Zero Padding or Cyclic Prefix: Evaluation for Non-Orthogonal Signals

Abstract

The debate of using zero padding (ZP) instead of a cyclic prefix (CP) for enhancing channel estimation and equalization performance is a recurring topic. This is particularly true for orthogonal signals, such as orthogonal frequency division multiplexing (OFDM). Yet, there are far fewer studies evaluating the impact of ZP and CP in non-orthogonal systems. Such systems have the added complexity of self-induced interference rendering channel estimation and equalization more challenging. For this reason, this work proposes a new channel estimation and equalization technique for non-orthogonal systems, which combines ZP with an orthogonal demodulator. Results show that the multipath components that appear in the ZP part can be used to enhance performance when compared to the CP approach.

Alignment Signal Aided CP-Free SEFDM

“…. It should be noted that, unlike CP-OFDM symbols, CP-SEFDM symbols suffer of phase discontinuity between the CP part and the symbol [13]. Each received CP-SEFDM symbol consists of three parts, y cp , y and y isi , as depicted in step 5 in Fig.…”

“…The first part,ȳ cp ∈ C µ×1 , represents the CP part, which is affected by the ISI components coming from the previous symbol and hence it is neglected at the receiver; while the second part, y ∈ C Q×1 , denotes the received SEFDM symbol that is passed to the detection stage; and finally the third part, y isi ∈ C µ×1 , is the ISI components, which are added to the succeeding symbol. Having removed the CP, the demodulated CP-SEFDM symbol,r cp , will be given as [13]:…”

“…Similar as the OFDM signal, CP can be used for mitigate the ISI in the SEFDM signal. However, in CP-SEFDM signals, there is a discontinuity in phase between the CP portion and the SEFDM symbol leads to interference being introduced from the CP to the SEFDM symbol [13]. Thus, after removing the CP portion at the receiver, the SEFDM symbols are padded with zeros at the input of the discrete Fourier transform (DFT), which in turn yields a non-continuous signal in the time-domain, hence introducing additional interference to the subcarriers.…”

“…Thus, after removing the CP portion at the receiver, the SEFDM symbols are padded with zeros at the input of the discrete Fourier transform (DFT), which in turn yields a non-continuous signal in the time-domain, hence introducing additional interference to the subcarriers. As a result, CP-SEFDM often suffers the inaccurate channel equalisation because of the non-zero mean square error (MSE) floor in the channel estimation which has been reported in [13]. In addition to symbol interference introduced by CP, the CP-SEFDM also suffers the coarse channel estimation and equalisation caused by non-orthogonality between the subcarriers, which is also known as inter-carrier interference (ICI).…”

“…The solutions either cause high time-domain processing complexity or do not bring clear improvement in channel equalisation. Recently, ZP-SEFDM is proposed in [8], [13] to provide accurate channel estimation and equalisation performance in the frequency-domain.…”