System and method for reducing peak-to-average power ratio in orthogonal frequency division multiplexing signals using reserved spectrum
View Patent ↗A method and apparatus for reducing the peak to average power ratio of an OFDM signal. The method involves operating on the signal only after it is converted to the time domain where the signal is compared to a power threshold and any signal above the power threshold is considered undesirable. A reduced signal is produced by reducing the magnitude to the power threshold whenever it exceeds that threshold. The delta between the original signal and reduced signal is the clipped signal. The clipped signal is multiplied by a g-function, and then subtracted from the original signal to produce a first iteration of a peak-reduced signal. The process can then be repeated using the first peak-reduced signal in place of the original signal to produce the second iteration of a peak-reduced signal. This process can be repeated each time bringing the peaks closer to the power threshold. The g-function can be defined either to allow for per-symbol cyclic signal processing or non-cyclic processing that is symbol time independent.
1. A method of reducing a peak-to-average power ratio (PAPR) in communications signals using reserve tones, said method comprising:
obtaining a communications symbol;
for n iterations, where n is a positive integer,
subjecting said communications symbol to a hard limiter;
defining clip signal components of said communications symbol;
filtering said clip signal components using a g-function;
providing a delay in conjunction with said defining and said filtering steps so as to form a delayed communications symbol; and
subtracting an output of said filtering step from said delayed communications symbol so as to produce an output signal y(n) having a reduced PAPR;
wherein said output y(n) is derived from the iteration
y ( n )= y ( n− 1)−ξ· Gy clip ( n− 1)
where y clip (n) is one said clip signal component, ξ is a positive constant between 0 and 1, and G is a mapping matrix.
2. The method as claimed in claim 1 , wherein said mapping matrix G is a matrix where each row and column is expressed as a cyclic shift of a g-function vector.
3. The method as claimed in claim 2 , wherein ξ is a constant value of 0.5.
4. The method as claimed in claim 3 , wherein said g-function is formed by
establishing a high ratio between a main peak filter tap magnitude and an average filter tap magnitude, and
minimizing energy of any secondary peak filter tap magnitudes.
5. The method as claimed in claim 4 , further including a step of
providing one final iteration of cyclic processing with an all-pass g-function having a different clipping threshold than with said g-function so as to effect harder cut-off of symbol peaks.
6. The method as claimed in claim 5 , wherein said method is performed within a mobile station.
7. The method as claimed in claim 5 , wherein said method is performed within a base station.
8. The method as claimed in claim 5 , wherein defining said clip signal components includes controlling a degree of clipping such that signal quality is maintained within predetermined specifications.
9. The method as claimed in claim 3 , wherein said g-function is formed by assigning location and weights symmetrically to reserve tones so as to create a real-component-only g-function.
10. A filtering apparatus for use within a peak power regulator, said apparatus comprising:
an iteration control mechanism for introducing n iterations of a communications symbol, where n is a positive integer;
a hard limiter;
a means for forming clip signal components of said communications symbol;
a g-function filter for filtering said clip signal components;
a delay mechanism in cooperation with said means for forming clip signal components and said g-function filter, said delay mechanism providing a delayed communications symbol; and
a means for subtracting an output of said g-function filter from said delayed communications symbol so as to produce an output signal y(n) having a reduced PAPR;
wherein an output y(n) formed by said apparatus is derived from the iteration
y ( n )= y ( n− 1)−ξ· Gy clip ( n− 1)
where y clip (n) is one said clip signal component, ξ is a positive constant between 0 and 1, and G is a mapping matrix.
11. The apparatus as claimed in claim 10 , wherein said G is a matrix where each row and column is expressed as a cyclic shift of a g-function vector.
12. The apparatus as claimed in claim 11 , wherein ξ is a constant value of 0.5.
13. The apparatus as claimed in claim 12 , wherein said g-function is formed by
establishing a high ratio between a main peak filter tap magnitude and an average filter tap magnitude, and
minimizing energy of any secondary peak filter tap magnitudes.
14. The apparatus as claimed in claim 13 , further including a means for providing one final iteration of cyclic processing with an all-pass g-function having a different clipping threshold than with said g-function so as to effect harder cut-off of symbol peaks.
15. The apparatus as claimed in claim 14 , wherein said apparatus forms part of a mobile station.
16. The apparatus as claimed in claim 14 , wherein said apparatus forms part of a base station.
17. The apparatus as claimed in claim 14 , further including a means for defining said clip signal components so as to control a degree of clipping such that signal quality is maintained within predetermined specifications.
18. The apparatus as claimed in claim 13 , wherein said g-function is formed by assigning location and weights symmetrically to reserve tones so as to create a real-component-only g-function.