High-Speed Downlink Packet Access(HSDPA)
High-Speed Downlink Packet Access (HSDPA) (Sometimes known as High-Speed Downlink Protocol Access) is a 3G mobile telephony protocol in the HSPA family, which provides a roadmap for UMTS-based networks to increase their data transfer speeds and capacity. Current HSDPA deployments now support 1.8 Mbit/s, 3.6 Mbit/s, 7.2 Mbit/s and 14.4 Mbit/s in downlink.
고속 하향 패킷 접속 [高速下向-接續, high speed downlink packet access]
비동기식 3세대 이동 통신의 하향 링크에서 10Mbps 수준의 고속 패킷 데이터 서비스를 제공하는 전송 규격. 인터넷 통신은 주로 내려받기(download)가 많아 하향 링크의 고속화가 서비스의 필수 요소이며, 고속 데이터는 주로 정지 상태에서 사용되므로 이러한 조건을 최대한 수용하도록 하향 링크의 전송 규격을 개선한 것으로 WCDMA R5에 적용되었다. 기본 개념은 각종 물리 링크나 MAC 링크 기능을 기지국에서 수행하도록 하며, 단말기가 기지국에 가깝거나 정지 상태에서는 오버헤드를 줄여 처리 속도를 높인 것으로, 적응 변조 및 코딩(AMC), 하이브리드 자동 재송 요구(HARQ), 송수신 다중 다이버시티(multiple input multiple output), 고속 패킷 스케줄링 등의 기술이 적용되었다.
Further speed grades are planned for the near future. The networks are then to be upgraded to HSPA Evolved, which provides speeds of 42Mbit downlink in its first release.
In addition to supporting high data speeds, HSDPA greatly increases the capacity of the network. Current HSDPA networks have the capacity to provide each customer with 30 gigabytes of data per month in addition to 1000 minutes of voice and 300 minutes of mobile TV.
As of March 19 2007, 100 HSDPA networks have commercially launched mobile broadband services in 54 countries. Near to 40 HSDPA networks support 3.6 Mbps peak data throughput (downlink direction). A growing number are delivering 7.2 Mbps peak data throughput, leveraging new higher-speed devices coming into the market. One network has been declared as “14.4 Mbps (peak) ready” while several additional networks will have this capability by end 2007. The first commercial HSUPA (uplink direction) network is launched, with several more set to follow in 2007.
This protocol is a relatively simple upgrade where UMTS is already deployed.
CDMA-EVDO networks had the early lead on performance, and Japanese and South Korean providers were the highly successful benchmark for that technology. Lately the situations seems to be changing in favour of HSDPA as an increasing number of providers worldwide are adopting it. South Korea’s KTF and SK Telecom built nationwide HSDPA networks. KTF are selling HSDPA handsets to its customers on March 2007. SK Telecom announcing that by first half of 2007 it will be selling HSDPA handsets to its customers and that it will be cutting funding to its CDMA2000 network. A similar situation occurred in Australia, with Telstra announcing the closure of its CDMA-EVDO network and its replacement with a HSDPA network.
The HS-DSCH channel does away with two basic features of other W-CDMA 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 original 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.
Fast Packet Scheduling
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 often 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 CDMA2000 1xEV-DO, where data is sent to only one user at a time.
Adaptive Modulation and Coding
The modulation scheme and coding is changed on a per-user basis depending on signal quality and cell usage.
QPSK is the initial modulation scheme, however, in good radio conditions the introduction of 16QAM modulation will improve data throughput rates by approximately double that of QPSK. QPSK with 5 Code allocation will typically offer up to 1.8 Mbit/s peak data rates. 16QAM with 5 Codes will increase this to 3.6 Mbit/s. Additional Codes (e.g. 10, 15) can also be used to improve these data rates or extend the network capacity throughput significantly. Theoretically, HSDPA can give throughput up to 10.8 Mbit/s.
HSDPA is part of the UMTS standards from release 5 onwards, which also accompanies an improvement on the uplink providing a new bearer of 384 kbit/s (previous max bearer was 128 kbit/s).
As well as improved data rates that are associated with HSDPA one of the key benefits that are seen is the reduction on latency, improving the round trip time for applications.
Along with the HS-DSCH channel, three new physical channels are also introduced. One is the High Speed-Shared Control CHannel (HS-SCCH) which informs the user that data will be sent on the HS-DSCH 2 slots ahead. The second one is the Uplink High Speed-Dedicated Physical Control CHannel (HS-DPCCH), which carries acknowledgment information and current channel quality indicator (CQI) of the user. This value is then used by the Node-B in calculating how much data to send to the UE on the next transmission. The third downlink physical channel is the HS-PDSCH (High Speed-Physical Downlink Shared CHannel). This is the physical channel mapped to the above HS-DSCH transport channel that carries actual user data.