WCDMA code tree fragmentation and what to do about it

Managing the downlink code tree in WCDMA is an important Radio Resource Management function. As a WCDMA system starts accepting traffic, various branches of the code tree will be blocked. Ensuring that the code tree is as compact as possible enables the system to freely allocate higher branches (lower SF). For HSDPA heavy networks this is key in ensuring the scheduler has at any TTI the maximum SF16 available.

Most UTRAN vendors manage this during the set-up phase. So for example a CS12.2kbps voice call (SF128) will get the left most SF128 available. This ensures that the right hand side of the code tree can be allocated to HSDPA. In high traffic situations however, as calls continuously get set up and released, even this approach might lead to fragmentation. As an example assume that at time X, SF128,48 was the left most SF128 available and this get allocated to a UE. At time X+1 however, it might be that a number of connections have been released and SF128,30 is now available. If however the original call on SF128,48 is still ongoing that space between 48 and 30 cannot be utilised.

A solution to this problem is to use dynamic code tree management which is what this network is using. As the trace extract shows at RRC Radio Bearer Setup that UE was allocated SF128-37.

Then as time progresses, a number of calls get released and the RNC instructs the UE to switch to SF128-30 as this is the left most SF128 available. This switch is signalled to the UE using the RRC Physical Channel Reconfiguration message, as the spreading operation in WCDMA is covered by the physical layer.

This procedure can then further repeat itself, depending on how much traffic there is and how long the connection lasts.