Jung-Bin Kim

Comparison of tightly power-constrained performances for opportunistic amplify-and-forward relaying with partial or full channel information

In this letter, we present exact statistical expressions for opportunistic amplify-and-forward (AF) relaying under a tight power constraint, where a best relaying node is selected based on partial channel information (PCI) or full channel information (FCI), respectively. Based on the statistical analysis, we compare PCI with FCI strategy in terms of outage and average capacity. Numerical investigation shows that, as the number of candidate relays increases, the performances with FCI are continuously improved while those improvements with PCI are bounded. However, for the small number of the relays at a low SNR region, PCI and FCI achieve very similar performances. The numerical results also show that the relaxed power constraint that ignores the noise power at the relay leads to significant deviations from the tight constraint, which is the more reasonable one, especially in computing the outage with FCI.

Comparison of two SNR-based feedback schemes in multiuser dual-hop amplify-and-forward relaying networks

In this letter, we investigate amplify-and-forward relaying where a source is communicating with the best user through an intermediate relay that covers multiple users. To reduce the amount of feedback that is needed to select the best user, we consider two kinds of SNR-threshold based channel quality information (CQI) reporting: end-to-end SNR based reporting (ETE-R) and second-hop quality based reporting (SH-R). We derive the probability density function (pdf) and the cumulative density function (cdf) of the SNR received at the best user in Rayleigh fading channels, and compare the average capacity, the average number of feedback users and the feedback outage probability of the two reporting schemes, respectively. Numerical results show that SH-R can achieve the same capacity as an ideal full feedback system when the number of users is large while it reduces the amount of feedback greatly, but ETE-R provides relatively poor performance.

BER Analysis of Dual-Hop Amplify-and-Forward MIMO Relaying with Best Antenna Selection in Rayleigh Fading Channels

Combining relaying and multi-input multi-output (MIMO) transmission is a generic way to overcome the channel-fading impairments. Best antenna selection is a simple but efficient MIMO method that achieves the full diversity and also serves as a lower bound reference of MIMO performance. For a dual-hop MIMO system with an ideal amplify-and-forward (AF) relaying gain and best antenna selection, we provide a probability density function (PDF) of received signal-to-noise ratio (SNR) and an analytic BER equation when using M-ary PSK in Rayleigh fading channels. The analytic result is shown to exactly match with simulated one. Furthermore, the effect of link unbalance between the first hop and the second hop, due to differences in the number of antennas deployed in both hops as well as in the average power of channel coefficients, on the BER performance is numerically investigated and the results show that the links with better balance give better performance.

Exact and Closed-Form Outage Probability of Opportunistic Decode-and-Forward Relaying with Unequal-Power Interferers

In this letter, we study outage performance of opportunistic single relay selection (OSRS) in decode-and-forward (DF) relaying with unequal-power co-channel interferers under Rayleigh fading channels. With the interferers, signal-to-interference-plus-noise ratio (SINR) may not be so great when the interferers also transmit signals at a power level similar to the source. In this case, high signal-to-noise ratio (SNR) approximation often used in outage analysis is not suitable for giving an adequate outage expression. We provide an exact and closed-form outage expression. And using asymptotic analysis, we show that OSRS still achieves full diversity gain in the presence of a finite number of interferers whose transmission powers are finite. When a finite number of interferers also transmit signals at the power level proportional to the source, we show that the asymptotic outage decreases log-linearly as the density of nodes increases. Finally, we show that when the number of interferers is proportional to the node density, the higher density, though creating the greater number of potential relays, does not necessarily contribute to improving the outage performance but deteriorates the performance.

Performance of dual-hop amplify-and-forward beamforming and its equivalent systems in rayleigh fading channels

Combining a dual-hop relaying with multi-input multi-output (MIMO) transmission is a natural extension to overcome the channel impairments. Transmit beamforming (TBF) and maximal ratio combining (MRC) are widely accepted ones, which maximize the signal-to-noise ratio (SNR) at the receiver when channel state information is available. With these methods, there are four possible combinations in constructing dual-hop transmission: TBF-TBF, MRC-MRC, MRC-TBF, and TBF-MRC, respectively. We provide optimal amplify-and-forward (AF) weights at a relay, which maximize the end-to-end SNR for the four systems, respectively, and show the equivalence of the four systems in terms of the SNR. Using relaxed AF weights from the optimal ones, we provide a probability density function (PDF) and a moment generating function (MGF) for the end-to- end SNR per bit with an assumption of an equal number of diversity branch for each hop, which is used to obtain the BER performance for M-ary QAM and PSK constellations, respectively. Numerical results show that the BERs with the relaxed AF weights provide tight lower bounds for those with optimal AF weights. We also compare the BER performance of above AF relaying with that of dual-hop decode-and-forward (DF) relaying.

Cooperative System with Distributed Beamforming and Its Outage Probability

Conventional decode-and-forward (DF) relaying with multiple relays can increase the reliability by creating a virtual antenna array, but induces the capacity loss. We introduce a DF relaying with distributed beamforming (DBF) which has better reliability than conventional DF relaying as well as increases the capacity. Under the power constraints of each relay, not the total sum power of all relays, we find the optimal DBF weights, for each relay, which maximize the received signal-to-noise ratio (SNR) at the destination. We derive an approximated probability density function (PDF) of received SNR and analyze the outage probability in Rayleigh fading channels. Numerical results show that derived probability is very well matched with the simulation results.

Outage Probability and Achievable Diversity Order of Opportunistic Relaying in Cognitive Secondary Radio Networks

In cognitive radio networks, cognitive secondary users (CSUs) can interfere with the primary users (PUs) when they falsely occupy the PUs licensed spectrum. If CSUs in transmission temporarily form a cooperative relaying network, the interference from CSUs to PU can be reduced due to the cooperative diversity. In this paper, the outage performance as well as the achievable diversity order of opportunistic relaying used for cooperative CSUs is investigated. The cooperative relaying network of CSUs is referred to opportunistically relayed cognitive secondary network (ORCSN). Exact outage probabilities and then the diversity order by high SNR approximation are provided under Rayleigh and Nakagami-m fading channels, respectively. It is shown that a spectrum-sensing method used by the CSUs plays an important role in determining the diversity order in ORCSN. Under Rayleigh fading channels, a cooperative sensing enables ORCSN to achieve the full diversity order but a distributed sensing provides the diversity order smaller than one. Under Nakagami-m fading channels, the achievable diversity order is restricted by the Nakagami channel parameters between the PU and the CSUs and also depends on the sensing method. Moreover, the full diversity order can be achieved by a proper selection of the threshold used for spectrum sensing. Numerical investigation is also provided and used to verify the analysis.

End-to-End BER Performance of Cooperative MIMO Transmission with Antenna Selection in Rayleigh Fading

Combining a dual-hop relaying with multi-input multi-output (MIMO) transmission is a natural way to overcome the channel impairments. The antenna selection is a simple but efficient MIMO method since it enjoys the full diversity with moderate channel state information (CSI) from the receiver and no complicated spatial coding. For amplify-and-forward (AF) MIMO relaying with best antenna selection, we derive the probability density function (PDF) of the received signal-to-noise ratio (SNR) and provide a BER equation when using M-ary PSK in Rayleigh fading channels.

Capacity Scaling and Diversity Order for Secure Cooperative Relaying With Untrustworthy Relays

This paper presents an investigation of the secrecy performance of amplify-and-forward (AF) distributed beamforming (DBF) and opportunistic relaying (OR) through K untrustworthy relays. The capacity scaling and achievable diversity order of a secure communication are derived for K → ∞ and in high signal-to-noise-ratio (SNR) regimes, respectively. The DBF is shown to convey confidential messages with a capacity scaling of 1/2 log K regardless of the trustworthiness of the relays, even without intended jamming (IJ), where 1/2 log K corresponds to the maximal capacity scaling of cooperative relaying in non-eavesdropping environments. However, using multiple untrustworthy relays worsens the secrecy performance of the OR, and as K increases, the secrecy capacity converges to zero. When secrecy outage performance with IJ is considered, the achievable diversity order for both the DBF and OR is limited to one. However, only the DBF exploits the benefits using multiple untrustworthy relays, and its secrecy outage probability decreases exponentially as K increases.

Transmission optimization with a space-time filter at low SNR wireless environment

We study the capacity of M multiple antenna system, when the sources are in the far-field of the array, especially at low channel SNR, in an outdoor wireless environment. When the channel has flat fading, the capacity increases as M/sup 2/ times as that of a single antenna system. With a space-time filter, designed by singular-value-decomposition, the channel can be coordinated to obtain a higher receive SNR. With a coding scheme, the receive SNR is good enough to get coding gain, while the channel SNR is too low to obtain the benefits from coding.