Orthogonal frequency division multiplexing (OFDM) communications device and method that incorporates low PAPR preamble and variable number of OFDM subcarriers

• US 8,175,178 B2
• Filed: 10/01/2008
• Issued: 05/08/2012
• Est. Priority Date: 10/01/2008
• Status: Active Grant

• Current Assignee: Smartsky Networks Llc
• Original Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Inventors: Moffatt, Christopher D., Hoffmann, John E.
• Primary Examiner(s): Payne, David C.
• Assistant Examiner(s): Stevens, Brian J
• Application Number:
  US12/242,958
• Publication Number:
  US 20100080310A1
• Time in Patent Office: 1,315 Days
• Field of Search: 375/260, 375/259
• US Class Current: 375/260
• CPC Class Codes: H04L 27/2613 Structure of the reference signals
• CPC Class Codes: H04L 27/2618 Reduction thereof using auxiliary subcarriers
• CPC Class Codes: H04L 27/262 Reduction thereof by selection of pilot symbols

Abstract

A communications device includes communications data and a training sequence corresponding to a preamble. A modulation and mapping circuit modulates the communications data and training sequence into a plurality of multiple subcarriers that are orthogonal to each other to form an OFDM communication signal having modulated subcarriers carrying the communications data forming a data payload and modulated subcarriers forming the preamble. A circuit turns OFF and ON selected subcarriers based on an encryption algorithm to increase transmit power and signal-to-noise ratio and reduce the Inter-Carrier Interference (ICI). The modulation and mapping circuit applies a quadratic modulation to the subcarriers carrying the training sequences to produce a low peak-to-average power ratio (PAPR) preamble with a PAPR value of approximately 2.6 decibels (dB).

Claims

1. A communications device, comprising:

a source of communications data and a training sequence corresponding to a preamble;

a modulation and mapping circuit that modulates the communications data and training sequence into a plurality of variable subcarriers that are orthogonal to each other to form an Orthogonal Frequency Division Multiplexing (OFDM) communications signal having modulated subcarriers carrying the communications data forming a data payload and modulated subcarriers forming the preamble;

a circuit operating with the modulator and mapping circuit and configured to turn OFF and ON selected subcarriers pseudo-randomly based on an encryption algorithm to increase transmit power and signal-to-noise ratio per subcarrier and reduce the Inter-Carrier Interference (ICI); and

wherein said modulation and mapping circuit is configured to apply a quadratic modulation to each of the subcarriers carrying the training sequences and apply a specific phase angle to each sinuosoid forming the modulated subcarrier carrying the training sequence to produce a low peak-to-average power ratio (PAPR) preamble.

2. The communications device according to claim 1, and further comprising a circuit for spreading the selected ON subcarriers over the frequency domain to reduce average power for enhanced Low Probability of Interception/Low Probability of Detection (LPI/LPD) and allow for more transmit power within a spectral mask and reduce adverse effects of Frequency Selective Fading.

3. The communications device according to claim 1, and further comprising a pseudo-random sequence generator operative with the modulation and mapping circuit for generating pseudo-random signals to the modulation and mapping circuit based on an encryption algorithm for frequency hopping each subcarrier at an OFDM symbol rate.

4. The communications device according to claim 1, wherein said modulation and mapping circuit is operative for independently amplitude modulating each subcarrier carrying the data sequences.

5. The communications device according to claim 1, wherein the low PAPR preamble is independent of the number of subcarriers forming the OFDM communications signal.

6. The communications device according to claim 1, wherein said modulation and mapping circuit is operative for setting quadratic phases of sinusoids forming the subcarriers carrying the training sequences.

7. The communications device according to claim 6, wherein said modulation and mapping circuit is operative for setting the phases of the sinusoids to a specific angle.

8. The communications device according to claim 1, wherein the preamble includes a long sync sequence for channel and fine frequency offset estimation having a guard interval (GI).

9. The communications device according to claim 1, wherein said modulation and mapping circuit is operative for forming each of the subcarriers that carries the training sequences of the OFDM communications signal to have equal amplitude.

10. The communications device according to claim 1, wherein the PAPR preamble has a PAPR value of 2.6 dB.

11. A method of communicating data, comprising:

receiving communications data to be transmitted and a training sequence corresponding to a preamble;

generating an Orthogonal Frequency Division Multiplexing (OFDM) communications signal having modulated subcarriers carrying the communications data forming a data payload and subcarriers carrying the training sequences forming the preamble;

turning OFF and ON selected subcarriers pseudo-randomly based on an encryption algorithm to increase transmit power and signal-to-noise ratio per subcarrier and reduce the Inter-Carrier Interference (ICI);

applying quadratic phase modulation to each of the subcarriers carrying the training sequences at a specific phase angle to each sinusoid forming the modulated subcarriers carrying the training sequence to produce a low peak-to-average power ratio (PAPR) preamble; and

transmitting the OFDM communications signal along a communications channel to a receiver.

12. The method according to claim 11, which further comprises spreading the selected ON subcarriers over the frequency domain to reduce average power for enhanced Low Probability of Interception/Low Probability of Detection (LPI/LPD) and allow for more transmit power within a spectral mask and reduce adverse effects of Frequency Selective Fading.

13. The method according to claim 11, which further comprises generating pseudo-random signals to a modulation and mapping circuit based on an encryption algorithm for frequency hopping each subcarrier at an OFDM symbol rate.

14. The method according to claim 11, which further comprises equally spacing in frequency each of the subcarriers carrying the training sequences.

15. The method according to claim 11, which further comprises forming each of the subcarriers carrying the training sequences to have equal amplitude.

16. The method according to claim 11, which further comprises independently amplitude modulating each subcarrier carrying the data sequences.

17. The method according to claim 11, wherein the low PAPR preamble is independent of the number of subcarriers forming the OFDM communications signal.

18. The method according to claim 11, which further comprises measuring fluctuations within a flat-topped spectrum of the OFDM communications signal corresponding to the preamble to reflect the frequency response of the communications channel.

19. The method according to claim 11, which further comprises setting quadratic phases of sinusoids forming the subcarriers carrying the training sequences.

20. The method according to claim 19, which further comprises setting the phases of the sinusoids to a specific angle.

21. The method according to claim 11, which further comprises forming the PAPR preamble with a PAPR value of 2.6 dB.

22. A method of communicating data, comprising:

receiving communications data to be transmitted and a training sequence corresponding to a preamble;

generating within a modulation and mapping circuit an Orthogonal Frequency Division Multiplexing (OFDM) communications signal having modulated subcarriers carrying the communications data forming a data payload and subcarriers carrying training sequences forming the preamble, wherein the preamble includes a long sync sequence for channel and fine frequency offset estimation having a guard interval (GI);

turning OFF and ON selected subcarriers pseudo-randomly based on an encryption algorithm to increase transmit power and signal-to-noise ratio and reduce the Intercarrier Interference (ICI);

applying a quadratic phase modulation to each of the subcarriers carrying the training sequences at a specific phase angle to each sinusoid forming the modulated subcarriers carrying the training sequence to produce a low peak-to-average power ratio (PAPR); and

splitting the guard interval from the long sync sequence and processing into values that represent the inverse low PAPR preamble as plus or minus one (+/−

1) values in a real or imaginary component as adds and subtracts for channel estimation.

23. The method according to claim 22, which further comprises spreading the selected ON subcarriers over the frequency domain to reduce average power for enhanced Low Probability of Interception/Low Probability of Detection (LPI/LPD) and allow for more transmit power within a spectral mask and reduce adverse effects of Frequency Selective Fading.

24. The method according to claim 22, which further comprises generating pseudo-random signals to the modulation and mapping circuit based on an encryption algorithm for frequency hopping each subcarrier at an OFDM symbol rate.

25. The method according to claim 22, which further comprises using the “

n”

samples of the guard interval as a normal GI and portions of the long sync sequence for Fast Fourier Transform (FFT) processing for channel estimation.

26. The method according to claim 22, which further comprises forming the PAPR preamble with a PAPR value of 2.6 dB.