How Many Users Could Be Served Simultaneously In A UMTS Cell?
This post over at the radio access blog has made me think a bit of how many users in Cell-DCH state could be supported simultaneously on HSDPA. As the post analyzes correctly, each user requires a Dedicated Physical Control Channel in the downlink direction with a spreading factor of 256. That means that there are a maximum number of 256 such channels that could be used but unfortunately that wouldn't leave spreading codes for any other use, including the high speed data channels themselves. So in practice there must be significantly fewer users to still have enough codes for other purposes such as the HS channels and also for voice calls. So we end up with quite a small number and it will be more and more difficult to serve the 2000 subscribers with 3 sector base stations if there can only be a fraction of them active at the same time. So fast forward to the fractional DPCCH feature also described in the blog post above that bundles 10 control channels on a single spreading code and it's quite obvious how significant this feature will become in the future as smartphones and connected apps spread ever further.