can <= concatenate into a big message when use it multiple times?

constant preamble : STD_LOGIC_VECTOR (7 DOWNTO 0) := '5555555555555555';

constant dest_mac_addr : STD_LOGIC_VECTOR (5 DOWNTO 0) := '1078D2AD90CB';

constant src_mac_addr : STD_LOGIC_VECTOR (5 DOWNTO 0) := '001234567890';

constant payload : STD_LOGIC_VECTOR (7 DOWNTO 0) := '0001020304050607';

constant wholepacketlength : STD_LOGIC_VECTOR (1 DOWNTO 0) := '22';

variable tx_data : STD_LOGIC_VECTOR(31 DOWNTO 0);

variable CheckSumResult : STD_LOGIC_VECTOR (31 DOWNTO 0);

variable CheckSumValid : STD_LOGIC;

begin

tx_data <= preamble;

tx_data <= dest_mac_addr;

tx_data <= src_mac_addr;

tx_data <= payload;

H2:crcgen port map(

clk => CLOCK_50, --: IN STD_LOGIC;

data => payload, -- : IN STD_LOGIC_VECTOR (7 DOWNTO 0);

datavalid => enableit, --: IN STD_LOGIC;

empty : IN STD_LOGIC_VECTOR (1 DOWNTO 0);

endofpacket : IN STD_LOGIC;

reset_n : IN STD_LOGIC;

checksum => CheckSumResult, --: OUT STD_LOGIC_VECTOR (31 DOWNTO 0);

crcvalid => CheckSumValid --: OUT STD_LOGIC

);

tx_data <= CheckSumResult;