Spread Spectrum Modulation - Digital Communication: Principles and System Modelling - page 162

7.7.3 Principle of Operation

7.7.3.1 MUX

According to the figure above (Fig. 7.18 ), 4 channels are multiplexed into one trunk.
Bits sent from Ch0, Ch1, Ch2 and Ch3 are 1, 0, 1, and 1 respectively [ 1 ]. These
4 symbols are modulated (DSSS) using seq0 (1111), seq1 (1100), seq2 (1010)
and seq3 (1001) respectively, all are in BNRZ format. Therefore, the results of 4
modulators would be

(+1, +1, +1, +1) ∗ 1 Ch0_MOD → = ( +1, +1, +1, +1)

1) ∗ −

→ − − = − −

Ch0_MOD (+1, +1, 1, 1 ( 1, 1, +1, +1)

1) ∗ 1

→ − − = − −

Ch0_MOD (+1, 1, +1, (+1, 1, +1, 1)

1, +1) ∗ 1 Ch0_MOD → (+1, − 1, − = (+1, − 1, − 1, +1)

−

2, 2, 2). This is indeed
the multiplexed sequence. We can say, this sequence is the multiplexed 1, 0, 1 and 1
after pseudo-code modulation (DSSS). That's why this technique of modulation is
called as Code Division Multiplexing, and access to one code division multiplexed
line, against permission, is called as CDMA.

Adding all the above sequences bit by bit, we get, (2,

7.7.3.2 DMUX

At the de-multiplexer side, firstly, the multiplexed sequence (2, 2, 2, 2) goes to 4

different dispreading/demodulator block. The four demodulators are synchronized
with proper carrier sequences in order, i.e., seq0 (1111), seq1 (1100), seq2 (1010)
and seq3 (1001) respectively.
Therefore, the results of 4 demodulators (analog bit by bit product) would be

−

(+1, +1, +1, +1) ∗ (2,

Ch0_DMOD → − 2, 2, 2) = (+2, − 2, +2, +2)

1) ∗ (2,

Ch0_DMOD → (+1, +1, − 1, − − 2, 2, 2) = (+2, − 2, − 2, − 2)

1) ∗ (2, Ch0_DMOD → (+1, − 1, +1, − − 2, 2, 2) = (+2, +2, +2, − 2)

1, +1) ∗ (2, Ch0_DMOD → (+1, − 1, − − 2, 2, 2) = (+2, +2, − 2, +2)

Next, all the analog values are summed up and normalized to get the transmitted
exact bit.
This procedure has a nice advantage that, if even a single digit is erroneous in the
carrier chip sequence, the de-multiplexing cannot be done properly, because, spread
spectrum; hence the CDMA is highly secure transmission.

References

1. Das, A., “Line Communication System - Telecommunication Switch Approach (2nd Ed.)”,
New Age International, 2010