move openbsc/* to repos root

This is the first step in creating this repository from the legacy openbsc.git.

Like all other Osmocom repositories, keep the autoconf and automake files in
the repository root. openbsc.git has been the sole exception, which ends now.

Change-Id: I9c6f2a448d9cb1cc088cf1cf6918b69d7e69b4e7

1 files changed, 89 insertions, 0 deletions

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+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
+