Pulse or digital communications – Transceivers
Reexamination Certificate
1997-07-18
2001-03-27
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transceivers
C375S225000, C375S296000, C375S297000, C375S346000
Reexamination Certificate
active
06208686
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to digital microwave radios, and in particular relates to methods and systems for stabilizing FM deviation or occupied bandwidth of the transmit output signal of digital FM microwave radios.
BACKGROUND OF THE INVENTION
Digital radio terminals have become particularly advantageous in a number of key types of communication. High frequency point to point communications are used by, among others, cellular operators, telecommunications operators, private network operators, governments, and large telecommunications operations.
While many modulation techniques are available for use on microwave digital radios, such as QPSK, QAM and so on, cost and other issues have militated in favor of the use of direct modulated oscillators, or direct modulated FSK systems.
Operators of such microwave digital radios are typically assigned to specific frequencies, or channels, for their communications. Each channel is characterized by a center frequency and a spectrum emission mask or template which permits a higher energy level at the center frequency and decreasing energy levels as the transmitted frequency diverges (in either direction) from the center frequency. The spectrum emission mask, sometimes referred to simply as the “mask,” is defined by the federal government, and transmissions outside the mask can interfere with transmissions on adjacent channels as well as resulting in serious adverse consequences to the system operator. Such interference with neighboring channels is referred to “stepping on” those channels.
Superficially, it would seem to be straightforward to avoid stepping on adjacent channels simply by setting the direct modulated FSK occupied bandwidth of the system. However, this has been proven not to be correct. Most importantly, it is now recognized that the transmit occupied bandwidth—and therefore radio performance—can vary significantly with temperature or frequency in a direct modulated FM system. While temperature varies relatively slowly, it can vary over a significant range. This can cause a carefully tuned output spectrum to exceed the mask.
The historical approach to compensate for temperature variations and avoid stepping on adjacent channels has been to reduce the maximum bandwidth; however, this has the unacceptable effect of reducing the FM demodulated signal-to-noise ratio. This reduction in the demodulated signal amplitude can result in significantly poorer performance for the radio network. A common approach to representing such degradation is to perform a conversion of the occupied bandwidth from the frequency domain to the time domain. Where multiple digital modulation levels are used, the result of the time conversion is a plurality of random time-variant waveforms of different levels which are generally arcuate and, plotted together, take the general shape of an eye. This is frequently referred to as “the eye”, and such terminology will be used from time to time hereinafter. A reduction in the demodulated signal amplitude—and the corresponding reduction in the occupied bandwidth—basically is depicted in the eye by the arcuate waveforms which form the eye becoming less arcuate (i.e., flatter) and moving closer to one another, such that the overall impression is that the eye opening becomes smaller. An enhancement in the demodulated signal amplitude—and the corresponding increase in occupied bandwidth—is depicted by the waveforms becoming more arcuate and moving further apart. This is commonly referred to as the eye becoming larger. A larger eye is generally more desirable.
Variations in frequency, even with constant temperature, can also lead to significant variation in occupied bandwidth. Thus, for tunable systems which can be operated at any of a wide range of frequencies, undesirable occupied bandwidth changes can result from changes in selected channel. For many operators of microwave radio systems, the frequency of operation is chosen on-site. Thus, the occupied bandwidth of the system must be readily configurable outside of the manufacturing facility, and must take into account the variations in occupied bandwidth which can result from changes in frequency at even a stable temperature.
As a result, there has been a long-felt need for a system which dynamically maintains optimized occupied bandwidth over a significant range of operating temperatures and frequencies.
SUMMARY OF THE INVENTION
The present invention provides a system and method for dynamically optimizing the system transmitted occupied bandwidth of a radio system and maintaining that energy at a level closely approximating the mask. More specifically, in a first embodiment of the invention, in a system comprising a transmitter and a receiver, the energy received from the transmitter at the receiver is monitored periodically. The monitoring cycle is matched to the reasonable period for meaningful variation in the occupied bandwidth. The measured FM demodulator output voltage level (the recovered eye) is then compared to acceptable voltage levels and, if the difference exceeds a predetermined threshold, a control packet is fed back to the transmitter. The control packet is then supplied to the microprocessor on the transmitter side, and the amplitude of the transmitter waveform is adjusted either upward or downward to optimize occupied bandwidth, which typically involves maximizing occupied bandwidth while remaining within the mask. It can thus be seen that this embodiment of the invention comprises a form of digital feedback from the remote receiver side of one of a pair of transceivers to send a control message to the transmitter portion of the other one of the pair of transceivers to modify the output. It will be appreciated that, within a network of such transceivers, each transmitter portion will receive individualized feedback transmitted by each associated receiver portion, with the feedback path including the remaining transmitter/receiver portions of the transceiver pair.
In an alternative embodiment, the need for feedback from the receiver portion of the pair is not required. Instead, a method of predictive adjustment is implemented. In this embodiment, a data table is established reflecting the correlation between temperature and/or frequency and occupied bandwidth for the output waveform. Then, by monitoring both frequency and temperature at the transmitter, rather than relying on the outboard unit of the transceiver pair, the appropriate correction can be looked up in the data table and the correction made locally. As with the first embodiment, the correction is provided to the microprocessor of the transmitter to cause appropriate correction of the occupied bandwidth.
The invention may be better appreciated from the following Figures, taken together with the accompanying Detailed Description of the Invention.
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Cite: 47CFR2.985, Part 2, Sec. 2.985, http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi.
Castagna Peter
Lee Richard
McDonald Patric
McMillen Mark
Read Dick
Chin Stephen
Innova Corporation
Liu Shuwang
Squire Sanders & Dempsey
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