In this paper, we present a novel low-complexity technique, which improves the performance of single-antenna multi-carrier communication systems, suffering from in-phase and quadrature (I/Q)-imbalance (IQI) at the receiver. We refer to the proposed scheme as I/Q-imbalance self-interference coordination (IQSC). This technique not only mitigates the detrimental effects of IQI, but, through appropriate signal processing, also coordinates the self-interference terms produced by IQI in order to achieve second-order frequency diversity. However, these benefits come at the expense of a reduction in transmission rate. More specifically, IQSC is a simple transmit diversity scheme that improves the signal quality at the receiver by elementary signal processing operations across symmetric (mirror) pairs of subcarriers. Thereby, the proposed transmission protocol has a similar complexity as Alamouti's space-time block coding scheme and does not require extra transmit power nor any feedback. To evaluate the performance of IQSC, we derive closed-form expressions for the resulting outage probability and symbol-error rate. Interestingly, IQSC outperforms not only existing IQI compensation schemes but also the ideal system without IQI for the same spectral efficiency and practical target error rates, while it achieves almost the same performance as ideal (i.e., IQI-free) equal-rate repetition coding. Our findings reveal that IQSC is a promising low-complexity technique for significantly increasing the reliability of low-cost devices that suffer from high levels of IQI.

We employ the theory of multidimensional lattice constellations in order to evaluate the maximum-likelihood (ML) performance of a general multiple-input multiple-output MIMO system. We show that the lattice representation of the equivalent system model enables a mapping of the received signal set to a single point in a multidimensional constellation, with the generator matrix being equal to the equivalent channel matrix. With the aid of this geometrical interpretation, we derive a performance limit for a general MIMO system in terms of the symbol error probability, which can be written in closed form for various designs. Application of the proposed analysis to popular MIMO schemes reveals the tractability of the approach and validates that designs exploiting the available degrees of freedom in a better way approach the proposed performance limit.

We propose a generalized framework for handling Multiple-Input Multiple-Output (MIMO) schemes defined within the context of the Third Generation Partnership Project (3GPP) physical layer specifications. A detailed review of all the multi-antenna transmission techniques adopted by the 3GPP so far is firstly presented, starting from the Global System for Mobile (GSM) communications and passing through the High-Speed Packet Access (HSPA) releases to the very recent Long Term Evolution (LTE)-Advanced standard. Motivated by the outcome of this overview, we introduce the Transmit Matrix (TRAM) concept, that can include as special cases several MIMO systems (e.g. transmit diversity, spatial-multiplexing, etc.) regardless of the presence or not of a feedback channel. Our approach is based on the adoption of a novel measure called Necessary Transmit Information (NTI) and on its one-to-one correspondence with the TRAM. The last allows for a convenient alternative representation of a TRAM-based MIMO system model and serves the straightforward construction of NTI-codebooks for a wide range of transmission schemes. The indicative applications provided reveal the universal nature of the proposed methodology.

In this paper, a closed-loop wireless communication system employing Quasi-Orthogonal Space-Time Block Codes (QOSTBCs) is presented, where the total available power can be suitably allocated to the four transmit antennas, considering a very low-complexity decoding scheme at the receiver side. After deriving a particular optimization problem concerning the optimal choice of the power weights, we prove that the best strategy in terms of error probability is to switch-off three out of the four transmit antennas under the requirement of symbol-by-symbol maximum-likelihood decoding (MLD). Our proposed scheme requires a very low-data rate dedicated feedback link, since just one or two bits per block are sufficient for outperforming other existing closed-loop systems. The technique applies to all the well-known QOSTBCs with arbitrary number of receive antennas, while some special cases are taken indicatively for illustration purposes.

Monotonicity criteria are established for the generalized MarcumQ-function,QM(alpha,beta), the standard NuttallQ-function,QM,N(alpha,beta) , and the normalized NuttallQ-function,QM,N(alpha,beta), with respect to their real order indicesM,N. Besides, closed-form expressions are derived for the computation of the standard and normalized NuttallQ-functions for the case whenM,Nare odd multiples of 0.5 andMgesN. By exploiting these results, novel upper and lower bounds forQM,N(alpha,beta) andQM,N(alpha,beta) are proposed. Furthermore, specific tight upper and lower bounds forQM(alpha,beta), previously reported in the literature, are extended for real values ofM. The offered theoretical results can be efficiently applied in the study of digital communications over fading channels, in the information-theoretic analysis of multiple-input multiple-output systems and in the description of stochastic processes in probability theory, among others.