Self-Interference Cancellation for LTE-Compatible Full-Duplex Systems

This paper discusses the design of a single channel full-duplex wireless transceiver. The design uses a combination of RF and baseband techniques to achieve full-duplexing with minimal effect on link reliability. Experiments on real nodes show the full-duplex prototype achieves median performance that is within 8% of an ideal full-duplexing system. This paper presents Antenna Cancellation, a novel technique for self-interference cancellation. In conjunction with existing RF interference cancellation and digital baseband interference cancellation, antenna cancellation achieves the amount of self-interference cancellation required for full-duplex operation. The paper also discusses potential MAC and network gains with full-duplexing. It suggests ways in which a full-duplex system can solve some important problems with existing wireless systems including hidden terminals, loss of throughput due to congestion, and large end-to-end delays.

The advent and advancement of full-duplex (FD) technology in radio transceivers is expected to lead to full-duplex enabled multi-cell (FD-MC) networks in next generation wireless communication systems. Additional interferences between FD nodes and devices are the main challenges for the deployment of such networks. As in an early deployment scenario where the base stations (BS) operate in FD mode while the user equipments (UE) remain in the half-duplex (HD) mode, the self-interference (SI) within the FD-BS, the mutual interference (MI) between the FD-BS's and between the HD-UE's paired for FD scheduling are the major issues that could jeopardize the capitalization of FD system gains if they are not well addressed. In this paper, we present a system architecture for multi-stage cancellation of SI and joint cancellation of MI and residual SI in an FD-MC network. Multiple orthogonal pilots and their derived forms are utilized for channel estimation during a common training period. System-level delay calibration necessary for SI and MI channel estimation and cancellation is also provided.

In-band full duplex (IBFD) wireless is of utmost interest to future wireless communication and networking due to great potentials of spectrum efficiency. IBFD wireless, how- ever, is throttled by its key challenge, namely self-interference. Therefore, effective self- interference cancellation is the key to enable IBFD wireless. This paper proposes a real-time non- linear self-interference cancellation solution: Deep learning based Self-Interference Cancellation (DSIC) to enable IBFD wireless. In this solution, a self-interference channel is modeled by a deep neural network (DNN). Synchronized self- interference channel data is first collected to train the DNN of the self-interference channel. Afterwards, the trained DNN is used to cancel the self-interference at a wireless node. This solution has been implemented on a USRP SDR testbed and evaluated in real world in multiple scenarios with various modulations in transmitting information including numbers, texts as well as images. It results in the performance of 17dB in digital cancellation, which is very close to the self-interference power and nearly cancels the self- interference at a SDR node in the testbed. The solution yields an average of 8.5% bit error rate (BER) over many scenarios and different modulation schemes.

Full-duplex (FD) radio has been introduced for bidirectional communications on the same temporal and spectral resources so as to maximize spectral efficiency. In this paper, motivated by the recent advances in FD radios, we provide a foundation for downlink hybrid-duplex heterogeneous networks (HDHNs), composed of multi-tier networks with a mixture of access points (APs) operating either in FD mode or half-duplex (HD) mode. Specifically, we characterize the network interference from FD-mode cells, and derive the HDHN throughput by accounting for AP spatial density, self-interference cancellation (IC) capability, and transmission power of APs and users. By quantifying the HDHN throughput, we present the effect of network parameters and the self-IC capability on the HDHN throughput, which shows the superiority of FD mode for larger AP densities (i.e., larger network interference and shorter communication distance) or higher self-IC capability.

Full-duplex (FD) is an emerging technology to alleviate the spectrum crunch. Self-interference is the major issue needs to be solved to realize FD devices, especially for the bidirectional FD model wherein two FD featured devices communicate with each other directly. Beamforming is an effective technology to reduce the self-interference by optimally allocating transmission power. However, the beamforming based self-interference cancellation schemes are not applicable in bidirectional FD model when FD featured transceivers are far away from each other, since large transmission power is needed to guarantee the signal to be received successfully. To enable the beamforming based self-interference cancellation mechanism for bidirectional FD communications with large distance, we propose a relay selection based self-interference cancellation mechanism. By introducing relays, two links in bidirectional FD communications are separated, which enables successful self-interference cancellation with existing beamforming based mechanisms. Simulation provides the guidance of optimal relay deployment. It also shows our proposed relay selection enabled self-interference cancellation mechanism outperforms the existing bidirectional FD communication schemes.

Full duplex Cognitive Radio (CR) achieves more spectral efficiency if it operates in listen and talk mode. It also facilitates to find multiple free available spectrum in a wide band and fast frequency handoff in the case the secondary user uses the frequency occupied by the Primary User (PU). Full duplex CR works using two RF chain with employing the sensing by a dedicated RF chain and transmitting the data with other RF chain. The main challenge involved in full duplex sensing is the self-interference problem, particularly for energy detector based spectrum sensing. Energy detector based spectrum sensing is widely used because of less computational complexity and PU signal knowledge is not required which is an essential feature in wideband spectrum sensing. This work proposes an SDR implementation of full duplex CR using energy detector with self-interference cancellation using Wiener filter. The self-interference signal is predicted by Wiener filter and it is removed in the received signal before apply to the energy detector. The proposed scheme is simulated and validated through an experimental study on SDR platform. The results show the improvement in probability of detection of PU signal. Parallel and pipelined architectures are employed to implement the entire system using the Kintext 7 FPGA on NI USRP RIO 2943R SDR, which result in reduction of sensing time.

This paper considers the implementation and application possibilities of a compact full duplex multiple-input multiple-output (MIMO) architecture, where direct communication exists between users, e.g., device-to-device (D2D) and cellular link coexisting on the same spectrum. For the architecture of the compact full duplex radio, we combine an analog self-interference canceler-based dual polarization with high cross-polarization discrimination and long-term evolution (LTE)-based per-subcarrier digital self-interference canceler. While we consider the compactness and power efficiency of an analog solution, we focus on the digital canceler design with robustness to a frequency-selective channel and high compatibility with a conventional LTE system. For an over-the-air wireless experiment of full duplex testbed with a two-user-pair, we implement a full duplex MIMO physical layer, supporting 20-MHz bandwidth, on an Field-Programmable Gate Array-based software-defined radio platform. Furthermore, we propose a novel timing synchronization method to construct a more viable full duplex MIMO link. By having the full duplex link prototype fully operating in real time, we present the first characterization of the proposed compact full duplex MIMO performance depending on the transmit power of the full duplex node. We also show the link quality between nodes. One of the crucial insights of this paper is that the full duplex operation of a user is capable of acquiring the throughput gain if the user has self-interference capability with guaranteed performance.

Hybrid duplex wireless networks, use half duplex (HD) as well as full duplex (FD) modes to utilize the advantages of both technologies. This paper tries to determine the proportion of the network nodes that should be in HD or FD modes in such networks, to maximize the overall throughput of all FD and HD nodes. Here, by assuming imperfect self-interference cancellation (SIC) and using ALOHA protocol, the local optimum densities of FD, HD and idle nodes are obtained in a given time slot, using Karush–Kuhn–Tucker (KKT) conditions as well as stochastic geometry tool. We also obtain the sub-optimal value of the signal-to-interference ratio (SIR) threshold constrained by fixed node densities, using the steepest descent method in order to maximize the network throughput. The results show that in such networks, the proposed hybrid duplex mode selection scheme improves the level of throughput. The results also indicate the effect of imperfect SIC on reducing the throughput. Moreover, it is demonstrated that by choosing an optimal SIR threshold for mode selection process, the achievable throughput in such networks can increase by around 5%.

With self-interference cancellation (SIC), a wireless full duplex terminal is allowed to transmit and receive signal simultaneously in the same frequency band. This paper focuses on the analysis of desired receive signal impact on the radio frequency (RF) SIC performance. The full-duplex system model is rst given, and the principle of radio frequency self-interference cancellation is introduced. Then, from the perspective of the convex function, the in uence of the desired signal on the RF SIC is analyzed when the total power of the signal after RF SIC is taken as the objective function. It is found that when there is no desired signal, the objective function is a convex function, and the residual self-interference power can converge to the minimum value by the gradient descent method. If the desired signal exists and the channel is constant, the objective function is also a convex function, and the residual self-interference power can also converge to the minimum value. However, in the presence of the desired signal and the channel uctuates, the target is not a convex function, and the residual self-interference power cannot be converged to the minimum value by the gradient descent method.

One primary challenge in the high-speed train (HST) communication is that the wireless signal suffers from severe attenuation while it penetrates the sealed carriage of the train. In this letter, we show that such a nuisance can provide an opportunity of improving the uplink capacity when a full-duplex (FD) mobile relay (MR) is furnished on the train. To this end, by introducing the self-interference cancellation (SIC) level, we present an uplink channel model for the HST communication with FD MR. Then the uplink capacity is analyzed under either user equipment power constraint or total power constraint. By comparing with time division scheme, we can derive the required SIC level for the FD scheme to bring capacity improvement, which is confirmed by numerical results. Furthermore, theoretical analysis shows that, for a given SIC level, the uplink channel with severer carriage attenuation has higher capacity, which is also validated by numerical results.

The main contribution of the paper is demonstrating a ray tracing framework to extract the three dimensional (3D) channel parameters as input to stochastic models in studying small cell environments. We focus on Long Term Evolution (LTE) small cells, taking as reference outdoor small cells deployments providing services to indoor users. Many stochastic channel models model the channel parameters in two dimensions and ignore the variation in the elevation plane. They are designed to capture the characteristics of macrocellular networks where eNBs are placed hundreds of meters away from each other. In such scenarios the two-dimensional channel characteristics might be sufficient to capture the channel properties. In contrast, small cells are deployed much more closely to each other. The performance of small cells is highly dependent on 3D antenna patterns, the environment specific characteristics such as 3D geometry of the buildings, materials used for building construction and their specific propagation properties. Therefore in small cell environments the site specific modeling of channel parameters in three dimensions is needed. Measurements to determine the channel parameters may not be available for every environment of interest. Ray tracing could close the gap in extracting the 3D site-specific channel parameters pertaining to small cell environments.

Full duplex allows a device to transmit and receive simultaneously in the same frequency band, theoretically doubling the throughput compared to traditional half duplex systems. However, several limitations restrict the promised full duplex gain: non-ideal self-interference cancellation, increased inter-cell interference and traffic constraints. In this paper, we first study the self-interference cancellation capabilities by using a real demonstrator. Results show that achieving ∼110 dB of cancellation is already possible with the current available technology, thus providing the required level of isolation to build an operational full duplex node. Secondly, we investigate the inter-cell interference and traffic constraints impact on the full duplex performance in 5th generation systems. System level results show that both the traffic and the inter-cell interference can significantly reduce the potential gain of full duplex with respect to half duplex. However, for large traffic asymmetry, full duplex can boost the performance of the lightly loaded link.

Full-Duplex (FD) transceivers may be expensive in terms of complexity, power consumption, and price to be implemented in all user terminals. Therefore, techniques to exploit in-band full-duplex communication with FD base stations (BSs) and half-duplex (HD) users' equipment (UEs) are required. In this context, 3-node topology (3NT) has been recently proposed for FD BSs to reuse the uplink (UL) and downlink (DL) channels with HD terminals within the same cell. In this paper, we present a tractable mathematical framework, based on stochastic geometry, for 3NT in cellular networks. To this end, we propose a design paradigm via pulse-shaping and partial overlap between UL and DL channels to maximize the harvested rate gains in 3NT. The results show that 3NT achieves a close performance to networks with FD BSs and FD UEs, denoted by 2-node topology (2NT) networks. A maximum of 5$\%$ rate loss is reported when 3NT is compared to 2NT with efficient self-interference cancellation (SIC). If the SIC in 2NT is not efficient, 3NT highly outperforms 2NT. Consequently, we conclude that, irrespective to the UE duplexing scheme, it is sufficient to have FD BSs to harvest FD rate gains.

Spectrum sharing is proposed as one of the solutions to help the commercial cellular industry meet the 1000x mobile data challenge they are facing. Most of the valuable spectrum lies below 6 GHz and is currently being occupied by commercial communication and RADAR systems. Commercial communication systems of the future are expected to be based on 3GPP Long Term Evolution (LTE) standards. In this paper, we present an overview of some of the techniques that can be used for enabling efficient co-existence of commercial LTE and RADAR systems. The solutions outlined in this paper do not require any changes to the existing standards nor are they specific to any regional regulatory requirements.

This paper investigates self-interference cancellation (SIC) techniques for enabling full-duplex (FD) operations in mobile communication systems. We explore several SIC strategies: analog beamforming to suppress direct-path SI, digital baseband SIC based on channel estimation, SIC with power amplifier (PA) nonlinearity estimation, and digital beamforming for reflected-path SI nulling. Simulation results confirm that these combined approaches effectively suppress SI below the noise floor, demonstrating the feasibility of practical FD deployments in future mobile networks.