# US 2008/0187322 A1 · Optical high-rate pulse position modulation scheme and optical communications system based thereon

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- Number: US 2008/0187322 A1
- Document ID: US20080187322A1
- Status: Published application
- Kind: A1
- Application number: 11/971,274
- Filed: 2008-01-09
- Published: 2008-08-07
- Earliest claimed: 2007-02-06
- Assignees: Oerlikon Space AG
- Inventors: Michael Tuchler
- Web page: https://patentgrep.com/patent/US20080187322A1

## CPC

- H04B 14/026: {using pulse time characteristics modulation, e.g. width, position, interval}
- H04L 25/4902: {Pulse width modulation; Pulse position modulation}
- H04L 25/4917: {using multilevel codes}
- H04B: TRANSMISSION
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

It is an object of the present invention to provide a communications system for communication between two remote terminals, where even at very long distances between these two terminals and/or at very small signal-to-noise ratios a robust communication is possible. The invention achieves the goal set before by suggesting an inventive pulse position modulation scheme and a corresponding optical communications system. The system and method according to the invention are very well suited for communication with a satellite in deep space, for instance.

## Claims

### Claim 1

Method comprising the steps:
at a transmitting side of an optical channel providing bits for transmission across the optical channel,
processing the bits by performing a bit-to-symbol mapping in order to provide map said bits into PPM symbols, where
each PPM symbol has a symbol duration comprising D*M time slots and a blank time of blank time duration,
each time slot has a slot duration,
the blank time duration is D*B times the slot duration, with B being an integer number≧1
the parameter D is an integer number≧2 defining the number of pulses located at D of the D(M+B) time slots,
M defines the number of time slots of each PPM symbol, with M being an integer number≧4,
where each pulse has a pulse duration, and
where said slot duration is shorter than said pulse duration so as to provide for theoretically overlapping pulses inside each PPM symbol;
where a number P of different pulse patterns contained in each of said PPM symbols is defined by
P
=
(
D
·
M
D
)
.

### Claim 2 (depends on 1)

The method of claim 1,
where said theoretically overlapping pulses are provided inside each PPM symbol in order to provide for an increased raw data rate for the transmission of the bits across said optical channel, and
where an overlap ratio is defined by pulse duration/slot duration, said ratio preferably being equal to 2 or equal to 3.

### Claim 3 (depends on 2)

The method of claim 2, wherein an amplitude modulation is applied so that each of said pulse of a PPM symbol can take A different amplitude levels, where A is the number of available amplitude levels, said amplitude modulation being applied to further increase said raw data rate and where a number P of different pulse patterns contained in each of said PPM symbols is defined by
P
=
A
D
·
(
D
·
M
D
)
.

### Claim 4 (depends on 3)

The method of claim 3, wherein between two consecutive pulses of the PPM symbol there is at least one blank time and the last pulse of the respective PPM symbol is at least one blank time away from the end of the respective PPM symbol.

### Claim 5 (depends on 4)

The method of claim 4, wherein said symbol duration is constant and non-data dependent.

### Claim 6 (depends on 5)

The method of claim 5, wherein an asymmetric FEC encoding technique is employed on the transmitting side prior to performing said bit-to-symbol mapping.

### Claim 7 (depends on 6)

The method of claim 6, wherein a baseline PPM scheme is employed to generate a synchronization word.

### Claim 8 (depends on 7)

The method of claim 7, wherein said synchronization word together with a number of said PPM symbols are transmitted across said optical channel to a receiving side.

### Claim 9 (depends on 8)

The method of claim 8, wherein at the receiving side a matched filtering approach is employed for processing the PPM symbols received.

### Claim 10 (depends on 8)

The method of claim 8, wherein matched filter samples are produced at each slot time, these matched filter samples being ordered such that they correspond to the correct position of the pulses in the received PPM symbols.

### Claim 11 (depends on 8)

The method of claim 8, wherein at the receiving side data bits from the PPM symbols received are mapped to symbol positions and wherein said synchronization word is used to synchronize said receiving side with respect to said transmitting side.

### Claim 12 (depends on 8)

The method of claim 8, wherein a matched filter impulse response hΔt(n) of matched filters and a phase detector impulse response gΔt(n) of phase detectors are linearly approximated, which yields a very efficient implementation of the matched filters and the phase detectors using only four filters.

### Claim 13

Laser communications system comprising a first transceiver and a second transceiver for establishing two optical channels there between, wherein
said first transceiver comprises a first transmitter with a Laser light source for transmitting optical pulse position modulated signals into a first of said optical channels, said first transmitter comprising a bit interleaver followed by a symbol mapper mapping interleaved coded data bits into modulation symbols from a PPM symbol alphabet, said symbol mapper feeding a frame generator, followed by a pulse shaper so that an analog waveform is assigned to each modulation symbol, said pulse shaper providing said analog waveform for modulating said Laser light source,
said second transceiver comprises
a receiver for receiving said pulse position modulated symbols, said receiver comprising an all-digital PPM detector with a high-speed analog-to-digital converter followed by a frame and symbol detector, a symbol demapper mapping received pulse position modulated symbols to data bits, said symbol demapper forwarding said data bits to a bit deinterleaver.

### Claim 14 (depends on 13)

The Laser communications system of claim 13, wherein said first transmitter further comprises a forward-error-correction encoder that encodes data bits to code bits, said code bits being fed into said bit interleaver.

### Claim 15 (depends on 13)

The Laser communications system of claim 13, wherein said bit interleaver shuffles said code bits using the index permutation function n(n).

### Claim 16 (depends on 14)

The Laser communications system of claim 14, wherein said frame generator processes said modulation symbols to add further symbols before and after a payload section of the PPM frame, said further symbols being added in order to help the receiver to detect and identify each PPM frame.

### Claim 17 (depends on 14)

The Laser communications system of claim 14, wherein said receiver comprises an avalanche photo diode receiver, said avalanche photo diode comprising an avalanche photo diode coupled with a trans-impedance amplifier.

### Claim 18 (depends on 17)

The Laser communications system of claim 17, wherein said receiver further comprises a low-noise amplifier and a variable gain amplifier, these amplifiers being situated between said avalanche photo diode receiver and said high-speed analog-to-digital converter.

### Claim 19 (depends on 18)

The Laser communications system (10) of claim 18, wherein said receiver (R2) further comprises a low-pass filter (39) arranged between said variable gain amplifier (37, 38) and said analog-to-digital converter (31).

### Claim 20 (depends on 19)

The Laser communications system of claim 19, wherein said frame and symbol detector as well as said symbol demapper are implemented in the digital domain using an FPGA.

### Claim 21 (depends on 20)

The Laser communications system of claim 20, wherein said frame and symbol detector comprises matched filters.

### Claim 22 (depends on 21)

The Laser communications system of claim 21, wherein said frame and symbol detector is enabled to control the gain of the variable gain amplifier to ensure that the received signals are within a predefined voltage range fitting into an operating range of said high-speed analog-to-digital converter.

### Claim 23 (depends on 22)

The Laser communications system of claim 22, wherein said frame and symbol detector comprises a preamble detector implementing a preamble detection algorithm enabling the first receiver to find a preamble of a received PPM frame and to perform a coarse slot time synchronization between said first transmitter and said first receiver.

### Claim 24 (depends on 23)

The Laser communications system of claim 23, wherein said frame and symbol detector comprises a synchronization word detector filter enabling the first receiver to find a synchronization word of a received PPM frame.

### Claim 25 (depends on 24)

The Laser communications system of claim 24, wherein said first transmitter is carried aboard of a spacecraft or satellite.

### Claim 26 (depends on 25)

The Laser communications system of claim 25, wherein a user data interface is provided in said spacecraft or satellite in order to provide user data to be transmitted to said first transmitter.

### Claim 27 (depends on 25)

The Laser communications system of claim 25, comprising an FPGA in which control and signal processing functions of the first transmitter are implemented.

### Claim 28 (depends on 25)

The Laser communications system of claim 25, comprising a Laser diode module comprising said Laser light source, which serves as a seed Laser diode, and comprising an optical fiber amplifier pump module and an optical fiber amplifier.