In 5G NR, the preamble format and sequence length for the Random Access Channel (RACH) are critical components that influence the performance and reliability of initial access procedures. The preamble sequences are designed to accommodate various propagation conditions, cell sizes, and timing advance requirements. They are categorized into short and long sequences, each serving different purposes:

Short Sequence Formats:

Short sequences are primarily used in scenarios with typical cell sizes and delay spreads. They are designed for general purposes, including a wide range of deployment scenarios from urban to rural. The short sequence formats are identified as A1, A2, A3, B1, B2, B3, B4, C0, and C2, with each format tailored for specific conditions:

• A1, A2, A3: These formats are suitable for different coverage levels and small to medium cell radii. The variations among A1, A2, and A3 mainly address different levels of delay spread and timing advance requirements. For example, A1 is typically used for small cells with low delay spread, while A3 supports larger cells or higher delay spreads. The sequence length for these formats is 139, and they use a subcarrier spacing (SCS) of 1.25 kHz or 5 kHz, depending on deployment needs.
• B1, B2, B3, B4: Optimized for medium to large cell sizes, these formats offer a balance between coverage and capacity. The B-series caters to a broader range of cell sizes and delay spreads, with B4 specifically designed for larger delay spreads. The sequence length for B formats is also 139, but they use a higher SCS of 7.5 kHz, which allows for shorter preamble duration and is suitable for scenarios with higher frequency bands or more challenging radio environments.
• C0, C2: These are specialized formats, with C0 being tailored for specific use cases requiring very high timing accuracy, and C2 designed for extended coverage scenarios, such as in rural or deep indoor environments. The C formats use a sequence length of 139 and an SCS of 15 kHz, making them suitable for high-frequency deployments or scenarios where precise timing is critical.

The short sequence formats are defined in 3GPP TS 38.211, Table 6.3.3.1-2, and are mainly used for FR1 (Frequency Range 1, sub-6 GHz) deployments. The flexibility in sequence length and SCS allows operators to optimize RACH performance based on specific deployment scenarios.

Long Sequence Formats:

Long sequences, identified as formats 0, 1, 2, and 3, are used for very large cell radii and scenarios requiring extended coverage and high reliability. These formats are characterized by longer sequence lengths, providing better performance in terms of timing advance measurement and detection reliability in challenging conditions:

• 0, 1, 2, 3: The long sequence formats are designed to support scenarios with significant delay spreads and provide reliable performance over greater distances. They are particularly useful in rural deployments, where cells need to cover larger areas with minimal infrastructure. The sequence length for long formats is 839, and the SCS is typically 1.25 kHz or 5 kHz. The longer sequence length improves the ability to distinguish between different UEs and enhances detection in environments with high noise or interference.

The long sequence formats are also specified in 3GPP TS 38.211, Table 6.3.3.1-1, and are mainly used for FR1 deployments where large cell coverage is required, such as in rural or remote areas. The use of a longer sequence helps to accommodate larger timing uncertainties and provides robust performance even in challenging radio conditions.

Summary Table of Preamble Formats:

Format	Sequence Length	Subcarrier Spacing (kHz)	Typical Use Case
0, 1, 2, 3 (Long)	839	1.25 / 5	Large cells, rural, high delay spread
A1, A2, A3 (Short)	139	1.25 / 5	Small to medium cells, urban/suburban
B1, B2, B3, B4 (Short)	139	7.5	Medium to large cells, higher frequency bands
C0, C2 (Short)	139	15	Specialized, high accuracy, deep indoor

The choice between short and long sequence formats depends on specific network requirements, including cell size, expected delay spread, and coverage needs. The adaptability of these formats allows 5G networks to optimize the RACH process for efficiency and reliability across a wide range of deployment scenarios.

Further Readings

• 5G RACH