Multiple access in backscatter communication systems
A backscatter transmission scheme is provided in which a carrier signal is transmitted by a carrier communication device discontinuously as carrier signal bursts. The carrier signal, while it is on, is divided into carrier time segments. Each carrier time segment has a specified time. The different carrier time segments represent different times that backscatter devices can make backscatter transmissions. In addition, in some embodiments, a set of frequencies are available for backscatter transmission. The set of frequencies includes the carrier frequency and a set of frequencies with respective frequency shifts relative to the carrier frequency. The combination of a specific carrier time segment, a specific frequency shift, together constitutes a specific backscatter transmission opportunity.
1. A method comprising:
a backscatter device receiving a carrier signal from a carrier communication device, the carrier signal having a start time;
the backscatter device producing a backscatter carrier signal from the received carrier signal;
at a time offset relative to the start time, the backscatter device generating a backscattered signal by modulating data using the backscatter carrier signal and transmitting the backscattered signal using a transmission resource; and
wherein the time offset is predefined and at least in part defines a time segment of the transmission resource.
2. The method of claim 1 wherein:
the backscatter device producing a backscatter carrier signal from the received carrier signal comprises applying a frequency shift to the received carrier signal, the frequency shift being one of a set of possible frequency shifts, the set of possible frequency shifts including a zero frequency shift;
wherein the frequency shift is predefined and at least in part defines the transmission resource.
3. The method of claim 1 wherein:
modulating data comprises performing load modulation.
4. The method of claim 2 wherein generating the backscattered signal comprises:
performing scrambling of symbols output by said modulating data using a scrambling sequence, wherein a symbol alphabet of symbols output by scrambling is the same as a symbol alphabet of symbols output of said modulating data,
wherein the scrambling sequence is predefined and at least in part defines the transmission resource to be used by the backscatter device.
5. The method of claim 4 wherein said modulating data and said scrambling are performed in a single step using a load modulator.
6. The method of claim 2 wherein generating the backscattered signal comprises:
performing spreading of symbols output by said modulating data using a spreading sequence, wherein a symbol alphabet of symbols output by spreading is the same as a symbol alphabet of symbols output by modulating data,
wherein the spreading sequence is predefined and at least in part defines the transmission resource.
7. The method of claim 6 wherein said modulating and said spreading are performed in a single step using a load modulator.
8. The method of claim 3 further comprising:
the backscatter device using a resistive impedance in the load modulation to generate a zero output to achieve sparse spreading in the output of load modulation.
9. The method of claim 1 further comprising:
the backscatter device performing charging of a battery of the backscatter device using the received carrier during a time that does not overlap with the time segment of the transmission resource.
10. The method of claim 1 further comprising:
the backscatter device performing data reception using the received carrier during a time that does not overlap with the time segment of the transmission resource.
11. An apparatus comprising:
a processor and a memory, the apparatus configured to implement a method comprising:
receiving a carrier signal from a carrier communication device, the carrier signal having a start time;
producing a backscatter carrier signal from the received carrier signal;
at a time offset relative to the start time, generating a backscattered signal by modulating data using the backscatter carrier signal, and transmitting the backscattered signal using a transmission resource;
wherein the time offset is predefined and at least in part defines a time segment of the transmission resource.
12. The apparatus of claim 11 wherein:
producing a backscatter carrier signal from the received carrier signal comprises applying a frequency shift to the received carrier signal, the frequency shift being one of a set of possible frequency shifts, the set of possible frequency shifts including a zero frequency shift;
wherein the frequency shift is predefined and at least in part defines the transmission resource.
13. The apparatus of claim 11 comprising a load modulator, wherein said modulating the data is performed using the load modulator.
14. The apparatus of claim 12 wherein generating the backscattered signal comprises:
performing scrambling of symbols output by said modulating data using a scrambling sequence, wherein a symbol alphabet of symbols output by scrambling is the same as a symbol alphabet of symbols output of said modulating data,
wherein the scrambling sequence is predefined and at least in part defines the transmission resource to be used by the apparatus.
15. The apparatus of claim 14 further comprising a load modulator, wherein said modulating data and said scrambling are performed in a single step using the load modulator.
16. The apparatus of claim 12 wherein generating the backscattered signal comprises:
performing spreading of symbols output by said modulating data using a spreading sequence, wherein a symbol alphabet of symbols output by spreading is the same as a symbol alphabet of symbols output by modulating data,
wherein the spreading sequence is predefined and at least in part defines the transmission resource.
17. The apparatus of claim 16 further comprising a load modulator, wherein said modulating and said spreading are performed in a single step using the load modulator.
18. The apparatus of claim 13 , further configured to:
perform sparse spreading by using a resistive impedance in the load modulation to generate a zero output to achieve sparse spreading in the output of the load modulator.
19. The apparatus of claim 11 further configured to:
perform charging of a battery of the apparatus using the received carrier during a time that does not overlap with the time segment of the transmission resource.
20. The apparatus of claim 11 further configured to:
perform data reception using the received carrier during a time that does not overlap with the time segment of the transmission resource.