Phee Lep Yeoh

Transmit Antenna Selection for Security Enhancement in MIMO Wiretap Channels

We propose and analyze transmit antenna selection (TAS) to enhance physical layer security in a wiretap channel with NA antennas at the transmitter, NB antennas at the receiver, and NE antennas at the eavesdropper. We focus on the practical scenario where the transmitter does not have any channel state information (CSI) of the eavesdroppers channel. The transmitter selects a single antenna that maximizes the instantaneous signal-to-noise ratio (SNR) at the receiver. The receiver and the eavesdropper employ either maximal-ratio combining (MRC) or selection combining (SC) to combine the received signals. For the proposed protocols, we derive new closed-form expressions for the probability of non-zero secrecy capacity. We consider Nakagami-m fading with non-identical fading parameters of the main channel, mB, and of the eavesdroppers channel, mE. Next, we derive new closed-form expressions for the exact secrecy outage probability, based on which the ε-outage secrecy capacity is characterized. Based on the exact expressions, we derive the asymptotic secrecy outage probability which accurately reveals the secrecy diversity order and the secrecy array gain. We confirm that the proposed protocols achieve identical secrecy diversity orders of NANBmB. An interesting conclusion is reached that this diversity order is independent of NE and mE. Furthermore, we prove that under the proposed protocols, the secrecy outage probability and the ε-outage secrecy capacity improve with increasing NA.

Cognitive Relay Networks With Multiple Primary Transceivers Under Spectrum-Sharing

We examine the impact of multiple primary transmitters and receivers (PU-TxRx) on the outage performance of cognitive decode-and-forward relay networks. In such a joint relaying/spectrum-sharing arrangement, we address fundamental questions concerning three key power constraints: 1) maximum transmit power at the secondary transmitter (SU-Tx), 2) peak interference power at the primary receivers (PU-Rx), and 3) interference power at SU-Rx caused by the primary transmitter (PU-Tx). Our answers to these are given in new analytical expressions for the exact and asymptotic outage probability of the secondary relay network. Based on our asymptotic expressions, important design insights into the impact of primary transceivers on the performance of cognitive relay networks is reached. We have shown that zero diversity order is attained when the peak interference power at the PU-Rx is independent of the maximum transmit power at the SU-Tx.

Physical Layer Security of Maximal Ratio Combining in Two-Wave With Diffuse Power Fading Channels

This paper advocates physical layer security of maximal ratio combining (MRC) in wiretap two-wave with diffuse power fading channels. In such a wiretap channel, we consider that confidential messages transmitted from a single antenna transmitter to an M-antenna receiver are overheard by an N-antenna eavesdropper. The receiver adopts MRC to maximize the probability of secure transmission, whereas the eavesdropper adopts MRC to maximize the probability of successful eavesdropping. We derive the secrecy performance for two practical scenarios: 1) the eavesdroppers channel state information (CSI) is available at the transmitter and 2) the eavesdroppers CSI is not available at the transmitter. For the first scenario, we develop a new analytical framework to characterize the average secrecy capacity as the principal security performance metric. Specifically, we derive new closed-form expressions for the exact and asymptotic average secrecy capacity. Based on these, we determine the high signal-to-noise ratio power offset to explicitly quantify the impacts of the main channel and the eavesdroppers channel on the average secrecy capacity. For the second scenario, the secrecy outage probability is the primary security performance metric. Here, we derive new closed-form expressions for the exact and asymptotic secrecy outage probability. We also derive the probability of nonzero secrecy capacity. The asymptotic secrecy outage probability explicitly indicates that the positive impact of M is reflected in the secrecy diversity order and the negative impact of N is reflected in the secrecy array gain. Motivated by this, we examine the performance gap between N and N+1 antennas based on their respective secrecy array gains.

MIMO Wiretap Channels: Secure Transmission Using Transmit Antenna Selection and Receive Generalized Selection Combining

We propose and analyze transmit antenna selection with receive generalized selection combining (TAS/GSC) for physical layer security enhancement in multiple-input multiple-output wiretap channels. In this protocol, a single antenna out of NA antennas is selected at the transmitter and LB antennas out of NB antennas are combined at the legitimate receiver. We characterize the physical layer secrecy of TAS/GSC via our new closed-form expressions for the exact and the asymptotic secrecy outage probability. We demonstrate that the maximum secrecy outage diversity gain of NA NB is achieved.

Two-Way Relaying With Multi-Antenna Sources: Beamforming and Antenna Selection

We propose and analyze two multiple-input-multiple-output (MIMO) two-way relaying schemes with an amplify-and-forward protocol in Nakagami-m fading channels, where multi-antenna sources communicate via a single-antenna relay. Specifically, we present a new framework for the comparative analysis of beamforming and antenna selection with nonidentical fading parameter m in the two source-relay links. To facilitate the comparison, we derive new exact, approximate, and asymptotic expressions for the sum symbol error rate (SSER) with M-ary phase-shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM). Based on the asymptotic SSER, we prove that beamforming and antenna selection have the same diversity order. The diversity order is dominated by the weaker source-relay link, which is determined by the product of the number of source antennas and the fading parameter. We proceed to characterize the fundamental difference between the two schemes in terms of their array gains and average signal-to-noise ratios (SNRs). To obtain further insights, we address the key question of “How to allocate the total transmit power such that the SSER is minimized?” Our answer is given in the form of new concise expressions for the power-allocation factor that optimally distributes the total transmit power between the sources and the relay. A pivotal conclusion is reached that antenna selection offers the same SSER as beamforming when the source in the weaker link is equipped with a single antenna.

Multiuser MIMO Relay Networks in Nakagami-m Fading Channels

This paper proposes a low complexity protocol that preserves full diversity in multiuser amplify-and-forward relay networks with N_S antennas at the source, N_R antennas at the relay, and N_D antennas at each of the K destinations. In the proposed protocol, a two-fold diversity is guaranteed: 1) multi-antenna diversity via transmit antenna selection with maximal-ratio combining (TAS/MRC), and 2) multiuser diversity via opportunistic scheduling. Under perfect feedback with precise channel state information (CSI), we derive new exact and asymptotic symbol error rate (SER) expressions in closed-form for the general case of Nakagami-m fading. We prove that the full diversity order of N_SN_DKm_X + min{N_SN_Rm_Y, N_RN_DKm_Z} is guaranteed, where m_X, m_Y, and m_Z denote the fading parameters of the source-destination, source-relay, and relay-destination links, respectively. To examine the impact of delayed feedback, we next derive new exact and asymptotic SER expressions in closed-form. We prove that in the presence of delayed feedback, outdated CSI degrades the diversity order to N_Dm_X + min{N_Rm_Y, N_Dm_Z}. In addition, based on our asymptotic expressions, we determine the optimal power allocation between the source and the relay such that the SER is minimized. We show that optimal power allocation offers superior performance over uniform power allocation; highlighting a pivotal design choice for maximizing network performance without investing additional resources.

Transmit Antenna Selection for Interference Management in Cognitive Relay Networks

We propose transmit antenna selection (TAS) in decode-and-forward (DF) relaying as an effective approach to reduce the interference in underlay spectrum sharing networks with multiple primary users (PUs) and multiple antennas at the secondary users (SUs). We compare two distinct protocols: 1) TAS with receiver maximal-ratio combining (TAS/MRC) and 2) TAS with receiver selection combining (TAS/SC). For each protocol, we derive new closed-form expressions for the exact and asymptotic outage probability with independent Nakagami-m fading in the primary and secondary networks. Our results are valid for two scenarios related to the maximum SU transmit power, i.e., P, and the peak PU interference temperature, i.e., Q. When P is proportional to Q, our results confirm that TAS/MRC and TAS/SC relaying achieve the same full diversity gain. As such, the signal-to-noise ratio (SNR) advantage of TAS/MRC relaying relative to TAS/SC relaying is characterized as a simple ratio of their respective SNR gains. When P is independent of Q, we find that an outage floor is obtained in the large P regime where the SU transmit power is constrained by a fixed value of Q. This outage floor is accurately characterized by our exact and asymptotic results.

Selection Relaying with Transmit Beamforming: A Comparison of Fixed and Variable Gain Relaying

This paper presents a comparison and analysis of selection relaying with transmit beamforming as an effective approach to combat channel impairments in relay-assisted cellular networks. We consider the downlink scenario where the base station equipped with N antennas transmits to the mobile station either directly, or indirectly via a relay station, according to the link with the strongest received signal-to-noise ratio (SNR). We compare two amplify-and-forward protocols: i) fixed gain relaying which requires partial channel state information (CSI), and ii) variable gain relaying which requires full CSI. We present new exact closed-form expressions for the generalized moments of the end-to-end SNR to characterize the higher-order statistical properties of the SNR. We derive new exact closed-form expressions for the symbol error rate (SER), which are valid for a wide variety of modulations. Furthermore, we explicitly characterize the asymptotic behavior of the SER to obtain two key performance parameters: the array gain and the diversity order. Based on these, we reveal that the SNR advantage of variable over fixed gain relaying vanishes in the large N limit.

Unified Analysis of Transmit Antenna Selection in MIMO Multirelay Networks

We present a unified asymptotic framework for transmit antenna selection in multiple-input multiple-output (MIMO) multirelay networks with Rician, Nakagami-m, Weibull, and generalized-K fading channels. We apply this framework to derive new closed-form expressions for the outage probability and symbol error rate (SER) of amplify-and-forward (AF) relaying in MIMO multirelay networks with two distinct protocols: 1) transmit antenna selection with receiver maximal-ratio combining (TAS/MRC) and 2) transmit antenna selection with receiver selection combining (TAS/SC). Based on these expressions, the diversity order and the array gain with M-ary phase-shift keying and M-ary quadrature-amplitude modulation are derived. We corroborate that the diversity order only depends on the fading distribution and the number of diversity branches, whereas the array gain depends on the fading distribution, the modulation format, the number of diversity branches, and the average per-hop signal-to-noise ratios (SNRs). We highlight that the diversity order of TAS/MRC is the same as TAS/SC, regardless of the underlying fading distribution. As such, we explicitly characterize the SNR gap between TAS/MRC and TAS/SC as the ratio of their respective array gains. An interesting observation is reached that for equal per-hop SNRs, the SNR gap between the two protocols is independent of the number of relays.

Exact and Asymptotic SER of Distributed TAS/MRC in MIMO Relay Networks

We propose distributed transmit antenna selection with receiver maximal-ratio combining (TAS/MRC) for use in a two-hop multiple-input multiple-output (MIMO) relay network. The network under consideration is equipped with NS, NR, and ND antennas at the source, relay, and destination, respectively. First, we derive a new closed-form expression for the exact cumulative distribution function (cdf) of the end-to-end SNR. Based on this, we present a new closed-form expression for the exact symbol error rate (SER). Our analytical results are further evaluated in the high SNR regime, leading to practical design insights. Our asymptotic expressions are concise and have the added advantage of explicitly characterizing the diversity order and the array gain of the network. Our exact and asymptotic results are valid for general operating scenarios with distinct average received SNRs in each hop.