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EEE 264-2:Capacity Increase Techniques and Calculations

EEE 264-2:Capacity Increase Techniques and Calculations. Increasing capacity of cellular systems. Capacity (or number of users) of a cellular system can be enlarged using frequency reuse Capacity can also be improved using cellular layout and antenna design techniques such as:

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EEE 264-2:Capacity Increase Techniques and Calculations

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  1. EEE 264-2:Capacity Increase Techniques and Calculations

  2. Increasing capacity of cellular systems • Capacity (or number of users) of a cellular system can be enlarged using frequency reuse • Capacity can also be improved using cellular layout and antenna design techniques such as: • Cell splitting • Antenna sectoring

  3. Cell splitting design • Cell splitting subdivides a congested cell into smaller cells, each with its own base station. • Original large cell with radius R is split into medium cells with radius R/2. • Medium cell is further split into small cells with radius R/4.

  4. Cell splitting from radius R to R/2 and R/4 R R/2 Large cells R/4 Medium cells Small cells

  5. System capacity increase using cell splitting-Example • Consider a R - R/2 cell splitting system with R = 1 km. Suppose each base station is allocated 60 channels regardless of cell size. Find the number of channels contained in a 3 x 3 km2 area around (small ) cell “A” for the following cases: • Without cell splitting • With cell splitting

  6. Cell splitting example with R = 1 km R A R/2

  7. ( ) R R/2 2 Solution • To cover a 3 x 3 km2 area centered around cell A, we need to cover 1.5 km to right, left, top and bottom Number of large cells in this 3 x 3 km2 area ~ 4 Number of small cells in this 3 x 3 km2 area = x number of large cells = 4 x 4 = 16 small cells • With large cells, the number of channels = 4 x 60 = 240 • With 16 small cells, the number of channels = 60 x 16 = 960

  8. System capacity increase using directional antennas (sectoring) • In basic form, cellular antennas are omnidirectional • Directional antennas can increase the system capacity relative to that of omnidirectional antennas • Sectorization can be done in multiples of 60˚

  9. Antenna sectorization 2 1 1 3 2 6 3 4 2 1 1 3 5 2 6 4 3 5 a. 3 sectors of 120˚ each b. 6 sectors of 60˚ each

  10. ( ( ) ) S I S I 1 1 1 2 qk qk 3N ( ( ) ) S I S I omni omni Sectoring calculations • Signal to interference ratio (S/I) ( ) S I 1 6 , k = path loss exponent qk = omni q = = = 6 = = 3 , N = Number of cells/cluster 60˚ 120˚ • For 120 degrees sectoring • For 60 degrees sectoring †

  11. ( ) S I Example • For N = 7, base stations using omnidirectional antennas can just satisfy the 18 db requirement. • Determine if the use of 120˚ sectoring and N = 7 would better satisfy the 18 db requirement for path loss exponent k = 4.

  12. ( ) S I Solution Radio Propagation Models Since this is greater than 18 db, it will work. = 4.6 3 x 7 3 N q = = ( ) 1 2 1 2 = 220.5  23.4 db qk = = 4.6 4 120˚

  13. Handoff management • MSC is an appropriate device to oversee the handoff operation • Transfer of mobile control from current base station BS to new target BS • Initiation phase • Employs a decision making strategy based on measured received signal.

  14. Handoff management • Execution phase • Involves the allocation of new channels to mobile, and exchange of control messages. • MSC obtains status information for all base stations periodically • Intraswitch handoff — between cells controlled by same MSC • Interswitch handoff — between cells controlled by different MSCs.

  15. Handoff Strategies -MCHO • Mobile controlled handoff (MCHO) • Desirable since it does not burden the network. • However, it increases complexity of mobile terminal.

  16. Handoff Strategies -NCHO • Network controlled handoff (NCHO) • BSs or APs (Access points) monitor signal quality from mobile. • MSC then chooses the candidate BS or AP and initiates handoff. • Mobile plays passive role in process.

  17. Handoff Strategies - MAHO • Mobile assisted handoff (MAHO) • Employed by GSM system • Mobile records signal levels from various BSs using a periodic beacon generated by BSs Mobile relays power levels from different BSs to MSC via current BS • MSC makes handoff decision.

  18. Types of handoff - Hard • Hard handoff - (break before make) • Mobile has radio link with only one BS at anytime • Old BS connection is terminated before new BS connection is made.

  19. Types of handoff - Soft • Soft handoff (make before break) • Mobile has simultaneous radio link with more than one BS at any time • New BS connection is made before old BS connection is broken • Used by CDMA systems

  20. Types of handoff - Backward/Forward • Backward handoff • Handoff is predicted and initiated via the existing BS link • Loss of power in existing BS link is a problem • Forward handoff • Handoff is begun via the new BS radio link • Delay is a problem

  21. Intraswitch handoff process MSC Fixed Terminal AP_1 AP_0 Link beforehandoff Link afterhandoff MS

  22. Signaling sequence for intraswitch handoff MSC (3) SEQ_PKT (8) UP_READY NEW_AP_READY (4) HO_MUST (2) LAST_PKT (7) NO_MORE (6) LAST_UP AP_0 AP_1 (5) UP_NO_MORE FBK NEW_AP_READY (4) HO_MUST (5) READY MS

  23. Example handoff process • Mobile is currently located in cell served by AP_0 and is moving toward the cell being served by AP_1. • When mobile reaches the cell boundary of AP_0, the MSC initiates and executes handoff algorithm. • MSC knows that AP_1 has a channel available to accept the handoff.

  24. Handoff process: Step 1 • MSC directs the mobile to handoff to AP_1 • Send the message (1) NEW_AP_READY To the mobile via AP_0, with identity of candidate AP_1 included.

  25. Handoff process: Step 2 • AP_0 responds with message (2) LAST_PKT to the MSC • Message contains the sequence number of packet sent to the mobile • MSC send the sequence number of the following downlink packet to AP_1 using the message (3) SEQ_PKT

  26. Handoff process: Step 3 • The message (4) HO_MUST indicates the last downlink packet from MSC to AP_0 • When AP_0 receives this message, it flags the termination of connection by sending the message (5) VP_NO_MORE to MSC

  27. Handoff process: Step 4 • Mobile switches its operating frequency and sends the message (6) READY to AP_1 • This message contains the sequence number of the last packet correctly received by mobile

  28. Handoff process: Step 5 • AP_1 starts downlink transmission and buffers all uplink packets from mobile • It also sends the message (7) LAST_UP to MSC requesting approval of uplink transmission

  29. Handoff process: Step 6 • MSC waits for message(8) NO_MORE from AP_0 • Than MSC switches uplink connection from AP_0 to AP_1 and sends the message(9) UP_READY to AP_1

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