Alireza Mehrnia

Detailed OFDM modeling in network simulation of mobile ad hoc networks

In mobile ad hoc network (MANET) studies, it is imperative to use highly detailed device models as they provide high layer protocols with good prediction of underlying wireless communication performance. However, such studies often utilize abstract models for execution speed and simplicity. This paper first shows that physical layer variables including path loss, shadowing, multipath, Doppler have significant effects on the predicted overall networking performance. It then proposes an approach to simulate details of wireless propagation and radio characteristics in networking studies while still maintaining a reasonable simulation execution time. Through our runtime performance studies with detailed OFDM Simulink/MATLAB models and QualNet network simulator, it is shown that the proposed approach can improve the simulation runtime performance by three to four orders of magnitudes without compromising the fidelity of simulation results.

Incidence of Conversion to Active Waitlist Status Among Temporarily Inactive Obese Renal Transplant Candidates

Background Candidates may be active or temporarily inactive (status 7) on the kidney transplant waiting list. One reason candidates may be inactive is for a “weight currently inappropriate for transplantation.” We hypothesized that many of these candidates would not achieve active status. Methods Using OPTN/UNOS data from 2006 to 2012, we used competing risks methods to determine the cumulative incidence of conversion to active status (activation), death, and delisting before conversion among 1679 obese adult kidney candidates designated as status 7 because of a weight inappropriate for transplantation. Fine and Gray competing risks regression was performed to characterize factors associated with conversion to active status in the overall study population and of transplantation among a subgroup of activated candidates. Results At 6 years, the cumulative incidence of activation was 49%, of death before conversion was 15%, and of delisting was 21%. Higher body mass index (BMI) was strongly associated with a decreased subhazard of activation (BMI ≥45 versus 30–34.9, sHR: 0.22; 95% CI, 0.16–0.33). Female sex, diabetic end-stage renal disease, history of a previous transplant, panel reactive antibodies less than 80%, dialysis dependence at listing, and UNOS region 5 were negatively associated with activation. Among activated candidates, the cumulative incidence of transplantation at 6 years after initial waitlisting was 61%. Conclusion Our findings indicate that half of obese status 7 candidates with a weight inappropriate for transplantation will not achieve active waitlist status. BMI at listing had a strong association with conversion to active status; comorbid factors and regional variation also impact activation.

Improved survival among sickle cell kidney transplant recipients in the recent era

BACKGROUND Studies from older cohorts of kidney recipients have observed that recipients with sickle cell disease (SCD) have lower patient survival compared with age- and race-matched controls. We examined whether survival has improved among SCD recipients in the current era. METHODS Using Organ Procurement and Transplantation Network/United Network for Organ Sharing data, all black/African-American kidney recipients were stratified according to transplant year into an early (1988-99) and recent era (2000-11). Patient and allograft survival among SCD recipients and those with other diagnoses were compared (early era: SCD n = 67, others n = 20 694; recent era: SCD n = 106, others n = 34 428). A secondary-matched cohort analysis compared patient and allograft survival between SCD recipients matched to recipients with other diagnoses based on recipient and donor age, gender and donor type (deceased versus living). RESULTS Patient survival at 6 years was lower among SCD recipients in the early era compared with other diagnoses (55.7 versus 78.0%; P < 0.001). Six-year patient survival among sickle cell recipients improved in the recent era (69.8%; P versus early era = 0.04), although still trended toward lower survival compared with other diagnoses (80.0%; P = 0.07). Multivariate Cox proportional hazard models revealed an increased mortality risk with SCD in both eras [early: hazard ratio (HR) = 3.12; 95% confidence interval (CI): 2.15-4.54; recent: HR: 2.03; 95% CI: 1.31-3.16]. Patient survival among matched SCD recipients in the recent era was comparable to diabetic recipients (SCD: 73.1%, diabetes: 74.1%; P = 0.44). CONCLUSIONS Patient survival has improved among contemporary sickle cell recipients compared with an earlier cohort and is comparable to a matched cohort of diabetic kidney recipients. Appropriately selected SCD patients may receive kidney transplants with reasonable survival outcome.

Optimal Factoring of FIR Filters

New insights suggest that the most efficient FIR digital filters can be created by using a scaled sequence of stages, each representing a factor of the filters transfer function. A crucial capability for building such filters concerns finding the best FIR filter factors, then carefully scaling and sequencing them. The efficiency of the resulting structure depends heavily upon obtaining such optimal factors. We offer an algorithm to find, scale and sequence optimally factored FIR filters.

Minimum word-length requirements for MIMO-OFDM

The first step in the design and implementation of a MIMO-OFDM system is to analyze the characteristics of the baseband modulated signal. This is crucial in order to minimize the complexity of the digital signal processing. This paper demonstrates the generalized derivation of the minimum word-length requirements for the main data-path in OFDM systems based on the analytical evaluation of the normalized power distribution, clipping noise effect, and essential dynamic range. We have formulated the minimum word-length requirements for the main data path in both transmitter and the receiver. The theoretical analyses are also supported by system level simulations.

Further Desensitized FIR Halfband Filters

A halfband filters sensitivity to coefficient quantization can be significantly reduced by constructing the filter as a cascade of sections. The further desensitized FIR halfband filter presented here has an increased reduction in coefficient sensitivity. This is accomplished by doubling the number of extracted (1+z-1) blocks in the cascade filter structure. The relationship between the resulting further-desensitized halfband filters structure and the halfband filters transfer function is also demonstrated through reasoning, as well as showing the same for the previous desensitized halfband filter, for which the present filter represents an improvement. Further, it is shown how such insensitivity can be exploited in the filter-design process to yield halfband filters with reduced hardware complexity, leading to lower power consumption and/or higher operating speeds.

Can aminotransferase to platelet ratio index and other non-invasive markers effectively reduce liver biopsies for renal transplant evaluation of hepatitis C virus-positive patients?

BACKGROUND Advanced fibrosis or cirrhosis is still regarded as a contraindication for kidney transplantation alone by most centers. The value of aminotransferase to platelet ratio index (APRI) and other non-invasive markers has been less studied in hepatitis C virus (HCV)-positive patients with concurrent end-stage renal disease to predict hepatic fibrosis. Can these be used to effectively decrease the number of biopsies done in these patients being evaluated for transplantation? METHODS Our study population included 255 patients with liver biopsy data. All patient information was collected and reviewed from medical records. The diagnostic accuracy of the predictive models was analyzed by calculating sensitivity, specificity, positive predictive value and negative predictive value. RESULTS The variables associated with F3-F4 were aspartate aminotransferase (P = 0.007), bilirubin (P ≤ 0.001), platelet count (P = 0.01) and APRI (P ≤ 0.001). The use of any one laboratory abnormality to predict liver biopsy scores did not show high positive predictive values (22.6-72.7%). Having abnormal liver findings or cirrhosis on imaging was associated with high specificities (92.0-97.8%) but low sensitivities (31.4-42.9%). Using APRI levels of ≥0.40 and ≤0.95 as an indication for liver biopsy, 50% of patients with F3-F4 would have correctly avoided having a biopsy. However, 33% of patients with F3-F4 would have been mislabeled and not be indicated for biopsy. CONCLUSIONS Our data suggest that there may not currently be a simple and sufficiently accurate non-invasive test to replace liver biopsy in renal transplant workup for HCV-positive patients. The risks outweigh the benefits when it comes to using non-invasive markers like the APRI.

Efficient Halfband FIR Filter Structures for RF and IF Data Converters

Halfband filters used in multi-gigasample-per-second radio frequency/intermediate frequency data converters to interpolate or decimate the signal are responsible for a considerable portion of the power consumption. We now know that the sensitivity of halfband filters to coefficient quantization can be systematically reduced. This can be exploited in the filter-design process to yield halfband filters with reduced hardware complexity, leading to lower power consumption and/or higher operating speeds. The general derivation of such desensitized halfband structures and the relationship between their coefficients and those of conventional halfband filters are presented. Cadence implementation results show significant improvements.

FIR Filter Design via Extended Optimal Factoring

An improved optimal factoring and zero-grouping algorithm is presented that reduces the hardware needed to implement FIR filters in a cascade structure, with better factors obtained via an optimal grouping of both off-unit-circle as well as on-unit-circle transfer-function zeros. The concept, additional design techniques, and benefits are examined here using practical examples. It is shown that the extended approach offers access to more features that can be exploited to achieve a greater reduction in coefficient quantization levels, thereby creating filters having minimal hardware complexity.

FIR Filter Design Using Optimal Factoring: A Walkthrough and Summary of Benefits

Decomposing an FIR filter into a cascade of blocks, each representing an optimal factor of H(z), the filter?s transfer function, can start with the complex-conjugate pairs of zeros of H(z) and then creating optimal pairings of these zero-pairs. This leads to a general method for designing FIR filters having minimal hardware complexity. We illustrate this concept, further techniques, benefits, and performance improvements in this paper, highlighting the practical features of the design method. It is also shown that the resulting structure can achieve lower hardware complexity, when compared with the prior art.