16-QAM transmitter and receiver without channel

M = 16; % QAM order
fs = 16000; % sampling frequency in Hz
Ts = 1/fs; % sampling interval in s
fc = 1000; % carrier frequency in Hz (must be < fs/2 and > fg)
Rs = 100; % symbol rate
Ns = 20; % number of symbols

x = randint(Ns,1,M);
y = modulate(modem.qammod(M),x);

L = fs/Rs; % oversampling factor

% Impulse shaping
y_a = reshape(repmat(y', L, 1), 1, length(y)*L);

%% Modulation
Q=real(y_a);
I=imag(y_a);
t = 0 : Ts : (length(y_a) - 1) * Ts;
C1 = I .* sin(2*pi * fc * t);
C2 = Q .* cos(2*pi * fc * t);
s = C1 + C2;

%% Demodulation
r_I = s .* sin(2*pi * fc * t);
r_Q = s .* cos(2*pi * fc * t);

%% Filter

% Design filter with least-squares method
N     = 50;           % Order
Fpass = Rs/2;         % Passband Frequency
Fstop = 2*fc - Rs/2;  % Stopband Frequency
Wpass = 1;            % Passband Weight
Wstop = 1;            % Stopband Weight

% Calculate the coefficients using the FIRLS function.
b  = firls(N, [0 Fpass Fstop fs/2]/(fs/2), [1 1 0 0], [Wpass Wstop]);

% Filtering
w_I = filter(b, 1, r_I);
w_Q = filter(b, 1, r_Q);

%% Sampling
u_I = downsample(w_I, L, L/2);
u_Q = downsample(w_Q, L, L/2);

figure;
plot(r_I); hold on;
plot(w_I,'r'); hold off;
figure;
plot(u_I, u_Q, '.');