0
$\begingroup$

I am working on communication system in simulink and wondering, how can i separate the Even and odd part of the signal.

In my Transmitter block, I have :

$$ \boxed{\text{Binary Data signal}}{\longrightarrow}\boxed{\text{Serial to parallel}}^{\nearrow\boxed{\text{Even Part}}{\longrightarrow}\boxed{\text{Multiply PN code}}{\longrightarrow}\text{signal}}_{\searrow\boxed{\text{Odd Part}}{\longrightarrow}\boxed{\text{Multiply PN code}}{\longrightarrow}\text{signal}} $$ Not sure how to separate those two part from signal ? Also can i use below equations for channels I and Q. $$ I(t) = \sqrt{\frac{2E_s}{T} \cos(\omega t)\cos(45+n)} $$

$$ Q(t) = \sqrt{\frac{2E_s}{T} \sin(\omega t) \sin(45+n)} $$

$\endgroup$
1
$\begingroup$

The OP is mis-understanding how this QPSK signaling scheme operates.

There is a stream of input bits $b_0, b_1, b_2, b_3, \ldots,$ that is coming serially at the rate of one bit every $T$ seconds, so $b_0$ during $[0,T)$, $b_1$ during $[T,2T)$, and more generally, $b_n$ during $[nT, (n+1)T)$.. So, there is a clock signal at $\frac 1T$ Hz at the input.

The first thing that happens is the serial-to-parallel conversion where the serial bit stream gets converted into a stream of dibits (two bits in parallel), one bit on each of two individual wires and each bit lasting for $2T$ seconds (that is, the dibit lasts for $2T$ seconds. Thus, we have $$(b_0, b_1) \to \left[\begin{matrix}b_0\\b_1\end{matrix}\right], \quad (b_2, b_3) \to \left[\begin{matrix}b_2\\b_3\end{matrix}\right], \cdots \cdots (b_{2n}, b_{2n+1}) \to \left[\begin{matrix}b_{2n}\\b_{2n+1}\end{matrix}\right], \cdots$$ So immediately we need to understand that

  • The output of the serial-to-parallel converter is clocked at with a clock at $\frac{1}{2T}$ Hz, and this clock is synchronized with the input clock of the serial to parallel converter,
  • There is a delay of $2T$ seconds in that $\left[\begin{matrix}b_0\\b_1\end{matrix}\right]$ is the output of the serial-to-parallel converter during $[2T,4T)$, $\left[\begin{matrix}b_2\\b_3\end{matrix}\right]$ is the output during $[4T,6T)$, and so on,
  • The "even bits" have been sorted into appearing on the upper wire and the "odd bits" on the lower of the two wires, just as is envisioned in the OP's diagram. These are the Inphase and Quadrature branches of the transmitter.

The dibit enters into the direct-sequence modulator which says multiplication but this is if the bits ($0$ or $1$) have been converted into real numbers $+1$ and $-1$ respectively, and ditto the PN sequence. The logical operation with $0$s and $1$ is the XOR operation: the bit $b$ lasting $2T$ seconds is replaced by a serial stream of $N$ bits $(b\oplus \text{PN}_0, b\oplus \text{PN}_1, \cdots, b\oplus \text{PN}_{N-1})$ which last for a total of $2T$ seconds. Thus the PN sequence generators (usually different ones in the I and Q branches) are clocked at $\frac{N}{2T}$ Hz, and this clock is also synchronous with all the other clocks.

Everything is digital up to this point. To get the actual transmitted signal, the $0$s and $1$s are mapped to $\pm 1$, each rectangular pulse (of duration $2T/N$ seconds in the direct-sequence QPSK system) is replaced by the desired baseband pulse shape (e.g. root raised cosine) to get the complex baseband representation $I(t)+jQ(t)$ of the QPSK signal. The actual RF signals in the I and Q branches of the transmitter are $I(t)\cos(2\pi f_c t)$ and $-Q(t)\sin(2\pi f_c t)$ where $f_c \gg \frac{1}{2NT}$ is the RF carrier frequency. Thus the actual RF QPSK signal is of the form $$s(t) = I(t)\cos(2\pi f_c t)-Q(t)\sin(2\pi f_c t) = \text{Re}\big((I(t)+jQ(t))\exp(j2\pi f_ct)\big)$$ and results from modulating a (spread) dibit onto an RF carrier. Dibits consist of two bits; all Americans know that make two bits make a quarter, which explains why they think that the Q in QPSK stands for quaternary instead of quadrature or quadriphase.....

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.