Aerospace and Electronic Systems - May 2019 - 75

Guan et al.

Figure 11.
Examples of 3, 4, and 6 colors for HTS use.

FREQUENCY REUSE AND SPECTRAL EFFICIENCY
LEVERS FOR INCREASING THROUGHPUT
The remaining terms in (2) for transforming allocated
bandwidth into throughput are frequency reuse and spectral efficiency. A brief explanation of these terms is provided next. Frequency reuse factor is defined as follows:

Np Nb
FR
:
(3)
Nc
Np as noted previously is the number of polarizations (1 or
2), Nb the number of beams, and Nc the number of colors
used. This last term requires some explanation. When a
bandwidth ðBw Þ is allocated to a spacecraft, it is divided
into sub-bands, typically 2 or 3. Then right-hand or both
right- and left-hand polarization are used with the subbands. Each sub-band and polarization type constitute a
color. Figure 11 provides examples of 3, 4, and 6 colors.
Typically, a guard band of 5%-10% between sub-bands is
left unused for interference issues [the term hguard in (2)].
Each color is then assigned to a spot beam and is reused
as many times as needed/technically possible in nonoverlapping beams. The spot beams are then tessellated to cover a
desired geographic region. An example of 60-spot beam
coverage of parts of the United States is shown in Figure 12.

As seen in (3), the most effective leverage of throughout is the number of beams. Polarization is binary (1 or 2),
and the number of colors is a denominator term with a
lower bound constraint, Nc ! 3; which is required to
enable the reuse of colors in nonadjacent beams.
To increase the number of spot beams Nb covering a
given region, and consequently increase satellite throughput
(Rtotal), the beamwidth has to be reduced, assuming for the
time being that sufficient RF power is available. Since beamwidth is inversely proportional to frequency and antenna
reflector diameter, increasing Nb requires the use of higher
frequency bands and/or larger diameter antenna reflector.
The shift from Ku band to Ka and Q/V bands is already
underway with current and proposed HTS systems. The
remaining lever is to increase the size of the reflector. This is
constrained to some extent by the size of the current launch
vehicle fairings. As a result, the development of deployable
and/or self-assembled very large reflectors (VLDR) on orbit
would be a boon for HTS and can contribute to reducing its
cost per bit/s. A review of trends and challenges of antennas
for satellite communication is provided in Rahmat-Samii
and Densmore [25]. Spot beam isolation and beam-to-beam
interference7 become increasingly important as the number
of beams is increased to tessellate a given geographic area,
and they have to be carefully attended to. A discussion of
these issues can be found in Lutz [26] and Gayard [27].
The remaining term in (2) is the spectral efficiency,
b. Roughly speaking, spectral efficiency reflects how
many information bits per second can be carried in one
Hz. Analytically, spectral efficiency is given in (4)
b ¼ log2 ðM Þ
8
>
< M : number of amplitude=phase states
r : code rate or foreward error correction :
>
:
a : filter roll-off factor

The term log2 ðMÞ is referred to as the order of the modulation scheme. The code rate or forward error correction
term, r, is the ratio of the number of information bits

Figure 12.
Example of 60-spot beam coverage of parts of the United States
with 4 colors.

MAY 2019

(4)

A kind of self-jamming in HTS systems.

IEEE A&E SYSTEMS MAGAZINE

Aerospace and Electronic Systems - May 2019

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