Zhihang Yi

Joint optimization of relay-precoders and decoders with partial channel side information in cooperative networks

In this paper, we jointly optimize the relay-precoders and decoders with partial channel side information (CSI) in a cooperative network. Specifically, two different CSI assumptions are considered: 1) full CSI at the destination terminal and partial CSI at the relay terminals; 2) partial CSI at the destination terminal and the relay terminals. We show that the optimum relay-precoders and decoders work in a fashion of channel selection. It is demonstrated by numerical results that the proposed optimum relay-precoders and decoders improve the performance considerably

Diversity order analysis of the decode-and-forward cooperative networks with relay selection

In this paper, we focus on the decode-and-forward (DF) cooperative networks with relay selection. Many detection schemes have been proposed for the DF; but it has been shown that the cooperative maximum ratio combining (C-MRC) can achieve almost the same performance as the optimum maximum likelihood detector and has a much lower complexity. Therefore, we first combine the C-MRC with the relay selection and show that it achieves the full diversity order by deriving an upper bound of its average bit error rate (BER). In order to reduce the signaling overhead in the C-MRC with relay selection, a novel detection scheme, namely product MRC (P-MRC), is proposed for the DF and it achieves the same diversity order as the C-MRC. Then we combine the P-MRC with the relay selection and show that it achieves the full diversity order by deriving an upper bound of its average BER.

Outage Probability and Optimum Power Allocation for Analog Network Coding

We study the analog network coding (ANC), which is a well-known amplify-and-forward (AF)-based bidirectional protocol, for a bidirectional network consisting of two different sources and a relay. In this protocol, the two sources exchange information with the help of the relay during two time slots in a half-duplex mode. For this system, we first derive a tight lower bound of outage probability, which is very close to the exact outage probability in the whole signal-to-noise ratio (SNR) range irrespective of the values of channel variances. Using the tight lower bound, we obtain finite-SNR diversity-multiplexing tradeoff of the ANC protocol. Furthermore, we propose an optimum power allocation scheme, which simultaneously minimizes the outage probability and maximizes the total mutual information of the ANC protocol.

Single-Symbol ML Decodable Distributed STBCs for Cooperative Networks

In this correspondence, the distributed orthogonal space-time block codes (DOSTBCs), which achieve the single-symbol maximum likelihood (ML) decodability and full diversity order, are first considered. However, systematic construction of the DOSTBCs is very hard, since the noise covariance matrix is not diagonal in general. Thus, some special DOSTBCs, which have diagonal noise covariance matrices at the destination terminal, are investigated. These codes are referred to as the row-monomial DOSTBCs. An upper bound of the data-rate of the row-monomial DOSTBC is derived and it is approximately twice higher than that of the repetition-based cooperative strategy. Furthermore, systematic construction methods of the row-monomial DOSTBCs achieving the upper bound of the data-rate are developed when the number of relays and/or the number of information-bearing symbols are even.

Joint Optimization of Relay-Precoders and Decoders with Partial Channel Side Information in Cooperative Networks

In this paper, we jointly optimize the relay-precoders and decoders with partial channel side information (CSI) in a cooperative network. Specifically, two different CSI assumptions are considered: 1) full CSI at the destination terminal and partial CSI at the relay terminals; 2) partial CSI at the destination terminal and the relay terminals. We show that the optimum relay-precoders and decoders work in a fashion of channel selection. It is demonstrated by numerical results that the proposed optimum relay-precoders and decoders improve the performance considerably.

Outage Probability and Optimum Combining for Time Division Broadcast Protocol

Time division broadcasting (TDBC) is a well-known bidirectional protocol. In this protocol, two sources exchange information with the help of a relay terminal. For amplify-and-forward (AF)-based TDBC, we first derive a tight lower bound of the outage probability in closed-form, and it is very close to the exact outage probability in the whole signal-to-noise ratio (SNR) range irrespective of the values of channel variances. Using the tight lower bound, diversity-multiplexing tradeoff of the TDBC protocol is obtained for finite-SNR. Furthermore, we investigate how to optimize the TDBC protocol; specifically, an optimum method to combine the received signals at the relay terminal is developed. This method minimizes the outage probability and maximizes the total mutual information of the TDBC protocol at the same time.

Relay ordering in a multi-hop cooperative diversity network

In this paper, we first propose an optimum relay ordering algorithm for the multi-branch multi-hop cooperative diversity networks. This optimum algorithm has a high complexity that makes it hard to implement. Therefore, a suboptimum relay ordering algorithm, which considerably reduces the complexity, is then developed. Furthermore, for a cooperative network with two relays, we analytically evaluate the performance of the suboptimum algorithm by using an approximate end-to-end signal-to-noise ratio expression. Specifically, an approximate probability of wrong selection and an approximate expression of the symbol error rate are derived. The analysis and the numerical results demonstrate that the suboptimum algorithm performs very well as the optimum one at a much lower complexity.

Improved Opportunistic Beamforming in Ricean Channels

In order to improve the performance of opportunistic beamforming in Ricean channels, we propose an improved opportunistic beamforming scheme which forms beams intelligently to the users. First, a new concept of the generalized Ricean distribution is introduced. On the basis of this theory, we propose a maximum-likelihood (ML) estimator of the directions of arrival of the users, and we derive the Cramer-Rao lower bound to evaluate the performance of the ML estimator. Furthermore, the generalized Ricean K-factors are estimated by a moment-based method. With the same pilot overhead in the downlink and with the same feedback overhead in the uplink, the improved scheme considerably outperforms conventional opportunistic beamforming

SER Analysis and PDF Derivation for Multi-Hop Amplify-and-Forward Relay Systems

An amplify-and-forward, multi-branch, multi-hop relay system with K relays, in which the relays broadcast to other relays as well as the destination, is analyzed. An approximate symbol-error-rate (SER) expression, which is valid for any number of relays and for several modulation schemes, is found for the multi-hop system. Also, the cumulative density function (CDF) and probability density function (PDF) are found for the random variable, Z = XY/(X + Y + c), where X and Y are sums of independent, Erlang random variables, and c is a constant. The moment generating function (MGF) of Z is found for the special case in which c = 0. It is shown that these results are generalizations of previously published results for special cases of Z. The MGF of Z is used to develop the approximate SER expression. Results for the analytic SER expression are included and compared with simulation results for various values of K, for various modulation schemes, and for two choices of system parameters (channel variances). Results for the multi-hop system are also compared to results for the two-hop system (in which relays transmit only to the destination).

Optimum beamforming in the broadcasting phase of bidirectional cooperative communication with multiple decode-and-forward relays

This letter focuses on the broadcasting phase of bidirectional cooperative networks with multiple decode-and-forward relays. In this phase, the relays first combine the information-bearing symbols transmitted by the sources, and then broadcast them back to the sources in order to achieve bidirectional communications. Two different combining methods at the relays are considered. The first one is that the relays transmit linear combinations of the information-bearing symbols to the sources by beamforming. We develop an algorithm that can compute the optimum beamforming vector in closed form and this beamforming vector minimizes the outage probability of the bidirectional cooperative network. The second method is that the relays combine the information-bearing symbols by exclusive-or and then transmit them to the sources by beamforming. For this case, we show that the instantaneous signal-to-noise ratios at the sources depend on the values of the information-bearing symbols. Based on, the optimum beamforming vector is computed and it minimizes an upper bound of the outage probability of the bidirectional cooperative network.