Application of LTE technology for the VSAT-networks development
VSAT technology is widely used for a long time. There are numerous approaches to creating VSAT networks using different network topologies and technologies of the satellite channels [1,2]. Today is a special interest connected with VSAT technology development with regard to the use of multi-beam satellites HTS . Throughput HTS satellite capacity already reached more than a hundred Gbit / c. It is stated that the capacity of the new HTS satellites will reach 0.5-1 Tbit / s [3, 4]. At the same time the cost of a unit of information transfer today is comparable with fiber-optic, and the cost of subscriber VSAT station is 300-400 $. By 2020. VSAT is projected to decrease in the prices for HTS networks up to 100 $ (company data SatixFy Ltd).
The demand for network services based on the HTS satellites is growing, but this growth is not sufficient for the utilization of satellite capacity. The growth in demand for services is mainly due to market developments in North America (mainly in the US). For example, in the EU and Russia in demand in absolute terms a slight increase. However, there is a constant search for better technical solutions, including decisions related to the integration of satellite and terrestrial channels . These new solutions search activity related to the fact that the total volume of global resource HTS satellites today is significantly (several times) greater than the demand.  Figure 1 shows the data of the company Euroconsult. This situation is predicted in the studies and other analytical companies. In the period 2015-2024, the projected increase in the volume of services in networks with HTS 1.1 bn. $ To 4.9 billion. $ (Data Eurocosult). It is believed that the most promising options for utilization of satellite capacity in HTS networks is the organization of satellite channels (backhaul) to connect remote cellular base stations (VSAT + 3G / 4G)  and the use of VSAT-stations for the creation of collective access points Wi-Fi (VSAT + Wi-Fi). And the version of VSAT + Wi-Fi (unlike VSAT + 3G / 4G) allows the operator to provide satellite services in its network, if addressed subscriber billing problems. Judging by the results of analytical reports on the company's NSR backhaul market for fixed applications (marine, aviation and land mobile tools are not included), you can expect a need for 33 000 connections to 2023. (An increase of nearly 3 times compared to 2013.). Approximately the same amount of connections is forecasted to provide services to mobile vehicles. Approximately 49% of the connections scheduled using HTS satellite resources. The most promising areas for the use of multiple access points are considered to be Asian countries. However, despite the fact that the cost of the HTS resource and the cost VSAT stations continue to decline in the future, it should be noted the following problems:
- the high cost of central station VSAT network;
- uniqueness equipment of the central station VSAT network;
- uniqueness VSAT equipment.
In addition, the problem is to provide fast loading (utilization) satellite capacity HTS, which is directly connected with the activity of the demand on its resources.
In addition to traditional solutions of community access organizations can consider innovative solutions using LTE technology in FDD mode. In this case it is assumed direct retransmission of LTE signals via satellite.
The use OFDM signals for retransmission via satellite is a non-optimal solution in terms of spectral efficiency achieved. Moreover, LTE standards suggest that the signal propagation delay should be on the order of less than a satellite radio links based on geostationary satellites.
However, the application of OFDM signals and adaptation of LTE technology is being actively considered in relation to the use of the mobile-satellite service systems [9-14].
In this case, we consider the option of using LTE technology in HTS systems in the fixed-satellite service. It is quite obvious that the successful implementation of such solutions can extend the range of potential subscribers to the satellite network, ie, increase resource utilization HTS satellites. It can be assumed that the value of VSAT & LTE CAPEX ground segment systems and the cost of the subscriber connection can be significantly reduced against to traditional VSAT solutions.
To evaluate the technical and economic parameters of such a decision should first assess the technical and economic parameters of the existing traditional VSAT solutions based on HTS satellites.
The normalized price parameters of the Gateway VSAT station
Several Gateway stations are typically used in a network created on the basis of the HTS satellites (in promising projects several dozen). Gateway is equipped with an antenna 5-9m, which provides access to the nodes interface with Internet (POP) for connecting the satellite network with external terrestrial networks (Figure 2). Typically, Gateway supports multiple HTS satellite beams (4 to 8) which are permanently assigned to a particular. Gateway segment substantially affects the system CAPEX. Table 1 presents the data to assess the value of the Gateway in the network based on the HTS. These figures are approximate (according to news reports), but still reflect the price parameters of the Gateway. In the future, it is assumed that the prices for equipment will decline (about 20% over five years). Apparently for further analysis and comparison options should take approximately $ 4,000 related to the frequency range of 1 MHz in the forward channel for evaluating the normalized value Gateway (Table 2).
Another important factor may be the value of CAPEX, referred to the subscriber. To assess this indicator should be used in such a concept as a factor "oversubscription" [7, 8]. In this case, to take concrete factor "oversubscription" = 1: 5. Taking as a starting value of 1 MHz bandwidth, we find that the received "oversubscription" ratio on average per subscriber (registered user) will account for 200 kHz (1 MHz / 5 subscribers). However, with probability close to one each of the subscribers will be able to use the channel rate, which can be achieved in 1 MHz (for example, at about 3Mbps, 16APSK ¾).
VSAT<E and VSAT&Wi-Fi SAT for small settlements
In connection with the collective use of VSAT + Wi-Fi and VSAT + LTE uniquely evaluate the cost of the subscriber connection is not possible. Table 2 shows the average values for evaluating the price increase of the normalized cost of the end user connection. In this case, the price parameters of the connection using Wi-Fi adopted by BDUK pilot program data (national program of broadband in the UK https://www.gov.uk/guidance/broadband-delivery-uk). These evaluation program BDUK, appears to account for the subscriber units rate subscriber and providing at least 2 Mbps, but are exaggerated with respect to conventional commercial estimated at about two times.
Table 2 shows the average price range of values for multiple access points providing will support approximately 100 concurrent connections subscribers. LTE base station prices are based on the cost of equipment, licenses and installation of equipment (eNodeB of the target cost of the equipment is about 60%).Assuming as for the Gateway coefficient "oversubscribed" 1: 5, in the 20MHz band can support up to 100 simultaneous connections for LTE.
As can be seen from Table 2 solutions for providing shared access to subscribers using Wi-Fi or LTE costs reduced to one subscriber are close to each other (it can be assumed that the LTE using a slightly higher cost). For the average ratings based on the fact that subscribers will not be about 50% of the number of potentially possible, should take overestimate cost of $ 250 per subscriber in the organization of the access point based on the coefficient of "oversubscription” ξ = 0.2. In general, the cost of connecting a subscriber depends on the adoption (appointed on the basis of statistical analysis) the value ξ. The cumulative cost to the subscriber can be estimated as the sum of the costs presented in Figure 3a and 3b. And the value is not necessarily equal ξg = ξv. However, with public access it is advisable to adhere to the organization ratio ξg> ξv.
It is recalled that in this case, the unit cost of the subscriber connection does not take into account the costs of creating a space segment.
Retransmission of LTE signals via a geostationary satellite MSS
In addition to the above-mentioned decisions of subscribers accessing using backhaul satellite channels it is possible to directly transmit subscriber LTE signals via satellite (ITU-R M.2047-0 data), ie, use forvardhaul satellite channels. An example of the technical implementation is LightSquard network based on multibeam geostationary satellite Sky Terra 1 (due to EMC conflict with the GPS system operation is suspended). The technical solution of the ground segment disclosed in several official documents in relation to the technical systems of mobile satellite communications [9-13]. According to these documents, many functional LTE standards procedures have continuity with LTE technology for use forvardhaul satellite channels. All the problems associated with high latency and signal propagation properties of multisignal OFDM. Problems multisignal mode and associated noise in OFDM transmission at the simplest case can be minimized by providing a linear transponders of the satellite. Although there are numerous studies that include more sophisticated methods [10-12].
The problem in high latency satellite links requires Gateway software revision and most importantly software user devices that are used in the LTE network. In this connection it should be noted that these solutions are formulated for mobile satellite networks that operate in L- and S-band . But all these solutions lead to the fact that it takes a unique Gateway and dual-mode subscriber units. Such devices can work in the terrestrial cellular network, and if necessary switch MSS to operate in a satellite network (satellite subscriber terminals in the network LightSquard are based on Qualcomm company MDM9600 chipset). Network solutions are based on the group BMSat standards, which are now developing China Communications Standards Association (CCSA) . It should be noted that BMSat standards  are based on 3GPP standards with maximum continuity LTE technology.
Retransmission of LTE signals via a geostationary HTS satellite FSS
Satellites such as HTS are FSS service. HTS multibeam satellite difference from geostationary MSS is used that Ku or Ka frequency band and a beam width substantially smaller (in the range 0.7-0.3 degrees). Accordingly, for work subscriber units through satellite HTS need a device which can linearly convert a signal from LTE standard range (band) in the range of HTS satellite operating (Figure 4) and is able to transmit and receive information in a range of the HTS subscriber beams (Figure .5). This device is similar to VSAT station, but as a "modem" to the subscriber station uses LTE terminal. Moreover, this "modem" is associated with the electronic part of the station over the air. For this purpose use a simple omni-directional antennas and provide EIRP rather obviously less 0 dBW as required by decision State Commission for Radio Frequencies of Russia for repeaters LTE (Annex number 1 for SCRF decision on December 11, 2013 № 13-22-06).
HTS Satellite is associated with the Gateway which has access to any external network. Gateway of usual VSAT network is unique, ie, the user equipment can be used only of the producer to which the central station equipment. But there is another possible solution which envisages the creation of a Gateway on the basis of several eNodeB. At the radio engineering part of the Gateway eNodeB signals are transferred to a band of feeder line (feeder beam) HTS satellite. If the formation of LTE signal using the bandwidth of 20 MHz then to operate in a single beam HTS satellite with 250 MHz can be placed 10-11 eNodeB. And each eNodeB serving customers located within the local area determined by the condition of maximum difference between the slant range to customers in FDD mode no more than 100 km with an arbitrary center inside the working HTS beam area (the condition of minimizing the delay of user signals in the standards 3GPP for Preamble Format 3 modes).
Also, obviously, it should be noted the requirement of linearity amplifier which retransmit OFDM signals. Regular VSAT direct channels of repeater mode (digital stream TDM) on the satellite HTS is close to saturation. In our case, the OFDM signal retransmission necessary to establish OBO about - 7dB - 8 dB from the saturation level (NPR reached the level of no worse than 20 dB, http://www.lintech.com/PDF/hpa.pdf). Accordingly, energy of subscriber beams on a Satellite-UE line in this case is below at least 6 dB. If we assume a typical EIRP for HTS of the subscriber beams about 65 dBW the flow rate in the forward link of 35-40 Mbps (20 MHz band) can be achieved when using the subscriber antenna 0.7 m under good weather conditions. If we assume the coefficient "oversubscription» ξg = 0.2, the speed available to the subscriber, will be less than about 1.5 times compared with the standard scheme VSAT network. As a result, with probability close to one, the average speed for the subscriber in the forward channel will exceed 2 Mbps, and the peak is not lower than 35 Mbps. Assuming that you want to create a central station that supports 6-beams of 250 MHz each, it would take 66 eNodeB. Whereas the equivalent cost of eNode about 30 000 $ (inclusive of all expenses and installation), and about 1 million $ on radio equipment, it turns out that the ratio of "oversubscription" ξg = 0.2 installation costs per subscriber will be approximately $ 450. This value is lower by about a factor of 2 than using a typical base station network based HTS. Comparison with a typical central station is illustrated in Figure 3a, depending on ξg. Naturally, the subscriber station (Figure 4) at cost less any VSAT station about 1.5-2. Moreover, this subscriber station is not connected to the signal processing, i.e. not closed on the specific VSAT manufacturer.
Broadcast mode in LTE satellite network
At the 3GPP LTE standards (rel.9) describes eMBMS broadcast mode (evolved Multicast / Broadcast Multimedia Services) cellular LTE networks. Given the limited frequency resources in LTE networks it is not commercially attractive (this option is considered by many operators). Testing has shown the technical feasibility of its implementation as early as 2006-2007, but hitherto widely broadcast mode in LTE networks is not received. However, work in this area are active throughout the world. In this case, when using LTE satellite network implemented on the basis of HTS, broadcast mode may be more attractive. Naturally, with multibeam satellite technology broadcast solution in identical packages in all the beams are ineffective. But it is possible to divide the broadcast packets and content across regions. In addition, as noted above (Figure 1) there is an excess of the resource HTS satellites.
Using satellite technology offers advantages due to the fact that the expensive frequency resource LTE network is not consumed. In this case the subscriber units capable of operating in a mode in eMBMS LTE network, seems to require a minimum software revision for the purpose parry increased delay in the satellite channel.
The above analysis of bachhaul technology use in HTS based satellite networks shows (Figure 3b) that is achievable cost of subscriber connection at times lower than with the use of fiber optic technology . It is possible to further reduce this figure through the integration of satellite and terrestrial cellular networks. Thus, the application of LTE technology to build a satellite network using forvardhaul channel allows users to reduce the cost of connection by 2 times (Fig. 3a).
But in this case, the satellite network operator should delegate some of its authority and its revenue for the cellular network operator. But this solution extends the range of potential satellite broadband users and mobile operators to ensures the presence in those regions, which today are commercially unprofitable.
It should be noted that a significant advantage of a satellite network using LTE technology is its openness (regardless of the VSAT equipment manufacturer and VSAT technology is unique). In this case, we can talk about import safety, as there is a very wide range of manufacturers and suppliers in the supply of system elements.
According to forecasts of analytical agencies, the number of subscriber units in LTE networks already in 2020 will be 2.0-2.5 billion. If the satellite networks LTE will need at least 0.1% of subscribers, it will amount to 2.0-2.5 million, which is higher than the actual operating VSAT-stations in the worldwide. Accordingly VSAT segment can already claim a share of massive market LTE.
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