A random-access channel (RACH) is so named because it refers to a wireless channel (medium) that may be shared by multiple UEs and used by the UEs to (randomly) access the network for communications. Given below are some of the use cases of RACH in wireless communication system.
RACH is used for call setup and to access the network for data transmissions.
RACH is used for initial access to a network when the UE switches from a radio resource control (RRC) connected idle mode to active mode, or when handing over in RRC connected mode. Moreover, RACH may be used for downlink (DL) and/or uplink (UL) data arrival when the UE is in RRC idle or RRC inactive modes, and when re-establishing a connection with the network.
RACH is used to request uplink scheduling if no dedicated scheduling-request resource has been assigned to UE.
3GPP release 15 defines a four step RACH procedure. Given below is an example four-step RACH procedure.
A first message (MSG1) is sent from the UE to gNB on the physical random-access channel (PRACH). MSG1 includes a RACH preamble. This preamble is designed such that it can be detected even when there is lack of accurate timing information.
gNB responds with a random-access response (RAR) message (MSG2) which includes the identifier (ID) of the RACH preamble, a timing advance (TA), an uplink grant, cell radio network temporary identifier (C-RNTI), and a back off indicator. MSG2 includes a PDCCH communication including control information for a following communication on the PDSCH.
In response to MSG2, MSG3 is transmitted from the UE to gNB on the PUSCH. MSG3 includes a RRC connection request, a tracking area update, and a scheduling request.
The gNB then responds with MSG4 which includes a contention resolution message.
The 3GPP in Release-16 has named legacy Random access procedure as 4-step RA type or Type-1 RA procedure and the new 2-step RA procedure as 2-step RA type or Type-2 RA procedure. As the name implies, the two-step RACH procedure effectively “collapses” the four messages of the four-step RACH procedure into two messages. Below given is an example two-step RACH procedure.
A first enhanced message (msgA) is sent from the UE to gNB. In this case, msgA includes a RACH preamble for random access and a payload over PUSCH, which effectively combines MSG1 and MSG3 described above. The msgA payload, for example, includes the UE-ID and other signaling information (e.g., buffer status report (BSR)) or scheduling request (SR). This message is transmitted repeatedly with step-wise increased power until a response (msgB) is received.
gNB responds with a random access response (RAR) message (msgB) which effectively combines MSG2 and MSG4 described above. For example, msgB includes the ID of the RACH preamble, a timing advance (TA), a back off indicator, a contention resolution messages, UL/DL grant, and a transmit power control (TPC) commands.
Given below are benefits of 2-step RACH procedure
Benefit of 2-step RACH procedure is seen when msgA is detected by gNB quickly without repeated transmissions. otherwise there is an additional overhead (in comparison with 4-step procedure) of transmitting MsgA payload.
Also, in case of operation in unlicensed spectrum, collapsed RACH procedure implies a reduced number of LBT (Listen Before Talk) operations with a corresponding reduction in overhead and delay.
2-step RACH requires less UE processing compared to 4-step RACH. Hence it has the benefit of power saving especially if a UE is under the scenario with small data traffic which requires the UE to wake up and transmit data intermittently.
Power Saving Techniques for 5G and Beyond: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9112193
TWO-STEP RANDOM ACCESS CHANNEL (RACH) PROCEDURE TO FOUR-STEP RACH PROCEDURE FALLBACK United States Patent Application 2020012071
Coexistence is very common word used to describe compatibility between two independent systems. Question really is : Can two independent system coexist without sharing resources with each other ? Each system has its own faiths, beliefs, cultures and rules. So, It is not always easy to coexist. However, sometimes shared interest makes system coexist and one can not deny importance of sharing in such systems.
5G-NR and 4G-LTE are two such systems. First of all, for the sake of coexistence, dropping 5G and 4G from name (There are systems who do not like this naming of 4G,5G etc.). Lets call it LTE-NR coexisting system. Like any other independent systems, LTE-NR coexistence is not an easy task, It requires sharing of resources. The fundamental resource that is shared between these two systems is Frequency Spectrum.
NR is designed to be deployed in wide arrays of spectrum from low frequency bands to mmWave frequency bands. Most of the bands in sub 6GHz are used by LTE. Before these LTE’s carrier can be refarmed for NR, both NR and LTE will need to be deployed in same band as LTE operation and NR-LTE coexistence should be considered in sub 6GHz.
These two systems also need to agree on some common minimum rules (Like “common Minimum Program” when two independent political parties try to coexist).
Coexistence requires that new belief systems and cultural traditions emerge and spread. Supplementary UL (SUL) is one such belief.
Supplementary uplink frequency is referred to an UL carrier supplemented to the NR carrier. The supplementary uplink frequency is a complimentary UL access link to NR carrier. The complimentary UL access link includes the UL shared channels, UL control channels and PRACH channels.
UL sharing between NR and LTE is illustrated in Figure 1. There is low frequency spectrum operating LTE system, e.g. FDD band (F1_UL and F1_DL) in 900MHz or 2GHz, and higher frequency spectrum operating NR system, e.g. TDD band in 3.5GHz or higher (F2). The LTE UL carrier, i.e. F1_UL, is shared between LTE and NR. DL carriers, F1_DL and F2 are dedicated for LTE and NR, respectively.
No new belief should emerge without associated benefits and SUL is no exception. It provides following benefits (Notice that it provides benefits for both the systems i.e. LTE and NR, a win-win belief)
Better resource utilization of LTE UL, Typically, the LTE FDD UL radio resource is not fully utilized due to DL-heavy traffic arrival. Therefore, utilizing LTE UL spare radio resource for NR UL can improve the resource utilization of UL spectrum. This also provides additional radio resource allocated for DL transmission for NR TDD in higher frequency band as shown in Figure 1.
Improve the NR UL coverage, If the deployment of NR gNB in higher frequency band and LTE eNB in low frequency band are co-located, there will be coverage gap between NR system and LTE system. The UL coverage issue for NR system will be more vulnerable comparing to LTE UL coverage since UE transmit power is limited and the limited number of UL slots in NR TDD. Therefore, it would be beneficial if the UL transmission for NR can be operated at UL carrier of LTE system in lower frequency in the standalone NR deployment in higher frequency band shared the cell site with LTE system.
In the LTE uplink, there is a half-tone (7.5 KHz) shift of sub-carriers to reduce the impact of the dc leakage to the DFT-spread-OFDM waveform. In the case of LTE and NR co-existing on the same carrier frequency, the uplink sub-carriers of the two RATs will therefore be dis-aligned relative to each other, leading to inter-sub-carrier interference. Although f-OFDM can be applied to each NR UE, no receiver filter can be adopted on LTE UEs since no impact is expected to the ability of legacy LTE devices to operate on the LTE carrier co-existing with NR. Hence, a 7.5-kHz shift is also required for the SUL bands, otherwise, the subcarriers of LTE and NR would not be orthogonal .
In LTE DL, Cell Specific Reference signal is an “Always On” signal and It is sent in every DL subframe. Therefore, when LTE-DL and NR-DL are coexisting it is needed that NR does not map DL data/control on LTE CRS REs. This is resolved by following ways:
Rate matching around LTE CRS for NR-PDSCH
The 4 OFDM symbol SSB using 30 KHz subcarrier spacing can be placed on OFDM symbols between two CRS Symbols. This Special SSB time pattern is called case-C in 3GPP Specification.
The starting OFDM symbol of the NR CORESET in a subframe can be indicated by the SSB so that can also be palced on the OFDM symbols not occupied by LTE CRS.