Get next and prev posts in Wordpress by alphabetical order.

next_prev_post.php

 <?PHP
 /*
  * Sort Next/Previous Post Link Buttons Alphabetically
  */
 function filter_next_post_sort($sort) {
    if (get_post_type($post) == 'portfolio_page') {
        $sort = "ORDER BY p.post_title ASC LIMIT 1";
    }
    else{
        $sort = "ORDER BY p.post_date ASC LIMIT 1";
    }
    return $sort;
}
function filter_next_post_where($where) {
    global $post, $wpdb;
    if (get_post_type($post) == 'portfolio_page') {
        return $wpdb->prepare("WHERE p.post_title > '%s' AND p.post_type = '". get_post_type($post)."' AND p.post_status = 'publish'",$post->post_title);
    }
    else{
        return $wpdb->prepare( "WHERE p.post_date > '%s' AND p.post_type = '". get_post_type($post)."' AND p.post_status = 'publish'", $post->post_date, $post->post_type );
    }
}
function filter_previous_post_sort($sort) {
    if (get_post_type($post) == 'portfolio_page') {
        $sort = "ORDER BY p.post_title DESC LIMIT 1";
    }
    else{
        $sort = "ORDER BY p.post_date DESC LIMIT 1";
    }
    return $sort;
}
function filter_previous_post_where($where) {
    global $post, $wpdb;
    if (get_post_type($post) == 'portfolio_page') {
        return $wpdb->prepare("WHERE p.post_title < '%s' AND p.post_type = '". get_post_type($post)."' AND p.post_status = 'publish'",$post->post_title);
    }
    else{
        return $wpdb->prepare( "WHERE p.post_date < '%s' AND p.post_type = '". get_post_type($post)."' AND p.post_status = 'publish'", $post->post_date, $post->post_type );
    }
}
add_filter('get_next_post_sort',   'filter_next_post_sort');
add_filter('get_next_post_where',  'filter_next_post_where');
add_filter('get_previous_post_sort',  'filter_previous_post_sort');
add_filter('get_previous_post_where', 'filter_previous_post_where');

Hi, using this code and it works great. Where can I add $excluded_terms so that I can have the prev/next skip posts in certain categories? Thanks in advance for you help.