High Speed Uplink Packet Access, HSUPA provides a major increase in data rate and overall performance for 3G UMTS networks.
As the name indicates, HSUPA applies to the uplink, and as such it is the companion to HSDPA which is applied to the downlink.
With both HSDPA and HSUPA active, the overall scheme is referred to as HSPA - high speed packet access.
Although when using HSUPA, there is a considerable increase in performance, the overall data rate in the uplink is not as fast as that available in the downlink. This is because the majority of data is passed in the downlink rather than the uplink.
In addition to this there are additional difficulties providing the same performance from the UE in view of some of the restrictions imposed by the fact that a large number of UEs are communicating with the NodeB.
3G UMTS HSUPA key characteristics
HSUPA brings enhanced performance through the addition of new features that sit on top of the existing UMTS technology.
The key specification parameters that are introduced by the use of HSUPA are:
- Increased data rate: The use of HSUPA is able to provide a significant increase in the data rate available. It allows peak raw data rates of 5.74 Mbps.
- Lower latency: The use of HSUPA introduces a TTI of 2 ms, although a 10ms TTI was originally used and is still supported.
- Improved system capacity: In order to enable the large number of high data rate users, it has been necessary to ensure that the overall capacity when using HSUPA is higher.
- Modulation order: Originally only BPSK modulation, that adopted for UMTS, was used. Accordingly it did not support adaptive modulation schemes. Higher order modulation was introduced in Release 7 of the 3GPP standards when 64QAM was allowed.
- Hybrid ARQ: In order to facilitate the improved performance the Hybrid ARQ (Automatic Repeat reQuest) used for HSDPA is also employed for the uplink, HSUPA.
- Fast Packet Scheduling: In order to reduce latency, fast packet scheduling has been adopted again for the uplink as for the downlink, although the implementation is slightly different.
With these specification parameters enable HSUPA to complement the performance of HSDPA, providing an overall performance improvement for systems incorporating HSPA.
3G HSUPA basics
At the core of HSUPA, High Speed Uplink Packet Access are a number of new technologies that are very similar to those used with HSDPA. However there are a few fundamental differences resulting from the different conditions at either end of the link.
- The uplink in UMTS, and HSUPA is non-orthogonal because complete orthogonality cannot be maintained between all the UEs. As a result there is more interference between the uplink transmissions within the same cells.
- The scheduling buffers are located in a single location (NodeB) for the downlink, whereas for the uplink they are distributed within several UEs for the uplink. This requires the UEs requiring to send buffer information to the scheduler in the NodeB so that it can then provide an overall schedule for the data transmission.
- In the downlink, the shared resource is the transmission power. In the uplink, the resource is limited by the level of interference that can be tolerated and this depends upon the transmission power of the multiple UEs.
- High order modulation techniques are able to provide higher data rates for high signal level links in the downlink. There is not the same advantage in the uplink where as there is no need to share channelisation codes between users and the channel coding rates are therefore lower, although higher order modulation was introduced under Release 7.
The addition of HSUPA to the 3G UMTS network enabled the uplink as well as the downlink to provide much improved performance. With both HSDPA (downlink) and HSUPA (uplink) active the complete package was called HSPA - high speed packet access. Although most of the data is passed int he downlink, many isers found the uplink without HSUPA very slow and it degraded the overall user experience. With HSUPA active the overall experience was much improved.
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