Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
2000-08-17
2004-05-18
Tran, Sinh (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S250100, C455S249100, C342S433000, C342S417000, C375S316000
Reexamination Certificate
active
06738609
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a receiver and a method of receiving and, in particular, but not exdusively, to a receiver and method of receiving for use in a wireless cellular telecommunications network.
BACKGROUND TO THE INVENTION
FIG. 1
illustrate a known wireless telecommunication network
2
. The area covered by the network
2
is divided into a number of cells
4
. Each cell
4
has associated therewith a base transceiver station
6
. Each base transceiver station
6
is arranged to communicate with terminals
8
located in the cell
4
associated with that base transceiver station
6
. The terminals
8
may be mobile stations which are able to move between the cells.
Each base transceiver station is, in the GSM standard (Global System for Mobile Communications), arranged to receive N frequency channels out of 125 available channels C
1
. . . C
1
25
, as illustrated in
FIG. 2
a.
The 125 frequencychannels C
1
. . . C
125
occupy a bandwidth of 25 MHz. Each frequency channel therefore has a width of 200 KHz. Each channel is divided into frames F one of which is shown in
FIG. 2
b
. Each frame is divided into eight slots S
1
. . . S
8
. The GSM standard is a time division multiple access (TDMA) system and accordingly different mobile stations will be allocated different slots. Thus, the base transceiver station will receive signals from different mobile stations in different time slots in the same frequency channel. N is usually much less than 125.
Reference is made to
FIG. 4
which shows part of a known base transceiver station
9
which is arranged to receive N frequency channels at the same time. For clarity, only the receiving part of the base transceiver station
9
is shown. The base transceiver station
9
has an antenna
10
which is arranged to receive signals from mobile stations in the cell served by the base transceiver station
9
. The base transceiver station comprises N receivers R
1
, R
2
. . . RN. Thus one receiver is provided for each frequency channel which is to be received by the base station
9
. All of the receivers R
1
-RN are the same and accordingly the components of the first receiver R
1
only are shown.
The first receiver R
1
comprises a first bandpass filter
12
which is arranged to filter out signals which fall outside the 25 MHz bandwidth in which the available channels are located. The filtered output is input to a first low noise amplifier
14
which amplifies the received signals. The amplified signal is then passed through a second bandpass filter
16
which filters out any noise, such as harmonics or the like introduced by the first amplifier
14
. The output of the second bandpass filter
16
is connected to a mixer
18
which receives a second input from a local oscillator
20
. The frequency of the output of the local oscillator
20
will depend on the frequency of the channel allocated to the particular receiver. The output of the second bandpass filter
16
is mixed with the output of the local oscillator
20
to provide a signal at an intermediate frequency IF, which is less than the radio frequency at which the signals are received. The intermediate frequency IF output by the mixer
18
of each receiver will be the same for all receivers and may, for example, be 180 MHz. For example, if the channel allocated to a given receiver has a frequency of 880 MHz then the local oscillator
20
of that receiver will be tuned to 700 MHz. On the other hand, if the channel allocated to a given receiver has a frequency of 900 MHz, then the local oscillator will be tuned to a frequency of 720 MHz.
The output of the mixer
18
is input to a third bandpass fitter
22
which filters out any noise introduced by the mixer
18
. The output of the third bandpass filter
22
is amplified by a second amplifier
24
and output to a surface acoustic wave (SAW) filter
26
or another filter of an appropriate type. The surface acoustic wave filter
26
filters out all signals except that of the channel allocated to that particular receiver. In other words, all the channels received by the antenna
10
with the exception of the channel allocated to the receiver will be filtered out by the surface acoustic wave filter
26
. The output of the
30
surface acoustic wave filter
26
is connected to an automatic gain control unit
28
which alters the gain of the signal so that it falls within the dynamic range of an analogue to digital converter
30
.
One problem with the known architecture is that it is necessary to provide a receiver for each frequency.
With the known networks, the base transceiver station is required to receive signals from mobile stations
8
which are very close to the base transceiver station as well as from mobile stations
8
which are on the edge of a cell. Accordingly, the strength of the signals received by the base transceiver station will vary a great deal, depending on the distance between the mobile station and the base station. In this regard, reference is made to
FIG. 2
c
which shows the signal received from eight mobile stations, on eight different channels at the same time. As can be seen, the signal from the fourth mobile station MS
4
is very much stronger than the signal from the second mobile station MS
2
. The variation In the amplitude of the received signals gives rise to difficulties in the receiver.
If a single receiver were to be used with signals from more than one channel, amplifiers would have to amplify all of the signals received by the same amount at a given time including the signals with the larger amplitude and those of a smaller amplitude. The larger signals may therefore fall outside the dynamic range of the analogue to digital converter which may cause the analogue to digital converter to become saturated which lead to distortion. Typically, the distortion will take the form of intermodulation distortion which generates intermodulation product signals. This interference can interfere with signals received on other frequencies. If a lower amplification is used, this may result in the smaller signals being lost or swamped by background noise.
It is therefore an aim of some embodiments of the present invention to reduce or at least mitigate these problems.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a receiver for receiving a plurality of different signals at the same time, said receiver comprising means for identifying at least one strongest signal of said plurality of different signals; and means for attenuating said at least one strongest signal with respect to the other of said plurality of signals wherein the said attenuating means comprises hybrid means.
Thus, as the, at least one, strongest signal is identified and attenuated, the range of amplitudes of the signals which are provided for subsequent processing is reduced. This can avoid the problem of signals falling outside the dynamic range of, for example an analogue to digital converter.
Preferably, the attenuating means are arranged to allow signals which are not to be attenuated to pass therethrough substantially without change. This allows a single path to be provided for all signals with only the at least one strongest signal being attenuated, the other signals remaining substantially unchanged.
The plurality of different signals are preferably at different frequencies.
The hybrid means may be coupled to at least one tunable filter means, said at least one tunable filter means being tuned to a frequency of the signal to be attenuated. The tunable filter can take any suitable form and may be mechanical or electric. The frequency to which the at least one tunable filter is tuned may be controlled by the output of the identifying means. Preferably, at least one resistive load is connected to the hybrid means. Variation in the value of the resistive load may allow the degree of attenuation provided by the attenuation means to be controlled. The degree of attenuation provided may also be controlled by the tunable filter means.
The identifying means is preferably arrang
Afshar Kamran
Nokia Corporation
Squire Sanders & Dempsey L.L.P.
Tran Sinh
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