They also have different properties resulting from the different modes of operation that are required. In view of these facts they were often treated as almost separate entities. Now they are generally rolled out together. The two technologies are summarised below:.
The upgrades and additional facilities were introduced at successive releases of the 3GPP standard. As such the 3G infrastructure usage was prolonged and enabled many operators to maximise the use of their investment before having to add the capability for 4G.
The system provides many advantages for users over the original UMTS system. As the 3GPP standards evolved, so did the performance available. This provides for maximum data rates of 14 Mbps in the downlink.
QPSK is still used in the uplink where data rates of up to 5. The data rates quoted are for raw data rates and do not include reductions in actual payload data resulting from the protocol overheads. Shorter Transmission Time Interval TTI : The use of a shorter TTI reduces the round trip time and enables improvements in adapting to fast channel variations and provides for reductions in latency.
Use of shared channel transmission: Sharing the resources enables greater levels of efficiency to be achieved and integrates with IP and packet data concepts.
Use of link adaptation: By adapting the link it is possible to maximize the channel usage. In R99 the physical channel carrying user data can be allocated a channelization code with SF ranging from 4 to Therefore all the DL channels within one cell are orthogonal at the point of transmission; at point of reception there will however be some interference, mainly due to multi-path fading.
A major feature in HSDPA is that the channelization codes can be shared not only through code multiplexing, as in R99, but also using time multiplexing.
Through time multiplexing the radio resources may be more efficiently utilized than with code multiplexing since in each TTI the resources can be allocated to the UE experiencing the best radio channel. Of course if the data to be delivered does not require all available codes, then code multiplexing can be used.
In HSDPA also soft combining is used, meaning that a received faulty TB will be stored in a receiving buffer and recombined with the retransmitted TB, in order to enhance likelihood of correct decoding. HARQ is used for both non-real-time as well as real-time services. In order to avoid stalling due to data processing times and possible retransmissions, up to 8 HARQ processes can work in parallel, see figure 3.
Hence, the DL physical channels will all be orthogonal- at least at the point of transmission, having channelization codes from the same code tree, and they will be sharing the available power in the NodeB.
In the UL it is not possible to get perfect orthogonality since the senders, UEs, are moving independently. Every UE has its own channelization code tree and its own battery. As in R99 long scrambling codes, with low correlation, are used to separate between channels from different transmitters, i.
In Release 6 the maximum channel rate for Enhanced UL is 5. For Enhanced UL scheduling is still carried out by the NodeB, now the receiving node, while transport format selection is done by the scheduled UEs, see figure 6.
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