remove the double quotes of a subquery

thanks for you help. i'm trying with the next code in the subquery.

$this->db->order_by('u.sem_codigo', 'ASC');
$this->db->order_by('u.udi_letra_nomina','ASC');
$this->db->order_by('m.mat_anio_academico', 'ASC');

equal i have the error.

ERROR: syntax error at or near "AS" LINE 7: ...d"."udi_id" DESC, "m"."mat_anio_academico"" DESC)" AS "mudi" ^

this happens in the subquery. In clausule: "from"

¿how i can remove the quotes doubles of clausule "from"?