Maximizing Throughput of The GEO Satellite At C/Ku Bands


 The need for broadband data has increased speedily but in underserved rural areas, the mobile connectivity of 3G and LTE is still a significant challenge. By looking at the historical trend, the data traffic and the internet are still expected to grow in these areas [1]. The next generation of satellites is trying to decrease the cost per MB having the advantage of higher throughput and availability. To maintain the performance of the link, choosing an appropriate frequency is evident. A multi-beam satellite system can fulfill the demand and performance over a coverage area. The high throughput satellites (HTS) fulfill this requirement using C and Ku bands. In this paper, we present the benefits of using Ku-band on the user site and the composite of C and Ku bands on the gateway site. This configuration has proved to be a cost-efficient solution with high performance over the traditional straight configuration. The data rate is improved five times both on upstream and downstream as compared to the existing available FSS system. Moreover, it has got an advantage to Ku-band user that they would enjoy the significant improvement in the performance without upgrading their systems.


I. Introduction
The new generation of satellites is high throughput satellites (HTS). It has many times more throughput of the traditional FSS satellites allocating the same amount of frequency on orbit [2]. The throughput of these satellites is high by taking advantage of frequency reuse and multiple spot beams. The cost per bit is reduced irrespective of spectrum choice [3]. ADS is replaced by FTTH for access performance as a new standard. [4]. Throughput in this scenario is information delivery speed (bits/sec) which depends upon: Bandwidth (MHz): It is increased by using frequency reuse phenomena. E ciency (bits/sec per MHz): This is the amount of error-free information in the allocated bandwidth, which can be achieved by using spot beams.
One of the fundamental differences in the architecture of HTS and traditional satellites is the use of multiple spot beams for the coverage of a speci c desired location, unlike the previous wide beams.
These spots beams have two major bene ts [5].
High Gain (Transmit/Receive): The directivity in spot beams is high so the gain is higher compared to traditional wide beams. A narrow beam results in increased power due to which user terminals become smaller and higher order modulation becomes possible, thus higher data rate transmission per unit is achieved.
Frequency Re-Use: The desired service location can be covered by multiple spot beams. The same frequency band and polarization can be used by several beams which mean an increase in the capacity of the satellite system for a speci c frequency band allocated to the system. Four color schemes are the best settlement between performance and system capacity however three and seven color scenario is also be implemented for frequency reuse [6]. Dividing no of spots of spots beam antenna by no of colors gives the frequency reuse factor of spot beam antenna. The capacity of spot beam antenna is frequency reuse factor times more than a large contour single beam.
This capacity increase is attained without raising DC power and RF but just with a little amendment in the satellite [7]. In Single feed per beam SFB antennas the multiple beams are produced by a single horn. Active or passive lenses and single oversized designed re ector are used to produce overlapping spot beams to evade holes in the coverage area [8]. Multiple spot beam antennas MSB uses arrays of small horns to generate beams so using only one re ector overlapping spots can be achieved which pointer reduction in cost and mass.
High throughput satellites (HTS) system is classi ed into different generation according to its capacity.
The rst generation of HTS gives around a few tens of Gbits/s while the 2nd HTS generation produces one hundred Gbits/s approximately e.g. KA-SAT, Via Sat 1, Echo-star etc. [6]. Several hundreds of Gbit/s will be offered by the third generation. It will boost the capability of the system and optimization of the satellite and system resources, providing quality close to ber to the home [7] [8] [9]. KA-SAT is the new generation satellite of Ka-band of HTS system. It provides services for broadband and TV broadcasting as well [9]. It has 82 spots covering a large area using frequency-reuse factor 20-times. This system uses small user terminals for Asymmetric digital subscriber line (ADSL). Its capacity is over 90 Gbit/s. It was the rst HTS Ka-band satellite launched with this much capacity on 26 Dec 2010 and its commercial operation was started on 31 May 2011 [10].

Ii. High Thoughtput Satellite Systems
The High throughput Satellite (HTS) system consists of two kinds of major links. A multi-sport antenna con guration is used to serve the service area of the user. The service area is formed by contiguous cells [11]. The circular beam provides the performance of each cell. These cells are usually established on systematic polygons. The performance of each cell is determined by EIRP, G/T, downlink C/I and uplink C/I. The use of several spots for an area boosts the antenna gain per beam which results in improvement in the following [9]. Uplink Performance i.e. G/T (It is the gure of merit of satellite receiving system) and capacity return. Downlink Performance, increased antenna gains either it increases the forward capacity or reduce the repeater RF power requirement. Since there are several cells so frequency reuse with on a given polarization is possible several times. For frequency reuse, each cell is separated su ciently so that intersystem interference is controlled. Usually, a 4-color scheme with 250 MHz assigned per color is used for ordinary frequency reuse scheme, Quarter of the available frequency capacity is utilized by each color the colors can be red, green, yellow and blue. To reduce the number of gateways, several cells are associated with each gateway so both the polarization can be used by frequency plan. It means a gateway frequency plan that consists of many channels on each polarization. Using 2.0 m parabolic dish antenna, 16 beams of 0.75° spots beams can be created [12].

Iii. Forward Link Of Hts
It is the path of the signal from the gateway to the user is called a forward link. As the downlink (user) service area is decomposed into hexagonal cells thus key elements of the exploration concentrated on the output of the cell. In the case of HTS, consider a cell is a hexagon which is totally bounded by a circle.
The effective isotropic power (EIRP) can be stated as a function of the normalized region [9].

Iv. Forward Link Budget Analysis
The forward link which is an important component of the link budget. Its performance is determined by the performance of downlink. If propagation effects are ignored, the uplink performance remains constant. The downlink is the function of an area where terminal exists in a given cell. Further interferences that degrade downlink is repeater C/I effects and inter-spot-interference (ISI) [9]. (C/I) FWD.RPTR is the Carrier to Interference Ratio due to the forward repeater in db.
(C/I) FWD.DL is the Downlink Carrier to Interference ratio due to the inter-spot in db.

V. Hybrid C/ku Band System
A hybrid system of C/Ku-bands of high throughput satellite system if designed will be the optimal system to increase the throughput of a conventional satellite system in tropical areas and overcome the atmospheric effects. This improvement increases the performance without the requirement to upgrade the user terminal site.
For a satellite link, the total C/N is represented by [14] (C/N) Total

Vi. Performance Of Forward Link
It is used to transmit information from the gateway to the user terminal, the customer premise equipment (CEP) should be simpler and less expensive. Ku and Ka are the two bands more suitable for this purpose.
To compare the performances of these two bands the values of EIRP, FSPL, G/T, and K is assumed as follow.  From the above results, it is clear that "Ku Band" forward link is 2 dB better than "Ka-Band" and spectral e ciency is also better than "Ka" So, for equal cost and same coverage areas "Ku" band forward link performance is better than Ka. Now according to our design, we need Ku downlink on user site so +6045 LO for C-Ku and -1530 for Ku will be used.
Adding LO to C band as shown below: C-Ku downlink range at forward gateway will become (12490MHz -12615MHz) and (12625MHz -12750MHz) Now Adding LO to Ku band as shown below: Ku-Ku downlink range at forward gateway will become (12220MHz -12345MHz) and (12355MHz -12480MHz) Return Gateway: The return link is divided into 8 channels of 90 MHz For C band: 1. 3400 -3490 2. 3500 -3590 Reverse Link Architecture

X. Conclusion
From the Link Budget calculations, the maximum data rate in downstream is 20 Mbps and in upstream 2 Mbps in FSS while in HTS the downstream data rate is up to 100 Mbps and the upstream data rate is 10 Mbps for EIRP of 40dBW and 8dB/K G/T. It means our proposed system improved the data rate signi cantly ve times. Since in this system the frequency used in one spot beam can be utilized several times in the whole location and as much smaller the beam spot, the higher the data throughput which means low cost per megabit. Hybrid multi-spot beam satellite performs much better than the xedsatellite service (FSS) both on receive and transmit. It improved the system without too much equipment changing cost, so it is good news for satellite operators to accommodate more HD channels. This system is envisaged to be more bene cial for existing Ku-band users e.g. TV channels because they are already using Ku band and very little changes will be required to upgrade the Ku-band terminal to overcome the weather obstacle in communication in tropical areas to a greater extent.