Fixed and Dynamic Assignment

8/13/2019 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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Fixed and Dynamic Assignment

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