Differential encoding has an infinite error propagation. You can switch it
off independently from the mapping function (see bit 3 of Register 6, on
page 4-8).
2.11 Square Root Nyquist Filter
This pulse-shaper module implements a programmable square-root
raised cosine filtering function with a default 15% roll-off factor. The
precision of the internal Nyquist filter computations, and the width of the
output data bus, are sufficient for QAM modulations up to 256. The filter
operates at four times the oversampling rate. Figure 2.19 illustrates the
structure of the pulse shaper.
Figure 2.19 Pulse Shaper Structure
From Coefficient Register (Register 0)
11 11 11 11 11 11 11 11 11
11 11 11 11 11 11 11 11 11
11 bit
offset
C23 C24 C25 C26 C27 C28 C29 C30
C0 C1 C2 C3 C4 C5 C6 C7 C8 C9
The big adder
Numbers represent coefficient
word width of MAC structures
QS[19:0]
IS[19:0],
to Shifter
Note that the 20 bits per channel is a result of the length of each
11
4
coefficient register (2 ) plus the length of INQ (2 ) plus the number of
5
stages (30 = 2 ). The total number of coefficients is 124 (there is one
coefficient set for each of the four phases multiplied by the MAC
structure, which contains 31 multipliers).
Each of the two I and Q branches has one filter, realized as polyphase
structures. Each filter consists of four filter branches, which compute
1-phase filter results at the symbol rate. Thus, the L64777 Nyquist filter
module generates the desired pulse shape by combining the outputs of
four identical filter branches for I and Q.
Square Root Nyquist Filter
2-27
ETC [ ETC ]
