GC1115_06 Datasheet, PDF (21/82 Pages) Texas Instruments – Crest Factor Reduction Processor

GC1115
www.ti.com
SLWS144C – FEBRUARY 2005 – REVISED JUNE 2006
Designing Cancellation Pulse Coefficients
Cancellation pulse coefficient design is identical to FIR filter design. The goal of designing cancellation pulses is
to mirror the signal energy of the input signal. If the input signal contains one carrier, the cancellation pulse’s
center frequency and bandwidth will have the same center frequency and bandwidth as that carrier. For
multi-carrier signals, multiple single-carrier cancellation pulses are added together to create a spectrum that is
identical to the spectrum of the multi-carrier input signal. The goal of cancellation pulse design is to place energy
ONLY in those frequency regions where the input signal has appreciable energy, i.e. in the carrier bands
themselves. This design method ensures that cancellation pulse energy is inserted only where input signal
energy is already present, and to avoid placing any additional energy outside those bands, since such
out-of-band energy would worsen ACLR performance.
Upon request, TI will provide Matlab code that demonstrates how cancellation pulses are designed. This Matlab
software requires the following input parameters:
• Input sampling rate
• Single-carrier bandwidth
• Carrier center frequencies
• Desired cancellation pulse length (an odd number; from 15 to 255 for real cancel pulses, and from 15 to 127
for complex cancel pulses)
The Matlab cancellation pulse design software then calls a standard Matlab FIR filter design function (such as
fir1, firls, or remez) to design a prototype cancellation pulse whose bandwidth matches the single-carrier
bandwidth input parameter. If the input signal contains two or more carriers, copies of this prototype filter will be
translated to the center frequency of each carrier and the copies summed together.
Cancellation Coefficient Shadow RAM
The GC1115 uses a derivative-based approximation method to calculate highly accurate phase shifts of the
cancellation pulse. The approximation method requires not only the cancellation coefficients themselves to be
stored in the GC1115, but also the coefficients’ first and second derivatives. The cancellation coefficient shadow
RAM contains 768 unique addresses that hold up to 256 unique cancellation coefficients and their first and
second derivatives.
NOTE:
Upon request, TI will provide a Matlab script that calculates the proper first and
second derivatives from user-designed cancellation coefficients.
Shadow RAM is physically distinct from canceler RAM. This distinction is fully explained in the section entitled
Shadow RAM and Canceler RAMs, below. The microprocessor or DSP controlling the GC1115 can only access
the shadow RAM.
Cancellation coefficient shadow RAM is used differently, depending on the CANCEL_MODE setting (real or
complex coefficients; unique or mirrored coefficients). Table 10, Table 11, and Table 12 illustrate how
cancellation coefficients are stored and accessed in the GC1115’s cancellation coefficient shadow RAM.
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