182 research outputs found
Linear Beamforming for the Spatially Correlated MISO broadcast channel
A spatially correlated broadcast setting with M antennas at the base station
and M users (each with a single antenna) is considered. We assume that the
users have perfect channel information about their links and the base station
has only statistical information about each user's link. The base station
employs a linear beamforming strategy with one spatial eigen-mode allocated to
each user. The goal of this work is to understand the structure of the
beamforming vectors that maximize the ergodic sum-rate achieved by treating
interference as noise. In the M = 2 case, we first fix the beamforming vectors
and compute the ergodic sum-rate in closed-form as a function of the channel
statistics. We then show that the optimal beamforming vectors are the dominant
generalized eigenvectors of the covariance matrices of the two links. It is
difficult to obtain intuition on the structure of the optimal beamforming
vectors for M > 2 due to the complicated nature of the sum-rate expression.
Nevertheless, in the case of asymptotic M, we show that the optimal beamforming
vectors have to satisfy a set of fixed-point equations.Comment: Published in IEEE ISIT 2010, 5 page
Capacity and Stable Scheduling in Heterogeneous Wireless Networks
Heterogeneous wireless networks (HetNets) provide a means to increase network
capacity by introducing small cells and adopting a layered architecture.
HetNets allocate resources flexibly through time sharing and cell range
expansion/contraction allowing a wide range of possible schedulers. In this
paper we define the capacity of a HetNet down link in terms of the maximum
number of downloads per second which can be achieved for a given offered
traffic density. Given this definition we show that the capacity is determined
via the solution to a continuous linear program (LP). If the solution is
smaller than 1 then there is a scheduler such that the number of mobiles in the
network has ergodic properties with finite mean waiting time. If the solution
is greater than 1 then no such scheduler exists. The above results continue to
hold if a more general class of schedulers is considered.Comment: 30 pages, 6 figure
Active Queue Management for Fair Resource Allocation in Wireless Networks
This paper investigates the interaction between end-to-end flow control and MAC-layer scheduling on wireless links. We consider a wireless network with multiple users receiving information from a common access point; each user suffers fading, and a scheduler allocates the channel based on channel quality,but subject to fairness and latency considerations. We show that the fairness property of the scheduler is compromised by the transport layer flow control of TCP New Reno. We provide a receiver-side control algorithm, CLAMP, that remedies this situation. CLAMP works at a receiver to control a TCP sender by setting the TCP receiver's advertised window limit, and this allows the scheduler to allocate bandwidth fairly between the users
Optimality of binary power-control in a single cell via majorization
This paper considers the optimum single cell power-control maximizing the
aggregate (uplink) communication rate of the cell when there are peak power
constraints at mobile users, and a low-complexity data decoder (without
successive decoding) at the base station. It is shown, via the theory of
majorization, that the optimum power allocation is binary, which means links
are either "on" or "off". By exploiting further structure of the optimum binary
power allocation, a simple polynomial-time algorithm for finding the optimum
transmission power allocation is proposed, together with a reduced complexity
near-optimal heuristic algorithm. Sufficient conditions under which
channel-state aware time-division-multiple-access (TDMA) maximizes the
aggregate communication rate are established. Finally, a numerical study is
performed to compare and contrast the performance achieved by the optimum
binary power-control policy with other sub-optimum policies and the throughput
capacity achievable via successive decoding. It is observed that two dominant
modes of communication arise, wideband or TDMA, and that successive decoding
achieves better sum-rates only under near-perfect interference cancellation
efficiency.Comment: 24 pages, 11 figure
Millimeter Wave Beam Alignment: Large Deviations Analysis and Design Insights
In millimeter wave cellular communication, fast and reliable beam alignment
via beam training is crucial to harvest sufficient beamforming gain for the
subsequent data transmission. In this paper, we establish fundamental limits in
beam-alignment performance under both the exhaustive search and the
hierarchical search that adopts multi-resolution beamforming codebooks,
accounting for time-domain training overhead. Specifically, we derive lower and
upper bounds on the probability of misalignment for an arbitrary level in the
hierarchical search, based on a single-path channel model. Using the method of
large deviations, we characterize the decay rate functions of both bounds and
show that the bounds coincide as the training sequence length goes large. We go
on to characterize the asymptotic misalignment probability of both the
hierarchical and exhaustive search, and show that the latter asymptotically
outperforms the former, subject to the same training overhead and codebook
resolution. We show via numerical results that this relative performance
behavior holds in the non-asymptotic regime. Moreover, the exhaustive search is
shown to achieve significantly higher worst-case spectrum efficiency than the
hierarchical search, when the pre-beamforming signal-to-noise ratio (SNR) is
relatively low. This study hence implies that the exhaustive search is more
effective for users situated further from base stations, as they tend to have
low SNR.Comment: Author final manuscript, to appear in IEEE Journal on Selected Areas
in Communications (JSAC), Special Issue on Millimeter Wave Communications for
Future Mobile Networks, 2017 (corresponding author: Min Li
Design and Analysis of Transmit Beamforming for Millimetre Wave Base Station Discovery
In this paper, we develop an analytical framework for the initial access
(a.k.a. Base Station (BS) discovery) in a millimeter-wave (mm-wave)
communication system and propose an effective strategy for transmitting the
Reference Signals (RSs) used for BS discovery. Specifically, by formulating the
problem of BS discovery at User Equipments (UEs) as hypothesis tests, we derive
a detector based on the Generalised Likelihood Ratio Test (GLRT) and
characterise the statistical behaviour of the detector. The theoretical results
obtained allow analysis of the impact of key system parameters on the
performance of BS discovery, and show that RS transmission with narrow beams
may not be helpful in improving the overall BS discovery performance due to the
cost of spatial scanning. Using the method of large deviations, we identify the
desirable beam pattern that minimises the average miss-discovery probability of
UEs within a targeted detectable region. We then propose to transmit the RS
with sequential scanning, using a pre-designed codebook with narrow and/or wide
beams to approximate the desirable patterns. The proposed design allows
flexible choices of the codebook sizes and the associated beam widths to better
approximate the desirable patterns. Numerical results demonstrate the
effectiveness of the proposed method.Comment: 30 pages, 13 figures, submitte
The Tourism Adaptation Classification (TAC) framework: An application to New Zealand's Glacier country
Alongside growing awareness of the significance of environmental change for glacier regions, and their tourism-dependent communities, is the realization of the need to adapt to changing conditions. Such adaptation is necessary for tourism operators, managers, and planners as well as the visitors themselves, and is part of building resilient tourism systems. This paper presents a conceptual framework for understanding the possible stages of adaptation in glacier tourism destinations. The Tourism Adaptation Classification (TAC) framework aligns three stages of adaptation (resilience, transition, and transformation) against adaptation strategies implemented by tourism stakeholders and identifies specific characteristics. Using a desk-based case study approach, the framework is illustrated with reference to Glacier Country in New Zealand's Westland/Tai Poutini National Park in relation to three core dimensions of the tourism system: tourism planning and governance; tourism business and operations; and visitor experience
A Kronecker-Based Sparse Compressive Sensing Matrix for Millimeter Wave Beam Alignment
Millimeter wave beam alignment (BA) is a challenging problem especially for
large number of antennas. Compressed sensing (CS) tools have been exploited due
to the sparse nature of such channels. This paper presents a novel
deterministic CS approach for BA. Our proposed sensing matrix which has a
Kronecker-based structure is sparse, which means it is computationally
efficient. We show that our proposed sensing matrix satisfies the restricted
isometry property (RIP) condition, which guarantees the reconstruction of the
sparse vector. Our approach outperforms existing random beamforming techniques
in practical low signal to noise ratio (SNR) scenarios.Comment: Accepted to 13th International Conference on Signal Processing and
Communication Systems (ICSPCS'2019
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