%<matlab> 使用移位寄存器生成m序列
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%                                                                        %
%   該函數根據輸入的擴頻因子生成相應的M序列，       %
%  其中擴頻因子限制在2的3次冪到2的12次冪之間。    %
%                                                                        %  
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function Sequence = m_sequence_generator(SF)
SFlog2 = log2(SF);
if rem(SFlog2,1) ~= 0         % rem函數進行mod運算，此處檢驗SFlog2是否為整數
    error('擴頻序列SF必須是2的整數次冪！'); % 即檢驗擴頻因子是否為2的整數次冪
end
%%
switch SFlog2
    case 3
        R = 3; instate = zeros(1,3); instate(R) = 1;    
        %instate為寄存器狀態，由于寄存器不能存在全0狀態，故設置相應的位為1
        N = 2^R - 1; generator = [1 1 3];
        %N為寄存器級數，generator為本原多項式，對應有連接器的位置
    case 4
        R = 4; instate = zeros(1,4); instate(R) = 1;
        N = 2^R - 1; generator = [1 1 4];
    case 5
        R = 5; instate = zeros(1,5); instate(R) = 1;
        N = 2^R - 1; generator = [1 2 5];
    case 6
        R = 6; instate = zeros(1,6); instate(R) = 1;
        N = 2^R - 1; generator = [1 1 6];
    case 7 
        R = 7; instate = zeros(1,7); instate(R) = 1;
        N = 2^R - 1; generator = [1 3 7];
    case 8
        R = 8; instate = zeros(1,8); instate(R) = 1;
        N = 2^R - 1; generator = [1 2 3 4 8];
    case 9
        R = 9; instate = zeros(1,9); instate(R) = 1;
        N = 2^R - 1; generator = [1 4 9];
    case 10
        R = 10; instate = zeros(1,10); instate(R) = 1;
        N = 2^R - 1; generator = [1 3 10];
    case 11
        R = 11; instate = zeros(1,11); instate(R) = 1;
        N = 2^R - 1; generator = [1 2 11];
    case 12
        R = 12; instate = zeros(1,12); instate(R) = 1;
        N = 2^R - 1; generator = [1 1 4 6 12];
    otherwise
        error('SF必須是2的整數次冪, 大于等于2^8并且小于等于2^12.')
end
%%
[Sequence] = ...           % 調用線性反饋移位寄存器產生擴頻m序列
    linear_feedback_shift_register(R,N,instate,generator);