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In today’s post we carry on with our blog series on RF imperfection and compensation by addressing the effect of phase noise on MIMO OFDM system performance. We have shed light on the effect of IQ imbalance where we have seen that this type of impairment results in a subcarrier being interfered with by its mirror image subcarrier. The issue is deemed to be more severe in fading channel, where the constellation in M-QAM will undergo random shift and noise.

In case of phase noise the effect is twofold depending on the cut off frequency of the local oscillator SSB (single sideband) phase noise power spectral density’s (PSD) -3dB bandwidth (SSB phase noise PSD’s -3dB BW).  As depicted in the figure below, the contribution of the low and high SSB phase noise PSD’s -3dB BW causing constellation rotational and additive effects is apparent. Equal to all subcarriers, the rotational effect is analytically isolated in a common phase error (CPE) term in the signal model while the additive effect is regarded as inter-carrier interference (ICI)

[1-2].

Effect of phase noise on QPSK symbols

Figure 1. Effect of phase noise on QPSK symbols: a) low -3dB bandwidth of the SSB phase noise PSD β and b) high -3dB bandwidth of the SSB phase noise PSD β

 

This is has also being addressed by [3] where it has been shown that the CPE and ICI are mainly dominated by the spectral content of the phase noise PSD below and above fs/Nc respectively, where fs is the sampling rate and Nc is the number of subcarriers.  The Radio420X reference design has a β = 1000Hz (with very tight loop filter) -3dB SSB phase noise PSD bandwidth. When applied to an LTE system, one would expect at 30.72MHz sampling rate that CPE will be the dominant factor.

The CPE term is the same for all subcarriers within one OFDM symbol interval and changes slowly from one symbol to another. If the phase noise level is low, CPE approximately equals the mean of the phase deviation of an oscillator within one OFDM symbol. The ICI term is a random process. Schemes which compensate phase noise in OFDM systems have been proposed in [4] and [5]. In Nutaq’s OFDM reference design such CPE estimation is taken into account as part of the channel estimation.  We can consider the performance of a MIMO OFDM receiver in a flat Rayleigh fading channel with β = 1000Hz. From the presence of the phase noise we will see that the BER degradation is substantial as witnessed by the limiting BER-floor. This will limit the overall system performance regardless of the level of the SNR. Please see the figure in reference [7] for further details.

The effect and compensation of phase noise is usually addressed jointly with IQ imbalance in the presence of fading channel. Theoretical analysis of performance degradation due to phase noise and IQ imbalance in OFDM direct conversion systems is discussed in [6]. On the other hand joint compensation of IQ imbalance and phase noise in OFDM wireless systems is derived in [8].

References

  1. T. Pollet, M. Bladel, and M. Moeneclaey, “BER sensitivity of OFDM systems to carrier frequency offset and Wiener phase noise,” IEEE Trans.Commun., vol. 43, no. 2/3/4, pp. 191–193, Feb./Mar./Apr. 1995.
  2. L. Tomba, “On the effect ofWiener phase noise in OFDM systems,” IEEETrans. Commun., vol. 46, no. 5, pp. 580–583, May 1998.
  3. [102] H. Steendam, “The effect of synchronisation errors on multicarrier systems,”Ph.D. dissertation, Universiteit Gent, 2000.
  4. S. Wu and Y. Bar-Ness, “A phase noise suppression algorithm for OFDM based WLANs,” IEEE Commun. Lett., vol. 6, no. 12, pp. 535–537, Dec. 2002.
  5. G. Liu and W. Zhu, “Compensation of phase noise in OFDM systems using an ICI reduction scheme,” IEEE Trans. Broadcast, vol. 50, no. 4, pp. 399–407, Dec. 2004.
  6. Yin Wangy, Ke Gong, Zhaowu Chen, “Theoretical analysis of performance degradation due to phase noise and IQ imbalance in OFDM direct conversion systems”
  7. Schenk Tim, “RF imperfections in high-rate wireless systems: impact and digital compensation,” Springer, pp. 115, figure 4.12, February 2008.
  8. Qiyue Zou, Alireza Tarighat, and Ali H. Sayed, “Joint compensation of IQ imbalance and phase noise in OFDM wireless systems”.

 

A MIMO radio with automatic IQ imbalance compensation.