This paper presents a method for estimating the in-phase/quadrature-phase (I/Q) mismatch in low-intermediate frequency receivers for orthogonal frequency division multiplexing (OFDM) signals in the presence of carrier-frequency offset. The proposed estimator operates on the repetitive training sequence of the OFDM packet preamble. To derive a practical maximum-likelihood estimator, an invertible mapping operation is applied on the observation vectors, so that a closed-form expression of the likelihood function can be obtained. The proposed estimator is shown to achieve the Cramer Rao lower bound at high image signal power levels. The error variance of the estimator is analyzed, which closely matches simulation results. The uncoded bit-error-rate is shown via simulation to be approximately that of an ideal receiver without I/Q mismatch.

We investigate the performance of orthogonal frequency division multiplexing (OFDM)-based dual-hop amplify-and-forward (AF) relay networks in the presence of phase noise (PHN). We show that the use of an AF relay may not be beneficial compared to a direct transmission in the presence of PHN. Using outage probability analysis, an upper bound on the allowable PHN level is derived which ensures that the dual-hop outperforms the direct transmission. To improve dual-hop transmission performance in the presence of PHN, we propose a reduced-complexity joint channel and PHN estimator using full-pilot OFDM symbols for AF relay transmission. The proposed approach achieves a lower mean squared error compared to the conventional channel estimator. In addition, we derive a joint data detection and PHN estimation scheme for comb-type data OFDM symbols by modifying the maximum ratio combining metric to account for the effect of PHN at the destination node.

In this paper, we propose a robust non-iterative pilot-aided channel estimator for multi-input multi-output orthogonal frequency division multiplexing systems in the presence I/Q imbalance and phase noise. We show that by exploiting the phase noise and I/Q imbalance properties, the joint cost function for estimating them reduces to separate cost functions, each of which can be readily solved. Mean-squared-error performance of the proposed estimator is evaluated for IEEE 802.11n wireless local area network applications.

In this paper, the bit error rate performances of multi-input multi-output orthogonal frequency division multiplexing transmission schemes for two and four transmit antennas are studied and compared in the presence of phase noise, I/Q imbalance and channel estimation errors. It is shown that for matched rates and same decoding complexity, maximum diversity schemes such as the Alamouti code, perform better than the maximum capacity schemes such as spatial multiplexing. The simulation environment comply with IEEE 802.11n standard.

The maximum likelihood estimate of the impulse response of a frequency-selective channel in the presence of phase noise and I/Q imbalance is derived. The complexity of the joint estimator is reduced using approximate cost functions for both phase noise and I/Q imbalance. The proposed estimator is first applied to OFDM transmission in a single-input single-output system and then generalized to multi-input multi-output OFDM systems. The bit error rate performance of popular space-time codes for two and four transmit antennas is evaluated under zero-forcing, minimum mean squared error and maximum likelihood detection rules. An expression for the residual inter-carrier interference variance after phase noise and I/Q imbalance compensation is derived and compared to the uncompensated case. Significant improvement in signal-to-interference-noise is obtained with the proposed algorithm.

A new method for detecting packet-based OFDM signals in the presence of phase noise is presented. The scattered pilots in each data OFDM symbol are used to non-iteratively estimate and mitigate the phase noise induced inter-carrier interference (ICI). A significant improvement in the bit error rate (BER) is achieved using this technique, especially for systems suffering from high phase noise.

In this paper, we derive the maximum likelihood (ML) channel estimate in the presence of phase noise (PHN) and I/Q imbalance for OFDM systems. Since it is estimated jointly with I/Q and PHN, the channel estimate is more robust to these RF impairments compared to the conventional schemes. The complexity of the joint estimator is reduced using an approximate cost functions for both PHN and I/Q imbalance. The performance of the proposed estimator in terms of both channel mean squared error (MSE) and bit error rate (BER) is also presented and compared to the case of perfect channel knowledge.

We design a novel class of space-time codes, called linear diversity-embedding space-time block codes (LDE-STBC) where a high-rate STBC is linearly superimposed on a highdiversity STBC without requiring channel knowledge at the transmitter. In applying this scheme to multimedia wireless communications, each traffic type constitutes a transmission layer that operates at a suitable rate-diversity tradeoff point according to its quality-of-service requirements. This, in turn, provides an unequal-error-protection (UEP) capability to the different information traffic types and allows a form of wireless communications where the high-rate STBC opportunistically takes advantage of good channel realizations while the embedded high-diversity STBC ensures that at least part of the information is decoded reliably. We investigate transceiver design issues specific to LDE-STBC including reduced-complexity coherent decoding and effective schemes to vary the coding gain to further enhance UEP capabilities of the code. Furthermore, we investigate the application of LDE-STBC to wireless multicasting and demonstrate its performance advantage over conventional equal-error-protection STBC.

We propose a new full-rate space-time block code (STBC) for two transmit antennas which can be designed to achieve maximum diversity or maximum capacity while enjoying optimized coding gain and reduced-complexity maximum-likelihood (ML) decoding. The maximum transmit diversity (MTD) construction provides a diversity order of 2Nrfor any number of receive antennas Nrat the cost of channel capacity loss. The maximum channel capacity (MCC) construction preserves the mutual information between the transmit and the received vectors while sacrificing diversity. The system designer can switch between the two constructions through a simple parameter change based on the operating signal-to-noise ratio (SNR), signal constellation size and number of receive antennas. Thanks to their special algebraic structure, both constructions enjoy low-complexity ML decoding proportional to the square of the signal constellation size making them attractive alternatives to existing full-diversity full-rate STBCs in [6], [3] which have high ML decoding complexity proportional to the fourth order of the signal constellation size. Furthermore, we design a differential transmission scheme for our proposed STBC, derive the exact ML differential decoding rule, and compare its performance with competitive schemes. Finally, we investigate transceiver design and performance of our proposed STBC in spatial multiple-access scenarios and over frequency-selective channels.

Diversity-embedded space-time block coding (DE- STBC) introduced in [1] is a class of non-orthogonal STBC which can provide multiple diversity orders and transmission rates for the information symbols. This is achieved by transmitting the information symbols in layers each characterized by a diversity-rate tuple according to its QoS requirement. This paper derives a closed-form performance bound for DE-STBC over flat fading channels using outage probability analysis. Furthermore, the effective coding gain and system throughput are analyzed and optimized based on the derived outage expressions.

Diversity-embedding space-time block coding (DE-STBC), introduced in the work of Diggavi et al. (2003), enables unequal error protection (UEP) using multiple transmit antennas. Even though these codes do not require channel state information (CSI) at the transmitter, they do need it at the receiver for decoding. A novel differential DE-STBC scheme is proposed in this paper to eliminate the need for channel estimation at the receiver which is especially costly with multiple transmit and receive antennas. Most previously proposed differential schemes in the literature are based on orthogonal STBC and hence are not applicable to the non- orthogonal family of DE-STBC considered in this paper.

A new information lossless, full-rate space-time block code is presented for 2 transmit antennas over 2 symbol periods. We show that our code is capacity-achieving and exploit its special algebraic structure to derive a reduced-complexity maximum-likelihood (ML) decoding algorithm. When the number of receive antennas is greater than 1, our code outperforms the Alamouti code. Comparing the performance of our code with state-of-the-art B2Phiand Golden codes in [4] and [7], demonstrates competitive performance for practical low-to-medium SNR ranges, for high spectral efficiencies, and as the number of receive antennas increases while requiring much lower ML decoding complexity.

A method is introduced for model order reduction of large circuits extracted from layout. The algorithm, which is based on balanced realization, can be used for reducing the order of circuits before circuit-level simulation. In contrast to Pade-based algorithms which match the reduced order system with original system in some given frequencies, balanced realization based model algorithms provide a nearly optimal matching over all frequencies. Hence the balanced realization method produces stable and more accurate results compared to the Pade-based algorithms for model reduction. In addition given an upper bound for error, it is possible to compute the minimum degree for the reduced order model a priori. A numerically efficient method for balanced truncation of large circuits using the Arnoldi algorithm is presented and experimental results are reported.