# Potential Benefits of Dynamic Beam Synthesis to Mobile Satellite Communication, Using the Inmarsat 4 Antenna Architecture as a Test Example

Authors: R. F. E. Guy
Journal: International Journal of Antennas and Propagation
 (2009)

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## Abstract

Present mobile satellite communication systems use large antennas to provide multiple high-gain beams. Each beam covers a fixed geographic cell on the earth. Spatial frequency reuse is provided by synthesising beams with low-power levels over all cells operating at the same frequency. The performance needs for future systems are steadily increasing, leading to higher-gain requirements, which are met by using larger antennas with narrower beams. So the antenna pointing errors become a significant loss factor. An alternative approach is to abandon the use of fixed beams and dynamically synthesise the beams to optimise the antenna performance in real time. This both increases user gain and lowers cofrequency interference whilst also reducing the effects of pointing errors. Simulations, using the Inmarsat 4 antenna architecture as a test example, show that the spatial isolation performance can be significantly improved by using Dynamic Beam Synthesis.


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## Body

## 1. Introduction

The fourth generation of
Inmarsat satellites provides a good example of an advanced high-capacity, high-gain fixed beam/cell mobile communication system [1]. The L-band Mobile satellite system utilises a 9 m deployable
reflector fed via a 120-element feed array. A digital signal processor (DSP) is
used to produce many different beam types that cover the earth and provide
different user services. The most demanding of these is the Personal Mobile
Communication (PMC) service. This uses ~200 high-gain narrow spot beams to
cover the earth. The beams provide spatial frequency reuse; so cocoloured beams are synthesised to have a high degree of
isolation between them. The earth is covered by contiguous cells  (nominally
hexagonal, as viewed from the satellite) that are fixed with respect to
geographic locations on the earth. The beams are changed throughout the day so
that they remain aligned to these fixed geographic cells even though the
satellite orbit is slightly inclined. Users are assigned to a beam that aligns
with the geographic cell they occupy.However, performance
requirements for mobile satellite communication systems are steadily
increasing, both in terms of higher antenna gain and frequency reuse capacity.
In order to provide the edge of cell edge of coverage (EOC) directivity and the
beam roll-off rate, required to achieve interbeam isolation, the beam size
will need to decrease. So, many more beam/cells will be required to cover a
given geographical area. This will lead to far larger reflectors and feed
arrays with many more radiating elements. For conventional fixed beam/cell
systems, the antenna pointing error is likely to become a dominant loss factor
in the link budget. An alternative is to abandon the fixed beam/cell scheme
and form beams that are optimised to individual user locations. This paper uses
simulations of a “dynamic beam synthesis” (DBS) process to highlight the
potential benefits of this approach.


## 2. Fixed Beam System

The Inmarsat 4
type of antenna system architecture is used as a baseline to compare the fixed
beam/cell system to the alternative of dynamically synthesising the beams
making use of the individual user locations. The following outlines the antenna
system.


### 2.1. Frequency Reuse

Spatial frequency reuse allocates adjacent beams different frequencies but
reuses the same frequency in more distant beams. To do this efficiently
requires the use of a regular hexagonal grid of beams. Figure1 shows  3,
4, and 7 colour schemes. Each colour
represents a different frequency. The colouring principal can be extended to higher-colour schemes such as 9, 12, 13, 16, and 19. An interesting property is that the
distance between common colour beam centres is the square root of the number of
colours. This relationship also holds for a square lattice.Figure 1
Frequency reuse schemes with 3, 4, and 7 colours.A typical PMC
beamset, which was designed for 7 colour frequency reuse, is shown in Figure2.  This has 228 contiguous, high-gain beams
(~38 dBi at edge of coverage). To achieve the isolation requirement the peak to
edge gain variation of the beams is ~3 dB.Figure 2
Example fixed beamset.


### 2.2. Future Trends

The trend for
next generation satellites will be to increase the gain requirements. A 6 dB
increase implies doubling the reflector diameter and quadrupling the number of
feeds. There will also be pressure to increase the frequency reuse factor.
Lowering the frequency colour scheme from 7 to 4 could require the peak to edge
gain delta to increase by a further 1 to 2 dB. The exact figure is dependent on
the isolation requirement.


### 2.3. Limitations of a Fixed Beam System

A fixed beam
system is limited by the following factors.(i)
Cells fixed at specific geographic location.(ii)
Pointing errors are accounted for by expanding
the geographic cells by the pointing error. This degrades both EOC gain and
isolation.(iii)
Orbit inclination distorts the fixed cell
grid, requiring resynthesis of the whole beamset and the use of new weights
many times a day.(iv)
The G/T figure of merit for the satellite is
limited by what can be achieved at the worst point at the edge of cell grid,
when distorted by the inclination and expanded by the pointing error. This is
further reduced by other effects such as vector weight errors.(v)
The EIRP figure of merit for the satellite is
limited by the same effects.


### 2.4. Consequences of Pointing Errors

The following
points highlight the effect of pointing error in a fixed beam system.(i)
For the example beamset using a 7-colour
reuse scheme,0.1° pointing error gives ~0.6 dB loss of gain at EOC.(ii)
For a scaled system with 6 dB more gain,0.1°
pointing error gives ~1.2 dB loss.(iii)
For a scaled system with 6 dB more gain, using
a 4-colour scheme,0.1° pointing error gives ~2 dB loss.
Assuming the
pointing error is not significantly reduced for the larger reflector diameters,
then as the gain and frequency reuse factor is increased, still further, the
pointing error will start to dominate the loss budget.


#### 2.5. Implications of Using a Fixed Beam System

If a fixed beam
system is used to meet future gain and isolation demands, the cell size must
reduce, and both the reflector diameter and number of feed elements must
increase. A 6 dB gain improvement requires doubling the reflector diameter and
quadrupling the number of feeds.If the reuse scheme falls
from 7 to 4 colour, then the additional losses increase by ~3 dB requiring a
~9 dB improvement, which implies about three times the diameter reflector and
eight times as many elements.


## 2.1. Frequency Reuse

Spatial frequency reuse allocates adjacent beams different frequencies but
reuses the same frequency in more distant beams. To do this efficiently
requires the use of a regular hexagonal grid of beams. Figure1 shows  3,
4, and 7 colour schemes. Each colour
represents a different frequency. The colouring principal can be extended to higher-colour schemes such as 9, 12, 13, 16, and 19. An interesting property is that the
distance between common colour beam centres is the square root of the number of
colours. This relationship also holds for a square lattice.Figure 1
Frequency reuse schemes with 3, 4, and 7 colours.A typical PMC
beamset, which was designed for 7 colour frequency reuse, is shown in Figure2.  This has 228 contiguous, high-gain beams
(~38 dBi at edge of coverage). To achieve the isolation requirement the peak to
edge gain variation of the beams is ~3 dB.Figure 2
Example fixed beamset.


## 2.2. Future Trends

The trend for
next generation satellites will be to increase the gain requirements. A 6 dB
increase implies doubling the reflector diameter and quadrupling the number of
feeds. There will also be pressure to increase the frequency reuse factor.
Lowering the frequency colour scheme from 7 to 4 could require the peak to edge
gain delta to increase by a further 1 to 2 dB. The exact figure is dependent on
the isolation requirement.


## 2.3. Limitations of a Fixed Beam System

A fixed beam
system is limited by the following factors.(i)
Cells fixed at specific geographic location.(ii)
Pointing errors are accounted for by expanding
the geographic cells by the pointing error. This degrades both EOC gain and
isolation.(iii)
Orbit inclination distorts the fixed cell
grid, requiring resynthesis of the whole beamset and the use of new weights
many times a day.(iv)
The G/T figure of merit for the satellite is
limited by what can be achieved at the worst point at the edge of cell grid,
when distorted by the inclination and expanded by the pointing error. This is
further reduced by other effects such as vector weight errors.(v)
The EIRP figure of merit for the satellite is
limited by the same effects.


## 2.4. Consequences of Pointing Errors

The following
points highlight the effect of pointing error in a fixed beam system.(i)
For the example beamset using a 7-colour
reuse scheme,0.1° pointing error gives ~0.6 dB loss of gain at EOC.(ii)
For a scaled system with 6 dB more gain,0.1°
pointing error gives ~1.2 dB loss.(iii)
For a scaled system with 6 dB more gain, using
a 4-colour scheme,0.1° pointing error gives ~2 dB loss.
Assuming the
pointing error is not significantly reduced for the larger reflector diameters,
then as the gain and frequency reuse factor is increased, still further, the
pointing error will start to dominate the loss budget.


### 2.5. Implications of Using a Fixed Beam System

If a fixed beam
system is used to meet future gain and isolation demands, the cell size must
reduce, and both the reflector diameter and number of feed elements must
increase. A 6 dB gain improvement requires doubling the reflector diameter and
quadrupling the number of feeds.If the reuse scheme falls
from 7 to 4 colour, then the additional losses increase by ~3 dB requiring a
~9 dB improvement, which implies about three times the diameter reflector and
eight times as many elements.


## 2.5. Implications of Using a Fixed Beam System

If a fixed beam
system is used to meet future gain and isolation demands, the cell size must
reduce, and both the reflector diameter and number of feed elements must
increase. A 6 dB gain improvement requires doubling the reflector diameter and
quadrupling the number of feeds.If the reuse scheme falls
from 7 to 4 colour, then the additional losses increase by ~3 dB requiring a
~9 dB improvement, which implies about three times the diameter reflector and
eight times as many elements.


## 3. Dynamic Beam System

In a dynamic beam system the beams are not fixed. The
beams are synthesised in “real time,” using the known user locations to both
maximise individual user gain and minimise interference from other cofrequency
users.


### 3.1. The Synthesis Process

The following outlines the dynamic beam synthesis
process.(1)
A fixed geographic cell lattice is used to assign frequencies to users,
for example, using 7-colour reuse.(2)
At any one time there are likely to be a number of users assigned the
same frequency.(3)
In any one cell only one user has the same frequency.(4)
A new user requests a channel via a global beam, giving their location,
(e.g., provided by a GPS receiver). Their geographic location is used to assign
an available channel frequency for that user cell.(5)
All other users with the same frequency are identified.(6)
The beams associated with every one of these users are resynthesised
every time a new user is assigned an existing frequency.(7)
Each beam is resynthesised to maximise power at the user location and
minimise power at the other user locations.(8)
The synthesis expands the user locations by the pointing error to
provide coverage and isolation in the presence of pointing errors.(9)
The pointing error allowance would be further expanded to provide extra
margin to cope with vector weight errors,and satellite inclination change.(10)
The weights would be resynthesised and updated on a periodic basis to
cope with gross inclination changes.


### 3.2. Simulation of the DBS Process

The simulation program, MAXIM, has
been written to simulate the dynamic beam synthesis process. It performs both
the synthesis and the analysis to produce plots of directivity and isolation
performance. The program works in the following order.(1)
Reads secondary array fed
reflector (AFR) element patterns.(2)
Reads cell lattice coordinates
and reuse colour scheme.(3)
Chooses a far-field grid point.(4)
Finds associated cell colour and
finds all cocoloured cell coordinates.(5)
Randomly assigns “N” other users in “N”
different cocoloured  cells at random points in each cell.(6)
Synthesises ideal weights for each cocoloured
user, using the inverse of the covariance matrix [2].(7)
Calculates beams from the synthesised weights
(this step can include vector weight errors if requested).(8)
Calculates the directivity of the user with
pointing errors.(9)
Calculates the isolation of the
user with pointing errors.(10)
Repeats simulation “M” times and selects requested percentage
confidence level performances.(11)
Repeats simulation steps (3)–(10) for all grid points.(12)
Outputs results.


### 3.3. Simulation Results

The DBS beams were
synthesised to maximise power over the user and minimise power over cocoloured
users. To illustrate this, the following example is used. A user is at (0.08°, 0.08°), and seven other cocoloured  user locations are chosen at random. The
closest user is in the first cocoloured  ring of cells surrounding the coverage
cell.  Figure 3 shows a maximum
directivity beam for (0.08°, 0.08°), and the locations of the other users are indicated by the vertices of
the straight white lines. The beam peak is 43.5 dBi, and the level at the closest
user is 25 dBi (i.e., 18.5 dB below the beam peak).Figure 3
Beam prior to DBS.Figure4 shows the
synthesised beam with deep nulls over the other user locations. The nulls are
below −50 dBi. The peak
directivity has fallen by about 0.25 dB. Figure 5 shows the effect of likely
vector weight errors. The nulls have filled in; for example, the level of the
closest user is −1 dBi (i.e., 26 dB lower than the maximum directivity case). All
nulls remain below 0 dBi over an area encompassing the pointing error
uncertainty margin and provide ~40 dB isolation.Figure 4
Synthesised beams with nulls, no errors.Figure 5
Synthesised beams with nulls, with  errors.A full simulation was carried
out over the earth using five trials at each grid point. The worst values, for
each grid point, of these five trials were selected. It included vector weight
errors, as for the “fixed beamset”, and pointing errors of0.16°. Satellite pointing error margin was 0.1°; the additional 0.06° is equivalent to a user location error of 40 Km at the
subsatellite point. GPS can give user location to tens of meters or better.
The directivity levels are shown in Figure 6. The received isolation over the
earth is shown in Figure 7. The received isolation was calculated as the ratio
of the sum of the powers received from the cocoloured  users to the power of
the user. The results presented in this paper are for a 7-colour frequency
reuse scheme, with eight cocoloured  users.Figure 6
Minimum directivity of DBS beams biased for isolation, 7 colour, with errors, (scale 35 to 43 dBi).Figure 7
Minimum isolation of DBS beams biased for isolation, 7 colour, with errors, (scale −31 to −10 dB).The results show higher
directivity levels than the fixed beamset. There are a few areas where the
directivity was lower than before. This was due to the synthesis procedure
achieving very low nulls at the expense of user directivity. Higher directivity
levels can be achieved by relaxing the null depth. Figures8 and 9 show the
directivity and isolation when the synthesis is biased more towards
directivity.Figure 8
Minimum directivity of DBS beams, biased for gain, 7 colour, with errors, (scale 35 to 43 dBi).Figure 9
Minimum isolation of DBS beams, biased for gain, 7 colour, with errors, (scale −31 to −10 dB).


## 3.1. The Synthesis Process

The following outlines the dynamic beam synthesis
process.(1)
A fixed geographic cell lattice is used to assign frequencies to users,
for example, using 7-colour reuse.(2)
At any one time there are likely to be a number of users assigned the
same frequency.(3)
In any one cell only one user has the same frequency.(4)
A new user requests a channel via a global beam, giving their location,
(e.g., provided by a GPS receiver). Their geographic location is used to assign
an available channel frequency for that user cell.(5)
All other users with the same frequency are identified.(6)
The beams associated with every one of these users are resynthesised
every time a new user is assigned an existing frequency.(7)
Each beam is resynthesised to maximise power at the user location and
minimise power at the other user locations.(8)
The synthesis expands the user locations by the pointing error to
provide coverage and isolation in the presence of pointing errors.(9)
The pointing error allowance would be further expanded to provide extra
margin to cope with vector weight errors,and satellite inclination change.(10)
The weights would be resynthesised and updated on a periodic basis to
cope with gross inclination changes.


## 3.2. Simulation of the DBS Process

The simulation program, MAXIM, has
been written to simulate the dynamic beam synthesis process. It performs both
the synthesis and the analysis to produce plots of directivity and isolation
performance. The program works in the following order.(1)
Reads secondary array fed
reflector (AFR) element patterns.(2)
Reads cell lattice coordinates
and reuse colour scheme.(3)
Chooses a far-field grid point.(4)
Finds associated cell colour and
finds all cocoloured cell coordinates.(5)
Randomly assigns “N” other users in “N”
different cocoloured  cells at random points in each cell.(6)
Synthesises ideal weights for each cocoloured
user, using the inverse of the covariance matrix [2].(7)
Calculates beams from the synthesised weights
(this step can include vector weight errors if requested).(8)
Calculates the directivity of the user with
pointing errors.(9)
Calculates the isolation of the
user with pointing errors.(10)
Repeats simulation “M” times and selects requested percentage
confidence level performances.(11)
Repeats simulation steps (3)–(10) for all grid points.(12)
Outputs results.


## 3.3. Simulation Results

The DBS beams were
synthesised to maximise power over the user and minimise power over cocoloured
users. To illustrate this, the following example is used. A user is at (0.08°, 0.08°), and seven other cocoloured  user locations are chosen at random. The
closest user is in the first cocoloured  ring of cells surrounding the coverage
cell.  Figure 3 shows a maximum
directivity beam for (0.08°, 0.08°), and the locations of the other users are indicated by the vertices of
the straight white lines. The beam peak is 43.5 dBi, and the level at the closest
user is 25 dBi (i.e., 18.5 dB below the beam peak).Figure 3
Beam prior to DBS.Figure4 shows the
synthesised beam with deep nulls over the other user locations. The nulls are
below −50 dBi. The peak
directivity has fallen by about 0.25 dB. Figure 5 shows the effect of likely
vector weight errors. The nulls have filled in; for example, the level of the
closest user is −1 dBi (i.e., 26 dB lower than the maximum directivity case). All
nulls remain below 0 dBi over an area encompassing the pointing error
uncertainty margin and provide ~40 dB isolation.Figure 4
Synthesised beams with nulls, no errors.Figure 5
Synthesised beams with nulls, with  errors.A full simulation was carried
out over the earth using five trials at each grid point. The worst values, for
each grid point, of these five trials were selected. It included vector weight
errors, as for the “fixed beamset”, and pointing errors of0.16°. Satellite pointing error margin was 0.1°; the additional 0.06° is equivalent to a user location error of 40 Km at the
subsatellite point. GPS can give user location to tens of meters or better.
The directivity levels are shown in Figure 6. The received isolation over the
earth is shown in Figure 7. The received isolation was calculated as the ratio
of the sum of the powers received from the cocoloured  users to the power of
the user. The results presented in this paper are for a 7-colour frequency
reuse scheme, with eight cocoloured  users.Figure 6
Minimum directivity of DBS beams biased for isolation, 7 colour, with errors, (scale 35 to 43 dBi).Figure 7
Minimum isolation of DBS beams biased for isolation, 7 colour, with errors, (scale −31 to −10 dB).The results show higher
directivity levels than the fixed beamset. There are a few areas where the
directivity was lower than before. This was due to the synthesis procedure
achieving very low nulls at the expense of user directivity. Higher directivity
levels can be achieved by relaxing the null depth. Figures8 and 9 show the
directivity and isolation when the synthesis is biased more towards
directivity.Figure 8
Minimum directivity of DBS beams, biased for gain, 7 colour, with errors, (scale 35 to 43 dBi).Figure 9
Minimum isolation of DBS beams, biased for gain, 7 colour, with errors, (scale −31 to −10 dB).


## 4. Conclusion

The “dynamic beam synthesis”
(DBS) process has the potential to increase both the gain and isolation
performance of multibeam, frequency reuse, and satellite communication systems. The
simulations show that the isolation performance of dynamic beam synthesis can be an
order of magnitude better than using the fixed beam case.Table 1ParameterValueTransmit1525–1559 MHzReceive1626.5–1660.5 MHZPolarisationRHCPNumber of antennasSingle Tx/Rx array fed reflectorAntenna typeOffset-fed paraboloidal reflectorDiameter9 metresF/D0.53Feed120 helical elements, array diameter = 2.2 metresFeed positionIn front of focal planeBeamformerDigital Signal Processor, (DSP) with 158×8 multiport
amplifier (MPA*) blocks on transmitBeam alignmentBeams aligned with fixed geographic cellsBeam reconfigurationBeams reconfigured throughout the day to compensate for
orbit inclinationFrequency reuseSpatial diversity provides frequency reuse between beamsThe* MPA blocks are used to distribute power more uniformly over the transmit amplifiers and hence increase their efficiency.



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*Source: 249602-2009-05-06.xml*