Hari Krishna Garg

Optimal power allocation for fading channels in cognitive radio networks: Ergodic capacity and outage capacity

A cognitive radio network (CRN) is formed by either allowing the secondary users (SUs) in a secondary communication network (SCN) to opportunistically operate in the frequency bands originally allocated to a primary communication network (PCN) or by allowing SCN to coexist with the primary users (PUs) in PCN as long as the interference caused by SCN to each PU is properly regulated. In this paper, we consider the latter case, known as spectrum sharing, and study the optimal power allocation strategies to achieve the ergodic capacity and the outage capacity of the SU fading channel under different types of power constraints and fading channel models. In particular, besides the interference power constraint at PU, the transmit power constraint of SU is also considered. Since the transmit power and the interference power can be limited either by a peak or an average constraint, various combinations of power constraints are studied. It is shown that there is a capacity gain for SU under the average over the peak transmit/interference power constraint. It is also shown that fading for the channel between SU transmitter and PU receiver is usually a beneficial factor for enhancing the SU channel capacities.

On The Han–Kobayashi Region for theInterference Channel

In this correspondence, we derive a simplified description of the Han-Kobayashi rate region for the general interference channel. Using this result, we establish that the recently discovered Chong-Motani-Garg rate region is a new representation of the Han-Kobayashi region. Moreover, a tighter bound for the cardinality of the time-sharing auxiliary random variable emerges from our simplified description.

Performance Analysis of Two Hop Amplify-and-Forward Systems with Interference at the Relay

We analyze the performance of a two hop channel state information (CSI)-assisted amplify-and-forward system, with co-channel interference at the relay. The systems outage probability and the average bit error rate (BER) in the presence of Rayleigh faded multiple interferers are investigated. We derive an exact expression for the outage probability and an accurate bound for the systems average BER. Simulation results show the validity of the analysis and point out the effect of interference.

Optimal Power Allocation Strategies for Fading Cognitive Radio Channels with Primary User Outage Constraint

In this paper, we consider a cognitive radio (CR) network where a secondary (cognitive) user shares the spectrum for transmission with a primary (non-cognitive) user over block-fading (BF) channels. It is assumed that the primary user has a constant-rate, constant-power transmission, while the secondary user is able to adapt transmit power and rate allocation over different fading states based on the channel state information (CSI) of the CR network. We study a new type of constraint imposed over the secondary transmission to protect the primary user by limiting the maximum transmission outage probability of the primary user to be below a desired target. We derive the optimal power allocation strategies for the secondary user to maximize its ergodic/outage capacity, under the average/peak transmit power constraint along with the proposed primary user outage probability constraint. It is shown by simulations that the derived new power allocation strategies can achieve substantial capacity gains for the secondary user over the conventional methods based on the interference temperature (IT) constraint to protect the primary transmission, with the same resultant primary user outage probability.

Optimal power allocation for OFDM-based cognitive radio with new primary transmission protection criteria

This paper considers a spectrum underlay network, where an OFDM-based cognitive radio (CR) system is allowed to share the subcarriers of an OFDMA-based primary system for simultaneous transmission. Instead of using the conventional interference power constraint (IPC) to protect the primary users (PUs) in the primary system, a new criterion referred to as rate loss constraint (RLC), in the form of an upper bound on the maximum rate loss of each PU due to the CR transmission, is proposed for primary transmission protection. Assuming the channel state information (CSI) of the PU link, the CR link, and their mutual interference links is available to the CR, the optimal power allocation strategy to maximize the achievable rate of the CR system is derived under RLC together with CR¿s transmit power constraint. It is shown that the CR system can achieve a significant rate gain under RLC as compared to IPC. Furthermore, the relationship between RLC and IPC is investigated, and it is shown that the rate gain is obtained by exploiting the additional CSI of the PU link. A more general case referred to as hybrid protection to PUs is then studied, by taking into account that some PU links¿ CSI is not available at CR.

Performance Analysis of Partial Relay Selection With Feedback Delay

We analyze the impact of outdated channel state information due to feedback delay on the performance of amplify-and-forward relays with the kth worst partial relay selection scheme. In our analysis, new expressions for the systems outage probability and the average bit error rate are derived. The effects of the rank of the relay chosen, the average SNR imbalance, and the correlation between the delayed and current signal-to-noise ratio (SNR) on the system performance are investigated. Additionally, simple and accurate outage and average BER approximations are also derived to quantify the performance at high SNR. We also give simulation results to support the theoretical study.

Maximum-likelihood detection in DWT domain image watermarking using Laplacian modeling

Digital image watermarks can be detected in the transform domain using maximum-likelihood detection, whereby the decision threshold is obtained using the Neyman-Pearson criterion. A probability distribution function is required to correctly model the statistical behavior of the transform coefficients. Earlier work has considered modeling the discrete wavelet transform coefficients using a Gaussian distribution. Here, we introduce a Laplacian model and establish via simulation that it can result in a better performance than the Gaussian model.

Distributed Power Control for Spectrum-Sharing Femtocell Networks Using Stackelberg Game

In this paper, we investigate the distributed power allocation strategies for a spectrum-sharing femtocell network, where a central macrocell underlaid with several femtocells. Assuming that the macrocell protects itself by pricing the interference from the femtocells, a Stackelberg game is formulated to jointly consider the utility maximization of the macrocell and the femtocells. Then, the Stackelberg equilibrium for the proposed game is studied, and an effective distributed interference price bargaining algorithm with guaranteed convergency is proposed to achieve the equilibrium. Numerical examples are then presented to verify the proposed studies. It is shown that the algorithm is effective in distributed power allocation and macrocell protection requiring minimal network overhead for spectrum-sharing-based two-tier femtocell networks.

Partial and Opportunistic Relay Selection with Outdated Channel Estimates

This paper investigates the impact of using outdated channel estimates for relay selection and signal amplification on the performance of amplify-and-forward (AF) relays under partial relay selection (PRS) and opportunistic relay selection (ORS). In practice, outdated channel state information (CSI) can occur due to feedback or scheduling delay. Both variable gain (VG) AF and fixed gain AF schemes are considered. Outage probability, the average bit error rate (BER) and simplified high signal-to-noise ratio approximations are derived. The effect of parameters such as the number of relays, the rank of chosen relay, and the correlation between the delayed and current channel state information are analyzed. Outdated CSI for computing relay gains in PRS causes about 2 dB loss. In ORS, a 3% reduction in correlation causes up to an order of magnitude increase in the outage probability.

Interference Channels With Common Information

In this paper, the interference channel with common information (ICC), in which two senders need deliver not only private messages but also certain common messages to their corresponding receivers, is investigated. An achievable rate region for such a channel is obtained by applying a superposition coding scheme that consists of successive encoding and simultaneous decoding. It is shown that the derived achievable rate region includes or extends several existing results for the interference channels with or without common information. The rate region is then specialized to a class of ICCs in which one sender has no private information to transmit, and a class of deterministic interference channels with common information (DICCs). In particular, the derived rate region is found to be the capacity region for this class of DICCs. Last, the achievable rate region derived for the discrete memoryless ICC is extended to the Gaussian case, in which a numerical example is provided to illustrate the improvement of our rate region over an existing result.