This document discusses different methods for allocating channels in wireless communication systems. It describes three main categories: fixed channel allocation, dynamic channel allocation, and hybrid channel allocation. Fixed allocation assigns channels to each cell permanently. Dynamic allocation selects channels for each call request considering factors like future blocking. Hybrid allocation combines fixed and dynamic approaches, allowing cells to borrow channels from neighbors when needed. The choice of allocation strategy impacts system performance, especially during handoffs between cells.

1. • A scheme for increasing capacity and minimizing interference is
required.
• 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.
• The choice of channel assignment strategy impacts the
performance of the system, particularly how a call is managed
when a mobile user is handed off from one cell to another.

2. • There are three major categories for assigning
these channels to cells (or base-stations). They
are
• Fixed (Static) Channel Allocation,
• Dynamic Channel Allocation and
• Hybrid Channel Allocation which is a combination of the
first two methods

3. • Each cell is assigned 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 the cell are occupied, the call is blocked .
(No service)
• In a variation of the fixed channel assignment, a cell can
borrow channels from its neighboring cell if its own channels
are full.
• For efficient operation, FCA systems typically allocate
channels in a manner that maximizes frequency reuse.

4. • The problem with FCA systems is quite simple and occurs
whenever the offered traffic to a network of base stations is
not uniform. (In such case the available channels are not
being used efficiently)

5. • Voice channels are not allocated to different cells
permanently.
• Each time a call request is made, the BS requests a
channel from the MSC.
• MSC allocates a channel to the requested cell using an
algorithm that takes into account
 The likelihood of future blocking,
 The frequency of use of the candidate channel,
 The reuse distance of the channel, and
 Other cost functions.

6. • To ensure the minimum QoS, the MSC only allocates a given
frequency if that frequency is not currently in use in the cell, or
any other cell which falls within the limiting reuse distance.
• DCA reduces the likelihood of blocking, thus increasing the
capacity of the system.
• DCA strategies require the MSC to collect real time data on
channel occupancy and traffic distribution on a continuous
basis.
• Frequency reuse is often not maximized unlike the case for FCA
systems.
• Often involve complex algorithms for deciding which available
channel is most efficient.

7. • This scheme include systems that are hybrids of fixed and
dynamic channel allocation systems.
• Several methods have been presented that fall within this
category (Channel Borrowing is one of the most straightforward
hybrid allocation schemes).
• Channels are assigned to cells just as in fixed allocation schemes.
and 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
neighbouring cells given that a channel is available and use of
this channel won't violate frequency reuse requirements. (It is
often categorized as a subclass of fixed allocation schemes.)

8. • The major problem with channel borrowing is that
when a cell borrows a channel from a neighbouring
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 neighbouring cells; i.e., the
neighbouring cells with the most unassigned channels.

9. 9
FCA DCA
 Performs better under heavy traffic
 Low flexibility in channel
assignment
 Maximum channel reusability
 Sensitive to time and spatial
changes
 Not stable grade of service per cell
in an interference cell group
 High forced call termination
probability
 Suitable for large cell environment
 Low flexibility
 Performs better under light/moderate
traffic
 Flexible channel allocation
 Not always maximum channel
reusability
 Insensitive to time and time spatial
changes
 Stable grade of service per cell in an
interference cell group
 Low to moderate forced call termination
probability
 Suitable in microcellular environment
 High flexibility

10. 10
FCA DCA
 Radio equipment covers all
channels assigned to the cell
 Independent channel control
 Low computational effort
 Low call set up delay
 Low implementation complexity
 Complex, labor intensive
frequency planning
 Low signaling load
 Centralized control
 Radio equipment covers the temporary
channel assigned to the cell
 Fully centralized to fully distributed
control dependent on the scheme
 High computational effort
 Moderate to high call set up delay
 Moderate to high implementation
complexity
 No frequency planning
 Moderate to high signaling load
 Centralized, distributed control
depending on the scheme

11. • Consider a system with:
• Total number of channels = 20
• Probability of blocking = 1%
• Approach 1: Divide 20 channels in 4 trunks of 5 channels
• Traffic capacity for one trunk (5 channels) = 1.36 Erlangs
• Traffic capacity for four trunks (20 channels) = 4 X 1.36=5.44 Erlangs
• Approach 2: Divide 20 channels in 2 trunks of 10 channels
• Traffic capacity for one trunk (10 channels) = 4.46 Erlangs
• Traffic capacity for two trunks (20 channels) = 8.92 Erlangs
• Approach 3: Use 20 channels without dividing them
• Traffic capacity for one trunk (20 channels) = 12.00 Erlangs
Allocation of channel has a major impact !!!

12. S