In the dynamic world of LTE networks, the Random-Access Channel (RACH) plays a vital role in establishing communication between UE and eNodeB. It’s the lifeline for UEs to gain initial access and transmit data efficiently. This article goes deeper into the functionalities of RACH and explores the specific scenarios where it comes into play.

Understanding RACH’s Functionalities or Why Rach?

Beyond its role as an uplink transport channel, RACH offers a multifaceted approach to uplink communication:

• Uplink Synchronization: Imagine two orchestras trying to play together. Without proper synchronization, the music would be chaotic. Similarly, in LTE, successful data exchange relies on synchronized uplink and downlink transmissions. RACH facilitates this synchronization by allowing the UE to align its uplink signal with the network’s downlink timing and frequency.

• Random Access Procedure: Unlike a dedicated lane on a highway, RACH operates on a “first-come, first-served” basis. This is called the Random-Access Procedure. When multiple UEs attempt to access the network simultaneously, they transmit a short signal called a preamble on the RACH channel. The network then decodes these preambles and assigns resources to the UEs based on a pre-defined mechanism (contention-based or contention-free) to avoid collisions and ensure successful communication.

• Scheduling Request Transmission (RA_SR): Not all UEs have dedicated resources (physical channels) for uplink data transmission all the time. When a UE needs to send data but lacks a dedicated channel, it utilizes RACH to transmit a scheduling request message called RA_SR (Random Access Scheduling Request). This informs the network about the UE’s need to transmit data, and the network allocates resources based on traffic conditions and pre-defined scheduling policies.

Scenarios Triggering the RACH Process or When RACH procedure occurs?

Now that we understand Why RACH, let’s explore the specific situations where a UE relies on this channel:

• Initial Access (RRC_IDLE Mode): When a UE powers on for the first time, it’s in a disconnected state (RRC_IDLE mode). To establish communication with the network, the UE initiates the RACH process by transmitting a preamble and sending connection setup information.

• RRC Reconnection Establishment: During network congestion or handover, a UE might lose connection with the network. To re-establish this connection, the UE again utilizes RACH to transmit a reconnection request and synchronize with the network.

• Loss of Uplink Synchronization: Maintaining synchronization is crucial for efficient data exchange. If the UE loses synchronization due to factors like signal fading or interference, it relies on RACH to regain synchronization with the network’s downlink signal.

• Handover with No Dedicated PUSCH Resources: During a handover, the UE might not have dedicated resources (PUSCH) allocated on the target cell. In such scenarios, the UE leverages RACH to request access and obtain the necessary resources for uplink data transmission on the new cell.

Optimizing RACH Usage

While RACH is essential for uplink access, excessive use can lead to network congestion and delays. To optimize RACH usage, various techniques are employed:

• Power Control: UEs adjust their transmission power based on network conditions to avoid overpowering other UEs and minimize collisions during the random-access procedure.
• Access Class Barring (ACB): The network can temporarily restrict UEs from accessing RACH under high traffic conditions to prevent congestion.
• Scheduling Requests: UEs can be configured to send scheduling requests (RA_SR) only when they have data to transmit, reducing unnecessary RACH usage.

Types of RACH – Contention-Based Random Access (CBRA) and Contention-Free Random Access (CFRA)

The two primary RACH procedures: Contention-Based Random Access and Contention-Free Random Access.

Understanding Preambles: The Signatures of Random Access

Before exploring the RACH procedures, it’s crucial to understand preambles. These act as unique “signatures” transmitted by the UE to the eNodeB to signal its access request. There are 64 preambles available in an LTE system, and UEs can randomly choose one during the access process.

Two Groups of Preambles:

Contention-Free Preambles: These are preambles specifically allocated to UEs undergoing handover between cells. This allocation ensures prioritized access for handover UEs, minimizing delays.

Contention-Based Preambles: The remaining preambles are used by UEs during the initial access procedure or when they lack dedicated resources for uplink transmission. These scenarios involve multiple UEs potentially contending for access, hence the name.

Contention-Based Random Access

As the name suggests, this procedure comes into play when multiple UEs attempt to access the network simultaneously. Here’s a breakdown of the steps involved:

• Step 1: UE Transmits Preamble: The UE selects a random preamble from the contention-based pool and transmits it on the RACH channel. The preamble selection also carries information about the required scheduling resources based on the data payload in Layer 3 (L3) packets the UE intends to send.

• Step 2: eNB Response and Collision Potential: The eNodeB attempts to decode the received preambles. If successful, it sends a Random-Access Response (RAR) to all UEs that transmitted a preamble, regardless of their identities at this stage. This RAR is broadcasted on the Downlink Shared Channel (DL-SCH) and includes an RA-RNTI (Random Access Response Temporary C-RNTI). The RA-RNTI identifies the resource block where the eNodeB decoded the preamble. Each UE is allocated a specific time slot to receive the RAR. If a UE doesn’t receive a response within its designated time window, it assumes a collision and retransmits a preamble with increased power in the next available slot. It’s important to note that if multiple UEs transmitted the same preamble and the eNodeB successfully decoded all of them, all UEs will receive an RAR, indicating they have a scheduled slot for sending an RRC (Radio Resource Control) connection request in the next step.

• Step 3: UE Sends RRC Request and Contention Resolution: Utilizing the resource block allocated in Step 2, the UE transmits an RRC connection request message on the Uplink Shared Channel (UL-SCH). This message includes the UE’s identifier, allowing the eNodeB to distinguish between UEs that might have collided in the previous step. Here’s where contention resolution takes place. If multiple UEs transmitted on the same resource block due to a collision, only the UE with the best radio conditions (strongest signal) will have its message successfully decoded by the eNodeB. The eNodeB then broadcasts a contention resolution message on the DL-SCH, which includes the identifier of the successful UE. UEs that cannot decode the message or whose identifiers don’t match back off and wait for the next access attempt. The UE that successfully transmitted its RRC request message can then proceed with data transmission on the allocated uplink resource block.

Contention-Free Random Access

In contrast to the contention-based approach, contention-free random access is initiated by the network, specifically during a UE handover scenario. Here’s a simplified view of the process:

• Step 1: eNodeB Reserves and Assigns Preamble: The serving eNodeB reserves a set of preambles dedicated to handover and assigns a specific preamble from this pool to the UE.

• Step 2: UE Responds with Preamble and ID: Since the eNodeB controls the process, there’s no contention here. The UE transmits the assigned preamble along with its identifier in response.

• Step 3: eNodeB Sends Random Access Response: Upon receiving the UE’s response, the eNodeB confirms the successful handover by sending an RAR that includes information about the resources allocated to the UE on the target cell.

What is RACH Preamble and preamble format?

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.

In LTE, the network controls when UEs transmit RACH.

The network plays a proactive role in managing RACH access. It broadcasts System Information Block 2 (SIB 2), which contains crucial parameters for RACH configuration. These parameters essentially tell the UE:

• When (Subframes): SIB 2 specifies the specific subframes within a frame where the UE is authorized to transmit its RACH preamble. Imagine a schedule where each subframe represents a time slot. The network dictates the permissible slots for UEs to initiate random access.
• How (Preamble Format): SIB 2 also defines the type of preamble signal (format) the UE should utilize during access attempts. Recall that different preamble formats exist, offering trade-offs between factors like resource utilization and robustness against noise. The network selects the appropriate format based on prevailing conditions.

This section could provide a more in-depth reference to the specific part of the 3GPP specification (36.331) that describes the RACH configuration information within SIB 2.

• There is total 64 preambles for each cell.
• Preamble available are divided into 2 groups. (Group A and Group B)
• Total 6PRBs are used to send RACH Preamble

How to Generate 64 PRACH Preamble Sequences?

This explanation dives into the process of generating PRACH preambles in LTE networks, focusing on the role of parameters broadcasted in System Information Block 2 (SIB 2). It also incorporates additional points for a more comprehensive understanding.

Key Parameters in SIB 2:

The eNodeB broadcasts three crucial parameters in SIB 2 to facilitate PRACH preamble generation by UEs:

a) rootSequenceIndex:

• Identifies the starting point for the Zadoff-Chu (ZC) root sequence library used to generate preambles. Think of it as a reference number in a codebook containing various ZC sequences.
• Additional Point: The actual ZC sequence library and its size (typically 64) are pre-configured in the network and known to both the UE and the base station.

b) highSpeedFlag:

• Indicates the presence of fast-moving UEs within the cell.
• When set to true, this flag restricts the number of cyclic shifts allowed for preambles to mitigate potential confusion between signals from slow and fast-moving UEs at the base station.
• Additional Point: This flag essentially enforces stricter timing requirements for fast-moving UEs to ensure their signals are distinguishable from those of slower UEs.

c) zeroCorrelationZoneConfig:

• Defines the maximum number of cyclic shifts a UE can generate from a single ZC root sequence.
• Cyclic shifts involve delaying the ZC sequence by a specific number of samples, resulting in different preamble variations.
• A larger zeroCorrelationZoneConfig value allows for more cyclic shifts, offering a wider range of preambles for the UE to utilize.
• Additional Point: This parameter balances the trade-off between the number of available preambles (increased diversity) and the risk of inter-cell interference, especially in large cells.

Example: Deriving the Base ZC Sequence (rootSequenceIndex = 22):

• The provided example states rootSequenceIndex = 22. However, directly obtaining the base ZC sequence from this index isn’t entirely accurate. Here’s a clarified explanation:
• The rootSequenceIndex points to the position of the base ZC sequence within the pre-defined library (of size 64) known to both the UE and the base station.
• In this case, rootSequenceIndex = 22 suggests that the 22nd ZC sequence in the library is considered the base sequence for generating preambles.
• The actual base ZC sequence itself cannot be directly derived from the index; it’s a pre-defined mathematical sequence stored in the library.

Preamble Generation Process:

1. The UE retrieves the rootSequenceIndex, highSpeedFlag, and zeroCorrelationZoneConfig values from SIB 2.
2. Based on the rootSequenceIndex, the UE accesses the corresponding base ZC sequence from the pre-defined library.
3. The UE generates multiple preambles by applying cyclic shifts to the base ZC sequence, up to the limit specified by zeroCorrelationZoneConfig.
4. If the highSpeedFlag is set, the number of cyclic shifts might be further restricted to avoid confusion with signals from fast-moving UEs.
5. The UE can then choose from this pool of generated preambles for its random-access attempt.

Why 6 PRBs use to send RACH message in LTE.

we know to send RACH message in LTE we require Preamble. we have total 4 preamble format we can refer table Number 36.211.

• total preambles type we have 0,1,2,3.
• Almost all operator uses preamble format 0.
• To send preamble we require 6 PRB in frequency domain and 1 msec in time domain why?
• Preamble is made of an 839 Zadd off-chu sequence.
• 1 preamble = 839 Zadd off-chu sequence [839 frequency sequence]
• 1 PRB=12 Subcarrier
• 6 PRB = 12*6 =72 Subcarrier
• Total occupied frequency
• 1.25Khz X839 = 1.048 Mhz =1.4Mhz
• T=1/f =1/1.25 Khz=0.8 msec =1 msec
• we know 1.4Mhz will have 6 PRB or 72 Subcarrier in frequency domain.

The difference in subcarrier spacing between PRACH preamble and uplink subframe (1.25 kHz vs 15 kHz) is likely due to historical reasons or specific design choices for the RACH process. It allows for simpler implementation of the PRACH signal with narrower subcarriers, potentially improving its robustness in noisy channels.

RACH Procedure in LTE

MSG 1

UEs initiate access requests to the network through the Random-Access Channel (RACH). Message 1 of RACH, also known as the Random-Access Preamble

UE sends RACH Preamble carrying RA- RNTI and eNB decode the preamble and get RA-RNTI

How UE decides the Power used for Rach request Transmission: –

Now UE need to decide the power which will be used for RACH Request transmission. Power is decided on the factors received in SIB2 as:-

preambleInitialReceivedtargetPower:- Power factor which will be used for first transmission of Rach Request.Value varies from -120dBm to -90 dBm .

powerRampingStep:- This is mainly used when eNodeB is not able to detect the Rach Request then UE will re transmit the RACH Request by increasing the power to powerRampingStep factor.

power used  for Rach Request transmission =
preambleInitialReceivedTargetPower + DELTA_PREAMBLE + (PREAMBLE_TRANSMISSION_COUNTER – 1) * powerRampingStep

DELTA_PREAMBLE = This is preamble format based delta offset. There are four formats available for preamble which are called as preamble formats.

MSG 2

In LTE networks, after transmitting the initial preamble (MSG 1), the UE waits for a response from the network (eNodeB) in the form of MSG 2.

This MSG 2 serves the purpose of contention resolution, especially when multiple UEs attempt random access simultaneously.

Random Access Response (RAR): This is the core part of MSG 2, carrying the network’s decision regarding the UE’s access request.

• If the network grants access, the RAR will include:
• Allocation information (e.g., resource blocks, power control parameters) for the UE’s upcoming data transmission.
• Timing information specifying when the UE can transmit its data.
• If the network rejects the access request due to congestion or other reasons, the RAR will indicate a NACK (Not Acknowledged) and might provide backoff instructions for the UE to retry later.
• Collision Detection: The network can detect collisions (overlapping preambles from multiple UEs) during MSG 1 reception. In such cases, MSG 2 might not include specific allocations for any UE and might instruct all UEs to backoff and retry later using a random-access procedure.

In short

• The eNodeB transmits the RA Response on the DL-SCH channel.
• Derives RA-RNTI
• Calculate TC-RNTI Calculate Timing advance.
• Uplink Resource Grant
• Hoping flag
• MCS
• back off indicator MAC header

MSG 3 (RRC Connection Request)

MSG 3 represents the actual data transmission phase initiated by the UE.

The UE adjusts its transmission power according to the power control parameters received in the RAR.

• UE saves TC-RNTI from RAR.
• Channel –UL-SCH
• UE does not have a permanent identity till now, so it will have a random number as the UE identity.
• The random UE identity is included in the RRC connection request.
• UE starts the T300 timer.

MSG 4 (RRC Connection Setup)

Following successful contention resolution (MSG 2) and data transmission by the UE (MSG  3), MSG 4 marks the establishment of an RRC (Radio Resource Control) connection between the UE and the network (eNodeB).

This message exchange involves RRC signaling to configure the connection parameters.

The eNodeB transmits an RRC Connection Setup message containing:

CRNTI (Cell Radio Network Temporary Identity):

• A unique identifier assigned to the UE for the RRC connection. This ID replaces the temporary RA-RNTI used earlier in the RACH procedure.
• System Information: Essential parameters about the cell configuration, neighboring cells, and other relevant information for the UE to operate within the network.
• Security Modes and Keys: Information to establish secure communication between the UE and the network.
• Scheduling Information: Uplink and downlink resource allocation details for the UE’s upcoming data transmissions.

MSG 5 (RRC Connection Setup Complete)

MSG 5 of RACH serves as confirmation from the UE that it has successfully established the RRC connection and received the configuration parameters from the network.

Some Brainstorming now, with a cup of Tea

• How the UE decides when and where it can send the RACH request?

Ans: By using the parameter PRACH config index and PRACH Freq Offset

• What if the UE doesn’t receive the RACH response in first trial?

Ans: Just retry and send PRACH

• If PRACH response is not received, do we transmit with same power again?

Ans: No, we need to increase the power by step of 2DB

• Difference between PRACH of WCDMA and LTE?

Ans: IN WCDMA PRACH can be used to transfer RRC messages and application data but in LTE PRACH doesn’t transfer RRC or application data