Fixed and Dynamic Assignment
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Transcript of Fixed and Dynamic Assignment
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FREQUENCY MANAGEMENT AND
CHANNEL ASSIGNMENT
BY HARPINDER KANG KHATTRA
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5. Reducing the load of invalid calls
a. Off-air call setupreducing the load of setupchannels
b. Voice storage service for No-Answer calls
c. Call forwardingd. Reducing the customers Keep-Dialing cases
e. Call waiting for Busy-Call situations
f. Queuing
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Frequency management
Frequency management refers to
designating setup channels and voice
channels
numbering the channels
grouping the voice channels into subsets
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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..
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Channel assignment strategies
For efficient utilization of the radio spectrum, afrequency reuse scheme is consistent with theobjectives of increasing capacityand minimizing
interference. A variety of channel assignment strategies has
been developed: fixedor dynamic.
The choice impacts the performance of the
system, particularly as to how calls are managedwhen a mobile is handed off from one cell toanother.
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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 interferencemeasurements
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Fixed channel assignment
Each cell is allocated a predetermined set ofchannels.
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 blockedand the subscriber does not
receive service. Several variations of the fixed assignment
strategy exists.
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Dynamic channel assignment
Channels are not allocated to different cells permanently.
Instead, each time a call request is made, the serving BSrequests a channel from the RNC.
The RNC then allocates a channel to the requested cell
following an algorithm that takes into account thelikelihood of future blocking within the cell, the frequencyof use of the candidate channel, the reuse distance of thechannel, 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 cellwhich falls within the minimum restricted distance offrequency reuse to avoid co-channel interference.
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Dynamic channel assignment
[continue]
Dynamic channel assignment strategies requirethe RNC to collect real-time data on channeloccupancy, traffic distribution, and radio signalstrength indications (RSSI)of all channels on a
continuous basis. This increases the storage and computational
load on the system but provides the advantageon increased channel utilization and decreased
probability of blocked call. There are also hybrid strategies (Flexible channel
assignment).
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Rules for channel assignment
Do not assign co-channels or adjacent channels atthe same cell site .
Do not assign co-channels in adjacent cell sites .
Do not mix and match channel groups in a cell orsector.
Avoid adjacent channel assignment in adjacentcell sites .
Maintain proper channel separation for anychannel assignment for a sector or site.
Maximize the distance between reusing cell sites.
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FIXED CHANNEL ASSIGNMENT
SCHEMES
Adjacent-Channel Assignment
Channel Sharing and Borrowing
Sectorization Underlay-Overlay Arrangement
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Adjacent-Channel Assignment
Adjacent channel assignment. (a) Omnidirectional-antenna cells;
(b) directional-antenna cells.
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Channel Sharing and Borrowing
Channel-sharing algorithm
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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 co-
channel 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
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Underlay-Overlay Arrangement
Overlaid Cells.
Underlaid-overlaid cell arrangements. (a) Underlay-overlay in
omnicell; (b) underlay-overlay in sectorized cells
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Implementation.
(c) two-level handoff scheme.
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Reuse Partition
1. The K range is 3 to 9; the operational callquality can be adjusted and more reusepatterns are available if needed.
2. Each channel set of old K = 9 systems is thesubset of new K = 3 systems. Therefore, theamount of radio retuning in each cell in thisarrangement is minimal.
3. When cell splitting is implemented, all presentchannel assignments can be retained.
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