Implement a QPSK modulator in VHDL

QPSK, or Quadrature Phase Shift Keying, is a digital modulation scheme that encodes two bits of data onto a single carrier wave by shifting the carrier wave's phase by one of four possible values (0, 90, 180, or 270 degrees).

Here is an example of VHDL code that implements a QPSK modulator:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity QPSK_modulator is
    Port ( clk : in  STD_LOGIC;
           data_in : in  STD_LOGIC_VECTOR (1 downto 0);
           carrier : in  STD_LOGIC_VECTOR (3 downto 0);
           qpsk_out : out  STD_LOGIC_VECTOR (1 downto 0));
end QPSK_modulator;

architecture Behavioral of QPSK_modulator is
    signal phase : std_logic_vector(3 downto 0);
begin
    process (clk)
    begin
        if rising_edge(clk) then
            case data_in is
                when "00" => phase <= carrier + "00";
                when "01" => phase <= carrier + "01";
                when "10" => phase <= carrier + "10";
                when "11" => phase <= carrier + "11";
            end case;
        end if;
    end process;
    
    qpsk_out <= phase(1 downto 0);
end Behavioral;

In this example, the data_in input is a 2-bit vector containing the data to be modulated, and the carrier input is a 4-bit vector containing the carrier wave. The qpsk_out output is a 2-bit vector containing the modulated QPSK signal. The phase signal is used to store the phase of the carrier wave, which is updated on each clock edge based on the value of the data_in input.