Method and apparatus for synchronization of the OFDM systems
A method and apparatus for the signal synchronization of an orthogonal frequency division multiplexing system includes a delay conjugate multiplication module, a phase processor and an edge detector. It provides estimates for the boundaries of inter-symbol interference free region by utilizing the characteristics of a guard interval in combination with the techniques of the delay conjugate multiplication module, phase differential operation, symbol-by-symbol average operation, and edge detection. The method determines a fixed optimal threshold for directly separating the inter-symbol interference region in a mobile environment to obtain an inter-symbol interference free version of symbol information.
1. A method for synchronization of orthogonal frequency division multiplexing (OFDM) systems, comprising the steps of:
(a) taking a normalized phase value to obtain a first output signal by utilizing the cyclic prefix characteristics of a guard interval to perform delay conjugate multiplication on a received signal;
(b) performing phase processing on said first output signal to obtain a second output signal indicating the location of an inter-symbol interference-free (ISI-free) region; and
(c) observing variations in said second output signal and using an edge detector to detect the location of said ISI-free region;
wherein at least one symbol-by-symbol average operation is used after said delay conjugate multiplication or before obtaining said second output signal.
2. The method as claimed in claim 1 , wherein said phase processing in said step (b) further comprises a phase differential step, an absolute value calculating step and a symbol-by-symbol average step.
3. The method as claimed in claim 1 , wherein said step (c) further comprises the steps of:
(c1) using a fixed optimal threshold to roughly separate an inter-symbol interference (ISI) region of said second output signal;
(c2) passing through a filter having a length of M sample periods to clearly separate said ISI region, wherein M is an integer number;
(c3) using a differential operation to find relative boundaries of said ISI region; and
(c4) using a mapping relation between said relative boundaries of said ISI region and boundaries of said ISI-free region to find an upper boundary ñ upper and a lower boundary ñ lower of said ISI-free region, wherein ñ upper and ñ lower are integer numbers.
4. The method as claimed in claim 1 , wherein said symbol-by-symbol average operation is to average every L input OFDM symbols to obtain an output OFDM symbol, where said L is an integer number representing the length of said symbol-by-symbol average operation.
5. The method as claimed in claim 3 , wherein said fixed optimal threshold is determined before system operation in accordance with the statistic characteristics of said second output signal.
6. The method as claimed in claim 3 , wherein in said step (c4), the location of said ISI-free region is transformed into useful information, including the location of middle point and length of said ISI-free region, symbol timing information required for estimating fractional carrier frequency offset, and information for estimating maximum delay spread of a channel.
7. The method as claimed in claim 5 , wherein the characteristics of said fixed optimal threshold include the mean and the standard deviation of said second output signal.
8. The method as claimed in claim 6 , wherein said middle point of said ISI-free region is determined by using mean value of said upper boundary ñ upper and said lower boundary ñ lower .
9. The method as claimed in claim 6 , wherein said length of said ISI-free region is determined by using the distance between said upper boundary ñ upper and said lower boundary ñ lower .
10. The method as claimed in claim 6 , wherein said maximum delay spread is determined by the difference between the number of sample points in the guard interval of an OFDM symbol and the length of said ISI-free region.
11. A synchronization apparatus for orthogonal frequency division multiplexing (OFDM) systems, comprising:
a delay conjugate multiplication module to perform the multiplication between a received signal and an N-sample delayed and conjugated version of said received signal, and a phase calculator to obtain a normalized phase value to generate a first output signal;
a phase processor for processing said first output signal to generate a second output signal indicating the location of an inter-symbol interference-free (ISI-free) region; and
an edge detector for observing variations in said second output signal and detecting the location of said ISI-free region;
wherein at least one symbol-by-symbol average operation is used after said delay conjugate multiplication module or before obtaining said second output signal in said phase processor.
12. The apparatus as claimed in claim 11 , wherein said phase processor further comprises a differential operation and an absolute value calculator.
13. The apparatus as claimed in claim 11 , wherein said edge detector further comprises:
a threshold calculator for determining a fixed optimal threshold to roughly separate an inter-symbol interference (ISI) region of said second output signal;
a filter for clearly separating said roughly separated ISI region;
a differential operation for finding relative boundaries of said ISI region; and
a mapper for building a corresponding relation between said relative boundaries of said ISI region and boundaries of ISI-free region to find an upper boundary and a lower boundary of said ISI-free region.