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How cell phone (mobile phone) works?

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    Default How cell phone (mobile phone) works?

    How cell phone (mobile phone) works?

    Millions of people around the world use cellular phones. They are such great gadgets, with a cell phone; you can talk to anyone on the planet from just about anywhere!

    Let's start with the basics:

    In essence, a cell phone is a radio. It picks up signals from towers, but if you wonder how the cell phone works and what makes it different from a regular household phone, read below as we go through the basics.

    The Cell Approach
    Each cell phone has a cellular system. When we say a cellular system, it means a division of a city into small cells. Each cell has a base station that consists of a tower and a small building containing the radio equipment this allows widespread frequency reuse across a city, so that millions of people can use cell phones concurrently. Each cell is typically sized at and covers about a 10 square miles radius.

    There is a requirement to have a large number of base stations in a city of any size to make cell phone use function conveniently. A typical large city can have hundreds of towers placed in certain regions to cover most of the areas completely. A central office called the Mobile Telephone Switching Office (MTSO) handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region. Each carrier in each city runs one.

    Cell Engineering
    In a typical analog cell-phone system in the United States, the cell-phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid, because cell phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent cells.

    Each cell has a base station that consists of a tower and a small building containing the radio equipment (more on base stations later).

    A single cell in an analog system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:

    A cell-phone carrier typically gets 832 radio frequencies to use in a city. Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically 395 voice channels per carrier. Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. With digital transmission methods, the number of available channels increases. For example, a TDMA-based digital system can carry three times as many calls as an analog system, so each cell has about 168 channels available.

    Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power.

    Low-power transmitters have two advantages:
    • The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, in the figure above, both of the purple cells can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city.
    • 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.

    The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using cell phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region.

    Cell Phone Codes
    All cell phones have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider. Letís say you have a cell phone, you turn it on and someone tries to call you.

    Here is what happens to the call:
    When you first power up the phone, it listens for an SID (System Identification Code (SID) - a unique 5-digit number that is assigned to each carrier by the FCC) on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel changing. If the phone cannot find any control channels to listen to, it knows it is out of range and displays a "no service" message.

    When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system.

    Along with the SID, the phone also transmits a registration request, and the MTSO keeps track of your phone's location in a database -- this way, the MTSO knows which cell you are in when it wants to ring your phone.

    The MTSO gets the call, and it tries to find you. It looks in its database to see which cell you are in. The MTSO picks a frequency pair that your phone will use in that cell to take the call. The MTSO communicates with your phone over the control channel to tell it which frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. You are talking by two-way radio to a friend!
    As you move toward the edge of your cell, your cell's base station notes that your signal strength is diminishing. Meanwhile, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) sees your phone's signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell. As you travel, the signal is passed from cell to cell.

    If the SID on the control channel does not match the SID programmed into your phone, then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of the phone you are using, is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds!

    Along Comes Digital
    Digital cell phones use the same radio technology as analog phones, but they use it in a different way. Analog systems do not fully utilize the signal between the phone and the cellular network. Analog signals cannot be compressed and manipulated as easily as a true digital signal. This is the reason why many cable companies are switching to digital, so they can fit more channels within a given bandwidth. It is amazing how much more efficient digital systems can be.

    Digital phones convert your voice into binary information (1s and 0s) and then compress it. This compression allows between three and 10 digital cell-phone calls to occupy the space of a single analog call.

    Many digital cellular systems rely on frequency-shift keying (FSK) to send data back and forth over AMPS. FSK uses two frequencies, one for 1s and the other for 0s, alternating rapidly between the two to send digital information between the cell tower and the phone. Clever modulation and encoding schemes are required to convert the analog information to digital, compress it and convert it back again while maintaining an acceptable level of voice quality. All of this means that digital cell phones have to contain a lot of processing power.

    Cellular Access Technologies
    There are three common technologies used by cell-phone networks for transmitting information:
    • Frequency division multiple access (FDMA)
    • Time division multiple access (TDMA)
    • Code division multiple access (CDMA)

    Although these technologies sound very intimidating, you can get a good sense of how they work just by breaking down the title of each one. The first word tells you what the access method is. The second word, division, lets you know that it splits calls based on that access method.

    FDMA puts each call on a separate frequency.
    TDMA assigns each call a certain portion of time on a designated frequency.
    CDMA gives a unique code to each call and spreads it over the available frequencies.
    The last part of each name is multiple access. This simply means that more than one user can utilize each cell.

    FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. To better understand FDMA, think of radio stations: Each station sends its signal at a different frequency within the available band. FDMA is used mainly for analog transmission. While it is certainly capable of carrying digital information, FDMA is not considered to be an efficient method for digital transmission.

    TDMA is the access method used by the Electronics Industry Alliance and the Telecommunications Industry Association for Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136). Using TDMA, a narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three time slots.

    Narrow band means "channels" in the traditional sense. Each conversation gets the radio for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. TDMA systems operate in either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.

    TDMA is also used as the access technology for Global System for Mobile communications (GSM). However, GSM implements TDMA in a somewhat different and incompatible way from IS-136. Think of GSM and IS-136 as two different operating systems that work on the same processor, like Windows and Linux both working on an Intel Pentium III. GSM systems use encryption to make phone calls more secure. GSM operates in the 900-MHz and 1800-MHz bands in Europe and Asia, and in the 1900-MHz (sometimes referred to as 1.9-GHz) band in the United States. It is used in digital cellular and PCS-based systems. GSM is also the basis for Integrated Digital Enhanced Network (IDEN), a popular system introduced by Motorola and used by Nextel.

    GSM is the international standard in Europe, Australia and much of Asia and Africa. In covered areas, cell-phone users can buy one phone that will work anywhere where the standard is supported. To connect to the specific service providers in these different countries, GSM users simply switch subscriber identification module (SIM) cards. SIM cards are small removable disks that slip in and out of GSM cell phones. They store all the connection data and identification numbers you need to access a particular wireless service provider.

    Unfortunately, the 1900-MHz GSM phones used in the United States are not compatible with the international system. If you live in the United States and need to have cell-phone access when you're overseas, the easiest thing to do is to buy a GSM 900MHz/1800MHz cell phone for traveling. You can get these phones from Planet Omni, an online electronics firm based in California. They offer a wide selection of Nokia, Motorola and Ericsson GSM phones. They don't sell international SIM cards, however. You can pick up prepaid SIM cards for a wide range of countries at

    CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.

    All of the users transmit in the same wide-band chunk of spectrum. Each user's signal is spread over the entire bandwidth by a unique spreading code. At the receiver, that same unique code is used to recover the signal. Because CDMA systems need to put an accurate time-stamp on each piece of a signal, it references the GPS system for this information. Between eight and 10 separate calls can be carried in the same channel space as one analog AMPS call. CDMA technology is the basis for Interim Standard 95 (IS-95) and operates in both the 800-MHz and 1900-MHz frequency bands.

    Ideally, TDMA and CDMA are transparent to each other. In practice, high-power CDMA signals raise the noise floor for TDMA receivers, and high-power TDMA signals can cause overloading and jamming of CDMA receivers.

    Dual Band vs. Dual Mode
    If you travel a lot, you will probably want to look for phones that offer dual band, dual mode or both. Let's take a look at each of these options:

    Dual band - A phone that has dual-band capability can switch frequencies. This means that it can operate in both the 800-MHz and 1900-MHz bands. For example, a dual-band TDMA phone could use TDMA services in either an 800-MHz or a 1900-MHz system.
    Dual mode- In cell phones, "mode" refers to the type of transmission technology used. So, a phone that supported AMPS and TDMA could switch back and forth as needed. It's important that one of the modes is AMPS -- this gives you analog service if you are in an area that doesn't have digital support.
    Dual band/Dual mode - The best of both worlds allows you to switch between frequency bands and transmission modes as needed.

    Changing bands or modes is done automatically by phones that support these options. Usually the phone will have a default option set, such as 1900-MHz TDMA, and will try to connect at that frequency with that technology first. If it supports dual bands, it will switch to 800 MHz if it cannot connect at 1900 MHz. And if the phone supports more than one mode, it will try the digital mode(s) first, then switch to analog.

    Sometimes you can even find tri-mode phones. This term can be deceptive. It may mean that the phone supports two digital technologies, such as CDMA and TDMA, as well as analog. But it can also mean that it supports one digital technology in two bands and also offers analog support. A popular version of the tri-mode type of phone for people who do a lot of international traveling has GSM service in the 900-MHz band for Europe and Asia and the 1900-MHz band for the United States, in addition to the analog service.
    Collection of my useful Threads - All in One

  2. #2
    Silver Member prasannaganesh's Avatar
    Join Date
    Nov 2008


    took lot of time to read..
    but worth it.
    i thank you for the info.
    keep posting.

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