Wireless technology has enormous potential to change the way people and things communicate. Future wireless networks will allow people on the move to communicate with
anyone, anywhere, and at any time using a range of multimedia services. Wireless communications will also enable a new class of intelligent home electronics that can interact with
each other and with the Internet. Wireless video will support applications such as distance learning and remote medicine, and self configuring wireless networks will provide the
baseline technology for widespread sensor networks and automated highways. There are many technical challenges that must be overcome in order to make this vision a reality.
These challenges transcend all levels of the overall system design, including hardware, communication link, network, and application design. In addition, synergies between the
design of these different system layers must be exploited to meet the demanding performance requirements of future wireless systems. Professor Goldsmith and her research group
are investigating many of these areas. The interdisciplinary research combines work in wireless channel modeling, information and communication theory, multiuser communications,
signal processing, and wireless network design. The specific research topics currently being addressed are described below.
- Capacity of Wireless Channels and Networks
One of the fundamental issues in the design of a wireless communication s
ystem is how fast data can be transmitted reliably. The theoretical upper bound
for speed of data
transmission over a channel is called the channel capacity. The capacity
of time-varying channels depends on what is known about the channel at the trans
receiver. It also depends on whether rate can be adaptive or constant, as
well as whether or not service outage can be tolerated.
We have investigated capacity and capacity regions of time-varying
multiuser channels, cellular systems, and ad-hoc networks with one
or more antennas at the transmitter(s) and receiver(s).
- Adaptive modulation and coding:
Wireless channels vary over time due to fading and changing interference conditions. Adaptive modulation and coding exploits these variations to
maximize the data rate that can be transmitted over such channels. Such schemes can also be designed in a prioritized manner so that high-priority (or low resolution) bits get
through in all channel conditions, and as the channel improves the lower-priority (high resolution) bits can be transmitted. We are currently looking at adaptive multimedia
modulation techniques, adaptive coded modulation, and adaptive rate,
power, and BER techniques for cellular and ad-hoc networks.
The goal of these adaptive schemes is to optimize performance in the
face of randomly changing channel and interference conditions.
- Multiantenna systems
Multiple antennas are known to significantly increase the capacity of wireless systems. We are investigating multiple facets of using multiple antennas in a wireless system
design. Specifically, we are looking at virtual MIMO in cellular systems with base station clustering as well as MIMO ad-hoc and mesh networks. One specific focus of our study is the best use of multiple antennas in wireless networks with frequency reuse, as in this setting antennas can be used for spatial multiplexing, diversity, and/or interference cancellation. We are characterizing the tradeoff regions for MIMO wireless networks and optimizing the operating point on these regions for specific applications.
- Multiple Access and Dynamic Resource Allocation
Sharing the limited spectrum within and between different multiuser systems can be done through a variety of techniques: the most common are frequency division, time
division (or multiplexing), spread spectrum coding, and hybrid combinations of these techniques. In addition, the spectrum can be more efficiently utilized by reusing
frequencies at spatially separated locations - this is the idea behind cellular communication systems, but can also be used in ad-hoc networks. Optimal ways to share the
spectrum and reduce interference through power control, dynamic rate adaptation, and dynamic channel allocation in both cellular and ad-hoc networks are also being looked
- Joint Source/Channel Coding
Another ongoing area of research is joint source/channel coding. Shannon's fundamental theorem showing that source coding and channel coding can be separated without
any loss of optimality does not apply to general time- varying channels, or to systems with a complexity or delay constraint (i.e. any real system). Since distortion by the
source encoder decreases with data rate, while channel errors increase with data rate, the joint source/channel coding problem reduces to allocating bits in an optimal way
between the source and channel encoders. This optimal allocation will depend on the source data (voice, video, images, etc.) as well as on the channel characteristics. Joint
source/channel coding techniques for multimedia data and distributed source coding for multiantenna systems are our current focus in this area. We are also looking at
developing a generalized source/channel separation theorem for time-varying channels.
- Wireless Networks
The nature of wireless channels suggests that the infrastructure to support cellular, personal, and indoor wireless communication systems might differ significantly from the
Internet and other hard-wired systems. Issues of access, resource allocation, routing, mobility management, network security, and network control may borrow from the
standards of wireline networks, but must also take into account the unique character of terminal mobility and the randomly-varying channel.
- Energy-Constrained Networks
Many types of wireless networks, like sensor networks, have nodes with a finite battery life. Once these batteries die away the node is gone, and can no longer perform its
intended function (e.g. data collection) nor participate in forwarding information from other nodes. This provides an interesting challenge in the optimization of such
energy-constrained networks, since energy-conservation is not typically included in network protocol design. We are investigating the capacity limits of
energy-constrained networks as well as optimal routing protocols that maintain full connectivity within the network for as long as possible.
- Wireless Communications for Control Applications
Many automated systems today require distributed control where the control commands and observations are transmitted via
wireless links (e.g. automated highway systems). However, most distributed controllers can experience catastrophic failure due to delayed or lost packets, which are common on
wireless links. In this research we are investigating optimal joint designs of distributed controllers and the wireless networks that connect the controllers. The goal is to insure
stability under all operating conditions and maintain the tightest control parameters given the operating constraints of the communications network.
- Cross-Layer Design in Wireless Networks
While a layered network protocol stack helps to reduce design complexity, it leads to highly suboptimal designs, especially when the underlying channels, networks, and sources are time-varying. We are investigating cross-layer design protocols that define a rich interface between protocol layers with dynamic information exchange and adaptivity optimized for system dynamics. This approach is particularly relevant to applications with stringent constraints such as video over wireless and sensor networks.