High-Speed Downlink Packet Access
High-Speed Downlink Packet Access (HSDPA) is a new mobile telephony protocol and is sometimes referred to as a 3.5G (or "3?G") technology. 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. 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.
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 Adaptive Modulation and Coding (AMC), fast scheduling at the Node B (Base Station) and fast retransmissions to deliver the improved downlink performance. The concept of incremental redundancy is used, 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.
The amount of the channelisation code tree, and thus network bandwidth, allocated to HSDPA users is determined by the network. The allocation is "semi-static" in that it can be modified while the network is operating, but not on a frame-by-frame basis. This allocation represents a tradeoff between bandwidth allocated for HSDPA users, versus that for voice and non-HSDPA data users. The allocation is in units of channelisation codes for Spreading Factor 16, of which 16 exist, and of which up to 15 can be allocated to HSDPA.
When the Node B decides which users will receive data on the next 2 ms frame, it also determines which channelisation code(s) will be employed for each user, and this information is sent to the user devices over one or more HSDPA "scheduling channels" (these scheduling channels are not part of the HSDPA allocation previously mentioned, but are allocated separately). Thus, for a given 2 ms frame, data may be sent to a number of users simultaneously, using different channelisation code. The maximum number of users to receive data on a given 2 ms frame is determined by the number of allocated channelisation codes. This differs from CDMA 1xEV-DO, where data is sent to only one user at a time.
Although 3G is not available in most countries, this protocol does have a chance to become the follow up to 3G where WCDMA is deployed. Its success as a GSM-replacement, vis-a-vis other contenders like CDMA2000 1x or CDMA2000 1xEV-DO and cellular datacommunication standards like WiMAX (IEEE 802.16) is still unclear, especially considering that KDDI's CDMA2000 is generally considered as being much more successful and smooth than DoCoMo's and Vodafone's UMTS/W-CDMA introduction in Japan, which are much slower than initially hoped.
List of telephony terms:
3gpp - a-law - abbreviated dialing - adsl - ani - answering machine - apn - automatic ring back - b-channel - baud - bell 202 modem - bit rate - bonding - bri - busy signal - cable modem - call-progress tones - call accounting - call capture - call forwarding - call originator - call park - call pick-up - call transfer - call waiting - call waiting deluxe - called party - caller id - caller id spoofing - calling party - carrier wave - cbr - ccitt - cdma - cdma2000 - cellular repeater - celp - channel - clec - clock rate - codec - collect call - conference - conference call - crc - csd - d-channel - data compression - device driver - dial-up - dial - dial tone - direct-inward-dialing - direct distance dialing - distinctive ring - dnis - dsl - dsp - dtmf - dtr - duplex - echo cancellation - edge - extension - fax - fcc - fdma - fidonet - follow-me - g.711 - g.723.1 - g.723 - g.726 - g.lite - gprs - gps - gsm - h.323 - harmonic - headphones - hscsd - hspda - iad - idsn - internet call waiting - isp - ivr - jack plug - local loop - long distance - microcontroller - mobile phone - modem - modulation - mu-law - music-on-hold - night service - off-hook - on-hook - pabx - pager - payphone - pbx - pcm - pots - prank call - precise tone plan - pstn - pulse dialling - push to talk - ring modulation - ring tone - ringback - ringing signal - rj11 - roaming - serial communications - serial port - signal noise - sim - simplex - sit - sms - softmodem - switchboard operator - tapi - tdma - telecommunications - telemarketing - telephone - telephone call - telephone card - telephone company - telephone exchange - telephone line - telephone number - telephone numbering plan - telephone operator - telephone switchboard - telephony - tts - twisted pair - umts - v.32 - v.32bis - v.34 - v.42bis - vbr - vertical service code - voicemail - voip - vox - wap - wav - wi-fi - wimax - wire
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