172 research outputs found
Why Does a Kronecker Model Result in Misleading Capacity Estimates?
Many recent works that study the performance of multi-input multi-output
(MIMO) systems in practice assume a Kronecker model where the variances of the
channel entries, upon decomposition on to the transmit and the receive
eigen-bases, admit a separable form. Measurement campaigns, however, show that
the Kronecker model results in poor estimates for capacity. Motivated by these
observations, a channel model that does not impose a separable structure has
been recently proposed and shown to fit the capacity of measured channels
better. In this work, we show that this recently proposed modeling framework
can be viewed as a natural consequence of channel decomposition on to its
canonical coordinates, the transmit and/or the receive eigen-bases. Using tools
from random matrix theory, we then establish the theoretical basis behind the
Kronecker mismatch at the low- and the high-SNR extremes: 1) Sparsity of the
dominant statistical degrees of freedom (DoF) in the true channel at the
low-SNR extreme, and 2) Non-regularity of the sparsity structure (disparities
in the distribution of the DoF across the rows and the columns) at the high-SNR
extreme.Comment: 39 pages, 5 figures, under review with IEEE Trans. Inform. Theor
Improving Bandwidth Efficiency in E-band Communication Systems
The allocation of a large amount of bandwidth by regulating bodies in the
70/80 GHz band, i.e., the E-band, has opened up new potentials and challenges
for providing affordable and reliable Gigabit per second wireless
point-to-point links. This article first reviews the available bandwidth and
licensing regulations in the E-band. Subsequently, different propagation
models, e.g., the ITU-R and Cane models, are compared against measurement
results and it is concluded that to meet specific availability requirements,
E-band wireless systems may need to be designed with larger fade margins
compared to microwave systems. A similar comparison is carried out between
measurements and models for oscillator phase noise. It is confirmed that phase
noise characteristics, that are neglected by the models used for narrowband
systems, need to be taken into account for the wideband systems deployed in the
E-band. Next, a new multi-input multi-output (MIMO) transceiver design, termed
continuous aperture phased (CAP)-MIMO, is presented. Simulations show that
CAP-MIMO enables E-band systems to achieve fiber-optic like throughputs.
Finally, it is argued that full-duplex relaying can be used to greatly enhance
the coverage of E-band systems without sacrificing throughput, thus,
facilitating their application in establishing the backhaul of heterogeneous
networks.Comment: 16 pages, 6 Figures, Journal paper. IEEE Communication Magazine 201
Exploiting Channel Diversity in Secret Key Generation from Multipath Fading Randomness
We design and analyze a method to extract secret keys from the randomness
inherent to wireless channels. We study a channel model for multipath wireless
channel and exploit the channel diversity in generating secret key bits. We
compare the key extraction methods based both on entire channel state
information (CSI) and on single channel parameter such as the received signal
strength indicators (RSSI). Due to the reduction in the degree-of-freedom when
going from CSI to RSSI, the rate of key extraction based on CSI is far higher
than that based on RSSI. This suggests that exploiting channel diversity and
making CSI information available to higher layers would greatly benefit the
secret key generation. We propose a key generation system based on low-density
parity-check (LDPC) codes and describe the design and performance of two
systems: one based on binary LDPC codes and the other (useful at higher
signal-to-noise ratios) based on four-ary LDPC codes
Key Generation Using External Source Excitation: Capacity, Reliability, and Secrecy Exponent
We study the fundamental limits to secret key generation from an excited
distributed source (EDS). In an EDS a pair of terminals observe dependent
sources of randomness excited by a pre-arranged signal. We first determine the
secret key capacity for such systems with one-way public messaging. We then
characterize a tradeoff between the secret key rate and exponential bounds on
the probability of key agreement failure and on the secrecy of the key
generated. We find that there is a fundamental tradeoff between reliability and
secrecy.
We then explore this framework within the context of reciprocal wireless
channels. In this setting, the users transmit pre-arranged excitation signals
to each other. When the fading is Rayleigh, the observations of the users are
jointly Gaussian sources. We show that an on-off excitation signal with an
SNR-dependent duty cycle achieves the secret key capacity of this system.
Furthermore, we characterize a fundamental metric -- minimum energy per key bit
for reliable key generation -- and show that in contrast to conventional AWGN
channels, there is a non-zero threshold SNR that achieves the minimum energy
per key bit. The capacity achieving on-off excitation signal achieves the
minimum energy per key bit at any SNR below the threshold. Finally, we build
off our error exponent results to investigate the energy required to generate a
key using a finite block length. Again we find that on-off excitation signals
yield an improvement when compared to constant excitation signals. In addition
to Rayleigh fading, we analyze the performance of a system based on binary
channel phase quantization.Comment: accepted for publication, IEEE Transactions on Information Theor
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