System and method for radio cell synchronization

1. A computer implemented method for radio cell synchronization, the method comprising:

receiving, from a base station, by a processor of a user equipment (UE), a wireless signal, the wireless signal comprising a plurality of synchronization signals identical to each other that repeat in predetermined time intervals, the synchronization signals comprising a plurality of substantially identical synchronization symbols;

for a plurality of coarse candidate synchronization points:

dividing, by the processor, the wireless signal into a plurality of signal segments, each equal or longer than the time interval in duration, and each including a plurality of sub-segments having same time duration as a symbol of the plurality of synchronization symbols;

calculating, by the processor, symbol-length cross-correlations between an expected symbol and the sub-segments;

calculating, by the processor, segmented symbol-wise correlations between the symbol-length cross-correlation results;

calculating, by the processor, a cost function based on the results of the symbol-wise correlations;

accumulating, by the processor, the cost functions across a plurality of signal segments; and

selecting, by the processor, a coarse synchronization point from the plurality of candidate synchronization points based on the accumulated cost functions.

2. The method of claim 1 , wherein accumulating the cost functions continues until a predetermined condition is met, and wherein determining the coarse synchronization point of the wireless signal comprises finding the accumulated cost function with a highest peak in its absolute value and selecting the coarse synchronization point based on a location of the highest peak.

3. The method of claim 1 , wherein calculating the symbol-length cross-correlations, calculating the auto-correlations, calculating the cost function and accumulating the cost functions are repeated for a plurality of frequency hypotheses.

4. The method of claim 3 , comprising estimating, by the processor, a coarse carrier frequency of the wireless signal based on a frequency hypothesis with the highest peak in its absolute value and on the phase of a value of the highest peak of the accumulated cost function.

5. The method of claim 4 , comprising:

performing estimation of a fine synchronization point and a fine carrier frequency using the coarse synchronization point and the coarse carrier frequency; and

establishing communication between the UE and the base station using, the estimated fine carrier frequency and fine synchronization point.

6. The method of claim 1 , wherein the symbol-length cross-correlations are calculated recursively.

7. The method of claim 1 , wherein the segmented symbol-wise correlations are calculated recursively, and wherein a product of a cover code is calculated in advance based on knowledge of the cover code.

8. The method of claim 1 , comprising performing, by the processor, decimation of the wireless signal prior to dividing the wireless signal to the plurality of signal segments using a varying decimation step.

9. The method of claim 1 , comprising performing, by the processor, decimation of the wireless signal prior to dividing the wireless signal to the plurality of signal segments.

10. The method of claim 1 , wherein the accumulation of the cost function is weighted using a decay factor.

11. The method of claim 10 , wherein the decay factor is determined based on the number of past accumulation or based on estimated time drift of the wireless signal.

12. The method of claim 1 , wherein the synchronization signal is a narrow-band primary synchronization signal (NPSS), and wherein the symbols are NPSS symbols with an orthogonal frequency-division multiplexing (OFDM) symbol length.

13. A computer implemented method for radio cell synchronization, the method comprising:

performing, by a processor, symbol-length cross-correlations between an expected narrow-band primary wireless signal (NPSS) symbol and sub-segments of a signal segment, the sub-segments having a time duration of the expected NPSS symbol;

performing, by the processor, symbol-wise correlations between the symbol-length cross-correlation results;

calculating, by the processor, a cost function based on the results of the symbol-wise correlations;

accumulating, by the processor, the cost functions across a plurality of signal segments; and

determining, by the processor, a coarse synchronization point and a carrier frequency of the synchronization signal based on the accumulated cost functions.

14. A device for performing radio cell synchronization, the device comprising:

a memory;

a processor configured to:

receive a wireless signal, the wireless signal comprising a plurality of synchronization signals substantially identical to each other that repeat in predetermined time intervals, the synchronization signals comprising a plurality of substantially identical synchronization symbols;

for a plurality of candidate synchronization points:

divide the wireless signal into a plurality of signal segments, each equal or longer than the time interval in duration, and each including a plurality of sub-segments having substantially same length as time duration as a symbol of the plurality of synchronization symbols;

perform symbol-length cross-correlations between an expected symbol and the sub-segments;

perform segmented symbol-wise correlations between the symbol-length cross-correlation results;

calculate a cost function based on the results of the symbol-wise correlations;

accumulate the cost functions across a plurality of signal segments; and

select a coarse synchronization point from the plurality of candidate synchronization points based on the accumulated cost function.

15. The device of claim 14 , wherein the processor is configured to continue accumulating the cost functions until a predetermined condition is met, and wherein the processor is configured to determine the coarse synchronization point of the wireless signal by finding the accumulated cost function with a highest peak in its absolute value and selecting the coarse synchronization point based on a location of the highest peak.

16. The device of claim 14 , wherein the processor is configured to repeat performing symbol-length cross-correlations, performing auto-correlations, calculating the cost function and accumulating the cost functions for a plurality of frequency hypotheses.

17. The device of claim 16 , wherein the processor is configured to estimate a coarse frequency of the wireless signal based on a frequency hypothesis with the highest peak in its absolute value and on the phase of a value of the highest peak of the accumulated cost function.

18. The device of claim 14 , wherein the processor is configured to perform decimation of the wireless signal prior to dividing the wireless signal to the plurality of signal segments, using a varying decimation step.

19. The device of claim 14 , wherein the processor is configured to perform decimation of the wireless signal prior to dividing the wireless signal to the plurality of signal segments.

20. The device of claim 14 , wherein the synchronization signal is a narrow-band primary synchronization signal (NPSS), and wherein the symbols are NPSS symbols with an orthogonal frequency-division multiplexing (OFDM) symbol length.