RACH is an Uplink Transport Channel which is used for initial Random Access. The major function of RACH is to allow the UEs to get Uplink Synchronization. RACH plays an important role in the transmission of Uplink Scheduling Request.
Data transmission happens in two ways from UE, once when the UE has a dedicated RRC (PUSCH resource available) and once when the UE needs to access the network and then begin data transmission.
When no dedicated connection is established, a Scheduling request will be transmitted on RACH called as RA_SR (Random Access Scheduling request).
The process of accessing the network when no dedicated RRC is established or when the UE Transmits for the first time ( after power-on ) is called Random Access and the Channel that plays the major role in this aspect is called RACH channel.
We can sequence the below scenarios when Random Access is required
- Initial Access: When the UE tries to access the network during RRC_IDLE or initial power on mode.
- During RRC _Reconnection establishment.
- When the UE losses the Uplink Synchronization from the network
- When no dedicated PUSCH resources available and during Handover
RACH in LTE Channel Structure:
The Fig 1: Shows the position of RACH in the Uplink LTE channel Structure. RACH is responsible for the Random-Access process. To access any network the UE needs to be synchronized both in the downlink and in the uplink. Downlink synchronization happens by cell selection and PLMN selection, but the uplink synchronization is taken care by the Random-Access process.
Random Access process takes place in two ways:
A. Contention Based Random Access
B. Contention Free Random Access.
Before we discuss about these two processes, we need to understand Preambles. Preambles are basically “Signatures” or specific patterns that is transmitted by UE to eNB.
There are in total 64 Preambles signatures that are available in a LTE call and UEs can randomly select any of the preambles. The preambles are orthogonal to each other and are divided into two groups.
One group is called contention free preambles which are allocated to UE which undergoes Handover and the other is contention-based preambles which are allocated to UEs which are going through the process of Random Access.
We will talk about in details about the Random-Access types below.
Contention Based Random Access:
As the name suggest this process is based on the scenario where there is a contention to access the network. And this contention happens when multiple UEs try to access the network at the same time.
The UE transmits the Preamble messages from the 64 Sequence available.
The preamble messages are grouped also based on the data payload in L3 Packets.
It gives the eNB an idea about the requirement of Scheduling resources.
The eNB now sends RA response to all the UEs which has sent RA preamble intimating them that resources have been reserved for them.
It can only send the response if it has correctly decoded the preamble. At this point the eNB doesn’t have any idea of the identity of the UE. This response is generated by the UE from the MAC layer and is transmitted in the DL-SCH. The Random-Access Response is addressed to an RA-RNTI which identifies the Resource Block on which the preamble was decoded.
There is a timer or time slot during which the UE is scheduled to receive the RAR. If the UE doesn’t receive the RAR in that specified time slot, it again sends RA preamble at the next available time with increased power. It is to be noted that if several UE has sent preambles in the same Random Access slot and all the preambles have been decoded by the eNB , all the UEs will receive RAR and will have the information that they have scheduled slot for sending RRC request as we will see in step3.
The UE now utilizing the resources allocated in Step 2, sends RRC connection request on UL-SCH. In the RRC messages the UE sends its identifier to the eNB to make it aware of its identity. This identifier is used to solve any contention resolution that can happen.
As we can see that all the UEs which sent the same preamble at the same time, while sending the RRC request message will collide and this will result in interference. Among those UEs the one which is having the best radio conditions, the message will be successfully decoded. In Contention resolution message the eNB send the UE identifier, so the UEs which can’t decode the message or whose identifier doesn’t match with the one sent, they back off and wait for the next turn to send the RA request. The UE which received the message and can decode its identify then starts transmitting on the Uplink resource block allocated to it.
Contention Free Random Access: In contention free Random Access the process is initiated by the network. It happens in case of a Handover of a UE from one EnB to another eNB.
Step1: The eNB reserves a set of preambles for this purpose and assigns a preamble from this and sends to UE.
Step 2: Since this entire process is controlled by the eNB there is no collision. The UE now sends back the response of Random-Access Preamble with the UE ID.
Step 3: The eNB then sends Random Access response with the message of resources allocated.
Summary in Brief:
Now let us explain what the significance of below terms is:
RA-RNTI: It is Random Access Radio Network Terminal Identifier. It basically contains the UE identifier.
T-C-RNTI: Temporary Cell RNTI. Used for scheduled unicast transmission. It is created by eNB and is used to identify a UE within the scope of the eNB during random access process and set up of RRC connection.
Importance of SIB2 in Random Access:
SIB2 carries the below information in LTE:
- RACH related parameters
- Idle mode paging configurations
- PUCCH and PUSCH configurations
- Uplink power control and Sounding Reference Signals
- Uplink carrier frequency and Bandwidth and Cell barring information
So, from the above points we can see that all parameters that are required for transmission of RACH are carried by SIB2. Now let us see what are parameters that are present in RACH, transmitted by SIB2. These are RACH configuration at the MAC level across cell
- Number of RA preambles: There are 64 preamble sequence available which can be used for contention and non-contention based Random Access. The parameter has a range from 4-64
- Size of RA preambles groupA: The 64 Preamble sequence available are divided into two groups , Group A and Group B. GroupA preambles are used when sending small packets and GroupB preambles are used when sending long packets . The Parameter has a range from 4 to 60.
- Preamble Initial Received Target Power: Initial UE transmit power for the PRACH which the eNB expects. The value starts from -120dBm to -90dBm with a step size of 2dBm
- Power Ramping Step:Multiple attempts to access the PRACH may fail due to unfavorable radio conditions so the UE increases power for random access preambles by a step specified by this parameter. The step can be 0 2 4 or 6 db.
- Power Ramping Parameters: Indicates the steps by which the UE should increase the transmit power where random access preambles is increased each time after a RACH access failure.
- MAC Contention Resolution Timer: The total time when a UE waits for Msg4 during a random-access procedure. This timer starts when a UE initially sends or resends Msg3.
- Max HARQ Msg3 Tx: Maximum number of Msg3 HARQ transmissions8.
- Preamble Trans Max: Maximum number of Preamble Transmissions, possible values are 3 4 5 6 7 8 10 20 50 100 200.
So we saw the information that are transmitted in SIB2 for RACH on MAC layer , let us know see the parameters which are configured in PRACH at the physical layer.
1. Root Sequence Index: Used to determine 64 physical RACH preamble sequences available in the cell. These preambles are generated by Zadoff Chu Sequence which has a series of root sequences. We can cyclic shift each sequence to obtain the preamble sequence. The Range of Rach Root Sequence is from 0 to 837.
2. PRACH Config Index: By this parameter we can define at what time – frequency grid the UE sends the RACH request to eNB.
3. High Speed Flag: This parameter determines what type of scenario the particular cell serves, whether high speed scenarios like catering to railway traffic or other scenarios
4.Zero Correlation Zone Config: This guarantees that the preambles generated are orthogonal to each other
5. PRACH Freq Offset: This parameter actually helps the cell to inform the UE and other neighbor cells about which PRB can be used for Random Access.
PRACH Preamble Format:
PRACH is the physical channel that initiates the exchange of information with ENB from UE end in the uplink transmission. The Preambles that is sent through this channel to Enb helps to establish the timing advance required for transmission. The signature or sequence that we mentioned earlier is basically the PRACH preamble.
PRACH Preamble = Cyclic Prefix with length TCP + Signature (Tpre)+GP
Cyclic Prefix is used to restrict inter symbol interference.
Guard Period (GP) is the unused portion of time at the preamble which is used for absorbing propagation delay
Ts=1/(15000*2048) which is equal to sampling rate of 30.72MHZ
The different Preamble formats are designed to cater to different cell radius and radio conditions
The requirement for generating RA preamble are good auto- correlation function (ACF) and good cross correlation function (CCF). Zadoff -CHU sequence suffices to both having good ACF and CCF.
Resource Block of RACH in LTE Frame Structure :
1 Subcarrier of PRACH preamble =1.25KHZ, whereas UL Subcarrier=15KHz.
So, 12 PRACH preamble subcarrier = 1 UL Subcarrier (12*1.25KHZ=15KHZ)
Now the Random Access preamble has 839 Subcarrier .
So total BW=839*1.25KHZ=1048.75.
Now there is a 15KZ guard band on either side, so total BW =(1048.75+30)KHZ
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