Multiplex communications – Wide area network – Packet switching
Patent
1990-10-30
1993-04-06
Kuntz, Curtis
Multiplex communications
Wide area network
Packet switching
370 75, 455 18, H04B 336
Patent
active
052009556
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
I. Field of the Invention
This invention relates to a mobile radio system, and in particular to a radio repeater (cell enhancer) for a digital time-division-multiple-access (TDMA) mobile radio system.
II. Prior Art and other Considerations
In a mobile radio system, there often exist areas of poor coverage within the service area of a base station, where poor signal reception by the base station from a mobile (and vice versa) results in unacceptable voice quality, or even the total loss of a call. One solution to this problem is the provision of a wideband repeater or re-radiator. This however suffers from certain disadvantages, viz (i) it would re-radiate all signals within the frequency range covered-including unwanted signals, for example from other radio systems which may be using the band; (ii) intermodulation and wideband noise; and (iii) the power level of each channel within the band would be amplified by an equal amount regardless of the actual signal strength.
A radio repeater is known (see the specification of our co-pending EP patent application no. 0274857) which utilizes a plurality of frequency-agile single-channel amplifier units for amplifying only traffic-carrying channels. This repeater is an analogue frequency-division-multiple-access (FDMA) system, and so is not suitable for digital systems such as the proposed pan-European GSM (Groupe Speciale Mobile) mobile radio system.
SUMMARY
The present invention provides a radio repeater for a frequency band having a plurality of RF channels for carrying a plurality of digital TDMA channels each having a sequence of time slots, comprising: a plurality of single RF-channel amplifier units; and control means for scanning the RF channels to identify an active RF channel and for assigning one of the amplifier units to an identified active channel, characterised in that each of the amplifier units has means for identifying an active time-slot of a TDMA channel, within an active RF channel, and for amplifying digital signals in that time slot.
Advantageously, each amplifier unit has, for duplex operation, forward and reverse amplifying paths, and the number of amplifier units is smaller than the number of the RF channels, the amplifier units being of selectable frequency, and the control means being arranged, in use, to select the frequency of each unit for assignment to an active channel.
In one preferred embodiment, each amplifier unit has, in each amplifying path, a first frequency changer for converting input signals down to an intermediate frequency, a band-pass filter, and a second frequency changer for converting the filtered signals up to the original frequency, the filter frequency being fixed, and the channel assignment of the unit being controllable by controlling the frequency changer local oscillator frequency. Conveniently, the intermediate frequencies for the forward and reverse paths of an amplifier unit differ by an amount corresponding to the transmit-receive frequency spacing of the speech channels, whereby a single local oscillator may be used for the unit. Preferably, the control means is common to all the amplifier units.
In another preferred embodiment, each amplifier unit has, in each amplifying path, a frequency changer for converting input signals to an intermediate frequency, a band-pass filter, a frequency discriminator for demodulating the digital signal, a regenerator for the recovered digital signal, a modulator driven by the regenerated digital signal and operating at the original frequency, the channel assignment of the unit being controllable by controlling the local oscillator frequencies supplied by the frequency changer and the modulator. The output of the modulator is amplified to produce the output of the amplifier unit.
Alternatively, the frequency changer may comprise two mixers both supplied with the input signal, the local oscillator for each mixer being at the same frequency but with a relative phase displacement of 90 degrees, with the local oscillator frequency being nominally the same as the
REFERENCES:
patent: 4152647 (1979-05-01), Gladden et al.
patent: 4490818 (1984-12-01), Otsuka
patent: 4713809 (1987-12-01), Mizota
Ball John R.
McFarlane David A.
British Telecommunications public limited company
Ghebretinsae T.
Kuntz Curtis
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