Distributed beamforming approach to full-duplex relay in multiuser MIMO transmission

The IEEE 802.16j mobile multi-hop relay (MMR) is studied to improve throughput, extend coverage, and increase capacity. Mobile relay stations attached to vehicles make arbitrary movements and have interference with other base stations or relay stations, thus lowering service functions. This study sets out to suggest an interference detection and avoidance method and evaluates its performance in order to help introduce a mobile relay station for vehicle mounting in a mobile multi-hop relay network. The proposed approach would be implemented by the addition of MAC management messages at a base or relay station instead of the change of mobile station.

Wireless Relay networks have become very important technologies in the future wireless systems. Current research works mostly focus on scenarios where relay stations are either stationary or mobile with uniform velocity. However, in many applications, relay stations are mobile with irregular patterns. In particular, when mobile relay stations (MRSs) are deployed to complement the cellular systems, many issues such as handoff should be carefully investigated. In this paper, we focus on the handoff problem and propose a new handoff decision algorithm based on the relative velocities of user equipment (UE) to the serving access point (AP) and the target AP. We have show that the proposed handoff algorithm can significantly improve the handoff successful rate when the mobile relay station changes its moving patterns.

In a cellular communication system, deploying a relay station (RS) is an effective alternative to installing a new base station (BS). A dual-hop network enhances the throughput of mobile stations (MSs) located in shadow areas or at cell edges by installing RSs between BSs and MSs. Because additional radio resources should be allocated to the wireless link between BS and RS, a frame to be transmitted from BS is divided into an access zone (AZ) and a relay zone (RZ). BS and RS communicate with each other through the RZ, and they communicate with their registered MSs through an AZ. However, if too many MSs are registered with a certain BS or RS, MS overloading may cause performance degradation. To prevent such performance degradation, it is very important to find the proper positions for RSs to be deployed. In this paper, we propose a method for finding the sub-optimal RS deployment location for the purpose of load-balancing and throughput enhancement. The advantage of the proposed method is the efficiency in find the sub-optimal location of RSs and its reliable tradeoff between load-balancing throughput enhancement. Since the proposed scheme finds the proper position by adjusting the distance and angle of RSs, its computational complexity lower than other global optimization approach or learning-based approach. In addition, the proposed scheme is constituted with the two stages of load-balancing and throughput enhancement. These procedures result in the appropriate tradeoff between load-balancing and throughput enhancement. The simulation results support these advancements of the proposed scheme.

This paper proposes a new uplink signalling to report the terminal's channel quality in multiple subbands to the base station (BS) using channel reciprocity in orthogonal frequency division multiple access / time division duplex (OFDMA/TDD) systems. In the proposed method, the terminal transmits uplink pilot signals in the subbands with different transmit power which is inversely proportional to the subband- based interference-plus-noise power. The BS can obtain knowledge of the terminal's received signal-to-interference-plus-noise power ratio (SINR) on subband basis, measuring the pilot signal power. In performance evaluation, accuracy of the channel quality reporting and amount of the control signalling are examined. From numerical results, it is found that the proposed method becomes effective as the number of subbands and terminals for channel quality reporting increases.

This paper addresses channel state information (CSI) estimation in a wireless relay network, which consists of a base station (BS), a relay station (RS), and a mobile station (MS). We propose a method for the BS to obtain the instantaneous CSIs of the three links (BS-RS, BS-MS, and RS-MS links) with minimized overhead. The CSI availability plays an important role for the BS to adapt its transmission strategies to the given channel conditions. Our proposed method exploits the fact that the link condition between fixed BS and fixed RS tends to be stable, and the frequent CSI update is not necessary for this link. In order for the BS to obtain the CSI of the distant link (i.e. RS-MS link), RS amplifies a pilot signal received from MS with a pre-defined amplification factor, and forwards it to BS. This enables the BS to obtain the CSI of the RS-MS link based on the received pilot signal and pre-knowledge on the CSI of BS-RS link. We show by computer simulation that our proposed method can reduce the required overhead for CSI estimation with little degradation on the accuracy of the obtained CSI.

A new scheme is proposed to compensate the multiple Doppler frequency offsets (DFOs) for high-mobility OFDM uplink transmissions in this paper. According to the characteristics of uplink transmission, we find it is a good choice to solve the multiple DFOs problem in the spatial-domain. The proposed scheme consists of a mobile relay station (MRS) between the base station (BS) and the mobile terminal (MT). The MRS is immobile respect to the MT, and hence the DFOs are only generated between the MRS and the BS. The MRS uses an antenna array to transmit signals through different paths. The DFOs are estimated during the downlink, and the DFO for each path is separately compensated before the particular transmit beamforming for this path. Therefore, all the signals in different paths are superimposed at the BS without DFO, and the BS does not have to be changed in this case of high-mobility scenarios. The effectiveness of the proposed scheme is verified by simulations.

In this paper, we consider transceiver design for multiuser cellular two-way relay network (cTWRC), where a multi-antenna base station (BS) exchanges information with multiple single-antenna mobile stations (MSs) with the help of a multi-antenna relay station (RS). We propose a low complexity two-way relaying scheme for cTWRC via signal space alignment. In our scheme, the precoders at the BS and MSs are jointly designed that the two data streams delivered to and from the same MS fall in the same spatial direction at the RS. With such signal space alignment, interference-free physical-layer network coding can be performed at the relay. We analyze the end-to-end sum bit error rate (BER) performance of the proposed scheme and investigate the optimal power allocation at the BS and RS to improve the BER performance. Numerical results demonstrate that our scheme is superior to the existing amplify-and-forward based signal alignment schemes.

In downlink relay systems, the conventional straightforward relaying scheme named decode-and-forward (DF) protocol is used at relay station (RS) to decode the data received from base station (BS) and forward the decoded data to the mobile station (MS) to improve the spatial diversity gain. However, conventional DF mode is spectral inefficient and not capacity optimal since the wireless channels in BS-to-RS, BS-to-MS, and RS-to-MS links are asymmetric, i.e., different channel qualities are obtained in different links in the practical scenario. In this paper, a distributed layered-modulation scheme is proposed to improve the DF mode performance in the presence of asymmetric links. With this proposed scheme, BS and/or RS transmit/re-transmit a layered modulated signal comprising a higher-order and a lower-order modulation components according to the different channel conditions in BS-to-RS and RS-to-MS links, respectively, and, therefore, the channel capacity of each link can be utilized efficiently. In addition, a two-layer demodulation is proposed at the destination by exploiting the two copies of transmitted packet from BS and RS, respectively. In order to further enhance the decoding performance, a soft combining method, in which the bit Log Likelihood Ratio (LLR) of the signals that received during the broadcast phase and the relaying phase are combined before turbo decoding, is used at MS receiver. Simulation results show that the proposed distributed layered modulation scheme obtains a significant performance gain in terms of block error rate (BLER) compared to the existing DF relaying scheme.

Air-based Multi-hop Sensor Network for the Localization of Persons

We consider the reciprocity of the information-theoretic channel of Time Division Duplex (TDD) Multi-User-Multiple Input Multiple Output (MU-MIMO) systems in the up- and downlink. Specifically, we assume that the transmit and receive chains are reciprocal. We take the mutual coupling between the antenna elements at the base station and at the mobiles into account. Mutual coupling influences how to calculate transmit power and noise covariance. The analysis is based on the Multiport Communication Theory, which ensures that the information-theoretic model is consistent with physics. It also includes a detailed noise model. We show that due to the coupling, the information-theoretic up- and downlink channels do not fulfill the ordinary reciprocity relation, even if the input-output relation of the transmit voltage sources and the receive load voltages, i.e., the channel which is estimated with the help of pilot signals in the uplink, is reciprocal. This is a fundamental effect that is not considered otherwise. We show via Monte Carlo simulations that both, using the ordinary reciprocity relation, and not taking the coupling into account, significantly decreases the ergodic rates in single-user and the ergodic sum rates in multi-user systems.

Block diagonalization (BD) is a practically favorable precoding technique that eliminates the interuser interference in downlink multiuser multiple-input multiple-output (MIMO) systems. In this paper, we apply BD to the downlink transmission in a cooperative multi-cell system, where the signals from different base stations (BSs) to all the mobile stations (MSs) are jointly designed with the perfect knowledge of the downlink channels and transmit messages. Specifically, this paper studies the BD precoder design to maximize the weighted sum-rate achievable for all the MSs. The associated optimization problem can be formulated in an auxiliary MIMO broadcast channel (BC) with a set of transmit power constraints equivalent to those for different BSs in the multi-cell system. Based on convex optimization techniques, this paper designs an efficient algorithm to solve this problem, and derives the structure of the corresponding optimal BD precoding matrix. Moreover, for the special case of single-antenna BSs and MSs, it is shown that the proposed solution leads to the optimal zero-forcing beamforming (ZF-BF) precoder design for the multiple-input single-output (MISO) BC with the per-antenna power constraints.

In cellular systems, deploying a large number of antennas at the base station (BS), also called massive multiple-input multiple-output (MIMO), can offer a huge improvement in system throughput. This performance gain achieved through coherent beamforming highly depends on having accurate channel state information (CSI) at the BS. Initially, massive MIMO was considered promising only for time division duplexing (TDD) systems because the downlink training overhead in frequency division duplexing (FDD) systems could be large. To reduce the overhead in FDD systems, we propose efficient eigenspace training and precoding. In the downlink training, the prebeamforming matrix is designed based on the spatial fading correlation to probe the channel and minimize the mean squared error (MSE) in channel estimation. In data transmission, the precoding matrix is designed based on instantaneous CSI to manage the inter-user interference. Our algorithms achieve significant savings in the downlink training because the overhead is associated with the dimension of prebeamforming matrix and no longer limited by the number of BS antennas. Compared with joint spatial division and multiplexing, our algorithms offer much lower MSE and, under some conditions, higher sum rates.

Recently, Coordinated Multi-Point (CoMP) transmission/Reception technology is introduced into the fourth-generation cellular mobile communication system. Time-division duplex (TDD) system is more suitable to apply CoMP when joint processing is needed because it can easily get the channel information through channel reciprocity. Actually, a crucial point to realise channel reciprocity is the antenna calibration, especially in the inter-base station (BS) case. Self-antenna calibration within the BS in the conventional system has been studied in previous works. In this paper, the authors investigate the impact of un-calibrated antennas of BSs and mobile stations (MSs) on the precoding matrix of two precoding techniques [block diagonal (BD) and zero forcing (ZF)] in the CoMP system. A theoretical model is set and analysed. The discussion of some properties gives a complete overview of the calibration problem in the CoMP system. In addition, the authors propose a practical scheme to solve inter-BS antenna calibration problem. System simulation results, which validate their analysis, are presented to show the necessity of antenna calibration.

This paper proposes a codebook based limited feedback zero-forcing(ZF) precoding scheme for multi-user multiple-input multiple-output (MIMO) downlink channels. In such system, each user needs to calculate a channel directional index (CDI) and a channel quality indicator (CQI), feedback them to the base station (BS). BS selects the set of active users, and calculates transmit beams through zero forcing method. In case of rank-one feedback, each user should combine the MIMO channel into a multiple-input single-output (MISO) channel through a receiver weight, then feedback CDI and CQI of this combined channel. Current existing schemes use the Hermitian of the left eigenvector associated to the maximum eigenvalue of the channel as the weight. The problem is that when there is multi-user interference, the feedback information is inaccurate. Focusing on this problem, we propose a new feedback scheme which uses a quasi minimum mean squared error (MMSE) weight to calculate CQI and CDI. By the simulations in 3GPP long term evaluation (LTE) environment, the proposed scheme is proved to achieve higher spectrum efficiency than the existing schemes.

Multiple transmit antennas can be employed in a downlink channel to serve multiple users under the assumption of perfect channel state information (CSI) of all users at the base station. In practice, however, the channel knowledge available at the base station is not perfect. For instance, in case of a time division duplex (TDD) system, the transmit CSI is the estimated uplink channel. On the other hand, in a frequency division duplex (FDD) system, feedback from the users is required to obtain transmit CSI. In this case, we assume the transmit CSI consists of a quantized version of the normalized estimated channel vector and the statistics of the channel norm of each user. In this work we consider the transceiver design of the downlink of a multiuser MISO system based on the minimum mean square error (MMSE) criterion with imperfect CSI as discussed above.