HSDPA (High Speed Downlink Packet Access)

HSDPA technology, also referred to as 3.5G, is a new packet-based mobile telephony protocol that enables data download speeds over 3G networks to increase by several times up to a potential 14.4 Mbit/s over a 5MHz bandwidth. An enhancement to the W-CDMA standard it is designed to increase available data rates several times over. Currently (as at July 2006) the obtainable downlink capacity stands at approximately 1.8 Mbit/s.

Using the data throughput capacity of 3G networks HSDPA delivers faster than current 3G download and streaming rates through increased spectral efficiency. HSDPA has a very fast transmission time interval (TTI) and can receive new packets every 2 milliseconds. This is five times faster when compared to the 10 milliseconds taken by regular UMTS data transfers. Any lost packets are also replaced much more quickly than in the standard 3G network. As an error free packet can be created from combining previous corrupt packets the user device saves any corrupted packet received. It combines these with any subsequent retransmissions to assemble an error-free packet as quickly and efficiently as possible.

A new channel, HS-DSCH (High-Speed Downlink Shared Channel), operating in a different way to from current (as at July 2006) W-CDMA channels is used for data downlink to the mobile phone. Logic functions such as that used to determine retransmission are now housed

Using channel-dependent scheduling mobile users share the HS-DSCH downlink channel, each transmitting data relating to their downlink quality (as often as 500 times per second due to the 2 ms TTI). Every TTI the MAC-hs (High Speed Medium Access protocol) at the Node B (Base transceiver station) checks the status of the flow and priority buffers. Depending on the scheduling algorithm it then determines which packets to send, how many, and to which users to send them on the next 2 ms frame. User devices that have reported a higher quality downlink signal are scheduled to receive larger amounts of data.

Also see UMTS and 3G

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