Boris Rankov

Spectral efficient protocols for half-duplex fading relay channels

We study two-hop communication protocols where one or several relay terminals assist in the communication between two or more terminals. All terminals operate in half-duplex mode, hence the transmission of one information symbol from the source terminal to the destination terminal occupies two channel uses. This leads to a loss in spectral efficiency due to the pre-log factor one-half in corresponding capacity expressions. We propose two new half-duplex relaying protocols that avoid the pre-log factor one-half. Firstly, we consider a relaying protocol where a bidirectional connection between two terminals is established via one amplify-and-forward (AF) or decode-and-forward (DF) relay (two-way relaying). We also extend this protocol to a multi-user scenario, where multiple terminals communicate with multiple partner terminals via several orthogonalize-and-forward (OF) relay terminals, i.e., the relays orthogonalize the different two-way transmissions by a distributed zero-forcing algorithm. Secondly, we propose a relaying protocol where two relays, either AF or DF, alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that both protocols recover a significant portion of the half-duplex loss

Achievable Rate Regions for the Two-way Relay Channel

We study the two-way communication problem for the relay channel. Hereby, two terminals communicate simultaneously in both directions with the help of one relay. We consider the restricted two-way problem, i.e., the encoders at both terminals do not cooperate. We provide achievable rate regions for different cooperation strategies, such as decode-and-forward based on block Markov superposition coding. We also evaluate the regions for the special case of additive white Gaussian noise channels. We show that a combined strategy of block Markov superposition coding and Wyner-Ziv coding achieves the cut-set upper bound on the sum-rate of the two-way relay channel when the relay is in the proximity of one of the terminals

Spectral Efficient Signaling for Half-duplex Relay Channels

We study two-hop communication protocols where one or two relay terminals assist in the communication between two transceiver terminals. All terminals operate in half-duplex mode, i.e., may not receive and transmit simultaneously at the same time and frequency. This leads to a loss in spectral efficiency due to the pre-log factor 1/2 in corresponding expressions for the achievable rate (capacity). We propose and analyze two relaying protocols that avoid the pre-log factor 1/2 but still work with halfduplex relays. Firstly, we consider a relaying protocol where two half-duplex relays, either amplify-and-forward (AF) or decodeand-forward (DF), alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that the protocol can recover a significant portion of the halfduplex loss. Secondly, we propose a relaying protocol where a bidirectional connection between two transceiver terminals is established via one half-duplex AF or DF relay (two-way relaying). It is shown that the sum rate of the two-way half-duplex AF relay channel achieves the rate of the one-way full-duplex AF relay channel, whereas the sum rate of the two-way half-duplex DF relay channel achieves the rate of the one-way full-duplex DF relay channel only in certain cases.

Distributed antenna systems and linear relaying for gigabit MIMO wireless

Spatial multiplexing is mandatory to achieve the extreme bandwidth efficiency of future gigabit/sec WLAN. Both distributed antenna systems (DAS) at the access point and cooperative relaying (the infrastructureless counterpart) have been recognized as means to meet coverage/range requirements and to enable spatial multiplexing in a low scattering environment. In this paper we evaluate three candidate schemes under a two-hop (relay) traffic pattern: (i) DAS with decode and forward in the access point (DDAS), (ii) DAS with linear processing in the access point (LDAS) and (iii) linear relaying without any information exchange between the relay nodes. We give lower bounds on the capacity of LDAS and DDAS. A main contribution of this paper is a systematic derivation of local gain allocation strategies for linear relaying with multi-antenna source and destination nodes, which are based on large system analysis and do not require global channel knowledge at the relays. We derive approximate expression for the ergodic capacity. We show for a source and destination with M antennas, that asymptotically (large number of relays) linear relaying with MN support nodes performs similar to LDAS with M distributed antenna elements. Finally we propose a zero forcing gain allocation, which enables spatial multiplexing of multiple single antenna source/destination pairs based on a small number of autonomous relays. The theory is supported by comprehensive performance results.

Large

In this correspondence, the cumulants of the mutual information of the flat Rayleigh fading two-hop amplify-and-forward multiple-input multiple-output (MIMO) relay channel under independent and identically distributed (i.i.d.) Gaussian input vectors are derived in the large array limit. The analysis is based on the replica trick and covers both spatially independent and Kronecker correlated fading. Beamforming at all terminals is restricted to weight matrices that are independent of the channel realization and constant over time. Expressions for mean and variance of the mutual information are obtained. Their parameters are determined by a nonlinear equation system. All higher cumulants are shown to vanish as the number of antennas per terminal, n, grows to infinity. In conclusion, the distribution of the mutual information I becomes Gaussian in the large n limit. In this asymptotic regime, it is completely characterized by the expressions obtained for mean and variance of I, which are in Theta(n) and O(1), respectively. Comparisons with simulation results show that the asymptotic results serve as excellent approximations for systems with only few antennas at each terminal.

n

Future wireless communication systems have to support high data rates. The capacity of these systems can be increased dramatically by using multiple antennas at the transmitter and the receiver working in a rich scattering environment. However, in a line-of-sight environment, the MIMO channel matrix is rank deficient, and therefore the capacity increase diminishes. Using cooperative amplify and forward relay nodes, it is possible to overcome this problem by increasing the rank of the MIMO channel. A special signaling scheme is necessary to achieve this increase of channel rank. Therefore the use of existing space-time algorithms is not straightforward. We show that a recently proposed class of linear space-time block codes exploits the offered capacity and achieves good performance results. The codes are able to use transmit diversity in combination with spatial multiplexing with reasonable and scalable computational complexity.

Analysis of Amplify-and-Forward MIMO Relay Channels With Correlated Rayleigh Fading

We study the impact of multiple amplify-and-forward relays on the capacity of wireless MIMO channels. For wireless networks with one source/destination pair (equipped with multiple antennas) and several single-antenna amplify-and-forward relays we determine the compound (over two time slots) channel matrix of the relay-assisted MIMO channel. We derive the asymptotic eigenvalues of the compound channel matrix by letting the number of transmit and receive antennas go to infinity (large-array limit). Then the asymptotic ergodic capacity of the amplify-and-forward transmission scheme is determined for Rayleigh and Rice fading. The results serve as tight approximation for a finite number of transmit and receive antennas and are accurate in the large-array limit.

Space-time processing for cooperative relay networks

We study a wireless network with one source-destination pair and several relays that forward the data from source to donation in a multi-hop fashion. We compare two schemes: i) The relays operate in an amplify-and-forward (AF) mode and establish a point-to-point MIMO channel between source and destination (MIMO tunnel), ii) The relays operate in a decode-and-forward (DF) mode and the data Is transmitted over different network paths (multi-path transmission). We show that the MIMO tunnel with AF relays outperform the multipath transmission with DF relays with respect to information rates.

On the capacity of relay-assisted wireless MIMO channels

In order to meet the increasing demand for data rate in modern multiprocessor systems optical transmission has become an important technology. Due to cost reasons only highly multimodal waveguides come into question for mass-producible board level interconnects. We present the idea of an optical multiple-input multiple-output (MIMO) channel by having more than one data source at the channel input and a multisegmented photodetector at the channel output. We develop channel models for the optical MIMO channel and show that orthogonality between modes in a highly multimodal waveguide can be exploited to submit independent data streams over different mode groups (modal multiplexing). Our performance results in terms of bit error rates show that the MIMO scheme outperforms the SISO scheme when the data rate is the same. Hence, optical MIMO schemes allows either to increase the data rate by keeping the amount of intersymbol interference (ISI) constant or to reduce the ISI by keeping the data rate constant.

Distributed spatial multiplexing in a wireless network

In this paper the ergodic mutual information of the i.i.d. Rayleigh fading amplify-and-forward MIMO relay channel without direct link between source and destination is derived in the large array limit. By means of the replica method an expression is obtained, whose parameters are determined by a nonlinear equation system. This system can be solved in closed form. Comparisons with Monte Carlo simulations show that the asymptotic expression serves as an excellent approximation for systems with only very few antennas at each node.