| Hybrid Channel Alloction Scheme |
The third category of channel allocation methods includes all systems that
are hybrids of fixed and dynamic channel allocation systems. Several methods
have been presented that fall within this category and in addition, a great deal
of comparison has been made with corresponding simulations and analyses [Cox,
Elnoubi, Jiang, Katzela, Yue, Zhang]. We will present several of the more
developed hybrid methods below.
Channel Borrowing is one of the most straightforward hybrid allocation
schemes. Here, channels are assigned to cells just as in fixed allocation
schemes. If a cell needs a channel in excess of the channels previously assigned
to it, that cell may borrow a channel from one of its neighboring cells given
that a channel is available and use of this channel won't violate frequency
reuse requirements. Note that since every channel has a predetermined
relationship with a specific cell, channel borrowing (without the extensions
mentioned below) is often categorized as a subclass of fixed allocation schemes.
The major problem with channel borrowing is that when a cell borrows a channel
from a neighboring cell, other nearby cells are prohibited from using the
borrowed channel because of co-channel interference. This can lead to increased
call blocking over time. To reduce this call blocking penalty, algorithms are
necessary to ensure that the channels are borrowed from the most available
neighboring cells; i.e., the neighboring cells with the most unassigned
channels.
Two extensions of the channel borrowing approach are Borrowing with
Channel Ordering (BCO) and Borrowing with Directional Channel Locking
(BDCL).
- Borrowing with Channel Locking was designed as an improvement over the
simpler Channel Borrowing approach as described above [Elnoubi]. BCO systems
have two distinctive characteristics [Elnoubi]:
- The ratio of fixed to dynamic channels varies with traffic load.
- Nominal channels are ordered such that the first nominal channel of
a cell has the highest priority of being applied to a call within the
cell.
The last nominal channel is most likely to be borrowed by neighboring
channels. Once a channel is borrowed, that channel is locked in the
co-channel cells within the reuse distance of the cell in question. To be
"locked" means that a channel can not be used or borrowed. Zhang and Yum
[Zhang] presented the BDCL scheme as an improvement over the BCO method.
From a frequency reuse standpoint, in a BCO system, a channel may be
borrowed only if it is free in the neighboring cochannel cells. This
criteria is often too strict.
- In Borrowing with Directional Channel Locking, borrowed channels are
only locked in nearby cells that are affected by the borrowing. This differs
from the BCO scheme in which a borrowed channel is locked in every cell
within the reuse distance. The benefit of BDCL is that more channels are
available in the presence of borrowing and subsequent call blocking is
reduced. A disadvantage of BDCL is that the statement "borrowed channels are
only locked in nearby cells that are affected by the borrowing" requires a
clear understanding of the term "affected." This may require microscopic
analysis of the area in which the cellular system will be located. Ideally,
a system can be general enough that detailed analysis of specific
propagation measurements is not necessary for implementation.
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