Concatenated Precoded OFDM for Residual CFO Effect Mitigation

Orthogonal frequency-division multiplexing (OFDM) is a popular transmission technique in wireless communications [1]-[3], primarily because it enables ef cient implementation by discrete Fourier transform (DFT) and easily avoids intersymbol interference by inserting suf ciently long guard intervals. However, spectrally overlapping data-modulated subcarriers make OFDM systems sensitive to carrier frequency offset (CFO) [4]-[5], which is often caused by Doppler shift or oscillator instability. The presence of CFO not only breaks the orthogonality among subcarriers, thus causing intercarrier interference (ICI), but also introduces complex multiplicative distortion (CMD) to all detected subcarrier signals. Therefore, estimation on CFO and channel response should be simultaneously considered and performed before data detection. In most communication systems, the CFO estimation usually consists of two stages, namely coarse and ne CFO estimation stages. With the aid of leading training blocks in each transmission frame, coarse CFO estimation is usually conducted in conjunction with channel estimation. Several coarse CFO estimation approaches using training blocks have been proposed in the literature [6]-[9]. These approaches operates on the received time-domain signal and make estimation on CFO with an accuracy of half the minimum frequency separation. Because of insuf cient accuracy during coarse estimation or sometimes due to the time varying nature in the transmission channel, there may exist residual CFO (RCFO) in received OFDM blocks even after CFO compensation with the coarse estimate. In order to mitigate the effect of RCFO, ne CFO estimation approaches have been proposed to estimate RCFO based on received OFDM data blocks, and then enable the suppression of ICI and CMD caused by RCFO [10]-[13]. When equipped with suf ciently many pilot symbols in each OFDM block, this stream of approaches can provide satisfactory performance at the cost of increased system overhead in processing several data blocks for attaining accurate RCFO estimation, and requiring considerable computational complexity as well. Another stream of approaches which apply signal coding at the transmitter to reduce the sensitivity to RCFO in the signaling format is widely conceived as low-complexity alternatives to ne CFO estimation approaches. All approaches in this stream have so far considered the phase-shift-keyed (PSK) format as the subcarrier modulation due to the dif culty in compensating CMD, which is dubbed as common phase error (CPE) [14] in the PSK context. A simple and CONCATENATED PRECODED OFDM FOR RESIDUAL CFO EFFECT MITIGATION 2 effective ICI self-cancellation approach was proposed by Zhao and Haggman in [15]. The ZhaoHaggman approach transmits each symbol over a pair of adjacent subcarriers with a 180 degree phase shift at the cost of halving data throughput, and thus enables the receiver to demodulate robustly in the presence of ICI. Nevertheless, the CPE introduces unknown CFO-dependent distortion to all received subcarrier signals and thus degrades the demodulation performance signi cantly even though ICI can be largely suppressed. The symmetric conjugate symbol repetition (SCSR) approach was proposed in [16] to mitigate the joint effect of CPE and ICI only in some cases of RCFO. By transmitting the standard OFDM in the rst path and its conjugate with opposite polarity in the second path, the conjugate cancellation (CC) approach was shown in [17] to provide improved performance in ICI suppression. Although yielding better ICI suppression performance than the SCSR approach, the CC approach is devoid of CPE suppression. Like Zhao-Haggman approach, SCSR, and CC approaches achieve the suppression performance by employing data-symmetric subcarrier signaling formats and thus operate at the cost of halving data throughput. Without sacri cing data throughput remarkably, correlatively coded OFDM was proposed otherwise in [18]-[19] to suppress ICI. By applying correlative coding, exploited originally in bandlimited digital communications [20]-[21], correlatively coded OFDM reduces the ICI power effectively and permit data detection by Viterbi decoding. However, correlative coding is still sensitive to CMD, and provides less ICI reduction than data-symmetric signal-coding approaches. The aforementioned approaches were all designed under the assumption that the RCFO effect remain constant over many blocks. This assumption is, however, not realistic because the CMD effect is time-variant over OFDM blocks. In order to suppress the time-variant RCFO effect, a concatenated precoding approach is proposed in this poster presentation to enable CMD estimation and ICI suppression at the receiver. The concatenated precoder is constructed by concatenating an outer modi ed correlative precoder with an inner reduced Hadamard precoder to process data blocks prior to OFDM modulation and enable ICI suppression and CMD compensation at the receiver. With the aid of a few transmitted pilot symbols within each data block, the inner reduced Hadamard precoder makes possible the embedded estimation of CMD from the equalized received pilot signals in each block, and thus enables the receiver to compensate CMD during outer decoding. The outer modi ed correlative precoder endows the received equalized signal with ICI self-cancellation capability and permits data detection by Viterbi decoding. November 12, 2012 DRAFT CONCATENATED PRECODED OFDM FOR RESIDUAL CFO EFFECT MITIGATION 3