N TCH HO : number of TS TCH reserved for incoming HO. They are not allocated to call setup, unless ALLOC ANYWAY is enabled.
HO MARGIN DIST/LEV/QUAL : remove the candidate cells for outgoing HO if they do not satisfy the criteria PBGT(n) > HO MARGIN + HO MARGIN DIST/LEV/QUAL (depending on the cause of the HO : high distance, or low level, or poor quality)
they cannot really improve HO spectacularly. They are very fine tuning parameters, in order to fix some specific problems (Micro to Umbrella cell or 1800 to 900 cell situations). Basically, you can keep them to default values, it will work fine.
If you set them to a value to high, no more candidates can verify the condition >> HO cannot happen
if the value is too negative, the candidate are actually worse than the serving cell >> it will lead to ping pong scenarios.
What exactly are the values of your HO KPIs ?