Post-processing Signal-to-noise Ratio Research Articles

Heterogeneous Semi-Blind Interference Alignment in Finite-SNR Networks With Fairness Consideration

Standard blind interference alignment (sBIA) suffers from noise accumulation which severely deteriorates received signal-to-noise ratio (SNR) and significantly reduces transmission rate. A noise accumulation factor is proposed to describe the loss between the user’s received SNR, and the final post processing SNR which determines the performance of the decoding of the encoded data streams (EDSs). A heterogeneous semi-BIA (H-SBIA) framework where users with different noise accumulation factors can be flexibly allocated effective EDSs (E-EDSs) is constructed. Relying on the H-SBIA framework, a heuristic H-SBIA algorithm is designed for maximizing the overall E-EDSs considering both fairness and coherence time constraints. Extensive simulations demonstrate that H-SBIA produces great fairness performance improvement at a limited cost in the achievable sum rate. The Jain’s fairness index is about 2.2 times greater than that for SNR-SBIA proposed in previous work, at the cost of sacrificing 10% of the achievable sum rate.

Remarks on the Performance of the Linear Detectors in MIMO Spatial Multiplexing Systems

Linear minimum mean square error (MMSE) detector has been shown to alleviate the noise amplification problem associated with the conventional zero-forcing (ZF) detector. In this paper, we analyze the performance improvement by the MMSE detector in terms of the condition number of its filtering matrix, the post-processing signal to noise ratio (SNR) improvement, and the variance of the postprocessing SNRs. To this end, we derive explicit formulas for the condition numbers of the filtering matrices and the post-processing SNRs. Analytical and simulation results demonstrate that the improvement achieved by the MMSE detector over the ZF detector does not only depend on the noise variance and the condition number of the channel matrix, but also on how close the smallest singular values are to the noise variance. This conjecture is also proven correct via analyzing the post-processing SNR of both detectors.

Performance of Reconfigurable Antennas in a Below-Decks Environment

Reconfigurable antennas have been proposed for mitigating multipath interference and increasing channel capacity in wireless networks. The majority of studies which have investigated these claims do so either in simulation or through the use of software defined radios in lab or office environments. There has been little work in quantifying experimentally the performance gains of a reconfigurable antenna in highly metallic environments representative of military and industrial applications. This letter quantifies the performance gains provided by the use of electrically reconfigurable antennas in the place of omnidirectional antennas given varying environmental and system configurations. Wireless measurements for various 802.11-like physical layers were performed in a set of multideck, coupled compartments aboard Thomas S. Gates (CG 51), a decommissioned Ticonderoga-class U.S. Navy cruiser. The reconfigurable antennas were observed to provide higher channel capacities than omnidirectional antennas. When the bulkhead doors connecting the coupled compartments were left open, the postprocessing signal to noise ratio (PP-SNRs) of signals received from reconfigurable antennas were up to 4 dB higher than what was observed when using omnidirectional antennas. As the compartments became less electrically coupled (i.e., bulkhead doors were closed), the benefit provided via the antenna pattern diversity of the reconfigurable antennas diminished.

Maximum-SNR Antenna Selection Among a Large Number of Transmit Antennas

The recent increased interest in large-scale multiple- input multiple-output systems, combined with the cost of analog radio-frequency (RF) chains, necessitates the use of efficient antenna selection (AS) schemes. Capacity or signal-to-noise ratio (SNR) optimal AS has been considered to require an exhaustive search among all possible antenna subsets. In this work, we prove that, under a total power constraint on the beamformer, the maximum-SNR joint beamforming transmit AS problem with two receive antennas and an arbitrary number of transmit antennas N is polynomially solvable and develop an algorithm that solves it with quartic complexity, independently of the number of selected antennas. The algorithm identifies with complexity O(N 4 ) a cubic-size collection of antenna subsets that contains the one that maximizes the post-processing receiver SNR. From a different perspective, for any given two-row complex matrix, our algorithm computes with quartic complexity its two-row submatrix with the maximum principal singular value, for any number of selected columns. In addition, our method also applies to receive AS with two transmit antennas. Finally, if we enforce a per-antenna-element power constraint on the beamformer (i.e., constant-envelope transmission), then the set of transmit AS subsets that contains the optimal one is the same as in the total power constraint case. Therefore, our algorithm offers a practical solution to the maximum-SNR antenna selection problem when either the transmitter or the receiver consists of a large number of antennas.

Performance Analysis of Combining Space-Time Block Coding and Scheduling over Arbitrary Nakagami Fading Channels

In this paper, we investigate a combining space-time block coding (STBC) and opportunistic scheduling referred to as a joint diversity in a multiple-input multiple-output (MIMO) system. We study the performance analysis of the joint diversity over the arbitrary Nakagami fading channels including a fading index of real and integer values. The exact symbol error rate (SER) expressions of the joint diversity are derived for phase shift keying (PSK) and quadrature amplitude modulation (QAM) including quantification of both the diversity order and signal to noise ratio (SNR) gain. From analysis results, we show that the number of users and fading index improve the diversity order of the joint diversity for SER regardless of modulation types. On the other hand, the number of users and fading index degrade the SNR gain of the joint diversity for SER with both modulations. Further, this paper derives the exact average post-processing SNR, the exact outage probability and the exact ergodic capacity including the tight outage capacity of the joint diversity over the arbitrary Nakagami fading channels including the fading index of real and integer values. From analysis results, we show that both average post-processing SNR and ergodic capacity of the joint diversity is degraded as the fading index increases, while the outage capacity is improved.

Performance Analysis of Scheduled Cognitive Radio Systems With MIMO Transmission

In this paper, we investigate the performance analysis of cognitive radio (CR) multiple-input-multiple-output (MIMO) systems, including the scheduled transmit antenna selection (TAS) with maximal ratio combining (MRC) and the scheduled space-time block coding (STBC). Here, the scheduled MIMO systems transmit data of a user to provide the maximum instantaneous postprocessing signal-to-noise ratio (SNR) at the CR receiver. For the closed-form performance analysis, we derive approximated probability density functions (pdfs) of the instantaneous postprocessing SNR at the receiver for the scheduled TAS with MRC and the scheduled STBC systems, respectively. Based on the results, we derive the approximated closed-form expressions of the average postprocessing SNR and ergodic capacity for the scheduled CR systems with MIMO transmission. Through the analysis, we provide the achievable average postprocessing SNR and ergodic capacity of the proposed scheduled systems as a function of the peak interference power constraint. In addition, we present the impact of imperfect channel state information (CSI) on the ergodic capacity for the scheduled TAS with MRC and the scheduled STBC systems through arbitrary order statistics, respectively.

Robust ergodic rate transmission for time division duplex beamforming systems

In time division duplex (TDD) beamforming systems, the base station estimates the channel state information (CSI) at transmitter based on uplink pilots and then uses it to generate the beamforming vector in the downlink transmission. Because of the constraints of the TDD frame structure and the uplink pilot overhead, there inevitably exists CSI delay and channel estimation error between CSI estimation and downlink transmission channel, which would degrade system ergodic rate. In this paper, we propose a robust ergodic rate transmission scheme, in which the uplink pilot time interval (UPTI) of an active user is adaptively adjusted according to the changing channel conditions such as Doppler frequency shift, uplink pilot signal to noise ratio (SNR), to minimize the impact of CSI delay and channel estimation error on the ergodic rate of TDD beamforming systems. In order to get the optimal UPTI, we first derive the average post-processing SNR for TDD beamforming systems with channel estimation error and CSI delay. We then obtain the optimal UPTI, which maximizes the average post-processing SNR, given the normalized pilot overhead (the number of pilot symbols per data symbol). The numerical simulation results validate that the the proposed robust ergodic rate transmission scheme not only maximizes the average post-processing SNR but also maximizes the system ergodic rate. Moreover, the scheme can adapt well to the changing channel environments compared with the current fixed UPTI scheme. Especially our research is valuable for the uplink sounding reference signal design in long term evolution advanced (LTE-Advanced) system.

A Limited Feedback Joint Precoding for Dual-Hop MIMO Amplify-and-Forward Relay Systems

This paper investigates the limited feedback joint precoding design for dual-hop multiple-input-multiple-output (MIMO) amplify-and-forward (AF) relay systems. Assuming that full channel state information (CSI) of two hops is available and that a linear receiver is employed at the destination, a suboptimal joint precoding structure, which is based on the maximization of the minimum postprocessing signal-to-noise ratio (SNR), is presented. Based on a derived lower bound on the minimum postprocessing SNR, a joint codeword selection (JCS) scheme at the destination is proposed. To reduce feedback latency and computational complexity, the lemma of singular value inequalities of a product matrix and other lemmas are used to split the JCS into two independent codeword selections (ICSs). Thus, the codeword selections at the relay and the destination are concurrently carried out. Simulation results illustrate the effectiveness of the proposed limited feedback joint precoding design. They also show that the ICS scheme displays a bit error rate performance that is close to that of the joint scheme, while having shorter feedback latency and lower computational complexity.

Extended Detection for MIMO Systems with Partial Incremental Redundancy Based Hybrid ARQ

In this paper, an extended detection scheme is proposed for multiple-input multiple-output (MIMO) systems which employ partial incremental redundancy (IR) based hybrid automatic repeat request (HARQ). Based on the extended MIMO system model that interprets multiple retransmissions as a single transmission, the extended detection scheme estimates the entire transmitted symbols utilizing all information obtained up to the current retransmission. Compared with the conventional zero-forcing (ZF) detection scheme, the extended ZF detection scheme improves the post-processing signal-to-noise ratio (PSNR) of the newly transmitted symbols in every retransmission, as well as the repeatedly transmitted symbols. Simulation results verify that the extended detection scheme outperforms the conventional detection scheme. Simulation results also show that the extended minimum mean-square-error (MMSE) detection scheme can achieve a better error performance than the conventional maximum-likelihood (ML) detection scheme, and even the extended ZF detection scheme shows a comparable error performance to the conventional ML detection scheme.

Modeling ordered subcarrier SNR in MIMO-OFDM wireless links

Recent research has shown that the detected bit error-rate of coded and bit-interleaved multiple-input, multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) links is characterized by the ordered post-processing signal-to-noise ratio (SNR). In this paper, we show that the per-stream ordered SNR converges asymptotically (with delay spread) to the inverse, marginal cumulative distribution function (CDF) of post-processing SNR. We derive a new approximation of the post-processing SNR inverse marginal CDF through an approximation of the marginal CDF for ordered eigenvalues in Wishart random matrices. By evaluating the independence of subcarriers in terms of the power delay profile, we show that the inverse marginal CDF is also an important characterization of post-processing SNR in practice. We exploit this structure to improve principal component regression algorithms that empirically infer a low-dimension basis for ordered SNR. Numerical simulations show high accuracy of this basis with just three dimensions for arbitrary power delay profiles. The consequences of this work include high-resolution limited channel feedback, simpler channel models for system simulation and algorithm design, as well as the reduction of dimension in link quality metrics for link adaptation. Simulations demonstrate the application to link adaptation in IEEE 802.11n with constellations that may be adapted per spatial stream.

An efficient antenna shuffling scheme for a DSTTD system

A new antenna shuffling scheme for DSTTD (double space time transmit diversity) is proposed. The proposed method obtains the shuffling pattern directly from the estimated channel by maximizing minimum post-processing SNR (signal to noise ratio), while the conventional method minimizes channel correlation. Since the minimum post-processing SNR is directly related with error performance, the proposed method shows better bit error rate performance than the conventional method.