METHODS, APPARATUSES AND SYSTEM FOR ASYNCHRONOUS SPREAD-SPECTRUM COMMUNICATION

• US 20100054131A1
• Filed: 08/25/2009
• Published: 03/04/2010
• Est. Priority Date: 08/26/2008
• Status: Active Grant

• Current Assignee: Agence Spatiale Europeenne
• Original Assignee: Agence Spatiale Europeenne
• Inventors: De Gaudenzi, Riccardo, del Rio Herrero, Oscar
• Granted Patent: US 7,990,874 B2
• Time in Patent Office: Days
• US Class Current: 370/236
• CPC Class Codes: H04B 1/71072 Successive interference cancellation
• CPC Class Codes: H04B 2201/709709 Methods of preventing interference

Abstract

A method of receiving data packets asynchronously transmitted by a plurality of user terminals using a spread-spectrum medium access protocol, comprises a step of cancelling interferences between colliding packets according to an innovative “sliding window” processing algorithm. A gateway receiver is adapted for carrying out interference cancellation using this algorithm. Asynchronous packet transmission from a user terminal is controlled by estimating a parameter indicative of the quality of information transmission through a communication channel; and inhibiting or allowing data transmission depending on a comparison between said estimated parameter and an adaptively varying threshold. A user terminal comprises transmission control means adapted for carrying out such a method. A communication system comprises a plurality of mobile user terminal communicating with a gateway through a satellite channel using an asynchronous spread-spectrum medium access protocol without closed-loop power control, wherein the user terminals and said gateway are of the kind described above.

Claims

1. A method of receiving data packets asynchronously transmitted by a plurality of user terminals using an unslotted spread-spectrum medium access protocol, each data packet being constituted by a contiguous sequence of bits comprising at least a preamble and a payload, the method including a step of successive interference cancellation comprising:

f) storing received signal samples in a processing memory;

g) performing packet identification on all the signals stored in said processing memory within a sliding processing window;

h) selecting one among the identified packets, decoding it and, if decoding is successful, cancelling the interferences of said packet with the other received data inside the sliding processing window;

i) repeating step c) for all or part of the packets identified at step b); and

j) shifting the processing window by a shifting step which is shorter than the overall length of said processing window.

2. A method according to claim 1, wherein substeps b), c) and d) are iterated a predetermined number of times, greater than one.

3. A method according to claim 2, wherein substeps b), c) and d) are iterated the maximum number of times allowing real-time processing of the received data.

4. A method according to claim 1, wherein substep c) comprises selecting the identified packet having the higher signal-to-noise-and-interference ratio.

5. A method according to claim 1, wherein substep b) comprises performing packet identification by detecting known packet preambles.

6. A method according to claim 5, wherein substep c) comprises:

c1) performing data-aided channel estimation for the selected packet over the preamble;

c2) performing error-correction decoding of the selected packet;

c3) performing enhanced data-aided channel estimation over the whole packet;

c4) reconstructing said packet at baseband and performing interference cancellation.

7. A method according to claim 6, wherein enhanced data-aided channel estimation, packet reconstruction and interference cancellation are performed only if error-correction decoding has been successful.

8. A method according to claim 1, wherein the length of said sliding processing window (PW) is at least equal to two, and preferably three times the length of a physical layer packet emitted by said user terminals.

9. A method according to claim 1, wherein the length of said shifting step is between one third and one half of the length of said sliding processing window.

10. A gateway receiver (GWR) for receiving data packets asynchronously transmitted by a plurality of user terminals (ST) using a spread-spectrum medium access protocol, comprising:

a front-end unit (FEU), for demodulating and digitizing the received signals; and

a data processing unit (DPU) for performing spread-spectrum demodulation, decoding and interference cancellation;

wherein said data processing unit is adapted for carrying out an interference cancellation method according to claim 1.

11. A method of performing bi-directional asynchronous communication between a gateway receiver and a plurality of user terminals using an unslotted spread-spectrum medium access protocol, comprising a step of transmitting data packet from at least one said user terminal and a step of receiving transmitted data packet at said gateway receiver, wherein:

the step of receiving transmitted data packets is performed according to claim 1; and

the step of a transmitting data packet includes sub-step of controlling asynchronous packet transmission, comprising;

estimating a parameter (S{circumflex over (N)}R(t_k)) indicative of the quality of information transmission through the communication channel, on the basis of data emitted by said gateway and received by said user terminal through said channel; and

inhibiting data transmission by said user terminal whenever said estimated parameter is indicative of an insufficient transmission quality, according to an adaptively varying criterion.

12. A method according to claim 11, wherein said parameter indicative of the quality of information transmission through the communication channel is a signal-to-noise-and-interference ratio (S{circumflex over (N)}R(t_k));

and wherein data transmission is inhibited when the estimated signal-to-noise-and-interference ratio is lower than an adaptively varying threshold (SNR_th(t*)).

13. A method according to claim 12, wherein said adaptively varying threshold (SNR_th(t*)) is equal to an average of the N highest values of the signal-to-noise-and-interference ratios estimated within a predetermined sliding observation window, N being a predetermined integer, multiplied by a predetermined factor lower than one.

14. A method according to claim 12, wherein said signal-to-noise-and-interference ratio is estimated from a known pilot sequence of a received data packet.

15. A method according to claim 11 comprising:

when transmission is not inhibited, and if at least a data packet is ready for transmission, transmitting said data packet at a random time within a time window of a predetermined length.

16. A method according to claim 15, further comprising:

upon reception at the user terminal of a signal, emitted by said gateway and indicative of a channel congestion situation, increasing the length of said time window by a predetermined amount with a first probability, and upon lack of reception of said signal for a predetermined time, decreasing the length of said time window with a second probability.

17. A method according to claim 16, wherein said first probability decreases and said second probability increases as the length of said time window increases.

18. A method according to claim 16, wherein the length of said time window is scaled according to a priority level of the data packet to be transmitted.

19. A method according to claim 11, further comprising randomly varying the transmit power for each packet, independently from other user terminals communicating with a same gateway, according to a probability distribution adapted for optimizing a received power distribution at said gateway.

20. A communication system comprising a plurality of mobile user terminal (ST) communicating with at least one gateway through a satellite channel using an asynchronous spread-spectrum medium access protocol without closed-loop power control, characterized in that:

said user terminals comprise transmission control means adapted for carrying out a sub-step of controlling asynchronous packet according to claim 11; and

said or each gateway comprises a gateway receiver (GWR) for receiving data packets asynchronously transmitted by a plurality of user terminals (ST) using a spread-spectrum medium access protocol, comprising;

a front-end unit (FEU), for demodulating and digitizing the received signals; and

a data processing unit (DPU) for performing spread-spectrum demodulation, decoding and interference cancellation.

21. A communication system according to claim 20, wherein said gateway further comprises means for detecting a congestion situation of the satellite channel, and emitting a congestion signal accordingly;

and wherein said user terminals comprises transmission control means operable upon reception of a congestion signal emitted by said gateway, for increasing the length of said time window by a predetermined amount with a first probability, and upon lack of reception of said congestion signal for a predetermined time, for decreasing the length of said time window with a second probability*.

22. A communication system according to claim 20, wherein the link margin for said user terminals is sized in order to achieve a desired floor Packet Error Rate (PER).