In MATLAB, the conv2 function is used to perform 2D convolution between two matrices. The syntax of conv2 is as follows: 

C = conv2(A, B)

Here, A and B are input matrices, and C is the output matrix after convolution. When A and B have the same size, the convolution is called "full convolution," which means that the output matrix C has the same size as the sum of the sizes of A and B minus 1.

To perform full convolution using conv2, the input matrices A and B are first flipped both horizontally and vertically. Then, the flipped B matrix is slid over A matrix, and the dot product of the overlapped elements is computed and added up. This process is repeated for all possible positions of B on A. The resulting matrix is the full convolution output C.

Here is an example code that demonstrates how to perform full convolution using conv2 in MATLAB: 

% create input matrices
A = randn(5, 5);
B = randn(3, 3);

% perform full convolution using conv2
C = conv2(A, flipud(fliplr(B)), 'full');

In this example, A and B are two random matrices of sizes 5x5 and 3x3, respectively. The flipud and fliplr functions are used to flip B horizontally and vertically before performing the convolution. The 'full' parameter is used to specify that full convolution is required. The resulting matrix C has size 7x7, which is the sum of the sizes of A and B minus 1.