624 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 61, NO. 2, FEBRUARY 2012 A Null-Subcarrier-Aided Reference Symbol Mapping Scheme for 3GPP LTE Downlink in High-Mobility Scenarios Siva D. Muruganathan, Member, IEEE, Witold A. Krzymie´ n, Senior Member, IEEE, and Abu B. Sesay, Senior Member, IEEE Abstract—In this paper, a new reference symbol (RS) mapping scheme for the Third-Generation Partnership Project (3GPP) long-term evolution (LTE) and LTE-Advanced (LTE-A) downlink is proposed to improve channel estimation performance in high- mobility communication scenarios. The proposed scheme employs null subcarriers to guard RSs, which helps mitigate the effect of intercarrier interference (ICI) on subcarriers carrying RSs. Additionally, the proposed scheme allows the ICI gain parame- ters to be estimated via a simple frequency-domain estimator. Modified Cramer–Rao bound (MCRB) expressions are derived for the proposed scheme, as well as for the conventional RS mapping scheme defined in the 3GPP LTE and LTE-A standards to compare their performance at high mobile user speeds. These bounds, together with mean square errors obtained from sim- ulations, reveal superior performance achieved by the proposed scheme in high-mobility scenarios. Additionally, at high mobile user speeds, the proposed scheme offers significant bit-error-rate (BER) performance improvement over the standard RS mapping. Index Terms—Channel frequency response estimation, inter- carrier interference (ICI), modified Cramer–Rao bound (MCRB), orthogonal frequency-division multiplexing. I. I NTRODUCTION S UPPORTING high mobile user speeds is one of the key requirements of the Third-Generation Partnership Project’s (3GPP) long-term evolution (LTE) and LTE-Advanced (LTE-A) standards [1]. However, the time-varying nature of the radio channel in such high-mobility communication scenarios poses a significant challenge in achieving this goal. In the 3GPP LTE/LTE-A downlink, the mobile radio channel variations within the transmit duration of one orthogonal frequency- division multiplexed (OFDM) symbol lead to the loss of Manuscript received January 25, 2011; revised June 30, 2011 and October 20, 2011; accepted October 30, 2011. Date of publication December 7, 2011; date of current version February 21, 2012. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada, by TRLabs, and by the Rohit Sharma Professorship. This paper was presented in part at the IEEE VTC-Spring, Taipei, Taiwan, May 2010. The review of this paper was coordinated by Dr. G. Bauch. S. D. Muruganathan was with the Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada. He is now with Research in Motion Limited, Ottawa, ON K2K 3K2, Canada. W. A. Krzymie´ n is with the Department of Electrical and Computer Engi- neering, University of Alberta, Edmonton, AB T6G 2V4, Canada, and also with TRLabs, Edmonton, AB T5K 2M5, Canada (e-mail: wak@ece.ualberta.ca). A. B. Sesay is with the Department of Electrical and Computer Engineer- ing, University of Calgary, Calgary, AB T2N 1N4, Canada (e-mail: sesay@ ucalgary.ca). Digital Object Identifier 10.1109/TVT.2011.2178621 orthogonality between different subcarriers. This will cause in- tercarrier interference (ICI) at the mobile receiver, which needs to be mitigated to avoid severe performance degradation [2], [3]. Furthermore, due to the presence of ICI, channel estimation at the mobile receiver becomes a formidably challenging task. Recently, various practical schemes have been studied to estimate the time-varying channel in OFDM systems. In [4], a time-domain raised-cosine interpolator and a frequency- domain raised-cosine interpolator with adaptive rolloff fac- tor are proposed for channel estimation in a mobile digital video broadcasting handheld (DVB-H) receiver. A reduced- complexity channel estimator for DVB-H, which exploits the banded and sparse structures of the channel matrix in the fre- quency and time domains, respectively, is proposed in [3] and [5]. In [6], a channel estimation scheme combining minimum mean-square-error (MMSE) interpolation and time-domain windowing is proposed to estimate the time-varying channel in DVB-H systems. More recently, in [7], the medium access con- trol layer performance of various channel estimation algorithms has been studied in the context of the 3GPP LTE downlink. In this paper, we propose a new reference symbol (RS) mapping scheme to improve downlink channel estimation per- formance in high-mobility scenarios over the standard RS mapping scheme defined in the 3GPP LTE standard [8]. The proposed scheme employs null subcarriers to guard RSs, which helps mitigate the effect of ICI at subcarriers carrying RSs. In addition, the proposed scheme allows the ICI gain parameters to be estimated via a simple frequency-domain estimator. A major contribution of this paper is the derivation of modified Cramer–Rao bounds (MCRBs) to study the efficiency of the standard and the proposed RS mapping schemes in estimat- ing the channel frequency response (CFR) gains. Generally, the MCRB is a looser bound than the standard Cramer–Rao bound (CRB) [9]–[11]. However, in the presence of nuisance or unwanted parameters in the observed signal, the MCRB is much easier to evaluate than the standard CRB [9]–[11]. In this paper, we treat the discrete transmitted symbols as the nuisance or unwanted parameters and derive the MCRBs cor- responding to the standard and proposed RS mapping schemes under different user mobility scenarios. Noting that, for discrete nuisance parameters, the MCRB asymptotically approaches the standard CRB at high signal-to-noise ratios [11], we use the derived MCRBs to analytically demonstrate the performance gain achieved by the proposed RS mapping scheme over the 0018-9545/$26.00 © 2011 IEEE