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Faster 3G Easier Said Than Done: Page 5 of 9

Increasing Complexity
Obviously, the improved performance and processor power provided by HSDPA implies an increase in complexity. There are many high-speed feedback loops needed to implement HSDPA efficiently and provide users with the best data rates possible.

For example, the TTI used for modulation and coding selection for individual frames in the HS-DSCH is just 2 ms compared with a typical time of 10 ms (and up to 80 ms) for the TTI used for power control in the existing Release 99 shared channel. Further, the algorithms needed to make good use of the possibilities provided by fast scheduling will be more complex than those implemented by existing radio network controller (RNC) software, but those decisions have to be made within a millisecond.

When link errors occur, data packets can be retransmitted quickly at the request of the mobile terminal. In existing W-CDMA networks, these requests are processed by the RNC. As with fast scheduling, better responsiveness is provided by HSDPA by processing the request in the base station.

The hybrid automatic repeat request (HARQ) protocol developed for HSDPA allows efficient retransmission of dropped or corrupted packets. The protocol has been designed to allow the average delivered bandwidth of HSDPA to be higher than would be possible if more extensive forward error correction were to be used. However, it puts significant demands on the basestation as support for HARQ calls for low latency. The latency demanded for efficient HARQ support calls for retransmissions to be processed within 2 to 7 ms. But the feedback loop that allows HARQ to be implemented is not one that exists in Release 99 basestations, as that function sits in the RNC for existing DCH and DSCH transmissions. So, not only must things work faster, many functions are new, adding to the capabilities and intelligence of the Node B.

In addition to fast retransmissions, a number of techniques are used to provide the mobile terminal with a better chance of receiving the data correctly. For users with a high coding rate, simple chase combining may be used, which simply repeats the packet. For users with a low coding rate, incremental redundancy can be used. In this scheme, parity bits are sent to allow the mobile terminal to combine the information from the first transmission with subsequent retransmissions.