In this paper, training symbol designs for estimation of frequency selective channels and compensation of in-phase (I) and quadrature (Q) imbalances on orthogonal frequency division multiplexing (OFDM) transmitters and receivers are studied. We utilize cross entropy (CE) optimization techniques together with convex optimization to design training sequences having low channel estimate mean squared error (MSE) and minimum effects of I/Q mismatch, while lowering the peak power of the training signals. The proposed design provides better channel estimate MSE and bit error rate (BER) performances and is applicable to OFDM systems with and without null subcarriers. The efficacies of the proposed designs are corroborated by analysis and simulation results.

Conventional algorithms for the joint estimation of carrier frequency offset (CFO) and I/Q imbalance no longer work when the I/Q imbalance depends on the frequency. In order to correct the imbalance across many frequencies, the compensator needed is a filter as opposed to a simple gain and phase compensator. Although, algorithms for estimating the optimal coefficients of this filter exist, their complexity is too high for hardware implementation. In this paper we present a new low complexity algorithm for joint estimation of CFO and frequency dependent I/Q imbalance. For the first part, we derive the estimation scheme using the linear least squares algorithm and examine its floating point performance compared to conventional algorithms. We show that the proposed algorithm can completely eliminate BER floor caused by CFO and I/Q imbalance at a lesser complexity compared to conventional algorithms. For the second part, we examine the hardware complexity in fixed point hardware and latency of the proposed algorithm. Based on BER performance, the circuit needs a wordlength of at least 16 bits in order to properly estimate CFO and I/Q imbalance. In this configuration, the circuit is able to achieve a maximum speed of 115.9 MHz in a Virtex 5 FPGA.

In this paper, we study the channel estimation in the presence of the receiver in-phase and quadrature-phase (I/Q) imbalance and carrier frequency offset (CFO) for orthogonal frequency division multiplexing (OFDM) systems using pilot symbols. The concept of channel residual energy (CRE) [9] is used to solve the joint estimation problem. By minimizing the CRE, we can jointly estimate the receiver I/Q, CFO and channel response using the pilot symbols in one OFDM block. Simulation results show that the proposed method can provide good performance and also works well when applied to the terrestrial digital video broadcasting (DVB-T) systems.

It is known that a heterodyne multimode receiver implemented with a single RF (Radio Frequency) receiver suffers from image-band interference due to imbalance, i.e. the phase error and the gain imbalance of the RF Hilbert transformer. The blind image band interference canceler with deterministic imbalance estimation that has been proposed mitigates the image-band interference. This performance of the image-band interference canceler is analyzed theoretically in this paper. As a result, it is revealed that estimation accuracy of the deterministic imbalance estimation is improved slightly as the imbalance becomes greater. In addition, it is also shown that the deterministic estimation achieves better performance as the power of image-band interference increases. The performance is confirmed by computer simulation.

In OFDM systems, in-phase and quadrature (I/Q) imbalances generated in the analog front-end introduce inter-channel interference and, consequently, error performance degradation. This letter provides an exact expression involving the two-dimensional (2-D) Gaussian Q-function for the error probability of an arbitrary 2-D modulated OFDM signal with I/Q imbalances. The effects of I/Q imbalances on the distribution of an AWGN and the error performance are analyzed.

Due to the importance of maintaining the orthogonality among subcarriers, the estimation of carrier frequency offset (CFO) is a crucial issue in orthogonal frequency division multiplexing (OFDM) systems. The CFO estimation becomes complicated in OFDM direct-conversion receivers (DCRs), where additional analog impairments such as I/Q imbalance and time-varying DC offset (TV-DCO) exist. In this paper, we propose a novel joint estimation method for CFO and I/Q imbalance in the presence of TV-DCO. By using the linear property of the TV-DCO and employing a periodic pilot sequence, the desired estimates can be obtained in closed-form. Simulation results confirm the validity of the proposed method.

This letter proposes a compensation method that can alleviate the problem of I/Q mismatch generated in the direct-conversion receiver of OFDM systems. In the proposed method, the amount of I/Q mismatch is estimated using null-carriers in transmitted signals, and it is subtracted from received symbols to suppress I/Q mismatch effects. Simulations show experiments that the proposed method can effectively eliminate the I/Q mismatch effects.