See Ho Ting

Cooperative decode-and-forward relaying for secondary spectrum access

A new transmission protocol for joint cooperation between a primary and secondary user in a cognitive radio system is proposed. If successful decoding is achieved, the primary/secondary transmitters behave as relays for the other system. With more efficient use of the communication resource, performance is enhanced. The proposed scheme builds on the work and offers improved outage probabilities for both the primary and secondary systems. The paper provides a full characterization of the Markov Chain describing the protocol behavior and a closed form expression for the stationary distribution of the chain. Furthermore, exact computation or tight approximations for the outage probabilities for all states of the chain are derived. Simulation results exhibit a performance improvement between 22 and 48 percent for primary system and an uniform decrease of outage probability for secondary system.

Performance bounds for two-way amplify-and-forward relaying

In this paper, the average sum rate of two-way amplify-and-forward (AF) half-duplex relaying system is analyzed. To this end, we first derive the harmonic mean of two independent gamma distributed random variables which have the same shape parameter but different scale parameters. By deriving tight upper and lower bounds for the average sum rate of two-way relaying, we verify that two-way relaying can significantly recover the spectrum efficiency loss of one-way relaying. We also extend the two-way AF half-duplex relaying to the case where source and destination terminals both transmit Alamoutis orthogonal space time block code (OSTBC) utilizing two antennas and the relay has only one antenna. By deriving both upper and lower bounds for the average sum rate as well as an upper bound for the pairwise error probability (PEP) for the proposed two-way OSTBC scheme, we show that the average sum rate is further improved compared to the single antenna case and a diversity order of two is also achieved. Furthermore, optimal power allocations under a global power constraint for two-way relaying with single antenna and the proposed two-way OSTBC scheme are derived analytically.

Adaptive two-way relaying and outage analysis

This paper analyzes the outage performance of cooperative protocols for the two-way relaying model in which a half-duplex relay assists in the bi-directional communication of two terminals. We first define the outage events and derive closed form results for the outage probabilities of conventional nonadaptive amplify-and-forward (AF) and decode-and-forward (DF) two-way relaying protocols.We then consider a simple adaptive protocol in the two-way relaying scenario, which switches between AF and DF depending on the decodability of the two bi-directional data streams at the relay. We also derive the outage probability for this adaptive scheme in closed form expressions, and our results show that it always outperforms the non-adaptive schemes. All the closed form results derived in this paper are applicable to arbitrary channel gains between terminals.

Cognitive Spectrum Sharing With Two-Way Relaying Systems

Dynamic spectrum-sharing protocols are gaining attention due to the need for improved spectrum utilization. We propose a spectrum-sharing protocol with two-way relaying systems, where two licensed primary users A and B communicate with each other with the assistance of an unlicensed secondary transceiver C, which acts as a relay. The secondary transceiver C gains spectrum sharing by using a decode-and-forward (DF) relay protocol and superposing the secondary transmission on network-coded primary signals. We analytically derive the outage probabilities for both the primary and secondary systems under the proposed cognitive two-way relaying (CTR) protocol. Our results show that a spectrum-sharing region exists such that, as long as C is located within this region, there will be a power-allocation threshold above which the proposed CTR protocol is able to provide a better (or an equal) outage performance for the primary system and, at the same time, achieve secondary spectrum sharing.

Cooperative Spectrum Sharing Protocol with Secondary User Selection

In this paper, we propose a two-phase protocol based on cooperative relaying for a secondary system to achieve spectrum access along with a primary system. The primary system comprises of a transmitter-receiver pair PT-PR, and the secondary system comprises of M transmitters STi, i ∈ {1, 2,..., M} and a common receiver SR. The secondary transmitter STp which achieves the request target rate for the primary system is selected to serve as a decode-and-forward (DF) relay for the primary system. With the cooperation of STp, the primary system is able to tolerate interference lower than a certain threshold in the relaying phase, without degrading its outage performance. The secondary transmitter STs, which satisfies this interference constraint and provides the optimal outage performance for the secondary system, is then selected to access the spectrum band simultaneously when STp is relaying the primary signal. Theoretical and simulation results confirm the efficiency of the proposed spectrum sharing protocol, and we show that both primary and secondary systems are able to achieve better outage performance with increasing M.

Cooperative spectrum sharing via controlled amplify-and-forward relaying

We consider a spectrum sharing protocol in which a secondary system operates on the same spectrum as a primary system, without adversely affecting the rate of the primary system. The protocol comprises of a two-phase transmission. In the first phase, the primary transmitter transmits a primary signal to the primary receiver, which is also received at the secondary transmitter and receiver. The secondary transmitter amplifies the received primary signal and generates a linearly weighted combination of this signal and the secondary signal. The weight is a variable power allocation factor α (0 ⩽ α ⩽ 1). This composite signal is then broadcast in the second transmission phase. We analyze the achievable rates for the primary and secondary systems, and determine α such that the rate of primary system with this spectrum sharing protocol is no worse than that in the absence of the secondary system. We show that the spectrum sharing protocol can improve the rate of primary system by an appropriate choice of α, while at the same time achieve secondary spectrum access.

AF two-path half duplex relaying with inter-relay self interference cancellation: diversity analysis and its improvement

In this paper, we consider a network consisting of one source, one destination and two relays, under a half duplex constraint. We analyze an amplify-and-forward (AF) two-path relaying scheme where one of the relays additionally performs inter-relay interference cancellation. With this inter-relay self interference cancellation, we show that degradation in performance can be significantly reduced when inter-relay channel gain increases. By treating the AF two-path relaying as an equivalent MIMO space time code, we derive an upper bound for the pairwise error probability (PEP) of this system. Previous works indicate that only a diversity order of two can be achieved even though physically there are three possibly independent spatial paths to the destination. From our analysis of the PEP, we analytically prove and identify the reason to the limited diversity order of two. We then propose a simple but novel precoding scheme which allows the system to achieve a full diversity order of three while maintaining the spectral efficiency gain of the two-path relaying protocol. A robust precoding design to achieve full diversity order with high coding gain is also proposed.

Cooperative OFDM Relaying for Opportunistic Spectrum Sharing: Protocol Design and Resource Allocation

In this paper, we propose an opportunistic spectrum sharing protocol that exploits the situation when the primary system is incapable of supporting its target transmission rate. Specifically, the secondary system tries to help the primary system to achieve its target rate via two-phase cooperative OFDM relaying, where the secondary system acts as an amplify-and-forward relay for the primary system by allocating a fraction of its subcarriers to forward the primary signal. At the same time, the secondary system uses the remaining subcarriers to transmit its own signal, and thus gaining opportunistic spectrum access. As a part of the protocol, if the primary system finds that outage will occur even when the secondary system serves as a pure relay, the primary system will cease transmission and the secondary system will be granted access to the primary spectrum. We study the joint optimization of the set of subcarriers used for cooperation, subcarrier pairing, and subcarrier power allocation such that the transmission rate of the secondary system is maximized, while helping the primary system, as a higher priority, to achieve its target rate. Simulation results demonstrate the performance of the proposed spectrum sharing protocol as well as the win-win solution for the primary and secondary systems.

High Rate Two-Way Amplify-and-Forward Half-Duplex Relaying with OSTBC

In this paper, we derive tight upper and lower bounds for the average sum rate of two-way amplify-and-forward (AF) half-duplex relaying, which show that two-way AF half-duplex relaying can significantly mitigate the spectral efficiency loss of conventional one-way AF half-duplex relaying. We then extend the AF half-duplex two-way relaying to the case where source and destination terminals both transmit Alamoutis orthogonal space time block code (OSTBC) utilizing two antennas and relay has only one antenna. We derive both upper and lower bounds for the average sum rate as well as an upper bound for the pair-wise error probability (PEP) for the proposed OSTBC scheme. We also find the optimal power allocation for both two- way relaying schemes analytically. Our theoretical analysis and numerical simulation results show that higher average sum rate compared to the single antenna case and diversity order of two are achieved by the proposed OSTBC scheme.

Wireless Compressive Sensing for Energy Harvesting Sensor Nodes

We consider the scenario in which multiple sensors send spatially correlated data to a fusion center (FC) via independent Rayleigh-fading channels with additive noise. Assuming that the sensor data is sparse in some basis, we show that the recovery of this sparse signal can be formulated as a compressive sensing (CS) problem. To model the scenario in which the sensors operate with intermittently available energy that is harvested from the environment, we propose that each sensor transmits independently with some probability, and adapts the transmit power to its harvested energy. Due to the probabilistic transmissions, the elements of the equivalent sensing matrix are not Gaussian. Besides, since the sensors have different energy harvesting rates and different sensor-to-FC distances, the FC has different receive signal-to-noise ratios (SNRs) for each sensor. This is referred to as the inhomogeneity of SNRs. Thus, the elements of the sensing matrix are also not identically distributed. For this unconventional setting, we provide theoretical guarantees on the number of measurements for reliable and computationally efficient recovery, by showing that the sensing matrix satisfies the restricted isometry property (RIP), under reasonable conditions. We then compute an achievable system delay under an allowable mean-squared-error (MSE). Furthermore, using techniques from large deviations theory, we analyze the impact of inhomogeneity of SNRs on the so-called k-restricted eigenvalues, which governs the number of measurements required for the RIP to hold. We conclude that the number of measurements required for the RIP is not sensitive to the inhomogeneity of SNRs, when the number of sensors n is large and the sparsity of the sensor data (signal) k grows slower than the square root of n. Our analysis is corroborated by extensive numerical results.