NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite ASSIGNMENT 1 CELLULAR TECHNOLOGYAgdon, Berverlyn B. October 03, 2011Communications 1/ BSECE 41A1 Score: Engr. Grace Ramones Instructor
A mobile phone allows calls into the public switched telephone system over a radio link. Earlymobile phones were usually bulky and permanently installed in vehicles; they provided limitedservice because only a few frequencies were available for a geographic area. Modern cellular"cell" phones or hand phones make use of the cellular network concept, where frequencies arere-used repeatedly within a city area, allowing many more users to share access to the radiobandwidth. A mobile phone allows calls to be placed over a wide geographic area; generally theuser is a subscriber to the phone service and does not own the base station. By contrast, acordless telephone is used only within the range of a single, private base station.A mobile phone can make and receive telephone calls to and from the public telephone networkwhich includes other mobiles and fixed-line phones across the world. It does this by connectingto a cellular network provided by a mobile network operator.In addition to telephony, modern mobile phones also support a wide variety of other servicessuch as text messaging, MMS, email, Internet access, short-range wireless communications(infrared, Bluetooth), business applications, gaming and photography. Mobile phones that offerthese more general computing capabilities are referred to as smartphones.The first hand-held mobile phone was demonstrated by Dr Martin Cooper of Motorola in 1973,using a handset weighing 2 1/2 lbs (about 1 kg). In 1983, the DynaTAC 8000x was the first to becommercially available. In the twenty years from 1990 to 2010, worldwide mobile phonesubscriptions grew from 12.4 million to over 4.6 billion, penetrating the developing economiesand reaching the bottom of the economic pyramid
Structure of the mobile phone cellular networkA simple view of the cellular mobile-radio network consists of the following: A network of Radio base stations forming the Base station subsystem. The core circuit switched network for handling voice calls and text A packet switched network for handling mobile data The Public switched telephone network to connect subscribers to the wider telephony networkThis network is the foundation of the GSM system network. There are many functions that areperformed by this network in order to make sure customers get the desired service includingmobility management, registration, call set up, and handover.Any phone connects to the network via an RBS (Radio Base Station) at a corner of thecorresponding cell which in turn connects to the Mobile switching center (MSC). The MSCprovides a connection to the public switched telephone network (PSTN). The link from a phoneto the RBS is called an uplink while the other way is termed downlink.Radio channels effectively use the transmission medium through the use of the followingmultiplexing schemes: frequency division multiplex (FDM), time division multiplex (TDM), codedivision multiplex (CDM), and space division multiplex (SDM). Corresponding to thesemultiplexing schemes are the following access techniques: frequency division multiple access(FDMA), time division multiple access (TDMA), code division multiple access (CDMA), andspace division multiple access (SDMA).
Cellular handover in mobile phone networksAs the phone user moves from one cell area to another cell whilst a call is in progress, themobile station will search for a new channel to attach to in order not to drop the call. Once anew channel is found, the network will command the mobile unit to switch to the new channeland at the same time switch the call onto the new channel.With CDMA, multiple CDMA handsets share a specific radio channel. The signals are separatedby using a pseudonoise code (PN code) specific to each phone. As the user moves from onecell to another, the handset sets up radio links with multiple cell sites (or sectors of the samesite) simultaneously. This is known as "soft handoff" because, unlike with traditional cellulartechnology, there is no one defined point where the phone switches to the new cell.In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically beimpossible to test the target channel directly while communicating. In this case other techniqueshave to be used such as pilot beacons in IS-95. This means that there is almost always a briefbreak in the communication while searching for the new channel followed by the risk of anunexpected return to the old channel.If there is no ongoing communication or the communication can be interrupted, it is possible forthe mobile unit to spontaneously move from one cell to another and then notify the base stationwith the strongest signal.
Cellular frequency choice in mobile phone networksThe effect of frequency on cell coverage means that different frequencies serve better fordifferent uses. Low frequencies, such as 450 MHz NMT, serve very well for countrysidecoverage. GSM 900 (900 MHz) is a suitable solution for light urban coverage. GSM 1800(1.8 GHz) starts to be limited by structural walls. UMTS, at 2.1 GHz is quite similar in coverageto GSM 1800.Higher frequencies are a disadvantage when it comes to coverage, but it is a decidedadvantage when it comes to capacity. Pico cells, covering e.g. one floor of a building, becomepossible, and the same frequency can be used for cells which are practically neighbours.Cell service area may also vary due to interference from transmitting systems, both within andaround that cell. This is true especially in CDMA based systems. The receiver requires a certainsignal-to-noise ratio. As the receiver moves away from the transmitter, the power transmitted isreduced. As the interference (noise) rises above the received power from the transmitter, andthe power of the transmitter cannot be increased any more, the signal becomes corrupted andeventually unusable. In CDMA-based systems, the effect of interference from other mobiletransmitters in the same cell on coverage area is very marked and has a special name, cellbreathing.One can see examples of cell coverage by studying some of the coverage maps provided byreal operators on their web sites. In certain cases they may mark the site of the transmitter, inothers it can be calculated by working out the point of strongest coverage.
Coverage comparison of different frequenciesThe conceptIn a cellular radio system, a land area to be supplied with radio service is divided into regularshaped cells, which can be hexagonal, square, circular or some other irregular shapes, althoughhexagonal cells are conventional. Each of these cells is assigned multiple frequencies (f1 - f6)which have corresponding radio base stations. The group of frequencies can be reused in othercells, provided that the same frequencies are not reused in adjacent neighboring cells as thatwould cause co-channel interference.The increased capacity in a cellular network, compared with a network with a single transmitter,comes from the fact that the same radio frequency can be reused in a different area for acompletely different transmission. If there is a single plain transmitter, only one transmission canbe used on any given frequency. Unfortunately, there is inevitably some level of interferencefrom the signal from the other cells which use the same frequency. This means that, in astandard FDMA system, there must be at least a one cell gap between cells which reuse thesame frequency.In the simple case of the taxi company, each radio had a manually operated channel selectorknob to tune to different frequencies. As the drivers moved around, they would change fromchannel to channel. The drivers knew which frequency covered approximately what area. Whenthey did not receive a signal from the transmitter, they would try other channels until they foundone that worked. The taxi drivers would only speak one at a time, when invited by the basestation operator (in a sense TDMA).
Frequency reuseThe increased capacity in a cellular network, comparing to a network with a single transmitter,comes from the fact that the same radio frequency can be reused in a different area for acompletely different transmission. If there is a single plain transmitter, only one transmissioncan be used on any given frequency. Unfortunately, there is inevitably some level ofinterference from the signal from the other cells which use the same frequency. This meansthat, in a standard FDMA system, there must be at least a one cell gap between cells whichreuse the same frequency.The frequency reuse factor is the rate at which the same frequency can be used in the network.It is 1/n where n is the number of cells which cannot use a frequency for transmission.Code division multiple access based systems use a wider frequency band to achieve the samerate of transmission as FDMA, but this is compensated for by the ability to use a frequencyreuse factor of 1. In other words, every cell uses the same frequency and the different systemsare separated by codes rather than frequencies.Depending on the size of the city, a taxi system may not have any frequency reuse in its owncity, but certainly in other nearby cities, the same frequency can be used. In a big city, on theother hand, frequency reuse could certainly be in use.
Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of oneor several frequency bands, or channels. It is particularly commonplace in satellitecommunication. FDMA, like other Multiple Access systems, coordinates access betweenmultiple users. Alternatives include TDMA, CDMA, or SDMA. These protocols are utilizeddifferently, at different levels of the theoreticalOSI model.Disadvantage: Crosstalk may cause interference among frequencies and disrupt thetransmission.FREQUENCY DIVISION MULTIPLE ACCESSFDMA is distinct from frequency division duplexing (FDD). While FDMA allows multiple userssimultaneous access to a transmission system, FDD refers to how the radio channel is sharedbetween the uplink and downlink (for instance, the traffic going back and forth between amobile-phone and a mobile phone base station). Frequency-division multiplexing (FDM) is alsodistinct from FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on the other hand, is an accessmethod in the data link layer.FDMA also supports demand assignment in addition to fixed assignment. Demandassignment allows all users apparently continuous access of the radio spectrum by assigningcarrier frequencies on a temporary basis using a statistical assignment process. The firstFDMA demand-assignment system for satellite was developed byCOMSAT for use onthe Intelsat series IVA and V satellites.There are two main techniques: Multi-channel per-carrier (MCPC) Single-channel per-carrier (SCPC)
Time division multiple access (TDMA) is a channel access method for shared medium networks.It allows several users to share the same frequency channel by dividing the signal into differenttime slots. The users transmit in rapid succession, one after the other, each using its own timeslot. This allows multiple stations to share the same transmission medium (e.g. radio frequencychannel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellularsystems such as Global System for Mobile Communications (GSM), IS-136, Personal DigitalCellular (PDC) and iDEN, and in the Digital Enhanced Cordless Telecommunications (DECT)standard for portable phones. It is also used extensively in satellite systems, combat-netradio systems, and PON networks for upstream traffic from premises to the operator. For usageof Dynamic TDMA packet mode communication.TDMA is a type of Time-division multiplexing, with the special point that instead of havingone transmitter connected to one receiver, there are multiple transmitters. In the case ofthe uplink from a mobile phone to abase station this becomes particularly difficult because themobile phone can move around and vary the timing advance required to make its transmissionmatch the gap in transmission from its peers.Code division multiple access (CDMA) is a channel access method used by various radiocommunication technologies. It should not be confused with the mobile phonestandards called cdmaOne, CDMA2000 (the 3G evolution of cdmaOne) and WCDMA (the 3Gstandard used by GSM carriers), which are often referred to as simply CDMA, and use CDMAas an underlying channel access method.Space-Division Multiple Access (SDMA) is a channel access method based on creating parallelspatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by whichit is able to offer superior performance in radio multiple access communication systems. Intraditional mobile cellular network systems, the base station has no information on the positionof the mobile units within the cell and radiates the signal in all directions within the cell in orderto provide radio coverage. This results in wasting power on transmissions when there are nomobile units to reach, in addition to causing interference for adjacent cells using the samefrequency, so calledco-channel cells. Likewise, in reception, the antenna receives signalscoming from all directions including noise and interference signals. By using smartantenna technology and differing spatial locations of mobile units within the cell, space-divisionmultiple access techniques offer attractive performance enhancements. The radiation pattern ofthe base station, both in transmission and reception, is adapted to each user to obtain highestgain in the direction of that user. This is often done using phased arraytechniques.
The Major Advantages of Cellular TechnologyApps for mobile phones is one of the major advantages to being mobile and on the go,especially to places we are unfamiliar with, is what is known as the GPS, or global positioningsystem. Some of the more sophisticated cellular devices come with a GPS option, so you canalways find your way. Today’s GPS is like road maps used to be without all the clutter andinformation that you don’t need. Cellular technology is entertaining. Though you can tell a joke through texting or live during aphone call, the other possibilities are endless: music, music videos, movies, photographs, videogames, word games and television programs are just a few of the ways that you can keepyourself entertained. They key to this technology is how long your batter life is and keepingyour cellular device charged.None of this would have been possible without cellular technology, which is practicallysynonymous with being on the move. Being on the move is total freedom, not being tied to aspecific location for anything. In the past, vocations and careers stated that we sometimes hadto stay in one location and for some, that is still the case. But most of us can work, beentertained and socialize from the road thanks to cellular devices.