126 research outputs found
Multihop Diversity in Wideband OFDM Systems: The Impact of Spatial Reuse and Frequency Selectivity
The goal of this paper is to establish which practical routing schemes for
wireless networks are most suitable for wideband systems in the power-limited
regime, which is, for example, a practically relevant mode of operation for the
analysis of ultrawideband (UWB) mesh networks. For this purpose, we study the
tradeoff between energy efficiency and spectral efficiency (known as the
power-bandwidth tradeoff) in a wideband linear multihop network in which
transmissions employ orthogonal frequency-division multiplexing (OFDM)
modulation and are affected by quasi-static, frequency-selective fading.
Considering open-loop (fixed-rate) and closed-loop (rate-adaptive) multihop
relaying techniques, we characterize the impact of routing with spatial reuse
on the statistical properties of the end-to-end conditional mutual information
(conditioned on the specific values of the channel fading parameters and
therefore treated as a random variable) and on the energy and spectral
efficiency measures of the wideband regime. Our analysis particularly deals
with the convergence of these end-to-end performance measures in the case of
large number of hops, i.e., the phenomenon first observed in \cite{Oyman06b}
and named as ``multihop diversity''. Our results demonstrate the realizability
of the multihop diversity advantages in the case of routing with spatial reuse
for wideband OFDM systems under wireless channel effects such as path-loss and
quasi-static frequency-selective multipath fading.Comment: 6 pages, to be published in Proc. 2008 IEEE International Symposium
on Spread Spectrum Techniques and Applications (IEEE ISSSTA'08), Bologna,
Ital
Multiaccess Channels with State Known to Some Encoders and Independent Messages
We consider a state-dependent multiaccess channel (MAC) with state
non-causally known to some encoders. We derive an inner bound for the capacity
region in the general discrete memoryless case and specialize to a binary
noiseless case. In the case of maximum entropy channel state, we obtain the
capacity region for binary noiseless MAC with one informed encoder by deriving
a non-trivial outer bound for this case. For a Gaussian state-dependent MAC
with one encoder being informed of the channel state, we present an inner bound
by applying a slightly generalized dirty paper coding (GDPC) at the informed
encoder that allows for partial state cancellation, and a trivial outer bound
by providing channel state to the decoder also. The uninformed encoders benefit
from the state cancellation in terms of achievable rates, however, appears that
GDPC cannot completely eliminate the effect of the channel state on the
achievable rate region, in contrast to the case of all encoders being informed.
In the case of infinite state variance, we analyze how the uninformed encoder
benefits from the informed encoder's actions using the inner bound and also
provide a non-trivial outer bound for this case which is better than the
trivial outer bound.Comment: Accepted to EURASIP Journal on Wireless Communication and Networking,
Feb. 200
Writing on Dirty Paper with Resizing and its Application to Quasi-Static Fading Broadcast Channels
This paper studies a variant of the classical problem of ``writing on dirty
paper'' in which the sum of the input and the interference, or dirt, is
multiplied by a random variable that models resizing, known to the decoder but
not to the encoder. The achievable rate of Costa's dirty paper coding (DPC)
scheme is calculated and compared to the case of the decoder's also knowing the
dirt. In the ergodic case, the corresponding rate loss vanishes asymptotically
in the limits of both high and low signal-to-noise ratio (SNR), and is small at
all finite SNR for typical distributions like Rayleigh, Rician, and Nakagami.
In the quasi-static case, the DPC scheme is lossless at all SNR in terms of
outage probability. Quasi-static fading broadcast channels (BC) without
transmit channel state information (CSI) are investigated as an application of
the robustness properties. It is shown that the DPC scheme leads to an outage
achievable rate region that strictly dominates that of time division.Comment: To appear in IEEE International Symposium on Information Theory 200
Beamforming Codebook Compensation for Beam Squint with Channel Capacity Constraint
Analog beamforming with phased arrays is a promising technique for 5G
wireless communication in millimeter wave bands. A beam focuses on a small
range of angles of arrival or departure and corresponds to a set of fixed phase
shifts across frequency due to practical hardware constraints. In switched
beamforming, a discrete codebook consisting of multiple beams is used to cover
a larger angle range. However, for sufficiently large bandwidth, the gain
provided by the phased array is frequency dependent even if the radiation
pattern of the antenna elements is frequency independent, an effect called beam
squint. This paper shows that the beam squint reduces channel capacity of a
uniform linear array (ULA). The beamforming codebook is designed to compensate
for the beam squint by imposing a channel capacity constraint. For example, our
codebook design algorithm can improve the channel capacity by 17.8% for a ULA
with 64 antennas operating at bandwidth of 2.5 GHz and carrier frequency of 73
GHz. Analysis and numerical examples suggest that a denser codebook is required
to compensate for the beam squint compared to the case without beam squint.
Furthermore, the effect of beam squint is shown to increase as bandwidth
increases, and the beam squint limits the bandwidth given the number of
antennas in the array.Comment: 5 pages, to be published in Proc. IEEE ISIT 2017, Aachen, German
Cooperative Relaying with State Available Non-Causally at the Relay
We consider a three-terminal state-dependent relay channel with the channel
state noncausally available at only the relay. Such a model may be useful for
designing cooperative wireless networks with some terminals equipped with
cognition capabilities, i.e., the relay in our setup. In the discrete
memoryless (DM) case, we establish lower and upper bounds on channel capacity.
The lower bound is obtained by a coding scheme at the relay that uses a
combination of codeword splitting, Gel'fand-Pinsker binning, and
decode-and-forward relaying. The upper bound improves upon that obtained by
assuming that the channel state is available at the source, the relay, and the
destination. For the Gaussian case, we also derive lower and upper bounds on
the capacity. The lower bound is obtained by a coding scheme at the relay that
uses a combination of codeword splitting, generalized dirty paper coding, and
decode-and-forward relaying; the upper bound is also better than that obtained
by assuming that the channel state is available at the source, the relay, and
the destination. In the case of degraded Gaussian channels, the lower bound
meets with the upper bound for some special cases, and, so, the capacity is
obtained for these cases. Furthermore, in the Gaussian case, we also extend the
results to the case in which the relay operates in a half-duplex mode.Comment: 62 pages. To appear in IEEE Transactions on Information Theor
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