Asymptotics of Transmit Antenna Selection: Impact of Multiple Receive Antennas

Consider a fading Gaussian MIMO channel with $N_\mathrm{t}$ transmit and $N_\mathrm{r}$ receive antennas. The transmitter selects $L_\mathrm{t}$ antennas corresponding to the strongest channels. For this setup, we study the distribution of the input-output mutual information when $N_\mathrm{t}$ grows large. We show that, for any $N_\mathrm{r}$ and $L_\mathrm{t}$, the distribution of the input-output mutual information is accurately approximated by a Gaussian distribution whose mean grows large and whose variance converges to zero. Our analysis depicts that, in the large limit, the gap between the expectation of the mutual information and its corresponding upper bound, derived by applying Jensen's inequality, converges to a constant which only depends on $N_\mathrm{r}$ and $L_\mathrm{t}$. The result extends the scope of channel hardening to the general case of antenna selection with multiple receive and selected transmit antennas. Although the analyses are given for the large-system limit, our numerical investigations indicate the robustness of the approximated distribution even when the number of antennas is not large.Comment: 6 pages, 4 figures, ICC 201

On Robustness of Massive MIMO Systems Against Passive Eavesdropping under Antenna Selection

In massive MIMO wiretap settings, the base station can significantly suppress eavesdroppers by narrow beamforming toward legitimate terminals. Numerical investigations show that by this approach, secrecy is obtained at no significant cost. We call this property of massive MIMO systems `secrecy for free' and show that it not only holds when all the transmit antennas at the base station are employed, but also when only a single antenna is set active. Using linear precoding, the information leakage to the eavesdroppers can be sufficiently diminished, when the total number of available transmit antennas at the base station grows large, even when only a fixed number of them are selected. This result indicates that passive eavesdropping has no significant impact on massive MIMO systems, regardless of the number of active transmit antennas.Comment: 7 pages, 2 figures; To be presented in IEEE Global Communications Conference (Globecom) 2018 in Abu Dhabi, UA

Optimal Number of Transmit Antennas for Secrecy Enhancement in Massive MIMOME Channels

This paper studies the impact of transmit antenna selection on the secrecy performance of massive MIMO wiretap channels. We consider a scenario in which a multi-antenna transmitter selects a subset of transmit antennas with the strongest channel gains. Confidential messages are then transmitted to a multi-antenna legitimate receiver while the channel is being overheard by a multi-antenna eavesdropper. For this setup, we approximate the distribution of the instantaneous secrecy rate in the large-system limit. The approximation enables us to investigate the optimal number of selected antennas which maximizes the asymptotic secrecy throughput of the system. We show that increasing the number of selected antennas enhances the secrecy performance of the system up to some optimal value, and that further growth in the number of selected antennas has a destructive effect. Using the large-system approximation, we obtain the optimal number of selected antennas analytically for various scenarios. Our numerical investigations show an accurate match between simulations and the analytic results even for not so large dimensions.Comment: 6 pages, 4 figures, IEEE GLOBECOM 201

Remote preoperative tonic-clonic seizures do not influence outcome after surgery for temporal lobe epilepsy.

OBJECTIVES: Tonic-clonic seizures are associated with greater chance of seizure relapse after anterior temporal lobectomy. We investigated whether the interval between the last preoperative tonic-clonic seizure and surgery relates to seizure outcome in patients with drug-resistant mesial temporal lobe epilepsy (MTLE). METHODS: In this retrospective study, patients were prospectively registered in a database from 1986 through 2014. Postsurgical outcome was categorized as seizure freedom or relapse. The relationship between surgical outcome and the interval between the last preoperative tonic-clonic seizure and surgery was investigated. RESULTS: One-hundred seventy-one patients were studied. Seventy nine (46.2%) patients experienced tonic-clonic seizures before surgery. Receiver operating characteristic curve of timing of the last preoperative tonic-clonic seizure was a moderate indicator to anticipate surgery failure (area under the curve: 0.657, significance; 0.016). The best cutoff that maximizes sensitivity and specificity was 27months; with a sensitivity of 0.76 and specificity of 0.60. Cox-Mantel analysis confirmed that the chance of becoming free of seizures after surgery in patients with no or remote history of preoperative tonic-clonic seizures was significantly higher compared with patients with a recent history (i.e., in 27months before surgery) (p=0.0001). CONCLUSIONS: The more remote the occurrence of preoperative tonic-clonic seizures, the better the postsurgical seizure outcome, with at least a two year gap being more favorable. A recent history of tonic-clonic seizures in a patient with MTLE may reflect more widespread epileptogenicity extending beyond the borders of mesial temporal structures

Body image and meta-worry as mediators of body dysmorphic disorder

Objectives: Meta-worry and attitudes towards the body have been largely overlooked as potential risk factors for body dysmorphic disorder (BDD) despite theorizing that a negative body image may play a critical role in the development of this disorder. Participants: The purpose of this study was to evaluate the fit of a theoretical model specifying body image and meta-worry as mediators between cognitive, metacognitive beliefs and body dysmorphic disorder(BDD) in a nonclinical sample of 635 participants (304 male and 331 female). Results: The data supported the model, and meta-worry and body image significantly mediated the relationship between cognitive, metacognitive beliefs and BDD. These findings provide essential preliminary evidence that body image may represent a necessary but not sufficient risk factor for BDD and that treatment for BDD should consider targeting body-related pathology in addition to meta-worry. Conclusion: The model may prompt future research into body dysmorphic disorder

Ictal verbal help-seeking: Occurrence and the underlying etiology.

PURPOSE: Ictal verbal help-seeking has never been systematically studied before. In this study, we evaluated a series of patients with ictal verbal help-seeking to characterize its frequency and underlying etiology. METHODS: We retrospectively reviewed all the long-term video-EEG reports from Jefferson Comprehensive Epilepsy Center over a 12-year period (2004-2015) for the occurrence of the term help in the text body. All the extracted reports were reviewed and patients with at least one episode of documented ictal verbal help-seeking in epilepsy monitoring unit (EMU) were studied. For each patient, the data were reviewed from the electronic medical records, EMU report, and neuroimaging records. RESULTS: During the study period, 5133 patients were investigated in our EMU. Twelve patients (0.23%) had at least one episode of documented ictal verbal help-seeking. Nine patients (six women and three men) had epilepsy and three patients (two women and one man) had psychogenic nonepileptic seizures (PNES). Seven out of nine patients with epilepsy had temporal lobe epilepsy; six patients had right temporal lobe epilepsy. CONCLUSION: Ictal verbal help-seeking is a rare finding among patients evaluated in epilepsy monitoring units. Ictal verbal help-seeking may suggest that seizures arise in or propagate to the right temporal lobe

Factors associated with tonic-clonic seizures in patients with drug-resistant mesial temporal epilepsy.

PURPOSE: Among different seizure types, tonic-clonic seizures are more significant because they are more often associated with morbidity. No prior study has been done to investigate risk factors associated with tonic-clonic seizures in patients with mesial temporal epilepsy. METHODS: In this retrospective study, all drug-resistant mesial temporal epilepsy patients in the database of patients who underwent epilepsy surgery at Jefferson comprehensive epilepsy center were recruited. These patients were prospectively registered in a database from 1986 till 2014. Patients\u27 age, gender, epilepsy risk factors, age at seizure onset, and preoperative seizure type(s) were registered routinely. Potential risk factors associated with experiencing preoperative tonic-clonic seizures were investigated. RESULTS: Two hundred seventy-five patients (132 males and 143 females) were studied. Aura type was associated with experiencing perioperative tonic-clonic seizures. Patients with epigastric auras less frequently reported having tonic-clonic seizures compared with those who had other types of auras (odds ratio: 0.37 and 95% confidence interval: 0.19-0.70; p=0.001) and those who did not have any auras (odds ratio: 0.35 and 95% confidence interval: 0.16-0.77; p=0.008). CONCLUSION: Epigastric auras may indicate that a specific anatomic location is involved in epileptogenesis from which generalization is harder because of that location\u27s poor connections with other brain regions