Ground Segment Technologies for Ka-Band and Beyond
Authors:
Joseph Merchlinsky, Hughes Network Systems, LLC, 11717 Exploration Lane,
Germantown, MD, 20876, 301-428-7034, Joseph.Merchlinsky@hughes.com
Gregory Ernst, Hughes Network Systems, LLC, 11717 Exploration Lane,
Germantown, MD, 20876, 301-428-5940, Greg.Ernst@hughes.com,
Robert Kepley, Hughes Network Systems, LLC, 11717 Exploration Lane,
Germantown, MD, 20876, 301-428-1662, Robert.Kepley@hughes.com
Introduction
The latest Ka-band High Throughput Satellite (HTS) systems are technological marvels that
optimize the use of available spectrum to deliver high-speed Internet access and an expanding range
of value-added interactive services, such as VoIP, video conferencing, distance learning, and digital
signage. These systems are typically driven by satellite beam placement and frequency reuse to
provide service in a given region. As a consequence, ground segment architectures and capabilities
have naturally evolved to support these multiple beam systems. This paper explores some of the
innovative gateway ground segment technologies that have been developed to operate most efficiently
with these advanced Ka-band satellite systems.
System Architecture
Modern Ka-band geostationary satellite systems are typically architected with hub-and-spoke
connectivity between gateways and the user terminals. As illustrated in Figure 1, gateways are
high-capacity, large-antenna earth stations with connectivity to the terrestrial telecommunications
network and to the satellite feeder links. The Very Small Aperture Terminals (VSATs) are located at
end-user locations, providing two-way broadband connectivity over the satellite to the gateways. User
data passes from the VSAT to the gateway to the terrestrial telecommunications network and back.
Traffic over the satellite network tends to be asymmetric, with more aggregate forward traffic flowing to
the VSAT than return traffic flowing from the VSAT.
Figure 1 Satellite System Architecture
Ka-band spot beam satellite systems multiply their overall throughput by reusing allocated
spectrum in the spot beams that the satellite creates on the surface of its coverage area. Any specific
segment of spectrum is not used in adjacent VSAT cells to prevent interference in the receiver. As in
cellular networks, a simple repeating pattern of spectrum allocation allows a portion of the spectrum to
be used in any one VSAT cell. A large number of user beams (typically 48 or more) can be provided
by the satellite in geosynchronous orbit, and hence the sheer throughput of the satellite is limited only
by the achievable complexity of the antenna patterns built into the spacecraft.
For maximum spectrum usage in the VSAT coverage area, there must be a corresponding
spectrum usage in the gateways that serve them. In a typical system, it follows that for every four
VSAT spot beams there could be a dedicated gateway spot beam that makes use of the full allocated
spectrum. This drives the satellite design towards overlapping spot beams in dense service areas, with
the spectrum allocated to VSAT terminals, and towards geographically separated spot beams in less
dense service areas, where the spectrum is available for gateways. The gateway spot beams must be
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geographically separated enough to be non-adjacent with respect to the satellite antenna
performance.
A significant challenge in such a network is to find enough locations that have the characteristics
needed for a gateway. Each location must have adequate Internet fiber access, reliable electrical
power, and preferably be in a low-precipitation weather zone, facilitating high feeder link availability.
These sites also must be in low-density service areas to maximize gateway bandwidth. Often the
entire available spectrum in the spot beam is allocated to the gateway; alternatively, some of the
spectrum can be taken away from the gateway allocation and used by VSAT terminals that are in
proximity to the gateway.
For example, consider a Ka-band satellite network which provides 48 user spot beams, supports
500 MHz bandwidth per beam, and employs the 4X frequency reuse pattern as shown in Figure 1.
The available gateway uplink bands provided by the satellite are shown in Figure 2. Four 250 MHz
bands providing 1 GHz bandwidth per polarization for a total of 2 GHz uplink bandwidth are available
per gateway. Therefore, each gateway in this case can service four 500 MHz user spot beams. The
entire 48 user spot beam satellite network requires 12 gateways.
Figure 2 Uplink Satellite Channel Frequency Assignment
Gateway Ground Segment Architecture
Figure 3 provides a diagram of the major elements of the gateway and a typical physical
partitioning. There are two main locations for gateway equipment:
1. Indoor Sheltered Baseband Equipment
2. Antenna Hub and Outdoors RF Equipment
Baseband equipment: Baseband equipment includes the modulator and demodulator
equipment, system timing, IF distribution, switching, gateway servers, and interfaces to the terrestrial
Internet backbone. A typical data center environment is suitable for the indoor sheltered equipment.
The gateway modulators generate the DVB-S2 carriers that are transmitted into each spot beam.
The gateway demodulators handle the TDMA traffic that comes back from the VSATs. The system
timing subsystem distributes timing information to both the modulators and the demodulators so that
the VSATs are kept synchronized and are able to transmit their bursts at the proper time.
The gateways have servers that perform Internet Protocol (IP) processing and Web acceleration
functions. IP processing manages the allocation of satellite throughput to specific VSATs. Web
acceleration improves the user’s browsing experience by compensating for the long path latency of
geosynchronous satellite. The interface to the terrestrial Internet backbone is a fiber optic access point
leased from an Internet Service Provider (ISP). The data is distributed by high-end switches to the
servers, modulators, and demodulators that comprise the gateway.
All of the power distribution, switches, data and IF interfaces, modulators, demodulators, timing
distribution, and servers have redundancy, so there is no single failure that will disrupt service.
RF equipment: Key Ka-band feeder link parameters like EIRP and G/T are achieved through
careful design of the system components located in the hub. The majority of the IF and RF signal
elements are located within the antenna hub. This centralized location provides reduced length
waveguide and coaxial cable components and permits convenient end-to-end system RF performance
and maintenance.
The Block Up-Converter (BUC) is normally located close to the HPA and interfaces through a
waveguide Ka-band connection. The mounting of the Low Noise Amplifier (LNA) close to the antenna
feed interface has been a common practice to maximize G/T performance for most satellite reception
applications over all common frequency bands.
For reliable operation, RF systems generally rely on redundant signal paths to support
high-availability operation. A 1:2 path redundancy scheme is shown for both the uplink and downlink
directions; however, other redundancy schemes can be implemented based upon availability
requirements, physical restrictions, etc.
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Most of the antenna components are located outdoors, including reflectors, antenna structure,
elevation and azimuth motors and drive electronics, feed rain deviator system, de-icing capability, AC
power distribution, and radiometer, if needed.
Figure 3 Ka-band Gateway Earth Station Physical Partitioning
Baseband Equipment
The large number of geographically dispersed gateways in remote locations presents a new
challenge when compared to traditional gateways. These Ka-band gateways are at locations that
require unmanned operation and remote management, driving comprehensive fault detection, and
redundancy. The high-availability requirement means that there can be no single point of failure that
disrupts service and that failures are detectable, with automatic switching to redundant elements.
The increased throughput of the satellite system also presents a challenge with respect to the size
and cost of the gateway. The deployment of advanced processing and high-capacity modems has
resulted in an unprecedented improvement in packaging density and low cost per user.
Advanced Processing
To meet these needs, Hughes has partnered with Hewlett-Packard to integrate state-of-the-art
data center technology with state-of-the-art satellite communications technology. Together they form a
future proof modular and scalable architecture that is ideal for virtualization and the evolving
requirements of a growing network. The high density of the data center technology gives us a much
smaller footprint compared to Ku-band gateways.
HP’s data center hardware, based on blade system architecture, marries leading-edge processing
density and power efficiency with an infrastructure that delivers unprecedented management and
redundancy. Each HP enclosure can host up to 16 dual processor server blades to form an extremely
dense and reliable computing platform. The enclosure has fully redundant 40 GB Ethernet connections
between each server and a pair of non-blocking switches. Other redundancy features include 6 power
supplies and 10 fans.
The enclosure has redundant Onboard Administrators (OA) that manage the computing and
switching elements and facilitate the deployment of software updates. The OA also monitors the health
of these elements and provides a comprehensive map of temperatures in the system.
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High Ca
apacity Mod
dems
Hughes has integrated modu
ulators and demodulators
d
s directly into
o the HP encclosures. By doing
d
so,
these co
omponents benefit from all
a the manag
gement capa
abilities provid
ded by the OA
O and the
compreh
hensive redundancy for power,
p
coolin
ng, and switching.
Figure 4 Ka-band Gateway
G
Re
elative Perfo
ormance
The modulator and demodula
ator cards arre based upo
on the latest DSP/FPGA technology. This
modulator ca
ards to have much higherr throughput, and a lower cost
enables these modulator and dem
t Ku-band designs.
d
Bein
ng FPGA-bassed also mea
ans that theyy can accommodate
per userr, compared to
firmware
e updates tha
at expand ca
apabilities ove
er the life of these system
ms. These PCIe cards arre paired
with HP blade serverrs to provide the process
sing capabilities needed to
t support the
ese high thro
oughput
O gateway architecture
a
deploys over 1 Gbps of m
modulated outroute or ovver 100 Mbpss of
cards. Our
demodulated inroute
e into each en
nclosure bayy. These com
mponents are
e part of a rob
bust m for n
redundancy scheme that can be scaled to wh
hatever availability is requ
uired in each
h network. Th
he bays
not filled with modula
ators or demodulators or their hosts are
a populated
d with serverr blades that perform
essing, Web acceleration
a
ption and deccryption.
IP proce
, and encryp
Surrrounding this hybrid blade
e architecture is a fully re
edundant L-b
band and timing distribution
system. Like the elem
ments inside the blade syystem archite
ecture, these
e elements are fully mana
aged
pect to fault detection,
d
tem
mperature trracking, and software upd
dates. The L-band distrib
bution
with resp
and switching system
m loops backk all active modulator outputs in the gateway to wh
here they are
e
ed to verify th
hat the outroutes are hea
althy. The sysstem also en
nables the ve
erification of
monitore
redundant modulatorrs by routing their outputss to monitoring equipmen
nt to ensure that they are
e healthy
and read
dy to transmiit in the case
e of a failure. Redundant demodulatorrs are alwayss monitoring active
Inroutes to verify their health.
The combination
n of these red
dundancy an
nd health monitoring capa
abilities in a modular
m
systtem
ensures a high availa
ability throug
ghout the sysstem’s lifetim
me even as it grows and evolves.
e
RF Equiipment
Ka-b
band satellite
e systems req
quire gatewa
ay feeder linkk bandwidths
s of several GHz
G
comparred to
previouss Ku-band sa
atellite system
ms of nomina
ally several hundred
h
MHzz. Traditionall Ku-band sa
atellite
transpon
nder bandwid
dths of 36 MH
Hz or 72 MH
Hz were used
d. Today’s Ka
a-band satelllites employ
bandwidths of 250 MHz
M and beyo
ond. To supp
port these wider bandwid
dths at Ka-ba
and and grea
ater
frequenccies, certain fundamental
f
l gateway de
esign enhanccements are required, inc
cluding appro
oaches
for increa
ased gain fla
atness and phase linearitty, solutions that
t
addresss the effects of
o increased
waveguide insertion loss, and pre
ecision uplink fade mitiga
ation techniques over wid
der bandwidtths.
Enhanced Uplink Performance
P
e
Wide
er bandwidth
h carriers are
e used to take
e advantage
e of wider Ka-band satellite transpond
der
bandwidths. The wid
der bandwidth
h carriers ne
ecessitate very stringent amplitude
a
an
nd phase response
requirem
ments on the major uplinkk path system
m elements to
o maintain modulated
m
sig
gnal integrityy.
Amplitud
de, group dellay, and phase response
e requirements over the 250
2 MHz sate
ellite transpo
onder
apply to the entire ga
ateway path including IF and RF portions. Single carrier bandw
widths have been
optimize
ed for over 10
00 MHz operration.
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Representative gateway end-to-end requirements and resultant performance for amplitude, group
delay, and phase deviation from linear responses are shown in Figure 5.
Item
Amplitude Variation
Group Delay Response
Phase Deviation from Linear
Gateway Requirement
≤1.0 dB peak-to-peak over 250 MHz
≤ 0.4 nS peak-to-peak over 250 MHz
≤ 7ᵒ peak-to-peak over 250 MHz
Figure 5 Representative Gateway End-to-End Amplitude, Group Delay, and Phase Deviation
from Linear Performance
Careful attention during the design phase is required to ensure low amplitude and phase
distortion. Major components and their interconnections must be included in an overall path
performance allocation. For example:






Entire signal path modeled and performance allocated for every device.
All components, including upconverters and high power amplifiers, should have these critical
performance parameters specified.
Component interfaces should be designed for lowest VSWR.
Coaxial cables and waveguide runs should be designed for best amplitude flatness and phase
response.
Slope equalizers.
Individual components and the integrated system require 100% performance validation.
Waveguide Loss Reduction
As a best practice, the physical design should confine the Ka-band signal handling elements to the
area nearest the antenna feed interfaces. The physical location of the gateway RF equipment is critical
at Ka-band compared to lower operational frequencies, partly due to the increased insertion loss of
waveguide components.
The waveguide typically has a loss of 0.4 dB per meter at 30 GHz/Ka-band compared to a loss of
around 0.2 dB per meter at 14 GHz/Ku-band. The increased waveguide insertion loss and the need to
maximize EIRP to optimize Ka-band system performance drives the need to locate the High-Power
Amplifier (HPA) as close to the antenna feed interface as possible.
Waveguide technology provides a low loss solution to interconnect wideband Ka-band signals
between major components. The nominal insertion loss between the HPA and the antenna feed
interface for a typical earth station application is 0.8 dB. The signal path between the HPA and the
antenna feed interface contains several contributing elements, including:



Waveguide assemblies: 0.4 dB
Redundancy switch: 0.14 dB
Directional couplers: 0.26 dB
Ka-Band Uplink Fade Mitigation
Ka-band signal paths are subject to significant attenuation due to rain. The signal attenuation due
to rain is larger at Ka-band and beyond compared to lower frequency Ku-band satellite networks.
Gateway uplink power control methods have been developed that strive to maintain a constant uplink
power level when the feeder link is exposed to weather events as received by the satellite. Beacon
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signals are transmitted from the satellite and received by the gateway earth station. The gateway
uplink power control system then analyzes the received beacon signals and applies a correction to the
uplink gain to effectively compensate for the increased Ka-band attenuation caused by the rain event.
The intent of the uplink power control system is to maintain a constant level of uplink flux density as
received by the satellite.
The uplink power control methods can be designed to effectively normalize the satellite received
power; however, Ka-band operation offers the additional challenge of a wideband uplink. The uplink
Ka-bandwidth is nominally twice as large as a percentage of center frequency compared to Ku-band
systems. A depiction of this wideband effect is provided in Figure 6. Beacon signals are normally
contained within the allocated 20 GHz downlink band. Upon incurring beacon fade, the uplink power
control system measures the amound of signal reduction and makes an assessment of the amount of
gain change required for the uplink path. Due to the wideband nature of the uplink path, approximately
2 GHz, the amount of gain correction required to maintain constant uplink receive flux density at the
satellite varies with frequency across the uplink band. Power control techniques, which can provide
precision frequency compensated adjustment across the uplink band, can improve the uplink power
control accuracy by several tenths of a dB compared to a single common uplink band adjustment.
Figure 6 Ka-band Ground Segment Rain Effects
Conclusion
Exploiting the high-throughput potential of Ka spot beam satellite systems demands new and
innovative ground segment technologies. At the core is a new class of gateway architecture providing
high-density and incorporating fully redundant IF, RF, and timing distribution stages for high availability
and low operational cost. Enhanced RF performance is achieved through wideband design
considerations, physical layout, and improved uplink fade techniques. Combining a best-in-class data
center and leading-edge modem design yields a flexible and cost-effective solution for operators to
deliver a wide range of broadband satellite services to customers in all market sectors—over Ka-band
spectrum and beyond.
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