If one looks at the past, there is a clear trend of satellites being
developed with increasingly higher transponder power and RF signals using higher
frequencies that enable the use of this higher power. The balance is slowly
tilting from bandwidth-intensive satellites to the power-intensive ones. The
bandwidth crunch is hitting the satellite industry too, with more and more
bandwidth-hungry applications like broadband services, being operated on
satellites.
Paired Carrier Multiple Access (PCMA)
PCMA is a novel method for frequency re-use that allows two
different earth stations to use the same frequency, time slots, and/or Code
Division Multiple Access (CDMA) code at the same time. The benefit of PCMA is
that it may effectively double the throughput of satellite systems in terms of
bits/sec/Hz, in simple words ‘Bandwidth’.
CRMA Brings the Benefits of Spread Spectrum to Satellite
A combination of CRMA and Asymmetric Paired Carrier Multiple
Access (A-PCMA) brings the benefits of spread-spectrum technology to satellite
networking, for the first time. CRMA is a direct sequence spread-spectrum
technique, similar to conventional CDMA used in cellular telephony.
How CRMA Changes the Economics of Satellite Communications
CRMA technology offers several benefits that will enable
users and service providers the opportunity to keep in pace with the
communications trends, and take advantage of the declining costs for service and
terminals:
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CRMA is an enhanced CDMA: CRMA uses a single spreading
code for all subscriber terminals, allowing a single corelator to feed
multiple demodulators in the hub. This enhancement over the normal CDMA,
makes it highly economical as well as less complex–features that the
traditional CDMA lacked. -
Much more efficient use of bandwidth: CRMA provides much
more efficient use of the channel capacity than contention or reservation
schemes needed for multi-frequency TDMA-based systems, because it takes up
very little bandwidth for frequency overhead. Users gain access to the full
system burst rate. -
Next generation CRMA terminals cost less: The CRMA return
channel does not require the expensive, fast frequency-hopping synthesizers
needed for multi-frequency TDMA return-channel architectures. -
CRMA subscriber terminals reduce RF costs: The TDMA-based
systems achieve an actual throughput equal only to a fraction of their burst
rate, yet TDMA terminals must be sized to achieve the maximum burst rate.
With CRMA, terminal throughput equals burst rate for the duration of any
transmission. The CRMA terminal can therefore, achieve the same rate as the
TDMA terminal using the less powerful (and as a result, less costly) RF
equipment. Even with its advanced design, the CRMA modulator includes a
standard L-band interface that makes it compatible with commodity VSAT
Transmit Blocks. -
When CRMA is combined with PCMA, data transmissions
coming back to the hub from remote sites can be combined within the same
bandwidth as an outbound channel. Rather than requiring additional bandwidth
as current systems do, such advanced technology-based networks would need
only the space segment required by the outbound channel, to support two-way
satellite services. -
An expandable data pipe to reach customers who need a
higher value of broadband services. -
Less power required by spread-spectrum transmissions.
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Combined with PCMA, CRMA gives you forward and return
channels in the same satellite transponder bandwidth that used to provide
traffic only in one direction. In addition, the RF power efficiency and
reduction in hardware components provide a path to more savings, and future
price points that other technologies cannot achieve. CRMA and PCMA would be
the enablers to reach more markets through a combination of bandwidth
efficiency and hardware cost savings that change the economics of satellite
communications.
Feroz Khan, country manager, and Gaurav R Kharod, marketing
manager, India and South-East Asia, Viasat Satellite Networks.