Orthogonal frequency division multiplexing (OFDM) communications device and method that incorporates low PAPR preamble and receiver channel estimate circuit

• US 8,189,697 B2
• Filed: 10/01/2008
• Issued: 05/29/2012
• 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:
  US12/242,967
• Publication Number:
  US 20100080311A1
• Time in Patent Office: 1,336 Days
• Field of Search: 375/260, 375/132, 375/259, 375/130, 370/210
• US Class Current: 375/260
• CPC Class Codes: H04L 25/022 of frequency response
• 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
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

A communications device includes a demapping and demodulation circuit that demaps and demodulates an OFDM communications signal as modulated subcarriers carrying communications data and modulated subcarriers carrying a training sequence forming a preamble that includes a long sync sequence for channel and frequency offset estimation having a extended guard interval (GI). The OFDM subcarriers carrying the training sequence have a quadratic modulation to produce a low peak-to-average power (PAPR) preamble with PAPR of approximately 2.6 decibels (dB). A channel estimate circuit is positioned to receive signals after processing within a FFT circuit and subcarrier demapper circuit and estimates the channel characteristics of the communications channel based on splitting the extended guard interval from the long sync sequence and processing into values that represent the low PAPR preamble as plus or minus one (+/−1) values in a real or imaginary component as adds and subtracts.

Claims

1. A communications device, comprising:

a demapping and demodulation circuit as a Fast Fourier Transform (FFT) circuit and subcarrier demapper circuit for processing an OFDM communications signal that had been transmitted along a communications channel, wherein the OFDM communications signal comprises modulated subcarriers carrying communications data forming a data payload and modulated subcarriers carrying a training sequence forming a preamble that includes a long sync sequence for acquisition, detection, frequency and channel estimation and having a extended guard interval (GI), wherein the OFDM subcarriers carrying the training sequence are selectively turned ON and OFF pseudo-randomly and have an applied quadratic modulation to produce a low peak-to-average power ratio (PAPR) preamble; and

a channel estimate circuit positioned to receive signals after processing within said FFT circuit and subcarrier demapper circuit, wherein said channel estimate circuit is operative with said FFT and subcarrier demapper circuit for estimating channel characteristics of the communications channel based on splitting the extended 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.

2. The communications device according to claim 1, wherein said channel estimate circuit and FFT circuit and subcarrier mapper circuit are operative for using “

n”

samples of the extended guard interval as a normal GI and portions of the long sync sequence for FFT processing.

3. The communications device according to claim 1, wherein said channel estimate circuit is operative for measuring fluctuations within a flat-topped spectrum of the received OFDM communications signal corresponding to the preamble to reflect the frequency response of the communications channel.

4. The communications device according to claim 1, and to further comprising an acquisition and synchronization circuit that includes circuitry for acquisition, detection, frequency and channel estimation.

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

6. A communications system, comprising:

a transmitter that comprises a modulation and mapping circuit that modulates communications data and a training sequence corresponding to a preamble into a plurality of multiple subcarriers that are orthogonal to each other to form an Orthogonal Frequency Division Multiplexing (OFDM) communications signal having modulated subcarriers carrying the communications data and forming a data payload and modulated subcarriers forming the preamble, wherein the preamble includes a long sync sequence for acquisition, detection, frequency and channel estimation and having a extended guard interval (GI), wherein said 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; and

a receiver that receives the OFDM communications signal from the transmitter as transmitted along a communications channel and comprising a demapping and demodulation circuit as a Fast Fourier Transform (FFT) circuit and subcarrier demapper circuit for processing the OFDM communications signal to obtain the communications data, and further comprising a channel estimate circuit positioned within the receiver to receive signals after processing within said FFT circuit and subcarrier demapper circuit, wherein the channel estimate circuit is operative with the FFT and subcarrier demapper circuit for estimating channel characteristics of the communications channel based on splitting the extended 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 within the receiver.

7. The communications system according to claim 6, wherein said channel estimate circuit and FFT circuit and subcarrier mapper circuit are operative for using “

n”

samples of the extended guard interval as a normal GI and portions of the long sync sequence for FFT processing.

8. The communications system according to claim 6, wherein the channel estimate circuit is operative for measuring fluctuations within a flat-topped spectrum of the received OFDM communications signal corresponding to the preamble to reflect the frequency response of the communications channel.

9. The communications system according to claim 6, wherein said receiver further comprises an acquisition and synchronization circuit that includes circuitry for acquisition, detection, frequency and channel estimation.

10. The communications system according to claim 6, wherein said transmitter further comprises a circuit for turning OFF and ON selected subcarrier based on an encryption algorithm to increase transmit power and signal-to-noise ratio per subcarrier.

11. The communications system according to claim 6, wherein said transmitter further comprises a pseudo-random signal 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.

12. The communications system according to claim 6, wherein said modulation and mapping circuit is operative for independently amplitude modulating each subcarrier carrying the data sequences.

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

14. The communications system according to claim 6, wherein said modulation and mapping circuit is operative for equally spacing in frequency each of the subcarriers forming the preamble.

15. The communications system according to claim 6, 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.

16. The communications system according to claim 6, wherein the PAPR preamble has a PAPR value of about 2.6 dB.

17. 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 training sequences forming the preamble, wherein the preamble includes a long sync sequence for channel and fine frequency offset estimation having a extended guard interval (GI);

applying a quadratic phase modulation to subcarriers carrying the training sequences to produce a low peak-to-average power ratio (PAPR) preamble;

turning OFF and ON selected subcarriers based on an encryption algorithm;

transmitting the OFDM communications signal along a communications channel to a receiver; and

receiving the OFDM communications signal within the receiver and performing channel estimation of the communications channel by splitting the extended 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.

18. The method according to claim 17, which further comprises using “

n”

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

19. The method according to claim 17, which further comprises processing within a channel estimate circuit that is operative with a FFT circuit and subcarrier demapper circuit for estimating channel characteristics.

20. The method according to claim 17, which further comprises equally spacing in frequency each of the subcarriers that carries the training sequences of the OFDM communications signal.

21. The method according to claim 17, which further comprises measuring at the receiver fluctuations within a flat-topped spectrum of the OFDM communications signal corresponding to the preamble to reflect the frequency response of the communications channel.

22. The method according to claim 17, which further comprises performing acquisition, detection, frequency and channel estimation at the receiver for acquiring the communications signal.

23. The method according to claim 17, 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.

24. The method according to claim 17, which further comprises forming each of the subcarriers that carries the training sequences of the OFDM communications signal to have equal amplitude.

25. The method according to claim 17, which further comprises independently amplitude modulating each subcarrier that carries the data sequences.

26. The method according to claim 17, wherein the PAPR preamble has a PAPR value of about 2.6 dB.