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(1987). Applied Probability and Queues. Journal of the Operational Research Society: Vol. 38, No. 11, pp. 1095-1096.

https://link.springer.com/content/pdf/10.1057%2Fjors.1987.184.pdf

Applied Probability and Queues

As improved versions of the successive cancellation (SC) decoding algorithm, the successive cancellation list (SCL) decoding and the successive cancellation stack (SCS) decoding are used to improve the finite-length performance of polar codes. In this paper, unified descriptions of the SC, SCL, and SCS decoding algorithms are given as path search procedures on the code tree of polar codes. Combining the principles of SCL and SCS, a new decoding algorithm called the successive cancellation hybrid (SCH) is proposed. This proposed algorithm can provide a flexible configuration when the time and space complexities are limited. Furthermore, a pruning technique is also proposed to lower the complexity by reducing unnecessary path searching operations. Performance and complexity analysis based on simulations shows that under proper configurations, all the three improved successive cancellation (ISC) decoding algorithms can approach the performance of the maximum likelihood (ML) decoding but with acceptable complexity. With the help of the proposed pruning technique, the time and space complexities of ISC decoders can be significantly reduced and be made very close to those of the SC decoder in the high signal-to-noise ratio regime.

Improved Successive Cancellation Decoding of Polar Codes

We present a comparative study of the performance of various polar code constructions in an additive white Gaussian noise (AWGN) channel. A polar code construction is any algorithm that selects $K$ best among $N$ possible polar bit-channels at the design signal-to-noise-ratio (design-SNR) in terms of bit error rate (BER). Optimal polar code construction is hard and therefore many suboptimal polar code constructions have been proposed at different computational complexities. Polar codes are also non-universal meaning the code changes significantly with the design-SNR. However, it is not known which construction algorithm at what design-SNR constructs the best polar codes. We first present a comprehensive survey of all the well-known polar code constructions along with their full implementations. We then propose a heuristic algorithm to find the best design-SNR for constructing best possible polar codes from a given construction algorithm. The proposed algorithm involves a search among several possible design-SNRs. We finally use our algorithm to perform a comparison of different construction algorithms using extensive simulations. We find that all polar code construction algorithms generate equally good polar codes in an AWGN channel, if the design-SNR is optimized.

A Comparative Study of Polar Code Constructions for the AWGN Channel.

Polar codes have emerged as important channel codes because of their capacity-achieving property. For low-complexity polar decoding, hardware architectures for successive cancellation (SC) algorithm have been investigated in prior works. However, belief propagation (BP)-based architectures have not been explored in detail. This paper begins with a review of min-sum (MS) approximated BP algorithm, and then proposes a scaled MS (SMS) algorithm with improved decoding performance. Then, in order to solve long critical path problem in the SMS algorithm, we propose an efficient critical path reduction approach. Due to its generality, this optimization method can be applied to both of SMS and MS algorithms. Compared with the state-of-the-art MS decoder, the proposed (1024, 512) SMS design can lead to 0.5dB extra decoding gain with the same hardware performance. Besides, the proposed optimized MS architecture can also achieve more than 30% and 80% increase in throughput and hardware efficiency, respectively.

Architecture optimizations for BP polar decoders

We consider a bandit problem with K task types from which the controller activates one task at a time. Each task takes a random and possibly heavy-tailed completion time, and a reward is obtained only after the task is completed. The task types are independent from each other, and have distinct and unknown distributions for completion time and reward. For a given time horizon τ, the goal of the controller is to schedule tasks adaptively so as to maximize the reward collected until τ expires. In addition, we allow the controller to interrupt a task and initiate a new one. In addition to the traditional exploration-exploitation dilemma, this interrupt mechanism introduces a new one: should the controller complete the task and get the reward, or interrupt the task for a possibly shorter and more rewarding alternative? We show that for all heavy-tailed and some light-tailed completion time distributions, this interruption mechanism improves the reward linearly over time. Applications of this model include server scheduling, optimal free sampling strategies in advertising and adaptive content selection. From a learning perspective, the interrupt mechanism necessitates learning the whole arm distribution from truncated observations. For this purpose, we propose a robust learning algorithm named UCB-BwI based on median-of-means estimator for possibly heavy-tailed reward and completion time distributions. We show that, in a K-armed bandit setting with an arbitrary set of L possible interrupt times, UCB-BwI achieves O(Klog(τ)+KL) regret. We also prove that the regret under any admissible policy is Omega(Klog(τ)), which implies that UCB-BwI is order optimal.

https://dl.acm.org/doi/pdf/10.1145/3341617.3326158

Learning to Control Renewal Processes with Bandit Feedback

We study the problem of serving randomly arriving and delay-sensitive traffic over a multi-channel communication system with time-varying channel states and unknown statistics. This problem deviates from the classical exploration-exploitation setting in that the design and analysis must accommodate the dynamics of packet availability and urgency as well as the cost of each channel use at the time of decision. To that end, we have developed and investigated two policies, one index-based (UCB-Deadline) and the other Bayesian (TS-Deadline), both of which perform dynamic channel allocation decisions that incorporate these traffic requirements and costs. Under symmetric channel conditions, we have proved that the UCB-Deadline policy can achieve bounded regret in the likely case where the cost of using a channel is not too high to prevent all transmissions, and logarithmic regret otherwise. In our numerical studies, we also show that TS-Deadline achieves superior performance over its UCB counterpart, making it a potentially useful alternative when fast convergence to optimal is important.

Learning for serving deadline-constrained traffic in multi-channel wireless networks

The theory of discrete-time online learning has been successfully applied in many problems that involve sequential decision-making under uncertainty. However, in many applications including contractual hiring in online freelancing platforms and server allocation in cloud computing systems, the outcome of each action is observed only after a random and action-dependent time. Furthermore, as a consequence of certain ethical and economic concerns, the controller may impose deadlines on the completion of each task, and require fairness across different groups in the allocation of total time budget $B$. In order to address these applications, we consider continuous-time online learning problem with fairness considerations, and present a novel framework based on continuous-time utility maximization. We show that this formulation recovers reward-maximizing, max-min fair and proportionally fair allocation rules across different groups as special cases. We characterize the optimal offline policy, which allocates the total time between different actions in an optimally fair way (as defined by the utility function), and impose deadlines to maximize time-efficiency. In the absence of any statistical knowledge, we propose a novel online learning algorithm based on dual ascent optimization for time averages, and prove that it achieves $\tilde{O}(B^{-1/2})$ regret bound.

/pdf/group-fair-online-allocation-in-continuous-time-1meossp83u.pdf

Group-Fair Online Allocation in Continuous Time

We investigate nonbinary polar-coded modulations, which achieve a significant performance gain of at least 1 dB compared to binary counterparts at a short block-length of 2048 bits.

Nonbinary Polar Coding for Multilevel Modulation

We consider a budget-constrained bandit problem where each arm pull incurs a random cost, and yields a random reward in return. The objective is to maximize the total expected reward under a budget constraint on the total cost. The model is general in the sense that it allows correlated and potentially heavy-tailed cost-reward pairs that can take on negative values as required by many applications. We show that if moments of order $(2+\gamma)$ for some $\gamma > 0$ exist for all cost-reward pairs, $O(\log B)$ regret is achievable for a budget $B>0$. In order to achieve tight regret bounds, we propose algorithms that exploit the correlation between the cost and reward of each arm by extracting the common information via linear minimum mean-square error estimation. We prove a regret lower bound for this problem, and show that the proposed algorithms achieve tight problem-dependent regret bounds, which are optimal up to a universal constant factor in the case of jointly Gaussian cost and reward pairs.

Semih Cayci

Papers

Learning to Control Renewal Processes with Bandit Feedback

Learning for serving deadline-constrained traffic in multi-channel wireless networks

Group-Fair Online Allocation in Continuous Time

Nonbinary Polar Coding for Multilevel Modulation

Budget-Constrained Bandits over General Cost and Reward Distributions