90 lines
2.7 KiB
Plaintext
90 lines
2.7 KiB
Plaintext
Ideas about a handover algorithm
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======================================================================
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This is mostly based on the results presented in Chapter 8 of "Performance
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Enhancements in a Frequency Hopping GSM Network" by Thomas Toftegaard Nielsen
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and Joeroen Wigard.
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=== Reasons for performing handover ===
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Section 2.1.1: Handover used in their CAPACITY simulation:
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1) Interference Handover
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Average RXLEV is satisfactory high, but average RXQUAL too low indicates
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interference to the channel. Handover should be made.
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2) Bad Quality
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Averaged RXQUAL is lower than a threshold
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3) Low Level / Signal Strength
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Average RXLEV is lower than a threshold
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4) Distance Handover
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MS is too far away from a cell (measured by TA)
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5) Power budget / Better Cell
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RX Level of neighbor cell is at least "HO Margin dB" dB better than the
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current serving cell.
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=== Ideal parameters for HO algorithm ===
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Chapter 8, Section 2.2, Table 24:
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Window RXLEV averaging: 10 SACCH frames (no weighting)
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Window RXQUAL averaging: 1 SACCH frame (no averaging)
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Level Threashold: 1 of the last 1 AV-RXLEV values < -110dBm
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Quality Threshold: 3 of the last 4 AV-RXQUAL values >= 5
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Interference Threshold: 1 of the last AV-RXLEV > -85 dBm &
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3 of the last 4 AV-RXQUAL values >= 5
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Power Budget: Level of neighbor cell > 3 dB better
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Power Budget Interval: Every 6 SACCH frames (6 seconds ?!?)
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Distance Handover: Disabled
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Evaluation rule 1: RXLEV of the candidate cell a tleast -104 dBm
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Evaluation rule 2: Level of candidate cell > 3dB better own cell
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Timer Successful HO: 5 SACCH frames
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Timer Unsuccessful HO: 1 SACCH frame
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In a non-frequency hopping case, RXQUAL threshold can be decreased to
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RXLEV >= 4
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When frequency hopping is enabled, the following additional parameters
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should be introduced:
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* No intra-cell handover
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* Use a HO Margin of 2dB
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=== Handover Channel Reservation ===
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In loaded network, each cell should reserve some channels for handovers,
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rather than using all of them for new call establishment. This reduces the
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need to drop calls due to failing handovers, at the expense of failing new call
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attempts.
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=== Dynamic HO Margin ===
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The handover margin (hysteresis) should depend on the RXQUAL. Optimal results
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were achieved with the following settings:
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* RXQUAL <= 4: 9 dB
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* RXQUAL == 5: 6 dB
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* RXQUAL >= 6: 1 dB
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== Actual Handover on a protocol level ==
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After the BSC has decided a handover shall be done, it has to
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# allocate a channel at the new BTS
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# allocate a handover reference
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# activate the channel on the BTS side using RSL CHANNEL ACTIVATION,
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indicating the HO reference
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# BTS responds with CHAN ACT ACK, including GSM frame number
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# BSC sends 04.08 HO CMD to MS using old BTS
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