Why Satellite Networks Can't Replicate Conventional Mobile Networks
Integrating Satellite Communication into a Conventional Mobile Device Isn't a Complementary Choice
The recent launch of the Huawei Mate 60 Pro, equipped with satellite calling capabilities, has generated significant excitement. At first glance, this development might suggest that satellite communication is poised to become more potent, potentially even rivaling the capabilities of traditional terrestrial mobile networks in the near future. However, this note aims to delve into the intricacies and challenges that make it extremely difficult, if not nearly impossible, to equip mobile phones with satellite calling and data capabilities that can fully replicate the functionality of conventional terrestrial networks.
GSO or NGSO?
Satellite networks can be designed to work in two different configurations of satellites - Geostationary and Non Geostationary. The former revolves around the earth at its rotational rate of 24 hours and therefore appears stationary to an observer. Whereas the latter revolves around the earth at much faster rates and therefore appears in motion. A satellite in a Geostationary Orbit (GSO) needs to be at a height of 36,000 km and those in NGSO can be anything as low as 300 km to something even higher.
It takes 1.5 hours for an NGSO satellite located at a height of 300 km to make one revolution of the earth. This means it will take just 5 minutes for it to traverse an arc of 20 degrees. Hence, achieving and maintaining a stable connection with NGSO satellites for reliable voice communication would challenging (maybe impossible), especially for handheld mobile phones without specialized equipment and tracking mechanisms.
Hence, handheld mobile phones are typically configured to work with satellites located in Geostationary orbits, as communicating with them will need no specilized tracking mechanism.
Low or High Frequency Bands?
Since satellites located in GSO orbits are located at 36,000 km, for mobile phone signals to reach such heights they need to transmit in frequency bands that have better propagation characteristics, wider coverage, improved signal penetration, and are less affected by rain and other atmospheric conditions.
Only low-frequency bands satisfy these attributes, and hence mobile phones with satellite reception and transmit capabilities are typically configured to operate in such bands.
Huawei Mate 60 Pro
Huawei Mate 60 Pro is understood to be linked to a Chinese GSO satellite called Tiatong operating in S-band (1980-2010/2170-2200 MHz). This is an FDD band with a bandwidth of 2x30 MHz.
Now free space path loss of an RF signal is given by Friis Transmission Equation and is calculated as below.
Note that normally mobile phones are configured for transmitting at 26 dBm (for conforming to regulations to mitigate EMF), which translates to -4 dB (dBm = dB +30).
Hence, if we take this into consideration then we have the constant term in the above equation replaced by 193.58 dB - pulling the received power at the satellite antenna receiver down further. But we understand that the Huawei Mate 60 Pro is configured to transmit at 33 dBm (3 dB), and therefore the calculation shown in the box holds true.
Data Rates
Now data rates that we will be able to experience are dependent on the amount of bandwidth that we have for transmitting and receiving. For Huawei Mate 60, this is just 30 MHz. This is extremely low bandwidth given the fact that we will just have a few satellites serving large numbers of users - compared to millions of ground-based BTS for the terrestrial networks.
Also, note that increasing the bandwidth to a greater value may not be possible, as it will cause two major problems - 1) It will increase background noise significantly, thereby nullifying the gain on account of increased bandwidth (Channel Capacity = BW x log(1+S/N); 2) It will significantly reduce the effective aperture, thereby increasing overall path loss significantly. Why? The reason is that large bandwidths are only available at high-frequency bands, which will drive down the effective aperture of the receiver as the same is proportional to the square of the wavelength, and wavelength is inversely proportional to frequency.
Hence, per-user data speeds will be extremely low, maybe in kilobits. This is coupled with the fact it is not possible to increase speeds by using high bit rate codecs due to extremely low received signal at the satellite receivers due to high path loss as shown above.
Latency
The other major challenge is increased latency due to the fact that the RF signal needs to travel a distance of 36000 km. This translates “two-way” delay of 240 milliseconds. This delay is further aggravated due to the aggregation of system processing delay on top of this - making voice communication unnatural and uncomfortable.
The increased latency has an impact on Data too - making internet browsing slow and frustrating. Data browsing speeds will decrease further due to the need to use robust coding (with lower bit rates padded with a large number of redundant error-correcting codes) to prevent the retransmission of Data triggered by repeated packet losses.
Hence, low speeds and high latency make the usage of mobile phones with satellite capabilities an option ONLY where no coverage of a conventional network is available.
How are SpaceX’s, and OneWeb’s Services Different?
They are different because these services operate with ground-based terminals which are fixed but capable of automatically tracking the satellites using beam-steering techniques. Such a configuration enables us to design the system with very high gain for both transmit and receive antennas. Path loss is also drastically mitigated due to the reduced height of the satellites from the ground (order of hundreds of km).
This reduced path loss, along with high receive and transmit gain makes the RF link budge quite comfortable for the system to operate at high frequencies and leverage large bandwidth without overwhelming the Channel Capacity with increased Signal to Noise Ratio.
Hence, such systems are capable of operating at reasonably low latency and large data speeds, but these metrics can never match those that are capable of being delivered by conventional gound based terrestrial networks.
Conclusion
From the analysis above, it's evident that satellite networks cannot match the capabilities of conventional mobile networks, whether in terms of data speeds or latency. At best, they serve as supplementary solutions in areas where deploying traditional mobile networks may not be economically viable. Therefore, satellite networks are unlikely to pose a competitive threat to terrestrial networks, both now and in the foreseeable future.