Patent Application
20050271125
The present invention pertains generally to communications systems. More particularly, the present invention pertains to wireless communications links between line-of-sight stations in a communications system.
Wireless communications links are well known in the prior art. An important advantage of wireless systems is that they do not require the laying or stringing of cables between stations. Two types of wireless systems that are finding many new applications are free space optical systems, which carry communications signals on light beams, and microwave systems, which carry communications signals on microwave beams in the 3 GHz to 30 GHz spectral range (wavelengths of 10 cm to 1 cm).
Laser data links are capable of handling high data transmission rates, which are in the range of several Gigabits per second (Gbps). Certain types of atmospheric conditions, however, can adversely affect laser data links. For instance, it can be shown that haze, fog, or heavy snow conditions will cause severe attenuation of the laser beam. This attenuation is due to scattering and when it happens, the laser data link becomes unreliable.
Microwave links have certain shortcomings different from those associated with laser links. Specifically, microwave data links generally have a lower data transmission rate than laser systems. Typically, rather than transmitting data at rates of Gbps, the data transmission rate for a microwave data link is less than a few hundred Megabits per second (Mbps). Further, microwave links tend to have large beam divergences, which can cause interference if multiple links are operating in the same geographical region. Thus, microwave frequencies used for communications are either allocated via expensive licenses or, where unlicensed, may be subject to interference from other users.
Recently, the Federal Communications Commission (FCC) in the United States has established rules that allow for commercial communication systems in a new part of the electromagnetic spectrum, i.e., from 71-76 GHz and 81-86 GHz. This allocated spectrum is part of the millimeter wave spectrum (30 GHz to 300 GHz frequency, 10 mm to 1 mm) and more specifically part of a spectral range designated as the “E-Band” (60-90 GHz). Use of these spectral ranges for communications provides some of the advantages associated with free space optics (laser communications) systems and microwave communications systems, while eliminating some of the disadvantages. Specifically, the available spectral ranges are large enough to accommodate high simultaneous send and receive data rates of 2.5 Gbps and even 10 Gbps like laser communications systems, but the transmission suffers much lower attenuation in haze, fog, or snow as compared to laser communication links. Furthermore, with antennas having diameters on the order of two feet, the beams become so narrow that interference between different links is not normally an issue. This has allowed the FCC to provide for a simplified and inexpensive site licensing procedure for communications systems in this band.
In light of the above, it is an object of the present invention to provide a wireless E-band millimeter wave communication link capable of send and receive data rates of 2.488 Gbps or higher that can operate within the FCC allocated spectrum from 71-76 GHz and 81-86 GHz. Another object of the present invention is to provide a wireless millimeter wave communications system that is available for effective data transfer between line-of-sight stations at high availability in a variety of atmospheric conditions. Another object of the present invention is to provide wireless millimeter wave communications links that are compact and space efficient by using frequency division duplexing, with a single antenna at each end of each link combining both transmit and receive functions. A further object of the present invention is to provide a wireless millimeter wave communications system that is relatively simple to manufacture, easy to install and use and comparatively cost effective.
The present invention provides a high data rate free space communication link operating at millimeter wave frequency ranges. Links include two transceivers, the first transceiver transmitting at a first frequency range and receiving at a second frequency range and a second transceiver transmitting at the second frequency range and receiving at the first frequency range. Each of the two transceivers has a primary tunable oscillator providing a basic frequency signal that is precisely the same for both transceivers. Preferably the primary tunable oscillator in one of the transceivers, the slave oscillator, is slaved to the primary tunable oscillator, the master oscillator, in the other transceiver and the two transceivers are locked in frequency and phase. Also preferably the master oscillator is frequency controlled to maintain a constant number of wavelengths in the millimeter wave radio beams between the two transceivers, at least for periods of time permitting substantial data transmission without change in the number of wavelengths. In both transceivers a center frequency is generated by frequency multiplication and mixing of harmonics of the basic frequency signal generated by the transceiver's primary tunable oscillator. Preferred embodiments are designed to operate at frequencies above 60 GHz. In a particular preferred embodiment the center frequency for the first transceiver is about 73.5 GHz and the center frequency for the second transceiver is about 83.3 GHz. Embodiments of the present invention include automatic transmit power control, (preferably about 20 db of it, permitting operation at about 1 percent to 100 percent of maximum transmit power) for assuring adequate signal transmission in a wide variety of atmospheric conditions but not excessive power that might interfere with other links at the same frequencies. The narrow beam widths of these transceivers at about 0.5 degrees using a two-foot diameter antenna and the above transmit power control permit a large number of these transceivers to operate in the same region using the same frequencies.
In this preferred embodiment the center frequency is modulated in a phase modulator, which imposes a 2.488 billion bits per second (Gbps) digital signal onto the center frequency through phase shift keying. The signal is amplified and filtered to restrict the resultant signal to be within the 71-76 GHz (or 81-86 GHz) allowed pass band. A diplexer in the first transceiver is designed to transmit signals in the 71-76 GHz band and receive signals in the 81-86 GHz band. At the other transceiver, the diplexer is designed to transmit signals in the 81-86 GHz band and receive signals in the 71-76 GHz band. After reception, the signal passes through a band pass filter (to discriminate against transmitted signals), a low noise amplifier and another band pass filter. The received signal is then mixed down to a lower frequency centered at 9.8 GHz (which is the difference between the transmitted and received frequency) and combined with a steady 9.8 GHz reference signal in order to demodulate the original data. The steady 9.8 GHz reference signal is kept in phase with the received data through the use of a phase locked loop. To eliminate ambiguity that would normally occur with regard to which phase corresponds to a digital zero and which phase corresponds to a digital one, the original data is encoded such that a transition in phase corresponds to a digital one, and no transition in phase (at the time when a transition could occur) corresponds to a digital zero.
Preferred embodiments include automatic transmit power control electronics that provide for continuous communication between transceivers of each transceiver's received power level. A digital processor in each transceiver monitors received signal strength. The received power level is communicated from one transceiver to the other using a separate (from the main data channel) communications channel. In preferred embodiments this separate communications channel is implemented by imposing a relatively slow amplitude modulation onto the transmitted signal, of only a few percent of the total transmitted power. This relatively low frequency modulation is used to form a continuous serial link between digital processors at each transceiver at 56 thousand bits per second, and is separate from the high-speed data that is transmitted over the link, which operates at 2.488 billion bits per second. Automatic gain control circuitry is used to sense this low-frequency modulation of the amplitude of the received signal to detect transmit power information imposed on the main signal at the other transceiver. In these preferred embodiments amplifiers, that are used to adjust the total transmitted power level for transmit power control, are also used to transmit digital information regarding received signal amplitude.
The data can be modulated onto the millimeter wave center frequency by a number of methods including on-off keying, simple amplitude modulation, higher order amplitude modulation, frequency modulation, and phase modulation. In one preferred embodiment of the present invention the data is modulated onto the carrier using dual phase shift keying (DPSK) whereby the phase of the carrier is varied between two settings that are 180 degrees out of phase. In another preferred embodiment, higher data rates, such as 10 Gbps, could be transmitted in the available bandwidth by using higher order phase modulation such as quaternary phase shift keying (QPSK) where the phase of the carrier varies between four settings that are 90 degrees apart.
Transceivers forming each end of the link may be mounted on the outside of and near the top of a building. The mounts holding the transceivers and the attachment locations on the building are typically chosen so as to minimize movement with changing weather conditions so that the transceivers will maintain pointing to each other within a fraction of their beam widths, which are typically less than a degree. Active tracking may also be provided. In alternate preferred embodiments, the transceivers could be mounted inside a building behind windows, on permanent towers, or on temporary towers erected for providing high bandwidth communications for a short period of time. For example, in one preferred embodiment the invention could be used to restore long haul telecommunications traffic between sections of a fiber that have been severed by a natural disaster, such as a bridge across a river collapsing during a flood. For example, four different wavelengths propagating through an optical fiber and carrying four separate date streams at 2.5 Gbps each could be separated out and sent to four transceivers mounted on temporary towers on one side of the river. The four data channels could then be sent across the river to opposite transceivers using the E-band millimeter wave communications. After reception on the other side of the river, the four different fiber wavelengths could be regenerated with the four data channels, and combined into the optical fiber continuing on that side of the river.
In accordance with the present invention, it is desirable to maintain high link availability (or uptime) at a maximum link range in varying weather conditions. The primary weather condition that affects link range for the E-band millimeter wave communications system is heavy rain. Within the United States, rain rate distributions have been partitioned into several different regions called Crane regions. Of primary commercial concern is the Crane region that includes the largest cities on the eastern seaboard such as Washington, New York and Boston, along with Chicago in the Midwest. In this Crane region, the preferred embodiment has a weather availability of 99.999% at a distance of 0.68 miles, which is far higher than any other wireless technology can provide at this distance at a 2.488 Gbps data rate. In other, drier regions, the operating distance at 99.999% availability is even further. Similarly, at lower availabilities such as 99.99% or 99.9%, the preferred embodiment will operate at significantly longer distances in all Crane regions.
A preferred antenna is a two-foot diameter parabolic dish antenna with a feed through the primary mirror and a small secondary mirror, and an electronics box mounted to the antenna. The circuitry that transmits and receives the modulated millimeter wave signals is contained within the electronics box. The transceiver connects to a user in the building on which it is mounted through a fiber optic cable, containing separate fibers for sending and receiving data. In addition, there is a power connection to provide power (for instance 110 V A.C.) to the transceiver. Within the electronics box there is both digital data forming and diagnostic circuitry, and microwave and millimeter wave circuitry.
Referring initially to
As shown in
Continuing in
Our description of
After leaving the Voltage Controlled Oscillator 60, the 4.9 GHz signal enters coupler 62, which splits the voltage onto two paths. On the upper path, the 4.9 GHz signal enters divider 64 (part #HMC434 manufactured by Hittite Microwave Corporation, Chelmsford, MA), which divides the frequency by 8 down to 612.5 MHz. This signal passes through filter 66 (part #SALF680 manufactured by Mini-Circuits Corporation, Brooklyn, N.Y.), and then into 3 dB splitter 68, where half of the output is used later in phase locking circuitry 130 (key component is phase detector part #HMC439QS16G manufactured by Hittite Microwave Corporation, Chelmsford, Mass.), and half continues on to frequency divider 78 (part #HMC432 manufactured by Hittite Microwave Corporation, Chelmsford, MA), which divides the frequency by 2 down to 306.25 MHz. Now looking at the lower path leaving coupler 62, the 4.9 GHz voltage signal enters coupler 70, where some of the voltage is split off to amplifier 72 (part #MNA-7 manufactured by Mini-Circuits Corporation, Brooklyn, N.Y.), and some is split off to amplifier 80 (part #MNA-7 manufactured by Mini-Circuits Corporation, Brooklyn, N.Y.). For now we continue with the signal from amplifier 80, which next enters mixer 84 (part #HMC498MS8G manufactured by Hittite Microwave Corporation, Chelmsford, Mass.), where it is mixed with the 306.25 MHz signal from the upper path. The output from mixer 84 is at a frequency of 4.9 GHz-306.25 MHz, or 4.59375 GHz. (The other output frequency of 4.9 GHz+306.25 MHz=5.20625 GHz would be used for an 81-86 GHz transmitting system). The 4.59375 GHz signal now passes through bandpass filter 86 and amplifier 88 (part #MNA-7 manufactured by Mini-Circuits Corporation, Brooklyn, N.Y.) and amplifier 90 (part #MNA-7 manufactured by Mini-Circuits Corporation, Brooklyn, N.Y.) before entering frequency doubler 92 (part #HMC368LP4 manufactured by Hittite Microwave Corporation, Chelmsford, Mass.), which generates an output frequency of 9.1875 GHz. This 9.1875 GHz frequency signal is filtered by filter 94 and amplified by amplifier 96 (part #HMC411LP3 manufactured by Hittite Microwave Corporation, Chelmsford, Mass.) before entering frequency doubler 98 (part #HMC283LM1 manufactured by Hittite Microwave Corporation, Chelmsford, Mass.), which generates an output frequency of 18.375 GHz.
After passing through bandpass filter 100, this 18.375 GHz voltage sine wave passes through 15 dB Coupler 102. About 3% of the signal enters Phase Lock Circuitry 130, and the other 97% enters frequency doubler 104 (part #HMC283LM1 manufactured by Hittite Microwave Corporation, Chelmsford, Mass.), which generates a frequency of 36.75 GHz at the output. This 36.75 GHz output signal then passes through bandpass filter 106 and enters coupler 108, which separates off some of the signal to be amplified by amplifier 138 (part #HMC283LM1, manufactured by Hittite Microwave Corporation, Chelmsford, Mass.) and used by the receiver circuitry starting at second harmonic mixer 140 (manufactured by Hittite Microwave Corporation, Chelmsford, Mass.). Saving that for later, we continue on from coupler 108 to amplifier 110 (manufactured by Hittite Microwave Corporation, Chelmsford, Mass.), whose 36.75 GHz output enters frequency doubler 112 (manufactured by Hittite Microwave Corporation, Chelmsford, Mass.) to generate a carrier frequency at 73.5 GHz. (If the circuit was for a transmitter in the 81-86 GHz band, the steps above would have led to frequencies of 10.4125 GHz at doubler 92, 20.825 GHz at doubler 98, 41.65 GHz at doubler 104, and 83.3 GHz at doubler 112, all as shown in
After the 73.5 GHz carrier frequency is generated by frequency doubler 112, it is amplified by amplifier 114 (made by Raytheon Corporation, Andover, Mass.) up to a power level of 10-50 mW. The power out of this amplifier can be varied for the purposes of changing the transceiver transmit power depending on weather conditions as will be discussed in more detail later.
After leaving amplifier 114, the 73.5 GHz carrier is modulated by Modulator 116 to impose a phase shift keyed data signal on it at a data rate of 2.488 Gbps. The nature of this modulation can be understood by reference to
In
Let us now move on to the receiving part of the transceiver in
As was mentioned previously, some of the power at a frequency of 36.75 GHz from the transmit carrier chain was split off by Coupler 108 and amplified by Amplifier 138. Both this 36.75 GHz reference signal and the amplified received signal at 83.3 GHz enter Second Harmonic Mixer 140 (manufactured by Hittite Microwave Corporation, Chelmsford, Mass.). The output of the Second Harmonic Mixer is centered at a frequency of 9.8 GHz, corresponding to 83.3 GHz minus 2×36.75 GHz. This output signal also contains the received data and is amplified by Amplifier 142 (manufactured by Hittite Microwave Corporation, Chelmsford, Mass.). The spectrum of the received signal after it has been down-converted in frequency can be seen in
After the down-converted signal at 9.8 GHz is amplified by Amplifier 142, it enters automatic gain control circuitry 132 (key components are part #'s HMC346LP3 and HMC441LP3, manufactured by Hittite Microwave Corporation, Chelmsford, Mass.). For the high data rate receive signal, this circuitry serves the purpose of signal voltage control, to set the data signal to a set voltage range, independent of the received signal strength at the antenna. (This circuitry also monitors the incoming signal and if it is too low or too high, the circuitry provides information for transmittal to transceiver 17, all as discussed in a section below entitled “Automatic Transmit Power Control”.)
After this signal voltage control step, the data signal enters Balanced Demodulator 134 (manufactured by Hittite Microwave Corporation, Chelmsford, Mass.), which extracts the NRZI-encoded data that was modulated onto the carrier by the transmitter at the other end of the link. It extracts this data by mixing the data signal with a reference signal that is kept in phase with the data signal. The reference signal is generated as part of the frequency multiplication chain of the transmitter, and originates from the same Voltage Controlled Oscillator 60. Part of this 4.9 GHz signal passes through Coupler 62 to Coupler 70, from which it passes to Amplifier 72. The frequency of this amplified signal at 4.9 GHz is then doubled to 9.8 GHz by Frequency Doubler 74 (part #HMC283 manufactured by Hittite Microwave Corporation, Chelmsford, Mass.). This 9.8 GHz reference signal is filtered by Band Pass Filter 76, and then enters Balanced Demodulator 134 for mixing with the down-converted received data signal. For this demodulation to work, it is critical that the reference signal at 9.8 GHz maintain a constant phase relationship with the down-converted receive data signal at 9.8 GHz. The result of mixing of the reference and data signals is shown in
As mentioned previously, it is critical that the reference signal maintain a constant phase relationship with the down-converted receive data signal. This phase locking function is accomplished by Phase Lock Circuitry 130 (
The output of the Phase Frequency Detector is sent to Voltage Controlled Oscillator 60, closing a feedback loop, which attempts to vary the frequency of VCO 60 so that no phase shift is detected relative to the incoming data signal in the Phase Frequency Detector. This phase locking has to work simultaneously in the transceivers at both ends of the link. For this purpose, oscillator 60B in one of the transceivers (in this case transceiver 17 transmitting at 81-86 GHz shown in
Since we are dealing with wavelengths of only about 3 mm and expect the transceiver will be mounted on buildings or towers that will move more than 3 mm with wind and temperature changes, preferred embodiments are designed to modify the frequency of the master oscillator (60B in transceiver 17,
Automatic transmit control circuitry in each transceiver keeps the output transmit power of each transceiver adjusted so that the received signal at each transceiver is within a desired range of about 100 nWatts. To do this each transceiver must keep the other transceiver informed of the strength of the signals being received so that the transmitted power of each transceiver can be appropriately adjusted for varying atmospheric conditions. This is accomplished utilizing gain control circuitry 132, processor 158 and amplifiers 114 and 118 and amplitude modulator 116A in transceivers 16 and 17 at both ends of a link. For example the automatic gain control circuitry 132 of transceiver 16 on building A (refer to
The operation of the gain control can be further understood by reference to
Referring to
The data rate is 2.488 Gbps (shown as 2.5 Gbps in
An important advantage of the present invention utilizing a two-foot (or greater) diameter antenna to keep the beam width less than 0.5 degrees is that multiple data links can be set up in close proximity to each other without causing interference between the different links. An example of how this attribute of the technology might be used in an emergency communications restoration application is shown in
While the particular millimeter wave communications link as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of constructions or design herein shown other than as described in the appended claims. For example, many other millimeter wave frequency ranges from as low as 30 GHz to over 100 GHz could be utilized using the concepts of the present invention. Demodulation could be accomplished at many frequencies other than about 9.8 GHz as described although doing the demodulation at less than 15 GHz has some definite advantages. Instead of using a single oscillator in each transceiver to generate the transmit frequency and the local oscillator frequency for the receiver demodulation, separate oscillators could be used for transmit and receive functions. For example, separate oscillators should preferably be used if one or more of the transceivers are on a moving platform such as an aircraft, because in that case it would probably not be practicable to control the frequencies such that the number of wavelengths along the path remains constant. A separate oscillator for generating the receiver demodulation frequency would allow for frequency and phase locking in that application. If the link range is long enough, the receiver signal may never be high enough to necessitate automatic transmit power control. Alternatively, if interference between different radio links is not an issue, the receive power can be controlled with a variable attenuator in front of the receive low-noise amplifier. If automatic transmit power control is implemented based on the strength of the receive signal at the opposite transceiver, the necessary control, information could be signaled by a methods other than the amplitude modulation described in the preferred embodiment. For example, extra bits cold be added to the high data rate digital communication signal, or a separate wireless or even a hard-wired telephone line could be used. At some sacrifice in signal to noise ratio, an alternate method could be used to demodulate the phase shift keying in the receive signal that would not involve a phase locked loop to generate a frequency locked local oscillator. For example, the receive signal could be mixed with itself after a set delay of an integer number of wavelength periods which is close to the bit period, achieving the demodulation and NRZI decoding functions in one step. Therefore, the scope of the present invention should be determined by the appended claims and their equivalents and not by the examples that have been given.
