CARRIER PAIRING, A TECHNIQUE FOR INCREASING
INTERACTIVE SATELLITE SYSTEMS CAPACITY. AN
ASSESSMENT OF ITS APPLICABILITY TO DIFFERENT
by G. Gallinaro(1), R. Rinaldo(2), A. Vernucci(1)
(1) Space Engineering S.p.A. - Rome, Italy
(2) European Space Agency - ESTEC - Noordwijk, Holland
The Carrier Pairing technique, i.e. the sharing of the same frequency band for
both Forward Link and Reverse Link carriers in a star or multi-star satellite
network, is here discussed. In particular a possible mechanization and its
performance in some reference scenarios are discussed to understand merits
and limitations of such technique.
It appears that, in multibeam satellite systems, carrier pairing is a viable
approach for increasing the capacity of a few selected hot spots. However, the
use of carrier pairing in all the beams, in a systematic way, may lead to a
lower overall spectral efficiency, with respect to alternative system
approaches, when the number of beams in the satellite coverage is large.
The Carrier Pairing technique was originally conceived with the aim to
increase the spectral efficiency of interactive satellite systems comprising a
Hub station and a great number of User Terminals (UTs), by allocating a
common band segment to signals transmitted by the Hub and the UTs.
Generally speaking, in the Forward-link (FL, Hub UTs) it is feasible to
manage interference resulting from signals spectral overlap, thanks to the
much higher level of the signal transmitted by the Hub compared to those
transmitted by the UTs. In the Reverse-link (RL, UTs Hub) the otherwise
intolerable interference caused by the Hub-transmitted signal can be
mitigated, at the Hub receive side, by locally adding to the received
composite signal a suitably modified replica of the signal transmitted by the
Hub itself into the FL. Carrier Pairing is a known technique that has already
been adopted for commercial equipment mainly intended for operation in
global-coverage satellite systems.
The urgent need to improve satellite systems competitiveness is leading
research to conceive solutions permitting to increase their capacity, thus
increasing economy of scale and ultimately permitting to reduce service
tariffs. The road being followed is that of proposing and assessing new
payload architectures on the one hand (e.g. multi-beam), and, on the other
hand, investigating new high-performance access solutions which Carrier
Pairing is an example of. At this regard an important issue to be dealt with is
the consistency between advanced payload architectures and the enhanced
access solution. For instance, for the Carrier Pairing case, it would be
important to assess its advantages in different system scenarios (number of
beams, traffic distributions, frequency reuse factor), so as to understand to
which extent the advantages stemming from the adoption of advanced
architectures can be added-up to those deriving from the optimization of the
The subject paper, which is based on some of the results obtained in the
course of an on-going contract awarded by ESA to Space Engineering, begins
introducing briefly the Carrier Pairing concept and its possible
mechanizations, and discussing the issues to be kept under control for
maximizing the technique effectiveness. Then, after defining some reference
system scenarios, the performance of Carrier Pairing in those scenarios is
discussed, showing the applicable results of a comprehensive simulation
campaign carried out in the context of the cited ESA study.
It appears that, in multibeam systems, carrier pairing is a viable approach for
increasing the capacity of a few selected hot spots. However, the use of
carrier pairing in all the beams may not be advantageous when the number of
beams in the satellite coverage is large as the intra-beam interference
becomes the limiting factor.
The paper is organized as follows. Next section contains a brief introduction
to the carrier pairing techniques and the related interference cancellation
scheme used at the GW side for recovering the RL signals. Section 3 shows
the BER /FER performances achievable on a non-linear satellite channel at
the RL GW demodulator.
Section 4 gives finally the overall system throughput which could be
achieved in a multi-beam system scenario using this technique and compares
it with that achievable with a conventional approach in which separate
frequency bands are utilized for the FL and RL. The comparison is done
assuming that the same total bandwidth and on-board power are used in both
approaches to eventually assess if carrier pairing is a viable choice for
improving the spectral efficiency of next generation broadband multimedia
2. CARRIER PAIRING TECHNIQUE
In a satellite system implementing a conventional star network architecture
(i.e. with a conventional frequency plan) we need for each carrier two
different frequency bands: one for up-link and one for down-link. In the end,
for a bidirectional circuit we need four frequency bands as shown in the
Figure 1 Forward/Reverse Link frequencies in conventional systems
For example, assuming Ka-band system operation, each of the four links may
be accommodated within the bandwidth shown below:
FL up-link (from GW to satellite): 27.5 ÷ 28.0 GHz
FL down-link (from satellite to User Terminals): 19.7 ÷ 20.2 GHz
RL up-link (from User Terminals to satellite): 29.5 ÷ 30 GHz
RL down-link (from satellite to GW): 18.3 ÷ 18.8 GHz
To reduce the occupied system bandwidth it is possible to share the same
bandwidth for FL and RL up-link as well as for FL and RL down-link (see
Figure 2 Carrier Pairing approach
With this approach only two frequency bands are required instead of four.
There is thus no distinction between user beams and GW beams. As a
consequence, a GW can only serve a single beam.
Frequency Reuse 1 /3
Figure 19 Cumulative distribution and probability density of up-link C/I in the
selected coverage with 0.5° beamwidth. Three colour pattern frequency reuse.
Statistics computed for the most peripheral beam. Perfect power control
A power ratio of 10 dB is thus only useful if an asymmetric traffic is expected
between FL and RL.
For symmetric traffic load a FL/RL power repartition of 4 to 1 would be
required to get an acceptable RL throughput. A total throughput of 14.8 Gbit/s
and 15.4 Gbit/s in fact results for that repartition case respectively for the FL
and RL. These numbers have to be compared against 14.4 Gbit/s and 18.0
Gbit/s which would be achievable respectively for the FL and RL of a
conventional system using separate FL and RL transponders (and frequency
There is a small improvement of the FL capacity paid by a reduction in the
Availability is instead slightly reduced (particularly for the RL) with respect
to the performance figures achieved in the reference scenario (99.9% and
In conclusions, carrier pairing does not appear particularly attractive in the
present context at least for symmetric (FL to RL) traffic. The following
considerations are however to be done.
In our evaluation we have assumed that the RL carriers are fully used with a
filling factor for the TDMA frame of 100%. This in practice never happens.
So we could design the system with a nominal power ratio between FL and
RL, e.g. 4 to 1, but the actual interference experienced by the FL carrier
would be somewhat smaller due to the fact that, statistically, not all RL
carriers are simultaneously active. At this regard, to control latency in packet
application on the RL, enough free capacity should be available to exploit the
free capacity assignment mechanism in DVB-RCS. Whilst unused free
capacity assignments are a complete waste in traditional systems, in this case
there is a partial compensation with an increase of the FL efficiency which
can be exploited to further reduce the FL to RL nominal power allocation
ratio. However, with this approach interference on the FL carrier could be
quite unpredictable especially when a small number of beams is considered.
In conclusion, it is our feeling that, apart for possible cases with very
unbalanced FL to RL traffic, Carrier Pairing may find useful usage when only
a few beams needs to operate in carrier pairing mode (either because the
coverage is based on few beams or because there are a few hot spots in the
For example Table 6 shows the results when only 8 out of 88 Beams at the
center of the coverage are used in carrier pairing mode, whilst the other
beams are used for RL only.
Table 6 Performance using carrier pairing in 8 hot spot-beams. A FL/RL
Power Allocation Ratio of 7 to 1 was used.
The FL throughput is the one achieved in the 8 beams used in carrier pairing
(to be added to the throughput provided in the normal FL transponders. The
RL throughput is the sum of throughput of all 88 RL transponders including
the 8 used in carrier pairing. For the transponders not used in carrier pairing
the characteristics of the conventional RL transponders are used.
An analysis of the performance of Carrier Pairing was performed to evaluate
if it can provide an improvement of system spectral efficiency in next
generation broadband satellite systems. It appears that in systems with large
number of beams the capability of carrier pairing to improve spectral
efficiency is quite limited at least when symmetric FL and RL traffic is
expected. However, it is expected that carrier pairing may be a possible
solution to improve the FL capacity if asymmetric traffic is expected
particularly when only a few hot spots are expected.
 ETSI EN 301 790, “Digital Video Broadcasting (DVB); Interaction
channel for satellite distribution systems, V1.3.1, 2002
 ETSI EN 302 307 v.1.1.1, “Digital Video Broadcasting; Second
generation framing structure, channel coding & modulation systems for
Broadcasting, Interactive Services, News Gathering and other broadband
satellite applications, V1.1.1., 2005
 M Debbah, G. Gallinaro, R. Muller, R. Rinaldo, A. Vernucci,
“Interference Mitigation for the Reverse-Link of Interactive Satellite
Networks,” submitted to Globecom 2006.
 S. Benedetto, R. Garello, G. Montorsi; C Berrou, C. Douillard et al,
“MHOMS: High-Speed ACM Modem for Satellite Applications”, IEEE
Wireless Comm., April 2005.