Full-duplex Case Research Articles

Cooperative Non-Orthogonal Multiple Access Transmission Through Full-Duplex and Half-Duplex Relays

We investigate the beamforming for cooperative non-orthogonal multiple access (NOMA) transmission through a decode-and-forward relay either with full-duplex (FD) mode or with half-duplex (HD) mode. The beamforming vectors at the source and at the relay are optimized to maximize the end-to-end sum rates of the NOMA transmission. The optimal beamforming for the second-hop of the NOMA transmission is present, from which the beamforming algorithms for the two relay modes are derived. For the FD case, the removal of self interference from the feedback loop is important and a two dimensional (2-D) search algorithm based on semi definite relaxation (SDR) is provided, where the 2-D search range is narrowed down as the algorithm iterates. For the HD mode, no iteration is necessary and only one implementation of SDR is required since the SI suppression is not needed. Simulation results show the performance advantage of the proposed FD mode system over that of HD mode in various parameter settings. This justifies the cost and complex hardware circuitry of FD mode.

A Double Threshold-Based Cooperative Spectrum Sensing With Novel Hard-Soft Combining Over Fading Channels

In this letter, a double threshold-based log-likelihood ratio (LLR) detector with a novel hard-soft combining rule (DT-LLR-HSC) is proposed for spectrum sensing in cooperative cognitive radio networks (CRNs) under the block Rayleigh fading scenario. A novel weighted hard-soft combining scheme is adopted at the fusion center to combine local binary/energy information resulting in global binary decision. A trust-based novel weight vector estimation considering both the sensing and reporting channel signal-to-noise ratio is also proposed. Further, the proposed DT-LLR-HSC scheme is adopted in full-duplex cooperative CRNs under Nakagami-m fading channel. The analytical expressions for the average probability of detection are derived and verified for both the half-duplex and full-duplex cases.

Secure Spectrum-Sharing Wiretap Networks With Full-Duplex Relaying

In this paper, we investigate the secrecy performance of dual-hop randomize-and-forward (RaF) cognitive wiretap networks over Rayleigh fading channels, in which the RaF relay is considered both as half-duplex (HD) and full-duplex (FD) operations. Specifically, for the HD, maximal-ratio combining/maximal-ratio transmission (MRC/MRT) is adopted at the RaF relay. For the FD, the RaF relay can simultaneously receive the signal from the source and transmit jamming signals to eavesdropper, and the selection combining-zero forcing beamforming/maximal-ratio transmit (SC-ZFB/MRT) and SC-ZFB/ZFB schemes are respectively adopted at the FD relay. To thoroughly assess the secrecy performance achieved from the proposed schemes, the closed-form expressions for the secrecy outage probability (SOP) of the dual-hop RaF cognitive wiretap channels with MRC/MRT, SC-ZFB/MRT and SC-ZFB/ZFB schemes are derived, respectively. In addition, we also provide simple asymptotic approximations for the SOP under two distinct scenarios, depending on the quality of the main and wiretap channels. Additionally, our analytical results and numerical simulations show the efficiency of our proposed solutions for both SC-ZFB/MRT and SC-ZFB/ZFB with FD cases and show considerable performance gains over MRC/MRT with HD scheme. Finally, for the proposed schemes, the SC-ZFB/ZFB scheme is beneficial to improve secrecy performance when the eavesdropper’s channel is better than the main channel and the RaF relaying strategy achieves better secrecy performance than that of the decode-and-forward (DF) relaying strategy for dual-hop cognitive wiretap networks.

An Auxiliary Antenna Based Inter-User Interference Mitigation Approach in Full-Duplex Wireless Networks

When a full-duplex (FD) base station (BS) serves an uplink and a downlink half-duplex (HD) users, the uplink user would produce the inter-user interference (IUI) on the downlink user. In this letter, an IUI suppression scheme is proposed based on the auxiliary antenna at the downlink user. A problem of maximizing the sum achievable rate of the uplink and downlink is formulated by adjusting the weighted vector at the downlink user with limited uplink and downlink transmit power. This problem is solved by two steps. First, the optimal weighted vector at the downlink user is derived. Second, the optimal downlink power is given for a fixed uplink power and the optimal uplink power is obtained by a search method. The single-cell simulation results show that the average achievable rate of the proposed IUI suppression scheme significantly outperforms the existing schemes especially when the cell radius is small and can nearly approach the performance of the ideal FD case with IUI perfectly suppressed.

Secrecy Rate Maximizing Beamforming Schemes for the DF Relay Wiretap Channels

We consider beamforming schemes for the multi-antenna decode-and-forward (DF) relays that maximize the secrecy rate (SR) in the relay wiretap channel. Both full-duplex (FD) and half-duplex (HD) relays are considered, where the FD relay fights the loop-back self-interference while maximizing the SR. The HD relay beamforming is derived from the FD case as a byproduct. The facts that, the weaker link among the two hops in the DF relay network dominates, the throughput and the first hop is shared among the destination and the eavesdroppers make us divide the beamformer solution space into three separate regions. Since only two of the three regions effectively produce non-negative SR, separate beamformer designs for these two effective regions are provided. Numerical results reveal performance trade off of FD and HD modes in various channel configurations, where the two challenges of self-interference suppression and the protection from eavesdropping ask different requirements for the beamforming.

Secrecy Outage Analysis for Downlink Transmissions in the Presence of Randomly Located Eavesdroppers

We analyze the secrecy outage probability in the downlink for wireless networks with spatially (Poisson) distributed eavesdroppers (EDs) under the assumption that the base station employs transmit antenna selection (TAS) to enhance secrecy performance. We compare the cases where the receiving user equipment (UE) operates in half-duplex (HD) mode and full-duplex (FD) mode. In the latter case, the UE simultaneously receives the intended downlink message and transmits a jamming signal to strengthen secrecy. We investigate two models of (semi)passive eavesdropping: (1) EDs act independently and (2) EDs collude to intercept the transmitted message. For both of these models, we obtain expressions for the secrecy outage probability in the downlink for HD and FD UE operation. The expressions for HD systems have very accurate approximate or exact forms in terms of elementary and/or special functions for all path loss exponents. Those related to the FD systems have exact integral forms for general path loss exponents, while exact closed forms are given for specific exponents. A closed-form approximation is also derived for the FD case with colluding EDs. The resulting analysis shows that the reduction in the secrecy outage probability is logarithmic in the number of antennas used for TAS and identifies conditions under which HD operation should be used instead of FD jamming at the UE. These performance trends and exact relations between system parameters can be used to develop adaptive power allocation and duplex operation methods in practice. Examples of such techniques are alluded to herein.

To Code in Space and Time or Not in Multihop Relay Channels

Multihop relay channels use multiple relay stages, each with multiple relay nodes, to facilitate communication between a source and destination. Previously, distributed space-time coding was used to maximize diversity gain. Assuming a low-rate feedback link from the destination to each relay stage and the source, this paper proposes end-to-end antenna selection strategies as an alternative to distributed space-time coding. One-way (where only the source has data for destination) and two-way (where the destination also has data for the source) multihop relay channels are considered with both the full-duplex and half-duplex relay nodes. For the full-duplex case, end-to-end antenna selection strategies are designed and proven to achieve maximum diversity gain by using a single-antenna path (using single antenna of the source, each relay stage and the destination) with the maximum signal-to-noise ratio at the destination. For the half-duplex case, two paths with the two best signal-to-noise ratios in alternate time slots are used to overcome the rate loss with half-duplex nodes, with a small diversity gain penalty. A multiple stream end-to-end antenna selection strategy for full-duplex multihop relay channel is also proposed to obtain a lower bound on the diversity multiplexing tradeoff of multihop relay channels. Finally, to answer the question of whether to code in space and time or not in a multihop relay channel, end-to-end antenna selection strategy and distributed space-time coding is compared with respect to several important performance metrics such as encoding/decoding complexity, rate of transmission, latency, bit-error-rate performance, and resource requirements.

Optimized Power Allocation for Pairwise Cooperative Multiple Access

Multiple access schemes in which the transmitting nodes are allowed to cooperate have the potential to provide higher quality of service than conventional schemes. In the class of pair-wise cooperative multiple access schemes in which channel state information is available at the transmitters, the allocation of transmission power plays a key role in the realization of these quality of service gains. Unfortunately, the natural formulation of the power allocation problem for full-duplex cooperative schemes is not convex. It is shown herein that this non-convex formulation can be simplified and recast in a convex form. In fact, closed-form expressions for the optimal power allocation for each point on the boundary of an achievable rate region are obtained. In practice, a half-duplex cooperative scheme, in which the channel resource is partitioned in such a way that interference is avoided, may be preferred over a full-duplex scheme. The channel resource is often partitioned equally, but we develop an efficient algorithm for the joint allocation of power and the channel resource for a modified version of an existing half-duplex cooperative scheme. We demonstrate that this algorithm enables the resulting scheme to attain a significantly larger fraction of the achievable rate region for the full duplex case than the underlying scheme that employs a fixed resource allocation.

Multiple-Antenna Cooperative Wireless Systems: A Diversity–Multiplexing Tradeoff Perspective

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We consider a general multiple-antenna network with multiple sources, multiple destinations, and multiple relays in terms of the diversity–multiplexing tradeoff (DMT). We examine several subcases of this most general problem taking into account the processing capability of the relays (half-duplex or full-duplex), and the network geometry (clustered or nonclustered). We first study the multiple-antenna relay channel with a full-duplex relay to understand the effect of increased degrees of freedom in the direct link. We find DMT upper bounds and investigate the achievable performance of decode-and-forward (DF), and compress-and-forward (CF) protocols. Our results suggest that while DF is DMT optimal when all terminals have one antenna each, it may not maintain its good performance when the degrees of freedom in the direct link are increased, whereas CF continues to perform optimally. We also study the multiple-antenna relay channel with a half-duplex relay. We show that the half-duplex DMT behavior can significantly be different from the full-duplex case. We find that CF is DMT optimal for half-duplex relaying as well, and is the first protocol known to achieve the half-duplex relay DMT. We next study the multiple-access relay channel (MARC) DMT. Finally, we investigate a system with a single source–destination pair and multiple relays, each node with a single antenna, and show that even under the ideal assumption of full-duplex relays and a clustered network, this virtual multiple-input multiple-output (MIMO) system can never fully mimic a real MIMO DMT. For cooperative systems with multiple sources and multiple destinations the same limitation remains in effect. </para>

Lower bounds on systolic gossip

Gossiping is an extensively investigated information dissemination process in which each processor has a distinct item of information and has to collect all the items possessed by the other processors. In this paper we provide an innovative and general lower bound technique relying on the novel notion of delay digraph of a gossiping protocol and on the use of matrix norm methods. Such a technique is very powerful and allows the determination of new and significantly improved lower bounds in many cases. In fact, we derive the first general lower bound on the gossiping time of systolic protocols, i.e., constituted by a periodic repetition of simple communication steps. In particular, given any network of n processors and any systolic period s , in the directed and the undirected half-duplex cases every s -systolic gossip protocol takes at least log ( n )/log (1/ λ ) − O (log log( n )) time steps, where λ is the unique solution between 0 and 1 of λ · p ⌊ s / 2 ⌋ ( λ ) · p ⌈ s / 2 ⌉ ( λ ) = 1 , with p i ( λ ) = 1 + λ 2 + ⋯ + λ 2 i − 2 for any integer i > 0. We then provide improved lower bounds in the directed and half-duplex cases for many well-known network topologies, such as Butterfly, de Bruijn, and Kautz graphs. All the results are extended also to the full-duplex case. Our technique is very general, as for s → ∞ it allows the determination of improved results even for non-systolic protocols. In fact, for general networks, as a simple corollary it yields a lower bound only an O (log log( n )) additive factor far from the general one independently proved in [Proc. 1st ACM Symposium on Parallel Algorithms and Architectures (SPAA), 1989, p. 318; Topics in Combinatorics and Graph Theory (1990) 451; SIAM Journal on Computing 21(1) (1992) 111; Discrete Applied Mathematics 42 (1993) 75] for all graphs and any (non-systolic) gossip protocol. Moreover, for specific networks, it significantly improves with respect to the previously known results, even in the full-duplex case. Correspondingly, better lower bounds on the gossiping time of non-systolic protocols are determined in the directed, half-duplex and full-duplex cases for Butterfly, de Bruijn, and Kautz graphs. Even if in this paper we give only a limited number of examples, our technique has wide applicability and gives a general framework that often allows to get improved lower bounds on the gossiping time of systolic and non-systolic protocols in the directed, half-duplex and full-duplex cases.