omnis for capacity ? omnis are not really for capacity, they are mostly for cheap coverage (initial roll out, rural areas, etc…)
what i recommend you to do is to make dualband concentric cells :
for each site, you’ll have 1 or 2 chained BTS, with a mix of 900MHz and 1800MHz TRX.
For instance, you can have a 3×2 in 900 and a 3×4 in 1800. For each cell:
Cell’s outer zone = 2 x 900MHz TRX (BCCH + SDCCH + TCH)
Cell’s inner zone = 4 x 1800MHz TRX (only TCH)
And you connect each cell to a dualband crosspolar antenna (900/1800). So in the end, you have only 3 BCCH (=3 sectors), only 3 antennas, and a 3×6 site.