What Happens After UE Sends a RACH Request to eNodeB in LTE

What Happens After UE Sends a RACH Request to eNodeB in LTE

The Random Access Procedure (RACH) in LTE is critical for establishing initial synchronization between the UE (User Equipment) and the eNodeB. When a UE sends a RACH preamble, the eNodeB responds with a Random Access Response (RAR) containing multiple fields required for uplink resource scheduling and synchronization. Below is a step-by-step, detailed view of what happens inside the eNodeB.

1. Temporary C-RNTI Assignment

After detecting a valid RACH preamble, the eNodeB assigns a Temporary Cell Radio Network Temporary Identifier (TC-RNTI) to the UE.

Example:

Suppose UE transmits Preamble ID 23. The eNodeB includes this ID in the RAR subheader and assigns a TC-RNTI like 0xABCD.

This TC-RNTI will be used:

• In the uplink grant (Msg3 transmission)
• For scheduling further messages until the final C-RNTI is assigned

Reference: 3GPP TS 36.321 Section 6.1.5

2. Uplink Timing Synchronization via Timing Advance (TA)

The eNodeB calculates the Timing Advance (TA) to align the UE’s uplink timing so that the transmission arrives at the eNodeB at the expected time.

Calculation:

• Subcarrier spacing = 15 kHz
• FFT size = 2048
• Basic time unit Ts​=1/15000×2048​≈32.55 ns

TA is an 11-bit value, ranging from 0 to 1282.

The effective advance is:

NTA​=TA×16

Total timing offset=(NTA)×Ts

Example:

Let’s assume TA = 100

Then:

NTA=100×16=1600

Offset=1600×32.55 ns=52.08 μs

This means the UE should advance its transmission by 52.08 microseconds to ensure proper alignment.

• TA is applied starting from subframe n+6

Reference: 3GPP TS 36.213 Section 4.2.3, TS 36.211 Section 8.1

3. Uplink Resource Allocation – UL Grant Explained

The eNodeB provides the UE with an UL Grant in the RAR, which is a 20-bit field divided into multiple subfields:

Hopping Flag (1 bit)

In LTE uplink, Resource Block (RB) allocation is contiguous, meaning that when a UE is assigned resources, it usually gets a continuous set of frequency subcarriers (e.g., RB10 to RB14). This is different from downlink, where distributed resource allocation is more flexible.

However, to improve frequency diversity (i.e., reduce the impact of frequency-selective fading), LTE uses a technique called Frequency Hopping in the uplink.

Value: 1 enables Frequency Hopping

Frequency hopping introduces diversity, especially helpful in fading conditions.

Configured in SIB2 with hopping parameters (e.g., hop offset)

How It Works:

The eNodeB enables frequency hopping by setting parameters in SystemInformationBlock2 (SIB2).

These parameters include:

• freqHoppingEnabled (true/false)
• hoppingOffset
• type of hopping (intra-subframe or inter-subframe)

The hopping pattern is predefined, and the UE applies this while transmitting across different slots/subframes.

Reference:

3GPP TS 36.211, Section 5.3.4

3GPP TS 36.331, SIB2 Configuration Parameters

Example:

Suppose an eNodeB allocates 5 Resource Blocks (RBs) to a UE for uplink transmission. Let’s say:

• In Subframe 0, the UE transmits on RBs 10 to 14
• In Subframe 1, due to hopping, the UE transmits on RBs 20 to 24
• In Subframe 2, the UE hops again and transmits on RBs 5 to 9

Even though the UE was assigned 5 contiguous RBs each time, the center frequency shifted (hopped) across subframes.

This way, if RBs 10–14 are experiencing bad channel conditions in one subframe, the UE gets a chance to transmit on better frequencies in subsequent subframes.

Subframe 0: UE transmits on RBs 10 to 14

Subframe 1: UE hops to RBs 20 to 24

Subframe 2: UE hops again to RBs 5 to 9

This figure clearly shows how the UE keeps the number of RBs constant (5 RBs) but changes the frequency position (center frequency) over time to achieve uplink frequency diversity and combat bad channel conditions.

Resource Block Assignment (10 bits)

This tells the UE where and how many RBs to use in the uplink (PUSCH).

Example Calculation:

Let:

• Total uplink RBs in system NULRB=100
• 10-bit value received from RAR = 1111111111 (decimal: 1023)

We calculate:

b=⌈log2​(100×101/2)⌉=⌈log2​(5050)⌉=13

Since b > 10, we need to prepend 3 leading zeros to create a 13-bit value. Then:

• Effective RB Assignment value: 127
• Number of RBs allocated:

⌊127/100⌋+1=2

• Starting RB index:

127mod  100=27

UE will use 2 contiguous RBs starting from RB#27

Reference: 3GPP TS 36.213 Section 6.1.1

Modulation and Coding Scheme (4 bits)

Modulation is the process of converting digital data into radio signals by varying signal properties like amplitude, phase, or frequency to transmit information over the air.

The Modulation and Coding Scheme (MCS) is a 4-bit field in the UL Grant that tells the UE which modulation type (like QPSK, 16QAM) and coding rate to use for data transmission. It directly affects the data rate and reliability of the uplink transmission.

• Value range: 0 to 15 (indexed MCS)
• Refer Table 8.6.1.1 in TS 36.213
• Example:
  • MCS = 10 – QPSK with ~1/2 code rate
  • MCS = 15 – 16QAM with higher code rate

Power Control Command (TPC – 3 bits)

The Transmit Power Control (TPC) field in the UL Grant controls how much power the UE should use when sending its data on the Physical Uplink Shared Channel (PUSCH). This is crucial because:

• If the UE transmits with too much power, it can cause interference to other UEs.
• If the UE transmits with too little power, the eNodeB might not receive the signal clearly, especially if the UE is far from the cell.

The TPC field is 3 bits long, allowing values from 0 to 7, and each value corresponds to a specific adjustment in decibels (dB), such as -6 dB, 0 dB, +2 dB, etc.

This adjustment tells the UE to either increase or decrease its transmit power compared to the previous transmission.

If TPC = 100 → UE increases power by 2 dB.

UL Delay (1 bit)

The UL Delay is a 1-bit flag included in the Uplink Grant (part of the Random Access Response). It is used by the eNodeB to indicate whether an uplink scheduling delay is applied to the UE’s PUSCH transmission.

• 0 = No delay (No additional delay is introduced for the PUSCH transmission. The UE can transmit in the earliest allowed subframe as per timing rules.)

• 1 = UE should apply UL delay (specific slot alignment) (A delay is introduced, and the UE must postpone its PUSCH transmission by one or more subframes (as per MAC layer scheduling constraints)

Example:

Let’s say the RAR is received at subframe 5:

If UL Delay = 0, UE may transmit Msg3 in subframe 8 (after applying timing advance rules).

If UL Delay = 1, UE may delay Msg3 to subframe 9 or later, based on scheduling configuration.

CSI Request (1 bit)

The CSI Request is a 1-bit field included in the Uplink Grant within the Random Access Response or in regular uplink scheduling messages. It indicates whether the UE should include Channel State Information (CSI) such as CQI, PMI, and RI in its upcoming PUSCH transmission.

• The CSI feedback helps the eNodeB understand the current radio channel quality from the UE’s perspective.
• Based on CSI, the eNodeB can optimize downlink decisions, such as choosing better MCS, beamforming parameters, and resource allocation.
• CSI reporting is aperiodic when triggered via this bit, and periodic when configured via RRC.

Example:

If a UE receives an uplink grant with CSI Request = 1, it may include:

• CQI (Channel Quality Indicator): tells how good the downlink channel is
• PMI (Precoding Matrix Indicator): helps with MIMO beamforming
• RI (Rank Indicator): indicates the number of spatial layers the UE can support

These are embedded within the MAC payload alongside the user or control data.

4. Timing of Random Access Response (RAR)

The RAR is sent within a RAR window configured in SIB2, typically 5 subframes.

• UE expects RAR in subframes following its preamble transmission.
• If RAR isn’t received → UE initiates a backoff using Backoff Indicator from SIB2

Reference: 3GPP TS 36.321 Section 5.1.4

5. Additional 3GPP-Mandated Procedures