Chan Dai Truyen Thai

Coordinated Direct and Relay Transmission With Linear Non-Regenerative Relay Beamforming

Joint processing of multiple communication flows in wireless systems has given rise to a number of novel transmission techniques, notably the two-way relaying, but also more general traffic scenarios, such as coordinated direct and relay (CDR) transmissions. In a CDR scheme the relay has a central role in managing the interference and boosting the overall system performance. In this letter we consider the case in which an amplify-and-forward relay has multiple antennas and can use beamforming to support the coordinated transmissions. We focus on one representative traffic type with one uplink user and one downlink user. Two different criteria for relay beamforming are analyzed: maximal weighted sum-rate and maximization of the worst-case weighted SNR. We propose iterative optimal solutions, as well as low-complexity near-optimal solutions.

Coordinated Direct and Relay Transmission with Interference Cancelation in Wireless Systems

Two-way relaying schemes in wireless systems obtain throughput gain by utilizing two features (1) jointly serve two communication flows, thus implementing network coding and (2) use of information that is a priori known to cancel interference and obtain the desired signal. Based on these principles, we propose other schemes that bring throughput gains in wireless cellular systems, where relayed and direct transmissions are carried out in coordinated way. The results show that the coordinated transmission exhibit throughput improvement similar to the two-way relaying schemes.

Multi-Flow Scheduling for Coordinated Direct and Relayed Users in Cellular Systems

There are two basic principles used in wireless network coding to design throughput-efficient schemes: (1) aggregation of communication flows and (2) interference is embraced and subsequently cancelled or mitigated. These principles inspire design of many novel multi-flow transmission (MFT) schemes. Such are the Coordinated Direct/Relay (CDR) schemes, where each basic transmission involves two flows to a direct and a relayed user. Usage of MFT schemes as building blocks of more complex transmission schemes essentially changes the problem of scheduling, since some of the flows to be scheduled are coupled in a signal domain and they need to be assigned a communication resource simultaneously. In this paper we define a novel framework that can be used to analyze MFT schemes and assess the system-level gains. The framework is based on cellular wireless users with two-way traffic and it sets the basis for devising composite time-multiplexed MFT schemes, tailored to particular optimization criteria. Those criteria can be formulated by adapting well-known schedulers in order to incorporate MFT schemes. The results show rate advantages brought by the CDR schemes in pertinent scenarios. Another key contribution is the proposed framework, which can be used to evaluate any future multi-flow transmission scheme.

Coordinated Transmissions to Direct and Relayed Users in Wireless Cellular Systems

The ideas of wireless network coding at the physical layer promise high throughput gains in wireless systems with relays and multi--way traffic flows. This gain can be ascribed to two principles: (1) joint transmission of multiple communication flows and (2) usage of a priori information to cancel the interference. In this paper we use these principles to devise new transmission schemes in wireless cellular systems that feature both users served directly by the base stations (direct users) and users served through relays (relayed users). We present four different schemes for coordinated transmission of uplink and downlink traffic in which one direct and one relayed user are served. These schemes are then used as building blocks in multi--user scenarios, where we present several schemes for scheduling pairs of users for coordinated transmissions. The optimal scheme involves exhaustive search of the best user pair in terms of overall rate. We propose several suboptimal scheduling schemes, which perform closely to the optimal scheme. The numerical results show a substantial increase in the system--level rate with respect to the systems with non--coordinated transmissions.

Interference cancelation schemes for uplink transmission in femtocells

We present techniques that solve the near-far problems in femtocell networks. When a co-channel configuration is used, a transmission in the macrocell may cause interference to a femtocell. We introduce methods to cancel the interference caused by the macrocell to a femtocell by using the highrate link between the macrocell base station (BS) and the femtocell BS. In such a way, the interference from macrocell transmitters is completely cancelled when uplink transmission is used in the femtocell. Therefore, the allocation of time slots for uplink/downlink in a femtocell can be done by trying only to minimize the interference that a femtocell terminal experiences during a downlink transmission.

Diversity-Multiplexing Trade-off for Coordinated Direct and Relay Schemes

The recent years have brought a significant body of research on wireless Two-Way Relaying (TWR), where the use of network coding brings an evident advantage in terms of data rates. Yet, TWR scenarios represent only a special case and it is of interest to devise similar techniques in more general multi-flow scenarios. Such techniques can leverage on the two principles used in Wireless Network Coding to design throughput-efficient schemes: (1) aggregation of communication flows and (2) embracing and subsequently cancel/mitigate the interference. Using these principles, we investigate Coordinated Direct/Relay (CDR) schemes, which involve two flows, of a direct and a relayed user. In this paper we characterize a CDR scheme by deriving/bounding the Diversity-Multiplexing Trade-off (DMT) function. Two cases are considered. In the first case a transmitter knows the Channel State Information (CSI) of all the links in the network, while in the second case each node knows only CSI of the links towards its neighbors. The results show that the new CDR scheme outperforms the reference scheme in terms of DMT characterization. Several interesting features are identified with respect to the impact of the CSI knowledge to the improvement in diversity or multiplexing brought by the CDR scheme.

Beamforming design for coordinated direct and relay systems

Joint processing of multiple communication flows in wireless systems has given rise to a number of novel transmission techniques, notably the two-way relaying based on wireless network coding. Recently, a related set of techniques has emerged, termed coordinated direct and relay (CDR) transmissions, where the constellation of traffic flows is more general than the two-way. Regardless of the actual traffic flows, in a CDR scheme the relay has a central role in managing the interference. In this paper we investigate the novel transmission modes, based on amplify-and-forward, that arise when the relay is equipped with multiple antennas and can use beamforming. We focus on one representative traffic type, with two downlink users and consider three different beamforming criteria: egoistic, altruistic, and sum-rate maximization. The sum-rate criterion leads to a non-convex problem and we introduce an iterative solution, as well as derive an upper performance bound. The numerical results demonstrate a clear benefit from usage of multiple antennas at the relay node.

Optimizing completion time and energy consumption in a bidirectional relay network

Consider a wireless network with multiple sources and destinations, where the amount of data of each source node is finite. An interesting question is what is the shortest completion time, i. e. the time required that all data from the sources gets to the respective destinations. A similar question arises for the minimal required energy. While the requirement for minimal energy consumption is obvious, the shortest completion time is relevant when certain multi-node network needs to reserve the wireless medium in order to carry out the data exchange among its nodes. The completion time/energy consumption required for multiple flows depends on the current channel realizations, transmission methods used and, notably, the relation between the data sizes of different source nodes. In this paper we investigate the shortest completion time and minimal energy consumption in a two-way relay wireless network. The system applies optimal time multiplexing of several known transmission methods, including one-way relaying and wireless network coding (WNC). We show that when the relay applies Amplify-and-Forward (AF), both minimizations are linear optimization problems. On the other hand, when the relay uses Decode-and-Forward (DF), each of them is a quadratic optimization problem. The results show that, for given channel realizations, there is an optimal ratio of the data packets at the sources to obtain minimal completion time or energy consumption. This can be used as a guidance for the nodes to apply traffic shaping. In most cases, DF leads to shorter completion time and energy consumption compared to AF.

Coordination of regenerative relays and direct users in wireless cellular networks

The area of wireless cooperation/relaying has recently been significantly enriched by the ideas of wireless network coding (NC), which bring substantial gains in spectral efficiency. These gains have mainly been demonstrated in scenarios with two-way relaying. Inspired by the ideas of wireless NC, recently we have proposed techniques for coordinated direct/relay (CDR) transmissions. These techniques embrace the interference among the communication flows to/from direct and relayed users, leveraging on the fact that the interference can be subsequently canceled. Hence, by allowing simultaneous transmissions, spectral efficiency is increased. In our prior work, we have considered CDR with non-regenerative relay that uses Amplify-and-Forward (AF). In this paper we consider the case of regenerative Decode-and-Forward (DF) relay. This refers also to joint decoding of the interfering flows received over a multiple-access channel. Our analysis shows that the assumption of regenerative relaying brings new parametrization of the CDR schemes, such that the the transmission times used by each node are subject to optimization. We show how these parameters should be chosen if the sum-rate needs to be optimized. Our results confirm the high gains in spectral efficiency over the reference schemes.

Rate regions for coordination of Decode-and-Forward relays and direct users

Recently, the ideas of wireless network coding (NC) has significantly enriched the area of wireless cooperation/relaying. They bring substantial gains in spectral efficiency mainly in scenarios with two-way relaying. Inspired by the ideas of wireless NC, recently we have proposed techniques for coordinated direct/relay (CDR) transmissions. Leveraging on the fact that the interference can be subsequently canceled, these techniques embrace the interference among the communication flows to/from direct and relayed users. Hence, by allowing simultaneous transmissions, spectral efficiency is increased. In our prior work, we have proposed CDR with Decode-and-Forward (DF) relay in two scenarios. In this paper, we extend the two existing regenerative CDR schemes and proposed for the other two scenarios such that all schemes benefit from the aforementioned principle of containing the interference. The parameters in the schemes are optimized to have the largest rate region or the highest sum-rate. Numerical results show that DF CDR is better than the reference scheme and almost better than AF CDR.