| Channel Allocation In Mobile Computing |
Channel allocation deals with the allocation of channels to cells in a
cellular network. Once the channels are allocated, cells may then allow users
within the cell to communicate via the available channels. Channels in a
wireless communication system typically consist of time slots, frequency bands
and/or CDMA pseudo noise sequences, but in an abstract sense, they can represent
any generic transmission resource. There are three major categories for
assigning these channels to cells (or base-stations). They are
- Fixed Channel Allocation,
- Dynamic Channel Allocation and
- Hybrid Channel Allocation which is a combination of the first two
methods.
Fixed Channel Allocation
Fixed Channel Allocation (FCA) systems allocate specific channels to specific
cells. This allocation is static and can not be changed. For efficient
operation, FCA systems typically allocate channels in a manner that maximizes
frequency reuse. Thus, in a FCA system, the distance between cells using the
same channel is the minimum reuse distance for that system. The problem with FCA
systems is quite simple and occurs whenever the offered traffic to a network of
base stations is not uniform. Consider a case in which two adjacent cells are
allocated N channels each. There clearly can be situations in which one
cell has a need for N+k channels while the adjacent cell only requires
N-m channels (for positive integers and m. In such a case,
k users in the first cell would be blocked from making calls while m
channels in the second cell would go unused. Clearly in this situation of
non-uniform spatial offered traffic, the available channels are not being used
efficiently. FCA has been implemented on a widespread level to date.
Dynamic Channel Allocation
Dynamic Channel Allocation (DCA) attempts to alleviate the problem mentioned
for FCA systems when offered traffic is non-uniform. In DCA systems, no set
relationship exists between channels and cells. Instead, channels are part of a
pool of resources. Whenever a channel is needed by a cell, the channel is
allocated under the constraint that frequency reuse requirements can not be
violated. There are two problems that typically occur with DCA based systems.
- First, DCA methods typically have a degree of randomness associated with
them and this leads to the fact that frequency reuse is often not maximized
unlike the case for FCA systems in which cells using the same channel are
separated by the minimum reuse distance.
- Secondly, DCA methods often involve complex algorithms for deciding
which available channel is most efficient. These algorithms can be very
computationally intensive and may require large computing resources in order
to be real-time.
|
