HSDPA Code allocation

Let talk about the basic resource allocation strategy in HSDPA.
HSDPA resource allocation strategy is based on the sharing of the available resources like code, power etc. In this particular post we will talk about the channelization code resources and its uses  in HSDPA Network.

Under one channelization code tree there could be maximum 15 codes (of SF16) that can be used for HSDPA users (HS-DSCH transmission). A service provider can opt for desired implementation if the same carrier is being shared between HSDPA and non HDSPA users.

An example scenario has been taken for illustration where 12 codes has been reserved for HS-DSCH transmission.

Code reservation can be dynamic or static but code allocation is always dynamic.
In static code reservation strategy, desired number of codes are reserved at the time of cell setup and can be  allocated or deallocated to the desired UEs as per the data requirement.
 But In dynamic code reservation strategy, codes are reserved dynamically as per HSDPA users availability in the cell which is good implementation from resource uses point of view.

Now will will take a look how channelization codes are reserved and allocated by Network.

Channelization Code Tree:

in the above scenario codes shown in red are reserved for HS-DSCH transmission and rest 4 green codes are left free for common/control or Non-HSDPA users.

• Once reserved these code cannot be allocated to non HSDPA users. 
• One or all twelve codes can be allocated to a UE being served in a TTI
•  Reserved twelve codes can be used to serve more than 12 UEs by time and code multiplexing but in different TTI.
•  Codes are allocated in a particular order. Pls refer to 3GPP 25.213  for Code group formation.

Code Allocation:

Node-B manages these resources autonomously at per TTI basis (2ms TTI). Reserved codes are allocated and deallocated to the UE as per data rate requirements and CQI basis.

Code sharing strategies:

Time Multiplexing

•  Every TTI, available resources are allocated to different set of UE(s), according to priorities.
•  Sequential allocation.

Code multiplexing:

•  Multi-code transmission, (multiple codes in same TTI)
•  Assign available codes to multiple UEs in same TTI
•  Max number of simultaneous UEs served in same TTI, is equal to number of HS-SCCH.
•  There are 4 HS-SCCH channel allocated to a cell so only 4 UE could be served per TTI.

Note:

• UE can monitor up to 4 HS-SCCH. (UE need to find its allocated code, modulation information and other control information from its corresponding HS-SCCH)
• If one UE has to be served per TTI then only one HS-SCCH transmission is required.

Snapshot of code allocation:

It’s a snapshot of 5 TTI in which 12 codes resources has been used to serve 15 UEs.

• Vertical plane represents the code and Horizontal plane represents the TTI
•  Different colors are used to represents different UEs, so there are 15 UEs served.
•  Four or less UEs have been served in a TTI ( four HS-SCCH assumed)
•  Total 15 UEs have been served in 10 millisecond period using 12 codes.

Note:
     Code reservation is performed by Radio Resource Manager in RNC which basically manages all the radio related resources. Whenever a new HSDPA RAB is allocated or de-allocated the RRM checks if it can optimize the use of code ie whether it can reserve or release some codes from the pool and then if yes it reserves/releasea and notify NodeB about it. Now nodeB has updated set for dynamic allocation or de-allocation.