HSDPA (High-Speed Downlink Packet Access) - 1.3.3.1

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The primary goal of HSDPA was to increase peak data rates and improve spectral efficiency for packet-switched data services. It achieved this through a set of sophisticated technical features:

1.  **Adaptive Modulation and Coding (AMC):** This is one of HSDPA's most fundamental features. Instead of using a fixed modulation scheme, AMC dynamically selects the optimal modulation and coding rate based on the real-time quality of the radio channel. When the mobile device is close to the base station and has a strong, clear signal, HSDPA can utilize **16-Quadrature Amplitude Modulation (16QAM)**, which allows it to transmit four bits of information per symbol. This is double the data rate of QPSK (Quadrature Phase Shift Keying), which was the primary modulation for data in original W-CDMA, transmitting only two bits per symbol. Conversely, if the signal quality degrades, the system can adapt to a more robust, but less spectrally efficient, QPSK modulation to maintain connection reliability. This continuous adaptation ensures that the highest possible data rate is used at all times under given channel conditions, maximizing throughput.

2.  **Hybrid Automatic Repeat Request (HARQ):** HARQ is a highly efficient error control mechanism. When a data packet is transmitted from the Node B to the mobile, the mobile checks for errors. If errors are detected, instead of requesting a complete retransmission of the packet, the mobile requests only the corrupted parts. Crucially, it **stores the initially received, even if erroneous, data**. When the retransmitted parts arrive, the receiver combines them with the stored data before attempting to decode the complete packet. This "soft combining" significantly increases the probability of successful decoding with fewer retransmissions, leading to lower latency and higher effective data throughput.

3.  **Fast Packet Scheduling at the Node B:** In the original W-CDMA, packet scheduling decisions were made by the Radio Network Controller (RNC), which is a centralized entity further away from the radio interface. HSDPA revolutionized this by moving the scheduling function directly to the **Node B (the base station)**. The Node B has immediate and highly accurate information about the instantaneous radio channel conditions for all active users within its cell. This allows for much faster and more intelligent scheduling decisions, typically made every **2 milliseconds (ms)**, which is the HSDPA Transmission Time Interval (TTI). This rapid scheduling enables the Node B to quickly allocate resources to users experiencing the best channel conditions at any given moment, maximizing overall cell throughput and reducing delays for interactive services.

4.  **Shorter Transmission Time Interval (TTI):** As mentioned, HSDPA reduced the TTI from 10 ms (in original W-CDMA) to **2 ms**. This shorter interval allows for quicker adaptation to changing radio channel conditions (via AMC), faster feedback to the scheduler, and more rapid retransmissions via HARQ. The increased responsiveness of the air interface is fundamental to achieving higher data rates and lower latency.

5.  **New Transport Channel (HS-DSCH):** HSDPA introduced a new downlink transport channel called the **High-Speed Downlink Shared Channel (HS-DSCH)**. This channel is specifically optimized for high-speed, bursty packet data. Unlike dedicated channels in W-CDMA that assign fixed resources to a user, the HS-DSCH is dynamically shared among multiple users in the cell. It uses a fixed spreading factor (typically 16) and relies on fast Node B scheduling and AMC for link adaptation, rather than fast power control for the data channel itself. This shared channel approach allows for very efficient multiplexing of users and flexible resource allocation.

The combined effect of these features enabled HSDPA to deliver a significant leap in 3G performance, offering theoretical peak downlink speeds that far surpassed original 3G capabilities and provided a truly transformative experience for mobile data users. It also offered operators a cost-effective upgrade path for their existing W-CDMA networks.

Key Concepts

• Downlink Focus: HSDPA specifically enhances download speeds.

• Channel Adaptation: AMC intelligently uses radio conditions.

• Efficient Retransmission: HARQ saves time and bandwidth.

• Node B Intelligence: Faster decisions for better throughput.

• Shared Resources: HS-DSCH for dynamic, efficient packet data.

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• Examples

• Faster Web Browse: Before HSDPA, loading image-rich websites on 3G could be slow. With HSDPA, pages loaded noticeably quicker, akin to a basic DSL connection.

• Quicker File Downloads: Downloading an email attachment or a small song file went from taking minutes to seconds with HSDPA.

• Dynamic Speed Variation: When you're standing right next to a 3G tower, your phone can download at very high speeds (using 16QAM). As you walk away and the signal weakens, HSDPA automatically switches to a more robust but slower mode (QPSK) to maintain the connection, rather than dropping it.

• Seamless Download Recovery: If a brief interference burst causes part of a file download to be corrupted, HARQ quickly requests only the missing pieces and stitches them back together, preventing a full restart of the download.

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• Flashcards

• Term: HSDPA

• Definition: High-Speed Downlink Packet Access, 3GPP Rel-5 enhancement for faster downloads.

• Term: AMC (HSDPA)

• Definition: Adaptive Modulation and Coding; adjusts modulation (QPSK/16QAM) and coding based on channel quality.

• Term: HARQ (HSDPA)

• Definition: Hybrid Automatic Repeat Request; combines retransmissions with soft combining for error recovery.

• Term: Fast Packet Scheduling (HSDPA)

• Definition: Resource allocation decisions moved to Node B for rapid, efficient user serving.

• Term: HS-DSCH

• Definition: High-Speed Downlink Shared Channel; new shared transport channel for HSDPA packet data.

• Term: 2ms TTI

• Definition: Shortened Transmission Time Interval in HSDPA for faster responsiveness.

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• Memory Aids

• "HSDPA: Has Super Downlink Power Acceleration": Helps remember its name and purpose.

• "AMC: All My Conditions": Reminds of Adaptive Modulation and Coding reacting to channel conditions.

• "HARQ: Hold And Repair Quickly": For Hybrid Automatic Repeat Request, combining and retransmitting.

• "Node B Knows Best": To remember fast scheduling moved to Node B.

• "HS-DSCH: High-Speed Data SHared Channel": Directly tells its function.

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