Performance Comparison of Handover mechanisms in mobile wireless communication networks for Broadband Wireless Access Systems

Mobility is the mobile communication characteristics which make it diverse and attractive for all. Wireless communication is now attractive to the whole world and provides the ability of users to communicate very active. Transferring the users from one radio network to another is accomplishing the IT. The process is called the handover process. The transfer takes place by adjusting the duration of the soft transfers, the area size and either by cell crossing or by deterioration in the signal quality of the current channel — the brief overview of handover, Handover in WiMAX and LTE, types of handover, handover types solutions, approaches used in literature are typically transfer parameters and I contemporary the convergent fact for continuance in the area of mobile and wireless communication Handover.


Introduction
Handover is a progression in mobile communications and telecommunications in which a connected a data session, or cellular calling shall be transferred from one cell site (base station) to another without session disconnection. Cellular services are based on handover and mobility, permitting the user to be progressed from one cell position range to another or to be moved to the nearest cell position for better action [1,2,3]. Handover is an essential element in deploying and planning cellular networks. It lets users create connect phone or data sessions calls on the move. This process keeps connected the calls and data sessions even if users move from one cell position to another. Mobility is the main features that has made cellular wireless communication an indispensable device. Handover is obligatory during signal quality deprivation or/and cellcrossing in the current channel. And also, we need handover for user performance and mobility. During the transition, users and networks are transferred to another network or base station [4]. The fundamental of 5 G is heterogeneous networks, and smooth handover is permanent in such Intl J Comp. Comm Inf, 25-37 / 26 networks. Handover phases are, decision, execution and: discovery. Network discovery helps to find a suitable network that satisfies the user on desired Quality of Services (QoS). Decision phase is where the handover should take place (handover initiation phase). These phases helps to regulate seamlessness handover. Wrong initiation time will lead to increased unnecessary handover or call drop rate resulting in poor QoS. Therefore, in order to ensure that QoS will not be affected and unnecessary transfers are avoided, handover take place at right time by the decision to trigger the handover taking due account of all the parameters.
2. Classification of Handover 2.1 Technology Assessment

Horizontal Handover
A horizontal handover is a traditional handover. Horizontal handovers are Layer-2 handovers. Only BS is changed here that maintains the IP information. Here, MS uses the hard handover (HHO, break-before-make handover) when the information is moved to another BS. Where connections are broken at every layers and no context information is shared between BSs. Latency is on the order of around 1000ms. Recent WiMAX standard supports the soft handover (SHO, make-before-break) and Fast BS Switching (FBSS) [5,6,7].

Vertical Handover
There are also intertechnology handovers where the connection of a call will be transferred from one access technology to another assess technology, e.g. a call is transferred from Group Special Mobile (GSM) to the Universal Mobile Telecommunications System (UMTS) or from Code-Division Multiple Access (CDMA IS-95) to cdma2000. This is called as inter-technology handover, and it's a transition that takes place between two different technology networks, e.g. 3 G to 4G. [8,9].

Protocol Layers
• Cross Layer handover

Soft Handover
Soft Handover is similar to the HHO with one significant difference; at the same time, an MS is registered with SHO on multiple BSs (the Active Set). [10,11].

Softer Handover
This is same as soft transfer that happens between two different sectors of the same cell.

Different Handover Mechanisms in the LTE Network
In Handover Parameter Optimization (HPO), LTE system Handover performance can be improved by (HPO). Load Balancing (LB) is an instrument that advances the user's fulfillment. LB may result in the rise of the frequency of the inter-cell handover and henceforth increase its opportunity of handover glitches leading to discount in handover performance. Also, the techniques to improve both HPO and LB do not consider the network permitted maximum radio link failure (RLF) ratio. Radio link failure would raise the possibility of call drop. The (DHA) dynamic hysteresis-adjusting method be able to be used to develop handover performance and the number of fulfilled users in LTE networks. Dynamic Hysteresis Adjusting uses the allowed extreme RLF ratio is considered as a key indicator [12].
In that respect, one of the main objectives of LTE or any wireless system is to provide seamless and rapid transfer from one cell (a source cell) to another (a target cell). During the handover period, the service should be maintained; data transmission would not be delayed or missed; otherwise, output would be significantly compromised.
The versatility for supporting various 4 G implementations and meeting the latency criteria on handover is important for the design of IMT-Advanced Systems. In the cellular network mobility management, handover is one of the primary mechanisms. The IMT-Advanced systems are designed with IEEE 802.16 m, based on next generation WiMAX networks and 3GPP LTE-Advanced. Different delivery schemes are developed, proposed and analyzed with IEEE 802.16 m and 3GPP to minimize the latency of delivery and to meet the quality of service requirement during delivery [13].

Handovers in Mobile WiMAX
A great need for a mobile device is to change its base station coverage if a mobile station (MS) is reached by another base station with the strongest signal strength. Handover is a process that provides continuous connection where a Mobile Station travels from one base station's air interface to another Base Station's (BS) air interface without troubling the existing connections. Supporting mobility requires transfers. For a transfer to take place, one must have at least two base stations: (i) base station serving (SBS) and (ii)base station target (TBS) serving. The transfers have typically measured a change in service to the base station and does not inherently mean that the BS needs to be changed [14,15].

Latency and Handover
Handover is a system for keeping user contact session uninterrupted during a user's travel from a place to another. When Mobile Station (MS) moves to a cell and conducts handover, the MS service packets are delayed, and the operation may be interrupted for a while. This delay is referred as Latency Time.
Latency time is not a problem for a non-real-time services like e-mail or file transfer. The delay prone applications like video streaming service, should however be provided within 20 to 25ms delay. If the real-time packet transmission delay will be longer than the playout delay, then those packets are discarded and the probability of packet loss will be increased. The Serving BS sends information message for a handover process, usually uses MOB NBR-ADV, which has information about neighboring BSs. MS regularly checks the neighboring BSs for selecting the target BS candidates by the consistency of the signals and message is sent to the serving BS with the order for handover. The Serving BS exchanges the messages with destination BSs candidates to select a target BS and sends the message of handover response to MS. After receiving the message, the MS breaks the link to the serving BS and connects to the target BS and performs the process of re-entering the network. The Target BS receives the MS ' security information from the serving BS. Upon re-entering the network MS may receive or send traffic. The re-entry process of the network involves synchronization with the new downlink, range and synchronization for uplink, re-authorization, and re-registration. The MS synchronizes with the new downlink of Target BS after breaking the connection with Serving BS to get parameters for DL and UL transmission. MS conducts process to acquire correct offset timing and power adjustments. Target BS uses the Fast-Ranging IE for this range process and provides the MS with a non-contention range opportunity. If MS do not receive the Fast-Ranging IE message then it conducts a contention-based range process by transmitting the CDMA codes.

Handover Mechanisms
HO latency remains a problem which affects applications in real time environment. Many researchers proposed methods to improve the mechanism for HO applications for real time application. This section gives a few suggestions for improving the HO mechanism.

Fast Handover Algorithm for IEEE 802.16e Broadband Wireless Access System
Lee introduced the IEEE 802.16e BWA quick HO algorithm for reducing the loss of wireless channel resources and latency of the HO. He suggested algorithm involves Target BS Selection (TBS), Quick Synchronization and Association (FSA), and OHIT. Here running time of HO is evaluated for four different scenarios of HO. This will result in increased system throughput and a certification of proposed algorithm's efficiency. Some parameters of this scheme depend on the traffic condition of the backbone network, the rate of cell charging, user mobility, and CINR.
• Type 1 -contention based ranging with pre-association.

Fast Base Station Switching in Soft Handover
A switching technique is considered to quick base station. In this process, a collection of diversity for each mobile station is preserved. Diversity set is maintained by mobile station as well as base station service. In fig. 7, a mobile station selects a base station from diversity set which is a collection of base stations that is chosen as target base station for handover. Diversity set is considered as the presenter base station which sends its current location to the controller of the base station for a handover decision. The base and mobile station serving monitor the nearby base stations which is added in a diversity set. Whenever there is a need for the base station controller to hand over to the mobile station the initiation message is sent. Depending on the implementation, the decision on the transfer is taken by the mobile station, base station or base station controller. • Fn-1: last filtered measurement result;

General Procedure of Handover Control
• Mn: latest Ec/Io or RSCP measurement result that is received from the physical layer; • A = 1/2(k/2), k means the "Filter coefficient," that includes the Measurement of Control message and is decided by UTRAN.

▪ Reporting
▪ The period report triggers the handover • Due to filtered measurement result ▪ Event report is the triggered handover • Due to the event ▪ Handover Algorithms o All handover processes containing soft and hard handover, and other algorithms are applied to the quantity-reported event decision.

Concepts of Handover
▪ Active Set: • A group of cells which has radio links to a particular mobile station.
• User information is directed from all the cells.
▪ Monitored Set: • A set of cells not in active set but which are tracked in compliance with the list of adjacent cells allocated by UTRAN.
▪ Detected Set: • A set of cells neither in active set nor in monitor set. Where di (k) is considered as the distance between BSi and MN at k th Sampling instant and Zi (k) is the shadow fading component at k th sampling instant, which is log-normally distributed and is described (in dB) as the Gaussian distribution with a zero mean and standard deviation σi .
Where ρi is correlation coefficient of {Zi(k)}. ηi(0,1) are standard random variables with a zero mean and variance as unity function. Parameter A represent the free space path loss and represented as A = 20log10 Where fi is considered as carrier frequency, and bhb as base station antenna height in meters. The empirical constants are well defined for suburban areas in Erceg model and is given in Table 1. PLF, PLhms, PLqms are the correction factors that are corresponding to frequency fi , MN height mHM, and MN antenna directivity q. PLqms is often referred as the antenna-gain reduction factor and helps in angular scattering reduction owing to the directivity of the antenna. α(hm) is the antenna correction factor in an urban area expressed in dB and is given as

Conclusion
I have offered a list of some methods that have been used in the field of mobile communication handover so far, and it is worth mentioning that Quality of Services (QoS) in mobile communication and is mostly knotted to success and how quickly the handover takes place. It is also well known that the soft computing-based algorithm performs better due to soft computing method's aptitude for gripping the uncertainty embedded in mobile wireless communication network. Consequently, selection of multiple and appropriate network parameters requires more seamless handover.