Channel Assignment Strategies
For efficient utilization of the radio spectrum, a frequency reuse scheme
that is consistent with the objectives of increasing capacity and minimizing
interference is required. A variety of channel assignment strategies have been
developed to achieve these objectives. Channel assignment strategies can be
classified as either fixed or dynamic. The choice of channel assignment strategy
impacts the performance of the system, particularly as to how calls are managed
when a mobile user is handed off from one cell to another
In a fixed channel assignment strategy; each cell is allocated a predetermined
set of voice channels. Any call attempt within the cell can only be served
by the unused channels in that particular cell. If all the channels in that cell are
occupied, the call is blocked and the subscriber does not receive service. Several
variations of the fixed assignment strategy exist. In one approach, called the borrowing
strategy, a cell is allowed to borrow channels from a neighboring cell if all
of its own channels are already occupied. The mobile switching center (MSC)
supervises such borrowing procedures and ensures that the borrowing of a channel
does not disrupt or interfere with any of the calls in progress in the donor
cell.
In a dynamic channel assignment strategy, voice channels are not allocated
to different cells permanently. Instead, each time a call request is made, the
serving base station requests a channel from the MSC. The switch then allocates
a channel to the requested cell following an algorithm that takes into account the
likelihood of fixture blocking within the cell, the frequency of use of the candidate
channel, the reuse distance of the channel, and other cost functions.
Accordingly, the MSC only allocates a given frequency if that frequency is
not presently in use in the cell or any other cell which falls within the minimum
restricted distance of frequency reuse to avoid co-channel interference. Dynamic
channel assignment reduce the likelihood of blocking, which increases the trunking
capacity of the system, since all the available channels in a market are accessible
to all of the cells.
For efficient utilization of the radio spectrum, a frequency reuse scheme
that is consistent with the objectives of increasing capacity and minimizing
interference is required. A variety of channel assignment strategies have been
developed to achieve these objectives. Channel assignment strategies can be
classified as either fixed or dynamic. The choice of channel assignment strategy
impacts the performance of the system, particularly as to how calls are managed
when a mobile user is handed off from one cell to another
In a fixed channel assignment strategy; each cell is allocated a predetermined
set of voice channels. Any call attempt within the cell can only be served
by the unused channels in that particular cell. If all the channels in that cell are
occupied, the call is blocked and the subscriber does not receive service. Several
variations of the fixed assignment strategy exist. In one approach, called the borrowing
strategy, a cell is allowed to borrow channels from a neighboring cell if all
of its own channels are already occupied. The mobile switching center (MSC)
supervises such borrowing procedures and ensures that the borrowing of a channel
does not disrupt or interfere with any of the calls in progress in the donor
cell.
In a dynamic channel assignment strategy, voice channels are not allocated
to different cells permanently. Instead, each time a call request is made, the
serving base station requests a channel from the MSC. The switch then allocates
a channel to the requested cell following an algorithm that takes into account the
likelihood of fixture blocking within the cell, the frequency of use of the candidate
channel, the reuse distance of the channel, and other cost functions.
Accordingly, the MSC only allocates a given frequency if that frequency is
not presently in use in the cell or any other cell which falls within the minimum
restricted distance of frequency reuse to avoid co-channel interference. Dynamic
channel assignment reduce the likelihood of blocking, which increases the trunking
capacity of the system, since all the available channels in a market are accessible
to all of the cells.
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