The advantage of sharing single carrier between HSDPA and R99 is the resource efficiency.Since HSDPA uses the transmission power of Node-B which is remained after the power assignment to R99 calls. Therefore, if the traffic of R99 call is low, the remaining power available for HSDPA will be high and high HSDPA performance will be expected. Another advantage is that sharing the single carrier could save the operator’s investment and running cost of Node-B. The disadvantage of this configuration is an impact on the performance of end-user experience. If the traffic volume of R99 call becomes high, the remaining power available for HSDPA will be reduced and as a result HSDPA performance received at end-user will also become low. In addition, HSDPA can become interferer to R99 call since HSDPA can use all the remaining transmission power. Such a high power transmission would increase the interference on R99 call and it would lead to the shrink of R99 service area.
Another disadvantage of single carrier configuration is the restriction on code allocation. In single carrier configuration, the channelization codes are also shared between HSDPA users and R99 users. If dynamic code allocation function is not implemented, the fixed channelization codes for HSDPA have to be reserved in advance and R99 users have to use the remaining channlization codes which belong to the same code tree. 
The following figure is the example that 15 channelization codes of SF16 are reserved for HS-DSCH and that FACH and PCH are mapped on to the different S-CCPCH. In this case, the channelization codes for DCH, CCH, HS-SCCH has to be allocated from the rest of code tree. For Common Channels, P-CPICH, P-CCPCH, AICH, and PICH use one channlization code of SF256 respectively and FACH uses one channlization code of SF64 and PCH uses one channelization code of SF128. Thus DCH and HS-SCCH can use the rest of the code tree.
The following figure is the example that 15 channelization codes of SF16 are reserved for HS-DSCH and that FACH and PCH are mapped on to the different S-CCPCH. In this case, the channelization codes for DCH, CCH, HS-SCCH has to be allocated from the rest of code tree. For Common Channels, P-CPICH, P-CCPCH, AICH, and PICH use one channlization code of SF256 respectively and FACH uses one channlization code of SF64 and PCH uses one channelization code of SF128. Thus DCH and HS-SCCH can use the rest of the code tree.
This example above shows that 3 channelization codes of SF128 are allocated for HS-SCCH. But in this code allocation, it is found that no channelization code is left for DCH if 15 channelization codes of SF16 are reserved for HS-DSCH and if FACH and PCH are mapped on to the different S-CCPCH and if 3 channelization codes of SF128 are allocated for HS-SCCH. In addition, from above, it is also found that up to 3 HS-SCCH can be allocated if 15 channelization codes of SF16 are reserved for HS-DSCH and if FACH and PCH are mapped on to the different S-CCPCH.
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