Diversity in orthogonal frequency division multiplexing systems

• US 6,807,145 B1
• Filed: 01/20/2000
• Issued: 10/19/2004
• Est. Priority Date: 12/06/1999
• Status: Expired due to Term

• Current Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Sayeed, Zulfiquar, Weerackody, Vijitha
• Primary Examiner(s): Pham, Chi
• Assistant Examiner(s): Ferris, Derrick W
• Application Number:
  US09/488,462
• Time in Patent Office: 1,734 Days
• Field of Search: 370/203, 370/210, 370/252, 455/91, 455/101, 375/346, 375/347, 375/348, 375/349
• US Class Current: 370/203
• CPC Class Codes: H04B 7/0613 using simultaneous transmission H04B7/0686 takes precedence
• CPC Class Codes: H04B 7/0682 using phase diversity (e.g. phase sweeping)
• CPC Class Codes: H04B 7/084 Equal gain combining, only phase adjustments antenna beam scanning or forming by phase or amplitude control H01Q3/26, e.g. phased arrays
• CPC Class Codes: H04L 1/0059 Convolutional codes
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

A system and method is disclosed for improving diversity in a multiple transmitter or multiple receiver orthogonal frequency division multiplexing (OFDM) system. Signal diversity is improved by introducing phase offsets at the transmitters and/or the receivers. The phase offsets are unique to each transmitter or receiver. The unique phase offsets are applied to individual symbols of the OFDM signal in the frequency domain.

Claims

1. A method for improving diversity in a multiple transmitter orthogonal frequency division multiplexing system, comprising the steps of:

receiving a data signal at a first transmitter and a second transmitter;

processing said data signal at said first transmitter and at said second transmitter to generate a plurality of differentially encoded complex symbols at said first and second transmitters; and

introducing first phase offsets to said plurality of differentially encoded complex symbols at said first transmitter in the frequency domain to generate a first stream of transmit symbols;

introducing second phase offsets to said plurality of differentially encoded complex symbols at said second transmitter in the frequency domain to obtain a second stream of transmit symbols;

transforming said first and second streams of transmit symbols from the frequency domain to the time domain; and

transmitting said first and second streams of transmit symbols in the time domain from said first and second transmitters, respectively, such that said first and second streams of transmit symbols at said first transmitter and at said second transmitter have unique phase offsets.

2. The method of claim 1, wherein said first phase offsets are zero degrees.

3. The method of claim 1, wherein said step of introducing first phase offsets comprises assigning a first unique phase offset value to said first transmitter and incrementally rotating each of said plurality of differentially encoded complex symbols more than a preceding one of said plurality of differentially encoded complex symbols by said first unique phase offset;

and

said step of introducing second phase offsets comprises assigning a second unique phase offset value to said second transmitter and incrementally rotating each of said plurality of differentially encoded complex symbols more than a preceding one of said plurality of differentially encoded complex symbols by said second unique phase offset, such that a phase of each of said plurality of differentially encoded complex symbols in said first transmitter is different from a phase of corresponding differentially encoded complex symbols in said second transmitter.

4. The method of claim 3, wherein said first unique phase offset and said second unique phase offset are between negative 10 degrees and positive 10 degrees.

5. The method of claim 4, wherein said first unique phase offset is zero degrees.

6. The method of claim 1, wherein said step of introducing first phase offsets comprises rotating each of said plurality of differentially encoded complex symbols by a first random phase offset chosen from within a first range of determined phase offsets;

and

said step of introducing second phase offsets comprises rotating each of said plurality of differentially encoded complex symbols by a second random phase offset chosen from within a second range of determined phase offsets, wherein said first random phase offset and said second random phase offset are unequal.

7. The method of claim 6, wherein said first and second ranges of determined phase offsets are between zero degrees and positive 10 degrees.

8. The method of claim 1, wherein said step of introducing first phase offsets comprises rotating each of said plurality of differentially encoded complex symbols by a first randomly chosen phase offset within a range between a first negative value and a first positive value;

and

said step of introducing second phase offsets comprises rotating each of said plurality of differentially encoded complex symbols by a randomly chosen phase offset within a range between a second negative value and a second positive value.

9. The method of claim 8, wherein said first and second determined phase offsets are between zero and 10 degrees.

10. A method for transmitting a digital data signal on a frequency division multiplex system using at least two transmitters, each transmitter corresponding to a communication channel, the method comprising the steps of:

applying a channel code to a digital signal to generate a sequence of sub-carrier symbols;

applying first phase offsets to said sequence of sub-carrier symbols in the frequency domain at a first transmitter, said offsets unique to said first transmitter, to obtain a first sequence of phase offset symbols;

applying second phase offsets to said sequence of sub-carrier symbols in the frequency domain at a second transmitter, said offsets unique to said second transmitter, to obtain a second sequence of phase offset symbols;

a transforming said first and second sequence of phase offset symbols from the frequency domain to the time domain; and

transmitting said first and second sequence of phase offset symbols in the time domain from said first and second transmitters, respectively, such that frequency diversity is increased in the frequency division multiplex system.

11. The method of claim 10, wherein said first and second phase offsets are introduced deterministically.

12. The method of claim 11, wherein the phase difference between adjacent sub-carrier symbols at each of said first and second transmitters is less than 10 degrees.

13. An orthogonal frequency division multiplexing transmission system for transmitting a data signal comprising:

a plurality of transmitters (i), each of said transmitters comprising;

a coder coupled to said data signal, said coder generating a differentially encoded complex signal comprising a plurality of complex symbols (n);

a rotator coupled to said coder for introducing a phase offset in the frequency domain, said offset unique to each of said plurality of transmitters, to each of said plurality of complex symbols (n), wherein said phase offset is unique for each of said plurality of transmitters (i);

an inverse Fourier transformer coupled to said rotator for transforming said differentially encoded complex signal from the frequency domain to the time domain to prepare said differentially encoded complex signal for transmitting; and

wherein said transmitted plurality of differentially encoded complex signals are capable of being received by a single receiver.

14. The transmission system of claim 13, wherein each of said coders comprises:

a convolutional coder coupled to said data signal;

a quantary phase shift keyer coupled to said convolutional coder;

an interleaver coupled to said quantary phase shift keyer; and

a differential coder coupled between said interleaver and said rotator, said differential coder passing said differentially encoded complex signal to said rotator.

15. The transmission system of claim 14, wherein said phase offset for the n^th symbol of the i^th transmitter is

16. The transmission system of claim 15, wherein said step-size is a value chosen between 0 and 10 degrees.

17. The transmission system of claim 14, wherein said phase offset for the n^th symbol of the i^th transmitter and said phase offset for a preceding symbol has a phase difference given by

18. The transmission system of claim 17, wherein said step-size is a value chosen between 0 and 10 degrees.

19. The transmission system of claim 14, wherein said phase offset for the nth symbol of the i^th transmitter and said phase offset for a preceding symbol has a phase difference given by

20. An orthogonal frequency division multiplexing receiver system for receiving a data signal comprising:

a plurality of receivers (i), each of said receivers comprising;

a receiver circuit coupled to receive a differentially encoded complex data signal comprising a plurality of complex symbols (n);

a Fourier transformer coupled to said receiver for transforming said differentially encoded complex signal from time domain to frequency domain;

rotator coupled to said Fourier transformer for introducing a phase offset in the frequency domain, said offset unique to said each of said plurality of receivers, to each of said plurality of complex symbols (n), wherein said phase offset is unique for each of said plurality of receivers (i); and

a combiner for combining said differentially encoded complex signal in the frequency domain after rotating; and

a differential decoder for decoding said differentially encoded complex signal in the frequency domain after rotating and combining.

21. The receiver system of claim 20, wherein said decoder comprises:

a differential decoder coupled to said combiner;

a de-interleaver coupled to said differential decoder;

a quantary phase shift keyer de-modulator coupled to said de-interleaver; and

a convolutional de-coder coupled to said quantary phase shift keyer de-modulator.

22. The receiver system of claim 21, wherein said phase offset for the nth symbol of the i^th receiver is

23. The receiver system of claim 22, wherein said step-size is a value chosen between 0 and 10 degrees.

24. The receiver system of claim 21, wherein said phase offset for the nth symbol of the ith receiver and said phase offset for a preceding symbol has a phase difference given by

25. The receiver system of claim 24, wherein said step-size is a value chosen between 0 and 10 degrees.

26. The receiver system of claim 21, wherein said phase offset for the n_th symbol of the i^th receiver and said phase offset for a preceding symbol has a phase difference given by

27. The receiver system of claim 26, wherein said step-size is a value chosen between 0 and 10 degrees.