distance measuring by encoder

i have one phase encoder ...

i wrote this program but got an errore about the multiplication, how can i alternate that by synthessisable way ??

library IEEE;

use IEEE.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

entity encoder is

port(

CLK : in std_logic;

sig : in std_logic ;

reset : in std_logic;

req_dist: buffer std_logic_vector(8 downto 0) -- till 5 meter.

);

end entity encoder;

architecture arc_encoder of encoder is

signal tmp_sig : std_logic;

signal counter : std_logic_vector(8 downto 0);

signal distance : std_logic_vector(8 downto 0);

begin

process(CLK)

begin

if (reset='1') then

counter <= (others => '0');

else

distance <= req_dist*24 and "111000000"; ------------------------------------------------------ HERE IS THE ERRORE

end if;

if rising_edge(sig) then

if (counter < distance) then

counter <= counter+1;

else counter <= counter;

end if;

end if;

end process;

end architecture arc_encoder;