mohatta2015.pdf

MORE MATH 17
transmitter
s 0 (m)
filter
T
x(f)
—►
sUm)
filter
T
Figure 1.13
Transmitter block diagram showing parallel multiplexing channels.
1.4.1.2 CDM For CDM, symbols are sent in parallel on different spreading waveforms.
The symbol waveform is formed from a spreading code or sequence of "chip"
values, i.e.,
JV..-1
Ό ) = (1/VÑc) Σ l,L(»W - nT c ), (1.35)
n=0
where
• N c is the number of chips used (the spreading factor),
• c];. m(n) is the nth chip value for the spreading code for symbol transmitted
on spreading code k of transmit antenna i during symbol period m, and
• p(t) is the chip pulse shape.
Chip values are assumed to have unity average energy and are typically unityamplitude
QPSK symbols. For transmitter i, the spreading codes are typically
orthogonal when time-aligned, i.e.,
JV„-1
E( c^,m(n)]*^), m (n) = JV c Ä(fc 1 -fc 2 ). (1.36)
A commonly used set of orthogonal sequences is the Walsh/Hadamard or Walsh
code set. There are K codes of length K, where K = 2 k a and alpha is the order.