Adrian Agustin

Linear Transceiver Design in Nonregenerative Relays With Channel State Information

This paper deals with the design of nonregenerative relaying transceivers in cooperative systems where channel state information (CSI) is available at the relay station. The conventional nonregenerative approach is the amplify and forward (A&F) approach, where the signal received at the relay is simply amplified and retransmitted. In this paper, we propose an alternative linear transceiver design for nonregenerative relaying (including pure relaying and the cooperative transmission cases), making proper use of CSI at the relay station. Specifically, we design the optimum linear filtering performed on the data to be forwarded at the relay. As optimization criteria, we have considered the maximization of mutual information (that provides an information rate for which reliable communication is possible) for a given available transmission power at the relay station. Three different levels of CSI can be considered at the relay station: only first hop channel information (between the source and relay); first hop channel and second hop channel (between relay and destination) information, or a third situation where the relay may have complete cooperative channel information including all the links: first and second hop channels and also the direct channel between source and destination. Despite the latter being a more unrealistic situation, since it requires the destination to inform the relay station about the direct channel, it is useful as an upper benchmark. In this paper, we consider the last two cases relating to CSI. We compare the performance so obtained with the performance for the conventional A&F approach, and also with the performance of regenerative relays and direct noncooperative transmission for two particular cases: narrowband multiple-input multiple-output transceivers and wideband single input single output orthogonal frequency division multiplex transmissions

Non-regenerative MIMO relaying with channel state information [cellular example]

This paper deals with the design of non-regenerative relays in cooperative transmission schemes. The conventional nonregenerative approach is the amplify and forward (AF) approach, where the signal received at the relay is simply amplified and retransmitted. In this paper, we propose an alternative design which maximizes the capacity for nonregenerative cooperative transmission, when channel state information is available at the relay station. Therefore, the relay units are not simple amplify and forward units, but still they are not required to demodulate or remodulate the symbols transmitted by the base station as in regenerative relaying schemes. We compare the performance so obtained with the performance for the conventional AF approach, and also with the performance of regenerative relays and direct noncooperative transmission.

Cooperative Wireless Networking Beyond Store-and-Forward

In future wireless networks devices may cooperate to form logical links. Each of these links may consist of several independent physical channels which are shared by the cooperating partners. Even without multiple antennas this cooperation provides diversity in time and space. This so-called cooperation diversity increases the robustness of the link vs. fading and interference. After surveying approaches in cooperation diversity we focus on optimizing its performance by combining several cooperation schemes and by integrating cooperation into space-time coding. For multiple scenarios, we further discuss the factors and benefits introduced by user cooperation and how cooperation-aware resource allocation can be employed to further increase the performance of cooperative networks. When it comes to implementation, the question arises how cooperation can be integrated efficiently into existing wireless networks. A case study for 802.11-based WLANs reveals the issues that need to be solved in order to deploy cooperative techniques. We provide an overview of the state of the art in implementing cooperation approaches, analyze how appropriate these approaches solve the issues, and, where appropriate, point out their deficiencies. We conclude with a road map for future research necessary to tackle these deficiencies for the practical implementation of cooperation in next generation mesh, WLAN, WMAN, and cellular standards.

Cellular capacity gains of cooperative MIMO transmission in the downlink

In this paper cooperation among users at the physical layer level is studied for the downlink. Distributed space time linear dispersion codes are proposed for the cooperative strategies amplify and forward (AF) and decode forward (DF), assuming no channel state knowledge at the transmitter. Both AF and DF strategies are evaluated in a cellular multiuser realistic scenario based on TDMA access, considering cellular reuse of the relay slot. Different configurations regarding the number of antennas at the relay and destination nodes are considered. The maximum achievable rate of cooperative schemes implemented by means of space time linear dispersion codes is compared with the theoretical capacity of the cooperative channel. Finally an analysis in terms of BER is given for different schemes.

A game theoretic approach for cooperative MIMO schemes with cellular reuse of the relay slot

An iterative game theory based algorithm is proposed to allocate the resources in cooperative schemes for the downlink. Both amplify-and-forward (AF) and decode-and-forward (DF) cooperative schemes are considered with cellular reuse of the relay slot. Multiple antennas can be used at the involved stations. Using the algorithm proposed, the simultaneous relay powers are decided in a decentralized way using mean channel level measures and mean values of noise and interference power. The cell capacity gain for the cooperative schemes using the decentralized algorithm is evaluated by means of simulation for both the AF and DF approaches.

Protocols and Resource Allocation for the Two-Way Relay Channel with Half-Duplex Terminals

The two-way relay channel (TWRC) describes the communication between two terminals sharing a common relay. In this work we compare different protocols for the TWRC with a half-duplex relay (forwarding and protocol I) and transmission schemes (Network Coding Relaying with random binning and XOR precoding), when the relay works in decode-and-forward and without channel state information at the transmitters. We provide the optimal resource allocation (in terms of phase duration, data rate and power allocation) when there is an individual or sum-average power constraint over the terminals. Closed-form solutions are derived whenever it is possible, while for others cases we show that the resource allocation can be formulated as a convex problem, which means that the solution can be obtained by low-complexity interior point methods.

Hybrid Turbo FEC/ARQ Systems and Distributed Space-Time Coding for Cooperative Transmission

Cooperative transmission can be seen as a “virtual” MIMO system, where the multiple transmit antennas are in fact implemented distributed by the antennas both at the source and the relay terminal. Depending on the system design, diversity/multiplexing gains are achievable. This design involves the definition of the type of retransmission (incremental redundancy, repetition coding), the design of the distributed space-time codes, the error correcting scheme, the operation of the relay (decode & forward or amplify & forward) and the number of antennas at each terminal. Proposed schemes are evaluated in different conditions in combination with forward error correcting codes (FEC), both for linear and near-optimum (sphere decoder) receivers, for its possible implementation in downlink high speed packet services of cellular networks. Results show the benefits of coded cooperation over direct transmission in terms of increased throughput. It is shown that multiplexing gains are observed even if the mobile station features a single antenna, provided that cell wide reuse of the relay radio resource is possible.

Improved Interference Alignment Precoding for the MIMO X Channel

We derive a linear transmit precoding scheme based on interference alignment at the transmitters and zero-forcing at the receivers, for the MIMO X channel based on the Generalized Singular Value Decomposition (GSVD) that is able to attain the outer bound on the degrees of freedom (DoF) region [2] for any antenna configuration, except when all terminals have one antenna. Additionally, the proposed scheme is able to enhance the transmission rate in terms of high SNR offset

Improper Gaussian signaling for the Z-interference channel

The optimal transmission scheme for a multiple-input multiple-output point-to-point channel (MIMO-P2P) is dependent on the type of received interference, which can be either proper or improper. We will show that the presence of improper interference is beneficial in terms of improving the achievable rate compared to a case with proper interference. These benefits are due to using widely linear precoder and receiver. Additionally, we investigate a cellular scenario where two types of users are coexisting, ones that are receiving proper interference while others which might be receiving improper interference, trying to devise the best transmission scheme to be adopted. We propose an improper Gaussian signaling-based scheme that allows controlling system performance and fairness in terms of bitrate through a single parameter.

Full exploitation of diversity in space-time MMSE receivers

A unified and general vision of different space-time processors is presented. Many popular receivers can be accommodated like V-RAKE receivers, weighted V-RAKE, or spatial narrowband beamforming. By making appropriate assumptions on the space-time characteristic of the interference it is possible to enhance the performance of the receiver through spatial/temporal pre-processors. These receivers are tested in the FDD mode of UTRA [ETSI-UTRA].