Handover

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In cellular telecommunications, handover, or handoff, is the process of transferring an ongoing Điện thoại tư vấn or data session from one channel connected to tướng the core network to tướng another channel. In satellite communications it is the process of transferring satellite control responsibility from one earth station to tướng another without loss or interruption of service.

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Terminology[edit]

American English uses 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 is more common in academic research publications and literature, while handoff is slightly more common within the IEEE and ANSI organisations.^{[original research?]}

Purpose[edit]

In telecommunications there may be different reasons why a handover might be conducted:^[1]

when the phone is moving away from the area covered by one cell and entering the area covered by another cell the Điện thoại tư vấn is transferred to tướng the second cell in order to tướng avoid Điện thoại tư vấn termination when the phone gets outside the range of the first cell;
when the capacity for connecting new calls of a given cell is used up and an existing or new Điện thoại tư vấn from a phone, which is located in an area overlapped by another cell, is transferred to tướng that cell in order to tướng free-up some capacity in the first cell for other users, who can only be connected to tướng that cell;
in non-CDMA networks when the channel used by the phone becomes interfered by another phone using the same channel in a different cell, the Điện thoại tư vấn is transferred to tướng a different channel in the same cell or to tướng a different channel in another cell in order to tướng avoid the interference;
again in non-CDMA networks when the user behaviour changes, e.g. when a fast-travelling user, connected to tướng a large, umbrella-type of cell, stops then the Điện thoại tư vấn may be transferred to tướng a smaller macro cell or even to tướng a micro cell in order to tướng không tính tiền capacity on the umbrella cell for other fast-traveling users and to tướng reduce the potential interference to tướng other cells or users (this works in reverse too, when a user is detected to tướng be moving faster kêu ca a certain threshold, the Điện thoại tư vấn can be transferred to tướng a larger umbrella-type of cell in order to tướng minimize the frequency of the handovers due to tướng this movement);
in CDMA networks a handover (see further down) may be induced in order to tướng reduce the interference to tướng a smaller neighboring cell due to tướng the "near–far" effect even when the phone still has an excellent connection to tướng its current cell.

The most basic size of handover is when a phone Điện thoại tư vấn in progress is redirected from its current cell (called source) to tướng a new cell (called target).^[1] In terrestrial networks 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 tướng 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 tướng maintain the Điện thoại tư vấn 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 tướng change one channel, which may be interfered or fading with a new clearer or less fading channel.

Types[edit]

In addition to tướng the above classification of inter-cell and intra-cell classification of handovers, they also can be divided into hard and soft handovers:^[1]

Hard handover: Is one in which the channel in the source cell is released and only then the channel in the target cell is engaged. Thus the connection to tướng the source is broken before or 'as' the connection to tướng the target is made—for this reason such handovers are also known as break-before-make. Hard handovers are intended to tướng be instantaneous in order to tướng minimize the disruption to tướng the Điện thoại tư vấn. A hard handover is perceived by network engineers as an sự kiện during the Điện thoại tư vấn. It requires the least processing by the network providing service. When the mobile is between base stations, then the mobile can switch with any of the base stations, sánh the base stations bounce the links with the mobile back and forth. This is called 'ping-ponging'.
Soft handover: Is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case the connection to tướng the target is established before the connection to tướng the source is broken, hence this handover is called make-before-break. The interval, during which the two connections are used in parallel, may be brief or substantial. For this reason the soft handover is perceived by network engineers as a state of the Điện thoại tư vấn, rather kêu ca a brief sự kiện. Soft handovers may involve using connections to tướng more kêu ca two cells: connections to tướng three, four or more cells can be maintained by one phone at the same time. When a Điện thoại tư vấn is in a state of soft handover, the signal of the best of all used channels can be used for the Điện thoại tư vấn at a given moment or all the signals can be combined to tướng produce a clearer copy of the signal. The latter is more advantageous, and when such combining is performed both in the downlink (forward link) and the uplink (reverse link) the handover is termed as softer. Softer handovers are possible when the cells involved in the handovers have a single cell site.

Handover can also be classified on the basis of handover techniques used. Broadly they can be classified into three types:

Network controlled handover
Mobile phone assisted handover
Mobile controlled handover

Comparison[edit]

An advantage of the hard handover is that at any moment in time one Điện thoại tư vấn uses only one channel. The hard handover sự kiện 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 tướng be capable of receiving two or more channels in parallel, which makes it cheaper and simpler. A disadvantage is that if a handover fails the Điện thoại tư vấn may be temporarily disrupted or even terminated abnormally. Technologies which use hard handovers, usually have procedures which can re-establish the connection to tướng the source cell if the connection to tướng the target cell cannot be made. However re-establishing this connection may not always be possible (in which case the Điện thoại tư vấn will be terminated) and even when possible the procedure may cause a temporary interruption to tướng the Điện thoại tư vấn.

One advantage of the soft handovers is that the connection to tướng the source cell is broken only when a reliable connection to tướng the target cell has been established and therefore the chances that the Điện thoại tư vấn will be terminated abnormally due to tướng failed handovers are lower. However, by far a bigger advantage comes from the mere fact that simultaneously channels in multiple cells are maintained and the Điện thoại tư vấn 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 Điện thoại tư vấn 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 is interfered or fading, soft handovers bring a significant improvement to tướng 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 tướng pay for soft handovers is use of several channels in the network to tướng tư vấn just a single Điện thoại tư vấn. This reduces the number of remaining không tính tiền 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 Điện thoại tư vấn reliability against the price of reduced capacity.

Possibility[edit]

While theoretically speaking soft handovers are possible in any technology, analog or digital, 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 tướng the need to tướng have multiple parallel radio-frequency modules) and those based on TDMA or a combination of TDMA/FDMA, in principle, allow not sánh expensive implementation of soft handovers. However, none of 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 tướng design not sánh 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 tướng 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.

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Implementations[edit]

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 tướng them. These potential target cells are called neighbors and 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 Điện thoại tư vấn one or more parameters of the signal in the channel in the source cell are monitored and assessed in order to tướng decide when 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, a target candidate may be selected among the cells which are not in the neighbor list. This is done in an effort to tướng reduce the probability of interference due to tướng 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 the captured voice-frequency band at the transmitter and assessing the size 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, 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 tướng 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 tướng tìm kiếm for signals from other cells. The mix of cells, whose signals are used during a soft handover, is referred to tướng as the active set. If the tìm kiếm finger finds a sufficiently-strong signal (in terms of high Ec/Io or RSCP) from a new cell this cell is added to tướng the active mix. The cells in the neighbour list (called in CDMA neighbouring set) are checked more frequently kêu ca the rest and thus a handover with a neighbouring cell is more likely, however a handover with others cells outside the neighbor list is also allowed (unlike in GSM, IS-136/DAMPS, AMPS, NMT, etc.).

Reasons for failure[edit]

There are occurrences where a handoff is unsuccessful. Much research has been dedicated to tướng this problem.^{[example needed]} 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 tướng another, a new frequency must be allocated for the Điện thoại tư vấn. If a user moves into a cell when all available channels are in use, the user's Điện thoại tư vấn must be terminated. Also, there is the problem of signal interference where adjacent cells overpower each other resulting in receiver desensitization.

Vertical handover[edit]

There are also inter-technology handovers where a call's connection is transferred from one access technology to tướng another, e.g. a Điện thoại tư vấn being transferred from GSM to tướng UMTS or from CDMA IS-95 to tướng cdma2000.

The 3GPP UMA/GAN standard enables GSM/UMTS handoff to tướng Wi-Fi and vice versa.

Handoff Prioritization[edit]

Different systems have different methods for handling and managing handoff request. Some systems handle handoff in same way as they handle new originating Điện thoại tư vấn. In such system the probability that the handoff will not be served is equal to tướng blocking probability of new originating Điện thoại tư vấn. But if the Điện thoại tư vấn is terminated abruptly in the middle of conversation then it is more annoying kêu ca the new originating Điện thoại tư vấn being blocked. So in order to tướng avoid this abrupt termination of ongoing Điện thoại tư vấn handoff request should be given priority to tướng new Điện thoại tư vấn this is called as handoff prioritization.

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There are two techniques for this:

Guard Channel Concept: In this technique, a fraction of the total available channels in a cell is reserved exclusively for handoff request from ongoing calls which may be handed off into the cell.
Queuing: Queuing of handoffs is possible because there is a finite time interval between the time the received signal level drops below handoff threshold and the time the Điện thoại tư vấn is terminated due to tướng insufficient signal level. The delay size is determined from the traffic pattern of a particular service area.

Inter and Intra System Handoff[edit]

Inter System Handoff

If during ongoing Điện thoại tư vấn mobile unit moves from one cellular system to tướng a different cellular system which is controlled by different MTSO, a handoff procedure which is used to tướng avoid dropping of Điện thoại tư vấn is referred as Inter System Handoff.
An MTSO engages in this handoff system. When a mobile signal becomes weak in a given cell and MTSO can not find other cell within its system to tướng which it can transfer the Điện thoại tư vấn then it uses Inter system handoff.
Before implementation of Inter System Handoff MTSO compatibility must be checked and in Inter System Handoff local Điện thoại tư vấn may become long-distance Điện thoại tư vấn.

Intra System Handoff

If during ongoing Điện thoại tư vấn mobile unit moves from one cellular system to tướng adjacent cellular system which is controlled by same MTSO, a handoff procedure which is used to tướng avoid dropping of Điện thoại tư vấn is referred as Intra System Handoff.
An MTSO engages in this handoff system. When a mobile signal becomes weak in a given cell and MTSO finds other cell within its system to tướng which it can transfer the Điện thoại tư vấn then it uses Intra system handoff.
In Intra System Handoff local calls always remain local Điện thoại tư vấn only since after handoff also the Điện thoại tư vấn is handled by same MTSO.

References[edit]

^ ^a ^b ^c Guowang Miao, Jens Zander, Ki Won Sung, and Ben Slimane, Fundamentals of Mobile Data Networks, Cambridge University Press, ISBN 1107143217, năm nhâm thìn.

External links[edit]

Look up handover in Wiktionary, the không tính tiền dictionary.

Intra-MSC GSM Handover Call Flow Archived September 22, 2020, at the Wayback Machine
Inter-MSC GSM Handover Call Flow Archived September 22, 2020, at the Wayback Machine
U.S. Patent 3,663,762: Cellular Mobile Communication System — Amos Edward Joel (Bell Labs), filed December 21, 1970, issued May 16, 1972