Minimizing the Access Time of Multi-item Requests in Wireless Data Broadcast Environments

Wireless data broadcast is an efficient way to disseminate data to a large number of users in mobile communication environments. In many applications, such as stock quotes, flight schedules and traffic news, the users may want to download multiple data items at one time and the applications may require the support of multi-channel architectures. This paper studies the problem of retrieving a set of data items from multiple wireless broadcasting channels such that all the requested data items can be downloaded with the least access time, namely Least Time Data Retrieval (LTDR). On one hand, we find that it is NP-hard to approximate this problem to within any non-trivial factor even if there are only two channels. On the other hand, we find that there exists a polynomial time (1+1/1-α)-approximation solution for an LTDR instance L if the optimal solution of L requirers τ time slots and (1-α)kT ≤ τ ≤ kT, where k is a positive integer, α is an arbitrary constant between 0 and 1, and T is the least common multiple of cycle lengths for all channels. We also derive a lower bound of polynomial time approximation for L matching that upper bound under the well known assumption P ≠ NP. To improve the practical efficiency of the approximation solution, a heuristic is exhibited and tested by simulations.