Interference Mitigation Technique for Coexistence of Pulse-Based UWB and OFDM

Hindawi Publishing Corporation EURASIP Journal on Wireless Communications and Networking Volume 2008, Article ID 285683, 11 pages doi:10.1155/2008/285683 Research Article Interference Mitigation Technique for Coexistence of Pulse-Based UWB and OFDM Kohei Ohno and Tetsushi Ikegami Department of Electronics and Communications, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan Correspondence should be addressed to Kohei Ohno, Received 31 May 2007; Revised 16 December 2007; Accepted 4 February 2008 Recommended by Ryuji Kohno Ultra-wideband (UWB) is a useful radio technique for sharing frequency bands between radio systems. It uses very short pulses to spread spectrum. However, there is a potential for interference between systems using the same frequency bands at close range. In some regulatory systems, interference detection and avoidance (DAA) techniques are required to prevent interference with existing radio systems. In this paper, the effect of interference on orthogonal frequency division multiplexing (OFDM) signals from pulsebased UWB is discussed, and an interference mitigation technique is proposed. This technique focuses on the pulse repetition cycle of UWB. The pulse repetition interval is set the same or half the period of the OFDM symbol excluding the guard interval to mitigate interference. These proposals are also made for direct sequence (DS)-UWB. Bit error rate (BER) performance is illustrated through both simulation and theoretical approximations. Copyright © 2008 K. Ohno and T. Ikegami. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION Spectrum sharing technologies are attractive since there is a real lack of frequency bands for radio systems. Cognitive radio is one approach to coexisting radio systems. Ultrawideband (UWB) is also able to share spectrum with other systems by spreading spectra extremely widely [1]. However, in UWB systems, a potential for interference exists when systems operate in the same frequency band. The Federal Communication Commission (FCC) allocated a frequency band for UWB from 3.1 GHz to 10.6 GHz and determined transmission power to be a maximum of −41.3 dBm/MHz in 2002 [2]. Detection and avoidance (DAA) techniques are required in both Japanese and European regulations to emit −41.3 dBm/MHz in the 4 GHz band [3, 4]. The effect of interference from UWB on narrow band systems has been evaluated by hardware experiments and computer simulations [5–7]. In multiband-orthogonal frequency division multiplexing (MB-OFDM) UWB systems, interference is detected using FFTs in the OFDM receiver. Null subcarriers are used for interfering bands [8]. Adaptive pulse waveform techniques are investigated as interference mitigation techniques in pulse-base UWB systems. UWB pulses consist of several narrow pulses that are combined to suppress an interfering band spectrum [9, 10]. Different interference characteristics are reported with changing the pulse repetition frequency and the center frequency of narrow band systems [7]. Low duty cycle (LDC)-UWB is recognized by European regulation as a DAA technique, since the average power is reduced by determining the maximum peak power [4]. Critical interference mitigation techniques are less favored. It is necessary to consider power consumption and transmitter-receiver hardware size for potential UWB system applications when DAA techniques are investigated. The effect of interference from UWB on various kinds of systems is investigated, and a multicarrier type template wave to mitigate the influence of IEEE802.11a interference is proposed [11]. The proposed template is effective not only for narrowband interference such as that produced by existing wireless LAN systems, but also for wideband interference such as that produced by MB-OFDM. This is achieved using a multicarrier template and hopping band detection [12]. The technique can be also applied to the DAA technique [13]. In this paper, a technique to mitigate interference on OFDM signals from pulse-based UWB (p-UWB) is examined using a physical layer approach. The proposed system focuses on pulse repetition interval in UWB assuming a 2 EURASIP Journal on Wireless Communications and Networking Tx Primary modulation mapping Data Serial to parallel IFFT Parallel to serial DAC BPF Carrier frequency Parallel to serial De-mapping Data FFT Serial to parallel Rx ADC BPF Carrier frequency Figure 1: OFDM transmitter-receiver structure. simple transmitter-receiver structure and a low-data rate personal area network (PAN) system. OFDM signals have a common modulation scheme for high-data rate wireless systems such as wireless LANs or mobile systems. In this paper, direct sequence (DS)-UWB is also discussed in relation to the effectiveness of the proposed mitigating methods. This paper is organized as follows. In Section 2, the system models of UWB and OFDM are explained. In Section 3, Section 3.1, simulation results for the pulse repetition cycle are shown. The mechanism for the proposed interference mitigation technique and discussion of simulation results are considered in Section 3.2. In Section 4, the proposed interference mitigation technique is applied to a DS-UWB system. 2. SYSTEM MODEL sofdm (t) = Re ∞   A pulse-based UWB (p-UWB) signal can be expressed: ∞    di s0 t − iTr , (1) i=0 where Tr is the pulse repetition interval, i denotes ith pulse, di is modulated data, and s0 (t) is the UWB pulse waveform, such as monocycle, sinusoidal wave enveloped with various waveforms, or differentials of Gaussian functions. Here, UWB pulse bandwidth is assumed to be wider than OFDM signals, and Bi-Phase modulation is adopted. Thus, di denotes +1 or −1. 2.2. OFDM OFDM is a common modulation scheme. It is used for many wireless systems, for example, wireless local area     sB t − hTSYM ·w t − hTSYM · exp − j2π fc t   , h=0 ⎧ ⎨1 w(t) = ⎩ 0   TGI < t < TFFT + TGI ,   t < TGI , t > TFFT + TGI , (2) sB 2.1. Pulse-based UWB suwb (t) = networks (LANs). OFDM is also expected to be a next generation mobile and wireless metropolitan area network (MAN) system since it has many advantages in bandwidth, transmission rate, and antimultipath effect, and so forth. A typical ODFM signal can be expressed: kTFFT N = N −1 l=0   dcl + jdsl exp j2πkl , N (3) where, N is the number of subcarriers, fc is a carrier frequency, and l and h are lth subcarriers in the hth symbol, respectively. dcl and dsl are transmitting data after primary modulation. TFFT and TGI are the IFFT/FFT period and guard interval duration, respectively. TSYM is symbol duration including TFFT , TGI , and a cyclic prefix duration Tcp . w(t) denotes a window function for IFFT. The window function is assumed to be rectangular, that is (...truncated)