Communication Technology  

 

 

 

Satellite-Mobile Communication

Imagine talking to someone far away like in deep forest or in the middle of ocean or even in another country, using your mobile phone. Have you ever wondered how your voice or message travels such long distances? A big part of the answer lies up in the sky with satellites (assuming that there is no roaming service).

Satellites are like big mirrors in space that bounce signals from one point on Earth to another, including the signal from your mobile phone. This is how we can send messages, make calls, and use the internet on our phones, even when we're in places where there are no phone towers nearby.

In this note, we're going to talk about how satellites and mobile phones talk to each other. It's a bit like playing catch with a ball, but instead of throwing a ball back and forth, we're sending invisible signals up to the sky and back down to Earth. This technology has made it possible for us to stay connected with people no matter where they are, even in the middle of the ocean or on top of a mountain.

We'll also look at what makes this communication challenging, how smart people are working to make it better, and what exciting things we can expect in the future. So, join us as we explore this amazing technology that keeps us connected to the world.

Challenges

Communicating between satellites and mobile phones, despite its widespread use and critical importance, faces several challenges. These challenges can affect how well and how reliably we can communicate, especially in remote areas or during critical situations.

  • Latency Issues: Latency is the delay before a transfer of data begins following an instruction for its transfer. In satellite communication, signals have to travel thousands of kilometers to space and back, which can cause a noticeable delay. This is particularly problematic for real-time applications like video calls or online gaming.
    • Implication on Implementation: The inherent delay in satellite communication due to the long distance signals must travel presents a challenge to meeting these latency requirements. To align with 3GPP standards, satellite communication systems need to implement techniques like onboard processing and optimized routing to reduce latency, ensuring compatibility with applications demanding low delay, such as VoLTE (Voice over LTE) and real-time gaming. ==> We may try hard to reduce the latency by data/signal processing on UE and Satellite side, but there wouldn't be much thing to do with the delay caused by propogation delay between UE and Satellite.
  • Doppler Shift : One of the significant challenges is the Doppler shift. This phenomenon occurs because the satellite and the mobile phone are moving relative to each other. As the satellite orbits Earth, its speed causes the frequency of the signals it sends and receives to change slightly. This effect can make it difficult for the mobile phone to accurately pick up the signal, as the frequency might shift out of the phone's expected range. Overcoming the Doppler shift requires sophisticated technology in both satellites and mobile phones to adjust the frequencies of the signals they send and receive.
    • Implication on Implementation: :The 3GPP standards include specific provisions for compensating frequency variations, including those caused by the Doppler effect, especially in high-speed scenarios like high-speed trains. For satellite communications, adapting these standards means developing more sophisticated Doppler shift compensation algorithms that can handle the relative movement between satellites and mobile phones. This ensures consistent signal quality and reliability across all mobile networks, including those extended through satellite links.
  • Delay Spread : Another challenge is delay spread, which refers to variations in signal travel times due to different paths the signal might take. In satellite communication, while the open space minimizes obstacles, the immense distances the signal travels can cause parts of it to arrive at slightly different times. This variation can distort the received signal, affecting the quality and clarity of the communication.
    • Implication on Implementation: : Delay spread, with its potential to distort signals, directly impacts the Orthogonal Frequency-Division Multiplexing (OFDM) technology used in LTE and 5G networks, as defined by 3GPP. OFDM is sensitive to timing discrepancies, which can cause inter-symbol interference. Addressing delay spread in satellite communications involves incorporating advanced equalization and timing correction techniques that are robust enough to handle the variations, thereby aligning with 3GPP standards that aim for high-quality, clear communication.
  • Transmission Power : The vast distance between satellites and mobile phones also poses a challenge for transmission power. Transmitting a signal across thousands of miles requires significant power, but satellites are limited by their size, weight, and the solar energy they can collect. Balancing the need for a strong, clear signal with these power constraints requires careful engineering and power management.
    • Implication on Implementation: : The 3GPP specifications detail power control mechanisms to optimize signal strength and quality while minimizing interference and power consumption. In the context of satellite communications, adhering to these specifications means developing satellite payloads and mobile devices that can operate effectively within the power constraints set by 3GPP. This involves leveraging highly efficient power amplifiers, smart power management systems, and adaptive transmission techniques to ensure that both uplink and downlink communications meet the power requirements and constraints defined by 3GPP.
  • Receiver Sensitivity : On the receiving end, mobile phones and satellite must have the sensitivity to detect the satellite's signal, which can be weak and prone to interference. Improving a mobile phone's receiver sensitivity without significantly increasing its power consumption or cost is an ongoing challenge in the field.
    • Implication on Implementation: : Receiver sensitivity is crucial for achieving the high-performance levels specified by 3GPP, particularly in challenging signal conditions. For satellite communications, enhancing receiver sensitivity to meet 3GPP standards involves deploying advanced noise reduction and signal processing technologies. This ensures that mobile phones and satellite receivers can detect and decode signals effectively, even at the low signal-to-noise ratios common in satellite links, thereby supporting the reliable, high-quality service expected from 3GPP-compliant networks.

How to Workaround the challenges ?

Addressing the challenges of satellite communication involves innovative workarounds and technologies, particularly when considering the integration of satellite systems with conventional User Equipment (UE) and Non-Terrestrial Network (NTN) capable UE, as defined by 3GPP standards.

NOTE : I guess most of the demo(pre-deployment ?) shown in 2020 or earlier is based on the satellite and non NTN UE (regular mobile phone without supporting NTN) because it is before the finalization of 3GPP NTN specification or UE modem chipset availability and they claim that the communication is possible with existing mobile phone without any modification. But almost no technical details in terms of protocol issues. So my comments on non-NTN devices are from my speculation.

  • Latency Issues
    • Satellite: I can think of a few options.
      • Choice of orbit (LEO or GEO). This would be the biggest factor.
      • Choice of eNB/gNB configuration : RACH Preamble type, RAR window size, Contention Resolution Window, HARQ related parameters etc.
      • Employing the advanced DSP to reduce the internal delay may do some help but not as much of orbit choice.
    • Conventional UE: According to the claims from various satellite solution provider, there wouldn't be any special tricks required.
    • NTN Capable UE: Compliance to NTN specification (e.g, handling large TA(Timing Advance), HARQ requirement)
  • Doppler Shift
    • Satellite: Design satellites and their payloads with advanced frequency compensation technologies that automatically adjust for Doppler shifts caused by their movement relative to the Earth. Doppler shift can be compensated both by Satellite and UE, but I think the compensation by Satellite would be the major factor.
    • Conventional UE: According to the claims from various satellite solution provider, there wouldn't be any special tricks required.
    • NTN Capable UE: How to handle Doppler shift is not directly specified by 3GPP, but advanced DSP(e.g CFO (Channel Frequency Offset) compensator, Advanced Channel Estimator / Equalizer) would be a great help.
  • Delay Spread
    • Satellite: Employ advanced modulation and coding schemes that are more resilient to the effects of delay spread, as well as smart beamforming technologies that can focus signals more directly to the intended receiver, reducing multipath interference.
    • Conventional UE: According to the claims from various satellite solution provider, there wouldn't be any special tricks required.
    • NTN Capable UE: NTN-capable UEs can use more advanced equalization and error correction protocols that are optimized for the unique challenges of satellite communication, including delay spread.
  • Transmission Power
    • Satellite: I think this should be handled mostly by Satellite using special techniques as below
      • High Power / High Gain Antenna. Due to this, the satellites for cellular communication has antenna with huge size
      • Advanced Beam Forming
    • Conventional UE: There wouldn't be any breakthrough on UE side because the max power from UE is strictly limited by 3GPP.
    • NTN Capable UE: There wouldn't be any breakthrough on UE side because the max power from UE is strictly limited by 3GPP
  • Receiver Sensitivity
    • Satellite: Like TX power, I think this should be handled mostly by Satellite using special techniques as below
      • High Power / High Gain Antenna. Due to this, the satellites for cellular communication has antenna with huge size
      • Advanced LNA.
      • The environment around satellite (very low temperature comparing to terrestrial environment) can be a strong positive factor (i.e, very low thermal noise)
    • Conventional UE: According to the claims from various satellite solution provider, there wouldn't be any special tricks required.
    • NTN Capable UE: Application of high quality LNA, advanced Equalizer etc can help.

Really Deployed ?

Satellite to cellular communication is not a wish-to-have technology any more. There are several solutions that are already deployed (or at least demonstrated / proved working). Followings are a list of these solution :

Starlink / Tmobile Direct to Cell

Source : SPACEX SENDS FIRST TEXT MESSAGES VIA ITS NEWLY LAUNCHED DIRECT TO CELL SATELLITES

Inmarsat

Source : Satellites - inmarsat

    • Satellite Orbit : GEO

    • Radio Access Technology : 4G

    • Frequency/Band : L-band (?)

AST/SpaceMobile

Source : AST/SpaceMobile

YouTube

Reference