Alamouti Coding for DFT Spreading-Based Low PAPR FBMC

Alamouti Coding for DFT Spreading-based Low PAPR FBMC

Abstract: Thus far, the solutions for the two major issues of FBMC/OQAM (filter bank multicarrier with offset quadrature amplitude modulation), i.e., 1) high PAPR and 2) incompatibility with MIMO have been devised exclusive of each other and thus, the signal formats and processing’s are also exclusive. Therefore, how to constructively combine their solutions in a single system is another challenging issue. As a specific solution to this, we extend the idea of DFT spreading-based Low PAPR FBMC scheme to the ICI-free Alamouti-coded FBMC. In order to inherit the merits of the two schemes, we first provide the required modifications for combining the two schemes. A mathematical analysis confirms that even with frequency reversal Alamouti code arrangement and sub block partitioning, the proposed scheme still achieves the single carrier effect of DFT spreading. The simulation results reveal that the proposed scheme achieves substantially lower PAPR than the previous Alamouti-coded FBMC scheme. Moreover, owing to the additional frequency diversity via DFT spreading, the proposed scheme with properly chosen sub block size achieves even lower BER than the theoretical Alamouti-coded BER with a diversity order of two. Considering the PAPR, BER,

and data rate loss, we conclude that the proposed scheme is a competitive solution for Alamouti-coded multicarrier system.

Existing system: Arranging the Alamouti-coded symbol in a frequency reversal manner and simply setting the phase shift terms for the frequency reversal counterpart to satisfy Hermitian symmetry, the intrinsic inter-antenna ICI (intercarrier interference) terms are self cancelled in the typical Alamouti decoding stage. Hence, FRACFBMC achieves near ICI-free performance with very simple signal modifications and negligible computation complexity compared to the existing Alamouti-coded FBMC schemes. In addition, FRACFBMC has negligible decoding latency because the Alamouticoded symbol pairs belong to the same FBMC symbol unlike. The signal structure proposed in has been further improved in removing the need for null insertion into the center subcarrier.

Proposed system: Instance, as FBMC1 is based on multicarrier modulation, it has a high PAPR (peak to average power ratio) as OFDM (orthogonal frequency division multiplexing) does. The practical high power amplifier in the transmitter has a limited linear range. Hence, in case of high amplification gain, a high PAPR could cause signal clipping or other nonlinearities in the FBMC signal. Various schemes to reduce the PAPR of FBMC were proposed in, but there exist some practical issues, such as considerable side information overhead, high computational complexity and data rate loss. Recently, in order to overcome these practical issues, a new type of FBMC called Low PAPR FBMC (LP-FBMC in short) was proposed.

Advantages: In this study, we are interested in applying Alamouti code to FBMC while maintaining the intrinsic advantages of FBMC over the OFDM-based scheme such as excellent OOB suppression capability.

Therefore, in this section, we focus on FBMC-based schemes for performance comparison in terms of PAPR and BER. For references, OFDM-based Alamouticoded schemes, i.e., Alamouti-coded versions of CPOFDM and DFT-S-OFDM are also included in performance comparison.

Disadvantages: This problem is more serious in more selective fading channels. However, note that by partitioning and decreasing the half sub block size P from 128, we can significantly decrease the BER. On the other hand, if the half sub block size is excessively smaller than the coherence bandwidth (like P=2 in Pedestrian- A channel), then the BER increases again due to the same reason we mentioned for the flat fading channel case above, i.e., more inter-half sub block interference.

Modules: Real multiplication: Note that in addition to the fundamental merits of FBMC such as excellent OOB suppression, no CP overhead or robustness against inter-user frequency/time synchronization, the proposed scheme achieves low PAPR which is inherited from LP-FBMC. On the other hand, it was shown that depending on the system configurations, FBMC modulation/demodulation require about 3 to 7 times higher complexity than that of OFDM modulation/demodulation in terms of number of real multiplications (RMs) (see Table 2 in [34]). Consequently, except in terms of complexity, the proposed scheme is attractive over the other schemes in all aspects considered in Table II. As analyzed in Section V, high complexity of the proposed scheme is due to multiple candidate signal generation in TX side for PAPR reduction. For fair comparison, if we consider the case when Alamouticoded CPOFDM adopts the typical PAPR reduction schemes.

Filter bank multicarrier: Although FBMC-OQAM (filter bank multicarrier with offset quadrature amplitude modulation) has a significant number of merits, there are still some issues that need to be resolved. For instance, as FBMC1 is based on multicarrier modulation, it has a high PAPR (peak to average power ratio) as OFDM (orthogonal frequency division multiplexing) does. The practical high power amplifier in the transmitter has a limited linear range. Hence, in case of high amplification gain, a high PAPR could cause signal clipping or other nonlinearities in the FBMC signal. Various schemes to reduce the PAPR of FBMC were proposed, but there exist some

practical issues, such as considerable side information overhead, high computational complexity and data rate loss. Recently, in order to overcome these practical issues, a new type of FBMC called Low PAPR FBMC (LP-FBMC in short) was proposed.

Sub blocks partitioning for frequency: Selective fading the self ICI cancellation property of the frequency reversal Alamouti-coded symbol arrangement shown in is valid only under the condition that each frequency reversal pair has an identical fading coefficient, i.e., flat fading within the frequency reversal half band pairs. In frequency selective fading environments, we can approximately achieve flat fading within the frequency reversal half band pairs by properly dividing the entire allocated band into multiple sub bands. Then, by separately applying the frequency reversal Alamouti coding to each sub band, we can restore the self ICI cancellation property. As the peak power reduction by DFT spreading is less effective.

Sub block: Fair comparison, the sub block partitioning in is employed to FRAC-FBMC as well as the proposed scheme. In all the three versions of the proposed scheme, as the number of partitions L increases the PAPR increases to some extent. This is because as shown, the number of pulse peaks in the single carrier formed pulse given in increases as L increases, and the summation of the pulse shaped symbols with different positions results in more fluctuation in the signal amplitude.

However, even with maximum sub block partitioning (L = 64), the increase in gap of PAPR remains less than 0.5 dB, and this is the case with all the three versions of the proposed scheme. In addition, for the versions with eight candidate selection, the PAPR rapidly converges as L increases from 1. On the other hand, in FRACFBMC, sub block partitioning has negligible impact on PAPR performance as there is no DFT spreading.