Research On Performance Enhancing Technologies For OFDM

OFDM(Orthogonal Frequency Division Multiplexing)is one of the existing modulation schemes based on multi-carrier technology.With high spectral efficiency and low complexity,it has been applied in many practical high-speed digital communication systems.However,OFDM has three main disadvantages: First,the receiver makes great demands on the accuracy of timing synchronization,because severe ISI(Inter-symbol Interference)and ICI(Inter-carrier Interference)can be resulted by even small synchronization errors of timing.Secondly,the performance of OFDM is very sensitive to the carrier frequency offset and the variance of the channel,which can destroy the orthognality among subcarriers and result in ICI.Thirdly,the OFDM signal has a high PAPR(Peak-to-average),which increases the cost of the power amplifier,and also the power assumption at the transmitter.The above disadvantages of OFDM make it very challenging to achieve reliable communicate in some scenarios,such as in underwater acoustic channels,or RF(Radio Frequency)channels between the base station and terminals in a high-speed train.In this paper,we investigate the performance-enhancing techniques for OFDM,including timing synchronization,power allocation,and index modulation.To achieve timing synchronization,at the sender,an auxiliary sequence termed preamble is always padded at the header of the OFDM frame.At the receiver,the position of the data frame is estimated by performing correlation operations on the preamble.The corresponding algorithms can be divided into two categories: 1)those performing self-correlation among different parts of the received sequence;2)those performing inter-correlation between the local preamble with the received sequence.In 2011,by combining the self-correlation and inter-correlation operations,Hamed and Mahrokh proposed a robust timing method(HM algorithm).This timing estimator is independent of the structure of the preamble,immune against the CFO(Carrier Frequency Offset),and working even at low SNR(Signal-to-noise Ratio).However,it suffers from high computational complexity.In this paper,based on HM algorithm,we propose a modified timing estimator,which can halving the computational complexity of HM algorithm without performance loss.OFDM-IM(OFDM with Index Modulation)is a kind of enhanced modulation schemes based on OFDM.In OFDM-IM,In OFDM-IM,the available sub-carriers are first divided into several sub-blocks.In each sub-block,only a fixed number of sub-carriers are activated,and the remaining sub-carriers keep silent.The position of the active subcarrier is variable sub-block by sub-block,and thus the indices of the activated subcarrier can convey some data bits,which is called index bits.Like the traditional OFDM,the constellation symbols transmitted on the active subcarriers deliver data bits,which is called symbol bits.In this paper,we propose a modulation scheme called ECI-OFDM-IM(Enhanced Coordinate Interleaved OFDM).In ECI-OFDM-IM,by increasing the minimum distance between different sub-carrier activation patterns,the indexbit error performance of the index bits is significantly improved;through performance-enhanced coordinate interleaving of data symbols on active sub-carriers,the symbol-bit error performance has also been significantly improved.Both theoretical analysis and Monte Carlo simulation results show that the error performance of ECI-OFDM-IM is significantly better than that of conventional OFDM and OFDM-IM at the same spectral efficiency.In mobile communication scenarios,many wireless channels can be described by MSML(Multi-scale Multi-lag)model,such as mobile UWA(Underwater acoustic)channels.Under a MSML channel,the received signal can be viewed as a superposition of many path components with different delays,attenuation factors,and time scales.For wide-band OFDM systems,the time-scaled signals always suffer from non-uniform frequency shifts across the sub-carriers.The resampling has been widely used to compensate the Doppler scaling induced non-uniform frequency shifts.However,in MSML channels,the ICI resulted by the Doppler distortion cannot be cleared out completely even with the optimal resampling factor,because the different pathscales cannot be compensated simultaneously with a single resampling factor.In this paper,given some statistics of the delays,attenuation factors,and path-scales of the channel paths,we derive the ICI coefficients of the sub-carriers in closed-form.It is also proved that the ICI coefficient increases with the sub-carrier frequency monotonously,i.e.,the sub-carriers of lower frequency are better than those of higher frequency.Based on this observation,we propose a novel power allocation scheme,in which the total subcarriers are divided into two consecutive parts with respect to the carrier-frequency.For the part of lower frequency,by considering that the ICI coefficients is negligible,all the sub-carriers are activated to make full use of the degrees of freedom of the channel.For the part of higher frequency,the subcarriers of odd indices keep silent to avoid the low energy efficiency resulted by ICI.The total power then is allocated uniformly across all the active sub-carriers.As shown in the simulation result,the proposed power allocation performs better than the existing statistical water-filling method and uniform power allocation in average achievable rate.