Physical downlink shared channel-related procedures

Article by Abhijeet Kumar

All information summarised it from 38.214 section 5, I summarize this information in a shorter version so that you can understand this document

Physical downlink shared channel related procedures" in the document discusses the detailed procedures and parameters involved in how User Equipment (UE) receives the Physical Downlink Shared Channel (PDSCH) in 5G New Radio (NR).

Transmission Schemes (5.1.1)

The document specifies that only one transmission scheme is defined for the PDSCH, which is utilized for all PDSCH transmissions. This simplifies the UE's job in configuring its receiver since it only needs to support this single scheme for handling PDSCH.

Key Information:

• Uniformity: The same transmission scheme is applied across all scenarios, ensuring consistency in how data is sent and processed.
• Simplicity: Reduces the complexity of the UE's design and operation.

2. Resource Allocation (5.1.2)

Resource allocation for PDSCH involves assigning time and frequency resources for downlink transmissions.

Time Domain Allocation:

• Slot Offsets and Symbols: Determines the specific timing for when the PDSCH should be transmitted.
• Allocation Length: Defines how long the PDSCH occupies the time-frequency grid.

Frequency Domain Allocation:

• Type 0 and Type 1 Allocations: These are the two main types of resource allocations.Type 0: Typically involves static or semi-static allocation across the bandwidth.Type 1: Allows for dynamic allocation, giving flexibility based on changing network conditions.

Key Information:

• Efficiency and Flexibility: Allocation types are chosen based on network needs and channel conditions, aiming to optimize resource usage and adapt to varying demands.

3. Modulation Order and Target Code Rate (5.1.3)

This section focuses on how modulation and coding parameters are adapted based on the channel quality.

Modulation and Code Rate:

• Adaptation: Parameters are dynamically adjusted to maintain data throughput and quality.
• Channel Quality Indicators (CQI): Used to select the appropriate modulation scheme and code rate.

Transport Block Size Determination:

• Based on Allocated Resources: The size of the data blocks is computed considering the modulation scheme, code rate, and number of Resource Blocks (RBs).

Key Information:

• Optimization: Ensures that the network efficiently uses available spectrum and power while maximizing data throughput.

4. PDSCH Resource Mapping (5.1.4)

Details the mapping of PDSCH data onto the physical resource blocks.

Mapping Granularity:

• RB Symbol Level and RE Level: Indicates the precision with which data is mapped to the physical resources, affecting how finely the system can control interference and optimize capacity.

Key Information:

• Precision and Control: Fine-grained mapping allows precise control over how data is transmitted, enhancing the ability to manage interference and optimize resource use.

5. Antenna Ports and Quasi Co-location (5.1.5)

Explains how antenna ports are configured and their relationship to reference signals.

Quasi Co-location:

• Relation to Reference Signals: Ensures that certain antenna configurations are aligned with reference signals for accurate channel estimation and signal decoding.

Key Information:

• Signal Integrity and Channel Estimation: Proper configuration and co-location are critical for accurate signal processing and channel estimation.

6. UE Procedure for Receiving Reference Signals (5.1.6)

Covers the procedures for UE to receive various reference signals necessary for demodulation and channel estimation.

Reference Signals:

• CSI-RS, DM-RS, PT-RS, SRS: Each plays a specific role in assisting the UE in understanding the channel conditions and correctly decoding the received signals.

Key Information:

• Robust Reception: Reference signals guide the UE in adapting to channel conditions, crucial for maintaining communication reliability and quality.

7. Code Block Group Based PDSCH Transmission (5.1.7)

Describes the segmentation of PDSCH data into code blocks and groups for efficient error handling and decoding.

Code Block Groups (CBGs):

• Segmentation and Grouping: Facilitates selective retransmission and efficient error correction, reducing the overhead in case of transmission errors.

Key Information:

• Efficiency in Error Correction: By grouping code blocks, the system can perform targeted retransmissions, enhancing overall transmission efficiency.

Lets Understand Resource allocation in Time and Frequency.

Time Domain Allocation

Overview: Time domain allocation determines when the PDSCH is transmitted in the context of 5G NR frames and slots. Each slot in 5G can carry a mix of downlink, uplink, and flexible parts, and time domain allocation specifies the exact timing and duration for PDSCH transmissions within these slots.

Key Elements:

• Slot Offsets: Defines the delay from the start of a frame to the transmission of the PDSCH. This helps in synchronizing the transmission between the gNB and UE.
• Start Symbols and Length: Identifies the starting symbol within a slot and the number of symbols over which the PDSCH extends. This precision allows the gNB to dynamically adapt the transmission based on immediate channel conditions and traffic demands.
• Mapping Type: PDSCH can be mapped using either Type A or Type B:
• Type A is typically used for more static and predictable traffic patterns, offering regular periodicity.
• Type B provides more flexibility and is used for traffic with higher variability and burstiness.

Allocation Table: The allocation for PDSCH in the time domain refers to a table indexed by a parameter from the DCI (Downlink Control Information). This table provides configurations such as the slot offset, the starting symbol, the length of the allocation, and the mapping type.

Procedure:

1. DCI Reception: UE receives DCI indicating how to decode the upcoming PDSCH.
2. Reading Allocation Table: UE uses the index provided in the DCI to look up the time domain allocation table.
3. Slot and Symbol Calculation: UE calculates the exact slot and symbols where the PDSCH will be transmitted based on the current frame structure and the table entry.
4. Ready to Receive: UE adjusts its receiver to start demodulating and decoding the PDSCH at the specified time.

Table 5.1.2.1-1: Valid S and L combinations

valid combinations of starting symbol SSS and length LLL for PDSCH transmissions under two types of PDSCH mapping (Type A and Type B) and two types of cyclic prefixes (normal and extended). These combinations define how data is arranged in time within a slot, which can have implications on the timing and efficiency of data transmission. Let's break down the table and then provide an example of time-domain mapping using this data.

Table Explanation:

1. PDSCH Mapping Type:

• Type A: Generally used for smaller allocation sizes.
• Type B: Used for larger and more dynamic allocation sizes, covering a broader range of slot configurations.

2. Cyclic Prefix:

• Normal Cyclic Prefix: Typically used in environments with lower delay spreads.
• Extended Cyclic Prefix: Used in environments with higher delay spreads to avoid inter-symbol interference.

3. Valid S and L Combinations:

• S: Starting symbol within the slot.
• L: Length of the transmission in symbols.
• S+L: Total span of the transmission, providing the end symbol within the slot.

Frequency Domain Allocation

Overview: Frequency domain allocation specifies how the available spectrum is divided and allocated for PDSCH transmissions. It determines which frequency resources (subcarriers) within a given bandwidth are used for PDSCH.

Types of Resource Allocation:

• Type 0 (Resource Allocation Type 0): This is a static or semi-static allocation where a set of contiguous resource blocks (RBs) is assigned to the PDSCH.
• Type 1 (Resource Allocation Type 1): Offers dynamic allocation capabilities, where non-contiguous RBs can be assigned to a PDSCH. This type is more flexible and can adapt to varying channel conditions and interference scenarios.

Key Elements:

• Resource Blocks (RBs): Basic units of frequency domain resources. An RB typically consists of 12 subcarriers.
• Resource Block Groups (RBGs): Groups of RBs that can be allocated together to improve management efficiency and adapt to UE capabilities.
• Precoding Information: Frequency domain allocation is closely linked with the precoding strategies used to optimize transmission for specific channel conditions.

Procedure:

1. Spectrum Assessment: gNB assesses the available spectrum and current network load.
2. DCI Configuration: gNB sends DCI with details about which RBs or RBGs are allocated for the upcoming PDSCH.
3. UE Decodes Allocation: UE uses the information from DCI to configure its receiver to focus on the specified frequency resources.
4. Reception and Decoding: UE receives the PDSCH on the allocated frequencies, applying the necessary precoding and modulation schemes to decode the data.

Let's learn in detail resource allocation in Frequency Domain

Type 0 (Resource Allocation Type 0)

Definition and Usage: Type 0 allocation is typically considered static or semi-static, meaning that the allocation patterns do not change frequently. This type of allocation assigns a set of contiguous Resource Blocks (RBs) to the PDSCH. An RB is the smallest unit of resources in the frequency domain that can be allocated to a user and consists of 12 subcarriers.

Characteristics:

• Contiguous Allocation: The resource blocks assigned to a transmission are consecutive in the frequency domain, forming a contiguous block of spectrum. This simplicity can be advantageous in environments with stable channel conditions and predictable traffic patterns.
• Simplicity in Signal Processing: Since the blocks are contiguous, the signal processing required for dealing with frequency-domain discontinuities (like guard bands between non-contiguous blocks) is reduced. This can lead to more efficient power usage and simpler receiver structures.
• Use Cases: Ideal for scenarios where traffic is consistent and channel conditions are stable, allowing for a set allocation that does not need to be frequently adjusted.

Type 1 (Resource Allocation Type 1)

Definition and Usage: Type 1 allocation is dynamic, offering the capability to assign non-contiguous resource blocks to a PDSCH. This flexibility is particularly useful in environments where channel conditions and interference patterns vary significantly across the spectrum.

Characteristics:

• Non-Contiguous Allocation: Allows the network to allocate resource blocks that are not sequentially adjacent in the frequency domain. This method can skip over parts of the spectrum that are experiencing high interference or poor channel conditions.
• Adaptability: Type 1 allocation can adapt more dynamically to real-time changes in the network, such as varying user demands and fluctuating channel quality. It enables the network to optimize resource use by avoiding bad parts of the spectrum while utilizing better-conditioned frequencies.
• Complexity in Signal Processing: The flexibility of non-contiguous allocation comes at the cost of increased complexity in signal processing. The transmitter and receiver must handle the variations in frequency allocation, potentially increasing the overhead for control signaling (to communicate the allocation to the UE).
• Use Cases: Best suited for dense urban environments with heterogeneous network deployments, where interference varies significantly across the spectrum and traffic demands fluctuate rapidly.