Multilevel pulse position modulation for efficient fiber optic communication

• US 20030072050A1
• Filed: 03/29/2002
• Published: 04/17/2003
• Est. Priority Date: 03/29/2001
• Status: Active Grant

• Current Assignee: Intersil Americas LLC (Renesas Electronics Corporation)
• Original Assignee: Quellan, Inc. (Renesas Electronics Corporation)
• Inventors: Vrazel, Michael G., Ralph, Stephen E., Hietala, Vincent Mark
• Granted Patent: US 7,149,256 B2
• Time in Patent Office: Days
• US Class Current: 398/9
• CPC Class Codes: H04B 10/5161 Combination of different modulation schemes
• CPC Class Codes: H04B 10/524 Pulse modulation
• CPC Class Codes: H04B 10/541 Digital intensity or amplitude modulation
• CPC Class Codes: H04L 25/4902 Pulse width modulation; Pulse position modulation
• CPC Class Codes: H04L 25/4917 using multilevel codes

Abstract

Decreasing the average transmitted power in an optical fiber communication channel using multilevel amplitude modulation in conjunction with Pulse Position Modulation (PPM). This multilevel PPM method does not entail any tradeoff between decreased power per channel and channel bandwidth, enabling a lower average transmitted power compared to On/Off Keying (OOK) with no reduction in aggregate data rate. Therefore, multilevel PPM can be used in high-speed Dense Wavelength Division Multiplexed (DWDM) systems where the maximum number of channels is traditionally limited by nonlinear effects such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Brillouin scattering (SBS), and stimulated Raman scattering (SRS). This modulation technique can enable an increased number of channels in DWDM systems, thereby increasing aggregate data rates within those systems.

Claims

1. A method for decreasing the transmitted power in a channel of an optical fiber communication system while maintaining a channel bandwidth, the method comprising the steps of:

receiving a digital input signal, comprising a series of input pulses, each input pulse having one of two pulse levels;

creating a digital input word having n bits from the digital input signal; and

converting each digital input word to a corresponding output symbol representing one of 2ⁿ distinct values;

wherein the output symbol comprises a multilevel pulse position modulated symbol.

2. The method of claim 1, wherein the output symbol is characterized by representing one of a plurality of unique amplitudes.

3. The method of claim 1, wherein the output symbol is characterized by representing one of a plurality of unique chip positions.

4. The method of claim 1, further comprising the step of generating an output signal comprising a series of output symbols.

5. The method of claim 4, wherein the output signal has an aggregate data rate equal to a data rate of the input signal.

6. The method of claim 5, wherein the output signal has a lower transmitted power than an on-off modulated signal transmitting a series of on-off modulated output symbols at the aggregate data rate.

7. The method of claim 6, wherein the transmitted power of the output signal is less than the transmitted power of the input signal by a factor of at least ½

.

8. An optical fiber communication system comprising:

a multilevel pulse position modulation transmitter for combining n input signals for transmission over an optical fiber communication link as an encoded output signal;

wherein the encoded output signal comprises a series of multilevel pulse position modulated symbols, each symbol representing one of 2ⁿ unique values.

9. The optical fiber communication system of claim 8, wherein a first transmission power required to transmit the encoded signal over the optical fiber communication link at a predetermined data rate is less than a second transmission power required to transmit an on-off modulated output signal over the optical fiber communication link at the predetermined data rate.

10. The optical fiber communication system of claim 9, wherein the first transmission power (P_FIRST) required to multiplex n on-off keyed input signals is a fraction of the second transmission power (P_SECOND) and is given by:

11. The optical fiber communication system of claim 8, further comprising a receiver operative to receive the encoded signal from the optical fiber communication link and having a decoder operative to decode the encoded output signal into n output signals.

12. The optical fiber communication system of claim 11, wherein the n output signals are substantially identical to the n input signals.

13. A method for encoding n on-off keyed input signals into a single multilevel pulse position modulated output signal, the method comprising the steps of:

combining at least two of the n input signals into a multilevel signal;

representing at least two other of the n input signals as chip positions in a four position pulse position modulation signal; and

modifying at least one amplitude level of the four position pulse position modulation signal in accordance with at least one amplitude level of the multilevel signal to generate the single multilevel pulse position modulated output signal.

14. The method of claim 13, wherein the step of combining at least two of the n input signals into a multilevel signal comprises the steps of:

attenuating a first signal of the at least two of the n input signals; and

adding the first signal to a second signal of the at least two of the n input signals.

15. The method of claim 14, wherein the first signal is attenuated by 6 dB.

16. The method of claim 14, further comprising the step of delaying the second signal for a first delay time, wherein the first delay time is equal to a second delay time imposed on the first signal by the attenuating step.

17. The method of claim 14, further comprising the step of delaying the at least two other of the n input signals for a third delay time, wherein the third delay time is equal to a fourth delay time imposed on the at least two of the n input signals by the attenuating step and the delaying the second signal step.

18. A transmitter for use in an optical fiber communications system, comprising:

a multilevel pulse position modulation encoding circuit operative to encode at least four on-off keyed input signals into a single multilevel pulse position modulated output signal;

a pulse shaping module for processing the output signal to maximize a dispersion distance of the output signal; and

an optical source for transmitting the output signal over a link of the optical fiber communications system.

19. The transmitter of claim 18, wherein the encoding circuit is operative to combine at least two of the on-off keyed input signals into a multilevel signal.

20. The transmitter of claim 19, wherein the encoding circuit is further operative to represent at least two other of the on-off keyed input signals as chip positions in a four position pulse position modulation signal.

21. The transmitter of claim 20, wherein the encoding circuit is further operative to modify at least one amplitude level of the four position pulse position modulation signal in accordance with at least one amplitude level of the multilevel signal.