Hybrid Automatic Repeat Request (HARQ) is an essential technology in 5G New Radio (NR), integrating error correction and retransmission mechanisms to ensure data reliability and efficient spectrum utilization. HARQ plays a critical role in enhancing data transmission performance, especially in challenging wireless environments. This article explores HARQ in 5G, covering its protocols, types, benefits, technical details, and its application in different 5G use cases.
Automatic Repeat Request (ARQ) Protocol
The foundation of HARQ lies in the Automatic Repeat Request (ARQ) protocol, a feedback-based error control method used in data communication systems. ARQ employs an error detection code, typically the Cyclic Redundancy Check (CRC), to identify errors in the received data, ensuring data integrity by retransmitting erroneous data blocks until they are correctly received.
ARQ Data Flow
1. CRC Calculation
The sender calculates a CRC value for the data block and appends it to the data. CRC, or Cyclic Redundancy Check, is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. By appending a CRC value to the data, the sender provides a means for the receiver to verify the integrity of the received data.
2. LDPC Coding
The data, along with the appended CRC, is encoded using Low-Density Parity-Check (LDPC) coding. LDPC is a type of error-correcting code that enables the detection and correction of errors that may occur during data transmission. LDPC coding involves adding parity bits to the data, which helps in error detection and correction at the receiver's end.
3. Modulation
The encoded data is then modulated for transmission over the communication channel. Modulation is the process of converting the digital data into a signal that can be transmitted over a medium. This step involves mapping the encoded bits to specific signal waveforms suitable for the transmission channel.
4. AWGN Channel
The modulated signal is transmitted through an Additive White Gaussian Noise (AWGN) channel. The AWGN channel introduces noise into the signal, simulating real-world conditions where transmitted signals are often subjected to interference and noise.
The equation 𝑦=𝑥+𝑛 represents the received signal y as the sum of the transmitted signal x and the noise 𝑛.
5. Demodulation
At the receiver end, the signal is demodulated to retrieve the encoded data. Demodulation is the reverse process of modulation, where the received signal is converted back into its original digital form. This step involves extracting the encoded bits from the received signal waveforms.
6. LDPC Decoding
The demodulated data is decoded using LDPC decoding. This step attempts to correct any errors introduced during transmission using the parity bits added during the LDPC coding step. The LDPC decoder uses the structure of the code to detect and correct errors in the received data.
7. CRC Validation
The receiver validates the decoded data using the CRC. The CRC value appended to the data at the sender's end is recalculated and compared with the received CRC value. If the CRC check passes, it indicates that the data has been received correctly without errors.
8. Decoded Data
If the CRC check passes, the data is considered error-free and is successfully decoded. The receiver can now use the decoded data as intended, ensuring that the information has been transmitted accurately.
Error-Free Reception
If no error is detected in the received data block, the data is declared error-free, and the transmitter is notified by sending a positive acknowledgment (ACK).
Error Detection
If an error is detected, the receiver discards the received data and notifies the transmitter by sending a negative acknowledgment (NAK). In response to a NAK, the transmitter retransmits the same information.
Hybrid Automatic Repeat Request (HARQ)
HARQ enhances ARQ by combining error correction and retransmission mechanisms. In HARQ, erroneously received blocks are not discarded but retained. The receiver requests retransmissions of corrupted packets, combining them with previously received versions to improve decoding success.
Types of HARQ in 5G
There are two primary types of HARQ used in 5G networks:
Chase Combining
Incremental Redundancy
Chase Combining (CC)
Chase Combining involves retransmitting the same set of coded bits as the original transmission. Each retransmission is combined with previous transmissions to improve the overall signal quality. This method is particularly useful when the initial transmission encounters errors due to poor channel conditions.
Process of Chase Combining:
1. Initial Transmission: The transmitter sends a coded message to the receiver.
2. Error Detection: The receiver checks for errors using a Cyclic Redundancy Check (CRC). If errors are detected, the receiver requests a retransmission.
3. Retransmission: The transmitter sends the same coded bits as the initial transmission.
4. Combination: The receiver combines the retransmitted bits with the previously received bits, enhancing the signal quality.
5. Decoding: The improved signal is decoded to retrieve the original message.
Example:
Scenario: A receiver initially receives a message with poor signal quality, resulting in errors.
Response: The receiver requests a retransmission.
Result: The receiver combines the new transmission with the initial one, resulting in an improved signal that can be decoded correctly.
Combined Signal Analysis
First Transmission: 𝑦1 = 𝑥+𝑛1
Where, 𝑦1 is the received signal from the first transmission,
𝑥 is the transmitted signal, and
𝑛1 is the noise.
Second Transmission: 𝑦2 = 𝑥+𝑛2
Where, 𝑦2 is the received signal from the second transmission, and
𝑛2 is the noise.
Combining Signals: 𝑦1+𝑦2 = 2𝑥+𝑛1+𝑛2
Effective Signal-to-Noise Ratio (SNR)
The effective SNR for the combined signal can be calculated as:
Effective SNR=2𝑃/𝜎2
where 𝑃 is the signal power and 𝜎2 is the noise variance.
Example:
1. Initial Transmission:
Received Signal: 𝑦1=𝑥+𝑛1
Example Values: 𝑥=5, 𝑛1=1
Calculation: 𝑦1=5+1=6
2. First Retransmission:
Received Signal: 𝑦2=𝑥+𝑛2
Example Values: 𝑥=5, 𝑛2=2
Calculation: 𝑦2=5+2=7
3. Combining Signals:
Combined Signal: 𝑦1+𝑦2=2𝑥+𝑛1+𝑛2
Calculation: 𝑦1+𝑦2=6+7=13
Simplified: 2𝑥+𝑛1+𝑛2=2(5)+1+2=10+3=13
4. Effective SNR:
Given: 𝑃=5, 𝜎2=1
Calculation: Effective SNR=2𝑃/𝜎2=2×5/1=10
Incremental Redundancy (IR)
Incremental Redundancy involves retransmitting additional redundant information that was not included in the original transmission. Each retransmission provides new redundancy, which helps in decoding the original message more accurately.
Process of Incremental Redundancy:
1. Initial Transmission: The transmitter sends a coded message containing only a portion of the redundant bits.
2. Error Detection: The receiver checks for errors using a CRC. If errors are detected, the receiver requests a retransmission.
3. Retransmission: The transmitter sends additional redundant bits.
4. Combination: The receiver combines the new redundancy with the previously received bits.
5. Decoding: The enhanced redundancy allows for more accurate decoding of the original message.
Example:
Consider a communication system where the initial transmission has a 10% chance of bit errors. Transmitting a 1000-bit message in a noisy environment: Using Incremental Redundancy can reduce the error rate significantly.
1. Initial Transmission:
Error rate: 10%
Bits sent: 1000 (e.g., 900 data bits + 100 redundant bits)
2. First Retransmission:
Additional 100 redundant bits sent
Combined redundancy improves error detection/correction
New error rate: 5%
3. Second Retransmission:
Additional 100 redundant bits sent
Further improvement in error detection/correction
New error rate: 2%
Components of the Diagram
Systematic Bits:
These are the original data bits that need to be transmitted.
Redundancy Versions (RV):
RV0: The initial transmission (first transmission) containing the systematic bits.
RV1: The second transmission with additional parity bits to aid error correction.
RV2: The third transmission with different redundancy bits to further aid error correction.
RV3: The fourth transmission with another set of redundancy bits.
1. 1st Transmission (RV0):
Colour: Blue. Systematic bits (original data bits) are sent first to transmit the original data. If the receiver cannot decode the data correctly, a retransmission will be requested.
2. 2nd Transmission (RV1):
Colour: Green. Additional redundancy bits (parity bits) are sent to provide extra information for error correction if the first transmission fails.
3. 3rd Transmission (RV2):
Colour: Red. Another set of redundancy bits different from RV1 is sent to offer more error correction information, increasing the chances of correct decoding by the receiver.
4. 4th Transmission (RV3):
Colour: Grey. Yet another set of redundancy bits is sent to maximize the error correction capability, ensuring the highest probability of successful data decoding.
Incremental Redundancy (IR) techniques example:
Step 1: Initial Transmission
Message Bits: 101
Basic Rate: 1/4 (for 3 message bits, output = 12 bits)
Transmission: Every third coded bit is sent.
Transmitted Bits: 1010 (4 bits)
Effective Code Rate: ¾
Step 2: First Retransmission
Additional Bits Sent: 4 bits
Effective Code Rate: 3/(4+4) = 3/8
Step 3: Second Retransmission
Additional Bits Sent: 4 bits
Effective Code Rate: 3/(4+4+4) = ¼
Step 4: Third Retransmission
Old Bits Retransmitted: 4 bits
Effective Code Rate Remains: 1/4
HARQ codebooks?
In the context of 5G Hybrid Automatic Repeat Request (HARQ), codebooks play a crucial role in managing the acknowledgment (ACK) and negative acknowledgment (NACK) feedback for transmitted data blocks. A User Equipment (UE) can receive multiple Code Block Groups (CBGs) or Transport Blocks (TBs) together, especially in scenarios involving Carrier Aggregation (CA), where multiple TBs may be received in a single Transmission Time Interval (TTI). The mechanism for determining the number of ACK/NACK bits to send and how to structure these bits is defined by a HARQ codebook.
1. CBG-Based HARQ-ACK Codebook
Configuration and Operation:
Maximum CBGs per TB: The RRC signaling specifies the maximum number of Code Block Groups (CBGs) that can be included in a Transport Block (TB).
CBG Feedback Indicator (CBGFI): Indicates which CBGs are present in the TB.
NACK for Remaining Bits: If a TB contains fewer CBGs than the maximum configured, the UE will send NACKs for the remaining positions. This ensures that the network is aware of the status of all expected CBGs, even if they were not transmitted.
Advantages:
Granularity: Provides fine-grained feedback at the CBG level, which can improve retransmission efficiency and reduce the need for full TB retransmissions.
Flexibility: Allows for more adaptive and efficient error correction strategies.
Use Case:
Enhanced Mobile Broadband (eMBB): Particularly beneficial in scenarios requiring high data rates and reliability, such as streaming high-definition video, where efficient retransmissions are critical.
1. Type-1 HARQ-ACK Codebook
Configuration and Operation:
Semi-Static Nature: The number of ACK/NACK bits is predetermined and remains constant regardless of the actual number of scheduled component carriers.
Fixed Bit Size: This fixed size can result in inefficient use of resources if the actual number of active carriers is less than the maximum configured.
Advantages:
Predictability: Simplifies the implementation and reduces the signaling overhead as the size of the codebook does not change dynamically.
Stability: Suitable for stable and predictable network environments with less fluctuation in the number of scheduled carriers.
Use Case:
Fixed Wireless Access (FWA): Useful in scenarios with stable connectivity and less variability in the number of active component carriers, such as in fixed broadband connections.
2. Type-2 HARQ-ACK Codebook
Configuration and Operation:
Dynamic Nature: The number of bits in the ACK/NACK report varies based on the actual number of scheduled component carriers.
Enhanced Dynamic Codebook (Release 16 onwards): Further optimizes feedback by adapting more effectively to the current network conditions.
Downlink Assignment Index (DAI): Helps the UE correctly infer the number of scheduled carriers, mitigating issues in poor channel conditions.
Advantages:
Efficiency: Reduces waste of resources by providing feedback only for scheduled carriers, optimizing uplink bandwidth usage.
Flexibility: Adapts to varying network conditions, improving overall system performance.
Use Case:
Dynamic Network Environments: Ideal for scenarios with frequent changes in the number of active component carriers, such as in highly mobile or dense urban environments.
3. Type-3 HARQ-ACK Codebook
Configuration and Operation:
OneShotFeedback: The UE sends a comprehensive ACK/NACK report covering all HARQ processes and all component carriers configured within a PUCCH group.
Full Scope Feedback: Provides a holistic view of the transmission status across multiple carriers and processes.
Advantages:
Comprehensive Feedback: Ensures that the network receives complete status information in a single report, facilitating more informed retransmission decisions.
Simplicity: Reduces the complexity of multiple, fragmented feedback reports.
Use Case:
Massive Machine Type Communications (mMTC): Suitable for applications requiring robust and comprehensive feedback mechanisms, such as in IoT networks with many devices and sensors needing reliable communication.
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