Ultrawideband Communications

At Oxford, we are currently working on applications for these very wideband signals in communications and medical imaging.

UWB Medical Imaging

Recently there has been a lot of interest in the development of novel medical imaging techniques based on UWB signals. The relative dielectric properties of living tissues are very varied offering large contrasts to this kind of imaging modlaity. In particular breast cancers have been shown to have dielectric constants and RF conductances that are more than a factor of 5 greater than normal healthy breast tissues.

The objectives of this project are

• To develop optimised antennas and transducers for medical imabing in appropriate immersion fluids (oils and saline solutions).
• To develop realistic UWB tissue simulant materials to enable construction of useful phantoms for further development of imaging.
• To demonstrate imaging acquisition using scanned antennas and switched antenna arrays with realistic phantoms.
• To explore physical optics based 3D imaging algorithms for conversion of measured phase and amplitude data into useful tomographic images.

UWB Propagation

The UWB propagation channel differs from the narrowband and wideband channel in its fundamental characteristics that are of great interest in communications. We are undertaking a collaborative EPSRC-funded research project on various aspects of the UWB communications channel. As a next step, we have applied this channel analysis to improved system design.

The major objectives of the propagation research are

• Diffraction, reflection, refraction and path loss characteristics in terms of time, frequency and range will be studied and models developed
• Polarisation behaviour through walls and structures of differing material content will be studied
• Spatio-temporal characteristics will also be studied with a view to identifying advanced antenna configurations for diversity, adaptive systems and MIMO type operation.
• Code families/pulse waveforms will be studied and simulated within the real channel data to ascertain an optimum waveform and modulation scheme.
• Capacity limits for communications will be ascertained based on the channel behaviour and the candidate modulation schemes.

UWB Communications Project

The Ultra Wide Band (UWB) field originated from work in time-domain electromagnetics that began in 1962 to fully describe the transient behaviour of a certain class of microwave networks through their characteristic impulse response. Once impulse measurement techniques were applied to the design of wideband, radiating antenna elements, it quickly became obvious that short pulse radar and communications systems could be developed with the same set of tools. The invention of a sensitive, short pulse receiver to replace the cumbersome time-domain sampling oscilloscope further accelerated system development. Prior to 1994, much of the early work in the UWB field, particularly in the area of impulse communications, was performed under classified programs. Since that time the development of UWB has greatly accelerated in the commercial arena, although there is a relative lack of fundamental research in support of it.

Ultra-wideband applications include through-the-wall imaging that can aid rescue workers and law enforcers. It can also be used for collision-avoidance radars to reduce auto accidents. For personal area networking, it can provide wireless links among camcorders, personal computers, DVD players, flat-screen television displays, printers, MP3 players and other devices, but UWB devices, which potentially support data rates in excess of 1 Gb/s, pose a range of regulatory concerns. In these realisations, the UWB transmission takes the form of noise-like signals at or below the background noise floor. They work across an unprecedented range of the radio spectrum, which is already licensed to a large number of government and commercial users. A key concern from other spectrum users is the interference that UWB transmissions potentially cause.

Thus, in support of the rapid development of UWB wireless technology, it is proposed here to investigate and model the behaviour of the ultra wide band propagation channel. This proposal deals with studying the behaviour of the frequency, time and spatial dependent characteristics of the channel. To date this behaviour has been neglected since transmissions have typically occupied a bandwidth of less than 3% of the carrier frequency. There is much recent interest in UWB radio systems that are characterised by having a transmission bandwidth of more than 500MHz and is larger than 20% of the centre frequency. Thus the wide band behaviour of the radio channel can no longer be neglected. To this end, a thorough investigation of this behaviour is required and an ultra wideband channel model needs to be constructed that encompasses this behaviour.

Currently, regulatory authorities around the world are reviewing the viability of UWB systems in the shared frequency spectrum. Consequently there is currently much interest in UWB and the technology is under scrutiny to justify the various claims that have been promulgated over the years. The Federal Communications Commission (FCC) approved limited deployment in February 2002. Because of the limits the FCC has placed on transmitting power, UWB signals can carry for only about 10m. This restricts the application of UWB technology to short range applications.

The objectives of the research are:

• Diffraction, reflection, refraction and path loss characteristics in terms of time, frequency and range will be studied and models developed
• Polarisation behaviour through walls and structures of differing material content will be studied
• Spatio-temporal characteristics will also be studied with a view to identifying advanced antenna configurations for diversity, adaptive systems and MIMO type operation.
• Code families/pulse waveforms will be studied and simulated within the real channel data to ascertain an optimum waveform and modulation scheme.
• Capacity limits for communications will be ascertained based on the channel behaviour and the candidate modulation schemes.