Capacity of MIMO channels in the presence of co‐channel interference

This paper presents analytical results on the capacity of multiple‐input‐multiple‐output (MIMO) fading channels in the presence of co‐channel interference (CCI). We consider the scenario in which the desired and CCI users are all subject to Rayleigh fading. We assume that channel realizations of both the desired and CCI users are known at the receiver. Moreover, we consider the case where the transmitter does not have any CSI and as such equal‐power allocation among transmit antennas is used. Given this setup, we derive the moment generating function (MGF) and the mean of the mutual information (MI). We then study the complementary cumulative distribution function of the MI using a Gaussian approximation. Finally, we present and discuss numerical examples to illustrate the mathematical formalism and to show the effect of various parameters on the capacity of MIMO channels in the presence of CCI. Copyright © 2006 John Wiley & Sons, Ltd.     

Soft handover overhead reduction by RAKE reception with finger reassignment

We propose and analyze in this paper a new finger assignment technique that is applicable for RAKE receivers when they operate in the soft handover (SHO) region. This scheme employs a new version of generalized selection combining (GSC). More specifically, in the SHO region, the receiver uses by default only the strongest paths from the serving base station (BS) and only when the combined signal-to-noise ratio (SNR) falls below a certain pre-determined threshold, the receiver uses more resolvable paths from the target BS to improve the performance. Hence, relying on some recent results on order statistics we attack the statistics of two correlated GSC stages and provide the approximate but accurate closed-form expressions for the statistics of the output SNR. By investigating the tradeoff among the error performance, the path estimation load, and the SHO overhead, we show through numerical examples that the new scheme offers commensurate performance in comparison with more complicated GSC-based diversity systems while requiring a smaller estimation load and SHO overhead.     

Performance comparison of different selection combining algorithms in presence of co-channel interference

In this paper, we present a unified approach for the computation of the outage probability, the level crossing rate (LCR), and the average outage duration (AOD) of selection combining (SC) in the presence of multiple cochannel interferences and under both minimum signal-to-interference ratio (SIR) and desired signal power constraints. We consider three selection algorithms, namely: 1) the best signal power algorithm; 2) the best SIR algorithm; and 3) the best total power (desired plus interference) algorithm. As a specific application example, we analyze the three algorithms for a low-complexity dual-branch SC receiver subject to multiple interferers over Rayleigh fading channels. When applicable, the new results are compared to those previously reported in the literature dealing with the outage probability, AOD, and LCR of 1) interference-limited systems and 2) power-limited systems. Numerical examples show that the minimum desired signal power constraint induces a floor to the outage probability, AOD, and LCR performance measures. They also show that the best SIR algorithm provides the best outage probability and AOD performance for low average SIR. On the other hand, the best signal power algorithm and the best S+I algorithm outperform the best SIR algorithm for high average SIR. It is also shown that the best SIR algorithm tends to have more outage level crossings.     

Performance Analysis of Joint Adaptive Modulation and Diversity Combining Over Fading Channels

Both adaptive modulation and diversity combining represent important enabling techniques for future generations of wireless communication systems. In this paper, capitalizing on recent developments in adaptive combining, we propose three joint adaptive modulation and diversity combining (AMDC) schemes. With these schemes, the modulation mode and diversity combiner structure are adaptively determined based on the fading channel condition and error-rate requirement. We accurately analyze these three AMDC schemes in terms of processing power consumption, spectral efficiency, and error-rate performance. Selected numerical examples show that the proposed AMDC systems meet the target error-rate requirement while achieving high spectral efficiency with low processing power consumption     

Performance analysis of switch‐based multiuser scheduling schemes with adaptive modulation in spectrum sharing systems

This paper focuses on the development of multiuser access schemes for spectrum sharing systems whereby secondary users are allowed to share the spectrum with primary users under the condition that the interference observed at the primary receiver is below a predetermined threshold. In particular, two scheduling schemes are proposed for selecting a user among those that satisfy the interference constraint and achieve an acceptable signal‐to‐noise ratio level. The first scheme focuses on optimizing the average spectral efficiency by selecting the user that reports the best channel quality. In order to alleviate the relatively high feedback required by the first scheme, a second scheme based on the concept of switched diversity is proposed, where the base station (BS) scans the secondary users in a sequential manner until a user whose channel quality is above an acceptable predetermined threshold is found. We develop expressions for the statistics of the signal‐to‐interference and noise ratio as well as the average spectral efficiency, average feedback load, and the delay at the secondary BS. We then present numerical results for the effect of the number of users and the interference constraint on the optimal switching threshold and the system performance and show that our analysis results are in perfect agreement with the numerical results. Copyright © 2014 John Wiley & Sons, Ltd.     

Postdetection switched combining - a simple diversity scheme with improved BER performance

This paper develops, analyzes, and optimizes a simpler form of dual-branch switch-and-stay combining (SSC), namely, one that relies on the output signal plus noise rather than the instantaneous signal-to-noise ratio (SNR) to trigger the switching between the diversity branches. Analysis supported by numerical results show that the newly proposed postdetection SSC scheme outperforms predetection SSC and this performance gain increases as the channel conditions improve in terms of average SNR and/or severity of fading. In brief, when simplicity of implementation is of primary concern, as is the case, for example, in mobile units, the proposed scheme offers an attractive low-complexity solution to mitigate the deleterious effects of multipath fading.     

Performance analysis of multibranch switched diversity systems

We investigate the performance of multibranch switched diversity systems. Specifically, we first derive generic formulas for the cumulative distribution function, probability density function, and moment-generating function of the combined signal power for both switch-and-stay combining (SSC) and switch-and-examine combining (SEC) schemes. We then capitalize on these expressions to obtain closed-form expressions for the outage probability and average error rate for various practical communication scenarios of interest. As a byproduct of our analysis we prove that for SSC with identically distributed and uniformly correlated branches, increasing the number of branches to more than two does not improve the performance, but the performance can be different in the case the branches are not identically distributed and/or not uniformly correlated. We also show that, in general, the SEC performance improves with additional branches. The mathematical formalism is illustrated with a number of selected numerical examples.     

Estimation of Nakagami-m fading channel parameters with application to optimized transmitter diversity systems

An optimum power loading algorithm for transmitter diversity systems over correlated and unbalanced Nakagami (1960) paths and its performance evaluation under perfect channel estimate conditions are derived. In addition, various online estimators of the required Nakagami channel parameters for optimized power loading and the comparison of their mean square error via Monte Carlo simulations are presented. Some of these estimators are used to obtain the performance of optimized transmitter diversity systems under imperfect channel estimation (ICE). These numerical results show that the diversity gain of these optimized systems compared with equipower systems increases as the severity of fading decreases and as the degree of branch imbalance increases even under ICE. On the other hand, in weakly correlated and (or) unbalanced branches, optimized transmitter diversity systems offer negligible gain or even losses compared with unoptimized systems because of the ICE.     

Probability of error for noncoherent M-ary orthogonal FSK with postdetection switched combining

The paper extends the notion of postdetection switched combining introduced by the authors to M-ary orthogonal modulation (Alouini and Simon, ibid., p.1591-1602, 2003) and proceeds to analyze and evaluate its average bit-error rate performance for the dual-branch case. The particular variation chosen for the switching strategy uses a different model than that previously used for the binary case and although it results in a slightly poorer performance, it nevertheless outperforms conventional (predetection) switched combining for all values of M. Results are obtained for a variety of popular channel models including Rayleigh, Rician, and Nakagami-m fading. Because of its relative simplicity of implementation, the proposed scheme once again offers a very attractive low-complexity solution for mitigating the deleterious effects of multipath fading.     

Effect of imperfect phase and timing synchronization on the bit-error rate performance of PSK modulations

A great deal of attention has been devoted in the recent literature to the study of the bit-error rate (BER) performance of phase-shift keyed (PSK) constellations, due to their high spectral efficiency. It is well known that an error in phase or timing synchronization affects the probability of correct decisions at the receiver. This problem becomes much more significant as data rates increase, and the corresponding symbol durations decrease. In this letter, we look into this problem and analyze the exact BER performance of generalized hierarchical PSK constellations under imperfect phase or timing synchronization over additive white Gaussian noise channels.