The effect of DC offset on multi-input multi-output (MIMO) direct transceivers with adaptive modulation (AM) is discussed in this paper. A variable-rate variable-power (VRVP) AM system with perfect channel state information (P-CSI) at both the transmitter and receiver in a MIMO scenario is considered. The DC offset is modeled as a zero mean complex Gaussian distributed random variable. By this modeling of the DC offset, the analytical expression for degraded bit error rate (BER) is derived. To derive this analytical expression, we establish a reasonable approximation. The good agreement between the analytical and simulation results shows that the approximation is valid and confirms the accuracy of the analytical expressions. Moreover, an approach to improve the degraded BER in these systems is introduced. For this purpose, we introduce a design for AM MIMO systems that takes account of DC offset and its effectiveness is confirmed. Throughput analysis for the AM MIMO system in the presence of DC offset is presented in this paper too. An analytical expression for throughput is derived and approximated to a simpler equation. At last, throughput results are compared to the simulation outcomes.

The accuracy of the comparator, which is often determined by its offset, is essential for the resolution of the high performance mixed-signal system. Various design efforts have been made to cancel or calibrate the comparator offset due to many factors like process variations, device thermal noise and input-referred supply noise. However, effective and simple method for offset cancel by applying additional circuits without scarifying the power, speed and area is always challenging. This work explores a dynamic offset control technique that employs charge compensation by timing control. The charge injection and clock feed-through by the latch reset transistor are investigated. A simple method is proposed to generate offset compensation voltage by implementing two source-drain shorted transistors on each regenerative node with timing control signals on their gates. Further analysis for the principle of timing based charge compensation approach for comparator offset control is described. The analysis has been verified by fabricating a 65 nm CMOS 1.2 V 1 GHz comparator that occupies 25 65 µm2 and consumes 380 µW. Circuits for offset control occupies 21% of the areas and 12% of the power consumption of the whole comparator chip.

One of the major drawbacks of orthogonal frequency division multiplexing (OFDM) systems is their vulnerability to synchronization errors. To remedy the inter-carrier interference (ICI) effect caused by carrier frequency offset (CFO) estimation errors, this paper proposes a weighted linear parallel ICI cancellation (WLPICIC) equalizer. The optimal weights in the WLPICIC scheme are derived in closed-form expressions by maximizing the average signal-to-interference ratio (SIR) at the WLPICIC output of each sub-carrier. The simulation results show that the WLPICIC equalizer significantly improves the performance of OFDM systems with frequency estimation errors in both AWGN channels and frequency selective fading channels.

In this letter, we propose a simple distributed space-frequency code with both timing errors and multiple carrier frequency offsets (CFO) in asynchronous cooperative communications. By employing both the Alamouti coding approach and the transmit repetition diversity technique, full diversity gain can be achieved by the fast symbol-wise maximum likelihood (ML) decoding at the destination node. Analysis and simulations demonstrate the effectiveness of the proposed method.

Carrier frequency offset may distort the orthogonality of the subcarriers in OFDM systems and it must be estimated and compensated to maintain the system performance. A blind carrier frequency offset estimator based on the histogram of the received signal's phase is proposed in this letter. The proposed estimator can operate under additive white Gaussian noise and multipath channels without known training signal, redundant guard interval, and virtual carrier. Compared to subspace-based blind estimators, the proposed estimator can provide better mean-square-error performance.

In next-generation mobile communications, it is important to improve the throughput of the cell edge as well as that of the whole cell. Multi-link transmission from two adjacent BSs has been studied for improving the throughput at the cell edge in OFDM-based cellular systems, which are major candidates for next-generation mobile communication systems. In multi-link transmission, an MS at a cell edge receives signals from both adjacent BSs by orthogonally multiplexing those signals in the frequency domain. Therefore, the cell-edge MS can utilize the frequency and power resources of both adjacent BSs, which improves the cell-edge throughput. However, when the received timing difference between the signals from both BSs exceeds the maximum permissible value, adjacent-channel interference is caused by the collapse of the orthogonality. In this paper, to resolve this issue, we propose a novel timing-offset interference canceller. To clarify the performance of the proposed canceller, this paper evaluates its performance with respect to the residual interference power and the BER by computer simulation.

When a zero offset reference sequence is defined, the i-bit shifted sequence has phase offset i with respect to the reference sequence. In this letter, we propose a new algorithm to compute phase offsets for a periodic binary sequence using the concept of order and index of an integer based on the number theoretical approach. We define an offset evaluation function that is used to calculate the phase offset, and derive properties of the function. Once the function is computed, the phase offset of the sequence is simply obtained by taking the index of it. The new algorithm overcomes the restrictions found in conventional methods on the length and the number of '0's and '1's in binary codes. Its application to the code acquisition is also investigated to show the proposed method is useful.

In this letter, a derivative constraint minimum output energy (MOE) receiver is proposed the offers enhanced robustness against carrier frequency offset (CFO). A theoretical analysis of the output signal-to-interference-plus-noise ratio (SINR) is presented to confirm its efficacy. Numerical results demonstrate that the proposed receiver basically offers the same performance as an optimal receiver with no CFO present.

Multipath is one of the major error sources that deteriorates tracking performance in global navigation satellite system (GNSS). In this letter, the orthogonal matching pursuit (OMP) algorithm is used to estimate multipaths which are highly correlated with the line of signal (LoS) signal. The estimated multipaths are subtracted from the received signal such that the autocorrelation function of the received signal is restored to optimize the tracking performance. The performance of the proposed technique is verified via computer simulations under the multipath environment of GNSS.

Video coding standard H.264/AVC employs transform coding to explore spatial correlation in inter picture prediction residue. This paper presents a block based DC offset to further explore the correlation in spatially neighboring blocks and provides H.264/AVC an enhanced coding efficiency performance. The proposed method applies DC offset to inter picture prediction residue, and encodes the offset compensated residual signal. The DC offset is derived from reconstructed residue in neighboring blocks. No additional bits are required for the DC offset representation. Simulation results report that the proposed method yields an average of 2.67% bit rate reduction for high resolution video over the H.264 baseline profile.

It is important to improve a cell-edge throughput of next generation mobile communication systems. Frequency reuse schemes such as three-cell reuse or fractional frequency reuse are suitable for achieving this goal. Another candidate is multi-link transmission; signals on different sub-carriers from adjacent base stations are received by a mobile. However, the orthogonality of these signals can collapse if a frequency offset between adjacent base stations is excessive; this loss triggers adjacent-channel interference. This paper proposes an interference canceller to solve this problem and confirms the effectiveness of the method through numerical analysis and computer simulations.

In this paper, a single-chip dual-mode 8-band 130 nm CMOS transceiver including A/D/A converters and digital filters with 312 MHz LVDS interface is presented. For a transmitter chain, linear direct quadrature modulation architecture is introduced for both W-CDMA/HSDPA (High Speed Uplink Packet Access) and for GSM/EDGE. Analog baseband LPFs and quadrature modulators are commonly used both for GSM and for EDGE. For a direct conversion receiver chain, ABB (Analog Base-Band) blocks, i.e., LPFs and VGAs, delta-sigma A/D converters, and FIR filters are commonly used for W-CDMA/HSDPA (High Speed Downlink Packet Access) and GSM/EDGE to reduce chip area. Their characteristics can be reconfigured by register-based control sequence. The receiver chain also includes high-speed DC offset cancellers both in analog and in digital stage, and the self-contained AGC controller, whose parameters such as time constant are programmable to be free from DBB (Digital Base-Band) control. The transceiver also includes wide-range VCOs and fractional PLLs, an LVDS driver and receiver for high-speed digital interface of 312 MHz. Measured results reveal that the transceiver satisfies 3GPP specifications for W-CDMA/HSPA (High Speed Packet Access) and GSM/EDGE.

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.

In this paper, the issue of carrier frequency offset (CFO) compensation in interleaved orthogonal frequency division multiple access (OFDMA) uplink system is investigated. To mitigate the effect of multiple access interference (MAI) caused by CFOs of different users, a new parallel interference cancellation (PIC) compensation algorithm is proposed. This scheme uses minimum mean square error (MMSE) criterion to obtain the estimation of interference users, then circular convolutions are employed to restore MAI and compensate CFO. To tackle the complexity problem of circular convolutions, an efficient MAI restoration and cancellation method is developed. Simulations illustrate the good performance and low computational complexity of the proposed algorithm.

This paper proposes a frequency asynchronous cross-polarization interference canceller for Vertical/Horizontal (V/H) polarization multiplexing satellite communications. In satellite communications, V/H polarization signals are likely to experience different frequency fluctuations, and so the cross-polarization undergoes two different frequency fluctuations. To cancel this cross-polarization interference, a new frequency asynchronous cross-polarization interference canceller that removes interference and frequency offsets is proposed. Computer simulations are carried out to evaluate its fundamental performance. The results show that the proposed canceller can remove the cross-polarization interference created by the two different frequency offsets, simultaneously.

Direct conversion receivers in orthogonal frequency division multiplexing (OFDM) systems suffer from direct current (DC) offset, frequency offset, and IQ imbalance. We have proposed an IQ imbalance estimation scheme in the presence of DC offset and frequency offset, which uses preamble signals in the time domain. In this scheme, the DC offset is eliminated by a differential filter. However, the accuracy of IQ imbalance estimation is deteriorated when the frequency offset is small. To overcome this problem, a new IQ imbalance estimation scheme in the frequency domain with the differential filter has been proposed in this paper. The IQ imbalance is estimated with pilot subcarriers. Numerical results obtained through computer simulation show that estimation accuracy and bit error rate (BER) performance can be improved even if the frequency offset is small.

If ranging processes are not frequent in an uplink OFDMA system, timing synchronization between the base and mobile stations may not be maintained and the performance may be degraded. This paper proposes a random access scheme in which a short OFDMA symbol is transmitted to maintain the orthogonality with timing offsets. A short symbol is constructed by inserting zero-padding to an OFDMA symbol.

In OFDM systems, the estimation/correction of carrier frequency offset (CFO) is crucial to maintain orthogonality among subcarriers. However, the CFO estimation suffers from DC offset (DCO) generated in low-cost direct-conversion receivers (DCRs). More seriously, in practice, DCO is time-varying due to the automatic gain control. In this paper, a novel CFO estimator in the presence of time-varying DCO is proposed. It is shown the residual DCO after high-pass filtering varies in a linear fashion. Based on this observation and the periodicity of the training sequence, we derive a CFO estimator independent of DCO. Also, the residual DCO can be estimated, using the obtained CFO. The validity of the proposed estimation method is demonstrated by simulations.

This letter presents recursive frequency offset estimation for MIMO (Multiple Input Multiple Output) system in flat-fading channels. With the recursive estimation, the frequency offset range in MIMO system can be extended compared with normal estimation. Simulation results show that the recursive frequency offset estimation is valid for large frequency offset.

In this letter, we present the impact of carrier frequency offset (CFO) in dual-hop orthogonal frequency division multiplexing (OFDM) systems with a fixed relay for frequency-selective fading channels. Approximate expressions of the average signal-to-noise ratios (SNRs) for both downlink and uplink are obtained and validated by simulations. It is shown that dual-hop systems have slightly worse average SNR degradation than single-hop systems. We also show that the average SNR degradation due to the CFO varies according to the gap between average received SNRs for the first and the second hop.