Radio Link Transfer & Roaming Management

Radio Link Transfer:

•          The wireless link transfer from one cell to another base station is classified into two categories. 

•         They are hard handoff and soft handoff The hard handoff is concerned with getting a new radio channel in the cell to which the mobile unit is moved in.

•        When the signal strength in the information flow of a communication link goes below -100 dBm, a new radio wireless link needs to be searched for a better- signal strength in the incumbent cell where the mobile unit has moved in.

•      This is an essential requirement to provide quality reception of the received signal. Otherwise, noise and interference will try to dominate to degrade the signal strength below the acceptable one.

•      In handoff only the neighbouring cell channels at the border of the whole cell are searched for freeness.

•      When a free channel is found in the incumbent cell handoff is initiated by the base station or mobile unit and mostly the MSC carries out the handoff.

•       The other radio link transfer technique is the soft handoff. 

•        Here when the signal strength goes below a threshold value, it takes more channel from cell which can improve the signal quality.    

•        Space diversity concept is utilized here with the help of more than one channel from nearby base station.

•       2/3 channels with space diversity can easily avoid fading to provide reliable communication. 

•       However, every information flow in this technique requires more number of channel and the bandwidth requirement is higher.

      However, the third generation mobile communication systems are capable of providing the higher bandwidth required for the soft handoff.

Roaming Management:

• The location of the moving mobile unit needs to be continuously recorded at the mobile switching centre (MSC). This is to enable it to switch the mobile unit to any other user in the system or the public telephone network.

• The home location register (HLR) and visiting location register (VLR) available at the mobile switching centre take up this responsibility.

• The location register in three different cases are discussed as follows:

Connection to Public Telephone Network:

•            Initially, the location details of the mobile. unit are available in the HLR. 

•          When a subscriber of a fixed telephone user wants to contact a mobile subscriber, the code routes the call to the correct MSC. 

•         From here a control signal will try to find out the location of the called user. The control signal will try to get the location information from HLR. 

•        Had the mobile unit moved away from the home location, the information will be retrieved by the HLR from VLR. Making use of the information retrieved, the switching unit of the MSC switches the calling user to the mobile unit.

         

                      Process of HLR and VLR

Connection from Mobile Unit to a Fixer User:

•              When the moble unit wants to get connected to a fixer user of a public network through a control channel, a path is established between mobile unitand the public switching network. As the location is fixed, there will not be any problew in locating the user.

•              Through the MSC the call is routed to the public switching network. The PSN will establish connection to the fixed user.

        

          

•          The need for tracking the location of the subscriber is desenbed in this chapter.

•          The different handoff techniques have been introduced in detail. 

•         Radio link transter strategies are hard handoff and soft handoff. 

•         Their relative merits and dements along with their protocols have been discussed.