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//
// Copyright 2015 Ettus Research
//
// Map symbols to gray encoded bits
// Note: Only use with power of 4 constellations (QPSK, QAM16, QAM64)
// TODO: Extend to other constellations
module symbol_to_gray_bits #(
parameter WIDTH_IN = 16,
parameter MODULATION_ORDER = 6, // QPSK = 2, QAM16 = 4, QAM64 = 6
parameter REVERSE = 0) // 0: Q/I mapping (i.e. QPSK symbol 10 Q = 1, I = 0), 1: I/Q mapping
(
input clk, input reset, input clear,
input [2*WIDTH_IN-1:0] i_tdata, input i_tlast, input i_tvalid, output i_tready,
output [MODULATION_ORDER-1:0] o_tdata, output o_tlast, output o_tvalid, input o_tready
);
// Truncate & invert sign bit
wire [MODULATION_ORDER/2-1:0] symbol_i = {~i_tdata[2*WIDTH_IN-1],i_tdata[2*WIDTH_IN-2:2*WIDTH_IN-MODULATION_ORDER/2]};
wire [MODULATION_ORDER/2-1:0] symbol_q = {~i_tdata[WIDTH_IN-1],i_tdata[WIDTH_IN-2:WIDTH_IN-MODULATION_ORDER/2]};
// Symbol to gray coded bits
reg [MODULATION_ORDER/2-1:0] gray_bits_i, gray_bits_q;
integer k;
always @(*) begin
for (k = MODULATION_ORDER/2-1; k >= 0; k = k - 1) begin
if (k == MODULATION_ORDER/2-1) begin
gray_bits_i[k] = symbol_i[k];
gray_bits_q[k] = symbol_q[k];
end else begin
gray_bits_i[k] = symbol_i[k] ^ symbol_i[k+1];
gray_bits_q[k] = symbol_q[k] ^ symbol_q[k+1];
end
end
end
wire [MODULATION_ORDER-1:0] gray_bits = REVERSE ? {gray_bits_q,gray_bits_i} : {gray_bits_i,gray_bits_q};
axi_fifo_flop #(.WIDTH(MODULATION_ORDER+1)) reg_gray_bits (
.clk(clk), .reset(reset), .clear(clear),
.i_tdata({i_tlast,gray_bits}), .i_tvalid(i_tvalid), .i_tready(i_tready),
.o_tdata({o_tlast,o_tdata}), .o_tvalid(o_tvalid), .o_tready(o_tready),
.space(), .occupied());
endmodule
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