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Thursday, February 3, 2011

Impact of HSDPA and HSUPA on UTRAN interfaces

The impacts of HSDPA and HSUPA in terms of data rates over the UTRAN air interface are well known and often are a focus of discussion. The impact on the operation of the other UTRAN interfaces requires attention as well. For the interface between the base station and RNC, the Iub interface, there are now larger data rates expected than with Release 99 terminals.

While Release 99 terminals are at most 384 kbps, the data rate on different interfaces, including the Iu-ps interface to the packet core network Serving GPRS Support Node (SGSN), is equal to the one used for radio. With HSDPA the situation has changed, however. Over the air interface, denoted as Uu in Third Generation Partnership Project (3GPP) terminology, there can be data rates up to 14.4Mbps over short (2-ms) periods. This, however, does not mean the same data rate being used on the Iub and Iu-ps interface for that particular user. From a single user point of view, the radio resource is time and code shared with other users in the cell. Thus , the average bit rate for a user in a loaded cell is clearly lower. Further, the peak rates of 10Mbps require extremely favorable radio conditions and it is unlikely that all the users in the cell would be able to get such a high data rate. Thus, the average traffic on the Iub interface is also less than the peak rate for radio. An example is illustrated below, which shows as a comparison the Release 99 384-kbps downlink and HSDPA for the 7.2-Mbps case.

The 384-kbps downlink will have equal data rate booking on all interfaces and will not exceed the 384-kbps limit. With HSDPA the radio interface peak rate supported by the terminal in the example is 7.2 Mbps. The service data rate over the Iu-ps and Iub interface could be limited, for example, to 1Mbps.The use of buffering in the BTS makes it possible to have the peak rate for the connection as high as terminal and BTS capabilities allow, while keeping the maximum bit rate over the Iub and Iu-ps in line with the QoS parameters received from the packet core. The buffer in the BTS – together with a scheduler that time-shares the resources – enables having a higher peak rate (over a short period of time) for radio than the average rate on Iub/Iu-ps. Having the transmission buffer in the BTS also requires flow control to be applied to avoid buffer overflow. In this way the user under better radio conditions may get more of the Iub resources, as the situation exists to get a lot of data through. The principle of flow control operation is shown in the Figure below.

The user under good radio conditions will get more Iub allocation (credits) as the data move fast from the radio point of view. On the other hand, when the buffer starts to get filled due to poor radio conditions (and resulting low data rate) the flow control will slow the data stream down for that kind of user, as shown in the Figure above.

For the network elements and the terminal itself the use of HSDPA and HSUPA will cause a lot of changes, especially on the physical layer as well as on the MAC and RLC layers. From the RNC point of view there is an impact which is due not only to the already mentioned flow control but also, on the other hand, to the changes in RRM. Mobility events cause some changes in addition to the opportunity to share Iub resources dynamically between all users. Further, the data rates increase from the practical maximum of 384 kbps for Release 99 devices up to 10Mbps and, theoretically, even up to 14Mbps. For the BTS and the terminal, the key changes with HSDPA are related to the addition of scheduling and retransmission functionalities at the BTS and, respectively, the needed packet combining functionality at the terminal. Additional channels are then needed for actual data transmission itself, including new modulation, as well as for signaling purposes to facilitate the new functionality. The key new functionalities due to HSDPA are illustrated in the figure below;

Similarly, with HSUPA new functionalities are needed as shown in the Figure below;

Here, scheduling is controlled from the BTS and there are needs for data flow handling in the other (uplink direction) as well as new signalling channels for HSUPA.The combining functionality is now at the base station and the new re-ordering functionality is added to the RNC. Uplink scheduling is required in addition to downlink scheduling for HSDPA.

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