Precoder design for asymmetric multi-user two-way AF relaying in cellular systems

In this paper, firstly, we introduce regenerative multi-way relaying. A half-duplex multi-antenna relay station (RS) assists multiple nodes which want to communicate to each other. Each node has a message and wants to decode the messages from all other nodes. The number of communication phases is equal to the number of nodes, N, such that when N = 2, we have the well known two-way relaying. In the first phase, multiple access (MAC) phase, all nodes transmit simultaneously to the RS and the RS decodes the data streams of all nodes. In the following (N - 1) broadcast (BC) phases, the RS transmits to each node the intended (N - 1) data streams from the other (N -1) nodes. Secondly, we propose a transceive strategy at the RS which couples the MAC and the BC phases. The strategy exploits the information about the achievable MAC signal to interference and noise ratio at the RS which enables the RS to transmit with the achievable MAC rate in each BC phase. As the strategy involves transmit beamforming at the RS, we intend to minimise the power at the RS. Since the power minimisation problem is NP-hard, we rewrite the problem into a semidefinite optimisation to solve it in reasonable time.

In the conventional two-way relay channel, two sources exchange information with help from a relay. We introduce a generalized two-way relay channel where each source additionally sends private information to the relay. For this channel, we consider a protocol that consists of a multiple- access (MA) phase and a broadcast (BC) phase and obtain achievable rate regions for both phases. The MA phase is related to computation over the multiple-access channel (MAC). We show that our scheme achieves rates that always lie within half bit of the capacity region for computation over MAC, and is capacity achieving in the BC phase. This near optimality in the MA phase can be achieved by time sharing a capacity-achieving code for the MAC and a superposition code that uses a lattice code as its component code.

In this paper, while considering the impact of antenna correlation and the interference from neighboring users, we study the problem of channel estimation and training sequence design in multi-input multi-output (MIMO) two-way relaying (TWR) systems. To this end, we propose to decompose the bidirectional transmission links into two phases, i.e., the multiple access (MAC) and the broadcast (BC) phases. By deriving the optimal linear minimum mean-square-error estimators, the corresponding training design problems for the MAC and BC phases are formulated and solved. Subsequently, algorithms and, in some special cases, closed-form solutions for obtaining the optimal training sequences for channel estimation in TWR systems are derived. Moreover, to further reduce channel estimation overhead, the minimum required length of the training sequences are determined. Simulation results verify the effectiveness of the proposed training designs in improving channel estimation performance in TWR systems.

Non-orthogonal multiple access (NOMA) and mmWave are considered to be promising techniques for 5G and beyond cellular network communication systems. While the mmWave band provides a very wide under-utilized bandwidth, NOMA enhances the spectral efficiency compared to orthogonal multiple access (OMA). The combination of these two techniques could be considered as the key solution to the high required data rates in next-generation communication systems. The implementation of NOMA is studied and verified under an ideal condition with perfect knowledge of the channel state information (CSI) at the base station (BS). However, under the practical conditions, the fluctuation of the wireless channel makes perfect CSI unachievable at the BS. Hence, we proposed to use the angle of departure (AoD) as feedback information from UEs to the BS. We assume that, mobile users (UEs) perfectly estimate the channel by detecting the pilot signals. Then, UEs quantize the AoD and feed back to the BS. Finally, the BS uses the AoD to perform user clustering, power allocation and beamforming. To reduce the feedback overhead further, we proposed a user clustering algorithm which uses one-bit feedback to determine the change of the AoD. Numerical results demonstrate that the proposed NOMA system outperforms the conventional orthogonal multiple-access (OMA) system with the same amount of feedback information.

The work presented considers a Physical Layer Network Coded (PNC) communication which is affected by Intersymbol Interference (ISI) due to Rayleigh fading channel. A pre-equalizer filter configured to combat the ISI is considered at the end nodes. The relay uses the Amplify and Forward (AF) strategy. The signal in the Multiple Access (MAC) phase is amplified at the relay and forwarded to the receiving nodes. The signal received by the end nodes in the broadcast (BC) phase is subjected to post-equalization by a receiver filter configured to overcome ISI. The performance of the PNC with only pre-equalizer applied, only post-equalization applied and the combined effect of pre-equalizer and post-equalizer is simulated .Simulation results indicate that the performance of PNC when only pre-equalizer applied , only post-equalization is applied are not much varying but better than the no equalization case. But the combined effect of pre-and post-equalization is much improved.

This paper proposes a novel physical layer security algorithm in interference alignment (IA) based multiuser communication system with a single amplify-and-forward (AF) relay in the coexistence of an eavesdropper that tries to hide its existence. Thus, the relay and the users cannot estimate the eavesdroppers channel state information (CSI). In the proposed algorithm, the users broadcasts artificial noise in the null of the relay at the multiple access (MAC) phase, and the relay jams the eavesdropper by broadcasting the artificial noise in the null of users at the broadcast (BC) phase. Therefore, the proposed algorithm is formulated as a joint optimization problem consisting of two semidefinite programming (SDP) problems aiming at maximizing the power of the artificial noise and preserving the quality- of-services (QoS) of the users. An iterative optimization algorithm is proposed to solve the joint problem. Extensive simulation results are provided to show the effectiveness of the proposed algorithm for IA based relaying networks.

Relaying techniques are a well investigated subject in literature. The simplest relay channel is a 3-node network and the communication is split into broadcast phase and Multiple Access (MAC) phase. The focus of this paper lies on the MAC part of the relay channel which is a superposition of the source's and the relay's signals. From information theory it is known that a trade-off between distributed Beamforming (dBF) and the classical MAC has to be found. Generally, the source transmits a superposition of a beamforming part and new information, where the power distribution between both parts has to be optimized. In this work, this conventional approach will be reinterpreted from a Multilevel Coding/Code (MLC) point of view, where the new information can be seen as a level on top of the information transmitted by the relay. In addition, Superposition Modulation (SPM) will be used as a joined mapping scheme to ensure coherent superposition of the dBF part. In this MLC view, the conventional scheme is extended to a more generalized scheme which allows to achieve the theoretical Decode and Forward (DF) rate of Gaussian relay channels. Furthermore, it is possible to realize a low complexity joint demapping of both signals at the destination with only minor performance degradation. Indeed, results show that the new scheme will outperform practical mapping schemes like Quadrature Amplitude Modulation (QAM).

In this paper, a joint resource allocation problem in amplify and forward (AF) OFDM-based two-way multiple-relay cognitive radio network is considered, where two transceiver nodes exchange information via a relay node. The full transmission happens in two phases: multiple access (MA) phase and broadcast (ВС) phase. Considering individual power and interference constraints, the power allocation, subcarrier pairing and relay selection are jointly optimized in order to maximize the sum-rate. The dual decomposition technique is applied to obtain the optimal solution. Additionally, an efficient suboptimal algorithm which drastically reduces the computational complexity of the optimal solution with a small performance degradation is proposed. Finally, simulation results are shown to demonstrate the performance gain of the proposed algorithms.

We consider the uplink three-cell single-user cellular systems with limited feedback interference alignment (IA). Considering the exchange of channel state information (CSI) between base stations (BSs), IA solution can be done at BSs. Then the indices of the quantized precoding vectors are fed back to users. A joint quantization strategy, which quantizes the precoding vectors for different users jointly, is proposed. The proposed quantization scheme enlarges the codebooks and quantizes precoding vectors jointly through the exchange of codebook information between BSs. The quantized precoding vectors for different users are set to have a same index. Only one of the three BSs is appointed to broadcast the same index so that the total feedback cost would not be increased. With the proposed quantization scheme, the quantization errors are mitigated and the system sum rate is increased. A quantization criterion, which we refer to as the maximum sum rate criterion, is also proposed. Compared with the general minimum chordal distance criterion, the proposed quantization criterion can achieve a higher sum rate. Performance improvement of the proposed scheme is analyzed. Simulations also confirm its effectiveness.

Two-way decode-and-forward (DF) relay technique is an efficient method to improve system performance in 5G networks. However, traditional orthogonal frequency division multiplexing (OFDM) based two-way relay systems only consider a per-subcarrier relay strategy, which treats each subcarrier as a separate channel, which results in significant sum rate loss, especially in fading environments. In this paper, a joint coding scheme over multiple subcarriers is involved for multi-pair users in two-way relay systems to obtain multiuser diversity. A generalized subcarrier pairing strategy is proposed to permit each user-pair to occupy different subcarriers during the two transmission phases, i.e., the multiple access and broadcast phases. Moreover, a low complexity joint resource allocation scheme is proposed to improve the spectrum efficiency with an additional multi-user diversity gain. Some numerical simulations are finally provided to verify the efficacy of our proposal.

Adaptive physical layer network coding (PNC) has been shown to be effective in two-way relay communications (TWRC). Given that the existing PNC methods were developed for frequency-flat fading channels, this paper studies adaptive PNC in OFDM systems operating over frequency-selective fading channels. Proposed and investigated are three methods to determine a common clustering for all subcarriers in order to reduce the overhead information required in the broadcast phase. The "Min-SER" method is given based on the analysis of the error event in the multiple access phase, while the "Max-fade-SER" and "Min-fade-distance" methods are recommended to further reduce the computational complexity in determining the common clustering. Considering the trade-off among performance, overhead and complexity, the "Max-fade-SER" method is the most attractive clustering method for applying adaptive PNC in OFDM systems.

Mobile WiMAX is a broadband technology that is capable of delivering triple play services (voice, data, and video). However, mobility in WiMAX system is still an issue when the mobile station (MS) moves and be handovered between base stations (BSs). This issue can cause unnecessary neighboring BS scanning and association, handover delay, and MAC overhead which may affect real-time applications. In this paper, the MS movement direction prediction (MMDP) based MS scanning to overcome the mobile WiMAX handover issues is presented. In this scheme, only potential BS candidates will be chosen for scanning purpose. Hence, the MS scanning process reputation will be reduced with these shortlisted BS candidates instead of scanning all neighbors BSs. Simulation results show that the proposed scheme reduces total scanning delay and scanning interval duration by 25% and 50% respectively.

Mobile WiMAX is a broadband technology that is capable of delivering triple play services (voice, data, and video). However, mobility in mobile WiMAX system is still an issue when the mobile station (MS) moves and its connection is handed over between base stations (BSs). In the handover process, scanning is one of the required phases to find the target BS. During the handover scanning process, the MS must synchronize with all the advertised neighbour BSs (nBSs) to select the best BS candidate for the incoming handover action. Without terminating the connection between the SBS and MS, the SBS will schedule the scanning intervals and sleep-intervals (also called interleaving interval) to MS for the handover scanning. However, during the scanning interval period, all the coming transmissions will be paused. Therefore, the redundant or unnecessary scanning of neighbouring BS cause delay and MAC overhead which may affect real-time applications. In this paper, the MS movement direction prediction (MMDP) based handover scanning scheme is introduced to overcome the mobile WiMAX handover scanning issue. It based on dividing the BS coverage area is into zones and sectors. According to the signal quality; there are three zones, no handover (No-HO), low handover (Low-HO) and high handover (High-HO) zones respectively and six sectors. In this scheme, only two BSs can become candidates; the two that the MS moves toward them will be chosen as the candidate for the handover scanning purpose. Hence, the handover scanning process repetition will be reduced with these two shortlisted BS candidates instead of scanning all nBSs. Thus, MMDP will reduce scanning delay and the number of exchange messages during the handover scanning comparing to the conventional scanning scheme. Although, the MMDP may need an extra computational time, the prediction and scanning process will be finished before the MS reach the High-HO zone, which mean the end-user's running application will be affected. Simulation results show that the proposed MMDP scheme reduces the total handover scanning delay and scanning interval duration by 25 and 50 % respectively. Also, the size of scanning message is reduced, which leads to reduced signalling overhead.

In modern wireless networks, dynamically optimizing the association between base stations (BSs) and users is a promising solution for performance improvement. However, a frequently changing BS-user association introduces heavy burden for network management. In this paper, we seek for balancing the flexibility and stability of the BS-user association in a green downlink MISO network. In addition to minimizing the transmit power and controlling the number of active BSs, we further consider reducing the users' switching frequency among BSs. This problem is a nonlinear mixed-integer program, and challenging to solve. As a compromise, we see for some efficient approximate solution to it by reformulating it as a convex sparse problem. An efficient algorithm is then developed to enhance the sparse solution, based on the iterative reweighted l 1 technique. The effectiveness of the proposed algorithm was validated by numerical simulations.

This paper studies the technique of bit-interleaved coded modulation with iterative decoding (BICM-ID) in a two-way relaying communication system. Iterative decoding based on the quaternary code representation is adopted at the relay for the multiple access (MA) phase. An upper-bound of the bit error probability (BEP) under the error-free (EF) feedback assumption for BICM-ID in the MA phase is obtained. Based on the obtained EF bound of the BEP, it is found that the multiple-access interference (MAI) is successfully eliminated by performing the iterative decoding and XOR based network coding at the relay. Simulation results are provided to corroborate the analysis and demonstrate the performance superiority of the proposed framework.