text section 5.13 의 내용
there are many types of SS systems. to be considered an SS system, a system must satisfy two criteria:
- the bandwidth of the transmitted signal
need to be much greater than that of the message
.
- the relatively wide bandwidth of
must be caused by an independent modulating waveform
called the spreading signal, and this signal must be known by the receiver in order for the message signal
to be detected.
the SS signal is
where the complex envelope is a function of both
and
. in most cases, a product function is used, so that
where
and
are the usual types of modulation complex-envelope functions. the SS signal are classified by the type of mapping function that are used for
.
we will deal about most common types of SS signals:
direct sequence (DS) and
frequency hopping (FH), and
hybrid techniques that include both DS and FH.
- direct sequence (DS). here, a DSB-SC type of spreading modulation is used [i.e.,
], and
is a polar NRZ waveform.
(FYI) 2. spread spectrum techniques: direct sequence systems
- frequency hopping (FH). here
is of the FM type where there are
hop frequencies determined by the
words obtained from the spreading code waveform
.
(FYI)
- hybrid techniques that include both DS and FH.
(FYI) 본 text,
digital and analog communication systems, 의 목적상 spread spectrum에 대한 내용은 전체적인 흐름만을 파악할 수 있을 정도의 내용만 다룬다. spread spectrum 이라는 특화된 주제에 대해서는 text,
spread spectrum systems with commercial applications' 을 기본교재로 정하며 이후에 연구되는 내용들은 이 text을 바탕으로 집약되어 정리될 수 있도록 한다.
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