Elsayed Ahmed

All-Digital Self-Interference Cancellation Technique for Full-Duplex Systems

Full-duplex systems are expected to double the spectral efficiency compared to conventional half-duplex systems if the self-interference signal can be significantly mitigated. Digital cancellation is one of the lowest complexity self-interference cancellation techniques in full-duplex systems. However, its mitigation capability is very limited, mainly due to transmitter and receiver circuits impairments (e.g., phase noise, nonlinear distortion, and quantization noise). In this paper, we propose a novel digital self-interference cancellation technique for full-duplex systems. The proposed technique is shown to significantly mitigate the self-interference signal as well as the associated transmitter and receiver impairments, more specifically, transceiver nonlinearities and phase noise. In the proposed technique, an auxiliary receiver chain is used to obtain a digital-domain copy of the transmitted Radio Frequency (RF) self-interference signal. The self-interference copy is then used in the digital-domain to cancel out both the self-interference signal and the associated transmitter impairments. Furthermore, to alleviate the receiver phase noise effect, a common oscillator is shared between the auxiliary and ordinary receiver chains. A thorough analytical and numerical analysis for the effect of the transmitter and receiver impairments on the cancellation capability of the proposed technique is presented. Finally, the overall performance is numerically investigated showing that using the proposed technique, the self-interference signal could be mitigated to ~3 dB higher than the receiver noise floor, which results in up to 76% rate improvement compared to conventional half-duplex systems at 20 dBm transmit power values.

Rate Gain Region and Design Tradeoffs for Full-Duplex Wireless Communications

In this paper, we analytically study the regime in which practical full-duplex systems can achieve larger rates than an equivalent half-duplex systems. The key challenge in practical full-duplex systems is uncancelled self-interference signal, which is caused by a combination of hardware and implementation imperfections. Thus, we first present a signal model which captures the effect of significant impairments such as oscillator phase noise, low-noise amplifier noise figure, mixer noise, and analog-to-digital converter quantization noise. Using the detailed signal model, we study the rate gain region, which is defined as the region of received signal-of-interest strength where full-duplex systems outperform half-duplex systems in terms of achievable rate. The rate gain region is derived as a piecewise linear approximation in log-domain, and numerical results show that the approximation closely matches the exact region. Our analysis shows that when phase noise dominates mixer and quantization noise, full-duplex systems can use either active analog cancellation or baseband digital cancellation to achieve near-identical rate gain regions. Finally, as a design example, we numerically investigate the full-duplex system performance and rate gain region in typical indoor environments for practical wireless applications.

Self-interference cancellation with nonlinear distortion suppression for full-duplex systems

In full-duplex systems, due to the strong self-interference signal, system nonlinearities become a significant limiting factor that bounds the possible cancellable self-interference power. In this paper, a self-interference cancellation scheme for full-duplex orthogonal frequency division multiplexing systems is proposed. The proposed scheme increases the amount of cancellable self-interference power by suppressing the distortion caused by the transmitter and receiver nonlinearities. An iterative technique is used to jointly estimate the self-interference channel and the nonlinearity coefficients required to suppress the distortion signal. The performance is numerically investigated showing that the proposed scheme achieves a performance that is less than 0.5dB off the performance of a linear full-duplex system.

A Beam-Steering Reconfigurable Antenna for WLAN Applications

A multifunctional reconfigurable antenna (MRA) capable of operating in nine modes corresponding to nine steerable beam directions in the semisphere space {-30°,0°, 30°}; φ ∈ {0°, 45°, 90°, 135°}) is presented. The MRA consists of an aperture-coupled driven patch antenna with a parasitic layer placed above it. The surface of the parasitic layer has a grid of 3 × 3 electrically-small square-shaped metallic pixels. The adjacent pixels are connected by PIN diode switches with ON/OFF status to change the geometry of the parasitic surface, which in turn changes the current distribution on the antenna, thus provides reconfigurability in beam steering direction. The MRA operates in the IEEE 802.11 frequency band (2.4-2.5 GHz) in each mode of operation. The antenna has been fabricated and measured. The measured and simulated impedance and radiation pattern results agree well indicating an average of ~ 6.5 dB realized gain in all modes of operation. System level experimental performance evaluations have also been performed, where an MRA equipped WLAN platform was tested and characterized in typical indoor environments. The results confirm that the MRA equipped WLAN systems could achieve an average of 6 dB Signal to Noise Ratio (SNR) gain compared to legacy omni-directional antenna equipped systems with minimal training overhead.

Self-interference cancellation with phase noise induced ICI suppression for full-duplex systems

One of the main bottlenecks in practical full-duplex systems is the oscillator phase noise, which bounds the possible cancellable self-interference power. In this paper, a digital-domain self-interference cancellation scheme for full-duplex orthogonal frequency division multiplexing systems is proposed. The proposed scheme increases the amount of cancellable self-interference power by suppressing the effect of both transmitter and receiver oscillator phase noise. The proposed scheme consists of two main phases, an estimation phase and a cancellation phase. In the estimation phase, the minimum mean square error estimator is used to jointly estimate the transmitter and receiver phase noise associated with the incoming self-interference signal. In the cancellation phase, the estimated phase noise is used to suppress the intercarrier interference caused by the phase noise associated with the incoming self-interference signal. The performance of the proposed scheme is numerically investigated under different operating conditions. It is demonstrated that the proposed scheme could achieve up to 9 dB more self-interference cancellation than the existing digital-domain cancellation schemes that ignore the intercarrier interference suppression.

Simultaneous transmit and sense for cognitive radios using full-duplex: A first study

In two-tier cognitive radio networks, spectrum sensing is important to minimize the interference to the primary users. With half-duplex radios, spectrum sensing has to be performed periodically between transmissions, leading to spectral inefficiencies. In this paper, we study the achievable rate gain and transmission range increase for in-band full-duplex transmissions using directional multi-reconfigurable antennas. The aim is to enable simultaneous transmission and sensing in cognitive radio networks. Our results show that directionality of multi-reconfigurable antennas can increase both the range and rate of full-duplex transmissions over omni-directional antenna based full-duplex transmissions.

On Phase Noise Suppression in Full-Duplex Systems

Oscillator phase noise has been shown to be one of the main performance limiting factors in full-duplex systems. In this paper, we consider the problem of self-interference cancellation with phase noise suppression in full-duplex systems. The feasibility of performing phase noise suppression in full-duplex systems in terms of both complexity and achieved gain is analytically and experimentally investigated. First, the effect of phase noise on full-duplex systems and the possibility of performing phase noise suppression are studied. Two different phase noise suppression techniques with a detailed complexity analysis are then proposed. For each suppression technique, both free-running and phase-locked loop-based oscillators are considered. Due to the fact that full-duplex system performance highly depends on hardware impairments that are difficult to fully model, experimental results in a typical indoor environment are presented. The experimental results performed on two different platforms confirm results obtained from numerical simulations. Finally, the tradeoff between the required complexity and the gain achieved using phase noise suppression is discussed.

Full-Duplex Systems Using Multireconfigurable Antennas

Full-duplex systems are expected to achieve 100% rate improvement over half-duplex systems if the self-interference signal can be significantly mitigated. In this paper, we propose the first full-duplex system utilizing multireconfigurable antenna (MRA) with ~90% rate improvement compared with half-duplex systems. MRA is a dynamically reconfigurable antenna structure that is capable of changing its properties according to certain input configurations. A comprehensive experimental analysis is conducted to characterize the system performance in typical indoor environments. The experiments are performed using a fabricated MRA that has 4096 configurable radiation patterns. The achieved MRA-based passive self-interference suppression is investigated, with detailed analysis for the MRA training overhead. In addition, a heuristic-based approach is proposed to reduce the MRA training overhead. The results show that at 1% training overhead, a total of 95 dB self-interference cancellation is achieved in typical indoor environments. The 95-dB self-interference cancellation is experimentally shown to be sufficient for 90% full-duplex rate improvement compared with half-duplex systems.

Semi-adaptive channel estimation technique for LTE systems

Channel estimation in orthogonal frequency division multiplexing (OFDM) systems is required for coherent demodulation. This paper deals with channel estimation in the OFDM based LTE system. In particular, two pilot-aided techniques are presented: the low-pass filter based estimator (LPFE), and the robust minimum mean square error estimator (MMSE). These techniques are characterized by their low complexity relative to other complex estimation techniques. A semi-adaptive channel estimator is proposed in this paper. In the proposed estimator, channel delay spread and Doppler frequency are used to adapt the interpolation filters in order to enhance the estimator performance while maintaining low complexity relative to the fully adaptive estimator. Simulation results show that there is performance improvement using the proposed adaptive scheme when applied to LTE systems.