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.
Figure 3. Signal spectrum with carrier pairing
The on-board HPA is operated in multi-carrier mode as it amplifies both the
FL and RL carriers. In practice the satellite HPA amplifies a single wideband,
high-power, FL carrier plus a multitude of low-power, narrowband, RL
The required power density of the FL carrier (Figure 3) is much higher than
that of the RL carriers due to the different G/T of GWs and UTs.
Approximately the required relative power density of the FL carriers with
respect to the RL carriers is equal to the ratio of the G/T between the GWs
and the UTs.
In a typical system scenario, for example, the clear sky G/T of the GWs and
UTs can be respectively 33.9 dB/k and 17.0 dB/k (see for example Table 1).
Hence, the power density difference between the FL carrier and the RL
carriers may be in the order of 17 dB. Actually, because there is some
difference in the Eb/No requirement of FL and RL, due to the larger
codeword length which is possible in the FL, such difference could be a little
lower (e.g. 12 or 13 dB).
Saturated EIRP 44.5 dBW
Antenna Gain (Tx / Rx) 45.1 dBi / 41.4 dBi
HPA Saturated Power
Minimum Operational OBO
Receiver Noise Figure
Clear Sky G/T
61.0 dBi / 57.5 dBi
120 W (for 4 carriers)
Table 1 GW and UT stations RF parameters assumed in this work
Such a power density ratio will allow the UTs to demodulate and decode the
FL carriers without any special processing. Even advanced Adaptive Coding
and Modulation (ACM) schemes, like the recently standardized DVB-S2, can
be used on the FL although operating modes with the highest spectral
efficiency would not be possible due the RL carrier interference floor.
On the other hand, the GW, in order to successfully demodulate and decode
the RL carriers, has to cancel its own transmitted signal. Being this signal
known at the GW, a conventional echo canceller can be used as shown in the
- io - L
Figure 4. FL Interference cancellation with an adaptive filter (echo canceller)
The adaptive filter may be implemented through the LMS (Least Mean
Square) algorithm trying to minimize the Mean Square Error (MSE) between
the recovered signal (received signal minus the echo signal estimated by the
adaptive filter) and the reference signal (i.e. the GW FL signal before
transmission to the satellite).
The required adaptive Filter length is depending on:
the degree of accuracy of bulk round trip delay estimation
the memory of the channel
The adaptive filter can be split into two independently adapted filters in case
of operation at two samples/symbol, obtaining a hardware complexity
reduction of a factor of two. Indicating with f(k) (k=0 or 1) the two filters, the
adaptation rule for computing the coefficients of the filter at iteration i+1 is:
Where y(k)[i] is the received signal at iteration i (the even or the odd sample
depending on the k value) and xi is the vector of reference signal samples
within the filter memory at iteration i.
The LMS adaptation step μ has to be optimized in order to guarantee good
performance and algorithm stability.