1118 IEEE TRANSACTIONS ON SIGNAL PROCESSING, VOL. 55, NO. 3, MARCH 2007 Impacts of Narrowband Interference on OFDM-UWB Receivers: Analysis and Mitigation Kai Shi, Yi Zhou, Burak Kelleci, Timothy Wayne Fischer, Erchin Serpedin, and Aydın ˙ Ilker Kars ¸ılayan Abstract—Orthogonal frequency-division multiplexing (OFDM)-based ultra-wide-band (UWB) transceivers hold the promise to revolutionize the next generation of short-range wireless networks and to be adopted in electronics products for both civil and military applications. For the UWB transceivers to coexist with nearby devices, it is necessary to design efficient UWB receivers whose operation is robust to narrowband interferences (NBI). This paper conducts an in-depth analysis to establish the impacts of NBI on the performance of an OFDM-UWB receiver. A comprehensive study to assess the effects of NBI on the quantization noise in the analog-to-digital converter (ADC), timing, and carrier acquisition is presented. The analytical results show that the efficiency of the ADC is degraded by NBI, although this problem could be slightly remedied by an adaptive autogain controller (AGC). It is also found that, compared with the conventional autocorrelation- based acquisition scheme, the pseudonoise (PN) sequence matched-filtering-based acquisition scheme presents higher robustness to NBI. Nevertheless, both these two acquisition schemes fail at high interference levels. As a conclusion, it is critical to develop novel and low-complexity NBI mitigation schemes for OFDM-UWB receivers that take into account the impacts introduced by NBI. Index Terms—Analog-to-digital converter (ADC), carrier fre- quency offset, narrowband interference, orthogonal frequency- division multiplexing (OFDM), quantization noise, timing syn- chronization, ultra-wide-band. I. INTRODUCTION T HE high-data-rate, low-power, huge spatial capacity, and high precision ranging of ultra-wide-band (UWB) [1], [2] communications promise to address the needs of the quickly growing home networking market that are not currently being met by the existing communication schemes [3]. According to the Federal Communications Commission (FCC) regulations [4], the transmitted power level of UWB systems is limited to 41.3 dBm/MHz and can be spread over a huge bandwidth 7.5 GHz. Such a noise floor operation enables the coexistence of UWB devices with other services such as GPS, IEEE 802.11 Manuscript received September 22, 2005; revised May 17, 2006. This work was supported in part by the NSF Award No. CCR-0092901 and Texas ATP Program. The authors are with the Department of Electrical and Computer En- gineering, Texas A&M University, College Station, TX 77843-3128 USA (e-mail: kaishi05@gmail.com; zhouyi@neo.tamu.edu; burak@ee.tamu.edu; tfischer@tamu.edu; serpedin@ece.tamu.edu; karsilay@ece.tamu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.org. Digital Object Identifier 10.1109/TSP.2006.887153 WLANs, IEEE 802.16 WiMax, etc. Due to their low transmis- sion power and huge reception bandwidth, UWB systems are subject to intentional and unintentional 1 narrowband interfer- ences (NBI) [5]. The scope of this paper is to analyze the impacts of NBI on the performance of an OFDM-UWB receiver [1], [2]. Our contri- butions are complementary to the results reported in [6], where a unifying performance analysis of UWB systems in the pres- ence of NBI was reported for UWB multiple-access schemes encompassing direct-sequence (DS), single-carrier and multi- carrier (SC/MC), and time-hopping (TH) modulations and Rake receivers. This paper provides an in-depth analysis of the impact of NBI on the signal-to-interference-and-noise ratio (SINR) loss at the output of analog-to-digital converter (ADC). The importance of this study is due to the fact that the SINR at the output of ADC serves as a good metric to evaluate the uncoded bit error rate (BER) performance of an OFDM-UWB receiver. It is found that NBI increases significantly the level of quantization noise at the output of ADC. To remedy the loss of effective ADC bit numbers, one might use higher precision ADC. However, due to the huge baseband bandwidth, UWB receivers usually employ a flash data converter, which prevents us from using high bit width. The NBI also introduces tremendous impacts on the timing and carrier synchronization. The author of [7] found that NBI introduces a significant error on the conventional auto- correlation-based timing and carrier frequency offset (CFO) estimators. Our analysis result shows that a PN matched-filter- based timing and CFO estimators present robust performance in case of mid-level NBI. However, as the interference level increases, the performance of the PN matched-filter method also becomes unacceptable. These results suggest us to de- velop novel and low-complexity NBI mitigation schemes for OFDM-UWB receivers that take into account the effects in- duced by NBI. An additional contribution of this paper is the development of a mixed interference mitigation scheme that consists of a digital NBI detector and an adaptive analog notch filter. Our study shows that such mixed schemes could be applied with success to combat strong NBI [i.e., interferences that give rise to signal-to-interference ratios (SIR) less than 0 dB] in OFDM-UWB receivers. Complete information about the circuit level implementation and chip design will be addressed in a separate paper [26]. 1 The radiation level of unintentional NBI is only limited by FCC Part 15 rule to not more than 41.3 dBm/MHz. 1053-587X/$25.00 © 2007 IEEE