Table B.1 present basic features introduced in UMTS architecture and protocols in order to support HSDPA. In addition, other capabilities like the multiple-input multiple-output (MIMO) receiver would be supported to provide further signal gain and higher throughputs. HSDPA introduces a new radio bearer in the UMTS system, the high speed downlink share channel (HSDSCH). This channel allows several users to be time-multiplexed SO that during silent periods the resources are available to other users. The HSDSCH uses 2 ms transmission time intervals (TTIs) and a fixed spreading factor of 16, which allows a maximum of 15 parallel codes for user traffic and signalling. In addition to accelerating service access for users and improving data transfers, this reduced TTI allows the system to adapt itself faster to changing conditions. The uplink data transmission of the HSDPA user initially relies on release 99 DCH with different available rates (i.e. 64, 128 or 384 kbps). A new MAC-hs entity in added on the BTS to handle all these new features needed for HSDPA traffic, as shown in Figure B.1. Layers above MAC-hs (for the high speed downlink shared channels), such as MAC-d (for the dedicated transport channels) and RLC, are similar to those in the release 99 networks. The adaptive modulation and coding (AMC) technique is used in order to compensate for variations in radio transmission conditions, while the transmission power remains constant. HSDPA-enabled user Main Changes in HSDPA Table B.1 Main HSDPA features Feature HSDPA MAC layer split Functionality moved to Node B to improve efficiency of packet scheduler and retransmissions Downlink frame size 2 ms TTI (3 slots) Channel feedback Channel quality reported at 2 ms rate (500 Hz) for CQI (channel quality indication), ACK (acknowledged)/ ACK, TPC (transmission power control) Data user multiplexing Time multiplexing/code multiplexing (optional) Adaptive modulation and coding (AMC) QPSK and 16-QAM mandatory scheme Fast layer 1 retransmission (improves RTT); chase or increm