Ha X. Nguyen

Signal Transmission With Unequal Error Protection in Wireless Relay Networks

This paper studies a relaying technique based on the signal-to-noise (SNR) threshold in cooperative networks using hierarchical modulation. Hierarchical modulation is useful in applications that require different protection classes of the information. In particular, a cooperative network with one source, one relay, and one destination is considered. Two different protection classes are modulated by a hierarchical 2/4-amplitude shift keying (ASK) constellation at the source. Based on the instantaneous received SNR at the relay, the relay decides to retransmit both classes by using a hierarchical 2/4-ASK constellation, or the more-protection class by using a 2-ASK constellation, or remains silent. Optimal thresholds are chosen to minimize the bit error rate (BER) of the less-protection class, whereas the BER of the more-protection class meets a given requirement. Numerical and simulation results are provided to verify the analysis. The results show that the optimal thresholds significantly improve the performance.

Amplify-and-Forward Relaying with M-FSK Modulation and Coherent Detection

This paper develops a detection scheme based on the implicit pilot-symbol-assisted architecture of M-ary frequency-shift-keying (FSK) modulation in amplify-and-forward (AF) multiple-relay networks. It is shown that, at the expense of the low spectral efficiency inherent in FSK modulation, every transmitted symbol from the source to the destination can be used as a pilot symbol. In the proposed detection scheme, the destination combines the signals from the source and the relays to perform detection by using the estimated channels obtained from the implicit pilot symbols. Using the Gaussian approximation for the effective noise at the destination, an upper bound of the approximated bit-error-rate (BER) is provided. Simulation results reveal that the proposed scheme can significantly improve the BER performance when compared to previously proposed schemes in a temporally-correlated fading environment. The simulation results also verify the obtained upper bound on the approximated BER.

Selection combining for noncoherent decode-and-forward relay networks

This paper studies a new decode-and-forward relaying scheme for a cooperative wireless network composed of one source, K relays, and one destination and with binary frequency-shift keying modulation. A single threshold is employed to select retransmitting relays as follows: a relay retransmits to the destination if its decision variable is larger than the threshold; otherwise, it remains silent. The destination then performs selection combining for the detection of transmitted information. The average end-to-end bit-error-rate (BER) is analytically determined in a closed-form expression. Based on the derived BER, the problem of choosing an optimal threshold or jointly optimal threshold and power allocation to minimize the end-to-end BER is also investigated. Both analytical and simulation results reveal that the obtained optimal threshold scheme or jointly optimal threshold and power-allocation scheme can significantly improve the BER performance compared to a previously proposed scheme.

Signal transmission with unequal error protection in relay selection networks

A relaying technique based on single-relay selection is studied for multiple-relay networks when hierarchical modulation is employed for unequal error protection. A single-relay selection scheme achieves a higher bandwidth efficiency while maintaining the same diversity order as when all the relays are selected in a multiple-relay network. Specifically, a cooperative network with one source, K relays, and one destination is considered in which two different protection information classes are modulated by a hierarchical 2/4-amplitude shift keying (ASK) constellation at the source. After selecting a relay to cooperate with the source, based on the instantaneous received signal-to-noise ratio, the selected relay decides to retransmit both classes by using a hierarchical 2/4-ASK constellation, or the more protection class by using a 2-ASK constellation, or remains silent. The approximated bit error rate (BER) of each information class is derived. Optimal thresholds are chosen to minimise the BER of one class while the BER of the other class satisfies a requirement. Numerical and simulation results are provided to validate the analysis. The results also show that the optimal thresholds can improve the error performance significantly.

Multi-Cell Multi-User Relaying Exploiting Overheard Signals

Two-way relay analog network coding improves the communication rate of a network with symmetric traffic between two end nodes by combining different transmission flows and exploiting the fact that the self-interference can be cancelled at each node. However, this scheme does not efficiently work in the case of asymmetric traffic between the nodes. We propose a novel scheme for a multi-cell multi-user shared-relay network with asymmetric traffic. By exploiting the overheard signals from the users with uplink messages to the users with downlink messages, we determine the precoding matrices at the base stations and the relay station, and use interference alignment such that all interference signal is suppressed. Numerical results show that the new schemes performance is better than a corresponding benchmark scheme in terms of sum-rate and user rates.

Throughput maximisation in non-coherent cooperative networks

An incremental relaying protocol based on the adaptive decode-and-forward relaying scheme is presented for a cooperative wireless network with binary frequency-shift keying modulation. To reduce error propagation and satisfy bit-error-rate (BER) requirement, the proposed protocol employs two thresholds. One threshold is used to select reliable relays: if a relay is requested to re-transmit, it will do so if its decision variable is larger than the threshold; otherwise, it remains silent. The other threshold is used at the destination as follows: the destination sends a request to re-transmit if the decision variable corresponding to a received signal is smaller than the threshold, otherwise, the destination sends a request to stop re-transmissions. The destination combines the signals from the requested relays and from the source to make the final decision. Very-tight closed-form upper bounds for both the average BER and throughput are derived for the proposed protocol. Based on the obtained BER and throughput expressions, the problem of choosing optimal thresholds to maximise the throughput while the BER meets a given constraint is investigated. Simulation results show that our proposed protocol leads to a considerable improvement in the performance of cooperative diversity systems.

Optimization of Linear Dispersion Codes for Wireless Relay Networks

A design method is introduced for linear dispersion (LD) space-time codes in wireless relay networks under Rayleigh fading channels. The codes are designed with a stochastic quasi-gradient algorithm to minimize the upper bound of the average pairwise error probability. Simulation results show that the optimized codes achieve a coding gain of about 2 dB over codes that are randomly generated based on the isotropic distribution.

Noncoherent decode-and-forward cooperative systems with maximum energy selection

This paper investigates the performance of a maximum energy selection receiver of an adaptive decode-and-forward (DF) relaying scheme for a cooperative wireless system. In particular, a close-form expression for the bit-error-rate (BER) is analytically derived when the system is deployed with binary frequency-shift keying (BFSK) modulation. The thresholds used at the relays to address the issue of error propagation are optimized to minimize the BER. While finding the optimal thresholds requires information on the average signal-to-noise ratios (SNRs) of all the transmission links in the system, the approximate threshold at each relay that requires only information on the average SNR of the source-corresponding relay is investigated. It is also shown that the system achieves a full diversity order with the approximate thresholds. Both analytical and simulation results are provided to validate our theoretical analysis.

Performance analysis of adaptive decode-and-forward relaying in noncoherent cooperative networks

This paper studies the bit error rate (BER) performance of an adaptive decode-and-forward (DF) relaying scheme for a cooperative wireless network operating on independent and identically distributed (i.i.d.) or independent and non-identically distributed (i.n.d.) Rayleigh fading channels. The considered network is with one source, K relays, and one destination in which binary frequency-shift keying modulation is employed to facilitate noncoherent communications. A relay decodes and retransmits the received signal to the destination only if its decision variable is larger than the corresponding threshold. Depending on the availability of the information of whether a particular relay retransmits or remains silent, the destination combines the received signals from all the relays and the received signal from the source or only the received signals from the retransmitting relays and the received signal from the source to detect the transmitted information. The average end-to-end BERs are determined in closed-form expressions. The thresholds employed at the relays are investigated to minimize the end-to-end BERs. Analytical and simulation results are provided to validate our theoretical analysis. The obtained results show that the studied scheme improves the BER performance significantly compared to the previous proposed piecewise-linear (PL) scheme. In addition, the information of whether a particular relay retransmits or remains silent at the destination does not really improve the BER performance of the network.

Wireless relay networks with Alamouti space-time code and M-FSK modulation

This paper presents a noncoherent detection scheme for a fixed-gain amplify-and-forward (AF) relay network, in which the source is equipped with two transmit antennas, while the multiple relays and destination are equipped with a single antenna. An Alamouti space-time block code is employed together with M-ary frequency-shift-keying (FSK). Detection at the destination is performed in two steps. First, the destination estimates the overall channels based on the pilot symbol inherent in FSK transmission. Then it performs the maximal ratio combining (MRC) to make a final decision. An upper bound on the error performance is derived in a closed-form expression and it is shown that the investigated scheme achieves the full diversity order. Simulation results are also presented to confirm the analysis in a temporally-correlated fading environment.