662,670 research outputs found
New super-orthogonal space-time trellis codes using differential M-PSK for noncoherent mobile communication systems with two transmit antennas
In this paper, we develop super-orthogonal space-time trellis codes (SOSTTCs) using differential binary phase-shift keying, quadriphase-shift keying and eight-phase shift keying for noncoherent communication systems with two transmit antennas without channel state information at the receiver. Based on a differential encoding scheme proposed by Tarokh and Jafarkhani, we propose a new decoding algorithm with reduced decoding complexity. To evaluate the performance of the SOSTTCs by way of computer simulations, a geometric two-ring channel model is employed throughout. The simulation results show that the new decoding algorithm has the same decoding performance compared with the traditional decoding strategy, while it reduces significantly the overall computing complexity. As expected the system performance depends greatly on the antenna spacing and on the angular spread of the incoming waves. For fair comparison, we also design SOSTTCs for coherent detection of the same complexity as those demonstrated for the noncoherent case. As in the case of classical single antenna transmission systems, the coherent scheme outperforms the differential one by approximately 3 dB for SOSTTCs as well
On Algorithmic Cache Optimization
We study matrix-matrix multiplication of two matrices, and , each of
size . This operation results in a matrix of size .
Our goal is to produce as efficiently as possible given a cache: a 1-D
limited set of data values that we can work with to perform elementary
operations (additions, multiplications, etc.). That is, we attempt to reuse the
maximum amount of data from , and during our computation (or
equivalently, utilize data in the fast-access cache as often as possible).
Firstly, we introduce the matrix-matrix multiplication algorithm. Secondly, we
present a standard two-memory model to simulate the architecture of a computer,
and we explain the LRU (Least Recently Used) Cache policy (which is standard in
most computers). Thirdly, we introduce a basic model Cache Simulator, which
possesses an time complexity (meaning we are limited to small
values). Then we discuss and model the LFU (Least Frequently Used) Cache
policy and the explicit control cache policy. Finally, we introduce the main
result of this paper, the Cache Simulator, and use it to
compare, experimentally, the savings of time, energy, and communication
incurred from the ideal cache-efficient algorithm for matrix-matrix
multiplication. The Cache Simulator simulates the amount of data movement that
occurs between the main memory and the cache of the computer. One of the
findings of this project is that, in some cases, there is a significant
discrepancy in communication values between an LRU cache algorithm and explicit
cache control. We propose to alleviate this problem by ``tricking'' the LRU
cache algorithm by updating the timestamp of the data we want to keep in cache
(namely entries of matrix ). This enables us to have the benefits of an
explicit cache policy while being constrained by the LRU paradigm (realistic
policy on a CPU).Comment: 20 pages, 3 figures, 2 table
Design and analysis of a distributed ECDSA signing service
We present and analyze a new protocol that provides a distributed ECDSA signing service, with the following
properties:
* it works in an asynchronous communication model;
* it works with parties with up to Byzantine corruptions;
* it provides guaranteed output delivery;
* it provides a very efficient, non-interactive online signing phase;
* it supports additive key derivation according to the BIP32 standard.
While there has been a flurry of recent research on distributed ECDSA signing protocols, none of these newly designed protocols provides guaranteed output delivery over an asynchronous communication network; moreover, the performance of our protocol (in terms of asymptotic communication and computational complexity) meets or beats the performance of any of these other protocols.
This service is being implemented and integrated into the architecture of the Internet Computer, enabling smart contracts running on the Internet Computer to securely hold and spend Bitcoin and other cryptocurrencies.
Along the way, we present some results of independent interest:
* a new asynchronous verifiable secret sharing (AVSS) scheme that is simple and efficient;
* a new scheme for multi-recipient encryption that is simple and efficient
Exponential Separation of Quantum and Classical Online Space Complexity
Although quantum algorithms realizing an exponential time speed-up over the
best known classical algorithms exist, no quantum algorithm is known performing
computation using less space resources than classical algorithms. In this
paper, we study, for the first time explicitly, space-bounded quantum
algorithms for computational problems where the input is given not as a whole,
but bit by bit. We show that there exist such problems that a quantum computer
can solve using exponentially less work space than a classical computer. More
precisely, we introduce a very natural and simple model of a space-bounded
quantum online machine and prove an exponential separation of classical and
quantum online space complexity, in the bounded-error setting and for a total
language. The language we consider is inspired by a communication problem (the
set intersection function) that Buhrman, Cleve and Wigderson used to show an
almost quadratic separation of quantum and classical bounded-error
communication complexity. We prove that, in the framework of online space
complexity, the separation becomes exponential.Comment: 13 pages. v3: minor change
A note on a problem in communication complexity
In this note, we prove a version of Tarui's Theorem in communication
complexity, namely . Consequently, every
measure for leads to a measure for , subsuming a result of
Linial and Shraibman that problems with high mc-rigidity lie outside the
polynomial hierarchy. By slightly changing the definition of mc-rigidity
(arbitrary instead of uniform distribution), it is then evident that the class
of problems with low mc-rigidity equals . As , this rules out the possibility, that had been
left open, that even polynomial space is contained in
Exponential Separation of Quantum Communication and Classical Information
We exhibit a Boolean function for which the quantum communication complexity
is exponentially larger than the classical information complexity. An
exponential separation in the other direction was already known from the work
of Kerenidis et. al. [SICOMP 44, pp. 1550-1572], hence our work implies that
these two complexity measures are incomparable. As classical information
complexity is an upper bound on quantum information complexity, which in turn
is equal to amortized quantum communication complexity, our work implies that a
tight direct sum result for distributional quantum communication complexity
cannot hold. The function we use to present such a separation is the Symmetric
k-ary Pointer Jumping function introduced by Rao and Sinha [ECCC TR15-057],
whose classical communication complexity is exponentially larger than its
classical information complexity. In this paper, we show that the quantum
communication complexity of this function is polynomially equivalent to its
classical communication complexity. The high-level idea behind our proof is
arguably the simplest so far for such an exponential separation between
information and communication, driven by a sequence of round-elimination
arguments, allowing us to simplify further the approach of Rao and Sinha.
As another application of the techniques that we develop, we give a simple
proof for an optimal trade-off between Alice's and Bob's communication while
computing the related Greater-Than function on n bits: say Bob communicates at
most b bits, then Alice must send n/exp(O(b)) bits to Bob. This holds even when
allowing pre-shared entanglement. We also present a classical protocol
achieving this bound.Comment: v1, 36 pages, 3 figure
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