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HSDPA is based on shared channel transmission, such that channel codes are

used as common resources which are dynamically shared between users both in

the code and time domains. This results in a more efficient use of available

resources (power and codes), especially when compared to UMTS Release 99.

Release 99 UMTS operates using a QPSK (Quadrature Phase Shift Keying)

modulation Scheme, which limits the theoretical throughput of the cell. In

addition to using QPSK, Release 5/6 HSDPA can also use 16QAM (16

Quadrature Amplitude Modulation) to provide higher data rates. Since 16QAM

operates at twice the peak rate of QPSK, i.e. four bits per symbol, it makes more

efficient use of available bandwidth. However, 16QAM also requires better 

radio channel conditions than those needed for QPSK operation and as such, it

is usually only seen closer to the base station.

The shared resource of HSDPA channel codes are dynamically allocated every

TTI (Time Transmission Interval); for FDD this is set to three slots, i.e. 2ms.

This use of a short TTI reduces the round-trip time, thus improving the tracking

of radio channel variations. In addition to sharing the HSDPA resources over 

time, there is also the possibility to share resources in the code domain. The fast

and fair scheduling feature determines which UE is allocated resources at any

given moment, typically with more data being sent to users with the best radio

conditions.

HSDPA relies on feedback information from the mobile to achieve more

efficient scheduling of data packet transmissions. Details provided include: the

channel quality, device capabilities and QoS (Quality of Service) requirements.

To accommodate fast feedback, the system includes additional MAC (Medium

Access Control) functionality located at the Node B, as well as CQI (Channel

Quality Indication) and ACK/NACK signalling for H-ARQ (Hybrid Automatic

Repeat Request) from the terminal. These are sent in an uplink HS-DPCCH

(High Speed - Dedicated Physical Control Channel) from each mobile. The

 Node B then gathers and utilizes the CQI from each active user to determine the

ongoing scheduling of resources.

The UE is able to use the H-ARQ mechanism to rapidly request the

retransmission of missing data. This mechanism enables the combining of 

information from the original transmission(s) with that of later transmissions,

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 before the mobile attempts to decode the message. This approach uses a method

known as IR (Incremental Redundancy), often referred to as soft-combining,

which increases the system capacity and provides additional robustness. If the

information is in error or missing, a NACK (Negative Acknowledgement)

indication is sent to the Node B. When the received data is correct an ACK (Acknowledgement) indication is sent.