Cognitive interference alignment for OFDM two-tiered networks

Abstract

In this contribution, we introduce an interference alignment scheme that allows the coexistence of an orthogonal frequency division multiplexing (OFDM) macro-cell and a cognitive smallcell, deployed in a two-tiered structure and transmitting over the same bandwidth. We derive the optimal linear strategy for the single antenna secondary base station, maximizing the spectral efficiency of the opportunistic link, accounting for both signal sub-space structure and power loading strategy. Our analytical and numerical findings prove that the precoder structure proposed is optimal for the considered scenario in the face of Rayleigh and exponential decaying channels.

A green approach to femtocells capacity improvement by recycling wasted resources

“…Let us start, by reviewing the conventional CIA [11]. In it, a primary (OFDM-based) and a secondary system share the spectrum at the same time avoiding interference to the primary receiver.…”

“…We have shown in [11] that, for perfect receiver synchronization and perfect channel state information at the secondary transmitter (CSIT), the optimal CIA precoder is given by a semi-unitary matrix structure such that…”

“…where, differently from [11], the secondary receiver does not discard the first L received symbols leading us to let r s = y s . Additionally, in (7), a decomposition of the channel for the CIA transmission into two components ∆ (l) (H s ) and ∆ (u) (H s ) represents the contribution of the channel that generates inter-symbol interference (ISI) and IBI, respectively [15].…”

“…Our technique is based on cognitive interference alignment (CIA), recently introduced in [11] to address the CR interference channel problem. In its classical form, CIA allows a cognitive transmitter to serve a secondary user by sharing the spectrum with a licensee OFDM primary macro-cell, protecting the primary OFDM receiver from undesired interference.…”

“…Note that the validity of the proposed approach is not restricted to femtocells, and can be seamlessly extended to any interference mitigation scenario. Unlike [11], this hybrid approach does not suffer from some issues inherent to CIA, such as synchronization at the primary receiver and interference channel knowledge acquisition. On the other hand, the simultaneous transmission of primary and secondary messages introduces a new challenge since the femtocell receiver is now subject to strong primary interference.…”