Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A TRANSMISSION SYSTEM FOR A
MOBILE COMMUNICATIONS SIGNAL
The present invention relates to a transmission system and a distribution
system for
a mobile communications signal, and in particular to a radio frequency (RF)
modem.
Base stations of cellular mobile telecommunications systems include
transceivers which
provide an air interface for the transmission and reception of RF mobile
communications
signals. The transceivers of the base stations are intended to communicate
with mobile
stations, or handsets, which move within the range of the cell provided by the
base station.
As with all RF communications systems, communication with the mobile stations
can be
compromised by physical obstacles, such as buildings and geographic features,
which affect
transmission of the RF signals. Accordingly to improve or extend coverage, the
RF signals
may be intercepted by an antenna and then distributed by a coax cable, or
other linear
analogue transmission media, to a site where a number of mobile stations may
be located. RF
repeaters may also be used to receive the RF signal, filter the signal and
amplify it for
retransmission at a higher signal level, again using analogue filtering and
amplification
techniques. These transmission and repeating techniques can be used to
transport the mobile
signal beyond the cell range of the base station to another location where it
can be distributed
directly to stations or retransmitted to provide another coverage area for the
base station.
The above transmission and RF repeating techniques, however, all operate on
the
modulated RF signal produced by the air interface of the base stations,
thereby requiring
linear analogue transmission media, and it is desired to be able to utilise
simpler, more
efficient and more economical transmission media, or at least provide a useful
alternative.
In accordance with the present invention there is provided a transmission
system for
a mobile communications signal, inchtding:
a demodulator at a first location for receiving and demodulating a first RF
mobile
communications signal into a digital sisnal: and
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a modulator at a second location for modulating said digital signal into a
second RF
mobile communications signal and transmitting said second signal.
Advantageously, said transmission system may include digital signal
transmission
media for transmitting said digital signal. The digital signal transmission
media may include
a microwave link, an optical fibre link, or a telecommunications line for
transmitting digital
data.
The present invention also provides a transmission system, including:
a first modem at a first location including a first demodulator for receiving
and
demodulating RF mobile communications signals into digital signals and a first
modulator for
modulating digital signals into RF mobile communications signals; and
a second modem ac a second location including a second demodulator for
receiving and
demodulating RF mobile communications signals into digital signals and a
second modulator
for modulating digital signals into RF mobile communications signals,
wherein the output of the first demodulator is connected to the second
modulator, and
the output of the second demodulator is connected to the first modulator, by
digital signal
transmission media.
Advantageously, the interfaces may be connected respectively to RF antennas,
or
alternatively one of the interfaces may be connected to the transceiver of a
base station.
The present invention also provides a mobile communications signal modem,
including:
a demodulator for receiving and demodulating RF mobile communications signals
into
serial digital data signals for transmission on a digital communications link;
and
a modulator for modulating serial digital data signals received on a digital
communications link into RF mobile communications signals.
A preferred embodiment of the present invention is hereinafter described, by
way of
example only, with reference w the accompanying drawings, wherein:
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Figure 1 is a block diagram of a preferred embodiment of a transmission
system;
Figure 2 is a block diagram of air interface modems of the transmission
system;
Figure 3 is a block diagram of a remote location retransmission system using
the
modems; and
Figure 4 is a block diagram of a multi-droplsimulcast system using the modems.
A transmission or repeater system 2, as shown in Figure 1, includes two GSM RF
modems 4 and 6 which are connected by a serial 270 kbitls duplex data link 8.
The modems
4 and 6 are each able to modulate and demodulate GSM 900/1800 megahertz RF
mobile
communications signals transmitted and received on antennas 10 and 12
respectively
connected to the modems 4 and 6.
The first modem 4 includes a first RF receiver danodulator and regenerator 14
which
is connected to the antenna IO to demodulate the downlink RF signal
transmitted by a GSM
base station. The downlink signal is a 935-960/1805-1880 megahertz, Gaussian
minimum shift
keying (GMSK) modulated, RF signal. The receiver 14 demodulates this into a
standard RS
422 data signal, or other data line protocol signal, having a rate of 270.833
kbitls. No
equalisation or error correction is applied by the receiver 14 to the
communications signal,
and non-coherent FM demodulation is employed for simplicity and cost
reduction. The data
stream produced by the receiver is carried by the link 8 to a second RF
transmitter modulator
18 of the second modem 6. The link 8 may be provided by a variety of digital
transmission
media, as discussed below. The second transmitter modulator 18 receives the
data stream
from the link 8, conditions the data stream and modulates the data on the
stream to produce
a GMSK modulated signal in the 935-960/1805-1880 megahertz band. The RF signal
is
amplified, filtered and fed to the antenna 12 for retransmission.
For the return path for uplink communications to the base station, the second
modem
6 has a second RF receiver demodulator regenerator 20, and the first modem 4
has a first RF
transmitter modulator 16, which are the same as the first receiver 14 and
second modulator
18, respectively, except the former operate at an RF frequency of 890-915!1710-
1785
megahertz for the uplink band. The modulators 16 and 18 produce an RF GSM
sienal which
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is in essence the same as the one received by the receiver 14 or 20 of the
other modem. Yet
the signals are delayed by the propagation delay of the system 2 and may have
a different
amplitude for the retransmitted signal of the second modulator 18, and will
normally have a
different amplitude for the first modulator 16 on the upiink path. The
amplitude of the
retransmitted signal on the uplink path is independent of the power of a
mobile handset or
station and any path loss between the handset and the second receiver 20. The
first modem
4 may be directly connected to the transceiver of a base station. Both of the
modems 4 and
6 provide a GSM air interface and a serial data interface.
The receiver 14, 20 of the modems 4 and 6, as shown in Figure 2, is
essentially a
superheterodyne receiver 22 which includes a data slicer to generate the
serial data signals.
The receiver 22 first filters and amplifies the incoming RF signal, and then
mixes this signal
with a local oscillator signal generated by a frequency synthesiser 24. The
frequency is
selected depending on whether it is operating in the downlink or uplink mode.
This produces
an intermediate frequency (IF) signal which is filtered by a surface acoustic
wave (SAV1~ filter
and then passed to a second mixer, where it is mixed by a crystal generated
local oscillator
signal of the receiver 22 to produce a second IF signal. The second IF signal
is filtered,
amplified and limited, and then passed to a quadrature demodulator. The signal
output of the
quadrature demodulator is an analogue baseband signal representing the
original Gaussian
filtered data contained in the RF signal. The voltage level of the analogue
signal is level
compared by the data sliver to generate high and low signals providing the
digital data stream
of 270.833 kbit/s in NRZ TTL format. To enhance the transmission distance of
the NRZ TTL
data, driverslconverters 26 are used to convert the data to a balanced
transmission format
which conforms to the Electrical Industrial Association (EIA) Standard EIA-422-
A. The
drivers 26 are capable of being set in a power down mode, which enables
multiple drivers to
be connected on the same line 28 of the data link 8 and operated at different
times without
affecting each other.
A modulator 16, 18 of the modems 4 and 6 includes a convenor 30, baseband
logic
32, a Gaussian filter 34, a quadrature modulator 36 and a clock recovery
circuit 38. The
convertor 30 receives the RS 422 data signals on the data link 8 and passes
them to the
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baseband logic circuit 32 in NRZ TTL format. The clock recovery circuit 38
includes a phase
locked loop (PLL) circuit which compares the phase of the NRZ format data with
that of a
local crystal oscillator. If the two signals have a phase error between them,
then the error is
used to cause a small shift in the frequency of the crystal oscillator until
the phases match.
S The oscillator is used to provide a continuous clock signal which is in-
phase with the
incoming data and can be used to clock the data at a precise rate so it can be
split into two
serial data streams by the baseband logic 32.
The baseband logic circuit 32 uses the recovered clock signal from the clock
recovery
circuit 38 and the incoming data from the convener 30 to generate serial in-
phase (I) and
quadrature (Q) data streams of 135 kbit/s for the quadrature modulator 36. The
logic circuit
32 conditions the two output streams so the correct rotation of the phase
vector out of the
quadrature modulator 36 occurs with the appropriate baseband data input. For
an incoming
high signal, the logic circuit 32 produces data on the I and Q lines to
generate a positive 90°
phase shift on the output of the quadramre modulator 36, whereas for a low
signal, the logic
circuit 32 produces data on the I and Q lines to generate a negative
90° phase shift.
The I and Q data streams output by the logic circuit 32 are filtered by the
Gaussian
filter 34 before being fed to the modulator 36. This is required to limit the
bandwidth of the
RF spectrum after modulation. The filtering requirements for GSM are specified
as being a
Gaussian filter with a bandwidth to bit ratio of 0.3, ultimately resulting in
0.3 Gaussian
minimum shift keying (GMSK). The bandwidth of the filter 34 is set at 81.25
kilohertz. The
filter 34 uses switch capacitive filters to generate a fourth order Gaussian
low pass filter
response.
The frequency synthesiser 24 is used to generate an RF signal for up
converting the
modulated signal back to a channe: in the GSM downlink or uplink band, and for
providing
an RF signal for the first mixers of the FM receivers 22 for channel
selection, as described
previously. For each frequency required for the RF signal, the synthesiser 24
includes a low
phase noise PLL, a high stability reference oscillator, and a low noise
voltage controller
oscillator (VCO). On the basis of channel frequency data loaded into the PLL,
the PLL drives
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the VCO to generate the required RF signal. The PLL uses the reference
oscillator to provide
a reference frequency. The channel frequency to be generated is loaded into
each PLL via a
serial data interface.
The quadrature modulator 36 includes two mixers and a phase shifter followed
by a
combiner. The mixers respectively mix the input I and Q streams with the
carrier signal from
the frequency synthesiser 24. The phase shifter imparts a phase shift on the
carrier signal
mixed with the Q stream based on the data on the I and Q data streams, and the
combiner
combines the twu signals to produce the 0.3 GMSK modulated RF signal, normally
generated
by the transceiver of a GSM base station. The output of the quadrature
modulator 36 is passed
to a power amplifier 40 for the downlink path in order to increase the power
of the signs! for
the retransmit antenna 12. Depending on the coverage area required, the output
power may
vary from tens of milliwatts to tens of watts. An RF duplexer 42 can be used
to enable both
the receive and transmit streams for a modem 4 and 6 to use one antenna 12 for
the air
interface.
The transmission system 2 is particularly advantageous as it can be used for
repeater
applications at a physical location where it is required to receive,
regenerate and retransmit
the GSM RF signal. The moderns 4 and 6 can also advantageously be placed at
different
locations on opposite sides of a physical obstacle with the data link being
used to effectively
transport the communications signal past the obstacle. The obstacle may be a
building or a
geographic feature, such as a mountain.
The modems 4 and 6 can also be used for digital transmission of mobile
communications signals to a remote location from a base station 50, as shown
in Figure 3. A
variety of different digital transmission media can be used to provide the
data link 8 to
transmit the 270 kbit/s data signal to a remote location, where it can then be
retransmitted.
if desired, using the second modem 6. The digital link may be provided by a
microwave link,
an optical fibre link, or a link provided by a standard twisted copper pair,
normally used to
transmit POTS signals. The microwave link may advantageously be used to pass
the
communications sienals to different buildings in a central business district.
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The first modem 4 may also be used to pass the communications signals to a
number
of second modems 6, which are located in different locations. The downlink
signals on the
data link 8 are simply replicated to different locations, by broadcasting or
multicasting using
multi-drop transmission. The uplink signals share a common data path to the
first modem 4,
and accordingly gating is required to allow multiple accesses for the received
GSM TDMA
data. The gating is executed based on a receive signal strength indicator
(RSSI) determined
by each receiver 22 of the second modems 6. The receivers 22 use the RSSI, and
any other
applicable information the receivers may derive, to determine if a valid
transmission has been
received from a handset, and thereby switch on and off the RS 422 driver 26
connected to the
l0 receiver, accordingly.
Many modifications will be apparent to those skilled in the art without
departing from
the scope of the present invention as herein described. For example, the
present invention is
applicable to any mobile communications system which employs digital
modulation
techniques, such as GSM (Group Special Mobile), DECT (Digital European
Cordless
Telephone). IS54 NADC (North American Digital Cellular) (or DAMPS (Digital
Advanced
Mobile Phone Svstem)), JDC (Japanese Digital Cellular), CT2 (Cordless
Telephone 2) and
digital paging systems such as that established by POCSAG (Post Office
Standard Advisory
Group) .
