Handoff

In cellular telecommunications, the term handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. In satellite communications it is the process of transferring satellite control responsibility from one earth station to another without loss or interruption of service. The British English term for transferring a cellular call is handover, which is the terminology standardised by 3GPP within such European originated technologies as GSM and UMTS.

Purpose

In telecommunications there may be different reasons why a handoff (handover) might be conducted:
* when the phone is moving away from the area covered by one cell and entering the area covered by another cell the call is transferred to the second cell in order to avoid call 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 call from a phone, which is located in an area overlapped by another cell, is transferred to that cell in order to free-up some capacity in the first cell for other users, who can only be connected to 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 call is transferred to a different channel in the same cell or to a different channel in another cell in order to avoid the interference;
* again in non-CDMA networks when the user behaviour changes, e.g. when a fast-travelling user, connected to a large, umbrella-type of cell, stops then the call may be transferred to a smaller macro cell or even to a micro cell in order to free capacity on the umbrella cell for other fast-travelling users and to reduce the potential interference to other cells or users (this works in reverse too, when a user is detected to be moving faster than a certain threshold, the call can be transferred to a larger umbrella-type of cell in order to minimise the frequency of the handoffs due to this movement);
* in CDMA networks a soft handoff (see further down) may be induced in order to reduce the interference to a smaller neighbouring cell due to the "near-far" effect even when the phone still has an excellent connection to its current cell;
* etc.

The most basic form of handoff (handover) is when a phone call in progress is redirected from its current cell (called "source") and its used channel in that cell to a new cell (called "target") and a new channel. 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 as two "sectors" on that cell site). Such a handoff, in which the source and the target are different cells (even if they are on the same cell site) is called "inter-cell" handoff. The purpose of inter-cell handoff is to maintain 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 handoff. Such a handoff, in which the cell is not changed, is called "intra-cell" handoff. The purpose of intra-cell handoff is to change one channel, which may be interfered or fading with a new clearer or less fading channel.

Types of Handoff

In addition to the above classification of "inter-cell" and "intra-cell" classification of handoffs, they also can be divided into "hard" and "soft" handoffs:
* A hard handoff 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 the source is broken before the connection to the target is made -- for this reason such handoffs are also known as "break-before-make". Hard handoffs are intended to be instantaneous in order to minimize the disruption to the call. A hard handoff is perceived by network engineers as an event during the call.
* A soft handoff 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 the target is established before the connection to the source is broken, hence this handoff 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 handoff is perceived by network engineers as a state of the call, rather than a brief event. A soft handoff may involve using connections to more than two cells, e.g. connections to three, four or more cells can be maintained by one phone at the same time. When a call is in a state of soft handoff the signal of the best of all used channels can be utilised for the call at a given moment or all the signals can be combined to 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 handoff is termed as "softer". Softer handoffs are possible when the cells involved in the handoff have a single cell site.

Comparison of Handoff

An advantage of the hard handoff is that at any moment in time one call uses only one channel. The hard handoff event 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 handoff 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 handoff fails the call may be temporarily disrupted or even terminated abnormally. Technologies, which utilise hard handoffs, 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 a temporary interruption to the call.

One advantage of the soft handoffs 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 a failed handoff are lower. However, by far a bigger advantage comes from the mere fact that simultaneously channels in multiple cells are maintained and 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 one the same moment in all channels is very low. Thus the reliability of the connection becomes higher when the call is in a soft handoff. Because in a cellular network the majority of the handoffs occur in places of poor coverage, where calls would frequently become unreliable when their channel is interfered or fading, soft handoffs 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 handoffs is use of several channels in 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 handoffs 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.

Possibility of Handoff

While theoretically speaking soft handoffs are possible in any technology, analog or digital, the cost of implementing them for analog technologies is prohibitively high and none of the commercially successful in the past technologies (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 handoffs. 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 handoffs. On one hand, this is facilitated by the possibility to design not so expensive phone hardware supporting soft handoffs for CDMA and on the other hand, this is necessitated by the fact that without soft handoffs CDMA networks may suffer from substantial interference arising due to 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 handoff is realised by changing the pair of used transmit/receive frequencies.

Implementations of Handoff

For the practical realisation of handoffs in a cellular network each cell is assigned a list of potential target cells, which can be used for handing-off calls from this source cell to them. These potential target cells are called "neighbours" and the list is called "neighbour list". Creating such a list for a given cell is not trivial and specialised 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 a handoff may be necessary. The downlink (forward link) and/or uplink (reverse link) directions may be monitored. The handoff 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 neighbouring cell. The phone and the BTSs of the neighbouring cells monitor each other others' signals and the best target candidates are selected among the neighbouring cells. In some systems, mainly based on CDMA, a target candidate may be selected among the cells which are not in the neighbour list. This is done in an effort to reduce the probability of interference due to the afore-mentioned "near-far" effect.

In analog systems the parameters used as criteria for requesting a hard handoff 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 form of these tones at the receiver). In non-CDMA 2G digital systems the criteria for requesting hard handoff 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 handoff is "Ec/Io ratio" measured in the pilot channel (CPICH) and/or RSCP.

In CDMA systems, when the phone in soft or softer handoff 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 handoff 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 a soft handoff, 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 handoff with a neighbouring cell is more likely, however a handoff with others cells outside the neighbor list is also allowed (unlike in GSM, IS-136/DAMPS, AMPS, NMT, etc.).

Vertical Handoff

There are also inter-technology handoffs 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.

ee also

* Cellular network
* Roaming
* Mobility management
* Radio resource management
* Voice call continuity
* Interleaving

External links

* [http://www.eventhelix.com/RealtimeMantra/Telecom/GSM_Handover_Call_Flow.pdf Intra-MSC GSM Handover Call Flow]
* [http://www.eventhelix.com/RealtimeMantra/Telecom/GSM_Inter_MSC_Handover_Call_Flow.pdf Inter-MSC GSM Handover Call Flow]
* [http://www.ylesstech.com/terminology.php?letter=all&id=1 Handoff protocols]