HSDPA

HSDPA (High Speed Downlink Packet Access) is a new standard that extends W-CDMA to increase the downlink communication rate. It was established by the 3GPP (Third Generation Partnership Project) and enables a data speed of approximately 14Mbps. NTT DOCOMO has positioned it as one of the mainstay technologies of what is generally referred to as the 3.5G standard.

HSDPA is an extension of W-CDMA and is defined in 3GPP release 5. It has been positioned as a 3.5G service and functions as a bridge between W-CDMA, generally known as 3G services, and the 4G mobile phone service slated for future introduction. HSDPA is divided into twelve categories according to supported modulation and error-correcting methods, and available data rates range from 0.9 Mbps to approximately 14 Mbps. In Europe, card-type HSDPA terminals have been developed offering PC connectivity and they are currently attracting attention as the vanguard of high-speed, PC-based mobile communications.

NTT DOCOMO began development in 2002, and conducted outdoor experiments in FY2004 to confirm basic characteristics and connectivity. On top of this, further research was conducted to reduce handset size and improve quality culminating in the joint development with Fujitsu, Motorola and NEC of mobile phone type prototypes with a maximum downlink speed of 3.6 Mbps.

The development of these mobile phone type handsets is critical to the introduction of future 4G services, as early provision of 3.5G services through a mobile phone handset - i.e. the ability to download large amounts of data at high speeds to independent handsets - makes it possible to deliver more valuable services to users. In addition, since HSDPA improves frequency utilization efficiency by a factor of approximately three, it is expected that service quality will also rise.

The main technical characteristics of HSDPA include the following four characteristics:

• (1) AMCS (Adaptive Modulation and Coding Scheme)
  This scheme assesses the status of continuously varying radio wave propagation paths while maintaining a constant transmission power and then automatically selects the optimal modulation method. Further, the system also improves communication efficiency and speed by selectively communicating with the terminal via the base station's cell with the best communication conditions.
• (2) Hybrid ARQ (Automatic Repeat reQuest)
  Hybrid ARQ improves on the conventional ARQ that is essential to wireless communications under constantly varying conditions. Unlike the conventional ARQ, which requests repeatedly for a packet data with a decoding quality when an error is found, Hybrid ARQ combines retransmitted data with received data that was not decoded in the past, to improve the reception quality and achieve a more efficient transmission.
• (3) Shared channel
  In addition to the current W-CDMA, HS-DSCH (High Speed-Downlink Shared CHannel) for HSDPA is newly provided as a common resource. This resource is dynamically time- and code-multiplexed and allocated to multiple users' data transmissions, resulting in more efficient allocation of wireless resources.
• (4) BS scheduling
  In the HSDPA environment, The Base Station applies a scheduler algorithm every 2 ms to select between all users on a shared channel and allocate data transmission to those with relatively good wireless communication conditions. Compared to systems that allocate users regardless of wireless communication conditions, this system improves cell throughput (the throughput of all users connected simultaneously to the Base Station).
  NTT DOCOMO launched HSDPA-compliant services on August 31, 2006, as a major step towards future 4G services.