Il-Min Kim

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

Joint Secure Beamforming Design at the Source and the Relay for an Amplify-and-Forward MIMO Untrusted Relay System

An amplify-and-forward (AF) multiple-input multiple-output (MIMO) relay network composed of a source, a relay, and a destination is considered, where transmit beamforming is employed both at the source and at the relay. The relay is a user who is willing to help the communication from the source to the destination. In our paper, however, the relay is untrusted in the sense that it may make a passive security attack; that is, it may decode messages of the source. We consider two ways to transmit confidential information of the source to the destination: noncooperative secure beamforming and cooperative secure beamforming. In the noncooperative scheme, the relay is simply treated as an eavesdropper, and does not participate in communication. In the cooperative scheme, the relay is asked to relay signals from the source to the destination. In this paper, the source and relay beamforming is jointly designed to maximize the secrecy rate in the cooperative scheme. The conditions under which the cooperative scheme achieves a higher secrecy rate than the noncooperative scheme are derived in the low and high signal-to-noise ratio (SNR) regimes of the source-relay and relay-destination links. The performance of the secure beamforming schemes is compared through extensive numerical simulations.

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.

Error Performance Analysis of BPSK Modulation in Physical-Layer Network-Coded Bidirectional Relay Networks

We analyze the error performance of the physical-layer network coding (PNC) protocol without channel coding in bidirectional relay networks for binary phase shift keying (BPSK) over Rayleigh fading channels. It is assumed that a bidirectional relay network consists of two sources and a relay, where each node has a single antenna and operates in a half-duplex mode, and the PNC over finite GF(2) is employed. In this system, since the maximum-likelihood (ML) detection metric of the multiple access channel (MAC) at the relay is given by the sum of two exponential functions, it is not possible to utilize the classical Euclidean distance rule. To make the performance analysis tractable, we approximate the ML detection metric by adopting the max-log approximation. Then we derive tight upper and lower bounds in closed form for the average symbol error probability of the MAC at the relay. Finally, we obtain tight upper and lower bounds in closed form for the end-to-end average bit-error rate (BER).

Relay Selection with ANC and TDBC Protocols in Bidirectional Relay Networks

We study relay selection (RS) with the analog network coding (ANC) and time division broadcast (TDBC), which are two major amplify-and-forward (AF)-based protocols in bidirectional relay networks. We consider a bidirectional network consisting of two different end-sources and multiple relays, where each terminal has a single antenna and operates in a half-duplex mode. In this network, a single best relay is selected depending on channel conditions to help bidirectional communication between the two end-sources. Specifically, we first consider RS schemes for the ANC and TDBC protocols based on a max-min criterion to minimize the outage probabilities. Then, for the RS in the ANC protocol, we derive a closed-form expression of the outage probability; for the RS in the TDBC protocol, we derive a one-integral form of the outage probability and its lower bound in closed-form. Numerical results confirm that the closed-form expression of the ANC protocol and the one-integral form of the TDBC protocol are very accurate, and that the closed-form lower bound of the TDBC protocol is also tight.

Existence and construction of noncoherent unitary space-time codes

We consider transmission using N transmit and reception using M receive antennas in a wireless environment assuming that neither the transmitter nor the receiver knows the channel coefficients. For the scenario that the transmission employs noncoherent T /spl times/ N unitary space-time codes and for a block-fading channel model where the channel is static during T channel uses and varies from T channel uses to the other, we establish the bound r /spl les/ min(T-N, N) on the diversity advantage rM provided by the code. In order to show that the requirement r /spl les/ min(T-N, N) cannot be relaxed, for any given R, N, T, and r /spl les/ min(T-N, N), we then construct unitary T /spl times/ N space-time codes of rate R that guarantee diversity advantage rM. Two constructions are given that are also amenable to simple encoding and noncoherent maximum-likelihood (ML) decoding algorithms.

Exact BER analysis of OSTBCs in spatially correlated MIMO channels

The bit-error rate (BER) performance of orthogonal space-time block codes (STBCs) in correlated multiple-input multiple-output (MIMO) channels is studied. We first derive closed-form exact BER equations for pulse amplitude modulation, quadrature amplitude modulation, and phase-shift keying constellations in a correlated Rayleigh MIMO channel. The BER expressions can be easily evaluated without any numerical methods. Then we consider correlated MIMO channels where the line of sight components exist and they suffer from shadowing, namely, correlated shadowed Rician MIMO channels. For three practical channel scenarios, the BER is analyzed, and closed-form BER equations are presented

Performance Analysis of Bidirectional Communication Protocols Based on Decode-and-Forward Relaying

We study and compare the performance of three very typical bidirectional communication protocols based on the decode-and-forward relaying: time-division broadcast (TDBC), physical-layer network coding (PNC), and opportunistic source selection (OSS). We first derive the exact closed-form outage probabilities for the PNC and OSS protocols and an exact but one-integral form outage probability for the TDBC protocol. To gain more insight, we also derive closed-form asymptotic outage probability expressions for all protocols, with which we obtain further analytical results on the asymptotic optimal power allocation, the asymptotic optimal relay location, and the performance gain of the OSS protocol over the TDBC protocol in the high signal-to-noise ratio (SNR) regime. Finally, we study the diversity-multiplexing tradeoff (DMT) for each protocol both in the finite and infinite SNR regimes.

Optimal Power Allocation for Secure Multicarrier Relay Systems

We study power allocation for secrecy rate maximization in a multicarrier decode-and-forward relay system, where an eavesdropper exists. We consider three transmission modes: in no communication, the source and the relay do not transmit at all; in direct communication, the source broadcasts signals during the first time slot and the relay does not forward any signal during the second time slot; in relay communication, the source broadcasts signals during the first time slot and the relay forwards the reencoded signals to the destination during the second time slot. Determining the transmission strategy adaptively on each subcarrier, the optimal source power and the optimal relay power over all subcarriers are derived to maximize the sum secrecy rate under a total system power constraint. In addition, a suboptimal power allocation scheme is proposed to substantially reduce the computational complexity. It is shown that the proposed suboptimal solution is asymptotically optimal in the limit as the number of subcarriers goes to infinity. Extensive numerical results are presented for various scenarios. In particular, the performance of the suboptimal scheme is very close to that of the optimal scheme even if the number of subcarriers is moderately small.