Peter Shu Shaw Wang

Cross Layer Design for OFDMA Systems Using the Beta-Min-Sum Belief Propagation Algorithm

This paper investigates the problem of resource allocation and scheduling for an orthogonal frequency division multiple access (OFDMA) broadband wireless network We formulate a utility-based cross-layer resource management framework that exploits instantaneous channel state information at the base station in order to schedule traffic during each frame. During each decision epoch, the objective is to find an efficient and fair rate and sub-carrier allocation policy that maximizes the average network utility subject to certain constraints. We develop an equivalent graph theoretical model for the cross-layer assignment problem and show its equivalence to the classical problem of finding the maximum weight matching (MWM) on a bipartite graph. This is a well-studied problem in classical graph theory and several well-known solutions exist. For the optimal assignment, we consider the Hungarian algorithm - which always achieves the maximal network utility - and compare its performance to a sub-optimal modified min-sum belief propagation algorithm. Through simulations, we demonstrate the negligible difference in performance between the two.

Utility-based cross layer optimization for OFDMA systems using the β-min-sum belief propagation algorithm

This paper investigates the problem of resource allocation and scheduling for an orthogonal frequency division multiple access (OFDMA) broadband wireless network. We formulate a utility-based cross-layer resource management framework that exploits instantaneous channel state information at the base station in order to schedule traffic during each frame. During each decision epoch, the objective is to find an efficient and fair rate and sub-carrier allocation policy that maximizes the average network utility subject to certain constraints. We develop an equivalent graph theoretical model for the cross-layer assignment problem and show its equivalence to the classical problem of finding the Maximum Weight Matching (MWM) on a bipartite graph. This is a well-studied problem in classical graph theory and several well-known solutions exist. For the optimal assignment, we consider the Hungarian algorithm - which always achieves the maximal network utility — and compare its performance to a sub-optimal modified min-sum belief propagation algorithm. Through simulations, we demonstrate the negligible difference in performance between the two.

A full rate dual relay cooperative approach for wireless systems

Cooperative relaying methods have attracted a lot of interest in the past few years. A conventional cooperative relaying scheme has a source, a destination, and a single relay. This cooperative scheme can support one symbol transmission per time slot, and is called full rate transmission. However, existing full rate cooperative relay approaches provide asymmetrical gain for different transmitted symbols. In this paper, we propose a cooperative relaying scheme that is assisted with dual relays and provides full transmission rate with the same macro-diversity to each symbol. We also address equalization for the dual relay transmission system in addition to addressing the issues concerning the improvement of system performance in terms of optimal power allocations.

The Impact of Imperfect Channel State Information on QRD-Based Precoded MIMO-OFDM System

A new closed-loop preceding method has been proposed for MIMO-OFDM systems with limited feedback by using the QR decomposition to the time-domain channel impulse response matrix. In contrast to the other methods where the receiver has to feed back full precoder index of each subcarrier or reduced index for a group of subcarriers to the transmitter, the GMD-based scheme can substantially reduce the amount of feedback information by returning only one precoder index to the transmitter, regardless of the number of subcarriers and channels. With this approach, the amount of required feedback for the proposed scheme does not grow with the number of subcarriers. This paper considers imperfect channel state information in the transmitter due to a feedback delay in the system.

Error Rate Performance in OFDM-Based Cooperative Networks

Cooperative relay networks have been shown to improve performance in wireless communication systems as a form of spatial diversity. In this paper, we investigate the error rate performance in a single path relay network and a multiple path relay network using orthogonal frequency division multiplexing (OFDM) signals. Using the amplify-and-forward and decode-and-forward relay algorithms, we derive input-output relations for the two networks. For the amplify-and-forward case, we consider two relay power allocation schemes. The first is constant gain allocation, where the amplifying gain is constant for all subcarriers. The second is equal power allocation, where each subcarrier transmits the same power. The former scheme does not require channel state information (CSI), while the latter one does. For the decode-and-forward case, the transmitter and each relay are assumed to have uniform power allocations. We simulate the word error rate (WER) performance for the two networks. For the single path relay network, amplify-and-forward gives very poor performance, because as we increase the distance between the transmitter and receiver (and thus, add more relays), more noise and channel distortion enter the system. Decode- and-forward gives significantly better performance because noise and channel distortion are eliminated at each relay. For the multiple path relay network, decode-and-forward again gives better performance than amplify-and-forward. However, the performance gains are small compared to the single path relay network case. Therefore, amplify-and-forward may be a more attractive choice in this case due to its lower complexity.

Equalization for symmetric cooperative relay scheme for wireless communications

Cooperative relaying methods have attracted a lot of interest in the past several years. A conventional cooperative relaying scheme has a source, a destination and a single relay. This cooperative technique can support one symbol transmission per time slot, called full rate transmission. However, existing full rate cooperative relay approaches have asymmetrical diversity gains for different symbols. Another approach is to use dual relays to overcome these effects. In this paper, we propose the design of equalizers for the cooperative relaying scheme that is assisted with dual relays providing full transmission rate and same macro-diversity to each symbol. The equalization of received symbols is carried out using a minimum mean squared error (MMSE) linear equalizer (LE) and decision feedback equalizer (DFE) which provide inter-symbol-interference (ISI) cancellation capabilities.

A dual relay transmission system for wireless communications: Power allocations and channel capacity

Cooperative relaying methods have attracted a lot of interest in the past several years. A conventional cooperative relaying scheme has a source, destination and a single relay. To overcome the effects of asymmetrical nature of one relay network, we have proposed a dual relay system that achieves significant system performance in terms of bit-error rate, signal-to-noise ratio and channel capacity over the conventional one relay and direct transmission systems. This paper presents the protocol of the dual relay system and addresses the issues of optimal power allocations at the terminals and the increase of system performance in terms of channel capacity.

Distributed LDPC coders in cooperative relay networks

In this paper, a new distributed LDPC coder is developed in the cooperative relay networks. Although the proposed scheme requires to use the same bandwidth as the conventional approach over OFDM based transmission, the use of the hybrid ARQ can be reduced. In addition, the overall performance can be improved using the proposed approach.

Optical filter elements based on waveguide gratings

The guided-mode resonance properties of planar dielectric waveguide gratings are presented and explained. It is shown that these structures function as filters producing efficient exchange of energy between forward and backward propagating diffracted waves with smooth lineshapes and arbitrary filter linewidths. Simple expressions based on classical slab waveguide theory give a clear view and quantification of the inherent TE/TM polarization separation and the free-spectral ranges of the filters. Furthermore, the resonance regimes, defining the parametric regions of the guided-mode resonances, can be directly visualized. It is shown that the linewidths of the resonances can be controlled by the grating modulation amplitude and by the degree of mode confinement (refractive-index difference at boundaries). Examples presented of potential uses for these elements include a narrow-line polarized laser, tunable polarized laser, photorefractive tunable filter, and an electro-optic switch. The guided-mode resonance filter represents a basic new optical element with significant potential for practical applications.

Power allocation for OFDM-based cooperative relay systems

Cooperative relays can provide spatial diversity and improve performance of wireless communications. In this paper, we study subcarrier power allocation at the relays for orthogonal frequency division multiplexing (OFDM)-based wireless systems. For cooperative relay with amplify-and-forward(AF) and decode-and-forward (DF) algorithms, we investigate the impact of power allocation to the mutual information between the source and destination. From our simulation results on word-error-rate (WER) performance, we find that the DF algorithm with power allocation provides better performance than that of AF algorithm in a single path relay network because the former is able to eliminate channel noise at each relay. For the multiple path relay network, however, the network structure is already resistant to noise and channel distortion, and AF approach is a more attractive choice due to its lower complexity.