Parent and Student Voices on the First Year of the DC Opportunity Scholarship Program

In the 50 years since economist Milton Friedman published “The Role of Government in Education” scholars and policy makers have been debating how parental choice through market mechanisms can and does operate in education. Market “optimists” argue that education is a service that can be produced under a variety of arrangements and that parents are natural education consumers. Market “pessimists” argue that education is a public good that should be produced in government-run schools, and that school choice programs suffer “market failure” because only advantaged families will have the resources and experience to choose effectively

Family Reflections on the District of Columbia Opportunity Scholarship Program: Final Summary Report

During the spring of 2004, the first federally funded voucher program – the District of Columbia Opportunity Scholarship Program (OSP) - was established. The School Choice Demonstration Project (SCDP) recognized that publicly-funded school vouchers represent a relatively new and unstudied approach to school choice and education reform. To address this need, the SCDP requested and received funding from the Annie E. Casey Foundation to capture the “Parent and Student Voices on the OSP.” A total of 110 families, representing 180 students, that applied during the first two years of the Program volunteered to participate in this study. As the last installment in a four-part annual series that began in 2005, this report summarizes key findings from the previous reports and provides a general overview of the respondents’ “reflections” upon their three or four years in the Program. Using a phenomenological approach, which includes focus groups, personal interviews and keypad polling information gathering techniques, participants were given multiple opportunities to share or describe their experiences. A consumer framework was often used to contextualize the families’ experiences. Their insights continue to shape the scope and direction of the OSP, and they will help inform other efforts to provide low income families with access to quality school options

Distributed space-time-frequency block code for cognitive wireless relay networks

In this study, the authors consider cooperative transmission in cognitive wireless relay networks (CWRNs) over frequency-selective fading channels. They propose a new distributed space-time–frequency block code (DSTFBC) for a two-hop non-regenerative CWRN, where a primary source node and multiple secondary source nodes convey information data to their desired primary destination node and multiple secondary destination nodes via multiple cognitive relay nodes with dynamic spectrum access. The proposed DSTFBC is designed to achieve spatial diversity gain as well as allow for low-complexity decoupling detection at the receiver. Pairwise error probability is then analysed to study the achievable diversity gain of the proposed DSTFBC for different channel models including Rician fading and mixed Rayleigh–Rician fading

Efficient cooperative spectrum sensing for three-hop cognitive wireless relay networks

This paper is concerned with cooperative spectrum sensing (CSS) mechanisms in three-hop cognitive wireless relay networks (CWRNs). The data transmission from a source to a destination is realised with the aid of two layers of cognitive radio (CR) users which are in the transmission coverage of two primary users. In this paper, we first propose a new CSS scheme for a layer of CR users to improve the spectrum sensing performance by exploiting both local decisions at the CR users and global decisions at the fusion centre. Particularly, we derive the probabilities of missed detection and false alarm for a practical scenario where all sensing, reporting, and backward channels suffer from Rayleigh fading. The derived expressions not only show that our proposed CSS achieves a better sensing performance than the conventional scheme but also characterise the effects of the fading channels on the sensing reliability. Furthermore, we propose a CSS scheme for two CR layers in a three-hop CWRN using binary XOR operator to help reduce one phase of sensing for a higher system throughput

Cooperative retransmission for wireless regenerative multirelay networks

This paper investigates retransmission (RT) mechanisms in wireless regenerative multirelay networks. Conventionally, the RT can be realized in a cooperative manner with the assistance of all available relays. However, this may result in high overall power consumption due to the RT of the same packets across the nodes, particularly when the number of relays is large. We propose a cooperative RT (CR) scheme based on relay cooperation (RC) and binary xor operations to significantly reduce the number of packets retransmitted to produce a more power-efficient system with nonoverlapped RTs. Significantly, we also derive the error probability of RT decisions at the source and relays and show that the proposed CR scheme improves the reliability of the RTs. Furthermore, by deriving the average number of packets to be retransmitted at the source and relays, we not only show that the proposed CR scheme reduces the number of RTs and removes overlapped retransmitted packets but determine the optimized number of relays used for the RT phase as well. Finally, simulation results are presented to demonstrate the validity of the analytical expressions

Network coding-based block acknowledgement scheme for wireless regenerative relay networks

This study is concerned with block acknowledgement (ACK) mechanisms in wireless regenerative relay networks. In an N-relay network, a total of (2N + 1) block ACK packets is required to acknowledge the data transmission between source and destination nodes via the N-relay nodes. In this study, the authors propose a block ACK scheme based on network coding (NC) to significantly reduce the ACK overheads by N block ACK packets. In addition, this achieves a reduction of N(N–1) computational operators. Particularly, we derive the error probability of the determination of the packets to be retransmitted at the source and relays, which shows that the NC-based scheme also improves the reliability of block ACK transmissions. Furthermore, asymptotic signal-to-noise (SNR) scenarios for forward links are considered and a general expression of error probability in multi-relay networks is derived for each SNR scenario. Finally, simulation results are presented to verify the analytical findings and demonstrate a lower number of data retransmissions for a higher system throughput

An efficient retransmission strategy for multi-source multi-destination relay networks over Rayleigh flat fading channels.

This paper considers the reliable transmission for wireless multisource multidestination relay networks where multiple sources want to distribute information to a set of destinations with the assistance of a relay. Simply, applying automatic repeat request (ARQ) protocols for retransmission of lost or erroneous packets may cause a considerable delay with a significantly increased number of retransmissions when multiple sources and multiple destinations are taken into consideration. To solve this problem, we propose a new ARQ protocol based on network coding (NC) to significantly reduce the number of retransmissions of the lost packets. In our proposed NC-based ARQ protocol, the relay detects packets from different sources, then combines and forwards the packets which are lost at the destinations using NC. In order to guarantee that all lost packets are retransmitted in an efficient way, we propose two packet-combination algorithms for the retransmissions at the relay and sources. Furthermore, we analyze the transmission bandwidth of different ARQ protocols over Rayleigh flat fading channels and provide the numerical results to demonstrate the superior performance of the proposed NC-based ARQ protocol over some existing schemes