An Experimental Study of ICI Cancellation in OFDM Utilizing GNU Radio System

ICWMC 2011 : The Seventh International Conference on Wireless and Mobile Communications An Experimental Study of ICI Cancellation in OFDM Utilizing GNU Radio System Miftahur Rahman, Sajjadul Latif, and Tasnia Ahad Dept. of Electrical Engineering and Computer Science North South University Dhaka, Bangladesh e-mails: [email protected], [email protected] and [email protected] Abstract— Inter-Carrier-Interference (ICI) is an effect that technique used; Section VIII explains software and hardware noticeably degrades the quality of the Orthogonal Frequency setup; Section IX describes the data retrieval process; Division Multiplexing (OFDM) signal. In this work, several Section X analyses the results; Section XI discusses the proposed ICI cancellation schemes have been tested using open future scope of the technology and Section XII makes the source Software Defined Radio (SDR) named GNU Radio. The concluding remarks. GNU Radio system used in the experiment had one Universal Software Radio Peripheral (USRP) module connected to a II. OFDM MODULATION & DEMODULATION computer. The USRP had two-daughterboard (RFX-400) for both transmission and reception of radio signals in the 400 OFDM is a technique as shown in Fig. 1, where the input MHz band. The input data to the USRP was prepared in data is converted to parallel bits and mapped according to compliance with IEEE-802.11b specification. The experimental predefined standard [9]. Inverse Fast Fourier Transform results were compared with the theoretical results of the (IFFT) is a vital part to convert signal from frequency proposed Inter-Carrier Interference (ICI) cancellation domain to time domain. After IFFT the parallel data is again schemes. The comparison of the results revealed that the new converted to serial data. Cyclic prefix is also added before it schemes are suitable for high performance transmission. The gets converted from digital to analog data. The input data results of this paper open up new opportunities of using should be prepared maintaining specific standard. According OFDM in 400MHz band, where channels are heavily to IEEE-802.11b specification [9], the data is transferred congested. The new ICI cancellation schemes can be used for using 64 subcarriers. In the IFFT mapping, the total 64 Digital TV applications or for secured government subcarriers in frequency domain are converted to time communication services in the 400MHz to 500MHz band. domain. In order to preserve the orthogonality of OFDM signal, preamble bits [9] are added. Also the cyclic prefix Keywords-OFDM; USRP; GNU radio; ICI cancellation; enables synchronization as the bits are used to detect the 400MHz, software defined radio; beginning and end of each frame and it appends the OFDM symbols one after another [9]. I. INTRODUCTION The received signal is demodulated according to the steps shown in Fig. 2, which is the opposite of OFDM modulation The OFDM is a technique, where a large number of technique shown in Fig. 1. orthogonal, overlapping subcarriers are transmitted in parallel, dividing the available transmission bandwidth into Serial narrowband sub-channels [1, 2, 3]. The separations of the Input Signal To IFFT subcarriers are kept as minimal as possible to get a very Data Mapper Parallel compact spectral utilization. Due to the use of narrow bandwidth, each sub channel requires a longer symbol period. The orthogonality of the OFDM is lost when there Digital To Add Parallel are channel impairments and frequency mismatch in the Analog (D/A) Cyclic To Converter Extension Serial transmitter and receiver. This leads to inter-carrier interference, phase rotation and performance degradation of Figure 1. OFDM Modulation the channel [4]. To reduce the effect of ICI, different cancellation schemes [5, 6] have been proposed and were tested in the GNU radio system. GNU radio is an open Analog to Remove Serial Input source software toolkit for building software radios by Digital (D/A) Cyclic To Signal making use of software that defines the transmitted Converter Extension Parallel waveforms and demodulators instead of hardware components [7]. The paper is organized as follows: Section II illustrates the Parallel to Signal OFDM modulation and demodulation techniques; Section III FFT describes different ICI cancellation schemes; Section IV Serial De-Mapper explains the need of the experiment, Section V discusses the experimental setup; Section VI explains the binary data preparation; Section VII is about the unique modulation Figure 2. OFDM Demodulation Copyright (c) IARIA, 2011. ISBN: 978-1-61208-140-3 94 ICWMC 2011 : The Seventh International Conference on Wireless and Mobile Communications III. ICI CANCELLATION SCHEMES To retrieve the data at receiving side, 1st and 4th sub-carriers Different cancellation schemes are used in order to reduce are added and 2nd and 3rd sub-carriers are subtracted as the effect of Inter-Carrier-Interference. Among the stated in equation (3) [1], X(K)+X(K)-(-X(K))-(-X(K))=4X(K). established models, self-cancellation and modified self- So the original data values become prominent. cancellation techniques [11] are considered for the D. New ICI cancellation scheme–2 experiment. Also, two other new schemes have been In this scheme each data bit is sent through four adjacent developed and tested during the research [11]. sub-carriers, each with 90o phase shift [4]. This gives A. ICI self-cancellation symmetry to the signal and greatly reduces the effect of the In this scheme each data bit is sent through two adjacent inter carrier interference. j j sub-carriers, one with weight ‗+1‘ and another with ‗-1‘ [11]. Y'(K) Y(K) 2 Y(K 1) Y(K 2) 2 Y(K 3) e e Y'(K) Y(K) Y(K 1) 3S(0) 2S(2) S(2) X(K)[S(1) 2S(0) S(1)] j X(K) 2 2S(1) 3S(1) S(3) N2 e X(l)[S(l K 1) j j 2 S(3) S(1) S(0) S(2) l0 e e leven lK 3S(l K) 2S(l K 2) S(l K 2) 2S(l K) S(l K 1)] j 2S(l K 1) 3S(l K 1) 2 (1) N4 [w(K) w(K 1)] (1.1) e S(l K 3) X(l) In order to cancel the ICI at the receiver, adjacent sub- l0 j 2 S(l K 3) S(l K 1) carriers are subtracted, X(K)-(-X(K))=2X(K) [11]. This ll4 e lK enhances the data value and reduces the noise level as given j S(l K) S(l K 2) in equation (1). e {w(K) w(K 1) w(K 2) w(K 3)} (4) B. ICI modified self-cancellation (1.4) . In the receiving end, the 3rd and 4th are phase shifted by 90o In this scheme each data is sent through two sub-carriers st nd one with weight ‗+1‘ and another with ‗-1‘ [7]. The Kth and and added to 1 and 2 sub-carriers respectively. Then the nd th o (N-1-K)th subcarriers are used. results of 2 and 4 phases are again shifted by 180 to get the original data as shown in equation (4). Y'(K) Y(K) Y(N 1 K) X(K)[2S(0) S(N 1 2K) S(2K N 1)] IV. THE NEED FOR THE EXPERIMENT N 1 In OFDM technique a large number of orthogonal 2 subcarriers are transmitted in parallel dividing the available X(l)[S(l K) S(N 1 l K) transmission bandwidth into narrowband sub-channels [1,2, l0 lK 3]. The separations of the subcarriers are kept as minimal as S(l N 1 K) S(l K)] possible to get a very compact spectral utilization. The {w(K) w(N 1 K)} ( 2 ) (1.2) channel impairments and frequency mismatch in the transmitter and receiver, sometimes lead to inter-carrier Similar to ICI self-cancellation, the interfering components interference (ICI) and phase rotation, resulting in th th are removed by subtracting the K and (N-1-K) sub-carriers performance degradation of the channel [4]. In order to in the receiving end as shown in equation (2). reduce the effect of ICI, different cancellation schemes [5,6] C. New ICI cancellation scheme-1 have been proposed. These schemes help to reduce the effects of ICI by cancelling out the interference from carrier. In this scheme each data is sent through four adjacent sub-carriers. The Kth and (K+3)th sub-carrier with weight As the data itself is used to cancel the interference, no extra ‗+1‘ and (K+1)th and (K+2)th with weight ‗-1‘ [1]. encoding is needed for error correction. The results from hardware simulations give better insight about the real life Y'(K) Y(K) Y(K 1) Y(K 2) Y(K 3) problems than does software simulations. The GNU radio is X(K)[4 S(0) S(1) 2 S(2) S(3) S(1) a cost effective and flexible implementation platform that 2 S(2) S(3)] can be used to verify the functionalities as well as the N4 performance of advanced models of wireless technologies in X(l)[4 S(l K) S(l K 1) a real time setup. The advent of high speed processors with l0 increased computational capability is making the software ll4 lK and hardware of GNU radio much closer to the antenna. 2 S(l K 2) S(l K 3) One of the main advantages of SDR is that it is software S(l K 1) 2 S(l K 2) S(l K 3)] reconfigurable, which leads to significant design {w(K) w(K 1) w(K 2) w(K 3) (3 (1.3)) simplification. Copyright (c) IARIA, 2011. ISBN: 978-1-61208-140-3 95 ICWMC 2011 : The Seventh International Conference on Wireless and Mobile Communications V.

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