be. If you start to elongate the loop so it becomes an elipse, the distance becomes shorter in some directions and longer in others so in the long axis of the elipse, the signal reaches a longer distance. The more you stretch the loop in one direction the further it will reach that way and it gets progressively shorter in all other directions. The analogy is the stick being a monopole antenna which is not directional in itself (at least in the horizontal plane) and adding other elements or reflectors around it modifies the radiation pattern to concentrate it in one direction.

Brian.

thanks again Brian

How did you establish a 10GHz microwave antenna to transmit signals more than 200km with less than 10 mW RF power?
how did you calculate the required power? formulas will be helpful.
I know that the more the directivity and gain, the more the distance the signals are transmitted, but how long distances, this is the questions

How did you establish a 10GHz microwave antenna to transmit signals more than 200km with less than 10 mW RF power?

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It was an experiment and not intended to be used commercially, we just wanted to see what was possible to achieve using inexpensive equipment. The locations were decided before the tests and were both at sites >500m above sea level. The transmitter was a modified Gunn oscillator adapted from a mechanism for automatically opening shop doors, it was designed to have a maximum range of about 5metres! The receiver was a modified domestic satellite LNB down converter with it's local oscillator re-tuned so it could receive slightly lower frequencies (10.3GHz instead of 10.75GHz). Both ends used 90cm domestic satellite dishes. We sucessfully sent about 30 minutes of live video and sound using FM at very high quality.

To be honest, we did not calculate the power but the DC input power at the transmitter was about 6V at 50mA = 0.3W and it was maybe 2.5% efficient so the RF output would be in the region of 7.5mW. The effective gain of the dish would about 30dB so the ERP was about 0.25Watts.

Brian.

Maybe there is a translation problem. We call a microwave frequency phone a cell phone, not a mobile phone. About 36 years ago I bought a wireless phone (a mobile phone?) that operated on 27MHz that was called a high frequency. It used a whip antenna about 50cm long to transmit and receive between the base station in the house and the wireless phone throughout the house and in the yard. A few years later it was replaced with a 49MHz phone.

Audioguru said:

Maybe there is a translation problem. We call a microwave frequency phone a cell phone, not a mobile phone. About 36 years ago I bought a wireless phone (a mobile phone?) that operated on 27MHz that was called a high frequency. It used a whip antenna about 50cm long to transmit and receive between the base station in the house and the wireless phone throughout the house and in the yard. A few years later it was replaced with a 49MHz phone.

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To be more precise and clear GSM has two bands GSM 900 and GSM 1800
Discussion is anout GSM communication and its handsets are mobile phones, cell phones, I phones.
I am not sure but the set you are saying as wireless is called cordless at some places as the frequency you are mentioning is not GSM frequency. Please correct if I am wrong

nomigoraya said:

To be more precise and clear GSM has two bands GSM 900 and GSM 1800
I am not sure but the set you are saying as wireless is called cordless at some places as the frequency you are mentioning is not GSM frequency. Please correct if I am wrong

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You are not wrong, that is exactly what i meant by mobile phone. i mentioned it also mobile station, it is called MS in GSM (900, 1800, 1900) or cell phone.

home phones are "cordless"
all others to base station are "mobiles"