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Channel Assignment Strategies

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					                 Channel Assignment Strategies



• Introduction


• Channel assignment strategies for coordination-based systems


• Channel assignment strategies for measurement-based systems




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                               2-1                      Wireless Comm. Lab
                   Introduction


• Frequency allocation should be carefully planned to avoid

 degradation caused by co-channel interference

• Fixed channel assignment, dynamic channel assignment,

 and hybrid channel assignment

• Coordination-based (Planning-based) vs. measurement-based




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Channel Assignment Strategies for Coordination-based Systems

 • Under the control of a central processor
 • Classification
    Fixed Channel Assignment
    Dynamic Channel Assignment
    Hybrid Channel Assignment
    FCA with Borrowing
    Directed Retry
    Load Sharing

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                 Fixed Channel Assignment


Channels can only be used in designated cells

Different groups of radio channels may be assigned to adjacent

cells, but the same groups must be assigned to cells separated by a

certain distance (reuse distance) to reduce co-channel interference

The easiest one, but provides the worst channel utilization

Advanced Mobile Phone System (AMPS) [1]
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                               2-4                         Wireless Comm. Lab
                 Dynamic Channel Assignment

Channels are temporarily assigned for use in cells for the duration
of the call
After the call is over, the channel is returned and kept in a central
pool
To avoid co-channel interference, any channel that in use in one
cell can only be reassigned simultaneously to another cell in the
system if the distance between two cells is larger than minimum
reuse distance
Needs more tranceivers for each base station
Behaves worse performance than FCA under heavy loads
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Channel Assignment Strategies for Coordination-based Systems


   • Channel selection Algorithms for DCA (see Fig. 1)
       First available (FA)
                             1 n 2
       Mean square (MSA) : ∑ D j        D ≤ D j ≤ 2D
                             n j =1
       Nearest neighbor (NN)

       Nearest neighbor plus one (NN+1)




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Channel Assignment Strategies




    Fig.7 Examples illustrating switching strategies
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Channel Assignment Strategies for Coordination-based Systems

 • Hybrid channel Assignment
     Divide the total number of channels into two groups , one
    of which is used for fixed allocation to the cells , while the
    other is kept as a central poor to be shared by all users.
    Mixes the advantages of FCA and DCA.
    Needs fewer transceivers than DCA.
    Saves the CPU time.
    Still performs worse than FCA for heavy traffic load.[3]
    The optinum ratio of dynamical channels to fixed channels
    depends on the traffic load.
    Can be performed with ReAssignment strategy


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Channel Assignment Strategies for Coordination-based Systems



 • FCA with Borrowing
    First proposed by L.G. Anderson.[4]
    Three algorithms ( see Figs. 2-5 ).
    Ordered channel borrowing with switching strategies ( see
    Figs. 6-7 ) proposed by Elnoubi et al.[5-6]




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Channel Assignment Strategies for Coordination-based System




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Channel Assignment Strategies for Coordination-based System




                   Fig. 4 Channel assignment Algorithm II
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Channel Assignment Strategies for Coordination-based Systems




                                                      Search sequence




     Fig5. Channel assignment Algorithm III.
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Channel Assignment Strategies




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Channel Assignment Strategies




 Fig.7 Examples illustrating switching strategies
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Channel Assignment Strategies for Measurement-based Systems


• A distributed measurement-based method can alleviate the
 processing time of the central processor
• QFCA was proposed by J. C-I Chang (1991) [7] for PACS.
• In QFCA, frequency assignment and portable access are separately
 performed
• Flow chart of frequency assignment (see Fig. 8)




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  Frequency assignment for QFCA




Fig.8


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Channel Assignment Strategies for Measurement-based Systems


• Portable access procedure
   Radio-port selection and channel (time-slot) selection
   The former is based on maximun received power
   The latter is based on Erlang B-type random idle time-slot
    selection
   New algorithms, including LIBTA, FABTA, and HIBTA,
   have been studied for the latter[8]
   Adaptive threshold concept has been proposed for time-slot
   selection (see Fig.9) [8]
   Flow chart of portable radio access (see Fig.10)


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Algorithm with adaptive threshold




      9

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Portable Access Procedure




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Channel Assignment Strategies for Measurement-based Systems

 •Directed Retry
   Proposed by Eklundh(1986)[9]
  (overlapped probability is achieved by change of transmitted
    power)
   Portable keeps a decreased-order list for port IDs in received
    signal strength
   Access procedure (see Fig.10)




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Channel Assignment Strategies for Coordination-based Systems




                           Fig. 11

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Channel Assignment Strategies for Measurement-based Systems


 •Load Sharing
   Also proposed by Eklundh and Karlsson (1989) [10]
   An extension of directed retry
   During conversation, portable must periodically measure
   the signal strength for all carrier frequencies, and ranks
   them according to the received carrier power
   The received carrier power from the optimum radio port
   besides the current communicating port should be sent to
   the RPCU periodically
   Access procedure (see Fig.12)


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The Flowchart of Load Sharing




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Channel Assignment Strategies for Measurement-based Systems

 •Portable Selection Algorithms in Load Sharing Systems

    HCPATA (Highest Carrier Power Above Threshold
    Algorithm)
    HCPA (Highest Carrier Power Algorithm)
    FAATA (First Available Above Threshold Algorithm)
    FAA (First Available Algorithm)




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                            2-25                          Wireless Comm. Lab
Channel Assignment Strategies for Measurement-based Systems


•Two Time-solt Selection Algorithms[11-12]

  LIBTA (Least Interference Below Threshold Algorithm)
  Hybrid Time-solt Selection
  HIBTA with adaptive threshold for the first access
  LIBTA with adaptive threshold for the remaining access




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Simulation Results




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Simulation Results




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Simulation Results




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Simulation Results




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Simulation Results




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Simulation Results




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       Comments for Measurement-band Systems

•Load sharing system outperforms the other two systems
 significantly, especially in terms of blocking rate

•Hybrid time-slot selection algorithm yields better grade
 of service

•With hybrid time-slot selection algorithm, load sharing system is
 better than directed system by around 0.7 Erlangs and better than
 QFCA system by around 2 Erlangs



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                           References
[1]EIA/TIA ANSI 533, Mobile station-land station compatibility
   specification.
[2]D. C. Cox and D. O. Reudink, “Dynamic channel assignment in
   high capacity mobile communication systems,” Bell Syst. Tech.
   J., pp. 1833-1875, July/Aug. 1971.
[3]D. C. Cox and D. O. Reudink, “Increasing channel occupancy in
   large-scale mobile radio systems: dynamic channel
   reassignment,” IEEE Trans. Veh. Tech., vol. VT-22, pp. 218-222,
   Nov, 1973.
[4]L. G. Anderson, “A simulation study of some dynamic channel
   assignment algorithms in a high capacity mobile
   telecommunication system,” IEEE Trans. Veh. Tech., vol. VT-22,
   pp. 210-217, Nov. 1973.
                                                                 NCCU
                               2-34                       Wireless Comm. Lab
                         References
[5]S. M. Elnoubi, R. Singh, and S. C. Gupta, “A new frequency
   channel assignment algorithm in high capacity mobile
   communication system,” IEEE Trans. Veh. Tech., vol. VT-31, pp.
   125-131, Aug. 1982.
[6]S. S. Kuek and W. C. Wong , “Ordered dynamic channel
   assignment scheme with reassignment in highway microcells,”
   IEEE Trans. Veh. Tech. vol. VT-41, no. 3, Aug. 1992.
[7]J. C.-I. Chuang, “Autonomous adaptive frequency assignment for
   TDMA portable ratio systems,” IEEE Trans. Veh. Tech. vol. VT-
   40, pp. 627-635, Aug. 1991.




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                              2-35                      Wireless Comm. Lab
                           References
[8]J. H. Wen, W. J. Chen and S. Y. Lin, “Time-slot selection
   algorithms for quasi-fixed frequency assignment TDMA cellular
   systems”, J. of the Chinese Institute of Electrical Engineering, vol.
   5, no. 3, pp. 223-233, Aug. 1998.
[9]B. Eklundh, “Channel utilization and blocking probability in a
   cellular mobile telephone system with directed retry, “ IEEE
   Trans. Comm., vol. COM-34, pp. 530-535, May 1989.
[10]J. Karlsson and B. Eklundh, “A cellular mobile telephone
   system with load sharing-an enhancement of directed retry,”
   IEEE Trans. Comm., vol. COM-37, pp. 530-535, May 1989.




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                         References
[11]J. H. Wen, W. J Chen and J. K. Ho, “Time-slot selection
   algorithm with directed retry/load sharing for personal
   communication systems,” Proceedings of the NSC, part A:
   Physical Science and Engineering, vol. 21, no. 6, pp. 631-636,
   Nov. 1997.
[12]J. H. Wen, W. J. Chen, S. Y. Lin and K. T. Huang,
   “Performance evaluation of LIBTA/hybrid time-slot selection
   algorithm for cellular systems,” IASTED International
   Conference Modeling, Simulation and Optimization, Singapore,
   pp. 178-181, Aug. 1997.




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