Nowadays when we see specification of a new smart phone then we usually come across a term HSDPA. HSDPA mean High Speed Downlink Packet Access. HSDPA is actually an enhancement or upgraded version to the 3G technology by which you can increase the download data rates from 384Kbps (theoretically 2Mbps) to 10Mbps (theoretically 14Mbps). Please Note: here Kbps stands for Kilo bits per second and Mbps stands for Mega bits per second not KiloBytes or MegaBytes. The idea is similar to EDGE means to increase the spectral efficiency for enhanced data rates. HSDPA delivers higher capacity through improved spectral efficiency, which provides higher data rates, shorter response times and better Qos (Quality of Service).

What exactly is done to increase the data rates in HSDPA??  In fact to increase the data rates a new channel called HS-DSCH (High speed downlink shared channel) was defined. This was somewhat similar to the DSCH defined in the release 99 specs. This channel maps to HS-PDSCH (high speed physical downlink shared channel). To support this data channel a control channel HS-SCCH (High speed shared control channels). The HS-DSCH is transmitted over the entire cell or over only part of the cell using e.g. beam-forming antennas. The increased data rates are supported through the introduction of Fast and Complex channel-control mechanisms based on a short fixed packet TTI (transmission time interval), AMC (adaptive modulation and coding) and L1 HARQ (layer1 hybrid automatic repeat request).
As it is an enhancement of 3G network so people usually call it as 3.5G. 4G mobiles will use LTE.
So, we can upgrade the current 3G systems to HSDPA. HSDPA is a power-hungry technology, and many of the base stations out there aren’t up to the job. The key piece of technology is the power amplifier, which has to be a full power, 45-watt model, to handle the extra data output. Having said that, most of the new newly designed RNC and Node B's are now capable of handling not just an upgrade from 3H to HSDPA and HSPA+ but also to LTE.

There could be many uses of this technology. We can use it as Broadband connections at home. It would be possible to watch extremely good quality real time Videos, download complete song tracks in less than a minute. Do real time video conferencing with more than one user using very high quality video. Eventually the wireless operators plan that instead of having wired broadband at home, people will move to wireless broadband. The list is non exhaustive. Imagine you are watching a live match on your phone in HD(high definition) by streaming and without a pause  :) .