VSAT NETWORK EARTH STATIONS

VSAT station
Figure 1.20 illustrates the architecture of aVSATstation. As shown in
the figure, a VSAT station is made of two separate sets of equipment:
VSAT station equipment
the outdoor unit (ODU) and the indoor unit (IDU). The outdoor unit
is the VSAT interface to the satellite, while the IDU is the interface
to the customer’s terminals or local area network (LAN).
1.7.1.1 The outdoor unit (ODU)
Figure 1.21 shows a photograph of an outdoor unit, with its antenna
and the electronics package containing the transmitting amplifier,
the low-noise receiver, the up- and down-converters and the frequency
synthesiser. The photograph in Figure 1.22 provides a closer
look at the electronics container.
For a proper specification of the ODU, as an interface to the
satellite, the following parameters are of importance:
– the transmit and receive frequency bands;
– the transmit and receive step size for adjusting the frequency
of the transmitted carrier or for tuning to the received carrier
frequency;
– the equivalent isotropic radiated power (EIRP), which determines
the performance of the radio frequency uplink. The EIRP
depends on the value of the antenna gain, and hence its size and
transmit frequency, and on the transmitting amplifier output
power (see Chapter 5, section 5.2);
– the figure of merit G/T,which determines the performance of the
radio frequency downlink. The G/T ratio depends on the value
of the antenna gain, and hence its size and receive frequency,
and on the noise temperature of the receiver (see Chapter 5,
section 5.3);
– the antenna sidelobe gain variation with off-axis angle which
controls the off-axis EIRP and G/T, hence determining the levels
of produced and received interference.
Operating temperature range, wind loading under operational
and survival conditions, rain and humidity are also to be considered.
Table 1.6 displays typical values for the ODU of a VSAT. LNA
typical noise temperature of today’s VSAT receiver is 50 K at Cband
and 120 K at Ku-band. Advances in HEMT FET technology
now make possible uncooled LNAs having noise temperatures of
35 K at C-band and 80 K at Ku-band.
1.7.1.2 The indoor unit (IDU)
The indoor unit installed at the user’s facility is shown in Figure 1.23.
In order to connect his terminals to the VSAT, the user must access
the ports installed on the rear panel of the outdoor unit, shown in
the photograph in Figure 1.24.
For a proper specification of the IDU, as an interface to the user’s
terminals or to a local area network (LAN), the following parameters
are of importance:
– number of ports;
– type of ports: mechanical, electrical, functional and procedural
interface. This is often specified by reference to a standard, such
as those mentioned in section 1.6.2 and in Appendix 3;
– port speed: this is the maximum bit rate at which data can be
exchanged between the user terminal and the VSAT indoor unit
on a given port. The actual data rate can be lower.
Coherent modulation schemes such as biphase shift keying (BPSK)
or quadrature phase shift keying (QPSK) are used. For acceptable
Table 1.6 Typical values for the ODU parts of a VSAT station
Transmit frequency band 14.0–14.5GHz (Ku-band)
5.925–6.425GHz (C-band)
Receive frequency band 10.7–12.75GHz (Ku-band)
3.625–4.2GHz (C-band)
Antenna
Type of antenna Offset, single reflector, fixed mount
Diameter 1.8–3.5m at C-band
1.2–1.8m at Ku-band
TX/RX isolation 35 dB
Voltage Standing Wave
Ratio (VSWR)
1.3:1
Polarisation Linear orthogonal at Ku-band
Circular orthogonal at C-band
Polarisation adjustment ±90◦ for linear polarised antenna
Cross polarisation isolation 30 dB on axis, 22 dB within 1 dB beamwidth
17 dB from 1◦ to 10◦ off axis
Sidelobe envelope 29 − 25 log θ
Azimuth adjustment 160 degrees continuous, with fine adjustment
Elevation travel 3 to 90 degrees
Positioning Automatic positioning optional
Tracking None
Wind speed:
operation 75 to 100km/h
survival 210km/h
Deicing Electric (optional) or passive (hydrophobic
coating)
Power amplifier
Output power 0.5W to 5W SSPA at Ku-band
3–30W SSPA at C-band
Frequency steps 100 kHz
Low noise receiver
Noise temperature 80–120K at Ku-band
35–55K at C-band
General characteristics
Effective Isotropic Radiated
Power (EIRP)
44 to 55dBW at C-band
43 to 53dBW at Ku-band
Figure of merit G/T 13 to 14 dBK−1 at C-band
19 to 23 dBK−1 at Ku-band (clear sky)
14 to 18 dBK−1 at Ku-band (99.99% of time)
Operating temperature −30◦C to +55◦C
performance, transmission rate on the carrier should be higher than
2.4 kbs−1, otherwise phase noise becomes a problem. For lower
data transmission rate values, phase shift keying is avoided and
frequency shift keying (FSK) is used instead.
1.7.2 Hub station
Figure 1.25 shows a photograph of a hub station and Figure 1.26
displays the architecture of the hub station with its equipment.
Apart from the size and the number of subsystems, there is little
functional difference between a hub and a VSAT, so that most of the
content of the above section applies here. The major difference is that
the indoor unit of a hub station interfaces to either a host computer
or to a public switched network or private lines, depending on
whether the hub is a dedicated or a shared one (see above section on
VSAT network options). Typical ODU hub station parameters are
indicated in Table 1.7.
One can note in Figure 1.26 that the hub station is equipped with a
network management system (NMS). The NMS is a mini-computer
or a work station, equipped with its an dedicated software and
displays, and used for operational and administrative functions.
This mini-computer is connected to each VSAT in the network
by means of permanent virtual circuits. Management messages
Table 1.7 Typical values for the ODU parts of a hub station
Transmit frequency band 14.0–14.5GHz (Ku-band)
5.925–6.425GHz (C-band)
Receive frequency band 10.7–12.75GHz (Ku-band)
3.625–4.2GHz (C-band)
Antenna
Type of antenna Axisymmetric dual reflector (Cassegrain)
Diameter 2 to 5m (compact hub)
5 to 8m (mediumhub)
8 to 10m (large hub)
TX/RX isolation 30 dB
Voltage Standing Wave
Ratio (VSWR)
1.25:1
Polarisation Linear orthogonal at Ku-band
Circular orthogonal at C-band
Polarisation adjustment ±90◦ for linear polarised antenna
Cross polarisation isolation 35 dB on axis
Sidelobe envelope 29 − 25 log θ
Azimuth travel 120 degrees
Elevation travel 3 to 90 degrees
Positioning 0.01◦/s
Tracking Steptrack at Ku-band if antenna larger than 4m
Wind speed:
operation 50 to 70km/h
survival 180km/h
Deicing Electric
Power amplifier
Output power 3–15W SSPA at Ku-band
5–20W SSPA at C-band
50–100 TWT at Ku-band
100–200 TWT at C-band
Power setting 0.5 dB steps
Frequency steps 100 kHz to 500 kHz
Low noise receiver
Noise temperature 80–120K at Ku-band
35–55K at C-band
Operating temperature −30◦C to +55◦C
are constantly exchanged between the NMS and the VSATs and
compete with the normal traffic for network resources.
1.7.2.1 Operational functions
Operational functions relate to the network management and provide
the capability to reconfigure the network dynamically by
adding, or deleting, VSAT stations, carriers and network interfaces.
Operational functions also include monitoring and controlling the
performance and status of the hub and each VSAT station, and all
associated data ports of the network. This entails operational management
tools which provide real-time assignment and connectivity
of VSATs, and management and control of new installations and
configurations.
The network control software allows automatic dynamic allocation
of capacity to VSATs with bursty interactive traffic and
to VSATs that will occasionally be used for stream traffic (see
Chapter 4, section 4.3). No operator intervention is required to effect
this temporary capacity reallocation.
The NMS notifies the operator in the case of capacity saturation,
which prevents more VSAT users from entering the service. The
NMS also handles all aspects related to alarm and failure diagnosis.
In particular, in case of any power interruptions at the VSAT
stations, the NMS downloads all the relevant software and system
parameters for operation restart.
1.7.2.2 Administrative functions
Administrative functions deal with inventory of equipment, records
of network usage, security and billing.
The NMS keeps an account of the VSAT stations installed and
operated, the equipment configuration within the hub and each
VSAT station, and the port configuration of each network interface.
This information is available on request by the operator, along with
statistical information on traffic, number of failures, average time
of data transmission delay, etc. The information can be analysed
and printed on a daily, weekly or monthly basis as well as being
stored on magnetic tape for future reference. It forms the basis
for traffic and trend performance analysis, cost distribution based
upon usage, etc.
The above long and diverse list of functions to be performed by
the NMS shows its important role for the network. Actually, the
adequacy of the NMS’s response to the user’s needs makes the