1. HSDPA

i. High Speed Downlink Packet Access HSDPA is based on 3GPP Release 5 specifications to provide data rates up to approximately 8–10 Mbps to support packet-based multimedia services.

ii. HSDPA strives to give the maximal bandwidth to the user with the best channel conditions. The data rates achievable with HSDPA are more than adequate for supporting multimedia streaming services.

iii. For example, with chip rate = 3.84Mcps, frame size = 3 slots, 15 codes and coding rate = ¾;

Throughput using 16 QAM = 10.8Mbps; Throughput using QPSK = 5.4Mbps

2. Reasons for high speed

i. HSDPA focuses on scheduling of data packet transmission and processing of retransmissions (in case of transmission errors).

ii. It uses a short frame length to further accelerate packet scheduling for transmission.

iii. It employs incremental redundancy for minimizing the air-interface load caused by retransmissions.

iv. It uses shorter transmission time interval (TTI) (2 ms) between two consecutive transmissions to the UE.

v. It uses channel sharing scheme to support transmission to multiple users.

vi. It uses adaptive modulation and coding according to the quality of the radio link.

vii. HSDPA thus shortens the round-trip time between the network and terminals and reduces variance in downlink transmission delay.

3. New channels

i. The new channels introduced in HSDPA are high-speed downlink shared channel (HS-DSCH), high-speed shared control channel (HS-SCCH), and high-speed dedicated physical control channel (HS-DPCCH).

ii. HS-DSCH enables sharing of radio resources. The spreading factor is 16, there is time domain multiplexing where each TTI consists of three time slots. Within each 2 ms TTI, a maximum of 15 parallel codes allocated to HS-DSCH. Codes may all be assigned to one user, or may be split across several users. The number of codes allocated to each user depends on cell loading, QoS requirements, and UE code capabilities (5, 10, or 15 codes). There are 15 categories of UE.

iii. The HS-SCCH (a fixed rate 960 kbps, SF = 128) is used to carry downlink signaling between Node B and UE before the beginning of each scheduled TTI. It includes UE identity, HARQ-related information and the parameters of the HS-DSCH transport format selected by the link-adaptation mechanism. Multiple HS-SCCHs can be configured in each sector to support parallel HS-DSCH transmissions. A UE can be allocated a set of up to four HS-SCCHs, which need to be monitored continuously.

iv. The HS-DPCCH (SF = 256) carries ACK/NACK signaling to indicate whether the corresponding downlink transmission was successfully decoded, as well as a channel quality indicator (CQI) to be used for the purpose of link adaptation. The CQI is based on a common pilot channel (CPICH) and is used to estimate the transport block size, modulation type, and number of channelization codes that can be supported at a given reliability level in downlink transmission. The feedback cycle of CQI can be set as a network parameter in predefined steps of 2 ms.

4. Basic operation

i. The basic operational principles behind HSDPA are relatively simple. The RNC routes data packets destined for a particular UE to the appropriate Node B.

ii. Node B takes the data packets and schedules their transmission to the mobile terminal over the air interface by matching the user’s priority and estimated channel operating environment with an appropriately chosen coding and modulation scheme (that is, 16-QAM vs. QPSK).

iii. The UE is responsible for acknowledging receipt of the data packet and providing Node B with information regarding channel condition, power control, and so on.

iv. Once it sends the data packet to the UE, Node B waits for an acknowledgment. If it does not receive one within a prescribed time, it assumes that the data packet was lost and retransmits it.

5. Backward compatibility with WCDMA

i. HSDPA is evolved from and backward compatible with Release 99 WCDMA systems.

ii. HSDPA marks a similar boost for WCDMA that EDGE does for GSM. It provides a two-fold increase in air interface capacity and a five-fold increase in data speeds in the downlink direction.

iii. HSDPA is particularly suited to extremely asymmetrical data services, which require significantly higher data rates for the transmission from the network to the UE, than they do for the transmission from the UE to the network.

iv. From an architectural perspective, HSDPA is a straightforward enhancement of the UMTS Release ’99 (R99) architecture, with the addition of a repetition/scheduling entity within the Node B that resides below the R99 media-access control (MAC) layer.

v. To support HSDPA, two changes must be made to the channel card. First, the downlink chip-rate ASIC must be modified to support the new 16-QAM modulation schemes and new downlink slot formats associated with HSDPA. In addition, the downlink symbol-rate processing section must be modified to support HSDPA extensions.

vi. The next change requires a new processing section, called the MAC-hs, which must be added to the channel card to support the scheduling, buffering, transmission, and retransmission of data blocks that are received from the RNC. This requires the introduction of a programmable processing entity together with a retransmission buffer.

vii. Since the channel card already contains both a general-purpose processor and a DSP, the MAC-hs is realized using DSP.

viii. From a cellular-network perspective, all R99 techniques can be supported in a network supporting HSDPA, since HSDPA mobile terminals (UEs) are designed to coexist with R99 UEs.

HSDPA is thus based on the same set of technologies as high data rate (HDR) to improve spectral efficiency for data services - such as shared downlink packet data channel and high peak data rates - using high-order modulation and adaptive modulation and coding, hybrid ARQ (HARQ) retransmission schemes, fast scheduling and shorter frame sizes.