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Compressed Pulse Position Modulation Encoding Method for Infrared Local Area Networks

IP.com Disclosure Number: IPCOM000038747D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 35K

Publishing Venue

IBM

Related People

Bustamanate, C: AUTHOR [+3]

Abstract

A technique is described whereby compressed pulse position modulation (CPPM) encoding provides signal symmetry, when used in conjunction with edge-coupled infrared local area network (LAN) data transmission circuitry. A method is described to reduce losses in edge-coupled systems, where losses of 6-8 dB are encountered [*]. In prior art, signal symmetry problems can occur when infrared transmission of data is influenced by devices, such as high frequency fluorescent bulbs, which produce infrared electrical noise in the 70 KHz range. The transmission signal rise and fall time may vary as much as 30%, causing the differentiated waveform to vary significantly in amplitude, as shown in Fig. 1. For this variance of rise times (30%), bit error rates may change within one order of magnitude.

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Compressed Pulse Position Modulation Encoding Method for Infrared Local Area Networks

A technique is described whereby compressed pulse position modulation (CPPM) encoding provides signal symmetry, when used in conjunction with edge-coupled infrared local area network (LAN) data transmission circuitry. A method is described to reduce losses in edge-coupled systems, where losses of 6-8 dB are encountered [*]. In prior art, signal symmetry problems can occur when infrared transmission of data is influenced by devices, such as high frequency fluorescent bulbs, which produce infrared electrical noise in the 70 KHz range. The transmission signal rise and fall time may vary as much as 30%, causing the differentiated waveform to vary significantly in amplitude, as shown in Fig. 1. For this variance of rise times (30%), bit error rates may change within one order of magnitude.

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Using CPPM encoding, one pulse encodes four bits by detecting the arrival of a pulse in one of sixteen possible time slots. In effect, the present pulse becomes the time reference for the next pulse. The CPPM encoded information is stored in the time frame between positive pulses. Typically, pulse width 10, as shown in Fig. 2, would be on the order of 500 ns. This is followed by guard time 11 of typically 1500 ns. After the guard time elapses, the next pulse is expected in one of the sixteen time slots 12, which are each typically 500 ns slots. By not allowing two consecutive pulses to be closer than 2000 ns, more time is allocated between pulses to discharge the capacitances in the filter stage of the circuitry. This allows an under-differentiation to occur. Three levels of under-differentiation are shown in Fig.
3. Using CPPM encoding, the discharge time is four times longer, enabling capacitors to be four times larger. The smaller differentiation results in less attenuation, providing a two-times improvement in the signal-to-noise ratio. (It...