doc/handover.txt

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