Efficient relaying strategies for mitigating fading and interference

Relaying is considered as a key strategy to mitigate fading and interference in wireless systems. The interference mitigation in a relay-assisted two-user Z-channel is investigated in this work. The two-user Z-channel, where only one receiver is interference-limited, models the interference in heterogeneous cellular networks and ad-hoc wireless networks. In the Z-channel, the transmission strategies based on the probability of system outage may not be optimal for the interference-free user as the outage event is dominated by the interference-limited user. Instead, the diversity gain region (DGR) is an appropriate performance metric as it captures the trade-off between achievable diversity gains at both the users. The aim of this work is to analyze the DGR of the relay-assisted Z-channel. The achievable DGR of the full-duplex (FD) and half-duplex (HD) decode-and-forward (DF) relay-assisted Z-channels for both the single-user (SU) and multi-user (MU) encoding schemes are analyzed. Mitigation of the fading in a point-to-point channel with the HD DF relay is also investigated. Achievable DGR with both the SU codes and MU codes are presented for the direct transmission scheme with two antennas at the interfering transmitter. If the interference-free receiver is constrained to operate at the optimal diversity multiplexing tradeoff (DMT), the direct transmission scheme with MU codes is shown to achieve the same DGR as that of the single-user (SU) codes with the interference ignorant (II) detector. The receiver treats the interference as additive white Gaussian noise with the II detector. Then, the achievable DGR of the FD DF relay-assisted Z-channel with the SU codes is analyzed. It is shown that the FD DF relay clustered with the interfering source does not achieve the same DGR as that of the direct transmission scheme with two antennas at the interfering source. Hence, from the DGR perspective, it is better to have two co-located antennas at the interfering transmitter than a clustered FD DF relay. The present work studies whether the non-clustered FD DF relay achieves better DGR than the direct transmission scheme with single antenna at all the terminals. The achievable DGR of the MU codes based transmission schemes, namely FD partial decode-and-forward (PDF) and the simplified Han Kobayashi (SHK) schemes are investigated. Furthermore, the achievable DMT of the interference-limited receiver is optimized under the constraint that the interference-free receiver achieves the optimal DMT. It is observed that the FD PDF and the SHK schemes provide better DGR than that of the direct transmission scheme at lower interference levels only. This is because, as the interference level increases, the interfering transmitter should reduce the power of the private message to improve the achievable DMT at the interference-limited receiver. This reduction in the power may lead to the outage of the private message at the interference-free receiver for the higher interference levels. Closed-form expressions for the achievable DGRs with the half-duplex (HD) dynamic decode-and-forward (DDF) and HD selective DDF (SDDF) protocols are obtained. At the lower multiplexing gains, the HD DDF and the HD SDDF protocols are shown to achieve better DGR than that of the FD DF protocol. This is because, the interference-limited destination can use the joint decoder which is robust than the successive interference canceler (SIC). Next, the closed-form expressions for the DGR and the symbol error rate with SIC for the HD DF and HD amplify-and-forward (AF) relaying are derived. The minimum source-to-relay and relay-to-destination channel gains required to achieve the full diversity are also obtained. For a three terminal relay channel with an interference-free destination, selective link adaptive regenerative relaying (SLAR) is proposed. The SLAR scheme is shown to achieve full diversity while improving the power and spectral efficiency. In the SLAR scheme, the relay needs full instantaneous channel state information (CSI), which may need additional transmission bandwidth and power. SLAR with limited feedback (SLAR-LFB) protocol is proposed to reduce the costs associated with the CSI feedback. In SLAR-LFB, the relay needs only the average CSI, which incurs little additional cost. The SLAR-LFB protocol is also shown to achieve full diversity while improving the power and spectral efficiency.Doctor of Philosophy (SCE

Interference cancellation through interference forwarding in relay-assisted systems

Interference forwarding and multiuser (MU) codes are potential solutions to manage moderate interference. In particular, MU codes based simplified Han-Kobayashi (SHK) scheme is known to achieve sum-rate within one bit of outer bound for a two-user interference channel. However, if each user has its own quality-of-service requirements, it is unknown whether the SHK scheme performs better compared to that of single-user (SU) codes. We investigate whether the SHK scheme with full-duplex decode-and-forward relay can achieve better diversity gain than SU codes for altruistic and selfish modes. In the altruistic mode, the interfering transmitter lets the interference-limited receiver operate at the interference-free diversity order, and in selfish mode, the interfering transmitter optimizes its receiver's diversity order. In both modes, the SHK scheme is shown to provide better diversity gain region (DGR) than the SU codes based transmission scheme only if the interference is weak and the multiplexing gain of the interference is high. Moreover, if the self-interference is not eliminated, relay based SHK protocol cannot operate in selfish mode and also, the multiplexing gain region where the relay based SHK scheme achieves better DGR than other schemes reduces.MOE (Min. of Education, S’pore

On the diversity gain region of the dynamic decode-and-forward relay-assisted Z-channel

In an interference-limited system, the interference forwarding by a relay enhances the interference level and thereby enables the cancellation of the interference. In this work, interference forwarding by a half-duplex dynamic decode-and-forward (HD DDF) relay in a two-user Z-channel is considered. In the two-user Z-channel, one user is interference-limited while the other user is interference-free. The diversity gain region (DGR), which characterizes the tradeoff between the achievable diversity orders between the two users, is an appropriate performance metric for the Z-channel. Closed-form expression for the achievable DGR with the interference forwarding by the HD DDF relay is presented. The multiplexing gain regions (MGRs) where the HD DDF protocol achieves better DGR over the direct transmission scheme, full-duplex decode-and-forward (FD DF) and FD partial DF relay assisted Z- channel are identified. The HD DDF protocol is shown to achieve better DGR than the FD DF and FD PDF relay for a large range of MGR. The achievable DGRs for the HD DDF, FD DF, and FD PDF relay-assisted Z-channel and direct transmission scheme are presented for various interference levels and multiplexing gain pairs

BER analysis of interference-limited synchronous wireless networks with line-of-sight links

The bit error rate (BER) of a Rician-faded and interference-limited synchronous wireless network with binary phase shift keying (BPSK) modulation is investigated. The probability density function (PDF) of the phase of the Rician-faded channel is unwieldy. However, the phase of the Rician-faded channel given the magnitude follows Von Mises distribution. This conditional PDF is exploited to derive an accurate expression for the BER of BPSK-modulated signals with an interference ignorant detector. Moreover, if either the desired signal or the interfering signal is Rayleigh-faded, the BER expression based on the uniform distribution assumption for the phase difference in decision statistic is shown to be accurate

On the DMT of RF energy harvesting-based dynamic decode-and-forward relaying

A multiple relay-based system, where all the relay nodes powered by radio frequency-based energy harvesting (EH), is considered. The present letter characterizes the diversity-multiplexing tradeoff (DMT) for EH-based dynamic decode-and-forward (EH-DDF) protocol for independent quasi-static Rayleigh faded channels. It is shown that the achievable diversity gain of EH-DDF improves with the reduction in the EH intervals. However, it is observed that the performance difference exists between classical DDF and EH-DDF even for infinitesimal harvest duration in lower multiplexing gain region.Accepted versio

Cognitive radio for aeronautical communications : a survey

Novel air traffic management (ATM) strategies are proposed through the Next Generation Air Transportation and Single European Sky for ATM Research projects to improve the capacity of the airspace and to meet the demands of the future air traffic. The implementation of the proposed solutions leads to increasing use of wireless data for aeronautical communications. Another emerging trend is the unmanned aerial vehicles. The unmanned aerial systems (UASs) need reliable wireless data link and dedicated spectrum allocation for its operation. On-board broadband connectivity also needs dedicated spectrum to satisfy the quality of service requirements of the users. With the growing demand, the aeronautical spectrum is expected to be congested. However, the studies revealed that the aeronautical spectrum is underutilized due to the static spectrum allocation strategy. The aeronautical communication systems, such as air-air and air-ground communication systems, inflight infotainment systems, wireless avionics intra-communications, and UAS, can benefit significantly from the introduction of cognitive radio-based transmission schemes. This paper summarizes the current trends in aeronautical spectrum management followed by the major applications and contributions of cognitive radio in solving the spectrum scarcity crisis in the aeronautical domain. Also, to cope with the evolving technological advancement, researchers have prioritized the issues in the case of cognitive radio that needs to be addressed depending on the domain of operation. The proposed cognitive aeronautical communication systems should also be compliant with the Aeronautical Radio Incorporated and Aerospace Recommended Practice standards. An overview of these standards and the challenges that need immediate attention to make the solution feasible for a large-scale operation, along with the future avenues of research is also furnished.Published versio