| Ideas about a handover algorithm |
| ====================================================================== |
| |
| This is mostly based on the results presented in Chapter 8 of "Performance |
| Enhancements in a Frequency Hopping GSM Network" by Thomas Toftegaard Nielsen |
| and Joeroen Wigard. |
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| === Reasons for performing handover === |
| |
| 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 |
| 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 |
| current serving cell. |
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| === Ideal parameters for HO algorithm === |
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| Chapter 8, Section 2.2, Table 24: |
| |
| Window RXLEV averaging: 10 SACCH frames (no weighting) |
| Window RXQUAL averaging: 1 SACCH frame (no averaging) |
| Level Threashold: 1 of the last 1 AV-RXLEV values < -110dBm |
| Quality Threshold: 3 of the last 4 AV-RXQUAL values >= 5 |
| Interference Threshold: 1 of the last AV-RXLEV > -85 dBm & |
| 3 of the last 4 AV-RXQUAL values >= 5 |
| Power Budget: Level of neighbor cell > 3 dB better |
| Power Budget Interval: Every 6 SACCH frames (6 seconds ?!?) |
| Distance Handover: Disabled |
| Evaluation rule 1: RXLEV of the candidate cell a tleast -104 dBm |
| Evaluation rule 2: Level of candidate cell > 3dB better own cell |
| Timer Successful HO: 5 SACCH frames |
| Timer Unsuccessful HO: 1 SACCH frame |
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| In a non-frequency hopping case, RXQUAL threshold can be decreased to |
| RXLEV >= 4 |
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| When frequency hopping is enabled, the following additional parameters |
| should be introduced: |
| |
| * No intra-cell handover |
| * Use a HO Margin of 2dB |
| |
| === Handover Channel Reservation === |
| |
| In loaded network, each cell should reserve some channels for handovers, |
| rather than using all of them for new call establishment. This reduces the |
| need to drop calls due to failing handovers, at the expense of failing new call |
| attempts. |
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| === Dynamic HO Margin === |
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| The handover margin (hysteresis) should depend on the RXQUAL. Optimal results |
| were achieved with the following settings: |
| * RXQUAL <= 4: 9 dB |
| * RXQUAL == 5: 6 dB |
| * 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 |
| |
| # allocate a channel at the new BTS |
| # allocate a handover reference |
| # activate the channel on the BTS side using RSL CHANNEL ACTIVATION, |
| indicating the HO reference |
| # BTS responds with CHAN ACT ACK, including GSM frame number |
| # BSC sends 04.08 HO CMD to MS using old BTS |
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