GC1115_06 datasheet(8/82 Pages) TI

Part #	GC1115

Description	Crest Factor Reduction Processor

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Manufacturer	TI [Texas Instruments]

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CFR PERFORMANCE METRICS FOR CDMA SIGNALS: CCDF, ACLR, PCDE, cEVM

GC1115

SLWS144C – FEBRUARY 2005 – REVISED JUNE 2006

3. After PDC Stage 2

4. After PDC Stage 3

5. After PDC Stage 4

When operating in histogram mode, the snapshot RAMs generate a histogram of the real part, the imaginary

part, or the power of each complex sample. In histogram mode, millions of samples can be characterized using

the snapshot RAM’s 32-bit bin counters, thus providing a statistically significant number of events for CCDF and

related magnitude distribution measurements.

Each of the GC1115’s PDC stages can be bypassed by clearing a corresponding bit in the CONTROL register.

Although bypassing a stage removes that stage’s peak cancellation capability, it also removes the latency

introduced by the peak detection and cancellation process itself, and reduces power consumption as well.

The GC1115 includes an on-chip test signal generator that can create DC levels, sawtooth waveforms, and a

filtered random number generator with Gaussian-like peak-to-average statistics (i.e. approximately 10 dB PAR).

In conjunction with a CRC generator, the test signal generator can be used to verify proper, expected behavior

of the GC1115 without applying an input signal to the GC1115 IN_A and IN_B ports. The CRC generator

processes the GC1115 output samples and generates a periodic, 16-bit checksum. Since both the test signal

generator and the CRC generator can be synchronized to the same sync source, the presence of a predictable,

periodic value in the CRC register indicates that the GC1115 internal circuitry is operating as expected. TI

provides several GC1115 configurations (starting test generator register values, sample periods, and expected

CRC value at the end of each period) that enable GC1115 users to test for proper GC1115 internal operation.

The test signal generator can also be used without the CRC generator, providing a suite of general-purpose test

signals (DC, sawtooth, pseudo-LFSR) to exercise ICs attached to the GC1115 output ports, OUT_A and

OUT_B.

The GC1115 reduces peaks by subtracting spectrally shaped waveforms from detected peaks in the input signal

whose interpolated magnitude is above a user-specified output PAR threshold. Researchers in peak reduction

techniques use a graph called the complementary cumulative distribution function, or CCDF, to display the

probability that a particular sample has a given magnitude. The x-axis of a CCDF curve begins at 0 dB, defined

as the average power of the signal, and extends to the peak value of the waveform. The y-axis of a CCDF curve

lists the probability (usually on a log scale) that a given complex sample has a certain magnitude. Plotting the

before and after CCDF curves on the same graph demonstrates that the GC1115 peak reduction algorithms

have achieved their primary purpose: reducing peaks to a user-specified level. In Figure 6, the input CCDF

curve is shown in blue and the output CCDF curve (after GC1115 peak reduction) is shown in red.

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