MIMO SLOTTED ALOHA (MSA) SYSTEM

• US 20090207954A1
• Filed: 02/19/2009
• Published: 08/20/2009
• Est. Priority Date: 02/19/2008
• Status: Active Grant

• Current Assignee: Elbit Systems of America LLC (Elbit Systems Limited)
• Original Assignee: Innovative Cocepts Incorporated
• Inventors: Dai, Qingyuan, Woloszynski, Charles H.
• Granted Patent: US 8,553,824 B2
• Time in Patent Office: Days
• US Class Current: 375/345
• CPC Class Codes: H04J 3/0605 Special codes used as synchronising signal
• 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 27/2657 Carrier synchronisation
• CPC Class Codes: H04W 56/001 Synchronization between nodes

Abstract

A method including receiving at a synchronizing node a first reference frame from a first reference node at a first time and storing a first time value representing the first time, and calculating a timing estimator by subtracting a minimum time value, representing the distance from the synchronizing node to the first reference node, from the first time value. The method includes receiving at the synchronizing node a second reference frame at a second time and transmitting from the synchronizing node to the first reference node a short timing contention time frame. The method includes receiving at the synchronizing node from the first reference node an arrival time value representing the time at which the first reference node received the short timing contention frame and calculating a time drift from the first arrival time value and the second time value and adjusting the timing estimator based on the time drift.

Claims

1. A method comprising:

receiving at a synchronizing node a first reference frame from a first reference node at a first time and storing a first time value representing the first time;

calculating a timing estimator by subtracting a minimum time value, representing the distance from the synchronizing node to the first reference node, from the first time value;

receiving at the synchronizing node a second reference frame at a second time and storing a second time value representing the second time;

transmitting from the synchronizing node to the first reference node a short timing contention time frame;

receiving at the synchronizing node from the first reference node an arrival time value representing the time at which the first reference node received the short timing contention frame;

calculating a first time drift from the first arrival time value and the second time value;

adjusting the timing estimator based on the first time drift; and

transmitting from the synchronizing node a frame according to a timing determined from the timing estimator.

2. The method of claim 1, wherein calculating the time drift is performed according to the equation:

first time drift=(first arrival time value−

second time value)/2.

3. The method of claim 1, further comprising:

determining a reference frequency of the transmission of the second reference frame; and

adjusting an oscillator at the synchronizing node based on the reference frequency.

4. The method of claim 1, further comprising:

receiving a third reference frame from a second reference node at a third time and storing a third time value representing the third time;

transmitting the short timing contention frame

receiving at the synchronizing node from the second reference node a second arrival time value representing the time at which the second reference node received the short timing contention frame;

calculating a second time drift from the second arrival time value and the third time value;

determining an average time drift of the first time drift and the second time drift; and

adjusting the timing estimator based on the average time drift.

5. The method of claim 4, further comprising:

determining a first reference node carrier frequency offset from the second reference frame;

determining a second reference node carrier frequency offset from the third reference frame;

determining an average carrier frequency offset of the first reference node carrier frequency offset and the second reference node carrier frequency offset; and

adjusting an oscillator at the synchronizing node based on the average carrier frequency offset.

6. The method of claim 5, wherein determining the first reference node carrier frequency offset comprises analyzing two frequency tone fields in the second reference frame and determining the second reference node carrier frequency offset comprises analyzing two frequency tone fields in the third reference frame.

7. A method comprising:

receiving at a first node a plurality of frames from a plurality of reference nodes;

measuring and recording frame timing values for each the plurality of frames representing the arrival times of the plurality of frames;

calculating carrier frequency offset values for each of the plurality of frames;

determining an average carrier frequency offset value of the plurality of frames;

adjusting a local oscillator at the synchronizing node;

transmitting a frame to the plurality of reference nodes;

receiving second frame timing values from the plurality of reference nodes representing the times the respective reference nodes received the short timing contention frames;

determining an average time drift for the arrival time values, wherein the time drift for each of the plurality of reference nodes equals;

(second timing value−

timing value)/2;

adjusting the frame timing according to the average time drift; and

transmitting data from the synchronizing frame according to the estimated frame timing.

8. The method of claim 7, further comprising:

for each of the plurality of frames, calculating an automatic gain control value from a frequency tone field of the frame.

9. The method of claim 7, wherein calculating the automatic gain control value comprises:

estimating the automatic gain control value as a maximum value;

comparing a receive signal strength indicator for the frame to a first threshold value;

if the receive signal strength indicator is greater than the first threshold value, decreasing the automatic gain control value by a first predetermined amount;

comparing the receive signal strength indicator for the frame to a second threshold value; and

if the receive signal strength indicator is less than the second threshold value, increasing the automatic gain control value by a second predetermined amount.

10. The method of claim 7, further comprising:

identifying a node associated with one of the plurality of frames by analyzing at least one frequency tone field of the frame,

wherein a frequency tone field of a frame comprises a randomly selected complex tone, and

wherein the analyzing comprises;

determining a complex tone power by performing a fast Fourier transform on the frequency tone field;

calculating a noise power; and

determining a signal to noise ratio of the complex tone power to the noise power.

11. The method of claim 7, wherein calculating a carrier frequency offset value for a frame comprises analyzing two frequency tone fields of the frame.

12. The method of claim 10, further comprising:

computing channel state information for each node by analyzing channel state information fields of the plurality of frames,

wherein each frame includes a channel state information field corresponding to each node, and

wherein the analyzing channel state information fields includes performing a fast Fourier transform.

13. The method of claim 12, further comprising analyzing a data field of the frame.

14. A method comprising:

receiving at a first node a plurality of signals from a plurality of nodes;

determining a plurality of carrier frequencies for the plurality of nodes;

determining a consensus of the plurality of carrier frequencies;

adjusting a local oscillator at the first node to match the consensus; and

transmitting data from the first node to one of the plurality of nodes according to timing determined by the local oscillator.

15. The method of claim 14, wherein computing the carrier frequency for a node comprises

receiving and analyzing two preambles.

16. The method of claim 14, further comprising estimating the phase offsets of the plurality of signals.

17. The method of claim 16, further comprising transmitting from the first node to the plurality of nodes an estimation of the phase offsets of the plurality of nodes.

18. The method of claim 17, further comprising:

determining an index for each node by analyzing a frequency tone field in the signal received from that node, wherein the index is a randomly-selected complex tone; and

computing channel state information for each of the plurality of nodes.

19. A computer-readable medium encoded with a data structure, the data structure comprising:

three identical preambles;

a plurality of channel state information estimation fields; and

a data field.

20. The computer-readable medium of claim 19, further comprising a separation field.

21. The computer-readable medium of claim 20, wherein each of the three preambles is a frequency tone field comprising a complex tone of 64 samples.

22. The computer-readable medium of claim 21, wherein each channel state information estimation field comprises 320 samples.

23. The computer-readable medium of claim 22, wherein:

the first preamble can be used for automatic gain control and timing estimation; and

the second and preambles can be used for carrier frequency offset estimation, node index detection, and signal to noise ration estimation.

24. The computer-readable medium of claim 22, wherein the plurality of channel state information estimation fields comprises a channel state information estimation field corresponding to each of the nodes in a network.