Techniques for transmitting positioning reference signals in an unlicensed radio frequency spectrum band

1. A method for wireless communication, comprising:

generating a positioning reference signal (PRS);

configuring the PRS in at least one downlink subframe among a plurality of downlink subframes, wherein the PRS is configured in the at least one downlink subframe in an unlicensed radio frequency spectrum band to be adjacent a clear channel assessment (CCA) exempt transmission (CET) in the unlicensed radio frequency spectrum band; and

transmitting the configured PRS in the at least one downlink subframe using the unlicensed radio frequency spectrum band.

2. The method of claim 1 , wherein a combined duration of the configured PRS and the CET is less than a maximum allowed duration of the CET in the unlicensed radio frequency spectrum band.

3. The method of claim 1 , wherein the transmitting the configured PRS comprises:

periodically transmitting the configured PRS adjacent the CET.

4. The method of claim 3 , wherein the periodically transmitting the configured PRS adjacent the CET comprises:

transmitting the configured PRS adjacent the CET according to a first periodicity, the first periodicity differing from a second periodicity at which the CET is transmitted.

5. The method of claim 4 , wherein the first periodicity is a variable periodicity.

6. The method of claim 1 , further comprising:

signaling a timing of the transmitting the configured PRS to a receiver.

7. The method of claim 1 , wherein the transmitting the configured PRS comprises:

transmitting the configured PRS contiguously with the CET.

8. The method of claim 1 , wherein the transmitting the configured PRS comprises:

time synchronizing the transmitting the configured PRS, by a first transmitter, with a transmission of at least a second PRS by at least a second transmitter.

9. The method of claim 1 , wherein the transmitting the configured PRS comprises:

transmitting the configured PRS, by a first transmitter, with a same periodicity as, and different phase offset than, a transmission of at least a second PRS by at least a second transmitter.

10. The method of claim 1 , wherein the transmitting the configured PRS comprises:

transmitting the configured PRS, by a first transmitter, with a different periodicity than a transmission of at least a second PRS by at least a second transmitter.

11. The method of claim 1 , wherein the transmitting the configured PRS comprises:

transmitting the configured PRS before the CET.

12. The method of claim 1 , wherein the transmitting the configured PRS comprises:

transmitting the configured PRS after the CET.

13. The method of claim 1 , further comprising:

associating the PRS with a muting parameter.

14. The method of claim 1 , further comprising:

associating the PRS with a variable cell-specific frequency shift parameter.

15. The method of claim 1 , wherein the transmitting the configured PRS comprises:

transmitting the configured PRS to occupy a portion of the unlicensed radio frequency spectrum band less than all of the unlicensed radio frequency spectrum band.

16. The method of claim 1 , wherein the transmitting the configured PRS comprises:

transmitting the configured PRS across a plurality of frequencies of the unlicensed radio frequency spectrum band.

17. The method of claim 1 , further comprising:

receiving at least one CCA clearance statistic related to at least one PRS measurement; and

determining to configure the PRS based at least in part on the at least one CCA clearance statistic related to the at least one PRS measurement.

18. The method of claim 17 , wherein the at least one CCA clearance statistic related to the at least one PRS measurement is received from at least one user equipment.

19. The method of claim 17 , wherein the at least one CCA clearance statistic related to the at least one PRS measurement is received from at least one evolved NodeB (eNB).

20. The method of claim 1 , further comprising:

transmitting an indication to a receiver to use prior RSTD measurements based at least in part on a prior PRS transmission.

21. The method of claim 1 , further comprising:

receiving a set of reference signal time difference (RSTD) measurements collected for each of a plurality of known locations; and

transmitting the received set of RSTD measurements to be stored in a database.

22. The method of claim 21 , further comprising:

receiving at least one RSTD measurement and at least one reference signal strength indicator (RSSI) associated with an unknown location; and

estimating a position of the unknown location based at least in part on the at least one RSTD measurement and the at least one RSSI associated with the unknown location, and the set of RSTD measurements stored in the database.

23. An apparatus for wireless communication, comprising:

means for generating a positioning reference signal (PRS);

means for configuring the PRS in at least one downlink subframe among a plurality of downlink subframes, wherein the PRS is configured in the at least one downlink subframe in an unlicensed radio frequency spectrum band to be adjacent a clear channel assessment (CCA) exempt transmission (CET) in the unlicensed radio frequency spectrum band; and

means for transmitting the configured PRS in the at least one downlink subframe using the unlicensed radio frequency spectrum band.

24. An apparatus for wireless communication, comprising:

a processor;

memory in electronic communication with the processor; and

instructions stored in the memory, the instructions being executable by the processor to:

generate a positioning reference signal (PRS);

configure the PRS in at least one downlink subframe among a plurality of downlink subframes, wherein the PRS is configured in the at least one downlink subframe in an unlicensed radio frequency spectrum band to be adjacent a clear channel assessment (CCA) exempt transmission (CET) in the unlicensed radio frequency spectrum band; and

transmit the configured PRS in the at least one downlink subframe using the unlicensed radio frequency spectrum band.

25. The apparatus of claim 24 , wherein a combined duration of the configured PRS and the CET is less than a maximum allowed duration of the CET in the unlicensed radio frequency spectrum band.

26. The apparatus of claim 24 , wherein the instructions to transmit the configured PRS are executable by the processor to:

periodically transmit the configured PRS adjacent the CET according to a first periodicity, the first periodicity differing from a second periodicity at which the CET is transmitted.

27. The apparatus of claim 24 , wherein the instructions to transmit the configured PRS are executable by the processor to:

time synchronize transmitting the configured PRS, by a first transmitter, with a transmission of at least a second PRS by at least a second transmitter.

28. The apparatus of claim 24 , wherein the instructions are executable by the processor to:

transmit the configured PRS, by a first transmitter, with a same periodicity as, and different phase offset than, a transmission of at least a second PRS by at least a second transmitter.

29. The apparatus of claim 24 , wherein the instructions to transmit the configured PRS are executable by the processor to:

transmit the configured PRS, by a first transmitter, with a different periodicity than a transmission of at least a second PRS by at least a second transmitter.

30. A non-transitory computer-readable medium storing computer-executable code for wireless communications, the code executable by a processor to:

generate a positioning reference signal (PRS);

configure the PRS in at least one downlink subframe among a plurality of downlink subframes, wherein the PRS is configured in the at least one downlink subframe in an unlicensed radio frequency spectrum band to be adjacent a clear channel assessment (CCA) exempt transmission (CET) in the unlicensed radio frequency spectrum band; and

transmit the configured PRS in the at least one downlink subframe using the unlicensed radio frequency spectrum band.