93-394: In cellular telecommunications , handover , or handoff , is the process of transferring an ongoing call or data session from one channel connected to the core network to another channel. In satellite communications it is the process of transferring satellite control responsibility from one earth station to another without loss or interruption of service. American English uses
186-487: A pseudonoise code (PN code) that is specific to each phone. As the user moves from one cell to another, the handset sets up radio links with multiple cell sites (or sectors of the same site) simultaneously. This is known as "soft handoff" because, unlike with traditional cellular technology , 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 be impossible to test
279-441: A 12-digit number sent by the handset to the cellular system for billing purposes, uniquely identified that phone on the network. The system then allowed or disallowed calls and/or features based on its customer file. A person intercepting an ESN/MDN pair could clone the combination onto a different phone and use it in other areas for making calls without paying. Cellular phone cloning became possible with off-the-shelf technology in
372-401: A cellular network, compared with a network with a single transmitter, comes from the mobile communication switching system developed by Amos Joel of Bell Labs that permitted multiple callers in a given area to use the same frequency by switching calls to the nearest available cellular tower having that frequency available. This strategy is viable because a given radio frequency can be reused in
465-519: A cooperative effort between Bell Labs and Motorola. It was officially introduced in the Americas on October 13, 1983, and was deployed in many other countries too, including Israel in 1986, Australia in 1987, Singapore in 1988, and Pakistan in 1990. It was the primary analog mobile phone system in North America (and other locales) through the 1980s and into the 2000s. As of February 18, 2008, carriers in
558-426: A different area for an unrelated transmission. In contrast, a single transmitter can only handle one transmission for a given frequency. Inevitably, there is some level of interference from the signal from the other cells which use the same frequency. Consequently, there must be at least one cell gap between cells which reuse the same frequency in a standard frequency-division multiple access (FDMA) system. Consider
651-513: A frequency reuse of 1. Since such systems do not spread the signal across the frequency band, inter-cell radio resource management is important to coordinate resource allocation between different cell sites and to limit the inter-cell interference. There are various means of inter-cell interference coordination (ICIC) already defined in the standard. Coordinated scheduling, multi-site MIMO or multi-site beamforming are other examples for inter-cell radio resource management that might be standardized in
744-499: A handover may be necessary. The downlink (forward link) and/or uplink (reverse link) directions may be monitored. The handover may be requested by the phone or by the base station (BTS) of its source cell and, in some systems, by a BTS of a neighboring cell. The phone and the BTSes of the neighboring cells monitor each other's signals and the best target candidates are selected among the neighboring cells. In some systems, mainly based on CDMA,
837-399: A large number of active phones in that area. All of the cell sites are connected to telephone exchanges (or switches), which in turn connect to the public telephone network . In cities, each cell site may have a range of up to approximately 1 ⁄ 2 mile (0.80 km), while in rural areas, the range could be as much as 5 miles (8.0 km). It is possible that in clear open areas,
930-468: A large number of users. In general terms, AMPS was very similar to the older "0G" Improved Mobile Telephone Service it replaced, but used considerably more computing power to select frequencies, hand off conversations to land lines , and handle billing and call setup. What really separated AMPS from older systems is the "back end" call setup functionality. In AMPS, the cell centers could flexibly assign channels to handsets based on signal strength, allowing
1023-468: A limited, shared resource. Cell-sites and handsets change frequency under computer control and use low power transmitters so that the usually limited number of radio frequencies can be simultaneously used by many callers with less interference. A cellular network is used by the mobile phone operator to achieve both coverage and capacity for their subscribers. Large geographic areas are split into smaller cells to avoid line-of-sight signal loss and to support
SECTION 10
#17328450595591116-650: A minimum of three channels, and three towers for each cell and greatly increases the chances of receiving a usable signal from at least one direction. The numbers in the illustration are channel numbers, which repeat every 3 cells. Large cells can be subdivided into smaller cells for high volume areas. Cell phone companies also use this directional signal to improve reception along highways and inside buildings like stadiums and arenas. Practically every cellular system has some kind of broadcast mechanism. This can be used directly for distributing information to multiple mobiles. Commonly, for example in mobile telephony systems,
1209-402: A mobile phone network manages handover). The most common example of a cellular network is a mobile phone (cell phone) network. A mobile phone is a portable telephone which receives or makes calls through a cell site (base station) or transmitting tower. Radio waves are used to transfer signals to and from the cell phone. Modern mobile phone networks use cells because radio frequencies are
1302-443: A new clearer or less fading channel. In addition to the above classification of inter-cell and intra-cell classification of handovers, they also can be divided into hard and soft handovers: Handover can also be classified on the basis of handover techniques used. Broadly they can be classified into three types: An advantage of the hard handover is that at any moment in time one call uses only one channel. The hard handover event
1395-455: A number of desirable features: Major telecommunications providers have deployed voice and data cellular networks over most of the inhabited land area of Earth . This allows mobile phones and mobile computing devices to be connected to the public switched telephone network and public Internet access . Private cellular networks can be used for research or for large organizations and fleets, such as dispatch for local public safety agencies or
1488-412: A number of frequency channels corresponding to a bandwidth of B/K , and each sector can use a bandwidth of B/NK . Code-division multiple access -based systems use a wider frequency band to achieve the same rate of transmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1, for example using a reuse pattern of 1/1. In other words, adjacent base station sites use
1581-568: A patent for this work in 1973. The first call on the prototype connected, reportedly, to a wrong number. While Motorola was developing a cellular phone, from 1968 to 1983 Bell Labs worked out a system called Advanced Mobile Phone System (AMPS), which became the first cellular network standard in the United States. The Bell system deployed ASTM in Chicago, Illinois, first as an equipment test serving approximately 100 units in 1978, and subsequently as
1674-516: A service test planned for 2,000 billed units. Motorola and others designed and built the cellular phones for this and other cellular systems. Louis M. Weinberg, a marketing director at AT&T, was named the first president of the AMPS corporation. He served in this position during the startup of the AMPS subsidiary of AT&T. Martin Cooper , a former general manager for the systems division at Motorola, led
1767-458: A soft handover, is referred to as the active set . If the search finger finds a sufficiently-strong signal (in terms of high Ec/Io or RSCP) from a new cell this cell is added to the active set. The cells in the neighbour list (called in CDMA neighbouring set ) are checked more frequently than the rest and thus a handover with a neighbouring cell is more likely, however a handover with others cells outside
1860-475: A target candidate may be selected among the cells which are not in the neighbor list. This is done in an effort to reduce the probability of interference due to the aforementioned near–far effect. In analog systems the parameters used as criteria for requesting a hard handover are usually the received signal power and the received signal-to-noise ratio (the latter may be estimated in an analog system by inserting additional tones, with frequencies just outside
1953-399: A taxicab company, as well as for local wireless communications in enterprise and industrial settings such as factories, warehouses, mines, power plants, substations, oil and gas facilities and ports. In a cellular radio system, a land area to be supplied with radio service is divided into cells in a pattern dependent on terrain and reception characteristics. These cell patterns roughly take
SECTION 20
#17328450595592046-465: A team that produced the first cellular handset in 1973 and made the first phone call from it. In 1983 Motorola introduced the DynaTAC 8000x , the first commercially available cellular phone small enough to be easily carried. He later introduced the so-called Bag Phone . In 1992, the first smartphone , called IBM Simon , used AMPS. Frank Canova led its design at IBM and it was demonstrated that year at
2139-430: A temporary interruption to the call. One advantage of the soft handovers is that the connection to the source cell is broken only when a reliable connection to the target cell has been established and therefore the chances that the call will be terminated abnormally due to failed handovers are lower. However, by far a bigger advantage comes from the mere fact that simultaneously channels in multiple cells are maintained and
2232-526: A time when invited by the base station operator. This is a form of time-division multiple access (TDMA). The history of cellular phone technology began on December 11, 1947 with an internal memo written by Douglas H. Ring , a Bell Labs engineer in which he proposed development of a cellular telephone system by AT&T. The first commercial cellular network, the 1G generation, was launched in Japan by Nippon Telegraph and Telephone (NTT) in 1979, initially in
2325-416: A user may receive signals from a cell site 25 miles (40 km) away. In rural areas with low-band coverage and tall towers, basic voice and messaging service may reach 50 miles (80 km), with limitations on bandwidth and number of simultaneous calls. Since almost all mobile phones use cellular technology , including GSM , CDMA , and AMPS (analog), the term "cell phone" is in some regions, notably
2418-696: A user moves into a cell when all available channels are in use, the user's call must be terminated. Also, there is the problem of signal interference where adjacent cells overpower each other resulting in receiver desensitization. There are also inter-technology handovers where a call's connection is transferred from one access technology to another, e.g. a call being transferred from GSM to UMTS or from CDMA IS-95 to CDMA2000 . The 3GPP UMA/GAN standard enables GSM/UMTS handoff to Wi-Fi and vice versa. Different systems have different methods for handling and managing handoff request. Some systems handle handoff in same way as they handle new originating call. In such system
2511-408: A wide geographic area. This enables numerous portable transceivers (e.g., mobile phones , tablets and laptops equipped with mobile broadband modems , pagers , etc.) to communicate with each other and with fixed transceivers and telephones anywhere in the network, via base stations, even if some of the transceivers are moving through more than one cell during transmission. Cellular networks offer
2604-598: Is a telecommunications network where the link to and from end nodes is wireless and the network is distributed over land areas called cells , each served by at least one fixed-location transceiver (such as a base station ). These base stations provide the cell with the network coverage which can be used for transmission of voice, data, and other types of content. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed service quality within each cell. When joined together, these cells provide radio coverage over
2697-1096: Is a generic term that applies to many 2G cellular systems). D-AMPS, commercially deployed since 1993, was a digital , 2G standard used mainly by AT&T Mobility and U.S. Cellular in the United States, Rogers Wireless in Canada, Telcel in Mexico, Telecom Italia Mobile (TIM) in Brazil, VimpelCom in Russia, Movilnet in Venezuela, and Cellcom in Israel. In most areas, D-AMPS is no longer offered and has been replaced by more advanced digital wireless networks. AMPS and D-AMPS have now been phased out in favor of either CDMA2000 or GSM , which allow for higher capacity data transfers for services such as WAP , Multimedia Messaging System (MMS), and wireless Internet access. There are some phones capable of supporting AMPS, D-AMPS and GSM all in one phone (using
2790-550: Is based on spread spectrum technology developed for military use during World War II and improved during the Cold War into direct-sequence spread spectrum that was used for early CDMA cellular systems and Wi-Fi . DSSS allows multiple simultaneous phone conversations to take place on a single wideband RF channel, without needing to channelize them in time or frequency. Although more sophisticated than older multiple access schemes (and unfamiliar to legacy telephone companies because it
2883-424: Is called the handover or handoff. Typically, a new channel is automatically selected for the mobile unit on the new base station which will serve it. The mobile unit then automatically switches from the current channel to the new channel and communication continues. The exact details of the mobile system's move from one base station to the other vary considerably from system to system (see the example below for how
Handover - Misplaced Pages Continue
2976-404: Is indeed very short and usually is not perceptible by the user. In the old analog systems it could be heard as a click or a very short beep; in digital systems it is unnoticeable. Another advantage of the hard handover is that the phone's hardware does not need to be capable of receiving two or more channels in parallel, which makes it cheaper and simpler. A disadvantage is that if a handover fails
3069-402: Is interfered or fading, soft handovers bring a significant improvement to the reliability of the calls in these places by making the interference or the fading in a single channel not critical. This advantage comes at the cost of more complex hardware in the phone, which must be capable of processing several channels in parallel. Another price to pay for soft handovers is use of several channels in
3162-466: Is more common in academic research publications and literature, while handoff is slightly more common within the IEEE and ANSI organisations. In telecommunications there may be different reasons why a handover might be conducted: The most basic form of handover is when a phone call in progress is redirected from its current cell (called source ) to a new cell (called target ). In terrestrial networks
3255-478: Is no problem with two cells sufficiently far apart operating on the same frequency, provided the masts and cellular network users' equipment do not transmit with too much power. The elements that determine frequency reuse are the reuse distance and the reuse factor. The reuse distance, D is calculated as where R is the cell radius and N is the number of cells per cluster. Cells may vary in radius from 1 to 30 kilometres (0.62 to 18.64 mi). The boundaries of
3348-480: The 2G (second-generation) technologies have this feature (e.g. GSM, D-AMPS / IS-136 , etc.). On the other hand, all CDMA based technologies, 2G and 3G (third-generation), have soft handovers. On one hand, this is facilitated by the possibility to design not so expensive phone hardware supporting soft handovers for CDMA and on the other hand, this is necessitated by the fact that without soft handovers CDMA networks may suffer from substantial interference arising due to
3441-521: The COMDEX computer-industry trade-show. A refined version of the product was marketed to consumers in 1994 by BellSouth under the name Simon Personal Communicator . The Simon was the first device that can be properly referred to as a "smartphone", even though that term was not yet coined. AMPS is a first-generation cellular technology that uses separate frequencies , or "channels", for each conversation. It therefore required considerable bandwidth for
3534-492: The GAIT standard). In 2002, the FCC decided to no longer require A and B carriers to support AMPS service as of February 18, 2008. All AMPS carriers have converted to a digital standard such as CDMA2000 or GSM. Digital technologies such as GSM and CDMA2000 support multiple voice calls on the same channel and offer enhanced features such as two-way text messaging and data services. Unlike in
3627-618: The UMTS system where it allows for low downlink latency in packet-based connections. In LTE/4G, the Paging procedure is initiated by the MME when data packets need to be delivered to the UE. Paging types supported by the MME are: In a primitive taxi system, when the taxi moved away from a first tower and closer to a second tower, the taxi driver manually switched from one frequency to another as needed. If communication
3720-506: The 1990s. Would-be cloners required three key items : The radio, when tuned to the proper frequency, would receive the signal transmitted by the cell phone to be cloned, containing the phone's ESN/MDN pair. This signal would feed into the sound-card audio-input of the PC, and Banpaia would decode the ESN/MDN pair from this signal and display it on the screen. The hacker could then copy that data into
3813-514: The 850 MHz Cellular band. For each market area, the United States Federal Communications Commission (FCC) allowed two licensees (networks) known as "A" and "B" carriers. Each carrier within a market used a specified "block" of frequencies consisting of 21 control channels and 395 voice channels. Originally, the B (wireline) side license was usually owned by the local phone company, and the A (non-wireline) license
Handover - Misplaced Pages Continue
3906-575: The Bell System , with cellular assets transferred to the Regional Bell Operating Companies . The wireless revolution began in the early 1990s, leading to the transition from analog to digital networks . The MOSFET invented at Bell Labs between 1955 and 1960, was adapted for cellular networks by the early 1990s, with the wide adoption of power MOSFET , LDMOS ( RF amplifier ), and RF CMOS ( RF circuit ) devices leading to
3999-454: The Oki 900 phone and reboot it, after which the phone network could not distinguish the Oki from the original phone whose signal had been received. This gave the cloner, through the Oki phone, the ability to use the mobile-phone service of the legitimate subscriber whose phone was cloned – just as if that phone had been physically stolen, except that the subscriber retained his or her phone, unaware that
4092-516: The US which prohibited the FCC type acceptance and sale of any receiver which could tune the frequency ranges occupied by analog AMPS cellular services. Though the service is no longer offered, these laws remain in force (although they may no longer be enforced). In 1991, Motorola proposed an AMPS enhancement known as narrowband AMPS (NAMPS or N-AMPS). Later, many AMPS networks were partially converted to D-AMPS , often referred to as TDMA (though TDMA
4185-709: The US, used interchangeably with "mobile phone". However, satellite phones are mobile phones that do not communicate directly with a ground-based cellular tower but may do so indirectly by way of a satellite. There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), cdmaOne , CDMA2000 , Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). The transition from existing analog to
4278-662: The United States were no longer required to support AMPS and companies such as AT&T and Verizon Communications have discontinued this service permanently. AMPS was discontinued in Australia in September 2000, in India by October 2004, in Israel by January 2010, and Brazil by 2010. The first cellular network efforts began at Bell Labs and with research conducted at Motorola . In 1960, John F. Mitchell became Motorola's chief engineer for its mobile-communication products, and oversaw
4371-476: The United States, the Canadian Radio-television and Telecommunications Commission (CRTC) and Industry Canada have not set any requirement for maintaining AMPS service in Canada. Rogers Wireless has dismantled their AMPS (along with IS-136 ) network; the networks were shut down May 31, 2007. Bell Mobility and Telus Mobility , who operated AMPS networks in Canada, announced that they would observe
4464-464: The base stations, paired with 416 frequencies in the 869–894 MHz range for transmissions from base stations to the mobile stations. Each cell site used a different subset of these channels than its neighbors to avoid interference. This significantly reduced the number of channels available at each site in real-world systems. Each AMPS channel had a one way bandwidth of 30 kHz, for a total of 60 kHz for each duplex channel. Laws were passed in
4557-467: The basis of 3G ). With FDMA, the transmitting and receiving frequencies used by different users in each cell are different from each other. Each cellular call was assigned a pair of frequencies (one for base to mobile, the other for mobile to base) to provide full-duplex operation. The original AMPS systems had 666 channel pairs, 333 each for the CLEC "A" system and ILEC "B" system. The number of channels
4650-459: The battery and/or antenna. The Oki 900 could listen in to AMPS phone-calls right out-of-the-box with no hardware modifications. AMPS was originally standardized by American National Standards Institute (ANSI) as EIA/TIA/IS-3. EIA/TIA/IS-3 was superseded by EIA/TIA-553 and TIA interim standard with digital technologies, the cost of wireless service is so low that the problem of cloning has virtually disappeared. AMPS cellular service operated in
4743-454: The call as the subscriber is moving out of the area covered by the source cell and entering the area of the target cell. A special case is possible, in which the source and the target are one and the same cell and only the used channel is changed during the handover. Such a handover, in which the cell is not changed, is called intra-cell handover. The purpose of intra-cell handover is to change one channel, which may be interfered or fading with
SECTION 50
#17328450595594836-498: The call could only fail if all of the channels are interfered or fade at the same time. Fading and interference in different channels are unrelated and therefore the probability of them taking place at the same moment in all channels is very low. Thus the reliability of the connection becomes higher when the call is in a soft handover. Because in a cellular network the majority of the handovers occur in places of poor coverage, where calls would frequently become unreliable when their channel
4929-399: The call may be temporarily disrupted or even terminated abnormally. Technologies which use hard handovers, usually have procedures which can re-establish the connection to the source cell if the connection to the target cell cannot be made. However re-establishing this connection may not always be possible (in which case the call will be terminated) and even when possible the procedure may cause
5022-515: The captured voice-frequency band at the transmitter and assessing the form of these tones at the receiver). In non-CDMA 2G digital systems the criteria for requesting hard handover may be based on estimates of the received signal power, bit error rate (BER) and block error/erasure rate (BLER), received quality of speech ( RxQual ), distance between the phone and the BTS (estimated from the radio signal propagation delay) and others. In CDMA systems, 2G and 3G,
5115-405: The case of a taxi company, where each radio has a manually operated channel selector knob to tune to different frequencies. As drivers move around, they change from channel to channel. The drivers are aware of which frequency approximately covers some area. When they do not receive a signal from the transmitter, they try other channels until finding one that works. The taxi drivers only speak one at
5208-493: The cells can also overlap between adjacent cells and large cells can be divided into smaller cells. The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12, depending on notation). In case of N sector antennas on
5301-418: The centers of the cells and were omnidirectional, a cellular map can be redrawn with the cellular telephone towers located at the corners of the hexagons where three cells converge. Each tower has three sets of directional antennas aimed in three different directions with 120 degrees for each cell (totaling 360 degrees) and receiving/transmitting into three different cells at different frequencies. This provides
5394-639: The control centers. Cellular companies who own an A or B license (such as Verizon and Alltel) were required to provide analog service until February 18, 2008. After that point, however, most cellular companies were eager to shut down AMPS and use the remaining channels for digital services. OnStar transitioned to digital service with the help of data transport technology developed by Airbiquity, but warned customers who could not be upgraded to digital service that their service would permanently expire on January 1, 2008. Telstra (formerly Telecom Australia) – operated an AMPS network in Australia from February 1987 until
5487-506: The cost of implementing them for analog technologies is prohibitively high and none of the technologies that were commercially successful in the past (e.g. AMPS , TACS , NMT , etc.) had this feature. Of the digital technologies, those based on FDMA also face a higher cost for the phones (due to the need to have multiple parallel radio-frequency modules) and those based on TDMA or a combination of TDMA/FDMA, in principle, allow not so expensive implementation of soft handovers. However, none of
5580-500: The desired service including mobility management, registration, call set-up, and handover . Any phone connects to the network via an RBS ( Radio Base Station ) at a corner of the corresponding cell which in turn connects to the Mobile switching center (MSC). The MSC provides a connection to the public switched telephone network (PSTN). The link from a phone to the RBS is called an uplink while
5673-458: The development and marketing of the first pager to use transistors. Motorola had long produced mobile telephones for automobiles, but these large and heavy models consumed too much power to allow their use without the automobile's engine running. Mitchell's team, which included Dr. Martin Cooper , developed portable cellular telephony. Cooper and Mitchell were among the Motorola employees granted
SECTION 60
#17328450595595766-615: The development and proliferation of digital wireless mobile networks. The first commercial digital cellular network, the 2G generation, was launched in 1991. This sparked competition in the sector as the new operators challenged the incumbent 1G analog network operators. To distinguish signals from several different transmitters, a number of channel access method s have been developed, including frequency-division multiple access (FDMA, used by analog and D-AMPS systems), time-division multiple access (TDMA, used by GSM ) and code-division multiple access (CDMA, first used for PCS , and
5859-589: The digital standard followed a very different path in Europe and the US . As a consequence, multiple digital standards surfaced in the US, while Europe and many countries converged towards the GSM standard. A simple view of the cellular mobile-radio network consists of the following: This network is the foundation of the GSM system network. There are many functions that are performed by this network in order to make sure customers get
5952-487: The end of 2000. As part of the introduction of mobile phone competition in Australia, the Australian government mandated GSM as the new standard for mobile networks, and required that Telstra close the AMPS network by 2000. However, GSM base stations could only serve a limited area. While this was OK for Europe, it meant that GSM could not cover large, sparsely populated rural areas of Australia cost effectively. Telstra deployed
6045-423: The final 832 (416 pairs per carrier). The additional frequencies were from the band held in reserve for future (inevitable) expansion. These frequencies were immediately adjacent to the existing cellular band. These bands had previously been allocated to UHF TV channels 70–83. Each duplex channel was composed of 2 frequencies . 416 of these were in the 824–849 MHz range for transmissions from mobile stations to
6138-445: The form of regular shapes, such as hexagons, squares, or circles although hexagonal cells are conventional. Each of these cells is assigned with multiple frequencies ( f 1 – f 6 ) which have corresponding radio base stations . The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent cells, which would cause co-channel interference . The increased capacity in
6231-593: The future. Cell towers frequently use a directional signal to improve reception in higher-traffic areas. In the United States , the Federal Communications Commission (FCC) limits omnidirectional cell tower signals to 100 watts of power. If the tower has directional antennas, the FCC allows the cell operator to emit up to 500 watts of effective radiated power (ERP). Although the original cell towers created an even, omnidirectional signal, were at
6324-439: The list is called neighbor list . Creating such a list for a given cell is not trivial and specialized computer tools are used. They implement different algorithms and may use for input data from field measurements or computer predictions of radio wave propagation in the areas covered by the cells. During a call one or more parameters of the signal in the channel in the source cell are monitored and assessed in order to decide when
6417-499: The metropolitan area of Tokyo . Within five years, the NTT network had been expanded to cover the whole population of Japan and became the first nationwide 1G network. It was an analog wireless network . The Bell System had developed cellular technology since 1947, and had cellular networks in operation in Chicago and Dallas prior to 1979, but commercial service was delayed by the breakup of
6510-638: The most common criterion for requesting a handover is Ec/Io ratio measured in the pilot channel ( CPICH ) and/or RSCP . In CDMA systems, when the phone in soft or softer handover is connected to several cells simultaneously, it processes the received in parallel signals using a rake receiver . Each signal is processed by a module called rake finger . A usual design of a rake receiver in mobile phones includes three or more rake fingers used in soft handover state for processing signals from as many cells and one additional finger used to search for signals from other cells. The set of cells, whose signals are used during
6603-410: The most important use of broadcast information is to set up channels for one-to-one communication between the mobile transceiver and the base station. This is called paging . The three different paging procedures generally adopted are sequential, parallel and selective paging. The details of the process of paging vary somewhat from network to network, but normally we know a limited number of cells where
6696-446: The neighbor list is also allowed (unlike in GSM, IS-136/DAMPS, AMPS, NMT, etc.). There are occurrences where a handoff is unsuccessful. Much research has been dedicated to this problem. The source of the problem was discovered in the late 1980s. Because frequencies cannot be reused in adjacent cells, when a user moves from one cell to another, a new frequency must be allocated for the call. If
6789-438: The network to support just a single call. This reduces the number of remaining free channels and thus reduces the capacity of the network. By adjusting the duration of soft handovers and the size of the areas in which they occur, the network engineers can balance the benefit of extra call reliability against the price of reduced capacity. While theoretically speaking soft handovers are possible in any technology, analog or digital,
6882-536: The number of subscribers per cell site, greater data throughput per user, or some combination thereof. Quadrature Amplitude Modulation (QAM) modems offer an increasing number of bits per symbol, allowing more users per megahertz of bandwidth (and decibels of SNR), greater data throughput per user, or some combination thereof. The key characteristic of a cellular network is the ability to reuse frequencies to increase both coverage and capacity. As described above, adjacent cells must use different frequencies, however, there
6975-431: The other way is termed downlink . Radio channels effectively use the transmission medium through the use of the following multiplexing and access schemes: frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA), and space-division multiple access (SDMA). Small cells, which have a smaller coverage area than base stations, are categorised as follows: As
7068-508: The phone had been cloned—at least until that subscriber received his or her next bill. The problem became so large that some carriers required the use of a PIN before making calls. Eventually, the cellular companies initiated a system called RF Fingerprinting, whereby it could determine subtle differences in the signal of one phone from another and shut down some cloned phones. Some legitimate customers had problems with this though if they made certain changes to their own phone, such as replacing
7161-501: The phone is located (this group of cells is called a Location Area in the GSM or UMTS system, or Routing Area if a data packet session is involved; in LTE , cells are grouped into Tracking Areas). Paging takes place by sending the broadcast message to all of those cells. Paging messages can be used for information transfer. This happens in pagers , in CDMA systems for sending SMS messages, and in
7254-441: The phone user moves from one cell area to another cell while a call is in progress, the mobile station will search for a new channel to attach to in order not to drop the call. Once a new channel is found, the network will command the mobile unit to switch to the new channel and at the same time switch the call onto the new channel. With CDMA , multiple CDMA handsets share a specific radio channel. The signals are separated by using
7347-510: The probability that the handoff will not be served is equal to blocking probability of new originating call. But if the call is terminated abruptly in the middle of conversation then it is more annoying than the new originating call being blocked. So in order to avoid this abrupt termination of ongoing call handoff request should be given priority to new call this is called as handoff prioritization. There are two techniques for this: Cellular network A cellular network or mobile network
7440-483: The receiving end to produce a somewhat normal-sounding voice at the receiver. TDMA must introduce latency (time delay) into the audio signal. As long as the latency time is short enough that the delayed audio is not heard as an echo, it is not problematic. TDMA is a familiar technology for telephone companies, which used time-division multiplexing to add channels to their point-to-point wireline plants before packet switching rendered FDM obsolete. The principle of CDMA
7533-415: The same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3. A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 ( GSM ). If the total available bandwidth is B , each cell can only use
7626-411: The same frequencies, and the different base stations and users are separated by codes rather than frequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one sector, but rather that the entire cell bandwidth is also available to each sector individually. Recently also orthogonal frequency-division multiple access based systems such as LTE are being deployed with
7719-477: The same frequency to be re-used, without interference, if locations were separated enough. The channels were grouped so a specific set was different of the one used on the cell nearby. This allowed a larger number of phones to be supported over a geographical area. AMPS pioneers coined the term "cellular" because of its use of small hexagonal "cells" within a system. AMPS suffered from many weaknesses compared to today's digital technologies. As an analog standard, it
7812-480: The same timetable as outlined by the FCC in the United States, and as a result would not begin to dismantle their AMPS networks until after February 2008. OnStar relied heavily on North American AMPS service for its subscribers because, when the system was developed, AMPS offered the most comprehensive wireless coverage in the US. In 2006, ADT asked the FCC to extend the AMPS deadline due to many of their alarm systems still using analog technology to communicate with
7905-527: The so-called near–far effect. In all current commercial technologies based on FDMA or on a combination of TDMA/FDMA (e.g. GSM, AMPS, IS-136/DAMPS, etc.) changing the channel during a hard handover is realised by changing the pair of used transmit/receive frequencies . For the practical realisation of handovers in a cellular network each cell is assigned a list of potential target cells, which can be used for handing over calls from this source cell to them. These potential target cells are called neighbors and
7998-401: The source and the target cells may be served from two different cell sites or from one and the same cell site (in the latter case the two cells are usually referred to as two sectors on that cell site). Such a handover, in which the source and the target are different cells (even if they are on the same cell site) is called inter-cell handover. The purpose of inter-cell handover is to maintain
8091-534: The target channel directly while communicating. In this case, other techniques have to be used such as pilot beacons in IS-95. This means that there is almost always a brief break in the communication while searching for the new channel followed by the risk of an unexpected return to the old channel. Advanced Mobile Phone System Advanced Mobile Phone System ( AMPS ) was an analog mobile phone system standard originally developed by Bell Labs and later modified in
8184-522: The term handoff , and this is most commonly used within some American organizations such as 3GPP2 and in American originated technologies such as CDMA2000 . In British English the term handover is more common, and is used within international and European organisations such as ITU-T , IETF , ETSI and 3GPP , and standardised within European originated standards such as GSM and UMTS . The term handover
8277-622: Was expanded to 416 pairs per carrier, but ultimately the number of RF channels limits the number of calls that a cell site could handle. FDMA is a familiar technology to telephone companies, which used frequency-division multiplexing to add channels to their point-to-point wireline plants before time-division multiplexing rendered FDM obsolete. With TDMA, the transmitting and receiving time slots used by different users in each cell are different from each other. TDMA typically uses digital signaling to store and forward bursts of voice data that are fit into time slices for transmission, and expanded at
8370-419: Was given to wireless telephone providers. At the inception of cellular in 1983, the FCC had granted each carrier within a market 333 channel pairs (666 channels total). By the late 1980s, the cellular industry's subscriber base had grown into the millions across America and it became necessary to add channels for additional capacity. In 1989, the FCC granted carriers an expansion from the previous 666 channels to
8463-438: Was interrupted due to a loss of a signal, the taxi driver asked the base station operator to repeat the message on a different frequency. In a cellular system, as the distributed mobile transceivers move from cell to cell during an ongoing continuous communication, switching from one cell frequency to a different cell frequency is done electronically without interruption and without a base station operator or manual switching. This
8556-471: Was not developed by Bell Labs ), CDMA has scaled well to become the basis for 3G cellular radio systems. Other available methods of multiplexing such as MIMO , a more sophisticated version of antenna diversity , combined with active beamforming provides much greater spatial multiplexing ability compared to original AMPS cells, that typically only addressed one to three unique spaces. Massive MIMO deployment allows much greater channel reuse, thus increasing
8649-455: Was susceptible to static and noise, and there was no protection from 'eavesdropping' using a scanner or an older TV set that could tune into channels 70–83. In the 1990s, an epidemic of "cloning" cost the cellular carriers millions of dollars. An eavesdropper with specialized equipment could intercept a handset's ESN (Electronic Serial Number) and MDN or CTN (Mobile Directory Number or Cellular Telephone Number). The Electronic Serial Number,
#558441