Woong Cho

Optimum Resource Allocation for Amplify-and-Forward Relay Networks With Differential Modulation

The optimum resource allocation is an important method to improve the error performance and energy efficiency of wireless relay networks. In this paper, we consider the resource allocation as a two-dimensional optimization problem; that is, the energy optimization and the location optimization. Differential modulation which bypasses the channel estimation at the receiver is investigated using the amplify-and-forward protocol for a relay system with arbitrary number of relays. At high signal-to-noise ratio (SNR), we first derive the average symbol error rate for systems with and without a direct link. Then, the optimum resource allocation schemes which minimize the system error are developed. The comparisons based on analytical and simulated results confirm that the optimized systems provide considerable improvement over unoptimized ones, and that the minimum error can be achieved via the joint energy-location optimization.

Optimum Resource Allocation for Relay Networks with Differential Modulation

In this paper, we investigate the resource allocation in a differentially modulated relay network. In addition to the energy optimization, we also consider location optimization to minimize the average symbol error rate (SER). The closed- form solution is derived for the single-relay case, and formulas allowing numerical search are provided for multiple-relay cases. Analytical and simulated comparisons confirm that the optimized systems provide considerable improvement over the unoptimized systems, and that the minimum SER can be achieved via the joint energy-location optimization.

Distributed Differential Schemes for Cooperative Wireless Networks

In cooperative wireless networks, virtual antenna arrays formed by distributed network nodes can provide cooperative diversity. Obviating channel estimation, differential schemes have long been appreciated in conventional multi-input multi-output (MIMO) communications. However, distributed differential schemes for general cooperative network setups have not been thoroughly investigated. In this paper, we develop and analyze two distributed differential schemes using both decode-and-forward (DF) and amplify-and-forward (AF) relaying protocols. For each scheme and relaying protocol combination, we derive the optimum maximum likelihood (ML) decision rule and its low-complexity suboptimum alternative. Simulations confirm that both schemes provide full diversity gain with either DF or AF relaying protocols. In addition, we carry out performance and rate comparisons between the two distributed differential schemes and preliminary investigations on the optimal relay positioning for the DF and AF relaying protocols

Joint Energy and Location Optimization for Relay Networks with Differential Modulation

The optimum resource allocation in communication systems is critical to enhance their performance and efficiency. In wireless networks, relay transmissions can enable cooperative diversity by forming virtual antenna arrays. In this paper, we consider resource allocation which minimizes the average system error rate not only by the power optimization, but also by the location optimization for systems with arbitrary number of relays. Differential modulation which bypasses the channel estimation at the receiver is investigated using the decode-and-forward protocol. Analytical and simulated comparisons confirm that the optimized systems provide considerable improvement over un-optimized ones, and that the minimum error rate can be achieved via joint energy-location optimization.

Resource Allocation for Amplify-and-Forward Relay Networks with Differential Modulation

The optimum resource allocation in relay networks has been treated as an important problem to improve the error performance and increase the energy efficiency. In this paper, a two-dimensional resource allocation, i.e., the energy optimization and location optimization, is carried out based on the average symbol error rate (SER) for the system with and without a direct link. Differential modulation which bypasses the channel estimation at the transceiver is investigated using amplify-and- forward protocol for the system with multiple relays. The benefits of optimization are validated by the analytical and simulated comparisons. We also show that the minimum error rate can be achieved via the joint energy-location optimization.

Optimum Energy Allocation for Cooperative Networks with Differential Modulation

The development of relay transmissions have recently attracted increasing interests in wireless communications since they provide cooperative diversity gains by forming virtual antenna arrays. In such relay networks, optimum energy allocation is critical to enhance their performance and efficiency. However, existing works on performance analysis and optimization mainly focus on single-relay systems. In this paper, an upper bound of the error performance is derived with arbitrary number of relays. Based on this bound, we then develop the optimum energy allocation schemes that minimize the average system error. Equally attractive is that our investigation is tailored for differential modulations, which bypass the channel estimation at the receiver and are particularly suitable for wireless relay networks. The performance improvement of the optimized systems over the un-optimized ones is confirmed by analytical and simulated comparisons

Performance analysis of cooperative networks with differential unitary space time coding

Multi-input multi-output (MIMO) scheme in communication systems enhances the system performance and capacity. In wireless communications, cooperative networks can provide spatial diversity gain by creating virtual antenna arrays. In this paper, we consider cooperative networks adopting the differential unitary space time code (DUSTC) which bypasses the channel estimation at the receiver. With high signal-to-noise ratio (SNR), the codeword error rate (CER) of these systems is analyzed using both decode-and- forward and amplify-and-forward relaying protocols. The effect of link quality on the error performance is also investigated. Using these results, the comparison between the STC-based and conventional cooperative networks, i.e, repetition-based cooperative system, is addressed.

TwinsNet: a cooperative MIMO mobile sensor network

A distributed sensor network with mobility provides an ideal system platform for surveillance and for search and rescue applications. We consider a system design consisting of a set of autonomous robots communicating with each other and with a base station to provide image and other sensor data. A robot-mounted sensor which detects interesting information coordinates with other mobile robots in its vicinity to stream its data back to the base station in a robust and energy-efficient manner. The system is partitioned into twin sub-networks in such a way that any transmitting sensor will pair itself with another nearby node to cooperatively transmit its data in a multiple-input, multiple-output (MIMO) fashion. At the same time, other robots in the system will cooperatively position themselves so that the overall link quality is maximized and the total transmission energy in minimized. We efficiently simulate the system’s behavior using the Transaction Level Modeling (TLM) capability of SystemC. The simulation results demonstrate the utility of our design and provide insights into performance of the system.

Optimum Energy Allocation in Cooperative Networks: A Comparative Study

Cooperative networks enjoy spatial diversity gains by forming virtual antenna arrays. To improve the performance of such cooperative networks, optimum energy (power) allocation has emerged as an important research topic. In our previous work, we have investigated the decode-and-forward (DF) protocol. In this paper, the more challenging amplify-and-forward (AF) scenario is considered. Differential modulation which bypasses channel estimation is adopted to reduce receiver complexity and communication overhead. For the AF relay system with and without a direct link between the source node and destination node, we first derive simple and general expressions of the average symbol error rate (SER), at reasonably high SNR. Based on these, the optimum energy distributions which minimize the average SER are obtained. The advantages of the energy optimization are confirmed by analysis and simulations. Using these results, we compare the systems with and without a direct link as well as systems with different relaying protocols.

Differential Modulation Schemes for Cooperative Diversity

Recent developments of wireless networks show that cooperative diversity can produce multi-input multi-output (MIMO) advantages by forming a virtual antenna array with distributed network nodes. Although differential schemes have long been appreciated in conventional MIMO communications since they provide the benefit of simplicity at the receiver by avoiding channel estimation, their performance and possible benefits in the distributed scenario have not been thoroughly investigated. In this paper, we examine two differential schemes for a cooperative diversity setup using a decode-and-forward relaying protocol. The first scheme (scheme I) is a direct extension of the conventional differential scheme for a single-input single-output (SISO) channel; whereas the second (scheme II) is a distributed cooperative counterpart of differential space-time coding (STC). We develop the maximum likelihood (ML) decision rules for two cases, in which the effect of the relay decision is captured by the transition probability. Simulations confirm that both schemes provide full diversity gain. Comparisons of the two schemes show that scheme I outperforms scheme II in terms of symbol error rate (SER); while scheme II can support a higher transmission rate, and this rate advantage increases as the number of relays increases