Jean Armstrong

Analysis of new and existing methods of reducing intercarrier interference due to carrier frequency offset in OFDM

Orthogonal frequency division multiplexing (OFDM) is very sensitive to frequency errors caused by frequency differences between transmitter and receiver local oscillators. This sensitivity is analyzed in terms of the complex weighting coefficients which give the contribution of each transmitter subcarrier to each demodulated subcarrier. Previously described windowing and self intercarrier interference (ICI) cancellation methods are analyzed in terms of these weighting coefficients. New ICI cancellation schemes with very much improved performance are described. A condition for orthogonality of windowing schemes is derived in terms of the discrete Fourier transform (DFT) of the windowing function.

New OFDM peak-to-average power reduction scheme

This paper describes a new peak-to-average power ratio (PAPR) reduction scheme for orthogonal frequency division multiplexing (OFDM). A time domain version of the OFDM signal is generated using an oversized inverse discrete Fourier transform (DFT). This results in trigonometric interpolation. The interpolated signal is clipped. To remove the resulting out-of-band components, the clipped signal is filtered through the use of a forward and inverse DFT. The filter passes the wanted in-band discrete frequencies while nulling the out-of-band components. The new scheme gives lower PAPR than other clipping techniques. Results are presented for the power spectral density and in-band distortion when the scheme is followed by a non-ideal amplifier. No change to the receiver is required so the scheme is compatible with existing communications standards.

Polynomial cancellation coding of OFDM to reduce intercarrier interference due to Doppler spread

Polynomial cancellation coding (PCC) is a coding method for orthogonal frequency division multiplexing (OFDM) in which the information to be transmitted is modulated onto weighted groups of subcarriers rather than onto individual subcarriers. It has previously been shown that PCC substantially reduces the sensitivity of OFDM to carrier frequency offset. In this paper, it is shown that PCC also reduces the intercarrier interference (ICI) due to Doppler spread. Results are presented for a frequency non-selective i.e. flat fading channel, subject to classical Doppler spread. By using weighted pairs of subcarriers the ICI due to Doppler spread can be reduced by approximately 15 dB. By using weighted groups of three subcarriers a further 15 dB reduction in ICI can be achieved.

Effects of clipping on the error performance of OFDM in frequency selective fading channels

Previous studies on the effect of the clipping noise on the error performance of orthogonal frequency-division multiplexing (OFDM) systems in frequency selective fading channels provide pessimistic results. They do not consider the effect of channel fading on the clipping noise. The clipping noise is added at the transmitter and hence fades with the signal. Here, the authors show that the bad subcarriers that dominate the error performance of the OFDM system are least affected by the clipping noise and, as a result, the degradation in the error performance of OFDM system in fading channels is very small.

Effects of phase noise on performance of OFDM systems using an ICI cancellation scheme

This paper investigates the effects of phase noise on the performance of orthogonal frequency division multiplexing (OFDM) systems using an intercarrier interference (ICI) cancellation scheme. In this case, the common phase error (CPE) and ICI caused by phase noise depend on the overall spectrum of each weighted group of subcarriers rather than on the spectrum of each individual subcarrier. This means that the system performance can be improved by filtering the phase noise to fit a particular spectrum. It is shown that the ICI cancellation scheme can significantly improve the bit error rate (BER) performance in the presence of phase noise.

Spectral analysis of OFDM signals and its improvement by polynomial cancellation coding

In orthogonal frequency division multiplexing (OFDM) systems, the subcarriers are generated by an inverse discrete Fourier transform. In the literature, the spectra of these subcarriers are often represented by a sequence of sinc functions of the same polarity. This is not consistent with the intercarrier interference (ICI) cancellation properties of polynomial cancellation coding (PCC) or similar self-ICI cancellation schemes in OFDM. This paper shows the spectra of these subcarriers are more accurately represented by a sequence of sinc functions with alternating polarity. It is shown that the spectral rolloff of OFDM signals can be improved by PCC and the resultant OOB power is much reduced.

Impulse noise mitigation for OFDM using decision directed noise estimation

The paper describes a new technique for mitigating the effects of impulse noise in OFDM. An estimate is made of the noise component in each received input sample. The estimates are based on the transmitted data. No pilot tones are required. When the estimate is large enough to indicate that impulse noise is present in the sample, the estimated noise component is subtracted from the input sample before final demodulation. Estimates of the noise are obtained from preliminary decisions based on the noisy signal. The technique is effective because the energy from each noise impulse is spread across the received spectrum. The technique can also be applied to multicarrier CDMA. Simulations show that, in cases of practical importance, the symbol error rate can be reduced by several orders of magnitude.

Performance of PCC-OFDM with overlapping symbol periods in a multipath channel

Polynomial cancellation coded orthogonal frequency division multiplexing (PCC-OFDM) with overlapping symbol periods is a modulation technique which overcomes many of the disadvantages of OFDM. PCC-OFDM is much less sensitive to frequency offset and Doppler spread than OFDM. Only the length of the equalizer used limits the delay spread that can be tolerated in a PCC-OFDM system. In this paper expressions are calculated for the intercarrier interference (ICI) and intersymbol interference (ISI) caused by timing errors in PCC-OFDM. Results are presented for simulations of PCC-OFDM in channels subject to frequency error and multipath. It is shown that good performance can be achieved using equalizers of only moderate complexity.

Multicarrier CDMA systems using PCC-OFDM

Polynomial cancellation coded orthogonal frequency division multiplexing (PCC-OFDM) with overlapping symbol periods is a modulation scheme with properties that make it well suited to high speed data transmission including mobile applications. PCC-OFDM is much less sensitive to frequency offset and Doppler spread than ordinary OFDM. Equalizers to counteract multipath transmission are much simpler for PCC-OFDM than for single carrier systems or for OFDM systems without a cyclic prefix. In this paper PCC-OFDM in combination with code division multiple access (CDMA) for a downlink is considered. It is shown that the proposed system gives lower bit error rate (BER) than OFDM for channels where the frequency offset or Doppler spread is significant, and for channels where the delay spread is more than a fraction of a symbol period. Increasing the delay spread has little effect on the performance of the PCC-OFDM system. The upper limit on the delay spread, which can be tolerated, is set by the span of the equalizer which can be a number of symbol periods long without excessive complexity. It is shown that in this application the two-dimensional equalizer can be simplified to a number of one-dimensional equalizers operating in parallel without significant loss in performance.

Blind frequency offset estimation for PCC-OFDM with symbols overlapped in the time domain

This paper presents a blind frequency offset estimator for Polynomial Cancellation Coded Orthogonal Frequency Division Multiplexing (PCC-OFDM) signals with symbols overlapped in the time domain (Overlap PCC-OFDM). The estimation is carried out in the frequency domain. A frequency offset estimate is obtained by using a pair of demodulated subcarriers at the Discrete Fourier Transform (DFT) output. As no pilot tones are required for the estimation there is no loss in bandwidth efficiency. Simulations show that this estimator is an approximately lineal function of frequency offset and has low variance.