GC1115_06 Datasheet, PDF (3/82 Pages) Texas Instruments – Crest Factor Reduction Processor

GC1115
www.ti.com
SLWS144C – FEBRUARY 2005 – REVISED JUNE 2006
INTRODUCTION
Figure 1 shows the typical usage of the GC1115 crest factor reduction processor in the transmit signal chain of a
wireless base station.
GC5016,
GC5316,
or
other DUC
GC1115
D/A
RF
upconversion
PA
Microproc.
or
TI DSP
Figure 1. Wireless System Using the GC1115
The GC1115 is initialized and controlled using an 8-bit address (A) and a 16-bit data (D) bus. These pins, along
with the read (RD), write (WR), and chip select (CS) pins, allow users to modify the control registers of the
GC1115.
Signals are provided to the GC1115 using two 18-bit input ports, IN_A[17:0] and IN_B[17:0], which are typically
driven by a digital upconverter such as the GC5016, GC5316, or similar. The GC1115 can accept either one or
two input channels.
When the GC1115 is configured to process two input channels, IN_A[17:0] carries channel 0’s multiplexed I & Q
samples, while IN_B[17:0] carries channel 1’s multiplexed I & Q samples. The GC1115 output samples are
provided to a D/A converter using two 18-bit output ports, OUT_A[17:0] and OUT_B[17:0]. The GC1115 output
may appear on one or both output ports, depending on which output mode (real or complex; parallel or
multiplexed; odd-even) is selected.
NOTE:
The D/A converter must not use the GC1115’s OUT_CLK signal. OUT_CLK is for
test purposes only. OUT_CLK’s phase is not aligned with the data on the OUT_A and
OUT_B ports. OUT_CLK may contain jitter in excess of that required to clock
high-speed D/A converters.
Why Cancel Peaks?
The purpose of the GC1115 is to lower the peak-to-average ratio (PAR) of composite digital communication
signals. Specifically, wideband code division multiple access (W-CDMA) and orthogonal frequency division
multiplexed (OFDM) signals have high peak-to-average ratios (PARs) ranging from 10 dB to 15 dB. After D/A
conversion, a power amplifier (PA) amplifies the resulting analog signal. In order to accommodate the high PAR
of W-CDMA and OFDM signals, the peak signal level must be at or below the 1 dB compression point of the PA,
which is usually accommodated by backing off the input drive to the power amplifier.
Figure 2 illustrates the effects of a decrease in peak-to-average ratio on a PA’s V_in vs V_out graph.
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