You can build character string sequences by memory manipulation just the same as numeric ones:
>> c='RCL'; % the initial characters to replicate
>> N=10; % the replication count for each
>> c=reshape(repmat(c.',1,N).',1,[]) % a pattern of N of the substring
c =
RRRRRRRRRRCCCCCCCCCCLLLLLLLLLL
>>
Now just replicate that as needed and concatenate with the rest into a cell array.
M=3; % say, whatever is multiplier need for final length
c=repmat(c.',M,1);
ADDENDUM
For brevity of presentation I purposely left the above as a row vector; the conversion to cellstr array should be obvious but just to make sure...
>> c=cellstr(reshape(repmat(c.',1,N).',[],1));
>> whos c
Name Size Bytes Class Attributes
c 30x1 1860 cell
>>
Is the column cellstring array needed... NB: reordered the reshape to '[],1' so cellstr is working on a column character string array.
