Denis Petrovic

Effects of Phase Noise on OFDM Systems With and Without PLL: Characterization and Compensation

In this paper, we propose an algorithm for suppressing intercarrier interference due to phase noise in coded orthogonal frequency division multiplexing (OFDM) systems. The algorithm approximates the phase-noise waveform by using a Fourier series approximation for the current phase-noise realization. Thereby, it cancels the effects of the phase noise beyond the standard common phase error correction used in contemporary OFDM standards. The algorithm requires that the correlation properties of the intercarrier interference are known. We calculate these properties in terms of the phase-noise spectral correlation matrix for both Wiener and Ornstein-Uhlenbeck phase-noise models, respectively. This modeling corresponds to a free-running oscillator, as well as a phase-locked loop realization of the local oscillator in orthogonal frequency division multiplexing transceivers. For both transceiver configurations, we investigate the performance of the proposed algorithm. It is demonstrated that the new algorithm achieves as much as one order of magnitude better performance in terms of packet/bit error rate when compared to a receiver with only the common phase error suppression.

Dirty RF: a new paradigm

The implementation challenge for new low-cost low-power wireless modem transceivers has continuously been growing with increased modem performance, bandwidth, and carrier frequency. Up to now we have been designing transceivers in a way that we are able to keep the analog (RF) problem domain widely separated from the digital signal processing design. However, with todays deep sub-micron technology, analog impairments-dirt effects-are reaching a new problem level which requires a paradigm shift in the design of transceivers. Examples of these impairments are phase noise, non-linearities, IQ imbalance, ADC impairments, etc. In the world of dirty RF we assume to design digital signal processing such that we can cope with a new level of impairments, allowing lee-way in the requirements set on future RF sub-systems. This paper gives an overview of the topic and presents analytical evaluations of the performance losses due to RF impairments as well as algorithms that allow to live with imperfect RF by compensating the resulting error effects using digital baseband processing

Dirty RF: A New Paradigm

The implementation challenge for new low-cost low-power wireless modem transceivers has continuously been growing with increased modem performance, bandwidth, and carrier frequency. Up to now we have been designing transceivers in a way that we are able to keep the analog (RF) problem domain widely separated from the digital signal processing design. However, with today’s deep sub-micron technology, analog impairments – “dirt effects” – are reaching a new problem level which requires a paradigm shift in the design of transceivers. Examples of these impairments are phase noise, non-linearities, I/Q imbalance, ADC impairments, etc. In the world of “Dirty RF” we assume to design digital signal processing such that we can cope with a new level of impairments, allowing lee-way in the requirements set on future RF sub-systems. This paper gives an overview of the topic and presents analytical evaluations of the performance losses due to RF impairments as well as algorithms that allow to live with imperfect RF by compensating the resulting error effects using digital baseband processing.

Intercarrier interference due to phase noise in OFDM - estimation and suppression

In this paper we provide an analysis of the intercarrier interference (ICI) due to phase noise in OFDM systems and present an algorithm for its suppression. We examine the general case where phase noise can take any values, thus the ”small” phase noise model is dropped. The statistical properties of the intercarrier interference are analyzed, showing that the ICI is generally a non-gaussian random process which has a large impact on the system performance. Closed form expressions which describe the correlation properties of the constituents of ICI are calculated. An MMSE approach for suppressing ICI in the frequency domain is presented. This approach avoids error propagation to which our previously proposed algorithm was prone. The performance of the suppression algorithm is shown, pointing out the limits for the ICI suppression algorithms in general.

Common phase error due to phase noise in OFDM-estimation and suppression

Orthogonal frequency division multiplexing (OFDM) has already become a very attractive modulation scheme for many applications. Unfortunately OFDM is very sensitive to synchronization errors, one of them being phase noise, which is of great importance in modern WLAN systems which target high data rates and tend to use higher frequency bands because of the spectrum availability. In this paper we propose a linear Kalman filter as a means for tracking phase noise and its suppression. The algorithm is pilot based. The performance of the proposed method is investigated and compared with the performance of other known algorithms.

Performance degradation of coded-OFDM due to phase noise

In this paper we investigate the effect of phase noise on coded OFDM transmission with parameters typical for present OFDM based WLAN standards. Two different scenarios are compared, namely when the local oscillator at the receiver is realized as a free running oscillator or as a PLL. System performance is evaluated as degradation of the effective SINR, using BER simulations and in terms of achievable cut-off rate for different M-QAM OFDM schemes in the presence of the phase noise.

Performance Comparison of OFDM Transmission Affected by Phase Noise with and without PLL

Oscillator phase noise present in OFDM systems limits the performance by producing a common phase error and additional intercarrier interference. While phase noise is most often modelled as a Wiener process corresponding to free running oscillators, we use a recently developed approach [1] to describe the stochastical behaviour of phase locked loops (PLL) to compare the performance degradation of OFDM-transmission for a receiver operating with a free running oscillator to one using a first order charge-pump PLL having a VCO with the same linewidth. In addition to the relative linewidth of the oscillator with respect to subcarrier spacing, a characteristic quantity determining the performance is the time constant of the PLL with respect to the OFDM symbol duration. Comparing the performance for different QAM constellations it is found, that a PLL will improve bit and symbol error rate, if its gain can be made high enough to obtain time constants smaller than the OFDM symbol duration.