**RACH and 3GPP TN/NTN**

# **RACH and 3GPP TN/NTN** ###### tags: `Project` [TOC] --- ## **RACH** <div style="text-align: center"><img src="https://i.imgur.com/SThv9Dl.png"/></div> :::spoiler Principle and characteristics - ++Purpose++ : Establish the timing synchronization between the receiver and transmitter is really important because we want to make sure that the transmitted and received messages will not be wrongly judged. - ++Functionality++ : - Achieve **Uplink synchronization between UE and gNodeB (gNB)**, UE = Transmitter and gNB = Reciever (eNB for 4G LTE and gNB for 5G NR) - Obtain the resource for Message 3 (Msg3), RRC Connection Request is one example of Msg3 - ++Uplink process criteria++ : - The synchronization process should happen **only when there is immediate necessity** - The synchronization should **be dedicated to only a specific UE** - ++Situations trigger RACH process++ : - Initial access from RRC_IDLE - RRC Connection Re-establishment procedure - Handover - DL or UL data arrival during RRC_CONNECTED when UL synchronisation status is "non-synchronised" - Transition from RRC_INACTIVE - To establish time alignment at SCell addition - Request for Other SI - Beam failure recovery ::: :::spoiler Types of RACH : Contention Based - ++Principle++ : Same as in LTE, when a UE transmit a PRACH Preamble, it transmits with a specific pattern called a "Signature". In each LTE cell, total 64 preamble signatures are available and UE select **randomly** one of these signatures. So, it has that probability the same PRACH preamble from multipe UE reaches the NW at the same time, and it will cause a collision. - ++Concept++ : This kind of PRACH collision is called "Contention" and the RACH process that allows this type of "Contention" is called "Contention based" RACH Process. In this kind of contention based RACH process, Network would go through **additional process at later step to resolve these contention and this process** is called **"Contention Resolution"** step. - ++Typical "Contention Based" RACH Procedure++ : - UE --> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size) - UE <-- NW : Random Access Response (Timing Advance, T_C-RNTI, UL grant for L2/L3 message) - UE --> NW : L2/L3 message - Message for early contention resolution ::: :::spoiler Types of RACH : NonContention Based - ++Principle++ : The contention can be prevented due to some reason (e.g, timing restriction). Usually in this case, the Network informs each of the UE of **exactly when and which preamble signature** it has to use. - ++Concept++ : Network will allocate these preamble signature so that it would not collide. This kind of RACH process is called "Contention Free" RACH procedure. To initiate the "Contention Free" RACH process, UE should **be in Connected Mode before the RACH process as in Handover case**. - ++Typical "Contention Free" RACH Procedure++ : - UE <-- NW : RACH Preamble (PRACH) Assignment - UE --> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size) - UE <-- NW : Random Access Response (Timing Advance, C-RNTI, UL grant for L2/L3 message) ::: :::spoiler Fundamental Difference from LTE RACH and NE RACH - Overall protocol sequence would be almost same in LTE and NR. The major difference between LTE RACH and NR RACH would lie just **before RACH Preamble gets transmitted**. - It is due to BeamForming which would be supported by default (especially in mmWave) in NR. So in case when NR is operating in Beamforming mode, UE need to **detect and select a best beam for RACH process**. - This beam selection process would be the fundamental difference between LTE RACH and NR RACH process. ::: --- ## **NB-IoT Random Access Procedure** <div style="text-align: center"><img src="https://i.imgur.com/H10mQZC.png"/></div> The RA procedure allows the NB-IoT UEs to **initiate uplink data transmission, achieve uplink synchronization, and obtain a permanent ID**. :::spoiler Message 1 - Preamble - After achieving downlink synchronization, the UEs will send a random access preamble to the serving BS, by using the physical random access channel (NPRACH). This allows the BS to estimate the RTD (Round Trip Delay) for each UE, based on the ToA (Time of Arrival) of the received signal. - Since NB-IoT employs a SC-FDMA (Single-carrier Frequency Division Multiple Access) for the uplink transmission, it is essential to align the received signals from multiple devices, both in time and frequency. The BS will use the ToA estimate, for determining a TA (Timing Advance) to be applied by each UE, for synchronizing their uplink transmissions. - What is worth emphasizing here is that at this step **the UEs will compete for the same NPRACH channel, hence packet collision may occur if two UEs randomly choose the same preamble configuration**. ::: :::spoiler Message 2 - RAR - If the BS detects an NPRACH preamble, it sends back a RAR (Random Access Response), also known as Message 2. - The RAR contains the TA parameter, which enables the time synchronization of the UEs. Besides, the RAR further encloses scheduling information pointing to the radio resources that the UEs has to use to transmit the subsequent messages. - Basically, from this step on, **the transmission of the data, either in downlink or uplink, is orchestrated by the BS**. This means that the **BS guarantees that different resources (time/frequency) are assigned to various UEs in order to avoid packet collision**. ::: :::spoiler Message 3 - Connection Request - In Message 3, the device will **include its identity as well as a scheduling request**. Furthermore, it will also **report its data volume status and power headroom**, to facilitate the base station scheduling and power allocation decision for subsequent transmissions. - At this step, **the HARQ (Hybrid Automatic Repeat Request) protocol is used by both, the UE and the BS, for the message exchange**. Basically, after each packet transmission, the UE has to wait for an ACK (Acknowledgment) or NACK (non-ACK) by the BS, or vice versa, to know whether the packet was correctly received. If not, the same packet is retransmitted. ::: :::spoiler Message 4 - Contention Resolution - In Message 4, **the network assigns a permanent ID to the UEs which had a successful RA procedure**. - At this point, the UEs make the **transition from RRC (Radio Resource Control) idle to RRC connected mode**. ::: --- ## **3GPP TN/NTN** <div style="text-align: center"><img src="https://i.imgur.com/dGsLaF0.png"/></div> :::spoiler Introduction - ++3rd Generation Partnership Project (3GPP)++ : A standards organization which develop protocols for mobile telecommunications. - ++Narrowband Internet of Things (NB-IoT)++ : A recent cellular technology standardized by 3GPP that aims to provide improved coverage for a massive number of low-throughput low-cost devices with low device power consumption in delay-tolerant applications. Simply put, it focuses specifically on **indoor coverage, low cost, long battery life, and high connection density**. - ++Satellite++ : It maximizes the inherent value of 5G networks **by solving coverage problems** and **difficult use-cases** that ground-based infrastructure alone cannot address. - ++Low Earth Orbit (LEO)++ : Also known as low-altitude earth orbit, it refers to the orbit where the space vehicle is **at a low altitude from the ground**. - ++Geostationary Orbit (GEO)++ : Refers to a circular orbit of 35,786km above the earth's equatorial plane, and the direction of the spacecraft on this orbit is the same as the direction of the earth's rotation. A spacecraft in a geostationary orbit takes the same time as the earth's rotation period (one sidereal day) to orbit the earth. Therefore, **in the eyes of ground observers, such a spacecraft is fixed in the sky**. - ++Enhanced/Extreme Mobile Broadband (eMBB)++ : It is one of the three defining characteristics of 5G. It will enable new data-driven experiences requiring high data rates, **resulting in a faster and better user experience**. - ++Ultra Reliable Low Latency Communications (URLLC)++ : It is one of the three defining characteristics of 5G. It is a primary enabler for a number of unique use cases in the areas of manufacturing, energy transmission, transportation and healthcare. **The delay budget for individual interfaces can be as low as 1ms**. - ++Massive Machine Type Communications (mMTC)++ : It is one of the three defining characteristics of 5G. It targets the cost-efficient and robust connection of billions of devices without overloading the network. **Critical success factors include : Coverage, Cost efficiency, Low power consumption, Longtime availability**. - ++LTE-M++ : It is a **low‑power wide‑area (LPWA) air interface** that lets you connect IoT and M2M devices with medium data rate requirements. **Key features include: Full mobility and in-vehicle hand-over, Low power consumption, Extended in-building range, Support of voice functionality via VoLTE (Voice over LTE)**. ::: :::spoiler Terrestrial Networks (TNs) and Non-terrestrial Networks (NTNs) - An NTN refers to a network, or segment of networks, using Radio Frequency (RF) resources on board of a satellite or Unmanned Aerial System (UAS) platform. - NTNs are a promising solution to complement TNs for **global coverage extension**. The primary role of NTN is to complement the TN services in under-served areas, to **improve the TN service reliability**, especially for mission-critical services, and to enable the network scalability by means of efficient multicast/broadcast resources for data delivery. - By integrating NB-IoT into an NTN, 3GPP aims to provide a standardized solution enabling global IoT operation anywhere on Earth. ::: :::spoiler NB-IoT problems for NTNs - The current NB-IoT RA preamble namely **Narrowband Physical Random Access Channel (NPRACH) cannot support the satellite channel impairments** (e.g. strong Doppler, large propagation delays) which are more severe compared to those faced by TNs. - One of the most important procedures worth analyzing for the NB-IoT NTN is the RA procedure, since it allows the NB-IoT UEs to initiate uplink data transmission, **achieve uplink synchronization and obtain a permanent ID** in the network. ::: --- ## **NB-IoT NTN reference scenarios** <div style="text-align: center"><img src="https://i.imgur.com/fTgWmHx.png"/></div> <div style="text-align: center"><img src="https://i.imgur.com/TQTyNyr.png"/></div> - Thses are two of the identified architecture options of a non-terrestrial network providing access to NB-IoT user equipments. They differ from each-other depending on the **payload, orbit, and cell type**. :::spoiler Payload type - Transparent payload : The satellite **acts as a relay**, and provides the link between the NB-IoT users and the serving BS (Base Station), which are on ground. - Regenerative payload : The **BS functionalities can be performed at the satellite**, and an ISL (Inter-satellite Link) can help for handover procedures. ::: :::spoiler Orbit type - It is agreed in 3GPP that as a starting phase **only the LEO and GEO satellite orbits will be considered**. - GEO satellite : It has an altitude of 35,786 km. - LEO satellite : It has two possible altitudes that are 600 km and 1200 km. Changing the altitude for the LEO satellite will not impact our analysis, but only certain parameters. ::: :::spoiler Cell type - Earth-fixed cells : It is fixed and all the UEs inside the cell will have **a certain coverage time by the satellite**. To increase this coverage time, each satellite has the capability to steer beams towards fixed points on Earth. This can be realized through a mechanically steerable beam or a BF (Beamforming) technique. - Earth-moving cells : It will move with the same speed as the satellite. In such a case, the location of the users inside the cell will change dynamically, and there will be constantly new users entering and going out of the cell. An advantage of this cell type is that it does not require a mechanical steering or BF, **resulting in a lower satellite cost**. ::: - The table of NB-IOT NTN scenario options : <div style="text-align: center"><img src="https://i.imgur.com/eRzUhT5.png"/></div> --- ## **Reference** - 5G/NR - Initial Access/RACH - http://www.sharetechnote.com/html/5G/5G_RACH.html - NB-IoT Random Access for Non-Terrestrial Networks - https://arxiv.org/pdf/2101.08079.pdf - On the Random Access Procedure of NB-IoT Non-Terrestrial Networks - https://orbilu.uni.lu/bitstream/10993/44187/1/PID6573949.pdf - Non-Terrestrial Networks 5G Integration - https://www.atis.org/initiatives/non-terrestrial-networks-ntn-5g-integration/ - Satellite and terrestrial network integration - https://core.ac.uk/download/pdf/53822257.pdf