An ultra wideband (UWB) communication is a recent hot technique in wireless communication fields. While almost all of the conventional wireless systems including mobile phones, wireless LAN, etc., use narrow frequency bands at most of a few dozen MHz, a UWB system occupies an extremely wide frequency band of more than 500 MHz (actually of GHz order). This is the origin of its name UWB. Therefore, its communication scheme is very different from the conventional ones. The following figure shows images of signal power spectra in UWB and conventional narrowband systems. In a conventional system, its frequency band needs to be narrow enough not to cause interference to/from other users and devices, and its signal power spectral density should be high in order to be robust to noise. On the other hand, in a UWB system, exploiting its very wide occupied bandwidth, the signal power spectral density is set to a significantly low level, which is generally lower than noise power spectral density. Such a low signal level thereby enables us to relieve interference to/from other wireless equipment, so that a UWB communication can be assumed as noise from other conventional systems.

Signal power spectra in UWB and narrowband systems.

UWB communications can be classified into two major methods: multiband-OFDM (MB-OFDM) and UWB impulse radio (UWB-IR). We can say that the former is an expansion of OFDM because it divides the given frequency band into several sub-bands and performs OFDM in each sub-band. Here we would like to explain the latter method UWB-IR. The figure below illustrates a simplified system structure of UWB-IR. At the transmitter, a fast pulse sequence is generated by a pulse generator. After modifying the sequence by variation such as on-off, pulse position, polarization, etc., according to input data, the transmitted signal is modulated by applying an impulse waveform considering the given frequency band to the sequence. A general impulse waveform for UWB has a pulse width of nano-second order, which results in a frequency bandwidth of several GHz. The received signal passes through a correlator with the same impulse waveform to detect the transmitted data. Both the transmitter and receiver in UWB can be simply designed because they do not require carrier modulation/demodulation. Also, fast pulse sequences make the data rate faster. These are benefits of UWB-IR. It should be noted that a UWB system is not intended for long-distance communications due to its low signal power density. So nowadays indoor applications in UWB have been mainly proposed, and a standard for connecting a PC and peripherals in wireless has already adopted a UWB technique.

Principle of UWB-IR.

In our laboratory, in order to integrate technical know-how in vigorously developed MIMO systems into UWB, interference cancellation schemes under multipath environments, channel estimation schemes, transmit timing control schemes, etc., have been studied. Moreover, a UWB system has drawn attention as a location detector because of its fast pulse transmission. We also have started to examine novel location detection schemes exploiting UWB properties.


M. Takanashi, Y. Ogawa, T. Nishimura, and T. Ohgane, "Studies on Channel Estimation Techniques in MIMO-UWB Communications," APWCS2006, pp. 332-336, Aug. 2006.

M. Takanashi, T. Nishimura, Y. Ogawa, and T. Ohgane, "Performance Evaluation of Transmit and Receive Timing Control in LOS MIMO-UWB Environments," WPMC2006, pp. 149-154, Sept. 2006.