Patent EA-003198-B1 (EA)
Title: Receiving system using ofdm
Inventor: MA STEVE
IPC: H04L25/03
Filed: 1999-02-25
Abstract: 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, 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; and b) detecting when said data is substantially identical at said two points of said sample stream, so as to determine said synchronization starting point; characterized in that each of said frames comprises an individual cyclic prefix 25; and c) said step of subtracting includes incrementing a subtraction window function with an increment value corresponding to at least one sample; d) said steps a) and c) are repeated until a minimum of a subtraction result is detected, corresponding to substantial identity of said data samples from said two points. 2. The method according to claim 1, characterized in that said data sample stream signal comprises alternate sequences of reference frames 34 and information frames 36. 3. The method according to claim 2, characterized in that each of said sequences of reference frames 34 comprises three consecutive reference frames, each said reference frame comprising identical data. 4. The method according to claim 3, characterized in that each of said reference frames 34 and said information frames 36 comprises an individual cyclic prefix 25 composed of a first and a second identical segments 26 of data, said first segment of data being part of a valuable portion 30 of data of said frame and being copied and appended to said valuable portion of data to generate said second segment of data. 5. The method according to claim 4, characterized in that a rough 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 d) comprises: - detecting a falling edge 39 of said minimum of a result of an application of said first subtraction on a series of data samples ranging 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 39 and ending with a rising edge; and - estimating a location of said frame synchronization starting point at a location of said falling edge 39. 6. The method according to 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; 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 corresponding to substantial identity of said data samples from said two points is detected, me location of said minimum being a precise frame synchronization starting point. 7. The method according to claim 1, characterized in that 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 d) comprises: - detecting a falling edge of said minimum of a result of an application of said first subtraction on a series of data samples ranging 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; and - estimating the location of said frame synchronization starting point at a location of said falling edge. 8. The method according to claim 7, characterized in that said individual cyclic prefix is 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 corresponding to substantial identity of said data samples from said two points is detected, said minimum being a precise frame synchronization starting point. 9. The method according to claim 8, further comprising the following steps which are performed prior to step a): h) 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 25 being composed of said two identical segments 26 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 framell. 10. An OFDM receiver suitable for determining a frame synchronization starting point within an incoming data sample signal having repeated data samples at predetermined intervals, comprising a subtracter for subtracting data samples from two points from different frames of said data sample stream signal separated by a predetermined number of data samples and for detecting when said data is substantially identical at said two points of said bit stream, so as to determine said synchronization starting point, characterized in that said receiver includes means for incrementing a subtraction window function with an increment value corresponding to at least one sample and for detecting a minimum of a subtraction result, corresponding to substantial identity of said data samples from said two points, each of said frames comprising an individual cyclic prefix. 11. The OFDM receiver according to claim 10, characterized in that said data sample stream signal comprises alternate sequences of reference frames and information frames. 12. The OFDM receiver according to claim 11, characterized in that each of said sequences of reference frames comprises three consecutive reference frames, each said reference frame comprising identical data. 13. The OFDM receiver according to claim 12, characterized in that 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, said first segment of data being part of a valuable portion of data of said frame and being copied and appended to said valuable portion of data to generate said second segment.