Pulse or digital communications – Pulse position – frequency – or spacing modulation
Reexamination Certificate
1998-03-17
2001-03-06
Pham, Chi H. (Department: 2631)
Pulse or digital communications
Pulse position, frequency, or spacing modulation
C329S312000
Reexamination Certificate
active
06198766
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for improving the communication reliability in high speed optical communication where pulse position modulation (PPM) is used. This has been found to yield particular benefits in the Infrared region of the spectrum now used for wireless communication between data processing devices.
BACKGROUND OF THE INVENTION
Optical communication in the Infrared region of the spectrum has been rapidly gaining ground as an important technique for communicating between data processing devices such as personal computers, printers, and other devices, for instance, in local area networks (LANs). Typically infrared optical transceivers connected to each processing device are used to send and receive data between the data processing devices. Because of the high speed that is desirable for communication between data processing devices, a 4 Mbps (Megabits per second) infrared communications standard using pulse position modulation (PPM) has been proposed by the IRDA Association. This standard, which is described in the Infrared Data Association Serial Infrared Physical Layer Link Specification which is available from the Infrared Data Association, requires very low noise and tight positioning of the pulses within transmissions. In communication between devices that are different distances apart, the portion of the optical transceivers used for receiving the infrared transmissions must be able to effectively handle different levels of signal strength, noise levels, and variation in pulse length or positioning and produce reliably consistent signal outputs. In the typical hardware configuration used to handle infrared transmissions at the present time, the output of the receiving portion of a transceiver is fed to a signal demodulator to decode the data contained within received transmissions. This data is intended to be fed to the data processing device to which it is to be connected.
It has been found that the signal pulses produced from transceiver outputs varied significantly for 4 Mbps PPM data streams. For lower speed communication, eg. under the 1.1 Mbps IRDA standard adopted previously, the effect of this variation has not been as significant due to the greater amount of forgiveness tolerated in the pulse positioning of IR pulses under that earlier standard. For these lower speed transmissions the critical factor was the minimum pulse width required for sufficient noise rejection. The 4 Mbps PPM data streams, on the other hand have very tight positioning requirements and very low noise allowances.
For the IRDA 4 Mbps PPM standard the pulses in a data stream are specified required to be either 125 ns. Iong or 250 ns. long. The data stream is composed of a number of transmission packets each of which consists of a preamble containing 125 ns pulses and a data portion of 125 or 250 ns pulses. It is important that a device used to process these pulses can distinguish between these two different pulse sizes. Unfortunately we have found that a 125 ns. optical pulse processed by a receiver or receiver portion of a transceiver may produce an electrical output pulse ranging from 20 ns. to 200 ns. in duration and that a 250 ns. optical pulse can produce an electrical pulse from 140 to 320 ns. In a circuit arrangement that we have employed the output of the receiver is decoded by a demodulator to determine the data received. It can be appreciated that there can be difficulties in data recognition if one or more single pulses are received and processed to yield a long pulse or other unanticipated output to be processed by the demodulator. The demodulator may yield an erroneous result.
We have also found that the pulse widths and noise produced by transceiver outputs have varied in response to the following factors:
distance from the transmitter of the optical pulses; ambient noise, variations in the release batches of a transceiver by a manufacturer, or different transceivers by different manufacturers.
It has been found that some transceivers produce output pulses that grow longer when the distance between communicating transceivers is increased, while other transceivers may produce shorter duration pulses, or pulses that increase in duration and then decrease in duration with increasing distance.
The task of designing signal processing circuitry, such as demodulators, that are to be used with Infrared transceivers under the circumstances described above has proven to be quite difficult.
For instance, pulse width acceptance can be set in a demodulation circuit of an IRDA compliant controller, but with the current variation among IRDA transceivers and with new transceivers being developed this could involve designing a specific demodulator circuit for each type or version of a transceiver. This would be very expensive and time consuming. We have also found that when a new IRDA transceiver is actually manufactured and tested, unexpected results are seen in the electrical pulse widths generated.
Accordingly a need therefore exists for compensating for the variations that may occur in the output signal pulses of a transceiver when it is sought to demodulate the signal in a reliable manner, while at the same time not requiring demodulation circuit redesign.
SUMMARY OF THE INVENTION
It has been found that adaptive pre-demodulation pulse shaping of the output signal of a transceiver receiver circuit before passing the output signal to a demodulator provides a significant advance in addressing the problem of pulse variation without the requirement of redesigning demodulators.
An adaptive pulse shaper using adaptive pre-demodulation pulse shaping logic and circuitry is provided by this invention which will decide if a pulse produced by a receiver to be sent to the demodulator should be lengthened (for instance by using an add operation) or shortened (e.g. by using a chop operation). The pulse shaping logic is preferably adapted to use the preamble phase of a 4 Mbps PPM packet to determine the appropriate add or chop level required for the remainder data carrying portion of the packet.
One aspect of the invention provides method of processing an PPM encoded input signal transmission of data pulses to a data processing apparatus by processing the pulses by lengthening or shortening the pulses to a length within the predetermined acceptance pulse duration range of the data processing apparatus comprising:
detecting when the input pulses of the input signal fall below the duration required and lengthening the pulses by a sufficient amount to bring them within the predetermined duration;
detecting when the input pulses of the input signal exceed the duration required and reducing the duration of the pulses by a sufficient amount to bring them within the predetermined duration.
Preferably in this method (as the PPM encoded input signal transmission comprises a packet of data pulses with a preamble portion of a sequence of repetitive pulses of known length and a data portion of data pulses)there is a provision for
detecting and identifying a pulse within the preamble portion of the packet,
determining whether the duration of the detected preamble pulse lies within the predetermined acceptance pulse duration, if the preamble pulse is shorter than the acceptance duration, determining an add amount to add that is sufficient to bring the pulse duration within the acceptance duration, and adding the add amount to all subsequent data pulses in the packet, and, if the preamble pulse exceeds the predetermined acceptance duration, determining the removal amount to remove that is sufficient to bring the pulse duration within the predetermined acceptance duration, and removing the removal amount from all subsequent data pulses in the packet, and, subsequently outputting the modified pulses to the data processing apparatus.
The encoded input signal may be received from the output of an optical receiving circuit and the modified pulses are transmitted to a PPM demodulator for demodulation.
Another aspect of the invention provides an adaptive pulse shaping pre-dem
Schuppe Raymond Walter M.
Varga Brian Daniel
Cameron Douglas W.
Dougherty Anne Vachon
International Business Machines - Corporation
Pham Chi H.
Tran Khai
LandOfFree
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