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diff --git a/openbsc/doc/handover.txt b/openbsc/doc/handover.txt deleted file mode 100644 index ac19e8725..000000000 --- a/openbsc/doc/handover.txt +++ /dev/null @@ -1,89 +0,0 @@ -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. - - -=== Reasons for performing handover === - -Section 2.1.1: Handover used in their CAPACITY simulation: - -1) Interference Handover - -Average RXLEV is satisfactory high, but average RXQUAL too low indicates -interference to the channel. Handover should be made. - -2) Bad Quality - -Averaged RXQUAL is lower than a threshold - -3) Low Level / Signal Strength - -Average RXLEV is lower than a threshold - -4) Distance Handover - -MS is too far away from a cell (measured by TA) - -5) Power budget / Better Cell - -RX Level of neighbor cell is at least "HO Margin dB" dB better than the -current serving cell. - -=== Ideal parameters for HO algorithm === - -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 - -In a non-frequency hopping case, RXQUAL threshold can be decreased to -RXLEV >= 4 - -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. - -=== Dynamic HO Margin === - -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 - - - -== Actual Handover on a protocol level == - -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 - |