The handover concept
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In mobile communication handovers is refer to the transformation of an ongoing call or data from one channel to another which are connected to the core network. It enables the users of cellular technology to receive their calls anywhere and at any time so this process provides the mobility to the users, making it possible to the users to roam seamlessly from one cell to another cell. It is performed when the link quality between the base station and the mobile station on the move is decreasing from certain level of threshold.
In this process the existing link or the connection is tear down and the link is replaced by the cell to which the cellular user is handed on the cell to which the user is handed over is the target cell in this case. The network controller decides from the measurement reports about the link quality that the hand over process is needed to another cell or not. The inability of the network to make a new connection to the target cell is the handover failure. It occurs only when there are no resources or the quality of the radio link is very less from some threshold value in the target cell.
The request of the handover is same as the new call, the utilization of the resource should be optimized which is severe in order to make less the call dropping and blocking probabilities. But it is admitted that force dropping of present call is more worthy then the new call blocking.
Requirements of handovers
The handover process is required when the following situations occurs.
- When the motion of the user equipment is very fast.
- The movement of the user's equipment from one cell to another during an ongoing session.
- The experience of interference phenomena by the user's equipment from the near cell.
These are some basic points due to which the network decides that the handover process is required.
The main aim of the handover process is to allow the mobile users to roam freely from one mobile network to another either the network are same or different. To achieve the load balancing in the different cell handover is also required. Also to maintain the good radio quality of the link between the mobile users and the serving base station and to minimize the interference level.
An effectual process of the handover is necessary in the UMTS network which assures effective mobility, providing of the maintaining of ongoing session and quality of services. In addition the freedom to move with in same or different network, the balancing of load and minimization of the interference level by allowing a good connectivity of the radio link to the base stations is main results of the UMTS handovers.
Handovers Types in UMTS
The following are the different handovers types in UMTS.
- Horizontal Handovers
- Vertical Handovers
- Soft Handovers
- Hard Handovers
- Softer Handovers
- Intra System Handover
- Inter System Handover
The transformation of an ongoing session from one cell to another cell having the same access technology. For example if user equipment is connected with the radio ink with the GSM network the horizontal handover must be from GSM to GSM. Similarly the handovers between two UMTS network is the horizontal handover.
The transformation of an ongoing session or call from one cell to another cell having different access technologies. For example when a mobile user is moving from GSM based network to the UMTS network, here the access technologies are changed so the handover in this case is the vertical handover.
Loose coupling and tight coupling are the two architectures' used in the vertical handovers between UMTS and WLAN.
In the hard handovers the old radio link is released first between the user equipment and the radio network controller before the new radio link is made between the user equipment and the radio network controller. Thus the source connection is broken first and then the target connection is made, so this type of handover is also called as “break before make". These handoffs are designed to be instant in order to less the breaking of call. The network engineers felt the hard handovers as an event on the ongoing call .
In GSM system each cell has different frequencies to operate with, so these types of handoffs are used there too. Mobile users when entering the new cells that have different frequencies has to broke down the original connection and t will make a new connection with the target base station. It uses a simple algorithm, when the signal strength from the current base station decreases from the nearby cell whose signal strengths are stronger than the current cell. In the UMTS to change the band of frequency between the user equipment and the UTRAN the hard handovers are used. The UMTS operator can demand for spectrum to increase the capacity so the many 5 MHZ band would used by one operator, the outcome here is the need of hard handovers between them. To change the cell that have the same frequency and when there is not the support of the micro diversity these hard handovers are used else when a user equipment is allocated a dedicated control channel, it move to the new and near cell of the UMTS network and when there is the possibilities of other handovers like soft or softer handover the hard handover is then an option and is performed .
Pros and Cons of Hard Handovers
There are many pros and cons of hard handovers which are discussed in detail in the below section.
- The hard handovers are simple and economical as the cellular phone hardware is not able to make connection with the two or more channels at the same time.
- Only one channel is used at any interval of time which makes it simple and easy.
- If the hardover process is not successfully executed then call may be terminated or ended.
In this type of handovers the user equipment communicates parallel from different Node-B's with more then one sectors so the link are added and it is deleted in such a way the mobile equipment and the UTRAN always keeps a link. The technique known as micro diversity is used in this type which is known as at the same time many radio links are working and active. This technique has many advantages which are shown in the bullets below .
- The near-far effect is reduced.
- The connections are more repellent to shadowing.
- It offers the chance to transmit data the other Node-B's and thus the communication is maintained.
The property of the CDMA that the same frequencies are used by the all Node-B's gives an edge to the soft handover. The user equipment connected to the Node-B is called as the User Equipment Active Set .
These handovers are also called as “make before break”. It is because the connection is made first to the other Node-B and releases the older connection after making the connection to the target.
Pros and Cons of Soft Handovers
There are also some pros and cons of the soft handover which are explained in the following section
- Sophisticated handover type in which the call dropping probabilities are low as compared to hard handovers.
- The connection to the target cell are more reliable as compared to the source connection at which the user equipment is connected first and after the handover procedure the target connection are more reliable.
- More than one radio links are used so the more complex hardware is needed for it in order to cope with the existing situation.
- More than one channel is used parallel in a single call so the handover process should be done in such a way that the dropping probabilities' should be low as possible.
It is the special type of the soft handover the communication moves parallel to the same Node-B's having over its different sector . The user equipment and RNC communicates with the two different air interface channels. So two different codes are required for downlink thus the user equipments can know the signal. Rake processing used in the user equipment can receive the two signals.
Inter System Handovers
There are different radio access techniques like UMTS uses WCDMA GSM uses CDMA etc, so the inter system handovers are that type of handovers which takes place between different cells having different radio access techniques.
Intra System Handovers
Only in the single system these handovers are found . The dual mode terminal FDD-TDD these handovers can be observed. The handovers occurs from the techniques FDD to TDD. There are two special types of the inter system handovers which are explained in the following sections.
Intra Frequency Handovers
In the WCDMA system if the intra system handover occurs with the cells having the carrier frequency same then this type of hand over is the intra frequency handovers.
Inter Frequency Handovers
In the WCDMA system if the intra system handover occurs with the cells having the carrier frequency different then this type of hand over is the inter frequency handovers.
Strategies of Handovers
The handover processes are carried out with different strategies each having its own pros and cons. The strategy is adopted and depends upon the quality of service which users require at that specific time and the cost of the network. The following are the different handover strategies.
Non prioritized strategy
Reserved channel strategy
In this type of handover strategy for the arrival of handovers calls some of the channels are specified for it.
RNC role in the Handovers Process
The soft handovers are easy if the Node-B's taking part in the hand over process if the RNC's are same. It becomes difficult if the Node-B's are in control of unlike RNC's. If the problems occurs in the Radio access network the core network is not permitted to be witting of the problem. Yet it is important if the communication between the RNC's is impossible directly to each other over the interface-Iur.