Pulse or digital communications – Pulse position – frequency – or spacing modulation
Reexamination Certificate
1998-01-06
2001-04-17
Vo, Don N. (Department: 2631)
Pulse or digital communications
Pulse position, frequency, or spacing modulation
C329S313000, C332S112000
Reexamination Certificate
active
06219380
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to communication systems and is particularly applicable to IR and RF wireless communication systems.
BACKGROUND OF THE INVENTION
Recently, demand has increased for providing mobile communications and easily deployed network communications. Much attention has been focused on RF and IF wireless communications systems and techniques for providing such types of communications. However, the RF communication spectrum is rather crowded. Therefore, many developers have recently focused on providing communications using the IR spectrum.
The IR communications channel proposes a number of challenges for the developer. Most notably, the environment in which a typical IR wireless communications system is to be deployed, such as an office building, warehouse or other enclosed structure, is fraught with interference sources such as the sun and lamps (fluorescent, filament, inverter fluorescent, etc.). In addition, it is desirable for some IR wireless communications systems to reduce the amount of power dissipated in the wireless communicating terminals. For example, in the context of portable transceivers, it is desirable to reduce the amount of power dissipated in transmitting information to conserve battery power.
The prior art has proposed pulse-position modulated transceivers for both wireless and wired communications. See H. K. Lu, T. H. Taur, K. C. Chen, C. K. Wang & M. T. Shih,
Prototyping an Indoor High Speed Diffuse Infrared Transceiver for Wireless Data Communications,
SINGAPORE ICCS/'94, p. 338-342 (1994); M. Audeh, J. Kahn & J. Barry, Performance of Pulse-Position Modulation on Measured Non-Directed Indoor Intrared Channels, IEEE Trans. On Comm., vol. 44, no. 6, June, 1996, p. 654-659; M. Rittler, F. Gfeller, W. Hirt, D. Rogers & S. Gowda,
Circuit and System Challenges in IR Wireless Communication,
ISSCC96, Feb. 10, 1996, SP 25.1; K. Yamazaki,
On a New Detection Scheme of Optical PPM Signal,
PROC. INT'L SYMP. INFOR. THEORY (1995); and U.S. Pat. No. 4,584,720. In such systems, the data to be communicated is formulated into a non-return to zero (NRZ) signal. Such an NRZ signal is pulse position modulated onto a carrier signal to produce a pulse position modulated (PPM) signal. The PPM signal includes variably spaced pulses, the relative location of which depends on transitions of the NRZ signal from a low level to a high level or from a high level to a low level. In particular, the PPM signal is divided into fixed length groups having 2
i
pulse positions or “slots” per group. Depending on the corresponding signal level of the NRZ signal, a pulse is selectively inserted into a particular slot of the group. That is, a train of pulses is produced in selected slots of the PPM signal, whose phase depends upon the polarity of the NRZ signal at the corresponding time.
The PPM signal thus formed may be inputted to a light emitting diode and transmitted in electromagnetic form to a receiver. The receiver has a photo diode which receives the electromagnetic PPM signal and converts it to electrical (i.e., voltage/current) form. The receiver amplifies the PPM signal, and may use an automatic gain control (AGC) circuit to vary the gain of the amplifier. A demodulator demodulates the NRZ signal from the PPM signal. The demodulator may employ a phase locked loop and/or slot clock to synchronize with the received PPM signal. The recovered NRZ signal may then be further processed, e.g., error corrected, processed, etc.
It is desirable to provide a simple and expandable PPM transceiver with a simple PPM encoder and a robust receiver that quickly locks onto, i.e., synchronizes with, a received PPM signal.
SUMMARY OF THE INVENTION
This and other objects are achieved by the present invention. According to one embodiment, a transmitter is provided with a PPM encoder having a frequency divider, slot selector, and mixer. Such a transmitter illustratively can PPM encode an NRZ signal as follows. The frequency divider divides the frequency of a clock signal to which the data of the NRZ signal are aligned to produce a half frequency clock signal. The slot selector selects pulses of the clock signal and the half frequency clock signal depending on logic values of the NRZ signal and a control signal to produce first and second slot selected signal. The mixer mixes the first and second slot selected signals to produce a PPM signal of the NRZ signal. A haze eliminator illustratively may also be provided for eliminating haze in the PPM signal.
According to another embodiment, a receiver is provided with an automatic gain control circuit, including a variable gain amplifier, a hysteresis comparator, an event detector, a timer, and a counter. Such an AGC circuit can regulate the signal level of a PPM signal as follows. The variable gain amplifier amplifies the PPM signal using a dynamically adjusted gain that depends on an inputted digital control value. The counter increments the inputted digital control value according to a clock signal outputted from the timer to increase the gain. The hysteresis comparator detects a signal level of the amplified PPM signal. The event detector causes the counter to decrease the inputted digital control value if the signal level is outside of a predetermined signal level range.
According to another embodiment, the receiver has a timing recovery circuit for synchronizing to an incoming PPM signal, including a frequency track, a slot locked loop and a phase locked loop. Such a timing recovery circuit can lock synchronization to an incoming PPM signal as follows. The frequency track generates a coarse clock fx having a frequency that depends on a frequency of the PPM signal. The slot locked loop generates a resampling clock signal having a frequency that depends on the course clock fx and having slots locked to slots of the PPM signal. The phase locked loop locks a phase of the coarse clock fx to a phase of the PPM signal.
Illustratively, the frequency track includes a frequency detector for counting the number N of clock pulses of a references clock between rising edges of a PPM signal on which a sequence of logic ‘1’ data bits is modulated. The frequency track may also have a programmable counter for generating the coarse clock fx having a frequency that depends on the frequency of the PPM signal on which the sequence of logic ‘1’ data bits is modulated.
Illustratively, the slot locked loop has a PPM encoder, a slot detector and a slot selection control signal generator. The PPM encoder is for generating the resampling clock signal with pulses of the same duration as the coarse clock signal fx inserted into specific slots as determined by dynamically varying slot selection signals. The slot detector is for comparing pulses of the resampling clock signal to pulses of the PPM signal to determine a phase error between the resampling PPM signal and the PPM signal. The slot selection control signal generator is for varying the slot selection signals if the phase error exceeds 90°.
Illustratively, the phase locked loop has a phase detector and a counter. The phase detector determines a polarity of a phase error between the coarse clock fx and the PPM signal. The counter inserts or deletes a pulse into the coarse clock fx depending on a polarity of the phase error.
According to a further embodiment, a transceiver is provided including the PPM modulator for transmitting NRZ signals, the AGC circuit for amplifying received PPM signals and the timing recovery circuit for synchronizing to the amplified received PPM signals.
REFERENCES:
patent: 5563916 (1996-10-01), Scarpa
patent: 5914989 (1999-06-01), Thapar et al.
patent: 5917372 (1999-06-01), Kakura et al.
patent: 5917865 (1999-06-01), Kopmeiners et al.
Dan Ten-Long
Huang Kuang-Hu
Wang Chorng-Kuang
Wang Wei-Chen
Industrial Technology Research Institute
Proskauer Rose LLP
Vo Don N.
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