Note: Descriptions are shown in the official language in which they were submitted.
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DUAL ANTENNA SYSTEM
Field of the invention
The invention relates to a mobile communication d.=vice
comprising at least two antennas.
Background of the invention
An increasing number of portable electrical equipment for
wireless communication is being produced. Not only mobile
phones, as in the preferred embodiment of the invention,
but also cordless phones and laptop computers with
electronic circuitry for transmitting and receiving EM-
signals.
Each year, a new generation of portable electrical
equipment is being introduced on the market.
In general, each new generation of portable electronic
equipment becomes smaller than the previous one. As the
equipment gets smaller, the distance between the handgrip
of the equipment and the transmitting/receiving antenna
herein, decreases. Therefore, all though the prob3.em of
user interference has always existed, the problem is now
increasing.
Prior to this invention, it was generally accepted that
portable electrical communication equipment had a poor
connection under some circumstances. If, for example, a
conventional mobile phone is laid on a metal surface or
if a hand is held around the antenna, it is known to have
a deteriorating effect on the radio connection. It: is the
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object of this invention to prevent or compensate for
this deterioration.
Summary of the invention
When a mobile communication device having a housing, said
device comprising antenna means and signal processing
means coupled to said antenna means, said antenna means
comprising at least two antennas having different
radiation patterns, an advantageous device has been
obtained.
Thus, the use of a selective radiation pattern provides
the possibility of increasing the signal quality o:E the
communication device, even if the device is used in
critical environments with respect to transmission and
reception.
Thus, the invention provides an efficient utilisat~.on of
multi-antenna systems as the antennas of the devicE: may
supplement each other, not only with respect to the'
different location within the housing of the device', but
also with respect to the directivity of the antennas. One
antenna radiation pattern may be suitable under certain
transmission conditions, while another radiation pattern
may be suitable under other transmission conditions..
If the hand of the user e.g. covers an antenna, another
antenna may be situated at the other end of the handset
and therefore uncovered. This advantage is of particular
interest if the utilised antennas are directive.
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An important aspect of the invention is that the signal
quality and the implemented correction by means of at
least two antennas are based on a recognition of the fact
that both transmission and receiving conditions have to
be evaluated and corrected.
Another important advantage of the invention is that both
of the at least two antennas may serve for transmitting
and receiving purposes. The result of the double :Function
antennas is that the overall performance of the mobile
device may be evaluated and adapted to certain receiving
and transmitting conditions.
It should be noted that a mobile device according to the
invention may not only detect undesired condition:3 when a
user actually uses e.g. a handset, i.e. in certain kinds
of active use. It may also detect and correct somewhat
static undesired passive conditions, for instance if the
handset is located on a table with metallic surface, or
partly covered by the surroundings, and thus blocking for
a quality transmission or receiving via the active
antenna.
In a preferred embodiment of the present invention, the
antennas are dual-band internal patch antennas, bath
operating at more than one band of frequencies.
In another embodiment of the invention, one or both
antennas are implemented as external antennas where
single or dual-band antennas are also a possibility.
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In a particularly preferred embodiment of the invention,
the directivity of the at least two antennas are in
opposite directions. Under these circumstance:, near
optimal and power saving transmission and receiving is
obtained as the signal transmission and receiving' may be
established in practically all directions around the
device.
This complementary radiation mode thus provides optimal
transmission quality with a minimum of power con~~umption
in contrast to e.g. omni-directional antennas, which have
to cover the total space around the mobile device, thus
wasting power on unused radiation directions.
It should be noted that the invention may be implemented
in a sequential manner in the sense that the receiving
and transmitting quality may be determined dynamically
after transmitting or receiving a burst.
It should moreover be noted that the algorithm measuring
and controlling switching is relatively slow as the
addressed signal quality is of a somewhat static
character contrary to e.g. diversity techniques
addressing fast changing receiving signal conditions at
the receiving site.
The term "different radiation pattern" implies that: e.g.
the radiation field of the antenna may be different:. A
different pattern may thus be obtained by a combination
of an omni-directional antenna combined with a
directional patch antenna or for instance a combination
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of two directional patch antennas having different:
orientation.
Another different radiation pattern may be obtained if
5 two individual antennas are mutually spaced in the
handset. Thus, a difference in radiation may be obtained
in the sense that the antennas radiate dynamically in
dependence of external local conditions. Conditions
providing different radiation pattern may for instance be
a total or partly covering of one of the antennas. Thus,
a dynamic and mutual change of radiation between two
antennas are thus obtained within the scope of the
invention if the antennas are spaced and/or orient<~ted
differently.
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Brief Description of the Drawings
Fig. 1 is a view of the preferred embodiment of a. mobile
communication device in accordance with the invention,
Fig. 2 shows a radiation characteristic of one directive
antenna,
Fig. 3 shows a radiation characteristic of two
directional antennas,
Fig. 4 shows a circuit diagram of the preferred
embodiment of a mobile communication device in accordance
with the invention,
Fig. 5 shows a flowchart of the algorithm selecting the
transmitting antenna based on the transmission quality of
that antenna, and
Fig. 6 shows a flowchart of the algorithm select=~ng the
receiving antenna based on the receiving quality of that
antenna.
Detailed Description
Fig. 1 shows a portable radio communication apparatus
comprising a handset 15 having a housing 14, a keypad 12
and a display 16.
Inside the handset, a printed circuit board (PCB) 3 with
the necessary electronic circuitry 16 is provided.
Shielded boxes 7-9 protect the electronic circuitry.
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Moreover, a microphone (not shown in fig. 1) is protected
y a shielding box 10.
Electrically connected to the PCB 3 by shielding walls 5
and 6 are a first and a second antenna 1, 2 with t:he same
operational frequencies. Also inside the handset and
electrically connected to the PCB are a battery 13 and a
loudspeaker 4.
The antennas 1, 2 are arranged at different positions and
with different directivity inside the housing :L4. The
antennas are placed apart from each other in sucih a way
that if one is being covered the other is most likely
uncovered. E.g. if one antenna is placed faced down on a
metal plate the other will be pointing upward. :Also if
one antenna is covered by the hand of the user, the other
antenna will be placed at the other end of the handset
15, and therefore likely be uncovered.
Means for detecting the de-tuning effect of the antennas
1, 2 caused by the user's hand from the closeness of the
user's head or other obstacles will be described in the
following.
Thus, the invention also comprises means for detecting
the connection quality of each antenna and a selection
algorithm that continuously selects the antenna that
provides the best connection.
In a preferred embodiment of the present invention, the
antennas are dual-band antennas, both operating at more
than one band of frequencies.
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In a further embodiment of the invention, one or both
antennas are implemented as external antennas. Single or
dual-band antennas are also a possibility.
Figure 1 shows the preferred embodiment of the dual
directive antenna system. Other more complex systE~ms with
PIN-diodes could be devised. One advantage o:E these
systems is the possibility of using one antenna/ressonator
for both directions and thereby save space.
In a preferred embodiment, each of the antennas i~~ having
an antenna gain lower than 0 dB in most directions.
However, when the dual antenna system is combined with a
selection algorithm, which selects the antenna with the
highest gain for each angle of orientation, the dual
antenna system will have a gain higher than 0 dB in most
directions.
It is a general rule of antenna design for handsets that
the lower the volume of the antenna, the lower the
efficiency. Therefore, if two antennas are implemented
using the same volume as a single conventional patch
antenna, both will have lower efficiency than a
conventional one. In a dual antenna system, the :handset
will continuously select the antenna, which is less
disrupted, as being able to choose between antennas is
better than having only one which might be disrupted.
Therefore, a dual directional antenna does not :require
more volume inside the housing than a single internal
antenna to achieve better performance.
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Fig. 2 shows the radiation characteristics of one
directive antenna.
FIG. 3 shows the radiation characteristics of a preferred
embodiment of the invention in which two directional
antennas are arranged, pointing in opposite directions.
It shows the directivity, in the horizontal planes, of
each of the two antennas at one operational frequency
band.
If the antenna arrangement is having more than one
operational frequency band, each of the frequency bands
will have a similar dual directivity.
It should be noted that the antennas complement each
other as for instance a bad or critical transmission or
receiving in the direction ~ - 0° may be elimin<~ted by
switching to the other antenna.
FIG. 4 shows the circuit diagram of a dual antenna front
end.
The output stage of the circuit comprises an amplifier 50
which may be coupled with two antennas 59a and 59b via an
antenna switch 58, an electromagnetic coupler 51 and a
selector switch 54. The electromagnetic coupler provides
two separate outputs. One output to a power detector 52,
TPD, and one output to a reflected power detector 57,
RPD.
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The circuit moreover comprises a low noise amplifier 56
which may also be coupled with the antennas 59a and 59b
via the antenna switch 58, a bandpass filter 5~~ and a
selector switch 54.
5
The circuit is in a transmitting mode when the ~~elector
54 is in position 54a, and in receiving mode when the
selector 54 is in position 54b.
10 The power detector 52, TPD, continuously measures the
transmitted power from the power amplifier 50 of the
output stage 50, while the power detector 57, RPD,
continuously measures the power of the signal transmitted
to the antennas 59a or 59b and reflected to the coupler
51 .
The power detector may comprise a diode detector.
However, according to the preferred embodiment of the
invention, the power detecting means comprises a
logarithmic amplifier. A logarithmic amplifier is
preferable due to the fact that this detector may easily
be integrated in the transceiver chip. The diode detector
would typically have to be an external discrete
component.
A method of measuring the transmitting quality, according
to one embodiment of the invention, will n.ow be
described.
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Under normal transmitting conditions, the impedance of
the output stage will be matched with respect to the
impedance of the connected antenna 59a or 59b in such a
way that the reflected signal measured in 57 will be
zero, or close to zero.
When the connected antenna 59a or 59b is completely or
partly covered, the resulting input impedance of the
antenna will change and the impedance matching of the
antenna with respect to the impedance of the output
amplifier will temporarily be lost. This lack of
adjustment will result in the connected antenna 59a or
59b reflecting part of the incoming signal back to the
amplifier 50.
This reflected signal will be detected by the power
detector 57, and a bad transmission signal quality has
thus been detected. It should be noted that the
possibility to measure the transmitting quality :is very
advantageous, as the a transmitting estimate represents a
very good estimate of the factual conditions because no
diversity phenomena will interfere with the measurement.
It should nevertheless be noted that the transmissions
signal quality may be determined in many other more or
less convenient ways.
The measuring of the receiving quality may e.g. be
obtained through a dynamically read-out of Rxqual ~;ralues,
such as bit error ratio, BER or frame error ratio, FER.
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Rxqual, FER and BER are values determined in the GSM-
standard.
FIG. 5 shows the flowchart of the algorithm for selecting
which antenna to use as a transmitting antenna for the
next transmitted burst. In a multiple band
implementation, the transmission antenna for each band is
selected separately by this algorithm.
The radiation properties of an antenna can be detected by
applying two methods: One method during transmission and
one during reception. To get the optimal performance,
both methods should be used. The receiving and
transmitting antenna may accordingly be ~~elected
independently.
During transmission, part of the transmitted power from
the PA module 50 of fig. 4 will be reflected from the
antenna. The amount of reflected power depends on how
much the antenna is disrupted by the user.
Therefore, the choice of transmitter antenna is based on
which of the antennas are reflecting less power.
In a conventional front end, both the transmitted and the
reflected energy is also being detected. Therefore, the
dual antenna system only increases the production price
by the cost of the antenna selection switch 58.
The selection algorithm of the antennas has to be divided
in two parts, one part that selects a TX antenna and one
part that selects an RX antenna. The choice of antenna is
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made on a burst to burst basis. The two parts of the
algorithm works separately because the bandwidth of each
antenna can be so narrow that it only matches either the
TX or the RX band.
The flowchart of the selection algorithm f:or the
transmission antenna is shown in figure 5.
The algorithm comprises four feedback loops in which a
value determining the transmission quality CTX will be
modified in dependence of the continuously rneasured
reflected power.
Before the first burst is transmitted, a counter, CTX, is
set to zero. Then, after each transmitted burst this
counter is increased by a certain number in accordance
with the reflected power. If little or no power is
reflected from the input terminal of the antenna, the CTX
is only increased by a small number. If the reflected
power is less than 1 dB, the CTX will be remain
unmodified. If more power is reflected, the counter CTX
increases by a larger number. When the counter exceeds a
certain limit, CTXmax, the other antenna is selected, and
the counter is set to zero.
This algorithm ensures that if the radiation properties
are very good for one antenna, this antenna will :be used
continuously. If the radiation properties are moderate
for one antenna but even worse for the other, both
antennas will be used over time, but the antenna with the
better radiation properties will be used more frequently.
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The algorithm ensures that both antennas are tested, but
at the same time it also ensures that the handset does
not toggle unnecessarily between the antennas.
FIG. 6 shows the flowchart of the algorithm for selecting
which antenna to use as receive antenna for tze next
burst.
The algorithm is based on the same principles as the
transmitting antenna selection algorithm described. above,
but, in this case, the assessment of a signal quality is
based on existing parameters which derive from i~he GSM
protocol.
During reception the RXqual for each burst is a clear
indicator of the reception quality. Therefore, during
reception, RXqual is used as a basis to select the
antenna providing the better connection. RXqual is
already used in conventional handsets as a basis for
channel selection.
In the embodiment of the invention described above,, a low
quality of the reception or the transmission will cause a
switching between the coupled antennas.
According to one embodiment of the invention, it should
be noted that different antennas may be selected for
receiving or transmitting purposes, respectively.
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According to one preferred embodiment of the invention,
one of the antennas 1, 2 is a directive antenna, said
directive antenna being the preferred antenna. This means
5 that the antenna is the main antenna, while the other
antenna serves primarily as a fall-back antenna being
selected only when absolutely necessary. Consequently, it
is not absolutely necessary to obtain two antennas with
equally high quality requirements, as the primary, and
10 best antenna, is intended to be the most frequently used
antenna.
It should moreover be noted that the invention focuses on
the somewhat static transmission conditions due to the
15 fact that the object of quality improvement relates to
improvement of long-term use and disregards short-term
transmitting/receiving interfering signal problems.
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