In multiple access OFDM systems (OFDMA) the separation of different users is achieved by exploiting the orthogonality of the subcarriers. However, a well-known problem in OFDM-based systems is the vulnerability to synchronization errors which leads to a loss of orthogonality. In this paper, a method is proposed to counteract the effect of different frequency offsets among users in an OFDMA uplink system with a frequency selective Rayleigh fading channel. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. This is performed by means of a successive or selective cancellation method. In particular, the performance of these cancellation schemes is analyzed by assuming either an ideal or estimated frequency offset. Simulation results show that it is possible to achieve, with the interference cancellation, a performance close to the ideal case, i.e., without frequency offset among users.

Multiuser interference cancellation receivers for OFDMA uplink communications with carrier frequency offset

We analyze the effect of Wiener phase noise on the capacity and signal-to-interference-plus-noise (SINR) ratio. Our analysis includes phase noise at the transmitter and receiver end of an OFDM communication link. We see that the capacity and SINR are random variables whose distribution depends on the phase noise and the fading channel. Using a Taylor series approximation, we show that the random variable, characterizing the phase noise, in the performance metrics can be expressed as a sum of correlated gamma variables with rank-deficient square-root normalized covariance matrix. The approximation holds well when the ratio between the subcarrier spacing and the 3dB bandwidth of the oscillator power spectral density is at least one order of magnitude, which for most practical oscillators is the case. In earlier literature, the probability density function of a sum of correlated gamma variables with full-rank square-root normalized covariance matrix was derived. We extend these results to the rank-deficient case and apply them to the random variable of our case. With the probability density functions characterizing the phase noise and the fading channel at hand, we proceed to obtain closed-form statistical measures of capacity and SINR. The simulations show the good agreement with our analytical expressions.

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Performance Analysis of OFDM with Wiener Phase Noise and Frequency Selective Fading Channel

Multiuser interference cancellation for OFDM systems with frequency offset

In orthogonal frequency division multiple access systems clusters of subcarriers are assigned to different users for parallel data transmissions. The subcarriers are overlapped, but orthogonal to each other such that there is no intercarrier interference (ICI). However, synchronization errors among users cause the loss of the orthogonality and introduce ICI resulting in multiple-access interference. Synchronization between users is particularly difficult in the uplink channel where the user signals are potentially asynchronous and affected by different frequency offsets due to misalignment in carrier frequencies and Doppler shifts. This paper proposes a method to lower the effects of different frequency offsets among user signals in an OFDMA uplink system. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. An approach based on the selective cancellation method, is proposed and its performance is analyzed by means of theoretical analysis and computer simulations. The effectiveness of the proposed system has been evaluated in the case of ideal and no-ideal frequency offset estimation and has been compared with that of the classical successive and parallel cancellation schemes. Simulation results show that the proposed approach allows performance close to the ideal case, i.e., with ideal frequency synchronization among users, with a low increase of the implementation complexity. Moreover, it is also highlighted here, that the successive cancellation method slightly outperforms the selective scheme, at the expense of a higher computational complexity and processing delay

Robust Multiuser Interference Cancellation for OFDM Systems With Frequency Offset

The performance of orthogonal frequency division multiplexing (OFDM) system degrades due to intercarrier interference (ICI) Recently, a simple ICI self-cancellation scheme has been proposed to reduce ICI due to frequency offset errors Despite its 50% throughput loss, it is very effective in cancelling ICI However, it does not remove the constant phase error, which has a considerable influence on the error performance of the OFDM system depending on the modulation scheme used We propose new cancellation schemes, namely adjacent conjugate symbol repetition (ASSR) and symmetric conjugate symbol repetition (SCSR), to eliminate this phase error while reducing ICI The proposed SCSR also eliminates the in-phase and quadrature (IQ) imbalance effects completely Analytical results show that SCSR offers an average carrier-to-interference ratio (CIR) gain of 10 dB for the normalized frequency offset of 01 over that of conventional scheme We evaluate the error performance of 16-QAM with various frequency offset and IQ-imbalance impairments and demonstrate SCSR's robustness to these impairments

A novel ICI cancellation scheme to reduce both frequency offset and IQ imbalance effects in OFDM

Orthogonal frequency division multiplexing (OFDM) is sensitive to the carrier frequency offset (CFO) and phase noise, which destroys orthogonality and causes intercarrier interference (ICI). Previously, two methods were available for the analysis of the resultant degradation in performance. Firstly, the statistical average of the ICI could be used as a performance measure. Secondly, the bit error rate (BER) could be approximated by assuming the ICI to be Gaussian. However, a more precise analysis of the performance (i.e. BER or SER) degradation is desirable. We propose a precise numerical technique for calculating the effect of the CFO and phase noise on the BER or symbol error rate (SER) in an OFDM system. In particular, closed form expressions for SER are derived using a Beaulieu (1990) series.

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Performance analysis of an OFDM system with carrier frequency offset and phase noise

Orthogonal frequency division multiplexing (OFDM) is sensitive to the carrier frequency offset (CFO). We introduce the peak interference-to-carrier ratio (PICR) to measure the resulting intercarrier interference (ICI). This paper shows that the PICR can be reduced by coding to select only those messages with low PICR as valid codewords. While explicit constructions of higher-rate, ICI-suppressing codes are rather difficult, we propose two new codes that map data onto three and four adjacent subcarriers. We also propose a new ICI reduction scheme, called tone reservation, that inserts optimized pilot tones in the data frame. Computing these pilot tones at the transmitter is a simple optimization technique.

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New ICI reduction schemes for OFDM system

This paper analyzes the peak-to-average power ratio (PAPR) problem in multicode-code division multiple access (MC-CDMA) systems. The statistical distribution of PAPR is derived and the achievable PAPR reduction for a given code rate is estimated. We show that the PAPR in MC-CDMA communications systems can be reduced by partial transmit sequence (PTS) and selected mapping (SLM) approaches. In PTS, the subblocks are multiplied by a set of phase factors that are optimized to minimize PAPR. In SLM, several independent data frames are generated and the data frame with the lowest PAPR is selected for transmission. We also show that the BER performance improves when the PAPR-reduced MC-CDMA signal is passed through a nonlinear amplifier. SLM reduces the error floor caused by amplifier nonlinearities and offers an SNR gain of 6 dB at 10/sup -5/ BER with an input back-off of 1 dB.

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K. Sathananthan

Papers

Performance analysis of an OFDM system with carrier frequency offset and phase noise

New ICI reduction schemes for OFDM system

Peak-to-average power ratio analysis in multicode CDMA