8,831 research outputs found
Information Theoretic Operating Regimes of Large Wireless Networks
In analyzing the point-to-point wireless channel, insights about two
qualitatively different operating regimes--bandwidth- and power-limited--have
proven indispensable in the design of good communication schemes. In this
paper, we propose a new scaling law formulation for wireless networks that
allows us to develop a theory that is analogous to the point-to-point case. We
identify fundamental operating regimes of wireless networks and derive
architectural guidelines for the design of optimal schemes.
Our analysis shows that in a given wireless network with arbitrary size,
area, power, bandwidth, etc., there are three parameters of importance: the
short-distance SNR, the long-distance SNR, and the power path loss exponent of
the environment. Depending on these parameters we identify four qualitatively
different regimes. One of these regimes is especially interesting since it is
fundamentally a consequence of the heterogeneous nature of links in a network
and does not occur in the point-to-point case; the network capacity is {\em
both} power and bandwidth limited. This regime has thus far remained hidden due
to the limitations of the existing formulation. Existing schemes, either
multihop transmission or hierarchical cooperation, fail to achieve capacity in
this regime; we propose a new hybrid scheme that achieves capacity.Comment: 12 pages, 5 figures, to appear in IEEE Transactions on Information
Theor
Fundamental Constraints on Multicast Capacity Regions
Much of the existing work on the broadcast channel focuses only on the
sending of private messages. In this work we examine the scenario where the
sender also wishes to transmit common messages to subsets of receivers. For an
L user broadcast channel there are 2L - 1 subsets of receivers and
correspondingly 2L - 1 independent messages. The set of achievable rates for
this channel is a 2L - 1 dimensional region. There are fundamental constraints
on the geometry of this region. For example, observe that if the transmitter is
able to simultaneously send L rate-one private messages, error-free to all
receivers, then by sending the same information in each message, it must be
able to send a single rate-one common message, error-free to all receivers.
This swapping of private and common messages illustrates that for any broadcast
channel, the inclusion of a point R* in the achievable rate region implies the
achievability of a set of other points that are not merely component-wise less
than R*. We formerly define this set and characterize it for L = 2 and L = 3.
Whereas for L = 2 all the points in the set arise only from operations relating
to swapping private and common messages, for L = 3 a form of network coding is
required
Interference Mitigation Through Limited Receiver Cooperation: Symmetric Case
Interference is a major issue that limits the performance in wireless
networks, and cooperation among receivers can help mitigate interference by
forming distributed MIMO systems. The rate at which receivers cooperate,
however, is limited in most scenarios. How much interference can one bit of
receiver cooperation mitigate? In this paper, we study the two-user Gaussian
interference channel with conferencing decoders to answer this question in a
simple setting. We characterize the fundamental gain from cooperation: at high
SNR, when INR is below 50% of SNR in dB scale, one-bit cooperation per
direction buys roughly one-bit gain per user until full receiver cooperation
performance is reached, while when INR is between 67% and 200% of SNR in dB
scale, one-bit cooperation per direction buys roughly half-bit gain per user.
The conclusion is drawn based on the approximate characterization of the
symmetric capacity in the symmetric set-up. We propose strategies achieving the
symmetric capacity universally to within 3 bits. The strategy consists of two
parts: (1) the transmission scheme, where superposition encoding with a simple
power split is employed, and (2) the cooperative protocol, where
quantize-binning is used for relaying.Comment: To appear in IEEE Information Theory Workshop, Taormina, October
2009. Final versio
Interference Mitigation Through Limited Receiver Cooperation
Interference is a major issue limiting the performance in wireless networks.
Cooperation among receivers can help mitigate interference by forming
distributed MIMO systems. The rate at which receivers cooperate, however, is
limited in most scenarios. How much interference can one bit of receiver
cooperation mitigate? In this paper, we study the two-user Gaussian
interference channel with conferencing decoders to answer this question in a
simple setting. We identify two regions regarding the gain from receiver
cooperation: linear and saturation regions. In the linear region receiver
cooperation is efficient and provides a degrees-of-freedom gain, which is
either one cooperation bit buys one more bit or two cooperation bits buy one
more bit until saturation. In the saturation region receiver cooperation is
inefficient and provides a power gain, which is at most a constant regardless
of the rate at which receivers cooperate. The conclusion is drawn from the
characterization of capacity region to within two bits. The proposed strategy
consists of two parts: (1) the transmission scheme, where superposition
encoding with a simple power split is employed, and (2) the cooperative
protocol, where one receiver quantize-bin-and-forwards its received signal, and
the other after receiving the side information decode-bin-and-forwards its
received signal.Comment: Submitted to IEEE Transactions on Information Theory. 69 pages, 14
figure
Channel Uncertainty in Ultra Wideband Communication Systems
Wide band systems operating over multipath channels may spread their power
over bandwidth if they use duty cycle. Channel uncertainty limits the
achievable data rates of power constrained wide band systems; Duty cycle
transmission reduces the channel uncertainty because the receiver has to
estimate the channel only when transmission takes place. The optimal choice of
the fraction of time used for transmission depends on the spectral efficiency
of the signal modulation. The general principle is demonstrated by comparing
the channel conditions that allow different modulations to achieve the capacity
in the limit. Direct sequence spread spectrum and pulse position modulation
systems with duty cycle achieve the channel capacity, if the increase of the
number of channel paths with the bandwidth is not too rapid. The higher
spectral efficiency of the spread spectrum modulation lets it achieve the
channel capacity in the limit, in environments where pulse position modulation
with non-vanishing symbol time cannot be used because of the large number of
channel paths
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