In a crisis, rapidly deployable mobile broadband services are vital: when ground communications are disabled, or the site of a humanitarian disaster is remote, then satellite communications (satcomms) can enable a mobile network to be set up quickly. Satcomms networks can provide wide service coverage and reduced vulnerability.
For example, during the Japanese tsunami in 2011, 1.9 million fixed lines and 29,000 base stations were damaged. Even where network equipment survived, there were disrupted links between the radio access network and the core (backhaul), which affected communications. Even if existing networks are still operating in this kind of crisis, sudden demand can still overload their capacity.
WHY LTE FOR SATELLITE COMMS?
There are different approaches to providing communications for rapid deployment, but LTE has multiple features that make it an ideal choice. For a start, LTE user equipment is widely available, with phones, laptops and tablets all being relatively low cost. Terrestrial networks with satellite backhaul can also use standard equipment.
For more specialized networks, mission-critical LTE is also part of the 3GPP specifications. It can provide a stable and robust communications platform for first responders, with the fundamental capabilities needed for mission-critical traffic handling including push-to-talk and proximity services.
LTE also provides native end-to-end security, including symmetric key cryptography and the snow 3G stream cipher that encrypts wireless transmissions. If the crisis is the result of hostile action, secure communications could be crucial to stabilizing the situation.
Finally, there is continued innovation by the global 3GPP community to drive new capabilities. For example, example Narrow-band Internet of Things (Nb-IoT) and CAT-M connectivity allow more efficient spectral usage, which helps amortize the cost of satellite spectrum and reduce costs.
SATELLITE NETWORK ARCHITECTURES
There are different satcomms/LTE system options available. Firstly, satellite backhaul can be used with a terrestrial LTE infrastructure (see Figure 1). This uses a standard 3GPP ground network with DVB-S2/S2X backhaul. It has the advantage that user equipment with standard mobile chipsets can be used unmodified, but the drawback is that ground base stations must be deployed. This can be difficult or impossible if a catastrophic event compromises ground movements and transportation.
This approach also requires an LTE license for the ground network in addition to satellite access. Unlicensed spectrum may be available, but this requires user equipment with radios in a non-standard LTE band. Unlicensed technologies such as Citizens Broadband Radio Service (CBRS) are options, but are only available in some areas and run the risk of being shut down under certain circumstances.
An alternative satellite architecture is "bent pipe" (see Figure 2). In this approach the LTE waveform is transmitted from the ground station and a satellite acts as a "bent pipe" RF repeater, retransmitting the waveform. Here, the satellite technology is waveform-agnostic, and does not require modification to support new waveforms, for example to move from DVB-S2 to 4G LTE to 5 G New Radio (NR). The satellite ground station integrates LTE eNodeB support, and each beam has a dedicated eNodeB at the ground station. Only one ground station is required per satellite, and the architecture allows co-location of equipment central-office type equipment.
A third option is to have modified, LTE-based eNodeBs carried on the satellite itself (see Figure 3), so that the satellite is now the base station. This does introduce new problems: equipment upgrade is difficult, although remote software modification is possible; and, base station synchronization is computationally intensive, since terrestrial GPS receivers do not work in a satellite context. The satellite is also more complex, because it must carry both eNodeB technology and DVB-S2/S2X backhaul circuitry.
As well as the architecture used, how satellites can achieve suitable coverage of the required land area must be considered. Looking first at Geostationary Earth Orbit (GEO) satellite broadcast LTE networks, where there are incumbent vendors who already own spectrum in the L-, S- and Ka bands. Satellites can be deployed to cover a single geography, using high throughput satellites with multi-spot beam technology, and steering the spot beams to focus capacity to where it is required.
An alternative approach is to use CubeSat satellites in Low Earth Orbit (LEO) for satellite broadcast LTE networks. CubeSat is reducing the cost of satellite deployment, for example SpaceX’s Falcon 9 recently deployed 64 cube satellites in one launch. Since launch costs are low, CubeSats can be built cheaply with little or no radiation-hardening, and be cost-effectively replaced every two or three years; so, the business case for adding cost to enable upgrades is weakened. As the satellites are in LEO, they can be lower power than for GEO, but more satellites are required to cover the same area; typically 50 to 80 are required in order to cover the Earth for LEO, compared to three for GEO.
SATELLITE LTE CHALLENGES
There are some technical issues to overcome with satellite LTE. First is increased delay to the transmitted signals. GEO satellites have a one-way delay of up to 280 ms. While LTE is designed for a maximum cell size of 100 km, or 0.35 msec delay, and the variations in the satellite path length increase the delay spread beyond 16.67 µs supported. Additionally, the LTE Random Access attach procedure requires substantial modification. This can be achieved using enhanced preamble channel (PRACH) processing in Layer 1, but the eNodeB needs to look for preamble over longer delays and larger delay spreads.
For CubeSats with eNodeB on board, in LEO, there are also increased Doppler effects to account for. The very high speed of the satellites causes strong Doppler effect conditions; for example, a CubeSat at an altitude of 200 km with a speed of 7.9 km/s can result in a Doppler shift up to +/- 25 ppm. Also, LTE macro cell handover is limited to a speed of 350 to 500 km/h depending on the frequency band limited by the LTE Random Access Procedure.
While the LTE standard has some flexibility, it is necessary to extend beyond the standard to use LTE over satellite. With deep knowledge and modification of the physical layer, protocol stack and hardware, product developers can take advantage of LTE economies of scale to achieve a specialized satcom product.
5G FOR NON-TERRESTRIAL NETWORKS
Satellite is an important technology for 5G. While LTE was all about terrestrial networks, 5G satellite applications are being considered while the standard is still evolving. 3GPP published a report on 5G satellite access (Technical Report 3GPP TR 22.822 V16.0.0 (2018-06)), which proposes several use cases. These include roaming between terrestrial and satellite nets, broadcast and multicast with a satellite overlay, the Internet of Things with a satellite network and indirect connection through a 5G satellite network.
The report also identifies potential requirements for 5G satellite applications. First, there is a need for meshed connectivity between satellites based on 5G radio access technology. Then, the satellite network must have quality of service (QoS) capability, with communication service availabilities greater than 99.99 percent. Massive machine type communications (mMTC) and/or Narrow-band Internet of Things (Nb-IoT) services should also be supported.
Another 3GPP report, "Study on New Radio (NR) to support non-terrestrial networks," looks at how 5G can be implemented using satellites, high-altitude platforms (HAPS) and Unmanned Aircraft Systems (UAS) (3GPP TR 38.811 V15.1.0 (2019-06)).
Finally, 3GPP report TS 22.261 includes specific requirements to support satellite access for 5G. This includes optimized delivery of content through caching, as well as system enhancements to handle the latencies introduced by satellite backhaul. It also considers the financial implications, and how satellite type access could be charged for.
Satellite based communications will form a key part of mobile network coverage, one that will offer increased resilience and rapid deployment to alleviate humanitarian crises, and 5G specifications will standardize these networks allowing greater interoperability of equipment designed for this market. However, questions remain as to the relative merits of the various satellite network architectures and the licensing of operational bands.
Looking at 5G timescales, these will be driven by the availability of satellite-enabled user equipment (UE). The "bent pipe" satellite infrastructure can be re-used initially, but inter-satellite link technology will require a new breed of satellite. This means that the roll-out of 5G satellite features can be expected to happen in phases, and alongside other non-terrestrial networks (NTNs).