High-speed point-to-point modem-less microwave radio frequency link using direct frequency modulation

ABSTRACT

A point-to-point microwave radio link that operates in a Frequency Division Duplex (FDD) mode using direct digital modulation with a Continuous Phase-Frequency Shift Keyed (CP-FSK) scheme. The transmit signal is generated by a circuit that uses a Voltage-Control Oscillator (VCO) operating in a microwave radio band. The VCO is deviated over a narrow frequency range that is reduced by a predetermined factor. The output of the VCO is then frequency multiplied by the predetermined factor to produce the modulated microwave output signal at the desired band. The deviation frequency of the VCO is thus chosen to be the reciprocal of the multiplication factor times the transport bit rate.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1 is a block diagram of a point-to-point wireless communications system that may make use of a direct conversion transmitter and receiver according to the invention. The system includes at least a pair of optical-to-microwave link interfaces , . A first optical-to-microwave link interface may be located, for example, at a central location such as a Network Access Point (NAP) that provides connections to a data network. In the illustrated example, the network connection is provided from an optical fiber that carriers a transport signal modulated in accordance with the OC-3 standard signaling format. The OC-3 optical signal carries an information signal having a data rate of 155.22 Megabits per second (Mbps). A similar optical-to-microwave converter unit is located at another remote location, such as a Network Termination Point (NTP). The unit also provides connectivity to a similar OC-3 optical transport connection. The units , may, for example, be located on the roofs of buildings in a campus environment to which it is desired to provide high-speed network connections between buildings.

In any event, both units and each have a transmitter and receiver . The transmitter and receivers operate in a Frequency Division Duplex (FDD) mode, such that transmitter-receiver pairs operate on distinct carrier frequencies. For example, in a downlink direction from unit towards unit , the transmitter in unit operates on the same microwave carrier frequency to which the receiver in unit is tuned. Likewise, the receiver in unit is tuned to the microwave carrier which the transmitter in unit operates.

Acceptable operating frequencies for the uplink and downlink may be in an unlicensed microwave band. For example, in the United States, appropriate unlicensed microwave radio bands occur in the various regions of the 40 to 320 GHz band.

It should be understood that the units and may be deployed at any short haul point-to-point locations, such that the specific locations are in effect network peers. It should also be understood that the invention may be used to carry data traffic between different types of locations and different types of network traffic.

Turning attention now to FIG. 2, an exemplary transmitter will be described in greater detail. The transmitter includes an optical to voltage transducer , a baseband filter , a direct modulator , a multiplier , a bandpass filter , a buffer amplifier , an output waveguide filter , and a transmit antenna . Optionally, a second-stage bandpass filter and multiplier may be utilized. The illustrated implementation is for a Continuous Phase-Frequency Shift Keyed (CP-FSK) implementation. As will be understood shortly, the signal radiated by the transmitter has a continuous phase and employs frequency modulation in order to communicate information to the receiver .

In operation, the input OC-3 formatted optical signal is fed to the optical to voltage transducer . The transducer produces at its output a raw transport bitstream. For an input optical signal of the OC-3 format, the transport bitstream is a digital signal at a 155.22 Mbps rate. The raw transport bitstream is then fed to a lowpass filter to remove any artifacts of the optical to voltage conversion process. It should be understood that other digital input signal types may be supported, such as OC-1, OC-12 or other optical range transport signals.

The modulator is preferably a Voltage Controlled Oscillator (VCO) of the Dielectric Resonator Oscillator (DRO) type. The modulator implements continuous phase Frequency Shift Key (FSK) type modulation shifting to, for example, a lower frequency to indicate a zero data bit and to a higher frequency to indicate a one data bit. The oscillator is implemented such that it preserves a continuous phase during the frequency shifts. The continuous phase nature of the oscillator further relaxes the requirements on the following filters , and buffer amplifier .

After being converted to a voltage from the optical carrier, the input baseband signal is directly fed to the control input of the VCO . The VCO provides a sub-deviated microwave carrier at its output, which shifts in frequency according to the logic state of the input signal. In the preferred embodiment, this deviation is set, however, to a relatively narrow range. For example, given an OC-3 input signal and a desired output signal in the range of 48-52 GHz, the deviation may be approximately over a range of only 38 MHz, in a carrier signal in the range of 10-13 GHz.

The sub-deviation amount is determined by the multiplication factor implemented by the following multiplier . In the illustrated embodiment, the multiplier implements a times four multiplication of the VCO output. In accordance with well-known communication theory, the spacing between the deviation frequencies in FM signals is dictated by the desired data rate. Thus, the ultimately transmitted signal must have a deviation of the desired 155.22 Mbps rate. However, the oscillators used in the VCO are not particularly narrow band or stable at such high operating ranges in the 40 GHz and above range. Thus, the approach here is to use a more stable VCO source at a lower range, such as in the 10-13 GHz range, and then to rely upon the multiplier to shift the VCO output up to the desired operating band.

The amount of sub-deviation is thus dictated by the specific multiplication factor implemented by the multiplier . In the case illustrated, where the desired output deviation is 155.52 MHz, the input deviation implemented by the VCO may be one-fourth of that or approximately 38.88 MHz. The output of the multiplier is thus a frequency-deviated signal carrying the digital information by the microwave frequency carrier in the desired unlicensed band. In the illustrated embodiment (number 1), this carrier is 50.000 Ghz, meaning that the VCO is centered at 1.25 GHz.

This raw microwave signal is then fed to the first-stage bandpass filter to remove artifacts of the direct modulation process. Unlike heterodyne receivers, no sidebands are created. Artifacts of the direct modulation process occur only at multiples of the 12.5 GHz VCO carrier and not at image frequencies of 155.52 MHz. No RF sidebands are generated. Thus, the first-stage bandpass filter need only remove the 12.5 GHz harmonics on either side of the 50 GHz carrier frequency. It therefore need not be a particularly sharp roll off filter.

A medium range buffer amplifier then receives the filtered signal and forwards it to an output waveguide filter .

The waveguide filter farther reduces the harmonics of the 10 GHz oscillator . It need not be an image-reject filter. Such image-reject filters, if they were needed, would further increase the cost. Elimination of the heterodyne stages, while not providing as bandwidth efficient an approach, does produce a less expensive radio.

Optionally, a second-stage multiplier and a bandpass filter may be included for operation at higher frequencies, such as in the 81 to 87 GHz band. In example 2, the microwave carrier is 856 GHz, generated from a 10.625 GHz VCO. In this instance, the input deviation may be even smaller, since the multiplication factor is times eight. Thus, the input deviation for the OC-3 signal may be in this instance in the range of only 9.72 MHz. The point is, as before, to make the deviation implemented by the VCO to be the reciprocal of the overall multiplication factor in the RF chain multiplied by the designed data rate. This ensures that after the frequency multiplication stages , , the carrier bandwidth is consistent with the data rate of the input signal.

Turning attention now to FIG. 3, an exemplary receiver will be described in greater detail. This receiver includes a receiving antenna , input waveguide filter , low-noise amplifier , bandpass filter , local reference generator , mixer , buffer amplifier , a pair of bandpass filters , , and associated detectors and , a differential amplifier and voltage-to-optical transducer .

The input signal provided to the receiving antenna is fed to the waveguide filter . This filter, having a center frequency in the 50 or 85 GHz range, as the case may be, filters the desired signal from the surrounding background information.

The low-noise amplifier may be implemented as a Monolithic Microwave Integrated Circuit (MMIC) feeding a planar bandpass filter in the 50 or 85 GHz range. The low noise amplifier typically has a 6-8 decibel (dB) noise figure and provides 10-20 dB of gain. The secondary filter may be implemented as needed prior to the down-converter mixer stage .

The local oscillator reference generator consists of a 12.5 GHz or 10.375 GHz oscillator , frequency multiplier and bandpass filter . The arrangement chain of components is identical to that used in the transmitter, namely the modulator , multiplier , and bandpass filter .

The down-converter uses a single mixer that provides the baseband information to a buffer amplifier . Thus, the resulting signal is the basic raw 155.52 MHz information modulated onto the microwave carrier output. For example, a logical bit one may be indicated by a 2.077 GHz frequency, namely 2 GHz plus one-half of 155.52 MHz and the logical one information may be associated with 1.923 GHz. Thus, the pair of bandpass filters and are tuned respectively to receive the frequencies indicating a data bit of zero or data bit of one, respectively.

The detector diodes and provide an output indication when energy is present in the output of the respective bandpass filters or . These detected signals are then fed to the differential amplifier to provide a resulting digital signal. This is then fed to the voltage-to-optical transducer to reconstruct the OC-3 format optical transport signal. The center frequencies of the two filters and differ by 155.52 MHz.

Down-conversion directly to the relatively high IF of 2 GHz provides for a simpler discriminator implementation, i.e., the respective bandpass filters may be at a microwave frequency rather than at baseband. This results from the fact that the resulting local oscillator signal fed to the down-convertor mixer is offset from the RF carrier by 2 GHz, and ensures that it is easier to reject images in the bandpass filters and .

The invention, therefore, provides for direct modulation of the input bitstream utilizing Continuous Phase Frequency Shift Keyed. No manipulation of the bitstream is required such as in the case of baseband modulation. Furthermore, because of the direct up-conversion to the desired microwave frequency carrier, multiple heterodyne stages are eliminated. Heterodyne stages, while providing for efficient filtering topologies, create interference and spurious noise problems, as well as increased cost in overall implementation.

By modulating the carrier source, such as provided by a voltage-control oscillator at a deviation frequency less than the desired baud rate by a factor of 1/n, with n being the multiplication factor in the up-conversion chain, the overall design is greatly simplified. Standard microwave component building blocks can be used in a highly-producible assembly as a result.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a point-to-point, optical to microwave link according to the invention.

FIG. 2 is a detailed circuit diagram of a Continuous Phase-Frequency Shift Keyed (CP-FSK) transmitter used in the link.

FIG. 3 is a detailed circuit diagram of a CP-FSK receiver.

CLAIMS

1. An apparatus comprising: a transport to microwave radio frequency adapter that accepts an input telecommunications transport signal on an input port and converts information in such signal to a desired microwave Radio Frequency (RD carrier, the input transport signal carrying information at an input bit rate, wherein the transport to microwave RF adapter further comprises: a voltage-control oscillator, coupled to receive the input transport signal, the voltage-control oscillator implementing a continuous phase frequency shift keyed deviation such that a first frequency is selected to indicate a first logical value for an input data bit in the transport signal and a second frequency is selected to indicate a second logical value for an input data bit in the transport signal, the deviation between the two frequencies selected to be equal to a predetermined fraction of the input bit rate; a first frequency multiplier connected to receive the output of the voltage-controlled oscillator and to multiply the output of the voltage controlled oscillator to the desired microwave RF carrier; and a microwave RF to transport adapter, to convert a received microwave RF signal to a transport signal carrying an output telecommunications transport signal, wherein the microwave RF to transport adapter further comprises: an oscillator, operating at a carrier frequency which is a predetermined fraction of a desired direct down-conversion frequency; a second frequency multiplier, connected to receive the oscillator output, and to multiply the oscillator output up to the desired direct down-conversion frequency; and a mixer, coupled to the frequency multiplier and the microwave RF signal, to provide a down-converted transport signal; and a pair of bandpass filters, a first bandpass filter tuned to a frequency which is equal to the down-conversion frequency plus one-half a data rate of the down-converted transport signal, and a second bandpass filter tuned to a frequency which is equal to the down-conversion frequency minus one-half the data rate of the down-converted transport signal.

2. An apparatus as in claim 1 wherein the telecommunications transport signal is provided on an optical physical medium.

3. An apparatus as in claim 2 additionally comprising: an optical-to-voltage transducer connected to receive the telecommunications signal and to provide a baseband electrical signal at an output.

4. An apparatus as in claim 1 wherein the first frequency multiplier implements a multiplication factor which is a reciprocal of the predetermined fraction used as the deviation in the voltage-controlled oscillator.

5. An apparatus as in claim 4 wherein the first frequency multiplier is implemented in a plurality of frequency multiplication stages.

6. An apparatus as in claim 1 wherein the voltage-controlled oscillator and first frequency multiplier perform a direct conversion of the input transport signal to the microwave RF carrier.

7. An apparatus as in claim 6 wherein the direct conversion is performed without using the input transport signal to modulate an intermediate carrier signal.

8. An apparatus as in claim 1 additionally comprising: a microwave bandpass filter connected to the output of the first frequency multiplier to filter harmonics of the carrier frequency of the voltage-controlled oscillator.

9. An apparatus as in claim 1 additionally comprising: a pair of detector diodes, each diode connected to a respective one of the bandpass filters, and to each provide a detected signal.

10. An apparatus as in claim 9 additionally comprising; a differential amplifier, connected to receive the two detected signals, and to provide the output transport signal.

11. An apparatus as in claim 10 additionally comprising: an electrical-to-optical transducer, coupled to the differential amplifier output, to provide an optical transport signal.

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