# Assignment4

###### tags: `Wireless Communications course`

reference:https://ieeexplore.ieee.org/document/9193893

## 為甚麼需要NTN(Non-Terrestrial Network)?
* 為了coverage Terrestrial Network(地面的基地台)所不能覆蓋的範圍.

* 因為透過提供multicast/broadcast resources讓data可以運送到network edges and user terminals, 5G NTN is expected to become a efficient solution to 擴展 network scalability

## Section2:
### partA(NTN General Description)
* NTN有不同的部署選擇,根據NTN platform,platform又分兩大類
* spaceborne(空運的)
* 取決於三個參數
* altitude
* beam footprint size
* orbit
* Geostationary Earth Orbiting (GEO)

* Medium Earth Orbiting (MEO)
* altitude varying from 7000 to 25000 km
* Low Earth Orbiting (LEO)
* 300 to 1500 km.
* airborne
* Unmanned Aircraft System(UAS) platform
* which are typically placed at an altitude between 8 and 50 km and include High Altitude Platform Systems (HAPS) at 20 km altitude.
* NTN terminal refers to either the 3GPP User Equipment (UE) or a specific satellite terminal.

* NTN gateway is a logical node connecting the NTN platform with the 5G core network.

* Service link is the radio link between the NTN terminal and the NTN platform.

* Feeder link is the radio link between the NTN gateway and the NTN platform.

### partB( 5G NTN Use Cases )
* 分三類:
* service continuity: 去提供TN沒辦法服務到的範圍.
* service ubiquity: 當天災破壞TN時,還可以有NTN可以補救
* service scalability: 當TN太忙時,可以幫忙分擔一些工作
#### Use Case Diagram:

* NTN supports all three usage scenarios defined by the International Telecommunication Union (ITU):
* Enhanced Mobile Broadband(eMBB)
* Massive Machine Type communication(mMTC)
* Ultra-Reliable and Low Latency communication(URLLC)
* satellite propagation delays + stringent(嚴格的)URLLC requirements of reliability, availability, and latency.

## Section3:
* An NTN platform may act as space mirror or gNB in the sky.
* two satellite-based NG-RAN(下一代的Radio access network) architectures are distinguished:
* transparent
* regenerative
* NTN platform may implement partial or full gNB functionalities depending on whether the gNB functional split is considered or not.

### partA: Satellite access architectures
* Transparent payload-based satellite

* NTN platform relays(中繼) the NR signal from the NTN gateway to the NTN terminal and vice versa.
* The NTN gateway can forward the NR signal of the NR-Uu interface to the gNB. (NTN gateway傳送NR-Uu的Signal給gNB)
* One or more transparent satellites may be connected to the same gNB on the ground.

* Regenerative payload-based satellite

* The NR-Uu interface is on the service link between the NTN terminal and the NTN platform.
The radio interface between the NTN platform and the 5G Core Network (5GC) is NG that is over SRI in the air path between the NTN platform and the NTN-gateway.
* . Inter-Satellite Links (ISL) are transport links between NTN platforms.
* Regenerative satellite-based gNB-DU

* The gNB-CU on the ground is connected via the F1 interface over SRI to the NTN platform, which acts as a gNB-DU.
* The NR-Uu is the radio interface between the NTN terminal and the gNB-DU onboard satellite.
* NG interface connects the gNB-CU on the ground to the 5GC.
* gNB-DU onboard different NTN platforms may be connected to the same gNB-CU on the ground.

### partB: Relay-like architectures
* (a) Transparent payload-based satellite

* the access network forwards the NR signal to the NTN terminal through a relay node, which receives it from the transparent payload-based satellite.

* (b) Regenerative payload-based satellite

* the regenerative payload-based satellite includes full of the gNB(衛星包含整個gNB)

* (c) Regenerative satellite-based gNB-DU

* 衛星包含gNB的功能切分,而不是整個gNB

### partC: Service continuity & multi-connectivity
* However, 5G systems do not support service continuity only between terrestrial NG-RAN and NTN NG-RAN, but also between two NTN NG-RANs.

* The ground terminal is simultaneously connected to the 5GC via both transparent NTN-based NG-RAN and terrestrial NG-RAN.
* The NTN gateway is located in the Public Land Mobile Network (PLMN) area of the terrestrial NG-RAN.

* This scenario may be used to provide services to UEs in un-served areas.
* In particular, LEO is be used to deliver delay-sensitive traffic since being characterized by lower propagation delay than GEO, which is, instead, used to provide additional bandwidth and, consequently, high throughput.(LEO被用來當作主要的,因為propagation delay比GEO低。GEO被用來當作提供額外的頻寬)

* This scenario may be used to provide services to UEs in under-served areas



* the NTN platform performs all gNB tasks(相比D)

## Section4:ROLE OF NON TERRESTRIAL NETWORKS IN CELLULAR COMMUNICATIONS

### SATELLITE NETWORK ROADMAP

* 衛星系統一開始(1G)企圖去提供第一代的類比服務(voice and other low data rate),主要在海上場景.
* 2G被用來提供飛機上的服務
* It is found to be expensive to compete with GEO and cellular networks
* the integration of satellites with the terrestrial network aims to potentiate the emerging third generation (3G) wireless system, also known as Universal Mobile Telecommunications System (UMTS)

### SATELLITES IN 3G UMTS SYSTEMS

* UMTS represents the first step towards the convergence of mobile and broadband systems by offering services to groups of users

### SATELLITES IN 4G SYSTEMS
* Long-Term Evolution (LTE) system is designed to support IP-based traffic and to achieve lower latency, higher data rates, and higher spectrum efficiency than UMTS.
* Mobile Satellite Systems (MSS) provide satellite communication services to mobile users and represent an attractive way to provide coverage at lower costs in places not (well) reachable by the cellular network
* FSS(Fixed Statellite System)被大氣衰減所影響,MSS則是因為non-Line-of-Sight 傳播衰減,因為障礙物與他們的不規則形狀

### NON-TERRESTRIAL NETWORK IN 5G SYSTEMS
* Until a couple of decades ago the satellites and the terrestrial network are considered to be independent and developed separately from each other.(直到幾十年前,衛星和地面網路被視為是可分開的)

* The NTN segment, when combined with the terrestrial network, plays a key role to achieve global coverage owing to the features of boosting capacity (as a result of high-frequency reuse, and precoding techniques) and ensuring service continuity even when traveling.

* The NTN segment, when combined with the terrestrial network, plays a key role to achieve global coverage owing to the features of boosting capacity(as a result of high-frequency reuse, and precoding techniques),and ensuring service continuity even when traveling.

* In summary, the 5G technology envisions the involvement of the NTN as a means to extend the terrestrial coverage area and to help the provision of broad claim of services whenever and wherever the traditional cellular network is overloaded or not available.

## section5:

### mobility management
* NGSO satellites are characterized by shorter propagation delay and higher data rates than GEO satellites and, therefore, considered an effective solution to extend the 5G terrestrial network

* However, the motion of both NGSO satellites around Earth globe and UEs in a given region lead to a time-varying NGSO channel.

* The dynamic nature of NGSO satellite links has an important impact on handover and paging procedures.

* The dynamic nature of NGSO satellite links has an important impact on handover and paging procedures.

* 三種handover
* intra-satellite handover
* inter-satellite handover
* inter-access network handover(vertical handover)

* The paging issue is mainly related to the tracking area (satellite coverage area)management
* it can be fixed (for both GEO and NGSO satellites) or moving (only for NGSO satellites).+
* The moving tracking area leads to a high paging load that is difficult to be managed by the network.
* The moving tracking area leads to a high paging load that is difficult to be managed by the network.

* In recent years, one of the main objectives is to find handover solutions over LEO satellite networks since handovers frequently occur owning to the LEO features<font color="#f00">(因為LEOs are positioned at low altitude, provide a limited coverage area, and speedily move around Earth)</font>

### propagation delay management
* The propagation delay has a profound impact on the system performance in non-terrestrial communications

* The propagation delay is defined as the delay either from the NTN gateway to the NTN terminal via space/airborne platform or from space/airborne platform to the NTN terminal

* the propagation delay depends on the NTN platform altitude, the NTN gateway position and elevation angle, and the NTN terminal position
* The <font color="#f00"> one-way propagation delay </font> considers the time needed by the information to travel from the NTN gateway to the NTN terminal through the NTN platform or from the NTN platform to the NTN terminal.
* The two-way propagation delay, also known as Round Trip Time (RTT), takes into account the time needed by the information to travel from the NTN gateway to the NTN terminal through the NTN platform and back or from the NTN platform to the NTN terminal and back.

### radio resource management
* Radio resource management is one of the major issues of 5G NR technology.
* efficient radio resource allocation is essential to avoid the following
* Intra-NTN inter-beam interference
* The success of HTS is driven by the multi-spot-beam technology that leads to boost in the capacity;
* efficient frequency re-use is needed to avoid interference between adjacent beams.\
* Inter-NTN interference.
* Inter radio access network interference.

## Section6 : Toward 6G satellite communications
* In addition to the 5G macro-categories (i.e., eMBB, mMTC, and URLLC),6G emerging(新興的) applications may be the following:
* Holographic Type Communications (HTC) require very high bandwidth to achieve excellent quality of hologram data transmitted from remote sites;

* Multi-Sense Networks involve not only acoustic, optical and tactile senses but also the sense of smell and taste for fully immersive experience;(多感官網絡不僅涉及聽覺、視覺和触覺，還涉及嗅覺和味覺，以實現完全沉浸式體驗)

* Time Engineered Applications, such as industrial automation, autonomous systems, and massive sensor networks, where the time factor is extremely important for real-time response;(時間工程應用，例如工業自動化、自治系統和大規模傳感器網絡，其中時間因素對於實時響應極為重要)

* Critical Infrastructure where critical safety operations are essential in emergency areas.

* A future vision of satellite communications may include the introduction of the following 6G enabling features:
* Holographic radio to control the physical space owing to Large Intelligence Surface (LIS) by improving the spectral efficiency and the network capacity.

* Non-Radio Frequency to compensate(補償) for wavelength distortion(失真) due to atmospheric phenomena and to offer ultra-low latency and high reliability.

* Artificial Intelligence to allow real-time satellite decisions and seamless satellite control to achieve high-level autonomous operations.


reference: