Permutation/Silence Coding in Partially Under-Utilized Cellular Networks

We consider a cellular system where (i) wireless spectrum is divided into resource blocks in time and frequency, (ii) each wireless receiver is assigned one or more resource blocks as the exclusive (primary) user of these blocks, (iii) some of the resource blocks are under-utilized due to the mismatch between the primary user´s application payload and the capacity of the dedicated resources, and (iv) there are users with backlogged traffic that cannot be fully served by only utilizing their own dedicated resource blocks. We show how partially utilized resource blocks in a given cell can be reutilized by conveying extra information to the backlogged users in the system (secondary users) by coding over the transmission patterns of the primary users. Specifically, we propose to create on-off transmission patterns (referred to as silence coding), where during on periods the base-station sends primary user´s symbols and during off periods the base-station keeps silent. Distinct transmission patterns in return are used to carry different messages for a given secondary user. We first analyze the achievable rate region for primary and secondary users from an information theoretical point of view for randomly generated codebooks. Then, we propose practical schemes that use permutation codes for silence coding with and without forward error correction. Finally, we evaluate the performance of these practical schemes.