Add next row to current row using AWK

File.txt :

a1
a2
a3
a4
a5
a6

Code :

awk '{array[NR]=$0} END {for (i in array) {if (i>1) {{print array[i-1]","array[i]}}}}' File.txt

Result :

a1,a2
a2,a3
a3,a4
a4,a5
a5,a6

After the all rows are imported to an array, it shows next row to current row under a condition of row > 1.