DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 395

TABLE 10.7 Sixteen-Level PAM Lookup
Table for Mapping

Symbol Block Level (in hex)

0000 0x7FFF

0001 0x6EEE

0010 0x5DDD

0011 0x4CCC

0100 0x3BBB

0101 0x2AAA

0110 0x1999

0111 0x0888

1000 - 0x0889

1001 - 0x199A

1010 - 0x2AAB

1011 - 0x3BBC

1100 - 0x4CCD

1101 - 0x5DDE

1110 - 0x6EEF

1111 - 0x8000

of masking and shifting. The first symbol block is obtained with masking of the four
least significant bits by anding the input sample with 0x000F. The second symbol
block is obtained through shifting the original input sample by four to the right and
masking the four LSBs. These steps are repeated until the end of the input sample
length and produce four symbol blocks. Assume that the input sample is 0xA52E.
In this case, 1110 (after masking the four LSBs) is mapped to 0x6EEF, as shown

in Table 10.7. Each symbol block is composed of 4 bits mapped into the 16 uniformly
spaced levels between

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0x8000 and 0x7FFF. The spacing between each level is
0x1111, selected for uniform spacing. The selected level is then transmitted as
a square wave. The period of the square wave is achieved by outputting the same
level many times to ensure a smooth-looking square wave at the output of the
transmitter.
The receiver is implemented with the assumption that the effects of the channel
and noise are neglected. As a result, the received sample is composed of individual
transmitted symbols or levels. Each transmitted symbol is a 4-bit segment, demod-
ulated by mapping it back to the original sequence of bits. The demodulated symbols
are then arranged in a buffer in order to reproduce the original transmitted
sequence. The least significant transmitted segment is placed in the least significant
received sequence (by adding and shifting). The first segment is shifted by 12 to the
left in order to place it at the most significant segment, and subsequently shifted by
4 to the right. The process is repeated until the four segments are in the right order
the way they were transmitted. The sample is then sent to the codec, and the orig-
inal waveform is reconstructed.

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