283,893 research outputs found
New Lower Bounds on the Capacity of Optical Fiber Channels via Optimized Shaping and Detection
Constellation shaping is a practical and effective technique to improve the
performance and the rate adaptivity of optical communication systems. In
principle, it could also be used to mitigate the impact of nonlinear effects,
possibly increasing the information rate beyond the current limit dictated by
fiber nonlinearity. However, this appealing idea is frustrated by the
difficulty of designing an effective shaping strategy that takes into account
the nonlinearity and long memory of the fiber channel, as well as the possible
interplay with other nonlinearity mitigation strategies. As a result, only
little progress has been made so far, while the optimal shaping distribution
and the ultimate channel capacity remain unknown. In this work, we describe a
novel technique to optimize the shaping distribution in a very general setting
and high-dimensional space. For a simplified block-memoryless nonlinear optical
channel, the capacity lower bound obtained by the proposed technique can be
expressed analytically, establishing the conditions for an unbounded growth of
capacity with power. In a more realistic scenario, the technique can be
implemented by a rejection sampling algorithm driven by a suitable cost
function, and the corresponding achievable information rate estimated
numerically. The combination of the proposed technique with an improved
(non-Gaussian) decoding metric yields a new capacity lower bound for the
dual-polarization WDM channel.Comment: Submitted to IEEE Journal of Lightwave Technology on November 30th,
202
A thermodynamic uncertainty relation for a system with memory
We introduce an example of thermodynamic uncertainty relation (TUR) for
systems modeled by a one-dimensional generalised Langevin dynamics with memory,
determining the motion of a micro-bead driven in a complex fluid. Contrary to
TURs typically discussed in the previous years, our observables and the entropy
production rate are one-time variables. The bound to the signal-to-noise ratio
of such state-dependent observables only in some cases can be mapped to the
entropy production rate. For example, this is true in Markovian systems. Hence,
the presence of memory in the system complicates the thermodynamic
interpretation of the uncertainty relation
When Two Choices Are not Enough: Balancing at Scale in Distributed Stream Processing
Carefully balancing load in distributed stream processing systems has a
fundamental impact on execution latency and throughput. Load balancing is
challenging because real-world workloads are skewed: some tuples in the stream
are associated to keys which are significantly more frequent than others. Skew
is remarkably more problematic in large deployments: more workers implies fewer
keys per worker, so it becomes harder to "average out" the cost of hot keys
with cold keys.
We propose a novel load balancing technique that uses a heaving hitter
algorithm to efficiently identify the hottest keys in the stream. These hot
keys are assigned to choices to ensure a balanced load, where is
tuned automatically to minimize the memory and computation cost of operator
replication. The technique works online and does not require the use of routing
tables. Our extensive evaluation shows that our technique can balance
real-world workloads on large deployments, and improve throughput and latency
by and respectively over the previous
state-of-the-art when deployed on Apache Storm.Comment: 12 pages, 14 Figures, this paper is accepted and will be published at
ICDE 201
Information Rates of ASK-Based Molecular Communication in Fluid Media
This paper studies the capacity of molecular communications in fluid media,
where the information is encoded in the number of transmitted molecules in a
time-slot (amplitude shift keying). The propagation of molecules is governed by
random Brownian motion and the communication is in general subject to
inter-symbol interference (ISI). We first consider the case where ISI is
negligible and analyze the capacity and the capacity per unit cost of the
resulting discrete memoryless molecular channel and the effect of possible
practical constraints, such as limitations on peak and/or average number of
transmitted molecules per transmission. In the case with a constrained peak
molecular emission, we show that as the time-slot duration increases, the input
distribution achieving the capacity per channel use transitions from binary
inputs to a discrete uniform distribution. In this paper, we also analyze the
impact of ISI. Crucially, we account for the correlation that ISI induces
between channel output symbols. We derive an upper bound and two lower bounds
on the capacity in this setting. Using the input distribution obtained by an
extended Blahut-Arimoto algorithm, we maximize the lower bounds. Our results
show that, over a wide range of parameter values, the bounds are close.Comment: 31 pages, 8 figures, Accepted for publication on IEEE Transactions on
Molecular, Biological, and Multi-Scale Communication
On Coding Efficiency for Flash Memories
Recently, flash memories have become a competitive solution for mass storage.
The flash memories have rather different properties compared with the rotary
hard drives. That is, the writing of flash memories is constrained, and flash
memories can endure only limited numbers of erases. Therefore, the design goals
for the flash memory systems are quite different from these for other memory
systems. In this paper, we consider the problem of coding efficiency. We define
the "coding-efficiency" as the amount of information that one flash memory cell
can be used to record per cost. Because each flash memory cell can endure a
roughly fixed number of erases, the cost of data recording can be well-defined.
We define "payload" as the amount of information that one flash memory cell can
represent at a particular moment. By using information-theoretic arguments, we
prove a coding theorem for achievable coding rates. We prove an upper and lower
bound for coding efficiency. We show in this paper that there exists a
fundamental trade-off between "payload" and "coding efficiency". The results in
this paper may provide useful insights on the design of future flash memory
systems.Comment: accepted for publication in the Proceeding of the 35th IEEE Sarnoff
Symposium, Newark, New Jersey, May 21-22, 201
Using Hashing to Solve the Dictionary Problem (In External Memory)
We consider the dictionary problem in external memory and improve the update
time of the well-known buffer tree by roughly a logarithmic factor. For any
\lambda >= max {lg lg n, log_{M/B} (n/B)}, we can support updates in time
O(\lambda / B) and queries in sublogarithmic time, O(log_\lambda n). We also
present a lower bound in the cell-probe model showing that our data structure
is optimal.
In the RAM, hash tables have been used to solve the dictionary problem faster
than binary search for more than half a century. By contrast, our data
structure is the first to beat the comparison barrier in external memory. Ours
is also the first data structure to depart convincingly from the indivisibility
paradigm
- …