Methods and apparatus for improved reference signal correlation characteristics
The auto-correlation properties of a reference signal or pilot pattern, such as a position reference signal (PRS) in a Long Term Evolution communication system, is improved by modifying the currently specified PRS patterns, and/or by PRS pattern shaping. Pattern shaping can result in creation of virtual PRS patterns, for example, by controlling the PRS transmitted or received power used by the correlator. PRS power shaping can be implemented differently according to the location where the PRS power is calculated, e.g., in a network node or in a user equipment.
1. A method of using reference signals (RS) in an orthogonal frequency division multiplex communication system in which the RS are organized in a RS pattern of resource elements (REs) that includes at least one of: a first plurality of columns corresponding to symbols and a second plurality of rows corresponding to subcarriers, and a set of power levels of RS REs, the method comprising:
forming a modified RS pattern based on a predetermined RS pattern by at least one of:
assigning respective different transmission power levels to selected REs of the predetermined RS pattern; and
adjusting received signal power levels of selected REs of the predetermined RS pattern;
wherein REs are selected and power levels are assigned or adjusted for maximizing a main peak of an autocorrelation of a received signal including the modified RS pattern.
2. The method of claim 1 , wherein the main peak is maximized by maximizing either a ratio of a squared power of a main autocorrelation peak to a total energy of the autocorrelation or a ratio of a metric that corresponds to an accumulated energy in the main autocorrelation peak to the total energy of the autocorrelation.
3. The method of claim 1 , wherein forming the modified RS pattern includes cyclically shifting REs in at least one column of the predetermined RS pattern; and REs in the at least one column of the predetermined RS pattern are cyclically shifted such that a number of REs per column and a space between REs in a column of the predetermined RS pattern is about the same as a number of REs per column and a space between REs in a column of the modified RS pattern.
4. The method of claim 1 , wherein at least one empty subcarrier of the predetermined RS pattern is filled with an RE of the modified RS pattern.
5. The method of claim 1 , wherein a density of RS REs over coherently accumulated segments of the modified RS pattern either is more uniform among subcarriers or varies more regularly among subcarriers relative to a density of RS REs over coherently accumulated segments of the predetermined RS pattern.
6. The method of claim 1 , further comprising sending, to another network node, an indication whether the method is implemented in a network node in the communication system.
7. The method of claim 1 , wherein an indication of the power levels is sent either to at least one of a base station and a positioning node, or to a user equipment according to either a Long Term Evolution Positioning Protocol or a Radio Resource Control protocol.
8. The method of claim 7 , wherein the indication is included in Observed Time Difference of Arrival assistance data.
9. The method of claim 1 , wherein the selected REs are selected by either an eNodeB or a positioning node in the communication system.
10. The method of claim 1 , wherein REs are selected by a receiving node configured to reshape received signals before correlating the received signals with a local replica of the modified RS pattern.
11. A reference signal generator in an orthogonal frequency division multiplex communication system in which reference signals (RS) are organized in a RS pattern of resource elements (REs) that includes at least one of: a first plurality of columns corresponding to symbols and a second plurality of rows corresponding to subcarriers, and a set of power levels of RS REs, the generator comprising:
an electronic processor configured to form a modified RS pattern based on a predetermined RS pattern by at least one of:
assigning respective different transmission power levels to selected resource elements of the predetermined RS pattern; and
adjusting received signal power levels of selected REs of the predetermined RS pattern;
wherein REs are selected and power levels are assigned or adjusted for maximizing a main peak of an autocorrelation of a received signal including the modified RS pattern.
12. The generator of claim 11 , wherein the main peak is maximized by maximizing either a ratio of a squared power of a main autocorrelation peak to a total energy of the autocorrelation or a ratio of a metric that corresponds to an accumulated energy in the main autocorrelation peak to the total energy of the autocorrelation.
13. The generator of claim 11 , wherein the modified RS pattern is formed by cyclically shifting REs in at least one column of the predetermined RS pattern; and REs in the at least one column of the predetermined RS pattern are cyclically shifted such that a number of REs per column and a space between REs in a column of the predetermined RS pattern is about the same as a number of REs per column and a space between REs in a column of the modified RS pattern.
14. The generator of claim 11 , wherein at least one empty subcarrier of the predetermined RS pattern is filled with an RE of the modified RS pattern.
15. The generator of claim 11 , wherein a density of RS REs over coherently accumulated segments of the modified RS pattern either is more uniform among subcarriers or varies more regularly among subcarriers relative to a density of RS REs over coherently accumulated segments of the predetermined RS pattern.
16. The generator of claim 11 , wherein the electronic processor is further configured to indicate the power levels to another node in the communication system.
17. The generator of claim 16 , wherein the indication is sent either to at least one of a base station and a positioning node, or to a user equipment according to either a Long Term Evolution Positioning Protocol or a Radio Resource Control protocol.
18. The generator of claim 17 , wherein the indication is included in Observed Time Difference of Arrival assistance data.
19. The generator of claim 11 , wherein the generator is included in either an eNodeB or a positioning node in the communication system.
20. An apparatus for a user equipment in an orthogonal frequency division multiplex communication system for using reference signals (RS) organized in a RS pattern of resource elements (REs), comprising:
a correlator;
a modified RS pattern generator configured to generate a modified RS pattern based on a predetermined RS pattern that is included in a received signal, wherein the modified RS pattern includes selected REs of the predetermined RS pattern that have adjusted signal power levels; and
an electronic processor configured to modify the received signal according to the modified RS pattern and form a modified received signal;
wherein the correlator is configured to correlate the modified received signal and the modified RS pattern and form a correlation result; and REs are selected and power levels are adjusted for maximizing a main peak of an autocorrelation of a received signal including the modified RS pattern.
21. The apparatus of claim 20 , wherein REs are selected and power levels are adjusted such that an autocorrelation of the modified RS pattern is optimized by maximizing either a ratio of a squared power of a main autocorrelation peak to a total energy of the autocorrelation or a ratio of a metric that corresponds to an accumulated energy in the main autocorrelation peak to the total energy of the autocorrelation.
22. The apparatus of claim 20 , wherein the RS are positioning reference signals.
23. The apparatus of claim 20 , wherein the apparatus is included in a user equipment.