FREQUENCY MANAGEMENT AND
CHANNEL ASSIGNMENT
BY HARPINDER KANG KHATTRA
Frequency –Spectrum Utilization
• 1. Increasing the number of radio channel
using narrow banding, spread spectrum, or
time
• division.
• 2. Improving spatial frequency-spectrum
reuse.
• 3. Frequency management and channel
assignment.
• 4. Improving spectrum efficiency in time
5. Reducing the load of invalid calls
a. Off-air call setup—reducing the load of setup
channels
b. Voice storage service for No-Answer calls
c. Call forwarding
d. Reducing the customers’ Keep-Dialing cases
e. Call waiting for Busy-Call situations
f. Queuing
Frequency management
Frequency management refers to
• designating setup channels and voice
channels
• numbering the channels
• grouping the voice channels into subsets
Channel assignment
• Channel assignment refers to the allocation of
specific channels to cell sites and mobile units
• Ideally channel assignment should be based
on causing the least interference in the
system..
Channel assignment strategies
• For efficient utilization of the radio spectrum, a
frequency reuse scheme is consistent with the
objectives of increasing capacity and minimizing
interference.
• A variety of channel assignment strategies has
been developed: fixed or dynamic.
• The choice impacts the performance of the
system, particularly as to how calls are managed
when a mobile is handed off from one cell to
another.
Fixed and Dynamic assignment
• Fixed frequency assignment: permanent
– certain frequencies are assigned to a certain cell
– problem: different traffic load in different cells
• Dynamic frequency assignment: temporary
– base station chooses frequencies depending on the
frequencies already used in neighbor cells
– more capacity in cells with more traffic
– assignment can also be based on interference
measurements
Fixed channel assignment
• Each cell is allocated a predetermined set of
channels.
• Any call attempt within the cell can only be
served by the unused channel.
• 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 exists.
Dynamic channel assignment
• Channels are not allocated to different cells permanently.
• Instead, each time a call request is made, the serving BS
requests a channel from the RNC.
• The RNC then allocates a channel to the requested cell
following an algorithm that takes into account the
likelihood of future blocking within the cell, the frequency
of use of the candidate channel, the reuse distance of the
channel, and other cost functions.
• Accordingly, the RNC 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
[continue]
• Dynamic channel assignment strategies require
the RNC to collect real-time data on channel
occupancy, traffic distribution, and radio signal
strength indications (RSSI) of all channels on a
continuous basis.
• This increases the storage and computational
load on the system but provides the advantage
on increased channel utilization and decreased
probability of blocked call.
• There are also hybrid strategies (Flexible channel
assignment).
Rules for channel assignment
• Do not assign co-channels or adjacent channels at
the same cell site .
• Do not assign co-channels in adjacent cell sites .
• Do not mix and match channel groups in a cell or
sector.
• Avoid adjacent channel assignment in adjacent
cell sites .
• Maintain proper channel separation for any
channel assignment for a sector or site.
• Maximize the distance between reusing cell sites.
FIXED CHANNEL ASSIGNMENT
SCHEMES
•
•
•
•
Adjacent-Channel Assignment
Channel Sharing and Borrowing
Sectorization
Underlay-Overlay Arrangement
Adjacent-Channel Assignment
Adjacent channel assignment. (a) Omnidirectional-antenna cells;
(b) directional-antenna cells.
Channel Sharing and Borrowing
Channel-sharing algorithm
Sectorization
• Comparison of Omnicells (Nonsectorized Cells) and Sectorized Cells
Omnicells:If a K = 7 frequency-reuse pattern is used, the frequency sets assigned in
each cell can be followed by the frequency-management chart. However,
terrain is seldom flat; therefore, K = 12 is sometimes needed for reducing cochannel interference. For K = 12, the channel-reuse distance is D = 6R, or the
co-channel reduction factor q = 6.
Sectorized Cell:There are three basic types.
1. The 120◦-sector cell
2. The 60◦-sector cell
3. The 120◦- or 60◦-sector cell
Underlay-Overlay Arrangement
• Overlaid Cells.
Underlaid-overlaid cell arrangements. (a) Underlay-overlay in
omnicell; (b) underlay-overlay in sectorized cells
• Implementation.
(c) two-level handoff scheme.
Reuse Partition
1. The K range is 3 to 9; the operational call
quality can be adjusted and more reuse
patterns are available if needed.
2. Each channel set of old K = 9 systems is the
subset of new K = 3 systems. Therefore, the
amount of radio retuning in each cell in this
arrangement is minimal.
3. When cell splitting is implemented, all present
channel assignments can be retained.
NONFIXED CHANNEL ASSIGNMENT
ALGORITHMS
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•
•
•
•
Fixed Channel Algorithm
Dynamic Channel Assignment
Hybrid Channel Assignment
Borrowing Channel Assignment
Forcible-Borrowing Channel Assignment