Time and frequency synchronization in Multi-Input, Multi-Output (MIMO) systems

• US 7,706,458 B2
• Filed: 06/07/2006
• Issued: 04/27/2010
• Est. Priority Date: 04/24/2001
• Status: Expired due to Term

• Current Assignee: American Patents LLC
• Original Assignee: Taffco Three Fund LLC (Intellectual Ventures LLC)
• Inventors: Mody, Apurva N., Stuber, Gordon L.
• Primary Examiner(s): Bayard, Emmanuel
• Application Number:
  US11/448,395
• Publication Number:
  US 20060239370A1
• Time in Patent Office: 1,420 Days
• Field of Search: 375/260, 375/299, 375/347, 375/267, 370/208, 370/209, 370/210
• US Class Current: 375/260
• CPC Class Codes: H04B 17/21 for calibration; for correcting measurements
• CPC Class Codes: H04B 7/04 using two or more spaced independent antennas
• CPC Class Codes: H04B 7/068 using space frequency diversity space-frequency coding H04L1/0606
• CPC Class Codes: H04L 27/2613 Structure of the reference signals
• CPC Class Codes: H04L 27/26134 Pilot insertion in the transmitter chain, e.g. pilot overlapping with data, insertion in time or frequency domain
• CPC Class Codes: H04L 27/2659 Coarse or integer frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/266 Fine or fractional frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/2663 Coarse synchronisation, e.g. by correlation
• CPC Class Codes: H04L 27/2665 Fine synchronisation, e.g. by positioning the FFT window
• CPC Class Codes: H04L 27/2675 Pilot or known symbols
• CPC Class Codes: H04L 27/2678 using cyclostationarities, e.g. cyclic prefix or postfix

Abstract

In a communication system, and in particular a wireless Orthogonal Frequency Division Multiplexing (OFDM) communication system, the present invention provides systems for synchronizing data transmitted across a channel. The present invention may be used in a Multi-Input, Multi-Output (MIMO) system in which the data is transmitted from any number of transmitting antennas and received by any number of receiving antennas. The number of transmitting and receiving antennas does not necessarily have to be the same. Circuitry is provided for synchronizing the data in both the time domain and frequency domain. Time synchronization involves coarse time synchronization and fine time synchronization. Frequency synchronization involves coarse frequency offset estimation, fine frequency offset estimation, and frequency offset correction.

Claims

1. An apparatus for synchronizing a communication system, the apparatus comprising:

a number (Q) of Orthogonal Frequency Division Multiplexing (OFDM) modulators, each OFDM modulator producing a frame having at least one inserted symbol, a plurality of data symbols, and cyclic prefixes;

Q transmitting antennas, each transmitting antenna connected to a respective OFDM modulator, the transmitting antennas configured to transmit a respective frame over a channel;

a number (L) of receiving antennas for receiving the transmitted frames; and

L OFDM demodulators, each OFDM demodulator corresponding to a respective receiving antenna, the L OFDM demodulators including a synchronization circuit that processes the received frame in order to synchronize the received frame in both time domain and frequency domain, wherein each of the L OFDM demodulators comprises;

a pre-amplifier;

a local oscillator;

a mixer having a first input and a second input, the first input connected to an output of the pre-amplifier, the second input connected to an output of the local oscillator;

an analog-to-digital converter (ADC) connected to an output of the mixer;

the synchronization circuit having one input connected to an output of the ADC;

a cyclic-prefix remover connected to an output of the synchronization circuit;

a serial-to-parallel converter connected to an output of the cyclic prefix remover; and

a discrete Fournier transform (DFT) stage connected to an output of the serial-to-parallel converter, an output of the DFT stage connected to another input to the synchronization circuit.

2. The apparatus of claim 1, wherein the cyclic prefixes protect the data symbols against Inter Symbol Interference (ISI).

3. The apparatus of claim 1, wherein the at least one inserted symbol has at least one pilot symbol inserted within the data symbols or at least, one training symbol inserted at the beginning of the frame.

4. The apparatus of claim 1, wherein the synchronization circuit comprises:

a circuit for finding the optimum time instant of the start of the demodulated frame;

a coarse time synchronization circuit; and

a fine time synchronization circuit;

wherein the coarse time synchronization circuit comprises a first circuit for performing auto-correlation on the received frame over a particular window and a second circuit for performing an automatic gain control operation.

5. The apparatus of claim 1, wherein each of the Q OFDM modulators comprises a transmitting local oscillator, each of the L OFDM demodulators comprises a receiving local oscillator, the synchronization circuit comprises a first frequency offset estimation circuit and a second frequency offset estimation circuit for estimating the frequency difference between the transmitting local oscillator and the receiving local oscillator, and the synchronization circuit further comprises a frequency offset correction circuit that utilizes the estimates from the second frequency offset estimation circuit and the first frequency estimation circuit to correct the frequency offset in discrete time;

wherein the second frequency offset estimation circuit is used to correct the frequency of the receiving local oscillator.

6. The apparatus of claim 1, wherein each of the Q OFDM modulators share a common transmitting local oscillator, each of the L OFDM demodulators share a common receiving local oscillator, and the synchronization circuit comprises first and second frequency offset estimation circuits for estimating the frequency difference between the transmitting local oscillator and the receiving local oscillator.

7. The apparatus of claim 1 further comprising OFDM encoder, wherein the OFDM encoder comprises:

a channel encoder;

a symbol mapper connected to an output of the channel encoder;

a space-time processor connected to an output of the symbol mapper, the space-time processor separating data into a plurality of sub-channels; and

a pilot/training symbol inserter, which inserts pilot symbols and training symbols onto the sub-channels.

8. The apparatus of claim 1, wherein each of the Q OFDM modulators comprises:

a serial-parallel converter;

an inverse discrete Fournier transform (IDFT) stage connected to an output of the serial-to parallel converter;

a cyclic prefix inserter connected to an output of the IDFT stage;

a parallel-to serial converter connected to an output of the cyclic prefix inserter;

a digital-to-analog converter (DAC) connected to an output of the parallel-to-serial converter;

a local oscillator;

a mixer having a first input and a second input, the first input connected to an output of the DAC, the second input connected to an output of the local oscillator; and

an amplifier connected to an output of the mixer.

9. The apparatus of claim 1, wherein each OFDM demodulator comprises a respective synchronization circuit.

10. The apparatus of claim 1, wherein the synchronization circuit comprises a first portion that includes individual circuits such that each individual circuit is used exclusively by a respective OFDM demodulator, and a second portion that includes circuits that are shared by all of the L OFDM demodulators;

wherein the second portion comprises a first frequency offset estimator circuit and a second frequency offset estimator circuit, and the first portion comprises a coarse time synchronization circuit, fine time synchronization circuit, and a frequency offset correction circuit;

wherein the second portion comprises a coarse time synchronization circuit, a third frequency offset estimator circuit, and a fourth frequency offset estimator circuit, and the first portion comprises a fine time synchronization circuit and a frequency offset correction circuit; and

wherein the first portion comprises coarse time synchronization circuits wherein the results of the different coarse time synchronization circuits are shared by the different OFDM demodulators.

11. The apparatus of claim 1 further comprising an OFDM decoder, wherein the OFDM decoder comprises:

a space-time processor that receives an output from each of the L OFDM demodulators;

a parameter estimator that receives an output from each of the L OFDM demodulators and estimates parameters of the channel;

a parallel-to-serial converter connected to an output of the space-time processor;

a symbol demapper having a first input and second input, the first input connected to an output of the parameter estimator, the second input connected to an output of the parallel-to-serial converter; and

a channel decoder connected to an output of the symbol demapper.

12. A method for synchronizing a Multi-Input Multi-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) system in time domain and frequency domain, the method comprising:

receiving a frame of data having an originally transmitted form and making use of a sub-carrier having a sub-carrier spacing transmitted from an OFDM transmitter, the frame of data including a training symbol having a synchronization component, the synchronization component aiding in synchronization, the frame data further including a plurality of data symbols and a plurality of cyclic prefixes;

demodulating the frame; and

synchronizing the frame with the transmitted frame from the OFDM transmitter such that the data symbols are synchronized in the time domain and frequency domain, wherein synchronizing in the frequency domain comprises;

estimating a frequency offset to within one half of the sub-carrier spacing;

repeating received samples of the received frame a number of times;

taking an N-point Fast Fourier Transform (FFT); and

performing a cross-correlation procedure in the frequency domain.

13. The method of claim 12, wherein synchronizing in the time domain comprises coarse time synchronizing and fine time synchronizing;

wherein coarse time synchronizing includes averaging the coarse lime synchronized results overtime.

14. The method of claim 12, wherein said training symbol comprises an orthogonal sequence for enhancing a fine time synchronization performance;

and

wherein the frame further comprises a preamble, cyclic prefixes contained within the preamble and in the data symbols such that the cyclic prefixes in the preamble are longer than the cyclic prefixes in the data symbols, thereby countering an extended channel impulse response and improving synchronization.

15. The method of claim 12, wherein the synchronization component comprises a preamble whose signal transmission matrix resembles an existing space-time block code;

wherein the synchronization component includes chirp-like sequences, the chirp-like sequences including at least one of Frank-Zadoff sequences, Chu sequences, Milewski sequences, Suehiro polyphase sequences, and Ng et al. sequences.

16. A receiver comprising:

means for obtaining a transmitted frame having an originally transmitted form and making use of a sub-carrier having a sub-carrier spacing, the frame comprising a training symbol that includes a synchronization component for synchronizing the frame with the originally transmitted form of the frame, the frame further comprising a plurality of data symbols and a plurality of cyclic prefixes;

means for demodulating the obtained frame; and

means for synchronizing the frame to the originally transmitted form to thereby synchronize the data symbols in both time domain and frequency domain, wherein the means for synchronizing comprises means for course time synchronization and means for fine time synchronization;

wherein the means for synchronizing further comprises means for estimating a frequency offset to within one half of the sub-carrier spacing, means for repeating samples of the obtained frame a number of times, means for taking an N-point Fast Fourier Transform (FFT), and means for performing a cross-correlation procedure in the frequency domain.

17. The receiver of claim 16, wherein the synchronization component comprises a preamble having a transmission matrix resembling an existing space-time block code, the synchronization component further comprising chirp-like sequences including a sequence selected from the group consisting of Frank-Zadoff sequences, Chu sequences, Milewski sequences, Suehiro polyphase sequences, and Ng et. al. sequences.

18. An apparatus for synchronizing a communication system, the apparatus comprising:

a number (Q) of Orthogonal Frequency Division Multiplexing (OFDM) modulators, each OFDM modulator producing a frame having at least one inserted symbol, a plurality of data symbols, and cyclic prefixes;

Q transmitting antennas, each transmitting antenna connected to a respective OFDM modulator, the transmitting antennas configured to transmit a respective frame over a channel;

a number (L) of receiving antennas for receiving the transmitted frames; and

L OFDM demodulators, each OFDM demodulator corresponding to a respective receiving antenna, the L OFDM demodulators including a synchronization circuit that processes the received frame in order to synchronize the received frame in both time domain and frequency domain, wherein the synchronization circuit comprises a first portion that includes individual circuits such that each individual circuit is used exclusively by a respective OFDM demodulator, and a second portion that includes circuits that are shared by all of the L OFDM demodulators;

wherein the second portion comprises a third frequency offset estimator circuit and a fourth frequency offset estimator circuit, and the first portion comprises a coarse time synchronization circuit, a fine time synchronization circuit, and a frequency offset correction circuit;

wherein the second portion comprises a coarse time synchronization circuit, a first frequency offset estimator circuit, and a second frequency offset estimator circuit, and the first portion comprises a fine time synchronization circuit and a frequency offset correction circuit; and

wherein the first portion comprises coarse time synchronization circuits wherein the results of the different coarse time synchronization circuits are shared by the different OFDM demodulators.

19. An apparatus for synchronizing a communication system, the apparatus comprising:

a number (Q) of Orthogonal Frequency Division Multiplexing (OFDM) modulators, each OFDM modulator producing a frame having at least one inserted symbol, a plurality of data symbols, and cyclic prefixes;

Q transmitting antennas, each transmitting antenna connected to a respective OFDM modulator, the transmitting antennas configured to transmit a respective frame over a channel;

a number (L) of receiving antennas for receiving the transmitted frames; and

L OFDM demodulators, each OFDM demodulator corresponding to a respective receiving antenna, the L OFDM demodulators including a synchronization circuit that processes the received frame in order to synchronize the received frame in both time domain and frequency domain; and

an OFDM decoder, wherein the OFDM decoder comprises;

a space-time processor that receives an output from each of the L OFDM demodulators;

a parameter estimator that receives an output from each of the L OFDM demodulators and estimates parameters of the channel;

a parallel-to-serial converter-connected to an output of the space-time processor;

a symbol demapper having a first input and second input, the first input connected to an output of the parameter estimator, the second input connected to an output of the parallel-to-serial converter; and

a channel decoder connected to an output of the symbol demapper.

20. A method for synchronizing a Multi-Input Multi-Output (MIMO) Orthogonal Frequency Division Multiplexing (QFDM) system in time domain and frequency domain, the method comprising:

receiving a frame of data transmitted from an OFDM transmitter, the frame of data including a training symbol having a synchronization component, the synchronization component aiding in synchronization, the frame data further including a plurality of data symbols and a plurality of cyclic prefixes;

demodulating the frame; and

synchronizing the frame with the transmitted frame from the OFDM transmitter such that the data symbols are synchronized in the time domain and frequency domain, wherein said training symbol comprises an orthogonal sequence for enhancing a fine time synchronization performance; and

wherein the frame further comprises a preamble, cyclic prefixes contained within the preamble and in the data symbols such that the cyclic prefixes in the preamble are longer than the cyclic prefixes in the data symbols, thereby countering an extended channel impulse response and improving synchronization.

21. A method for synchronizing a Multi-Input Multi-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) system in time domain and frequency domain, the method comprising:

receiving a frame of data transmitted from an OFDM transmitter, the frame of data including a training symbol having a synchronization component, the synchronization component aiding in synchronization, the frame data further including a plurality of data symbols and a plurality of cyclic prefixes;

demodulating the frame; and

synchronizing the frame with the transmitted frame from the OFDM transmitter such that the data symbols are synchronized in the time domain and frequency domain, wherein the synchronization component comprises a preamble whose signal transmission matrix resembles an existing space-time block code;

wherein the synchronization component includes chirp-like sequences, the chirp-like, sequences including at least one of Frank-Zadoff sequences, Chu sequences, Milewski sequences, Suehiro polyphase sequences, and Ng et al. sequences.

22. A receiver comprising:

means for obtaining a transmitted frame having an originally transmitted form and making use of a sub-carrier having a sub-carrier spacing, the frame comprising a training symbol that includes a synchronization component for synchronizing the frame with the originally transmitted form of the frame;

the frame further comprising a plurality of data symbols and a plurality of cyclic prefixes;

means for demodulating the obtained frame; and

means for synchronizing the frame to the originally transmitted form to thereby synchronize the data symbols in both time domain and frequency domain, wherein the synchronization component comprises a preamble having a transmission matrix resembling an existing space-time block code, the synchronization component further comprising chirp-like sequences including a sequence selected from the group consisting of Frank-Zadoff sequences, Chu sequences, Milewski sequences, Suehiro polyphase sequences, and Ng et al. sequences.