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M using Verilog to develop GMSK modulator based on FPGA under ISE platform

2022-07-22 01:58:00 【I love c programming】

Catalog

1. Algorithm description

Gaussian minimum frequency shift keying （Gaussian Filtered Minimum Shift Keying）, This is a GSM The modulation mode adopted by the system . Digital modulation and demodulation technology is an important part of the air interface of digital cellular mobile communication system .GMSK Modulation is in MSK（ Minimum frequency shift keying ） A Gaussian low-pass premodulation filter is inserted before the modulator .GMSK It improves the spectrum efficiency and communication quality of digital mobile communication .

GMSK Modulation technology is from MSK（Minimum Shift Keying） A digital modulation method developed on the basis of modulation , Its characteristic is that before the data stream is sent to the frequency modulator, it passes through a Gauss filter （ Premodulation filter ） Perform pre modulation filtering , In order to reduce the jump energy during the switching of two carriers with different frequencies , So that the channel spacing can become closer at the same data transmission rate . Because the digital signal is modulated Gauss Premodulation filtering , The modulated signal is not only phase continuous at the crossing zero , And smooth filtering , therefore GMSK The modulated signal has a compact spectrum 、 Good bit error characteristics , It has been widely used in digital mobile communication , Such as widely used GSM（Global System for Mobile communication） Mobile communication system is to use GMSK Modulation mode .
l979 Proposed by Nippon Telegraph and Telephone Corporation GMSK Modulation mode . It has good power spectrum characteristics , Better bit error performance , Especially the out of band radiation is small , It is suitable for working in VHF and UHF Band mobile communication system , It has attracted more and more attention .GMSK The theoretical research of modulation mode has been relatively mature , There are not many practical applications , It is mainly because there are still some difficulties in engineering in the design and manufacture of Gaussian filter . The minimum frequency shift keying of Gaussian filter before modulation is abbreviated as GMSK, The basic working principle is to shape the baseband signal through Gaussian filter , Then perform minimum frequency shift keying （MSK） modulation . Because the formed Gaussian pulse envelope has no steep edge , No inflection point , Therefore, the spectral characteristics are better than MSK Spectrum characteristics of signal .
Gaussian filter is usually used 3dB bandwidth B And input symbol width T The product of the BT Value as a main parameter in designing Gaussian filter .BT The smaller the value. ,GMSK The faster the high-frequency component of the signal power spectral density decays .

2. Partial procedure

`timescale 1ns / 1ps

module tops(
            i_clk,
				i_rst,
				o_binary_gen,
				o_chafen_encode,
				o_gauss_filter,
				o_adder,
				o_cos_I,
				o_sin_Q,
				o_cos,
				o_sin,
            o_GMSK_I,
            o_GMSK_Q,
            o_GMSK_R				
	        );
input              i_clk;
input              i_rst;
output             o_binary_gen;
output             o_chafen_encode;
output signed[15:0]o_gauss_filter;
output signed[31:0]o_adder;
output signed[15:0]o_cos_I;
output signed[15:0]o_sin_Q;
output signed[15:0]o_cos;
output signed[15:0]o_sin;
output signed[15:0]o_GMSK_I;
output signed[15:0]o_GMSK_Q;
output signed[15:0]o_GMSK_R;


reg[3:0]cnt;
wire    clk_16div;
always @(posedge i_clk or posedge i_rst)
begin
     if(i_rst)
	  begin
	  cnt <= 4'b0000;
	  end 
else begin
     cnt <= cnt + 4'd1;
     end
end
assign clk_16div = cnt[3];

binary_gen binary_gen_u(
							  .i_clk  (clk_16div), 
							  .i_rst  (i_rst), 
							  .o_dout (o_binary_gen)
							  );

chafen_encode chafen_encode_u(
							 .i_clk  (clk_16div), 
							 .i_rst  (i_rst), 
							 .i_din  (o_binary_gen), 
							 .o_dout (o_chafen_encode)
							 );

gauss_filter gauss_filter_u(
    .i_clk  (i_clk), 
    .i_rst  (i_rst), 
    .i_din  ({~o_chafen_encode,1'b1}), 
    .o_dout (o_gauss_filter)
    );

adder adder_u(
    .i_clk  (i_clk), 
    .i_rst  (i_rst), 
    .i_din  (o_gauss_filter), 
    .o_dout (o_adder)
    );
	 
nco_gen nco_gen_u(
    .i_clk(i_clk), 
    .i_rst(i_rst), 
    .i_adder(o_adder), 
    .o_cos(o_cos_I), 
    .o_sin(o_sin_Q)
    );	 
	 
	 
nco_gen2 nco_gen2_u(
    .i_clk(i_clk), 
    .i_rst(i_rst), 
    .o_cos(o_cos), 
    .o_sin(o_sin)
    );
	 
wire signed[31:0]I_tmps;
wire signed[31:0]Q_tmps;

multer multer_u1(
	.clk (i_clk),
	.a   (o_cos_I), // Bus [15 : 0] 
	.b   (o_cos), // Bus [15 : 0] 
	.sclr(i_rst),
	.p   (I_tmps)
	); // Bus [31 : 0] 

multer multer_u2(
	.clk (i_clk),
	.a   (o_sin_Q), // Bus [15 : 0] 
	.b   (o_sin), // Bus [15 : 0] 
	.sclr(i_rst),
	.p   (Q_tmps)
	); // Bus [31 : 0] 	 

assign o_GMSK_I = I_tmps[31:16];	 
assign o_GMSK_Q = Q_tmps[31:16];	 	 

assign o_GMSK_R = o_GMSK_I + o_GMSK_Q;

endmodule