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An iterative interference cancellation method for co-channel multicarrier and narrowband systems

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Abstract

Coexistence of narrowband (NB) and multicarrier technologies will be a major concern in next generation wireless communication systems due to the co-channel interference (CCI) problem. In this paper, an efficient CCI cancellation method is proposed that may be utilized for improved coexistence of NB and multicarrier technologies. The method treats both co-channel signals as desired signals and enhances them in an iterative manner. In every iteration, the signals are demodulated, regenerated, and subtracted from the received signal successively in order to obtain a better estimate of the other co-channel signal. Computational complexity of the proposed method is compared in detail with the joint demodulation technique. Through computer simulations, it is shown that the proposed method has lower complexity compared to joint demodulation, and it yields significant gains in the symbol error rate (SER) performance of both the NB and multicarrier systems.

Introduction

Transition from third generation (3G) to the fourth generation (4G) wireless systems is a major challenge that will be faced in the near future. Two different physical (PHY) layer technologies that have a high chance of being employed by next generation systems are Long Term Evolution (LTE) and WiMAX, both of which are multicarrier (MC) systems and can have a bandwidth up to 20 MHz. Relative to these technologies, 3G systems such as EDGE, DECT, CDMA-2000, and even W-CDMA with its 5 MHz bandwidth need to be considered as narrowband (NB) systems. During the transition phase from 3G to 4G, various multicarrier and NB systems might have to share the same spectrum, which will result in a severe performance degradation in both systems due to the co-channel interference (CCI).

Suppression of narrowband interference (NBI) in OFDM systems has already been considered in several works in the prior art [1], [2], [3], [4], [5], [6], [7], [8], [9]. In [1], linear minimum mean-square error (LMMSE) estimates of the interference are utilized. The proposed algorithm requires a priori information about the power spectral density of the NB signal. In [2], a normalized least mean squares (N-LMS) adaptive noise cancellation algorithm is introduced for suppressing NBI in pilot symbol assisted OFDM systems. NBI rejection via interferometry spreading codes is proposed in [3], whereas in [4], [5], a prediction error filter (PEF) is introduced in order to mitigate the effect of narrowband interference in the time domain. The NBI in an OFDM system has been addressed through successive interference cancellation methods in [6], [7]. In [6], assuming that the first subcarrier in consideration is interference-free, an error term is detected and used to mitigate the interference in subsequent subcarriers. This may result in error propagation in subsequent subcarriers in case of any error in the interference estimate. A generalization of the idea in [6] is discussed in [7] using soft decisions of the OFDM symbols. Two different NBI detection and cancellation algorithms using compressive sensing techniques have been proposed in [8], which show important gains in the OFDM bit-error-rate performance with respect to no cancellation. In [9], the NB signal is estimated over the unused OFDM subcarriers to cancel the NBI over the used OFDM subcarriers. The feasibility of this method is limited in practice due to the very few number of unused subcarriers in a well-designed OFDM based system.

In this paper, we treat both co-channel signals as desired signals and propose a method that combats CCI through enhancing both signals in an iterative manner. In the literature, iterative co-channel interference cancellation techniques have been considered in [10], [11], [12], [13], [14], [15], [16], which typically assume narrowband systems and consider that the interferer and victim both use the same technology. In [10], it is emphasized that by exploiting the differences in signal features such as their delays, initial signal separation can be obtained, which considerably increases the efficiency of iterative interference cancellation. In the current paper, we exploit the inherent initial signal separation that exists due to the multicarrier vs. single carrier natures of interfering signals as well as the fact that the information is in frequency domain for MC signal and in time domain for NB signal. The proposed method assumes availability of signal reception and transmission capabilities for both systems. At each iteration, each signal is demodulated and then regenerated based on the symbol decisions and the channel impulse response. This way, a better estimate of the signal is obtained. The regenerated signal is subtracted from the aggregate signal to obtain an estimation of the other co-channel signal. Through extensive simulations, it is proved that this method can provide a fundamental improvement in the performances of both systems in as few as three iterations. The relatively high computational burden (associated with multiple transitions between time and frequency domains) as well as the extra cost caused by the addition of a second system’s transceiver functionalities are compensated by the fundamental performance gain obtained. Our other contributions include a detailed comparison of the computational complexity of the proposed method with the joint demodulation technique and evaluation of the Gaussian approximation (GA) method for characterizing the interference from the other system.

The paper is organized as follows: Section 2 provides application examples and the system models for the MC and NB systems in consideration. Also, it shortly discusses the GA based symbol error rate (SER). Section 3 reviews the joint demodulation technique for the NB and MC signals, while Section 4 is a detailed description of the proposed CCI cancellation method. A complexity comparison of the joint demodulation and iterative interference cancellation approaches is made in Section 5, simulation results are presented in Section 6, and the last section concludes the paper.

Section snippets

Application examples

Earlier examples of coexistence studies in the prior art include [17], [18], which investigate the coexistence of code division multiple access (CDMA) and GSM systems. A contemporary example scenario, where coexistence of NB and multicarrier systems might be unavoidable, is the co-channel deployment of wideband CDMA (W-CDMA) based femtocells with LTE based macrocells, which has not been studied in the literature to the best of our knowledge. Femtocells [19], [20] are miniature cellular networks

Joint demodulation method

A well-known and efficient method for handling co-channel signals is to demodulate them jointly utilizing maximum likelihood estimation [36], [37]. For the coexistence scenario in consideration, ML estimation might be performed either in time domain or in frequency domain. However, time domain requires a smaller number of computations and it is more desirable to perform the ML estimation in time domain. This is due to the relationship between K and the number of NB symbols within the OFDMA

Iterative CCI cancellation method

Considering the apparently high complexity of the ML estimation based joint demodulation method, we propose an efficient but low complexity alternative, which we call iterative CCI cancellation method. The iterative cancellation method solves the co-channel interference problem through enhancing both Y(k) and z(n) in a successive manner in multiple iterations. The iterations get started by obtaining and using an initial estimate of either z(n) or Y(k), which will be denoted as zˆ(n) and Yˆ(k),

Computational complexity

Co-channel interference needs to be cancelled in real time by a mobile station or a base station that is affected by CCI. Therefore, the computational complexity of the cancellation algorithm employed is critical. This section aims to provide a comparison of complexities of the maximum likelihood and the proposed iterative interference cancellation algorithms in terms of the CPU cycle counts required by multiplication (MUL), addition (ADD), and comparison (CMP) operations.

Simulation parameters

Computer simulations are done to determine the performance of the proposed iterative canceler in different scenarios as well as to compare it with the joint demodulation method’s performance. For the simulations, a custom simulator prepared in MATLAB was utilized. The parameters of the OFDMA, NB, and CDMA systems employed in the simulations are presented in Table 3. The OFDMA symbol occupies 400 subcarriers out of 512 available ones due to the guard bands and empty subcarriers. The overlapping

Concluding remarks

In this paper, an iterative CCI canceler is proposed that mitigates the NB interference in multicarrier spectrum as well as the effect of MC signal on NB symbols. Application scenarios are provided where the proposed canceler might be very attractive such as the coexistence of CDMA and OFDMA based systems during the migration from 3G to 4G wireless technologies. It is shown that processing the whole MC band rather than only the overlapping band is more advantageous in spite of the increased

Mustafa E. Şahin received his B.S. degree in Electrical and Electronic Engineering from Boğaziçi University, Istanbul, Turkey, in 2004, and his M.S. and Ph.D. degrees in Electrical Engineering from the University of South Florida, Tampa, FL, USA, in 2006 and 2009, respectively. During his Ph.D. study, he collaborated with researchers in DOCOMO USA Communications Laboratories, Palo Alto, CA. He authored 20 international conference and journal papers and has 6 US patent applications. His research

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    Mustafa E. Şahin received his B.S. degree in Electrical and Electronic Engineering from Boğaziçi University, Istanbul, Turkey, in 2004, and his M.S. and Ph.D. degrees in Electrical Engineering from the University of South Florida, Tampa, FL, USA, in 2006 and 2009, respectively. During his Ph.D. study, he collaborated with researchers in DOCOMO USA Communications Laboratories, Palo Alto, CA. He authored 20 international conference and journal papers and has 6 US patent applications. His research interests include OFDMA-based co-channel femtocells, co-channel interference cancellation in OFDMA, MIMO implementation in WiMAX systems, and spectrum sensing in cognitive radios.

    Ismail Guvenc received his B.S. degree from Bilkent University, Turkey, in 2001, M.S. degree from University of New Mexico, Albuquerque, NM, in 2003, and Ph.D. degree from University of South Florida, Tampa, FL, in 2006 (with Outstanding Dissertation Award from USF Graduate School), all in Electrical Engineering. He was with Mitsubishi Electric Research Labs in Cambridge, MA, in 2005. Since June 2006, he has been with DOCOMO USA Labs, Palo Alto, CA, working as a research engineer. His recent research interests include femtocells, LTE systems, cognitive radio, and UWB communications and localization. He has published more than 50 international conference and journal papers, and several standardization contributions for the IEEE 802.15 and IEEE 802.16 standards. Guvenc has served in the organizing and technical program committees of several international conferences, and co-authored a book on ultra-wideband position estimation. He has over 15 pending US patent applications and he is a member of the IEEE.

    Hüseyin Arslan has received his Ph.D. degree in 1998 from Southern Methodist University (SMU), Dallas, Tx. From January 1998 to August 2002, he was with the research group of Ericsson Inc., NC, USA, where he was involved with several project related to 2G and 3G wireless cellular communication systems. Since August 2002, he has been with the Electrical Engineering Department of University of South Florida. In addition, he has worked as part time consultant for various companies and institutions including Anritsu Company, The Scientific and Technological Research Council of Turkey-TUBITAK, Lecroy, and XG technologies.

    Arslan’s research interests are related to advanced signal processing techniques at the physical layer, with cross-layer design for networking adaptivity and Quality of Service (QoS) control. He is interested in many forms of wireless technologies including cellular, wireless PAN/LAN/MANs, fixed wireless access, and specialized wireless data networks like wireless sensor networks and wireless telemetry. The current research interests are on UWB, OFDM based wireless technologies with emphasis on WIMAX and IMT-Advanced, and cognitive and software defined radio. He has served as technical program committee chair, technical program committee member, session and symposium organizer, and workshop chair in several IEEE conferences. He is a member of the editorial board for “Wireless Communication and Mobile Computing Journal” and “Research Letters in Communications”. Arslan is a senior member of IEEE.

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