OFDM receiving system

• US 6,151,295 A
• Filed: 02/26/1998
• Issued: 11/21/2000
• Est. Priority Date: 02/26/1998
• Status: Expired due to Term

• Current Assignee: Marvell Asia Pte Limited (Marvell Technology Group Limited)
• Original Assignee: Wavesat Telecom Incorporated
• Inventors: Young, Donovan, Ma, Steve, Ma, Jiang Lei
• Primary Examiner(s): Olms, Douglas W.
• Assistant Examiner(s): VANDERPUYE, KENNETH N
• Application Number:
  US09/030,865
• Time in Patent Office: 999 Days
• Field of Search: 370/203, 370/208, 370/503, 370/509, 370/510, 370/512, 370/504, 370/514, 375/229, 375/260
• US Class Current: 370/203
• CPC Class Codes: H04L 1/0071 Use of interleaving interleaving per se H03M13/27
• CPC Class Codes: H04L 2025/03414 Multicarrier
• CPC Class Codes: H04L 2025/03522 Frequency domain
• CPC Class Codes: H04L 25/03159 operating in the frequency domain H04L25/03165, H04L25/03178 take precedence
• CPC Class Codes: H04L 27/2607 Cyclic extensions
• CPC Class Codes: H04L 27/2656 Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
• CPC Class Codes: H04L 27/2678 using cyclostationarities, e.g. cyclic prefix or postfix

Abstract

A method of frame synchronization for OFDM point-to-multipoint channels is provided according to a first aspect of the invention, wherein sequences of reference frames are inserted at prescribed intervals between sequences of information frames. Each frame comprises a cyclic prefix, composed of two identical segments of data, which is used for locating a frame synchronization starting point by applying a subtraction function on the two segments of data for locating the starting point at the location of the minimum of the function. A method of channel equalization for OFDM channels is also provided according to a second aspect of the invention, wherein the reference frames and data known to a receiver are used for creating an equalization vector for correcting broadcast errors of the information frames received at the receiver side.

Claims

1. A method of frame synchronization for OFDM channels, wherein a frame synchronization starting point is to be located within an incoming data sample stream signal having repeated data samples at predetermined intervals, said method comprising the steps of:

a) subtracting data samples from two points from different frames of said data sample stream signal separated by a predetermined number of data samples defined by a subtraction window function;

b) incrementing said window function with an increment value corresponding to at least one sample;

c) repeating said steps a) and b) until a minimum of a subtraction result thereof is detected said minimum corresponding to substantially identical date samples from said two points; and

d) obtaining said frame synchronization starting point from a result of step c).

2. The method of frame synchronization for OFDM channels as claimed in claim 1, wherein said data sample stream signal comprises alternate sequences of reference frames and information frames.

3. The method of frame synchronization for OFDM channels as claimed in claim 2, wherein each of said sequences of reference frames comprises three consecutive reference frames, each said reference frame comprising identical data.

4. The method of frame synchronization for OFDM channels as claimed in claim 3, wherein each of said reference frames and said information frames comprises an individual cyclic prefix composed of a first and a second identical segments of data, wherein said first segment of data is part of a valuable portion of data of said frame and is copied and appended to said valuable portion of data to generate said second segment of data.

5. The method of frame synchronization as claimed in claim 4, wherein a reference frame search is performed for estimating the location of a frame synchronization starting point, wherein

said step a) comprises performing a first subtraction on said data samples from said two points being separated by an integral number of frames defined by said subtraction window function;

said step c) comprises detecting a falling edge of said minimum of a result of an application of said first subtraction on a series of data samples including a range from a beginning of a first reference frame of said sequence of reference frames to at least a second reference frame of said sequence, said minimum beginning with said falling edge and ending with a rising edge;

said step d) comprises estimating the location of said frame synchronization starting point at a location of said falling edge.

6. The method of frame synchronization as claimed in claim 5, further comprising the steps of:

e) performing a second subtraction on data samples from two points of a frame separated by a length of said valuable portion of data defined by a further subtraction window function;

f) incrementing said further window function with an increment value corresponding to at least one sample;

g) repeating said steps e) and f) until a minimum of a subtraction result thereof is detected said minimum corresponding to substantially identical data samples from said two points; and

h) obtaining a precise location of said minimum detected in said step g) as being a precise frame synchronization starting point.

7. The method of frame synchronization as claimed in claim 1, wherein a frame synchronization is performed for estimating the location of said frame synchronization starting point in said incoming sample stream signal, said signal comprising OFDM frames, wherein

said step a) comprises performing a first subtraction on said data samples from said two points being separated by an integral number of frames defined by said subtraction window function;

said step c) comprises detecting a falling edge of said minimum of a result of an application of said first subtraction on a series of data samples including a range from a beginning of a first frame to at least a second frame, said minimum beginning with said falling edge and ending with a rising edge;

said step d) comprises estimating the location of said frame synchronization starting point at a location of said failing edge.

8. The method of frame synchronization as claimed in claim 7, wherein said data sample stream signal comprises OFDM frames, each of said frames comprising an individual cyclic prefix composed of a first and a second identical segments of data, wherein said first segment of data is an end part of a valuable portion of data of said frame and is copied and appended to said valuable portion of data to generate said second segment of data, said method further comprising the steps of:

e) performing a second subtraction on data samples from two points of a frame separated by a length of said valuable portion of data defined by a further subtraction window function;

f) incrementing said further window function with an increment value corresponding to at least one sample;

g) repeating said steps e) and f) until a minimum of a subtraction result thereof is detected said minimum corresponding to substantially identical data samples from said two points; and

h) obtaining a precise location of said minimum detected in said step g) as being a precise frame synchronization starting point.

9. The method of frame synchronization for OFDM channels as claimed in claim 8, further comprising the following steps which are performed prior to step a):

i) at a transmitter side, incorporating among said sequences of information frames to be transmitted sequences of reference frames at prescribed regular intervals, each of said sequences of reference frames comprising at least one reference frame of data known to a receiver, each of said at least one reference frame comprising identical data known to said receiver;

i) at said transmitter side, creating said individual cyclic prefix for each frame to be transmitted, said cyclic prefix being composed of said two identical segments of data, wherein either an ending portion of each said frame is copied and appended to a beginning of said each frame, or a beginning portion of said frame is copied and appended to an end of said each frame.

10. A method of channel equalization for point-to-multipoint OFDM channels performed at a receiver side of an OFDM channel, comprising the steps of:

a) receiving an incoming data sample stream signal comprising interleaved sequences of reference and information frames, each of said reference frame containing data known to a receiver;

b) detecting said reference frames in said incoming data sample stream signal of OFDM frames;

c) transforming said incoming stream of OFDM frames from a time domain to a frequency domain;

d) creating a correction vector using said sequence of reference frames and data known to a receiver for correcting broadcast errors created by distortions caused by said OFDM channel;

e) correcting each information frame from said sequences of information frames received at said receiver using said correction vector for countering a distortion effect of said OFDM channel on said frames.

11. The method of channel equalization as claimed in claim 10 further comprising prior to step c) a time domain correlation operation of each incoming pair of reference frames for obtaining an ε

factor, wherein said step d) comprises a correction of a frequency offset effect of said OFDM channel using said ε

factor.

12. The method of channel equalization as claimed in claim 10, wherein said interleaved sequences of reference and information frames are alternated and have a predetermined number of frames, and step b) is performed by keeping track of a previously located frame synchronization starting point.

13. The method of channel equalization as claimed in claim 12, wherein a frame synchronization operation is performed prior to step b) for locating a frame synchronization starting point within said incoming stream of OFDM frames, said frame synchronization comprising the steps of:

f) subtracting data samples from two points of said data sample stream signal separated by a predetermined number of data samples defined by a subtraction window function;

g) incrementing said window function with in increment value corresponding to at least one sample;

h) repeating said steps f) and g) until a minimum of a subtraction result thereof is detected said minimum corresponding to substantially identical data samples from said two points; and

i) obtaining said frame synchronization starting point from a result of step h).

14. The method of channel equalization as claimed in claim 13, wherein each of said sequences of reference frames comprises three consecutive reference frames, each said reference frame comprising identical data.

15. The method of channel equalization as claimed in claim 14, wherein each of said reference frames and said information frames comprises an individual cyclic prefix composed of a first and a second identical segments of data, wherein said first segment of data is part of a valuable portion of data of said frame and is copied and appended to said valuable portion of data to generate said second segment of data.

16. The method of channel equalization as claimed in claim 15, wherein a reference frame search is performed for estimating the location of a frame synchronization starting point, wherein

said step f) comprises performing a first subtraction on said data samples from said two points being separated by an integral number of frames defined by said subtraction window function;

said step h) comprises detecting a falling edge of said minimum of a result of an application of said first subtraction on a series of data samples including a range from a beginning of a first frame to at least a second frame, said minimum beginning with said falling edge and ending with a rising edge;

said step i) comprises estimating the location of said frame synchronization starting point at a location of said falling edge.

17. The method of channel equalization as claimed in claim 16, wherein said data sample stream signal comprises OFDM frames, each of said frames comprising an individual cyclic prefix composed of a first and a second identical segments of data, wherein said first segment of data is an end part of a valuable portion of data of said frame and is copied and appended to said valuable portion of data to generate said second segment of data, said method further comprising the steps of:

j) performing a second subtraction on data samples from two points of a frame separated by a length of said valuable portion of data defined by a further subtraction window function;

k) incrementing said further window function with an increment value corresponding to at least one sample;

l) repeating said steps j) and k) until a minimum of a subtraction result thereof is detected said minimum corresponding to substantially identical data samples from said two points; and

m) obtaining a precise location of said minimum detected in said step l) as being a precise frame synchronization starting point.

18. The method of channel equalization as claimed in claim 10, wherein said sequence of reference frames comprises at least one reference frame and wherein step d) comprises performing for at least one reference frame of said sequence of reference frames a division of a first reference frame from said sequence by said data known to said receiver stored at said receiver side and obtaining at least one quotient representing at least one equalization vector representing an inverse of a frequency response of said OFDM channel.

19. The method of channel equalization as claimed in claim 18, wherein when more than one equalization vector is obtained, an average of all equalization vectors is computed for obtaining an averaged equalization vector.

20. The method of channel equalization as claimed in claim 19, wherein

each of said reference and information frames comprises a cyclic prefix composed of a first and a second identical segments of data, wherein said first segment of data is an end part of a valuable portion of data of said frame and is copied and appended to said valuable portion of data to generate said second segment of data;

said step c) uses a frame synchronization starting point within said cyclic prefix for transforming said incoming stream of OFDM frames from a time domain to a frequency domain.

21. The method of channel equalization as claimed in claim 20, further comprising the step of:

verifying an accuracy of said average of all equalization vectors by following the steps of;

performing a multiplication of said averaged equalization vector with a last reference frame from said sequence of reference frames, said last reference frame having not being used for prior computing of said equalization vectors, wherein said result represents an equalized or corrected said last reference frame, whereby the corrected reference frame should be equal to said data known to said receiver;

comparing said result with said data known to said receiver for computing an accuracy of said averaged equalization vector;

moving said frame synchronization starting point within said cyclic prefix and using new data read from a new location for creating a new averaged equalization vector, using previous steps.

22. The method of channel equalization as claimed in claim 12 wherein said sequence of reference frames comprises three reference frames and each of said reference and information frames comprises a cyclic prefix composed of a first and a second identical segments of data, wherein said first segment of data is part of a valuable portion of data of said frame and is copied and appended to said valuable portion of data to generate said second segment of data, and wherein said step c) uses a frame synchronization starting point within said cyclic prefix for transforming said incoming stream of OFDM frames from a time domain to a frequency domain.

23. The method of channel equalization as claimed in claim 22 wherein step d) comprises the steps of:

dividing a first reference frame from said sequence of reference frames by said data known to said receiver stored at said receiver side and obtaining at a first quotient representing a first inverse of a frequency response of said OFDM channel which is a first equalization vector for said OFDM channel;

dividing a second reference frames from said sequence of reference frames by said data known to said receiver stored at said receiver and obtaining at a second quotient representing a second inverse of a frequency response of said OFDM channel which is a first equalization vector for said OFDM channel;

computing an average of said first quotient and second quotient, said average representing an equalization vector of said OFDM channel;

verifying an accuracy of said equalization vector by comparing a result of a multiplication of said vector with a third reference frame form said sequence of reference frames, with said data known to said receiver, said result representing an equalized or corrected reference frame, whereby said corrected reference frame should be equal to said data known to said receiver;

moving said frame synchronization starting point within said cyclic prefix and using new data read from a new location for creating a new averaged equalization vector, using previous steps.