High Speed Physical Downlink Shared Channel

Description

The High Speed Physical Downlink Shared Channel (HS-PDSCH) is the physical layer realization of the High Speed Downlink Shared Channel (HS-DSCH) in UMTS High-Speed Downlink Packet Access (HSDPA). It is the radio bearer over which the actual user data packets are transmitted from the Node B to the User Equipment (UE). Multiple HS-PDSCHs can be allocated to a single UE in a given Transmission Time Interval (TTI) to achieve higher data rates, and these channels are shared among all UEs in the cell on a TTI-by-TTI basis. The HS-PDSCH is characterized by its use of a fixed spreading factor (SF=16) and its operation on secondary scrambling codes, which distinguishes it from the primary scrambling code used for common and dedicated channels.

From a physical layer perspective, the HS-PDSCH carries the coded and modulated transport blocks of the HS-DSCH. The channel employs adaptive modulation, switching between Quadrature Phase Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16QAM), and later 64QAM (from Release 7), based on the Channel Quality Indicator (CQI) reported by the UE. The choice of modulation and the Transport Block Size (TBS) directly determines the instantaneous data rate. The channelization codes for the HS-PDSCH are drawn from a pool of codes with a spreading factor of 16, separate from the code tree used for dedicated channels (DCH). A UE can be assigned between 1 and 15 such codes in a TTI, depending on its capability (UE category), channel conditions, and scheduler decision. The set of codes is indicated to the UE via the associated HS-SCCH.

The transmission process is tightly synchronized. In each 2 ms TTI (subframe), the Node B scheduler decides which UE(s) to serve, selects the modulation and coding scheme (MCS), and allocates a specific set of channelization codes. This control information is sent on the HS-SCCH, which starts two slots (approx. 1.33 ms) before the corresponding HS-PDSCH transmission. This gives the UE time to decode the HS-SCCH and configure its receiver for the impending data transmission. The HS-PDSCH itself carries no explicit control information; all necessary decoding parameters are provided by the HS-SCCH. After attempting to decode the data, the UE sends a HARQ acknowledgment (ACK or NACK) on the uplink HS-DPCCH. The physical layer processing includes channel coding (Turbo coding), physical channel segmentation, and interleaving, as defined for the HS-DSCH transport channel, before mapping to the HS-PDSCH physical channel symbols.

Purpose & Motivation

The HS-PDSCH was created as the physical layer enabler for the high-speed shared channel concept of HSDPA. Prior to HSDPA, downlink user data in UMTS was primarily carried on the Dedicated Physical Channel (DPCH), which was inefficient for bursty packet data. The DPCH used a variable spreading factor and required permanent code allocation per user, leading to code tree exhaustion and limited peak rates. The HS-PDSCH addressed these limitations by adopting a fixed, low spreading factor (SF=16) and operating on secondary scrambling codes, which freed up the primary code tree for voice and signaling. This design allowed the system to allocate a large, contiguous block of channelization codes (up to 15) to a single user for a very short duration (2 ms), enabling very high peak data rates.

Its introduction solved the fundamental physical layer bottleneck for downlink throughput. By fixing the spreading factor, the chip rate per symbol was effectively increased, allowing more data bits per symbol when combined with higher-order modulation. The use of secondary scrambling codes created a parallel, dedicated resource pool for high-speed data that did not interfere with the operation of legacy Release 99 channels. Furthermore, the short TTI and the decoupling of control (HS-SCCH) from data (HS-PDSCH) allowed for the rapid, flexible scheduling and link adaptation that are hallmarks of HSPA. The HS-PDSCH is thus the physical workhorse that translated the HSDPA transport channel enhancements into tangible radio performance gains, making UMTS competitive with other broadband wireless technologies.