UWB (Ultra Wideband:超広帯域) 通信は、現行で利用されている無線通信のほとんど (携帯電話、無線LANなど) が狭帯域通信といわれているのに対して名付けられたもので、極めて広い帯域を使用する通信方式です。従来の無線通信では、その占有帯域がせいぜい数十MHzなのに対して、UWB通信では500MHz以上の帯域 (実際にはGHzのオーダー) を占有するため、従来の通信とはまったく異なる概念で通信を行います。下図に、UWB通信と狭帯域通信の信号電力スペクトルを示します。狭帯域通信では、占有帯域が狭い替わりに送信電力密度を大きくしています。これに対して、UWB通信では占有帯域が広いため非常に小さな送信電力密度となり、一般に雑音電力レベルよりも小さく設定されています。そのため、狭帯域通信では必須であった他の通信機器との干渉に対する配慮が不要で、既存の狭帯域通信からはUWB通信は単なる雑音にしか見えません。
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.
UWB通信と狭帯域通信の信号電力スペクトル
Signal power spectra in UWB and narrowband systems.
UWB通信を実現するための方式には、大きく分けてMB-OFDM (Multiband-OFDM) 方式とUWB-IR (Impulse Radio) 方式の2つがあります。前者は、占有帯域を複数の帯域 (サブバンド) に分け、各サブバンドでOFDM伝送を行う方式で、OFDMを拡張したものと言えます。ここでは後者のUWB-IR方式について説明します。UWB-IRの簡単な構成図を下に示します。送信機ではパルスジェネレータを用いて高速のパルス列を発生させます。このパルス列に送信データに応じた変化 (オンオフ、パルス位置、極性など) を加えることで送信信号を生成します。この際、占有帯域を考慮したインパルス波形を適用します。一般的なインパルス波形のパルス幅は数ナノ秒程度で、その帯域幅は数GHzになります。受信機では、送信機で用いたインパルス波形と受信信号との相関を取ることで復調が可能になります。狭帯域通信では必ず必要となるキャリア変調が必要がないため、送受信機ともに簡単な構成で済むうえ、高速パルスを使用することによる高速通信を実現できるのがUWB-IRの利点です。ただし、送信電力密度が小さいため遠距離の通信には向かず、主に室内でのアプリケーションが提案されています。既に、パソコンと周辺機器を無線で接続するための規格にUWB通信が採用されています。
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.
UWB-IRの原理
Principle of UWB-IR.
当研究室では、これまで精力的に研究が進められてきたMIMO技術などのノウハウをUWBに融合するべく、マルチパス環境下での干渉除去法、チャネル推定法、および送信タイミング制御法などについて検討しています。さらに、UWB通信では高速パルスを用いることから高精度の位置検出が可能であると言われており、その特性を利用した新しい位置検出手法についても検討を行っています。
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.
[文献]
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.
[Reference]
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.