Recently, the official account mentioned the technologies that engineers may need to pay attention to such as FPGA, ASIC, and signal processing algorithms in the fields of mobile communications and satellite communications, including 5G NTN, multiple access technology, and LEO communication satellites.
1 5G NTN
From the "5G NTN Technical White Paper: Heaven and Earth Integration, Mobile Phone Direct Connection":
5G NTN (5G Non Terrestrial Network) is an important evolution technology for new application scenarios such as satellite communications and low-altitude communications, marking the transition of 5G from the ground to space. Based on the 3GPP open standard, 5G NTN can realize the compatibility of satellite communication and terrestrial communication systems, and with the help of direct connection of mobile phones, it can make full use of and share the industrial chain and economies of scale of terrestrial 5G, and rapidly expand the scale of the satellite communication industry.
ZTE: The company has completed the verification of 5G NTN mobile phones directly connected to the satellite field and the sea area.
At the Mobile World Congress (MWC2023), MediaTek brought a breakthrough 5G NTN two-way satellite communication technology, which can once again enter the public eye, which can fill the gap in mobile communication network coverage.
For LEO mobile satellite systems, Doppler frequency compensation is a key technical point.
In 3GPP Rel-17 NTN, since the scenario is set to transparently relay satellites, Doppler changes affect the service link and the feed link. From the perspective of UE, the service link can calculate the corresponding Doppler change through the ephemeris information and the position information of the terminal, while for the feed link, due to the lack of location information of the ground gateway, this part of the Doppler offset needs to be compensated by the base station.
In Doppler compensation, the network needs to broadcast ephemeris information to the terminal, and the accuracy and format of the ephemeris are key factors. In the 5G NTN system, the time synchronization error needs to be within the range of 1/2CP (cyclic prefix), and the frequency error needs to be controlled within 0.1×10-6, so the ephemeris information needs to be updated periodically and maintain the necessary accuracy.
Considering that in NTN, the satellite-to-ground communication delay is too large, far exceeding the maximum indication range of the relevant timing parameters defined in the terrestrial network (such as PDSCH to HARQ feedback delay 1, uplink scheduling to PUSCH transmission delay 2, etc.), in order not to affect the compatibility of the standard, 3GPP Rel-17 introduces a new value offset (_offset) on the basis of the existing timing parameters, that is, a _offset is added to all influential timing relationships to cover the impact of satellite-ground propagation delay.
Specifically, it includes the timing relationship of DCI scheduling PUSCH transmission, RAR scheduling of PUSCH transmission, timing relationship of PDSCH-to-HARQ feedback, timing relationship of reference CSI resources, timing relationship of non-periodic SRS (sounding reference signal), effective time of TA commands carried by MAC CE (control element), and timing relationship of PDCCH scheduling PRACH transmission. The timing enhancement design introduces an offset K_offset that can be applied to modify the corresponding timing relationship, which can be different for different timing relationships.
For GEO and MEO networks, the number of HARQ processes is too large, resulting in limited UE caching capabilities. Therefore, 3GPP Rel-17 determines that the NTN has the ability to configure the UE to turn off the feedback and retransmission function of HARQ, and determines that only a maximum of 32 processes will be supported based on the capabilities of the terminal.
In the existing art, HARQ shutdown means that the UE cannot do a soft merge. When the PDSCH transmission fails, the RLC layer retransmission can also work, but compared with the HARQ retransmission at the MAC layer, the spectrum efficiency is low, and the UE cannot soft-merge the multiple retransmission results, and the second is the delay. In order to avoid retransmission at the RLC layer, NTN needs to improve the success rate of initial transmission by reducing spectral efficiency (such as repeated transmission, high BLER target, low MCS scheduling, etc.), but this also leads to low frequency efficiency of NTN.
(1) Condition switching
For low-orbit satellites, there are two modes of beam coverage: covering fixed beams and covering moving beams, the so-called covering fixed beam refers to the fixed area where the beam points to the ground, while covering the moving beam refers to the movement of the beam with the movement of the satellite.
Connectivity mode mobility management is divided into the following five specific scenarios based on UE mobile and satellite mobile:
Scenario 1: Feed-link handover for covering a fixed beam, including UE service link handover.
Scenario 2: Feed-link handover for covering the moving beam, including UE service link handover.
Scenario 3: Coverage Fixed Beam Service Link Handover Caused by Satellite Handover.
Scenario 4: Connection mode mobility of the override moving beam when the override moving beam is no longer serving the UE.
Scenario 5: Connection mode mobility due to UE movement, overlay movement and override fixed beam.
For the switching of the NTN system, the main consideration is how to use the ephemeris and the location information of the terminal to ensure the reliability of the switching. In 3GPP Rel-17 NTN, the technical scheme of conditional handover (CHO) is introduced, that is, based on the law of satellite movement, the terminal to the point is configured in advance according to a certain condition.
(2) Enhancement of measurement scheme
In traditional co-channel measurement and heterofrequency measurement, the transmission delay difference between different ground base stations and terminals is relatively small. For non-terrestrial networks, the transmission delay difference between satellites and UEs is large, especially the transmission delay difference between LEO and GEO to UE, which is even more than 100 milliseconds, and if the existing measurement configuration is used, the UE may not be able to detect the synchronization signal and PBCH block (SSB) of the target cell.
At the same time, due to the relatively fast movement speed of the satellite, the error rate of the measurement configuration may be much higher than that of the terrestrial network in the actual execution, so in 3GPP Rel-17, the measurement scheme is enhanced to fully consider the propagation delay difference between the target cell and the service cell to the UE, so that the UE can correctly detect the SSB of the target cell. At the same time, the moving speed of the satellite is comprehensively considered to improve the fault tolerance of the measurement configuration.
2 Multiple access technology
In high-speed data transmission, the design of FPGA multiple access detection system is essential to achieve conflict-free transmission between multiple users. The FPGA multiple access detection system mainly includes the selection and implementation of multiple access detection technology, as well as the analysis and reorganization of multi-user data.
FPGA implementation of sparse code multiple access system based on 5G wireless communication
On the basis of understanding the multiple access of wireless communication, a low-complexity FPGA implementation scheme of sparse code multiple access system based on 5G wireless communication is proposed.
Definition: A method in which multiple users directly use a common channel to communicate between users, also known as arbitrary location communication and multivariate connection. In the communication network, in order to reduce the number of direct lines between users, a switching center is usually set up, and each user only needs to have a pair of user lines connected to the switching center to realize communication between users; Multiple access communication is widely used in mobile communication networks, satellite communication networks, and computer area networks, which is a very promising communication mode
Multiple earth stations, no matter how far away, can be used for bilateral or multilateral communication through satellites as long as they are within the coverage area of the same satellite. Multiple access technology refers to the manner in which multiple earth stations in a system occupy their own channels to access and receive signals from satellites. At present, the main technologies used are frequency division multiplexing (FDMA), time division multiplexing (TDMA), code division multiplexing (CDMA), space division multiplexing (SDMA), and random multiple access (RA/TDMA).
3 LEO satellite LEO giant communication network
5G commercialization is in the ascendant, but the layout of 6G technology has already begun. Among the various 6G technology solutions, low-orbit communication satellites (LEO) have been regarded as a key part of the future era of efficient and intelligent interconnection.
The introduction of ultra-low orbit may change the paradigm of the Internet. Compared with traditional LEO or GEO satellites, the communication based on the VLEO mega constellation has the remarkable characteristics of low transmission delay, low propagation loss, high regional capacity, and low manufacturing and launch costs.