Coherent multiple-antenna block-fading channels at finite blocklength

In this paper we consider a channel model that is often used to describe the mobile wireless scenario: multiple-antenna additive white Gaussian noise channels subject to random (fading) gain with full channel state information at the receiver. Dynamics of the fading process are approximated by a piecewise-constant process (frequency non-selective isotropic block fading). This work addresses the finite blocklength fundamental limits of this channel model. Specifically, we give a formula for the channel dispersion -- a quantity governing the delay required to achieve capacity. Multiplicative nature of the fading disturbance leads to a number of interesting technical difficulties that required us to enhance traditional methods for finding channel dispersion. Alas, one difficulty remains: the converse (impossibility) part of our result holds under an extra constraint on the growth of the peak-power with blocklength. Our results demonstrate, for example, that while capacities of $n_t\times n_r$ and $n_r \times n_t$ antenna configurations coincide (under fixed received power), the coding delay can be quite sensitive to this switch. For example, at the received SNR of $20$ dB the $16\times 100$ system achieves capacity with codes of length (delay) which is only $60\%$ of the length required for the $100\times 16$ system. Another interesting implication is that for the MISO channel, the dispersion-optimal coding schemes require employing orthogonal designs such as Alamouti's scheme -- a surprising observation considering the fact that Alamouti's scheme was designed for reducing demodulation errors, not improving coding rate. Finding these dispersion-optimal coding schemes naturally gives a criteria for producing orthogonal design-like inputs in dimensions where orthogonal designs do not exist

A Beta-Beta Achievability Bound with Applications

A channel coding achievability bound expressed in terms of the ratio between two Neyman-Pearson $\beta$ functions is proposed. This bound is the dual of a converse bound established earlier by Polyanskiy and Verd\'{u} (2014). The new bound turns out to simplify considerably the analysis in situations where the channel output distribution is not a product distribution, for example due to a cost constraint or a structural constraint (such as orthogonality or constant composition) on the channel inputs. Connections to existing bounds in the literature are discussed. The bound is then used to derive 1) an achievability bound on the channel dispersion of additive non-Gaussian noise channels with random Gaussian codebooks, 2) the channel dispersion of the exponential-noise channel, 3) a second-order expansion for the minimum energy per bit of an AWGN channel, and 4) a lower bound on the maximum coding rate of a multiple-input multiple-output Rayleigh-fading channel with perfect channel state information at the receiver, which is the tightest known achievability result.Comment: extended version of a paper submitted to ISIT 201

Beta-Beta Bounds: Finite-Blocklength Analog of the Golden Formula

It is well known that the mutual information between two random variables can be expressed as the difference of two relative entropies that depend on an auxiliary distribution, a relation sometimes referred to as the golden formula. This paper is concerned with a finite-blocklength extension of this relation. This extension consists of two elements: 1) a finite-blocklength channel-coding converse bound by Polyanskiy and Verd\'{u} (2014), which involves the ratio of two Neyman-Pearson $\beta$ functions (beta-beta converse bound); and 2) a novel beta-beta channel-coding achievability bound, expressed again as the ratio of two Neyman-Pearson $\beta$ functions. To demonstrate the usefulness of this finite-blocklength extension of the golden formula, the beta-beta achievability and converse bounds are used to obtain a finite-blocklength extension of Verd\'{u}'s (2002) wideband-slope approximation. The proof parallels the derivation of the latter, with the beta-beta bounds used in place of the golden formula. The beta-beta (achievability) bound is also shown to be useful in cases where the capacity-achieving output distribution is not a product distribution due to, e.g., a cost constraint or structural constraints on the codebook, such as orthogonality or constant composition. As an example, the bound is used to characterize the channel dispersion of the additive exponential-noise channel and to obtain a finite-blocklength achievability bound (the tightest to date) for multiple-input multiple-output Rayleigh-fading channels with perfect channel state information at the receiver.Comment: to appear in IEEE Transactions on Information Theor

Saving lives in the time of COVID-19. Case study of harm reduction, homelessness and drug use in Dublin, Ireland.

Dublin has outperformed even best-case scenarios for COVID-19 mortality among homeless and drug using populations. The experience provides important lessons for policy discussions on the pandemic, as well as broader lessons about pragmatic responses to these key client groups irrespective of COVID-19. The overarching lessons is that when government policy is well coordinated and underpinned by a science-driven and fundamentally pragmatic approach, lives are saved. Within this, the importance of strategic clarity and delivery, housing, lowered thresholds to methadone provision, Benzodiazepine (BZD) provision and Naloxone availability were key determinants of policy success. Further, given the rapid collapse in policy barriers to these interventions that COVID-19 produced, it is important to secure and protect these improved policy responses into the post-COVID-19 era

A milestone in drug policy: saving the lives of people who use drugs and were homeless in Dublin during the covid-19 pandemic

Dublin has outperformed even best-case scenarios for COVID-19 mortality among homeless and drug-using populations, write Austin O’Carroll, Tony Duffin and John Collins (LSE). When government policy is well coordinated and underpinned by a science-driven and fundamentally pragmatic approach, lives are saved

LeaPi: Wireless Diagnostic Assistant

Nearly every person who usesWiFi on a daily basis has had trouble with a bad connection. Wireless connectivity issues are often difficult to diagnose and fix. Current solutions such as wired extenders, and MeshWiFi commercial packages are expensive and do not provide the user with a system that suggests placement of mesh units to maximize coverage. Our solution is an inexpensive and open-source diagnostic tool that maps out Wifi quality and informs the user of interference. With a simple, meaningful display, users will find trouble spots in their house, diagnose why IoT devices are not working, effectively place WiFi extenders and mesh nodes, and more

Causes of pleural effusions in horses resident in the UK

Pleural effusions (PE) reportedly occur most commonly secondary to bacterial pneumonia with neoplastic effusions contributing a minority of cases. The majority of reports originate from the USA and Australia, where long distance transport of horses, a recognised risk factor, may occur more frequently than in the UK. Anecdotally, a greater proportion of horses with PE are diagnosed with neoplasia in the UK than has been reported. The aim of this retrospective study was to describe the causes of PE in horses in the UK, and to identify markers that can help differentiate between septic and neoplastic causes of PE. Medical records from 4 equine hospitals in the UK were searched for horses diagnosed with PE. Information recorded included case background, admission physical examination and biochemical findings, and characteristics of the effusion (volume, cell count, total protein [TP] concentration). A total of 69 horses were identified, with 26 (38%) diagnosed with a neoplastic effusion. The remainder were categorised as septic, including 14/43 (32.5%) that had a history of international transport. Horses with septic effusions were significantly younger (8 vs. 13 years; P = 0.001) and had significantly smaller volumes of pleural fluid drained at admission (9.8 l vs. 32.2 l; P<0.001). Horses with septic PE had a significantly higher rectal temperature (38.6°C vs. 38.2°C; P = 0.03), fibrinogen concentration (7.8 g/l vs. 5.3 g/l; P = 0.01) and serum amyloid A concentration (230 mg/l vs. 59 mg/l; P = 0.02) than those with neoplastic effusions. Significantly higher pleural fluid cell count and TP concentration were identified in horses with septic PE (63.9 × 109/l vs. 8.6 × 109/l; P<0.001; 57.5 g/l vs. 35.9 g/l; P = 0.04). These results suggest that in the UK, neoplastic effusions account for a greater proportion of PE than previously reported. A large volume of PE in an older horse with a low cell count and relatively low TP concentration should increase the index of suspicion of neoplasia