LTE ANTENNA PAIR FOR MIMO/DIVERSITY OPERATION IN THE LTE/GSM BANDS
[0001] This invention relates to a pair of loop antennas for mobile handset applications,
and in particular to operation on the LTE network where more than one antenna is required
on each handset.
BACKGROUND
[0002] Long Term Evolution (LTE) is the latest standard under development for mobile
network technology. It is designed to enable wireless providers using both GSM and 3G
networks to transition to fourth generation (4G) networks and equipment. For consumers,
LTE will enable existing applications to run faster, and will also make available new mobile
phone applications. In order to obtain the higher data rates required for these new
applications, LTE has adopted multiple-input multiple-output (MIMO) technology, which will
require mobile phones to have two cellular radio antennas. LTE also uses lower
frequencies than the GSM band and mobile phone antennas will now have to have low
band performance extended down to 698 MHz (from 824 MHz at present). This
combination of needing two antennas and lower frequency performance presents
significant problems for the designer of antennas for mobile platforms.
[0003] In order for a pair of antennas to give good diversity performance or work
successfully in a MIMO system they need to sample, to a certain extent, different multipath
signals arriving at the equipment terminal. This means, in effect, that the antennas must
be different in some way by having different beam patterns, different polarisations, phase
responses or be physically well separated electrically (spatial diversity).
[0004] An indication of how similar two antennas are is given by the envelope correlation
coefficient p , which is a measure of how the radiation patterns of two antennas differ in
shape, polarization and phase. A ow correlation is very important for the performance of a
MIMO system because when pe = 1 the patterns are identical and no MIMO or diversity
gain is possible. However, when pe = 0 optimal MIMO gain is achieved it is important to
note that that the overall performance of the two antennas must be similar: good MIMO
performance cannot be achieved using one efficient antenna and one inefficient antenna.
Both must have similar efficiencies, but be different in one or more of the characteristics
listed above.
[0005] Recently it has been shown that loop antenna technology can be used for mobile
phone applications and, by means of switching or electronic tuning, can be configured to
cover the LTE bands as well as the GSM bands, for example as described in the present
Applicant's co-pending UK patent application no GB0914280.3. Recent developments
designed to improve bandwidth include multi-moding the loops, complex feed and
grounding arrangements and complex structural arrangements towards the centre of the
loop designed to improve the match to 50 ohms. Some of these developments are
described in detail in the present Applicant's co-pending UK patent application no
GB1 0 17472.0, the content of which is incorporated into the present application by
reference.
BRIEF SUMMARY OF THE DISCLOSURE
[0006] According to a first aspect of the present invention, there is provided a multipleinput
multiple-output (Ml MO) antenna system comprising first and second folded or
compacted loop antennas each having a longitudinal extent and mounted substantially
parallel to each other on a dielectric substrate having a conductive groundplane, wherein
the groundplane extends between the first and second antennas, but wherein the first and
second antennas are mounted on the substrate in areas where there is no groundplane,
and wherein the first and second antennas, in use, generate first and second radiation
patterns and also cause currents to flow in the groundplane between the antennas so as to
skew the first and second radiation patterns relative to each other by an angle greater than
zero.
[0007] The first and second antennas may be mounted relative to each other in a
manner similar to a pair of Helmholtz coils, although it is not essential or even necessarily
preferably that the antennas are spaced from each other by a distance similar to a radius
of each loop. However, it is preferred that the loops of the first and second loop antennas
are substantially co-axial. The greater the spacing between the first and second antennas,
the greater the diversity.
[0008] Each of the first and second loop antennas may be configured as described in co
pending UK patent application no GB1 0 17472.0, that is, each loop antenna may be
configured as a loop of conductive track that is formed on a dielectric substrate in a
compact manner by folding the loop over an edge of the substrate so to form first and
second patches. Alternatively, first and second patches may communicate galvanically
with each other by way of vias in the substrate so as to define a compacted loop. In other
embodiments, the loop may be compacted in a single plane by meandering or otherwise
folding the conductive track. In all embodiments, the expression "folded or compacted
loop antenna" is intended to signify a loop antenna formed by a conductive track in a
topologically loop-shaped configuration that encloses an area smaller than would be
enclosed by the conductive track if it were opened out into a circle. In most embodiments,
the enclosed area is smaller than that which would be enclosed by the conductive track if it
were to be opened out into a square or rectangle. This is because the compacted or
folded loop generally includes at least one re-entrant portion, typically where the loop
passes from one side of the substrate to the other.
[0009] Embodiments of the present invention make use of two loops disposed on a
mobile phone handset, USB dongle or other small platform in order to achieve MlMO or
diversity operation.
[0010] The first and second antennas may be identical to each other in construction
and/or performance, or may be different. Both loops may be mounted vertically with
respect to a horizontal substrate with a groundplane and parallel to each other. In
particularly preferred embodiments, the antenna system is arranged so that each loop can
be easily mounted vertically on a main PCB of a USB dongle.
[001 1] One end of each of the first and second loop antennas is connected to an F feed
for the appropriate signal. The other end of each loop antenna may be connected directly
to ground (for example by connecting to the groundplane), but advantageously the other
end of one or both of the loop antennas is respectively connected to ground by way of at
least one inductive component to adjust the effective length of the loop. In particularly
preferred embodiments, the other end of the one or each of the loop antennas is provided
with a switch allowing two or more different inductive components to be switched in
between the other end and ground, thereby allowing the electrical length of the loop to be
adjusted as required.
[0012] With the loops parallel to each other, and electrically closely spaced, it might be
thought that a low envelope correlation could not be achieved on a platform as small as a
dongle. However, inspection of the currents flowing in the groundplane of the dongle
shows that the two radiation patterns may be skewed so as to have an angular difference
between them. In currently preferred embodiments, the angle between the radiation
patterns is at least 20 degrees, preferably at least 35 degrees, and most preferably around
50 degrees or at least 50 degrees. An angular difference of 50 degrees can give rise to a
correlation coefficient of around 0.4, which is considered to be adequate for MlMO and
diversity applications.
[0013] In a typical dongle application there will be a requirement for a 'main' antenna
covering the LTE and GSM bands and a second antenna for LTE MlMO or diversity use.
This means that the two antennas do not need to have identical construction or
performance. Alternatively or in addition, they do not need to be electrically switched and
matched in the same way.
[0014] In one embodiment, both antennas may be identical but have electrical switching
circuits that may be used in identical or different ways. In some embodiments, three
switching states are provided but configurations with two, four or other numbers of states
are also possible. Measurements of the cross-correlation between the first and second
loop antennas shows that p < 0.5 or less across all the band used by the LTE protocol.
[0015] This result, combined with the good bandwidth and efficiency of the antennas,
means that they are suitable to meet the needs of LTE.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Embodiments of the invention are further described hereinafter with reference to
the accompanying drawings, in which:
Figure 1 shows a first prior art USB dongle antenna configuration for LTE;
Figure 2 shows a second prior art USB dongle antenna configuration for LTE;
Figures 3 and 4 show an embodiment of the present invention;
Figures 5 to 8 illustrate the theoretical background underlying embodiments of the
invention;
Figure 9 shows an exemplary connection scheme for an embodiment of the
present invention;
Figure 10 shows a plot showing input matching and isolation for an embodiment
of the present invention in isolation;
Figure 1 shows a plot showing input matching and isolation for an embodiment
of the present invention when plugged into a laptop computer;
Figure 12 shows a plot of antenna efficiency for an embodiment of the present
invention in isolation;
Figure 13 shows a plot of antenna efficiency for an embodiment of the present
invention when plugged into a laptop computer;
Figure 14 shows an isotropic 3D propagation plot showing the correlation
coefficient and cross-polarization power ratio values across different frequency bands of
an embodiment of the present invention in isolation; and
Figure 15 shows an isotropic 3D propagation plot showing the correlation
coefficient and cross-polarization power ratio values across different frequency bands of
an embodiment of the present invention when plugged into a laptop computer.
DETAILED DESCRIPTION
[0017] Figure 1 shows a first prior art MIMO USB dongle 1 in schematic form with the
housing removed. The dongle 1 comprises a USB connector 2, a PCB substrate 3, a main
antenna 4 and an orthogonally-disposed secondary antenna 5. An alternative
arrangement is shown in Figure 2, where the secondary antenna 5' is elevated above the
PCB substrate 3 and can be swivelled about a stalk 6 on which the antenna 5' is mounted.
For clarity, no other dongle components are shown, although it will be appreciated that the
PCB substrate 3 will be populated with various components such as memory and
processor circuits. MIMO USB dongles 1 of this type are intended for use as USB
modems that can be plugged into laptop or other computers, thereby allowing data
transmission and reception by way of an LTE mobile network. The main antenna 4 in each
case is generally dedicated to LTE, GSM and HSPA signals, while the secondary antenna
5, 5' provides spatial diversity for LTE signals. However, the secondary antenna 5 , 5' has
reduced performance relative to the main antenna 4, and therefore the USB dongle 1 as a
whole displays sub-optimal MIMO and a reduced data transfer rate.
[0018] Figures 3 and 4 show an embodiment of the present invention, again in schematic
form. A MIMO USB dongle 10 comprises a USB connector 2 , a PCB substrate 3 in the
form of a dielectric board such as FR4, a conductive groundplane 1, and a pair of folded
or compacted loop antennas 12, 12' disposed parallel to and opposite each other on the
longer edges of the PCB substrate 3 . It will be seen that the loop antennas 12, 12' are
vertically mounted with respect to a plane of the substrate 3 , and are surface mounted on
regions of the substrate 3 where no groundplane 1 is present. The groundplane 11 does,
however, extend between the antennas 12, 12'.
[0019] Each antenna 12, 12' comprises a loop formed of a conductive track 16, 16'
printed or otherwise formed on a dielectric substrate 13, 13'. In particular, each loop
antenna 12 may comprise a dielectric substrate 3 having first 14 and second 5 opposed
surfaces and a conductive track 16 formed on the substrate 13, wherein there is provided
a feed point 7 and a grounding point 18 adjacent to each other on the first surface 14 of
the substrate 13, with the conductive track 16 extending in generally opposite directions
from the feed point 17 and grounding point 18 respectively, then extending towards an
edge of the dielectric substrate 13, then passing to the second surface 15 of the dielectric
substrate 3 and then passing across the second surface 15 of the dielectric substrate 13
along a path generally following the path taken on the first surface 14 of the dielectric
substrate 13, before connecting to respective sides of a conductive arrangement formed
on the second surface 15 of the dielectric substrate 3 that extends into a central part of a
loop formed by the conductive track 16 on the second surface 15 of the dielectric substrate
13, wherein the conductive arrangement comprises both inductive and capacitive
elements. Instead of a conductive arrangement comprising both inductive and capacitive
components, a simple conductive loading plate may galvanically connect the two ends of
the conductive track 16 on the second surface 15, or the conductive track 16 may form a
continuous loop on the second surface 15. In another embodiment, instead of having both
a feed point 17 and a grounding point 18, the antenna 12 may have two grounding points
18, and be excited by a separate driven loop or monopole antenna (not shown) configured
to couple inductively or capacitively with the antenna 12.
[0020] In Figure 3, the dielectric substrates 13, 13' have central notches 19, 19' cut out
where the electric field will be highest during operation. This helps to improve efficiency.
[0021] The area 20 of the PCB substrate 3 and the groundplane 11 between the
antennas 12, 12' and the USB connector 2 can be populated with other circuit components
(not shown). Indeed, provided that they do not interfere too strongly with the antennas 12,
12', further circuit components may be mounted between the antennas 12, 12'.
[0022] It can be seen that the design of embodiments of the present invention is
symmetrical about a mirror plane along the centre line of the USB dongle 10, in contrast to
the illustrated prior art arrangements.
[0023] Figures 5 to 8 illustrate the theoretical background underlying embodiments of the
invention. With the loop antennas 12, 12' parallel to each other, and electrically closely
spaced, it might be thought that a low envelope correlation could not be achieved on a
platform as small as a dongle 10. However, inspection of the currents 30 flowing in the
groundplane 11 of the dongle 10, shows that the two radiation patterns 21, 22 may be
skewed so as to have a difference of 50 degrees between them. This angular difference
gives rise to a correlation coefficient of 0.4, which is considered to be adequate for MIMO
and diversity applications. In particular, it will be noted that locating the antennas 2 , 12'
on diagonally opposite corners of the PCB substrate 3 of the dongle 10 results in the
antennas 12, 12' operating in the same mode, which leads to high correlation and loss of
diversity. Diagonal modes, with the antennas 12, 12' on the same edge or end of the PCB
substrate 3, are required to give mid to low correlation and hence reasonable diversity.
[0024] Figure 8 shows the first 3 1 and second 32 radiation patterns generated by the
antenna system, and demonstrates that they are skewed relative to each other by 50
degrees, thereby providing reasonable diversity.
[0025] In a typical dongle application there will be a requirement for a 'main' antenna 12
covering the LTE and GSM bands and a second antenna 12' for LTE MIMO or diversity
use. This means that the two antennas 12, 12' do not need to have identical construction
or performance or they do not need to be electrically switched and matched in the same
way. In the illustrated embodiments, both antennas 12, 12' are identical but have electrical
switching circuits that may be used in identical or different ways, as shown Figure 9 .
[0026] Figure 9 shows the PCB substrate 3 with its groundplane 1, as well as two
islands or regions 23, 23' at opposed edges where no groundplane 11 is present. Each
antenna 12, 12' has an RF feed point 17, 17' to which is connected an RF feed port 24, 24'
and an antenna matching circuit 25, 25'. Each antenna 12, 12' also has a grounding point
18, 18' which connects to ground by way of a switch 26, 26' allowing switching between
three different ground connections 27, 27' with different inductances. The switches 26, 26'
are controlled by way of control lines 28, 28'. In the example shown, three switching states
are shown but configurations with two, four or other numbers of states are also possible.
Measurements of the cross-correlation between the antennas 12, 12' shows that p < 0.5
or less across the entire band used by the LTE protocol. This result, combined with the
good bandwidth and efficiency of the antennas 12, 12' means that they are suitable to
meet the needs of LTE.
[0027] Figure 10 shows a plot showing input matching and isolation, in two different
states (i.e. with different inductors switched in between the antennas and ground) across
four bands, namely the LTE 746-798 MHz band, the GSM band, the WCDMA band and
the LTE 2500-2690 MHz band for an embodiment of the present invention in isolation.
[0028] Figure 1 shows a plot corresponding to that of Figure 10, but with the dongle 10
plugged into a laptop computer.
[0029] Figure 12 shows a plot of antenna efficiency for an embodiment of the present
invention in isolation across four bands: 740-800 MHz, 820-960 MHz, 1710-2170 MHz and
2500-2690 MHz.
[0030] Figure 3 shows a plot corresponding to that of Figure 12, but with the dongle 10
plugged into a laptop computer.
[0031] Figure 14 shows an isotropic 3D propagation plot showing the correlation
coefficient and cross-polarization power ratio values across different frequency bands
(740-800 MHz, 820-960 MHz and 1710-2170 MHz) of an embodiment of the present
invention in isolation; with the cross-polarization power ratio between -15dB and +15dB. It
can be seen that the measured correlation coefficient p < 0.5 or less across all the bands,
and indeed is less than 0.4 across most of the spectrum.
[0032] Figure 5 shows a plot corresponding to that of Figure 12, but with the dongle 10
plugged into a laptop computer. Although the correlation coefficient is higher in the lower
bands than when the dongle 10 is in isolation, it is still sufficiently low to allow good MIMO
and diversity operation across the whole spectrum.
[0033] Throughout the description and claims of this specification, the words "comprise"
and "contain" and variations of them mean "including but not limited to", and they are not
intended to (and do not) exclude other moieties, additives, components, integers or steps.
Throughout the description and claims of this specification, the singular encompasses the
plural unless the context otherwise requires. In particular, where the indefinite article is
used, the specification is to be understood as contemplating plurality as well as singularity,
unless the context requires otherwise.
[0034] Features, integers, characteristics, compounds, chemical moieties or groups
described in conjunction with a particular aspect, embodiment or example of the invention
are to be understood to be applicable to any other aspect, embodiment or example
described herein unless incompatible therewith. All of the features disclosed in this
specification (including any accompanying claims, abstract and drawings), and/or all of the
steps of any method or process so disclosed, may be combined in any combination,
except combinations where at least some of such features and/or steps are mutually
exclusive. The invention is not restricted to the details of any foregoing embodiments.
The invention extends to any novel one, or any novel combination, of the features
disclosed in this specification (including any accompanying claims, abstract and drawings),
or to any novel one, or any novel combination, of the steps of any method or process so
disclosed.
[0035] The reader's attention is directed to all papers and documents which are filed
concurrently with or previous to this specification in connection with this application and
which are open to public inspection with this specification, and the contents of all such
papers and documents are incorporated herein by reference.
CLAIMS:
1. A multiple-input multiple-output (MIMO) antenna system comprising first and
second folded or compacted loop antennas each having a longitudinal extent and mounted
substantially parallel to each other on a dielectric substrate having a conductive
groundplane, wherein the groundplane extends between the first and second antennas,
but wherein the first and second antennas are mounted on the substrate in areas where
there is no groundplane, and wherein the first and second antennas, in use, generate first
and second radiation patterns and also cause currents to flow in the groundplane between
the antennas so as to skew the first and second radiation patterns relative to each other by
an angle greater than zero.
2. An antenna system as claimed in claim 1, wherein the first and second antennas
are mounted opposite each other on the substrate.
3. An antenna system as claimed in claim 1 or 2, wherein loops of the first and
second loop antennas are substantially co-axial.
4. An antenna system as claimed in any preceding claim, wherein each of the first
and second loop antennas is configured as a loop of conductive track that is formed on a
dielectric substrate in a compact manner by folding the loop over an edge of the substrate
so to form first and second patches.
5. An antenna system as claimed in any one of claims 1 to 3, wherein each of the
first and second loop antennas is configured as a loop of conductive track that is formed
on a dielectric substrate in a compact manner by forming first and second patches that are
galvanically connected by way of vias in the substrate so as to define a compacted loop.
6. An antenna system as claimed in any one of claims 1 to 3, wherein each of the
first and second loop antennas is configured as a loop of conductive track that is formed
on a dielectric substrate in a compact manner in a single plane by meandering or
otherwise folding the conductive track.
7. An antenna system as claimed in any preceding claim, wherein the first and
second antennas are be identical to each other in construction and/or performance.
8. An antenna system as claimed in any one of claims 1 to 6 , wherein the first and
second antennas are different to each other in construction and/or performance.
9. An antenna system as claimed in any preceding claim, wherein a first end of each
of the first and second loop antennas is connected to an RF feed.
10. An antenna system as claimed in any preceding claim, wherein a second end of
each of the first and second loop antennas is connected to ground.
11. An antenna system as claimed in any one of claims 1 to 8 , wherein both a first
end and a second end of each of the first and second loop antennas is connected to
ground, and further comprising a separate driving antenna for each of the first and second
loop antennas.
12. An antenna system as claimed in claim 10 or 11, wherein the second end of at
least one of the first and second antennas is connected to ground by way of an inductive
component.
13. An antenna system as claimed in claim 10 or 11, wherein the second end of at
least one of the first and second antennas is connected to ground by way of a switch that
allows at least two different inductive components to be selectively switched in between
the second end and ground.
14. An antenna system as claimed in any preceding claim, wherein a correlation
coefficient pe between the first and second antennas is no greater than 0.5 across
predetermined frequency bands of operation.
15. An antenna system as claimed in any preceding claim, wherein, in use, the first
and second radiation patterns are skewed relative to each other by an angle greater than
20 degrees.
16. An antenna system as claimed in any preceding claim, wherein, in use, the first
and second radiation patterns are skewed relative to each other by an angle greater than
35 degrees.
17. An antenna system as claimed in any preceding claim, wherein, in use, the first
and second radiation patterns are skewed relative to each other by an angle of
substantially 50 degrees.
18. An antenna system substantially as hereinbefore described with reference to or
as shown in Figures 3 to 15 of the accompanying drawings.
19. A dongle for connection to a computer, the dongle comprising an antenna system
as claimed in any preceding claim.
20. A mobile phone handset comprising an antenna system as claimed in any
preceding claim.