L64777 LSI Logic Corporation, L64777 Datasheet - Page 40

• Manufacturer Part Number: L64777
• Description: DVB Qam Modulator
• Manufacturer: LSI Logic Corporation

2.10 Differential Encoder and QAM Mapping
Figure 2.18 Differential Encoder and QAM Mapping
Interleaver
2-26
8
From
M-tuple
Byte to
This block performs differential encoding and mapping for 16 and 64
QAM, as specified in the Digital Broadcasting Systems for Television
Sound and Data Services: Framing Structure, Channel Coding and
Modulation Cable Systems , the baseline document, and its extensions.
The QAM 256 mapping is taken from the DVB document 1190.
The encoder performs differential encoding on the two most significant
bits of each symbol, as shown in the block diagram in Figure 2.18 and
specified in equations 2.1 and 2.2.
To clarify the underlying concepts in Figure 2.18, here are two examples:
1. If m = 4, A
2. If m = 6, A
Equation 2.1
Equation 2.2
Mapping performs a table look-up for the concatenation of the (m
least significant bits of each symbol with the differentially generated bits
I
In 64 QAM mode, the mapping block of the differential encoder maps the
6-tuples to two 3-bit values for I and Q output. In 16 QAM mode, it maps
the 4-tuples to two 2-bit values. For lower QAM modes, it aligns the I and
Q output values to the MSB and stuffs the least-significant bit (LSB) with
ones.
Modulator Architecture
k
and Q
MSB = A
MSB
LSB
B
k
1
k
k
.
Differential
k
k
Encoder
I
Q
is bit 3, B
is bit 5, B
k
m
k
=
=
2
A
A
k
k
k
k
is bit 2, and the LSB = (m
is bit 4, and the LSB = (m
I
Q
k
B
k
B
k
k
A
Mapping
B
k
k
I
k
Q
–
k
1
–
1
+
Q[3:0]
I[3:0]
+
A
k
A
k
B
k
B
2) bits = bits [1:0].
2) bits = bits [3:0].
k
A
k
B
k
Q
k
I
k
–
1
–
1
2)