Prevent negative image with BPSK modulation in a complex signal
An actual BPSK signal can be expressed as
$$\begin{align}
s(t) &amp;= \operatorname{Re}\left[\sum_{n=-\infty}^\infty
(-1)^{b_n}p(t-nT)e^{j(2\pi f_c t + \theta)}\right]\\
&amp;= \left[\sum_{n=-\infty}^\infty
(-1)^{b_n}p(t-nT)\right]\cos(2\pi f_c t + \theta)
\end{align}$$
where 


$b_n \in {0,1}$ is the $n$-th data bit (note that $(-1)^{b_n} = \pm 1$),
$p(t)$ is the baseband pulse shape. Typically $p(t)$ is nonzero only for
$t \in \left[-\frac{T}{2},\frac{T}{2}\right)$, e.g. 
$\operatorname{rect}\left(\frac{t}{T}\right)$.
$f_c$ is the carrier frequency which is sometimes chosen to be a harmonic of
the bit rate $T^{-1}$
$\theta$ is the carrier phase which might be arbitrary (depending
on initial conditions in the local oscillator when it is turned on) or carefully controlled to have 
value $0$ or $\pm \frac{\pi}{2}$ etc. by MATLABi people.


Notice that the term in square brackets in the second displayed equation
above is a series of nonoverlapping pulses $\pm p(t-nT)$ and that
the value of $b_n$ affects the BPSK signal phase only during 
$\left[nT-\frac{T}{2},nT+\frac{T}{2}\right)$.

With that as a preliminary setting of the stage, note that the negative
frequency image comes about from taking the real part in the first
displayed equation above.  We have
$$\operatorname{Re}\left[e^{j(2\pi f_c t + \theta)}\right]
= \frac{e^{j(2\pi f_c t + \theta)} + e^{-j(2\pi f_c t + \theta)}}{2}
= \frac{e^{j(2\pi f_c t + \theta)} + e^{j(2\pi (-f_c) t - \theta)}}{2}.$$
Furthermore,


  no filtering can remove the negative frequency image.


You might say "Well, let's not take the real part at all
since that's the source of the trouble!" and say
that
$$\sum_{n=-\infty}^\infty
(-1)^{b_n}p(t-nT)e^{j(2\pi f_c t + \theta)}$$ is a BPSK signal
containing positive frequencies only. Unfortunately, that is
a complex-valued signal that cannot be transmitted on a single
wire or radiated from a single antenna.  So you need to live with
the negative frequency image: it is there to make your complex
signal more real.