Conversion from Floating point representation to Fixed point representation
I think you need to use the floor() or round() functions in MATLAB, to emulate fixed-point variables and operations.  So you have to know the range of the fixed-point value and the precision of it.  The ratio of the range to the precision is the dynamic range and you get 6.02 dB and one bit of word width every doubling of that ratio.

If you convert:

x_fixed = precision*round(x_float/precision);
if x_fixed &gt; fixed_max
    x_fixed = fixed_max;
elseif x_fixed &lt; fixed_min
    x_fixed = fixed_min;
end;


then fixed_max - fixed_min = (2^N - 1)*precision  for N being the number of bits in your fixed-point word.  If you do this after every mathematical operation, you will be emulating fixed-point arithmetic.  

there might be better ways to round x_float than i shown.  sometimes the floor() is better than round().  and "fraction saving" is a better way to round an intermediate result with more bits to your N-bit fixed-point word, but requires an error state for each variable.

for every variable and do your arithmetic with their fixed-point values and round the result to a fixed-point value with code like above.

that's how to convert floating-point MATLAB to emulating fixed point math.