Operation of cellular
In a simple radio, both transmitters use the same frequency.
Only one can talk at once.
The standard cell phone of the first generation established a range of frequencies between 824 megahertz and 894 for analog communications. To face the competition and keep prices low, the standard established the concept of two carriers in each market, known as carriers A and B. Each carrier 832 is given voice frequencies, each with an amplitude of 30 Kilohertz. A pair of frequencies (one for sending and one for receiving) are used to provide a dual channel by telephone. The transmit and receive frequencies of each voice channel are separated by 45 Megahertz. Each carrier also has 21 data channels for use in other activities.The genius of cell phones is that a city can be divided into smaller "cells" (or cells) that allow you to extend the frequency throughout a city. This is what enables millions of users use the service in a large area without problems. Here's how it works. You can divide an area (a city) in cells. Each cell is typically a size of 10 square miles (about 26Km2). Cells were imagined as a hexagonal hexagons on a big field like this:However, the cell size can vary greatly depending on where you are. The base stations are separated from 1 to 3 km in urban areas, although they can be separated by more than 35 km in rural areas. In densely populated areas or areas with many obstacles (such as tall buildings), the cells can concentrate on smaller and smaller distances. Some technologies, such as PCS (Personal Communication Services), cells require very close together due to their high frequency and low power in which they operate. Buildings can, in turn, interfere with sending signals between cells that are more distant, so that some buildings have their own "microcell". The subways are typical scenarios where a microcell is needed. Microcells can be used to increase overall network capacity in densely populated centers such as the capital.Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in nonadjacent cells.Each cell in an analog system uses one-seventh of voice channels available.
That is, a cell, over the six cells that surround a hexagonal arrangement, each using one-seventh of the available channels so that each cell has a unique set of frequencies and there are no collisions between adjacent cells.This configuration can be seen graphically in the following figure:You can observe a group of cells numbered at the top.Thus, in an analog system, any cell can talk to 59 people on their cell phones simultaneously. With digital transmission, the number of available channels increases. For example, the TDMA digital system can carry three times more calls per cell, about 168 channels available simultaneously.Each cell has a base station that consists of a tower and a small building where you have the radio equipment. Each cell uses one-seventh of the 416 dual channels of voice. Thus leaving each cell approximately 59 available channels listed above.While the numbers may vary depending on the technology used in the place, the amounts used to show how this technology works, that in case of a modern generation can be extrapolated directly anyway.Cell phones have a low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most shortwave radios transmit at 5 Watts). The base station is also transmitting at low power. The low-power transmitters have two advantages:The energy consumption of the phone, usually battery operated, is relatively low. This means that under battery power requires small, and this makes it possible that there are phones that fit in your hand. Turn significantly increases the time that the phone can be used to recharge the battery.The transmissions from base stations and phones do not reach a distance beyond the cell. That is why in the above figure in each cell can use the same frequencies without interfering with each other.The transmissions from the central database and in the same cell phones do not leave it. Therefore, each cell can reuse the same 59 frequencies across the city.The cellular approach requires a large number of base stations for cities of any size. A typical large city can have hundreds of broadcast towers. But because so many people using cell phones, costs remain low for the user. Each carrier in each city has a central office MTSO call (PSTN in the diagram below). This office handles all telephone connections and base stations in the region.
Typical transmission tower cell phoneWhen the user wants to make a call, the cell phone sends a message to the tower requesting a connection to a specific phone number. If the tower has sufficient resources to enable communication, a device called a "switch" connects the signal from cell phones to channel the public telephone network. Call now takes a wireless channel and a channel in the public telephone network will remain open until the call is completed.The diagram below graph described above.Say you have a cell phone, turn it on and someone tries to call you. The MTSO gets the call and try to find it. From the early systems was the MTSO as activating your phone (using a control channel, since your phone is always listening) in each cell of the region to answer your phone. Then the phone base station and decide which of the 59 channels on your cell phone use. You are now connected to the base station and can begin to talk and listen.As you move into the cell, the base station will notice that your signal strength decreases. Meanwhile, the base station of the cell to which it is moving (which is hearing the signal) will be able to notice the signal becomes stronger. The two base stations coordinate themselves through the MTSO, and at some point your phone gets a signal telling you to change frequently. This change makes your phone move its signal to another cell.
In modern mobile systems expect a system identification signal (IDS) of the control channel when ignited. The phone also transmits a registration proposal and the network maintains some facts about its location in a database (so is that the MTSO knows which cell is ringing your phone if you want). As you move between cells, the phone detects changes in the signal, recorded and compared to those of the new cell when you change channels. If the phone can not find channels to listen to know that is out of range and displays a message "no service".This is, quite simply, the operation of mobile phones, ranging from the theoretical side in the division of geographical areas in cells to the electro magnetic waves exchange necessary to establish a simple communication between two mobile phones.While the topic can be approached in a much more technical, stopping more on aspects of frequency and amplitude of the waves for example, prefer to give a more general approach, giving it some specific technical data which we found most relevant to the general understanding of subject.
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