start with setting the request's fetch offset to 0 with a fetchLimit to 15 and just add 15 to the fetchOffset each time... Hope this helps
// first fetch
request.fetchOffset = 0;
request.fetchLimit = 15;
// second fetch
request.fetchOffset = 15;
request.fetchLimit = 15;
// third fetch
request.fetchOffset = 30;
request.fetchLimit = 15;