Signal transmission system and a transmitter and a receiver for use in the system

• US 4,768,187 A
• Filed: 06/30/1986
• Issued: 08/30/1988
• Est. Priority Date: 07/08/1985
• Status: Expired due to Term

• Current Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Original Assignee: US Philips Corporation (Koninklijke Philips N.V.)
• Inventors: Marshall, Christopher B.
• Primary Examiner(s): Olms, Douglas W.
• Assistant Examiner(s): Scutch, III, Frank M.
• Application Number:
  US06/880,399
• Time in Patent Office: 792 Days
• Field of Search: 370/69.1, 370/70, 370/120, 370/121, 370/20, 370/123, 375/39
• US Class Current: 370/343
• CPC Class Codes: H03C 1/00 Amplitude modulation H03C5/00, H03C7/00 take precedence
• CPC Class Codes: H03D 1/00 Demodulation of amplitude-modulated oscillations H03D5/00, H03D9/00, H03D11/00 take precedence
• CPC Class Codes: H04B 1/68 for wholly or partially suppressing the carrier or one side band
• CPC Class Codes: H04B 7/12 Frequency diversity
• CPC Class Codes: H04L 5/26 combined with the use of different frequencies

Abstract

A signal channel system which is particularly suitable for the employment of direct conversion receivers has a block of adjacent transmission frequency bands allocated to it. Each signal channel is split into two sub-channels or parts occupying mutually different frequency bands for transmission and the bands occupied by the sub-channels of the various channels are interleaved. In the receiver the local oscillator is tuned midway between the bands corresponding to the sub-channels of the wanted channel and the corresponding signals are recovered by bandpass filtering, phase-shifting, summing and differencing, demodulating and combining the frequency down-converted outputs of quadrature related mixers to which the received and the local oscillator signals are applied. Although d.c. block capacitors are used in the outputs of the mixers, these can have a low value because the notch formed in the transmission frequency characteristic does not affect the reception of the required signals, so that the receiver can settle rapidly at switch-on.

Claims

1. In a radio communication system comprising a plurality of transmitters transmitting signals to a plurality of receivers, the signals from respective transmitters being transmitted by modulating such signals on carrier waves having respective frequencies corresponding to respective message channels, each receiver receiving a signal in a message channel by direct conversion of the modulated signal in such channel to the frequency band of such signal, a method of operating of such system so as to separate the received signals from unwanted d.c. components produced by such direct conversion, such method comprising:

dividing the signal transmitted by each transmitter into complementary signals in respective frequency bands;

dividing the message channel of each transmitter into a pair of sub-channels in separate frequency bands symmetrically located above and below a carrier frequency assigned to such message channel, and modulating the respective complementary signals to translate them into the respective sub-channels;

assigning carrier frequencies to the respective transmitters in said system such that the sub-channels of any transmitter message channel are interleaved with a sub-channel of a message channel of least one of the other transmitter in said system;

tuning each receiver to the carrier frequency of a message channel to be received by such receiver;

each receiver directly recovering the complementary signals in the sub-channels of a received channel by converting the modulated signals in such sub-channels to the frequency bands of such complementary signals, and combining the frequency bands of the recovered complementary signals to derive a combined signal, the frequency band of such combined signal being separated from d.c.; and

band-pass filtering said combined signal to separate it from other signals in the received message channel, such filtering also blocking any d.c. component of said combined signal.

2. A method as claimed in claim 1, wherein the sub-channels of the message channel of any transmitter are interleaved with a plurality of sub-channels of message channels of other transmitters in said system, lower frequency sub-channels of the interleaved channels being in successive frequency bands below the carrier frequency of the message channel and upper frequency sub-channels of the interleaved channels being in successive frequency bands above the carrier frequency of the message channel.

3. A transmitter for use in a radio communication system operating as claimed in claim 1, comprising means for dividing an input signal into first and second complementary signals and means for translating said first and second complementary signals into first and second sub-channels, respectively, of a message channel corresponding to a carrier frequency of the transmitter;

said sub-channels occupying mutually different portions of said message channel frequency spectrum and which are situated symmetrically above and below the transmitter carrier frequency and separated therefrom by at least the frequency bandwidth of a sub-channel.

4. A transmitter as claimed in claim 3, wherein the frequency translating means comprise means for modulating said first and second complementary signals respectively onto first and second sub-carriers the frequencies of which are separated from the transmitter carrier frequency by half the sub-channel separation frequency, means for obtaining the sum and the difference of the modulated signals produced by the modulating means, means for producing a relative 90 degree phase-shift between the resulting sum and difference signals, means for frequency translating-up the relatively phase-shifted signals using quadrature related carrier signals, and means for combining the frequency translated signals.

5. A direct conversion receiver for use in a radio system operating as claimed in claim 1, comprising a pair of mixers for producing quadrature related output signals, a local oscillator for supplying to said mixers a signal having a frequency mid-way between the frequency bands occupied by the sub-channels of a selected channel, bandpass filtering means and phase-shifting means for filtering the output signals from the mixers to obtain signals lying within said frequency bands frequency-translated by said mixers and producing a relative 90 degree phase-shift between the outputs signals of said mixers, summing and differencing means for obtaining the sum of the difference between the filtered and relatively phase-shifted signals, respective demodulating means for the resulting sum and difference signals, and means for combining the output signals of said demodulating means to reconstitute the signals originally supplied to the selected channel.

6. A system for transmitting at least two signal channels comprising:

means for dividing each channel into first and second complementary sub-channels which occupy frequency bands symmetrical about a common subcarrier frequency;

means for adding signals in the sub-channels to form a first modulating signal;

means for subtracting signals in the sub-channels to form a second modulating signal;

means for phase shifting the second modulating signal from said means for substracting;

a carrier generator for generating first and second carrier signals of a different phase but identical frequency;

means for modulating said first carrier signal with the signal from said means for adding, and for modulating said second carrier signal with the signal from said means for phase sifting; and

means for combining the modulated first and second carriers from said means for modulating, whereby a single modulated carrier signal is produced containing said complementary sub-channels which are frequency symmetrical with respect to the frequency of said carrier signals.

8. The transmitter of claim 6 further comprising:

means for generating additional modulated carrier signals containing respective complementary sub-channels which are frequency symmetrical with respect to their respective carrier signals, said sub-channels having the same frequency offset with their respective carrier signals; and

,

means for interleaving said modulated carrier signals, whereby a plurality of interleaved sub-channels are produced.

7. A receiver for receiving a carrier frequency signal modulated with signals in first and second sub-channels which are frequency symmetrical with respect to said carrier frequency signal, said first sub-channel having a frequency position below said carrier signal frequency, said second sub-channel having a frequency position above said carrier frequency signal;

said receiver comprising;

a local oscillator producing first and second signals having the same frequency but different phase;

mixer means connected to receive said modulated carrier frequency signal having said first and second complementary sub-channels, and for receiving from said local oscillator said first and second signals having the same frequency, said mixer means producing first and second phase displaced output signals;

a signal adding circuit connected to said mixer means for adding the first and second phase displaced signals together to form a summation signal;

a signal substraction circuit for forming a difference signal from the first and second phase displaced signals produced by said mixer means;

demodulator means connected to said signal adding and subtracting circuits to receive said summation and difference signals and for forming therefrom first and second baseband signals containing information in said first and second sub-channels; and

a demultiplexer connected to receive said first and second baseband signals and form a single channel from said complementary sub-channels.