Analysis LEO-HTS systems and feasibility of phased array antennas for the subscriber terminals

V.R.
Anpilogov

Deputy General Director JSC “VSAT-TEL", Ph.D., associate professor

A.V.
Shishlov

Head of Department OJSC “Radiofizika", Ph.D., associate professor

A.G.
Eidus

General Director JSC “VSAT-TEL", Ph.D., associate professo

In 2012-2015 numerous reports on beginning development of multi LEO systems in the Ku and Ka-band were published, which got the name LEO-HTS. Their satellite constellations expected to create of hundreds or even thousands of small communications satellites with high capacity. Only from November 2014 to November 2015 was stated in ITU 20 satellite networks with a total number at hundreds of thousands of satellites in non-geostationary orbits LEO / MEO / HEO [1,2].

Of particular interest to such systems (judging by the statements of their investors) is associated with the ability to achieve high technical and economic indicators. Indeed, judging by the publication of preliminary data on the systems LEO-HTS, the resource cost price will be comparable with the future geostationary HTS systems or better [3]. Investors sure of satellite capacity demand is based on the active growth of cellular networks 4G and forecasts of active development of 5G. In addition, investors announced a number of projects directly connecting individual subscribers as the second main target.

It should be noted that in the early 2000 were known projects Teledesic (Ka), SkyBridge (Ku) and other like modern LEO-HTS. But none of the projects has reached implementation. Reporters are often asked investors of modern projects LEO-HTS. Why are contemporary systems better compared systems Teledesic and SkyBridge ? The answer is always the same - Today the satellite communications technologies have moved forward. Weight satellites can be reduced by several times while maintaining great potential of their informational capacity. Satellite productions are expected on the basis of their seriality. This allows to get the cost of 0.5 million $ per satellite and below. The cost of launching satellites decreases. As a result, the equivalent cost of Mbps is expected to significantly lower than for the systems of Teledesic and SkyBridge.

But one of the main arguments in favor of new systems LEO-HTS, which investors say, is to achieve extremely low price parameters of subscriber satellite terminals (approximately 100 - 300 $ in the SpaceX, [4, 10]) . As antennas with electrical beam scanning in a user terminal scheduled to use active phased array (APA).

The analysis of numerous publications in the field of phased arrays has shown that no single publication that would contain sufficient information to assess their feasibility in relation to this problems. Furthermore, as of November 2015 unknown no single publication that reveals the technical parameters of the systems LEO-HTS to uniquely identify the requirements for a phased array antenna of the subscriber terminals. In prior reports and publications only general data [1-5], that does not allow to clearly assessing the specific technical parameters. System specifications OneWeb partially disclosed in the publication [6] (based on the estimated parameters OneWeb for an analysis of EMC OneWeb and BSS). The results of this analysis are summarized in [1]. But it should be borne in mind that in projects rapidly transformed system parameters [7] in the process of designing.

Some of the common system parameters LEO-HTS (November 2015) are given in Table 1 below. As follows from [5.8, 9] information capacity of LEO-HTS systems is extremely high (up to tens Tbitps) with the low cost of transmission unit information. Forecasts of market development systems LEO-HTS inspire optimistic today [9]. However, to assess the validity of these optimistic forecasts, it should be appreciated that the problem of using bandwidth. In particular, an evaluation of the feasibility of the user terminal and price parameters of such terminals. Only when for the user terminal reasonable price will be $ 100-300 [4,10] you can count on the massive demand. The major costs of the user terminal are associated with the phased array antenna. To assess the feasibility of scanning antennas based on phased array antenna it is necessary to conduct preliminary analyzes of system and to formalize the exemplary parameters LEO-HTS. Then, need to create the initial requirements for a phased array antenna of the user terminal and determine its shape.

As already mentioned, planned today LEO-HTS systems have the space segment, which consists of a multiple satellites. The final parameters of the space segments are not published. General published data should be considered only as a purely preliminary (change as the project progresses). The most numerous group of satellites LEO-HTS announced in projects SpaceX and OneWeb (Table 1). General view of the satellites OneWeb are illustrated in Figure 1. Total estimated ratios to determine the orbital structure segment of the LEO-HTS are similar to used for low-orbit systems MSS [11].

The number of satellites, the service sector and the angular scan sector

The increase number of satellites in the space composition of constellation leads to some technical features of the system. Namely, the more satellites in orbit, the smaller the operating angle of elevation can be assigned. Figure 2 shows a diagram of the coverage area communications satellite. To ensure continuous global coverage area number of satellites required depends on the assumed operating elevation angle Θ and is defined by a known ratio

p - the number of orbital planes, q - number of satellites in the same orbital plane

Re - radius of the Earth (6371 km), h- is the height of the satellite's orbit.

The important factor is that in systems LEO-HTS slant range depends strongly on the angle of elevation and greatly affects the energy of radio signals, and changing delay signal

Figure 3 illustrates the evaluation of the required number of satellites in the orbit at an altitude of 1,200 kilometers (roughly equivalent to SpaceX and OneWeb According to 2015.) on condition a single coverage surface of the Earth. Fig. 4 illustrates an angular coverage area of satellite, depending on the elevation angle.

Data Analysis Figure 3 and 4 shows that increasing the number of satellites in orbit constellation composition allows to increase the operating elevation angle of the subscriber station. Accordingly, it reduces the scan angle required phased array antenna user terminal. At the same time reduced and the angular coverage area created by the satellite (Figure 4).

On the basis of Figure 4 can estimate the scanning angle (approximately) of the antenna array of the user terminal

Each satellite has to serve a relatively large angular area. Accordingly, to achieve the required budget of radiolink (and the specified bandwidth) each satellite has to use multibeam onboard antenna.

OneWeb. Figure 5 [6] gives an example of the design coverage area satellite OneWeb, which confirms the estimate of 2α in Figure 4.

Onboard satellite antenna with such angular beam parameters may be formed as a multi-beam phased array antenna (beam width LEO-HTS more than beam width HTS systems, more by an order).

The Ku - band available operating bandwidth of 500 MHz. In this case not be used by the polarization isolation between the beams, in each beam therefore possible to use not more than 125 MHz (four frequency letters or more). Given that beam groups of adjacent satellites NGSO may intersect we assume that the letter frequency 125 MHz (four frequency letters) or 62.5 MHz (eight frequency letters). However, Figure 5 illustrates the formation of a working coverage area OneWeb (from Example 4 satellites). Each satellite has 16 user beams, but the adjacent areas of overlap. Thus, to avoid interference the bandwidth of beam adopted 62.5 MHz (8 frequency letters).

SpaceX. As the data in Figure 4 in the coverage area SpaceX angular satellite service sector less (about 2 times) than the system OneWeb.

It is assumed that in the coverage area formed SpaceX narrower beams. It can be assumed that the working area of each beam has an angular sector of about 5 degrees. Under these assumptions, the satellite antenna forms a coverage area for at least 36 beams with a frequency band up to 125 MHz (optionally increase the number of beams and / or reduce the band of the beam to 62.5 MHz).

The capacity of the satellites

It is necessary to have the original technical specifications for the analysis and evaluation of the budget and satellite capacity. These data (as of November 2015) are fragmentarily published in the press, it are diverse and contradictory. We analyzed several options. One of the most likely options presented in the summary table. 11. Multivariate modeling involves an iterative process with the boundary conditions. The boundary conditions are including the parameters and information about OneWeb and SpaceX, which are relatively reliable. For simplicity it is assumed link budget "Gateway - KA" and "KA - Gateway" is significantly higher of subscriber link budget. The maximum achievable signal-code constructions adopted 16APSK, ¾ (according to the standard DVB-S2).

To minimize the antenna user terminal size and power consumption can be assumed that the primary signal-code design should take OPSK, 3/4, LDPC for the reverse channels. Consideration should be given substantial inter-beam interference (adjacent satellites), so further assessments are carried out for the beam bandwidth of 62.5 MHz, i.e., using 8 letters. Approximate satellite capacity in this case is given in Table 11.

Obviously, this is the maximum capacity of satellites (it can be assumed that at least 90% per year capacitance values can be achieved), with the proviso that the subscriber segment consists only of small individual subscriber terminals. It should be noted that these terminals can be used for multiple access (connection point with Wi-Fi hotspot or femtocell 4G / 5G). Moreover, the system provided for the use of more powerful terminals, designed to organizing transport channels (backhauls) for 4G networks, and possibly 5G. Taking into account the simplification of the value of satellite capacity can be regarded as a lower limit. Estimated parameters of subscriber terminals, taken in assessing the capacity of the satellites are analyzed below.

A phased array antenna of the user terminal

The values obtained for scan sector (4) and gain (directivity) of the antenna beam of the user terminal allow us to determine the minimum number of independently controlled channels in the performance of the antenna in the form of APA (PA) [28]. However, in this case, the antenna gain of the user terminal is not known beforehand. Therefore, in the simulation process to simultaneously perform link budget analysis LEO-HTS and establishes a relationship of complex system parameters and parameters of the antenna user terminal. When the received scan angle (4) as part of a phased array transducers are used faintly directed. Each emitter constitutes its active control channel if implemented APA. If implemented passive phased array (PA), each channel contains dividers and discrete phase shifters.

Based on the assessment of the sector scanning expedient beamwidth of separate phased array emitter (given the reduced gain in the sector boundary scan 3dB)

2ΔΘ_xi and 2ΔΘ_yi - beamwidth at - 3 dB level for a single emitter antenna array.
To simplify accept 2ΔΘ_xi = 2ΔΘ_yi = 2ΔΘ_i .

Estimated value of the gain of a separate element of phase array can be determined by the formula

2ΔΘ_i ≥ 36 degree for SpaceX , 2ΔΘ_i ≥ 74 degree for OneWeb

Accordingly, for the conditions of beam scanning, the proposed for systems SpaceX and OneWeb should be selected elements of phase array subscriber terminal to gain SpaceX Di ≤ 13 dB, OneWeb Di ≤ 7 dB.

The number of elements in the array (M) are associated with scanning angle

2Θ_x, 2Θ_y- angular sector scan in orthogonal planes;
2ΔΘ_x, 2ΔΘ_y - beamwidth antennas in orthogonal planes. In this case, assume 2ΔΘ_x =2ΔΘ_y≈2ΔΘ_o.

On the basis of these premises in view of the received data of the satellites OneWeb and SpaceX is possible to estimate achievable flow rates in radio links for the size and number of elements APA (fig.6-9). Similar estimates can be made for passive phased array (they are not shown), but in this case, due to losses in the receiving array elements and increase of thermal noise the potential capacity of the satellites will not be implemented on the line "down." Capacity of direct channels will decrease approximately 5-6 times.

There is opportunity estimate the size of phased array antennas of the user terminal (respectively, and the minimum number of elements M (7)] on the basis of the received signal-code constructions and alleged satellite parameters OneWeb and SpaceX . The results of this assessment are presented in Fig. 6-9 assuming budget a radio link with zero margin).

EMC problems

The problem of electromagnetic compatibility (EMC) systems LEO-HTS and existing FSS systems using geostationary satellites is extremely painful [6, 12-16]. LEO-HTS system may use only the Ku-band on a secondary basis. In addition, the already marked the future EMC problems between systems LEO-HTS [17]. The impetus for the start of EMC LEO-HTS served request of SpaceX to the FCC's to launch two experimental satellites MicroSat-1a and MicroSat-1b in 2016. [12].

The problem stems from international standards and restrictions for non geostationary satellites (note: in addition, we note that a number of rules can not be considered valid because they were taken for systems LEO-HTS beginning of the 2000s, which today complicates the situation even more [6] ) in the Ku and Ka-band. The level of power flux-density produced by GSO or NGSO satellites, limited regulations in the Radio Regulations (RR), depending on the elevation angle at which the satellite is visible.

In this case, these limits (summarized in tabl.3-5) does not depend on the angle of elevation, as is not provided to work with low elevation angles (details see. RR Table. 21-4). These data allow us to estimate the energy potentials of radio link and assess the problem of electromagnetic compatibility system LEO-HTS with other systems.. From the parameters presented in Table 3 shows that satellites in low circular orbits could potentially have a higher power (about 11dB, if not take into account the energy potential in the non-GSO satellite) than actually exist today satellites in geostationary orbit. However, to realize that this energy should be assessed beforehand a number of conditions.

For example, it can be concluded that the given parameters of satellites OneWeb and SpaceX restrictions (Table 3) are satisfied. The parameters transmitted by the terminal do not create problems in terms of constraints interference level indicated in Table. 4 in the direction of the GSO satellites (as well as for any HEO).

The main problem with the implementation of restrictions, briefly mentioned in the Table 5 (RR detail in table 22-1A, D). Indeed, if no action is taken, the level of interference from satellites LEO-HTS can significantly exceed the specified values. Particularly systems OneWeb and SpaceX create a level of power flux density in the direction of the receiving stations operating with satellites in geostationary orbit by orders of magnitude higher than that specified in the Radio Regulations.

Figure 9 provides an assessment of excess spectral power flux density, which create OneWeb satellites and SpaceX in the installation receiving earth stations in the overlapping frequency ranges for level limit - 146 dBW / m2 in 1 MHz and about the actual level of the received signal (Table. 3) These are periodically exceeded and mainly observed in the area of the equatorial latitudes. This problem is also noted in [6], which assessed that using receiving terminals with antenna 0.6m cumulative time of their defeat unacceptable interference for the year will amount to 78.6 minutes a day (equivalent to 0.945 probability readiness channel).

From Table 5, it follows that taking into account the temporary % exceeded even stronger (approximately + 20dB) than in Figure 10.

In order to correct this situation, for example in the OneWeb, it invited the whole beams group to lead at an angle (p from the nadir. This allows to create a guaranteed angle (p between the direction of the receiving stations from the GSO and the direction of the NGSO satellite.

It proposed to take the periodic step rotations of all satellites at the beginning of the passage of the equatorial zone. But while there is no answer to the following questions: Is such a scenario implementing the rotation of the satellite ?; Is it possible to achieve an acceptable level of side lobes from the onboard multibeam antenna?

Obviously, the problem of EMC LEO-HTS requires further study.

About the price parameters of phase array

In communication systems scanning antenna is usually done in the form of separate receiving and transmitting APA (PA). The reason is that otherwise required in each receiving channel AFAR install filters to suppress the signal of transmission channels, their radiation at frequencies of reception and out-band radiation, which would greatly complicate the design. Cost of APA (PA) in series production (excluding the settings and complete measurement of parameters) in the form of separate components (excluding the processor for implementation an aiming beam algorithm)

δ - the cost of emitting fabric, $ / cm² ;
Δ - the cost of construction, assembly, testing;
φ - the value of the phase shifter;
M_r - the number of receiving channels;
M_t - the number of transmission channels;
A_r - cost of Rx module;
A_t - cost of Tx module.

In accordance with the task must get the total cost of C_a = C_ar + C_at <$ 300. Unambiguous assessment of the parameters for (8) is impossible obviously. The reason for this is that AFAR hitherto practically not used for satellite communications systems above the C-band and the statistic of price parameters is absent. However, researches in this field are carrying out for decades. The most widespread APA received in radar systems of various purposes. In the technologies of radar systems APA progressed significantly as compared APA for communication systems. A number of elements APA for communication systems and radar systems for APA have much in common. But it should be noted that the direct use technologies of radar APA for communication systems is unacceptable.

In addition, some progress in the practical application of passive PA are in the field of satellite broadcasting in the Ku-band.

About the price parameters of radar APA. Often the idea of the high price parameters APA based on data from radar systems. For example, according to the company "Istok" (2006) APA price consists of the electronic components price , price of structure, assembly, tuning and testing APA (Figure 10). Roughly it can be assumed that the electronic elements base is 63-73% of the cost of the module APA with output power exceeding 1W. And already in 2005 was price of module $310 (X-band), i.e. two years the price is reduced almost 2-fold (Figure 10). In this case, taking into account the seriality APA can be expected that the share of the value of the element base for serial production will increase to 80% - 85% of the total price.

Figure 10 Cost of the transmitting module APA 2-10 GHz (according to the company "Istok")

Indirectly, this is confirmed by the data information (Table 6) of the price component APA sea-based radar [18] (included in the price AFAR digital to analog converters).

High price channel APA whether the data presented in the article [23] on the cost of 1 m for passive and active phased array. Data [23] refer to the radar with high radiation power (power of about 2 kW). Presented an overview shows that the experience of development and price estimates powerful radar APA be very critical use in the analysis of the achievable cost parameters APA for communication systems. At least flawed direct links to the high price parameters APA for communication systems based on the data of radar APA. Furthermore, one can assume significant progress in reducing the cost of APAA. For three years the price of the element base has declined by about two-fold (Figure 10).

About the price parameters of antenna arrays for receiving TV. The use phased arrays for satellite TV reception did not receive active development. However, examples of such antennas are known and provide additional information to our estimates. For example, in the late 90s the company Matsushima Electric Works offered on the market receiving passive arrays in Ku-band [20]. Today, such an antenna produces, for example, the company Selfsat.

Based on data from [20] to give the normalized valuation emitting fabric (without phase shifters). This value is approximately 8 = 0.021 $ 1 cm emitting fabric.

The simplest antenna is the size of 54.7 x 27.4 cm is 94 euros (including LNB). Excluding the cost of the LNB (about 20 euros), it is possible to assess the increase in costs associated with the design of the antenna and beam formers. The increase in prices due to the design and beam formers is A = $ 50 for small antennas (less than 0.6m) in series production.

As a result, the phased array antenna applied to a subscriber terminal LEO-HTS (receiving or transmitting) the cost of a radiating fabric based design, we will

C_n = C_nr + C_ut - the cost of emitting fabric with design, $.

In (9) it is assumed that the cost structure A uniformly divided between the receiving and transmitting APA (PA) because they are integrated into the user terminal in a single structure.

About the price parameters of phase shifters. The main element that is present in any conventional phased array antenna (APAA not considered digital or FAR, a company statement Kymeta [21, 22]) for any purpose, is a digital phase shifter. In [23] provides information about the price parameters of passive phased array used in large radar (radiated power of more than 2 kW). A special feature in this case is the condition of a sufficiently high capacity and lowest possible losses. Channels are made using the ferrite phase shifters and PIN-diode phase shifters.

As follows from the data value of the ferrite phase shifter between Ku / Ka of not less than $ 15-20, and the price to a PIN diode phase shifters significantly higher (Table 7). Additional pricing options are listed in the reports of research companies in the US [24] to develop phase shifter based ferroelectric and phase shifters such as MEMS (Table 8). About 10 years a number of organizations and companies in the US (US Army CERDEC, US Army Research Laboratory, US Army AMRDEC, Agile RF, Inc., Raytheon Space & Airborne Systems) conducted research on the creation and mass production of ferroelectric & MEMS phase shifters with respect to use in the AFAR . According to [24] achieved a reduction in the price of the phase shifter with $ 50 to $ 10. The total costs for research and development, including technological preparation of production amounted to 134 million. $. Thus production capacity demands exceed 10 times. It is noted that the MEMS phase shifters turned sufficiently low loss, allowing their use in passive phased array. Work continues in the framework of US military programs (Warfighter Information Network - Tactical (WIN-T) and Future Combat Systems (FCS) .These data can already be taken into account when assessing the achievable price parameters APA for communication systems.

As follows from Table 7, the most promising is the use of MEMS. This conclusion is confirmed by the data [25] where a description of APA X-band (10 GHz) and analysis of the components for its implementation. In particular, a comparison of embodiments of phase shifters (4 bit) and the estimation of their cost (Table 8).

As follows from these data [25], MEMS phase shifters are most beneficial to the technical parameters (including have almost zero energy consumption), and cost parameters. It should be noted that the MEMS phase shifters are discussed in detail in [26], where it is noted that they may be applied to 20 GHz while maintaining good technical parameters.

About the price parameters of the active channel. In [27], numerous embodiments of the APA applied to the problems of radar, but there is information about APA for communication systems. On the basis of the data [27] it can be concluded that the most promising area of technology APA for communication systems are based on MMIC SiGe. In Table 9 shows the value of the channel (transceiver module) APA depending on the power transmitting device. It was noted that for the minimum power in the long term the cost of $ 1 per channel formed on the basis of SiGe are achievable. In [27] represented of examples and experimental realization APA its elements into bands X, Ku, Ka, Q, W, including some examples relate to communication systems. And, for the receiving module noted that the achievable price of $ 1.

It is natural for the task of creating APA applied to LEO-HTS sufficient radiation power within 0.03W < P < 0.1W. Thus, there is reason to suppose that in the future is possible to obtain extremely low price parameters of active modules, but excluding the cost of phase shifters.

Cost of the antenna array. Taking into account the data presented in Fig.6-9, you can estimate the size of the antenna array of the user terminal. Based on these data and analysis of price parameters of elements APA (PA), you can get an idea of the cost of the reach ability of the user terminal, the stated project investors OneWeb and SpaceX. Obviously, if the reception and transmission grating in the form of APA, then talk about achieving cost $ 300 [7.10] has no reason to. In order to minimize the price it is advisable to transmit antenna array to perform a passive phased array using MEMS-phase shifters. For a numerical estimate parameters taken antenna array shown in Table 10 on the assumption that each beam bandwidth 62.5 MHz (minimization of inter-beam interference). Given Fig.11 data obtained on the basis of (9), provided that the receiving unit is worth $ 2, and $ 10 cost of the phase shifter (e.g., MEMS), the total cost of the antenna array system OneWeb about 30 times the above stated user terminal cost. Obviously, in this case advantageous to switch to a structure in which the scanning is carried out in elevation and azimuth scanning provided by mechanical rotation of the antenna system. This estimate applied to the system SpaceX littered with investors at least 3 times. We can expect that in the future large-scale production of phase shifters and the cost of the channel as a whole will decline. If the price drops shifter $ 10 to $ 2, then in this case the array antenna system the subscriber terminal SpaceX price can be achieved slightly below $ 300.

Results

System analysis shows that an increase of satellites in LEO-HTS space segment capacity of the system can be obtained, amounts to tens of Tbps. Dispose of this container investors plan, including by individual subscribers. If a segment in the subscriber terminals using only the individual, the capacity of the satellite is markedly lower than the declared investors (Table 11). However, the overall capacity of the system is comparable to the capacity of fiber optic links.

A particular problem is the practical realization of the user terminal and a key element of the problem is phased array.

Cost of subscriber terminal on the basis of modern technological features, high. Significantly higher than the investor of the project OneWeb and SpaceX state.

The main contribution to the cost of price indices makes phased arrays. Cost of the antenna system of the subscriber terminal provided on the basis of phased array at the present level, can not be achieved below $ 300. To achieve the performance declared by investors, it is necessary either to increase the number of the satellites or seek to create a channel modules (phase shifters + reinforcements) at the price of $ 1-2 per channel. However, the decision of the technological problems must be preceded by an adequate solution to the problem of EMC systems LEO-HTS with existing and planned satellite and terrestrial communication systems and broadcasting.

The authors are grateful to the Director of Business Development Sofia Connect Mr. Christian Frhr. von der Ropp for assistance in the review of information materials.

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