GC1115_06 Datasheet, PDF (6/82 Pages) Texas Instruments – Crest Factor Reduction Processor

GC1115
SLWS144C – FEBRUARY 2005 – REVISED JUNE 2006
www.ti.com
The GC1115 reduces the amplitude of peaks in a way that keeps the out-of-band (ACLR) energy well below
required levels. Users can determine how much in-band distortion is acceptable by monitoring the effects of
peak cancellation at a given output PAR, using two key in-band distortion metrics for CDMA signals:
1. peak code domain error, or PCDE, and
2. composite error vector magnitude, or EVM
Various CFR signal quality metrics (ACLR, PCDE, EVM, CCDF) will be further discussed in a subsequent
section.
Power spectral density: input (blue) & distortion (red)
100
80
Input Spectrum
60
Distortion Spectrum,
5 dB Output PAR
40
Distortion Spectrum,
8 dB Output PAR
20
0
−20
−4
−3
−2
−1
0
1
2
3
4
7
x 10
Figure 5. Spectral View of Distortion
SUMMARY OF GC1115 FEATURES
Refer to the GC1115 functional block diagram for the following discussion.
The GC1115 removes peaks from an input signal stream by subtracting user-designed, spectrally shaped
cancellation pulses from detected peaks in the input waveform that are above a user-specified detection
threshold. The cancellation pulse’s spectral shape matches that of the GC1115 input signal’s single- or
multi-carrier configuration. This cancellation pulse design methodology ensures that energy is only added into
frequency bands where signal energy is located, minimizing out-of-band energy that could otherwise increase
ACLR/ACPR.
Cancellation pulses may be real or complex. Real cancellation pulses are used when the input signal spectrum
is symmetric, while complex cancellation pulses are used when the input signal spectrum is asymmetric. A
single cancellation pulse reduces the peak amplitude of multiple samples around each signal peak while
maintaining both the in-band (PCDE, cEVM) and out-of-band (ACLR) signal quality requirements. Peaks are
cancelled to a user-specified level, which is called the output peak-to-average ratio, or PAR. The GC1115
provides four sequential peak detection and cancellation (PDC) stages to remove peaks. The PDC stage
thresholds are normally set so that earlier stages remove the largest peaks, while later stages remove smaller,
remaining peaks.
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