HSDPA Facts

High-Speed Downlink Packet Access (HSDPA) is a new mobile telephony protocol and is also known to as 3.5G (or “3½G”) technology which is one step higher for 3G. In this respect it extends WCDMA in the same way that EV-DO extends CDMA2000. HSDPA provides a smooth evolutionary path for Universal Mobile Telecommunications System (UMTS) networks allowing for higher data capacity (up to 14.4 Mbps in the downlink). It is an evolution of the W-CDMA standard, designed to increase the available data rate by a factor of 5 or more. HSDPA defines a new W-CDMA channel, the high-speed downlink shared channel (HS-DSCH) that operates in a different way from existing W-CDMA channels, but is only used for downlink communication to the mobile.

Technology

The HS-DSCH channel does away with two basic features of other WCDMA channels - the variable spreading factor and fast power control - and instead uses 1. Adaptive Modulation and Coding (AMC), 2. fast packet scheduling at the Node B (Base Station) and 3. fast retransmissions from Node B (known as HARQ-Hybrid Automatic Repeat Request) to deliver the improved downlink performance. The concept of “incremental redundancy” is used in HARQ, where retransmissions contain different codings of the user data, relative to the orignal transmission. When a corrupted packet is received, the user device saves it, and combines it with subsequent retransmissions, to formulate an error-free packet as quickly and efficiently as possible. Even if the retransmitted packet(s) is itself corrupted, the combination of the sum of the errored transmissions can yield an error-free packet.

The HS-DSCH downlink channel is shared between users using channel-dependent scheduling to take advantage of favourable channel conditions to make best use of available radio conditions. Each user device periodically transmits (as many as 500 times per second) an indication of the downlink signal quality. The Node B uses this information received from all user devices to decide which users will be sent data on the next 2 ms frame and, for each user, how much data should be attempted. More data can be sent to users which report high downlink signal quality.

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