Major Considerations for SATCOM Space Segment Subsystem Planning, Design to Drive 5G

By Andrew Aroh

New orbits: LEO & MEOs

• The advantages of the Geostationary are so overwhelming that it is unlikely that its popularity will decline in the foreseeable future in spite of the increasing difficulty of satisfying the demand for orbital positions.
• The geostationary satellite is not, however, the perfect solution to all requirements. Apart from the major issue of LATENCY, it has the disadvantage of being fairly low over the horizon in certain parts of the world, so that mountains, buildings and even trees can cause obstruction. This is frequently disturbing in user-oriented applications and particularly objectionable in land-mobile communications.

• For the above reasons, to provide continuity of service, an operational system would need several satellites describing the same inclined elliptical orbit but with different phases so that the relay functions can be taken over from one another. Such a concept called Loopus (LEO constellations) give the user the illusion that the same satellite is describing the same loop in-space indefinitely.

NEW FREQUENCY BANDS: OPERATING SPECTRUM

5G systems are designed to operate in the mmW band, however the current frequency band widely in use for satellite broadband communications is the 30/20GHz band where 3.5GHz are available in each direction.

• At these frequencies, the disturbances caused by the atmosphere, rain in particular, to the signal propagation are much more serious than at lower frequencies. Designing systems capable of giving a satisfactory level of service-quality in spite of these disturbances constituted a new challenge for communications engineers. However, a whole gamut of adaptive techniques is already available. The variable parameters in the adaptive process includes: the transmitted power, the transmission rate, the type of modulation & coding and even the frequency band itself.
• The choice of this band would be a long-term solution to the problem of accommodating all satellite mobile services in the same 1.5/1.6GHz band. Hence, it has become quite feasible to develop flat phased-array antennas suitable for installation even on aircrafts

OPERATING SPRECTRUM:

• Recall that the speed of wireless network is tied to how much spectrum you can use for it. The Mid-band air-waves have frequencies ranging from 3.5GHz to 7GHz that are slightly above current cellular bands but have quantities of spectrum (and speeds) that start to look like millimeter wave.
• Therefore, to get super-high, multi-gigabit speeds, carriers are first turning to newer, much higher frequencies, known as millimeter-waves. These band have been used before for backhaul connecting base stations to remote internet links. But have not been used for consumer devices before, because the handheld processing power and miniaturized antennas were not available, but now in wide production these days. These will enable new and emerging technologies such as virtual and augmented reality.
• At its most basic, Ka-band is a high frequency chunk of radio spectrum at 18.3GHz – 30GHz for uplink and 20GHz for the downlink; with a bandwidth of 3.5GHz. Q-band = 36GHz to 46GHz. V-band = 46GHz to 56GHz.
• Up at Ka-band, V-band and mmW-band there are big, broad swaths of spectrum available to create big channels for very high-speeds enabling the following three major categories of use case for 5G:
  • Massive Machine-to-Machine communications – also called the internet-of-things (IoT)
  • Ultra-reliable-low-latency communications for mission critical environments.
  • Enhanced Mobile Broadband for greater connectivity for people on the move, providing significantly faster data speeds and greater capacity keeping the world connected.

USERS AND NETWORK ORIENTED SYSTEMS

• Flourishing of new user-oriented applications followed by interesting prospects of sophisticated operational systems both in the fixed and the mobile services as a result of implementation of novel concept resulting in 3-D systems in which satellites and terrestrial links play a specific role in an integrated networks-oriented applications.

Terrestrial systems provide two dimensions and satellite third one, the satellite performing as an active communications mode rather than a simple relay.

• As a result of much R&D efforts already made, there are now the necessary space technology particularly in the field of antennas, space switching & on-board processing paving the way for development of revolutionary types of satellite systems, “the switchboard in space” which will eventually handle both fixed and mobile communications for 5G. This will give rise to substantial advantages in terms of earth station cost reduction, both in trunk call applications and in business services.
• It is believed that the basic problems will be to achieve the necessary level of reliability in space
• Future satellite systems will be characterized by a much greater capability to adapt their configuration to changing requirements, since these can be expected to vary considerably during the average time of a satellite in orbit.
• It has for sometime been possible to build antenna systems which generate very complicated beam contours. However, the beam structure cannot be altered once the satellite is launched, except to switch from one predetermined configuration to another. Beam forming networks with continuously variable elements now makes it possible to vary at will the beam structure in orbit.
• Antenna with reconfigurable beams associated with adequate frequency – returning capability of the transponders will enable a unified design to be adopted for all systems and a substantial saving to be realized. This technique will also be applied to low-power satellites, with two advantages: The first will be that their coverage can be modified as circumstances require; the second will be to permit shifts along the orbit to facilitate mutual co-ordination between different systems.
• For applications requiring continuous coverage (as distinct from spot coverage) but a higher antenna gain than is achievable with a single beam, the MULTIBEAM ACTIVE ANTENNA is the solution. This type of antenna is inherently capable of allowing multiple reuse of the frequency spectrum.
• Note that, the traffic is not evenly distributed; some beams would already be saturated, while others were still very lightly loaded. At the high frequencies used in the FIXED SERVICES, a possible answer is the beam – hopping technique which enables the satellite capacity to be time-shared between several areas with low traffic density. This technique must be associated with TDMA ACCESS TECHNIQUE. At lower frequencies, such as 1.5GHz, used by the mobile service, the solution is the phased-array with distributed power amplifiers. This technique allows the distribution of the antenna GAIN and the available TRANSMIT POWER in a very flexible manner in all required directions.

ACTIVE MULTI-BEAM ANTENNAS:

• A major growth area in regional and domestic satellite communications over the next two decades will be the demand by the business community for specialized services providing telephony, data transfer and video conferencing. Many of these services, especially video conferencing require high bandwidth allocations.

In addition, to enable the use of small earth-station antennas at Ka, V and subsequently mmW, it is necessary to increase the satellite EIRP over the operational area. In the field of maritime and land mobile communications at L-band, high EIRP is required to operate to small antennas on ships and mobiles respectively.

• These specifications of higher bandwidth and increased satellite EIRP has led to the requirement for a satellite antenna to radiate a large number of narrow beamwidth spot beams.
• The multiple beams positioned on a regular matrix and overlapping at a cross-over point close to their norminal half-power beamwidth afford contiguous coverage of the service area as typically illustrated in the diagram for the possible SATCOM Multibeam Active Antenna Coverage of Nigeria.

This can also offer the advantage of increasing the level of frequency re-use by spatial discrimination, whereby more than one non-adjacent beam can utilize a common frequency band. Polarization discrimination can be used in conjunction with spatial discrimination to realize an even greater level of frequency re-use. The coverage scenario also provides FLEXIBILITY as required for 5G in the traffic-to-beam allocation:

• Such multiple spot beam coverage could be achieved using a Flat plate phased-array antenna for LEO satellites.
• To obtain the necessary isolation between beams while maintaining minimum beam separation, it is required to generate each beam by a cluster of feed elements rather than a single element.
• An alternative approach would be the use of a DIRECT-RADIATING ARRAY where all the radiating elements contribute to each of the spot beams.
• The benefits associated with coverage reconfigurability and channel-to-beam switching can be greatly increased in the beam forming and switching networks by introducing amplifiers at the radiating element level and forming an active antenna
• The design of active antennas together with the development of the necessary amplifiers, variable phase and power dividers has made possible variable beam forming networks.
• A variable beam forming network allows any combination of MBAs ports to be illuminated with any desired distribution of RF amplitude and phase. It makes use of all the degree of freedom of the multiple beam antenna and yields numerous operational capabilities including continuous pattern shaping and adaptive nulling of interference sources.

THE APPLICATIONS OF THE RSBA SCHEME TO DIRECT LEO SATELLITE COMMUNICATIONS WITH MASSIVE MIMO SMARTBEAM FORMING

• Resource Shared Beam forming Access (RSBA) scheme can be applied to massive LEO Satellite communication systems operating at Ka-band for massive access using Non-orthogonal and grant-free access.
• Ka-band has already become the priority spectrum band for some LEO Satellite operators, which incorporate leading-edge technologies and features, such as sophisticated phased-array antennas on each Satellite to create multiple dynamic beams.
• RSBA leverages on the massive MIMO technology to achieve shapeable and steerable beams, which is crucial to materializing massive access for high-rate IoT systems. For example, surveillance and security systems that need to send multiple photos or high-volume sensor data could benefit from the RSBA technology in poor connectivity areas.
• RSBA could also play a role in maritime IoT to handle large data rates. As for the devices, the application scope should be circumscribed to advanced platforms with tracking antennas, providing the necessary gains at Ka-band.
• RSBA goes beyond the current deployments using Ka-band as the remarkable features are suitable for IoT. For instance, RSBA lies within the category of random access schemes that are able to perform blind user detection at the receiver. The link budget feasibility at Ka-band requires the necessity of compensating the delay and the Doppler shift effects to ease the detection.
• For the development of the new access scheme suitable for mMTCs in LEO Satellite communications systems, there is a need to allow spectral coexistence between Satellite and terrestrial systems and to improve beam management mechanisms. Feasibility analysis conducted from a regulatory characteristics, space segment link budget and system point of view, shows that the following are possible for more futuristic IoT ecosystem:
  1. Ability to obtain a beamformer in the direction of the target user, without neither acquiring channel state information nor carrying out an exhaustive search through multiple angles.
  2. RSBA can benefit from beamforming techniques to lower the collision probability within a large population of terminals transmitting simultaneously.
  3. Practical implementation aspect have been tackled, such as the estimation of the covariance matrices and the determination of the number of users.
  4. Simulations show that the proposed beamforming techniques is able to distinguish and separate users that are located in different spotbeams. Numerical result also reveal that performance gains can be achieved with respect to fixed beamforming networks.

All the above features give us the smart beamforming for Direct LEO Satellite Access of future IOT using mmWave.

LEO SATELLITE LINK BUDGET CALCULATIONS:

• Consider the critical points in a Satellite Link:
  • Available power at the Satellite
  • Available power at the ground station
  • Sensitivity of the Receiver
  • SNR at the Receiver
  • Reception level at the Earth to avoid interference
• Compute the received power at the entrance of the receiver. It must be higher than the sensitivity.
• Compute the noise received by the antenna relative to the reference point to compute the performance of the link budget. Typically at the entrance of the receiver or at the entrance of the LNA.
• Use the SNR and/or E_b/N_o methods to determine if we have achieved the required margins. Note the following in the computations:
  • Received Power:
    • Friis formula application to antennas, polarization and propagation
  • Noise computation:
    • Radio noise and antenna noise
  • Signal-to-Noise-Ratio computation:
    • C/No; Doppler Effect; E_b/N_o
  • Receiver sensitivity computation and interference analysis.

LEO SATELLITE CHARACTERISTICS:

• Altitude = 200-1400km (e.g. 1375km)
• Period = 90 Minutes
• A single satellite in LEO orbits is in view for approximately 20 minutes from AOS to LOS
• Light-weight, small (< 1m sq), inexpensive, easy to launch and smaller less powerful rockets are required to launch it
• Minimal delay (Ideal for Telephony)
• Short Life Span (5-7years) as compared with GEO satellites
• The LEO satellite move relative to the earth, and therefore communications are not practical unless there is a constellation of satellites.

SIMULATION & MODELING OF MODERN BROADBAND SATELLITE SYSTEM:

• Satellite systems and applications are expensive to design, test, deploy and operate.
• Mistake made at the development stage can cost millions of dollars in lost revenue if technology and services do not perform as originally intended.
• Satellite themselves and the networks & applications that they support are becoming increasingly complex and the potential cost of failure can be significant. Hence manufacturers and developers both for Networks & Applications are looking for ways to minimize the technical risk involved before building or deploying millions of dollars worth of space and ground infrastructure.
• Simulation and modeling techniques can provide relatively cheap and flexible way for developing new technologies and services without the cost and risks associated with building and developing real hardware.
• Simulation & modeling tools can be applied to the following areas:
  • Space segment subsystems design & development
  • Constellation & network topology
  • Network performance
  • Propagation path performance
  • Ground segment design & development
• Simulation & modeling is very critical for space segment subsystems planning, design and development. Typically for mobile broadband, the following suffix: Multibeam Active Antenna Coverage Areas; impact on network design; on-board Telemetry, Telecomm and control system (C&DH)

SUMMARY OF THE MAJOR CONSIDERATIONS

• The underlying trend in the design of communications satellite antennas has been and almost certainly continue to be towards more efficient usage of the basic resources, namely the frequency spectrum, orbital locations and the available satellite power.
• More effective use of the frequency spectrum has been achieved by re-using existing frequency bands by either spatial or polarization discrimination and more recently by the move towards the higher, less congested bands, such as 20/30GHz, V-band and the coming mmW-band.
• More effective use of the power is achieved with the implementation of contoured or multiple beam antennas concentrating their radiated energy into designated geographical areas in turn this leads to the use of smaller earth station antennas in much greater numbers.
• While future satellite antenna systems will seek to improve the usage of these basic resources, a further requirement will be that of reconfigurability of the coverage whilst in orbit. With this capability the antenna radiation characteristic will be subject to modification so as to:
  • Change the coverage area to meet different traffic demand
  • Modify the coverage area after the satellite has been relocated in a different orbital location
  • Provide discrimination against interference signals from varying locations.
• A major factor in the development of communication satellite for future missions will be increasingly stringent requirements in terms of beam-pointing accuracy that must be met by the on-board antenna subsystem.

For some missions, the required pointing accuracy cannot be met by the spacecraft attitude & orbit control system (AOCS) and it is necessary for the control loop to involve an RF sensor, integrated within the antenna to be pointed, which serves to lock the beam onto a ground beacon positioned within the required coverage zone. More sophisticated RF sensing techniques will become an integral part of the design of most satellite antennas.

Andrew Aroh is President of SSPI Nigeria