Two-Way MMDS Communications System

This invention relates generally to television signal transmission systems and specifically to so-called MMDS (microwave multipoint distribution systems).

MMDS distribution systems carry over-the-air television type signals in a restricted frequency range. The frequency range of the 6 MHz MMDS channels is 2150 MHz to 2686 MHz. A response signal arrangement is provided for in the channel frequency allocations whereby the MMDS receiving site may communicate back to the transmitting site. These systems are well known in the art and generally embody a pole-mounted block converter for downconverting the received analog MMDS channels to corresponding signal frequencies that the MMDS decoder is capable of processing. The downconverter includes a fixed frequency local oscillator operating at 2278 MHz that is generally in a crystal controlled phase locked loop (PLL) circuit arrangement for supplying a local oscillator output signal to a mixer that receives the input MMDS channels. The response signal is generated by a separate FM transmitter or may be carried over a separate telephone line. Ideally, the MMDS block converter oscillator is immune to environmental conditions to enable a highly stable 2278 MHz local oscillator output. In practice, the local oscillator output cannot be stabilized under the adverse environmental conditions in which it must operate. Even with a crystal controlled phase locked loop, and with the relatively reduced stability requirements of an analog signal system, difficulty is experienced with stability. Under these conditions, the oscillator is not sufficiently stable to permit generation of the response frequency, which is different from the MMDS signal frequency and must be precisely controlled. In the present analog signal environment, the art has failed to produce a simple, stable, low cost, two-way MMDS system that would permit the response signal to be radiated back to the transmission site via the receiver antenna. Further, in the rapidly forthcoming digital signal transmission systems with their much more stringent stability requirements, a need exists for a cost effective two-way MMDS communications system.

A principal object of the invention is to provide a novel two-way MMDS transmission system.

Another object of the invention is to provide a two-way MMDS system that is capable of operating with digital signals.

A further object of the invention is to provide a simple, low cost, stable two-way MMDS digital transmission system.

These and other objects and advantages of the invention will be apparent upon reading the following description in conjunction with the drawing, the single figure of which is a simplified block diagram of a two-way MMDS system constructed in accordance with the invention.

Referring to the drawing, an antenna 10 is coupled to a diplexer 12 which supplies a mixer 14 that also receives the output of a fixed frequency output local oscillator 16. The output of mixer 14 consists of a downconverted group of MMDS channel frequencies and is supplied to another diplexer 18, the output of which feeds a block 22 that includes an MMDS decoder and a data transmitter. The MMDS decoder in block 22 supplies a conventional television receiver 24 with the decoded MMDS channels. The transmitter in block 22 develops a reference that is BPSK (biphase shift key) modulated to form a data signal for transmission back to the transmitting site (not illustrated) in the form of a response signal of precise frequency. The data signal is coupled through diplexer 18 to a squelch circuit 26, the output of which is coupled to another mixer 28 which is also supplied with the output of fixed local oscillator 16 for converting the data signal to the frequency of the response signal. The response signal is supplied by the mixer 28, via a bandpass filter 30 and diplexer 12, to antenna 10 for radiation back to the transmitting site. The transmitter in block 22 generally includes a crystal oscillator to develop the stable reference which is coupled to a phase locked loop 20 to produce the actual control signal for fixed local oscillator 16.

As indicated on the drawing, the 6 MHz MMDS channel signals are in the frequency range of 2150 to 2686 MHz. The response frequency is indicated as 2686.0625 MHz, but as those skilled in the art will readily perceive, will differ depending upon the MMDS particular response frequency assigned to the decoder. The diplexer 12 is capable of passing the received MMDS channels to mixer 14 from antenna 10 and the converted data signal (response signal) from mixer 28 back to antenna 10. The circuits for performing these functions are well known in the art and require no further description here. Similarly, the structures of diplexer 18 and squelch circuit 26 are well known in the art as are PLL 20 and mixer 28.

The MMDS decoder in block 22 is also well known in the art. The crystal oscillator and BPSK transmitter of block 22 may conveniently consist of the oscillator and transmission module of a Z-view transmitter available from Zenith Electronics Corporation and fully described in U.S. Pat. Nos. 4,528,663; 4,553,161; 4,554,579 and 4,586,078. As indicated, the crystal oscillator produces a stable reference (carrier) of 408.625 MHz which is applied to PLL 20, and through diplexer 18, to squelch circuit 26. The data, in the form of biphase shift key modulation of the carrier, occurs for only a very short interval and consequently the phase modulation created does not result in "unlocking" of phase locked loop 20. At all times, except during modulation for the data signal, the level of the reference that is supplied to the phase locked loop 20 and to squelch circuit 26 is very low. While the level is high enough to control the PLL, it is too low to activate squelch circuit 26. Consequently, except during actual transmission of data, the squelch circuit 26 precludes the reference from being radiated from antenna 10. Yet, the reference signal is continuously available to stabilize phase locked loop 20 (and thereby control the fixed local oscillator 16).

In accordance with the invention, the MMDS decoder and the reference generator of block 22 are in a benign environment, i.e. located within a home or other dwelling where the temperature is controlled within fairly narrow limits and where the sensitive circuitry is shielded from the adverse effects of an outdoor environment. The remainder of the circuitry in the figure is mounted externally, generally as a part of the antenna structure, and is subject to environmental conditions that would seriously impair the stability of the local oscillator in prior art systems. The provision of the very stable reference within the benign MMDS decoder environment, overcomes the major stability problem associated with the prior art systems and also permits systems using the invention to operate with digital signals which, as is well known in the art, require very precise control of conversion frequencies.

While a specific frequency for the crystal oscillator is identified, there are systems in which a different frequency may be more convenient. For example, an 11 MHz oscillator is a viable alternative. In that event, those skilled in the art will readily perceive that the 11 MHz reference supplied from block 22 to PLL 20 would need to be multiplied to develop the 408.0625 MHz signal (or other signal dependent upon the particular response frequency desired) for mixing with the output of fixed local oscillator 16 in mixer 28. It will also be appreciated that power for the mixers and other MMDS components may be supplied by the decoder 22 along the cable via diplexer 18 as needed.

The result of the invention is a very compact environmentally stable two-way MMDS transmission system that is suitable for use with digital signals. It is recognized that numerous changes in the described embodiment of the invention will be apparent to those skilled in the art without departing from its true spirit. The invention is to be limited only as defined in the claims.