Think of a RACH preamble as a calling card for a UE. It’s a specific signal transmitted by the UE on the RACH channel to grab the attention of the eNodeB.

There are 64 preambles available in an LTE, and UEs can choose one randomly during the initial access or when requesting uplink resources.

Random Access Preamble Format

• CP (Cyclic Prefix): Represented by TCP in the text, the cyclic prefix is a crucial part of the preamble. It consists of a copy of the ending portion of the data sequence appended to the beginning of the actual data. This helps mitigate a phenomenon called inter-symbol interference (ISI) that can occur in wireless channels. ISI arises when signal reflections cause symbols to overlap and interfere with each other, leading to errors in data reception. By introducing the cyclic prefix, the receiver can effectively ignore the overlapping tails of symbols from previous transmissions, reducing ISI and enhancing signal reception.

Benefits of Longer CP:

Improved Tolerance in Fading Environments:

A longer CP helps mitigate the effects of multipath fading, a phenomenon where the signal reaches the receiver through multiple paths with varying delays. The CP absorbs these delays, preventing them from causing inter-symbol interference (ISI) and improving data integrity.

Reduced ISI even in highly fading environment: By absorbing the delayed signal components within the CP, longer CP minimizes ISI even in situations with severe fading.

• Sequence (TSEQ): Represented by TSEQ in the text, the sequence part is the core of the preamble that carries the unique identity for the UE. This sequence differentiates one preamble from another, allowing the eNodeB to distinguish between UEs attempting random access. The specific pattern of the sequence is predetermined and chosen based on higher layer control parameters.

Advantages of Longer T_SEQ (Formats 2 and 3):

Improved Decoding in Noisy Conditions: A longer sequence provides a larger correlation window for the receiver to detect the PRACH signal. This is particularly beneficial in noisy environments where the signal might be weak or distorted.

Enhanced Noise Resistance: The increased length offers a higher signal-to-noise ratio (SNR), making it easier for the eNodeB to distinguish the PRACH signal from background noise.

The details of random-access preambles used in LTE networks, focusing on Cyclic Prefix (CP), sequence length (T_SEQ), and the rationale behind different preamble formats.

Cyclic Prefix (CP) and its Calculation:

The formula for calculating the Cyclic Prefix (CP) in milliseconds (ms):

T_CP (in ms) = T_CP(in Ts) x 0.03255 x 1/1000

Here’s a breakdown of the formula:

• T_CP (in Ts): Represents the length of the CP in units of the sampling period (Ts). Ts is typically 1 microsecond (us) in LTE.
• 0.03255: Represents the conversion factor from Ts (microseconds) to milliseconds (ms).
• 1/1000: Converts the result from microseconds to milliseconds.

Guard Time and Cell Radius

Guard Time: This is an additional period inserted between transmissions to prevent interference between adjacent cells.

It’s calculated based on the cell radius and the time it takes for the signal to travel that distance.

Preamble Formats

There are preamble formats and the varying lengths of sequences (T_SEQ) associated with them:

• Formats 0 and 1: These formats have a single copy of the PRACH sequence converted to the time domain. This means the T_SEQ for these formats represents the length of a single PRACH sequence.
• Formats 2 and 3: These formats have two copies of the PRACH sequence concatenated (joined together). This results in a longer T_SEQ compared to formats 0 and 1.
• Format 4: preamble applicable only to Time Division Duplex (TDD) operation. This format is likely transmitted during specific subframes within the Downlink Pilot Time Slot (DwPTS) field, which is a dedicated control signalling period in TDD mode.

Why Multiple Preamble Format in RACH?

LTE utilizes multiple preamble formats in its Random-Access Channel (RACH) for a crucial reason: to strike a balance between reliable signal detection and efficient resource utilization.

Why there are different formats, focusing on the advantages of longer sequence length (T_SEQ) and Cyclic Prefix (CP) duration:

• Longer sequence (formats 2 & 3) helps find the signal better in noisy areas (like a longer phrase in a loud room).
• More sequence also improves dealing with background noise.
• Longer cyclic prefix (formats 1 & 3) helps reduce signal overlap in changing environments (like a bigger pause between sentences to avoid echo).
• This longer prefix also helps a lot in very difficult signal areas.
• But there’s a catch! Longer formats use more resources.
• So, the network picks the best format based on noise, traffic, and how strong your signal is.

LTE RACH Preamble Format Selection Strategies

This process involves reserving specific resource blocks for PRACH transmissions using two crucial parameters advertised in System Information Block 2 (SIB 2):

• PRACH Configuration Index
• PRACH Frequency Offset

1. PRACH Configuration Index:

Think of the PRACH configuration index as a codebook entry that defines two key aspects of PRACH transmission:

• Preamble Format: This specifies the specific type of preamble signal the UE will utilize during access attempts. Recall from previous discussions that different preamble formats exist, offering trade-offs between sequence length, resource consumption, and robustness against noise and fading. The configuration index dictates the appropriate format based on network conditions.
• Subframes for Random Access: This aspect outlines the specific subframes within a frame where the UE is authorized to transmit its PRACH preamble. Imagine a schedule where each subframe represents a time slot. The configuration index indicates the permissible slots within a frame for UEs to initiate random access attempts.

2. PRACH Frequency Offset:

While the configuration index manages the timing (subframes) of PRACH transmissions, the PRACH frequency offset plays a crucial role in the frequency domain allocation. Here’s how it functions:

• Resource Block Allocation: LTE data is transmitted in units called Resource Blocks (RBs). The PRACH frequency offset defines the specific RBs within the available bandwidth that will be dedicated to PRACH transmissions.
• Frequency Domain Location: This parameter essentially pinpoints the starting RB within the frequency domain where the UE’s PRACH signal will be located. Imagine a multi-lane highway; the frequency offset indicates the specific lane(s) reserved for PRACH transmissions within the overall spectrum allocated to the cell.

Example:

Consider a scenario where the PRACH configuration index is 5:

Interpretation: Based on the specific LTE system configuration, this index might indicate that the UE is allowed to transmit the PRACH preamble in subframe 7 of every frame. Additionally, it might specify the use of preamble format 0, which could be a shorter format suitable for situations without excessive noise or fading (depending on the network configuration).

Now, let’s say the PRACH frequency offset is 7: Interpretation: This offset suggests that the UE’s PRACH transmission will occupy resource blocks 7 to 12 within the frequency domain. These RBs will be exclusively reserved for UEs attempting random access using the format and timing dictated by the configuration index (subframe 7, preamble format 0 in this case).

In essence, the PRACH configuration index and frequency offset work in tandem to precisely coordinate the timing (subframes) and frequency domain location (resource blocks) for PRACH transmissions. This meticulous allocation scheme ensures efficient utilization of network resources and minimizes potential interference between UEs attempting random access.